Ruminant Physiology Digestion Metabolism, Growth
Ruminant Physiology Digestion Metabolism, Growth
Ruminant Physiology Digestion Metabolism, Growth
RUMINANT PHYSIOLOGY
Digestion, Metabolism, Growth and
Reproduction
Dedication
This volume is dedicated to the memory of the late Dr F.M.C. Gilchrist.
RUMINANT PHYSIOLOGY
Digestion, Metabolism, Growth
and Reproduction
Edited by
P.B. Cronj
Department of Animal and Wildlife Sciences
University of Pretoria
Pretoria
South Africa
Associate Editors
E.A. Boomker
P.H. Henning
W. Schultheiss
J.G. van der Walt
CABI Publishing
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CAB International 2000. All rights reserved. No part of this publication may be reproduced in any form or by any means, electronically, mechanically, by
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A catalogue record for this book is available from the British Library, London, UK
Library of Congress Cataloging-in-Publication Data
Ruminant physiology : digestion, metabolism, growth, and reproduction / edited by
P. Cronje ; assoc. editors, E.A. Boomker [et al.].
p. cm.
Includes bibliographical references and index.
ISBN 0-85199-463-6 (alk. paper)
1. Ruminants- - Physiology- - Congresses. I. Cronj, P. (Pierre) II. Boomker, E. A.
QL737.U5 R868 2000
571.1963- - dc21
00023661
00 Ruminant Prelims
Contents
Contributors
Foreword
Part I Regulation of Feed Intake
1 Integration of Learning and Metabolic Signals into a Theory of
Dietary Choice and Food Intake
J.M. Forbes and F.D. Provenza
2 Mathematical Models of Food Intake and Metabolism in Ruminants
A.W. Illius, N.S. Jessop and M. Gill
3 Control of Salivation and Motility of the Reticulorumen by the Brain
in Sheep
W.L. Grovum and J.S. Gonzalez
Part II Rumen Microbiology and Fermentation
ix
xiii
1
3
21
41
59
61
79
99
115
117
v
Contents
vi
131
149
169
171
187
205
225
227
237
255
273
275
295
311
329
353
Contents
vii
371
373
389
409
423
425
437
449
Index
463
Contributors
N.R. Adams, CSIRO Division of Animal Production and CRC for Premium Quality
Wool, Wembley, 6014 Western Australia
R.I. Aminov, Department of Animal Sciences and Division of Nutritional Sciences,
University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA
J.J. Bass, AgResearch, Ruakura Agricultural Research Centre, Private Bag 3123, Hamilton,
New Zealand
D.E. Bauman, Department of Animal Science, Cornell University, Ithaca, NY 14853,
USA
C.R. Baumrucker, Department of Dairy and Animal Science, Penn State University, 302
Henning Building, University Park, PA 16802, USA
A.W. Bell, Department of Animal Science, Cornell University, Ithaca, NY 14853, USA
F. Bocquier, Adipose Tissue and Milk Lipids Team, Herbivore Research Unit, INRATheix, 63122 St Gens Champanelle, France
J.M. Brameld, Division of Nutritional Biochemistry, School of Biological Sciences,
University of Nottingham, Sutton Bonington Campus, Loughborough, Leicestershire
LEI2 5RD, UK
B.H. Breier, Research Centre for Developmental Medicine and Biology, Faculty of
Medicine and Health Science, University of Auckland, Private Bag 92019, Auckland,
New Zealand
P.J. Buttery, Division of Nutritional Biochemistry, School of Biological Sciences, University
of Nottingham, Sutton Bonington Campus, Loughborough, Leicestershire LE12 5RD,
UK
A. Chesson, Rowett Research Institute, Greenburn Road, Bucksburn, Aberdeen AB21 9SB,
UK
Y. Chilliard, Adipose Tissue and Milk Lipids Team, Herbivore Research Unit, INRATheix, 63122 St Gens Champanelle, France
I.G. Colditz, CSIRO Animal Production, Pastoral Research Laboratory, Armidale, NSW
2350, Australia
ix
00 Ruminant Prelims
Contributors
P.B. Cronj, Department of Animal and Wildlife Sciences, University of Pretoria, Pretoria
0002, South Africa
J.M. Dawson, Division of Nutritional Biochemistry, School of Biological Sciences,
University of Nottingham, Sutton Bonington Campus, Loughborough, Leicestershire
LE12 5RD, UK
R.A. Ehrhardt, Department of Animal Science, Cornell University, Ithaca, NY 14853,
USA
D.L. Emery, CSIRO Animal Production, McMaster Laboratory, Prospect, NSW 2148,
Australia
A. Faurie, Department of Physiology and Brain Function Research Unit, University of the
Witwatersrand, 7 York Road, Parktown, Johannesburg 2193, South Africa
H.J. Flint, Rowett Research Institute, Greenburn Road, Bucksburn, Aberdeen AB21 9SB,
UK
E. Forano, Laboratoire de Microbiologie, INRA CR de Clermont-Ferrand Theix, 63122
St Gens Champanelle, France
J.M. Forbes, Centre for Animal Sciences, Leeds Institute of Biotechnology and Agriculture,
University of Leeds, Leeds LS2 9JT, UK
C.W. Forsberg, Department of Microbiology, University of Guelph, Guelph, Ontario
N1G 2W1, Canada
B.W. Gallaher, Research Centre for Developmental Medicine and Biology, Faculty of
Medicine and Health Science, University of Auckland, Private Bag 92019, Auckland,
New Zealand
M. Gill, NR International, Central Avenue, Chatham Maritime, Kent ME4 4TB, UK
J.S. Gonzalez, Department of Animal Production, University of Leon, 24071 Leon, Spain
W.L. Grovum, Department of Biomedical Sciences, Ontario Veterinary College, University
of Guelph, Guelph, Ontario, N1G 2W1, Canada
K.L. Houseknecht, Animal Health Drug Discovery, Pfizer Inc., Groton, CT 063408002, USA
A.W. Illius, Division of Biological Sciences, University of Edinburgh, West Mains Road,
Edinburgh EH9 3JT, UK
N.S. Jessop, Division of Biological Sciences, University of Edinburgh, West Mains Road,
Edinburgh EH9 3JT, UK
R. Kambadur, AgResearch, Ruakura Agricultural Research Centre, Private Bag 3123,
Hamilton, New Zealand
H. Laburn, Department of Physiology and Brain Function Research Unit, University of
the Witwatersrand, 7 York Road, Parktown, Johannesburg 2193, South Africa
S. Liu, CSIRO Division of Animal Production and CRC for Premium Quality Wool,
Wembley, 6014 Western Australia
G.E. Lobley, Rowett Research Institute, Greenburn Road, Bucksburn, Aberdeen AB21
9SB, UK
R.I. Mackie, Department of Animal Sciences and Division of Nutritional Sciences,
University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA
D.G. Masters, CSIRO Division of Animal Production and CRC for Premium Quality
Wool, Wembley, 6014 Western Australia
S.J. McClure, CSIRO Animal Production, McMaster Laboratory, Prospect, NSW 2148,
Australia
J.P. McNamara, Department of Animal Sciences, 233 Clark Hall, Washington State
University, PO Box 646351, Pullman, WA 99164-6351, USA
Contributors
xi
Foreword
xiv
Foreword
(WAAP Vice-president), Dr Heinz Meissner, Prof. Y Obara and Prof. Wolfgang von
Engelhardt. The committee formulated a number of guiding principles for future ISRP
meetings. These are: that the meeting should retain the character of previous symposia;
that the focus should remain on the physiology of livestock, reviewing advances over
the previous 5 years and setting directions for the next period; that comparative physiology and the impact of advances in physiology on products and sensitive consumer
issues are important; that the venue for meetings should be situated where there is a
core of established ruminant physiologists who could organize the symposium and, in
particular, attend to the scientific programme and publish the proceedings; that the
symposium should be easily accessible to young scientists and scientifically developing
communities. After considering several invitations and taking the principles agreed
upon into consideration, the hosting of the X ISRP was awarded to Denmark.
Norman H. Casey
(Chairman: Organizing Committee of the IX ISRP)
Introduction
The challenge of understanding how diet selection and food intake are controlled is
one that occupies an important place in the fields of nutrition, physiology and psychology. In the case of ruminant animals there are two special reasons for our interest in the
subject: the complexities of the digestive system and consequent metabolic peculiarities; and the agricultural and ecological importance of the sub-order. Despite several
decades of intensive study there is still no consensus on how intake is controlled
(Fisher, 1996), nor is there agreement about the way in which animals determine which
food(s) to eat when a choice is available. The past few years have seen the publication of
sufficient new evidence to allow us to advance our hypotheses about the control of food
intake and diet selection.
Firstly we review advances in our understanding of the role of learning in determining preferences and aversions for foods by ruminants; we then summarize the ways
in which the central nervous system (CNS) is informed about digestive and metabolic
processes; discuss the day-to-day variation in intake as an enabling factor in the linking
of learning with the physiological consequences of eating; and finally propose how
learning and metabolic information are brought together to provide testable hypotheses
of the control of diet selection and voluntary food intake. We take it as axiomatic that
long- and short-term regulation of intake are interwoven and do not attempt to differentiate between the two.
Ruminants learn preferences for a food flavour associated with infusions that correct
deficiency; the same nutrient given to excess leads to avoidance of the associated
flavour
In order to demonstrate unequivocally that such appetites are dependent on learned
associations between the sensory properties of the foods and their nutritive value it is
necessary to divorce the flavour of the food from its yield of nutrients. This can be done
by offering animals a distinctive food and at the same time giving a nutrient by a route
that bypasses the mouth, usually intraruminal infusion. In one such experiment with
lambs (Villalba and Provenza, 1997a) one flavour was paired with rumen infusion of
starch (2.59.4% of daily digestible energy (DE) intake) and another flavour with
control. Subsequent preference was strongly for the starch-paired flavour, even 8
weeks after infusions had stopped. Starch is rapidly fermented to volatile fatty acids,
predominantly propionic, in the rumen. Propionate absorption is likely to be insufficient for glucose synthesis in straw-fed animals so the hypothesis was tested that the
supply of this limiting nutrient would induce a preference for the flavour of food eaten
during supplementation (Villalba and Provenza, 1996). Even though the propionate
supplied was equivalent to no more than 1.4% of the daily metabolizable energy (ME)
intake, after 8 days of conditioning the sheep had developed a strong preference for
food flavoured with that flavour given during supplementation. It was shown that the
preference was induced by the propionate rather than the sodium or osmolality of the
infusions (Villalba and Provenza, 1996; Villalba and Provenza, 1997b).
We can conclude that a single nutrient can induce a preference or an aversion to
the flavour it was paired with during training, depending on the rate of administration
in relation to the animals requirements.
Ratio in which nutrients are supplied by different foods affects dietary choice
There are some situations in which the ratio of nutrients being absorbed from the digestive tract is such as to induce metabolic imbalance. When acetate and propionate were
infused together into the rumen of sheep, conditioned preferences were demonstrated
for the associated flavoured wheat straw but the preference was greater when the ratio of
acetate:propionate in the infusate was 55:45 than when it was 75:25 (Villalba and
Provenza, 1997b). It is likely that straw-fed lambs, with a high ratio of acetate:propionate produced by normal ruminal fermentation, would be deficient in glucose and this
would be better alleviated by the mixture with the higher proportion of propionate.
It has been proposed that certain types of diet provide imbalances between energy
and protein supply at different times of day, even though they may be balanced overall.
Kyriazakis and Oldham (1997) set out to test whether such asynchrony of nutrient
supply would influence diet selection, relative to a food designed to provide the same
nutrient supply in a synchronous manner. Foods were formulated to provide rapidlyor slowly-fermentable energy with high or low rumen degradable protein (RDP) all
foods had the same calculated contents of ME and metabolizable protein (MP). When
choices were offered, the proportion of the low-RDP food in the selected diet was
lower when the carbohydrate source was rapidly rather than slowly fermentable,
which is consistent with the hypothesis that ruminants learn to select a mixture of
foods that minimizes metabolic imbalance.
Lambs discriminate between the post-ingestive effects of energy and protein and
associate those effects with a foods flavour to modify food choices (J.J. Villalba and
F.D. Provenza, personal communication). Lambs acquire a preference for a poorly
nutritious flavoured food eaten during intraruminal infusions of energy (starch) or
protein (casein), and shortly after an intraruminal infusion of energy or protein (preload), lambs decrease their preferences for the flavour previously conditioned with
starch or casein, respectively. Preloads of casein decreased preferences for flavours previously paired with casein, and increased preferences for flavours paired with starch.
Preloads of energy had the opposite effect. These results show that lambs discriminate
between the post-ingestive effects of energy (starch) and protein (casein) and associate
the effects with specific external cues (i.e. added flavours) to regulate macronutrient
ingestion.
There is thus accumulating evidence that ruminants prefer to avoid a food, with
an adequate nutrient balance overall, that delivers different nutrients at different rates
and results in temporary imbalances.
wise they have no function), but also to be flexible (otherwise an animal might be
saddled with an unnecessary aversion for the rest of its life).
Trade-offs
There are many situations in which the animal must balance its intake or choice of
foods in order to trade off the intake of a toxin against the need to obtain nutrients
(bearing in mind that, in general, the only difference between a toxin and a nutrient is
the concentration in the diet). For example, lambs preferred barley to lucerne pellets in
the absence of added toxin (LiCl), but this preference was reversed the higher the content of LiCl in the barley (Wang and Provenza, 1997). Intake of foods containing a
constant concentration of LiCl increased as the proportion of barley in the food
increased, i.e. animals were better able to tolerate a higher concentration of toxin the
higher the yield of energy per kg. Likewise, sheep fed diets high in sagebrush, which
contains various terpenes, eat more sagebrush as the macronutrient content of their
diet increases (R.E. Banner, J. Rogosic, E.A. Burritt and F.D. Provenza, personal communication). When this finding is translated to the natural situation, in which both
toxin and nutrient contents of plants vary with season and location, it emphasizes how
nutrients interact to influence food intake. It also illustrates the ability of the body to
integrate signals from nutrients and those from toxins.
Many forages contain toxic phenolics but these are sometimes the available plant
species with the highest yield of digestible nutrients and grazing animals must trade
nutrients off against toxins. When mule deer were offered pairs of foods with different
phenolic:digestible energy ratios they ate high-energy foods when these were low in
phenolics, but low-energy foods when the high-energy ones were high in phenolics
(McArthur et al., 1993). Similarly, goats chose to eat more low-digestibility blackbrush
twigs when the high-digestibility twigs were high in toxin (either naturally-occurring
condensed tannins or added LiCl) (Provenza et al., 1994a). Also, lambs preferred foods
higher in readily-available carbohydrate but when a high-energy food had tannin added
to it then preference switched to the lower-energy food (C.H. Titus, F.D. Provenza,
E.A. Burritt, A. Perevolotsky and N. Silanikove, personal communication). When
polyethylene glycol (PEG) was given each morning lambs also shifted their preferences
to lower-energy foods as tannin levels increased, but to a significantly lesser extent than
their counterparts which did not receive the PEG supplement. Those supplemented
with PEG ate more than unsupplemented animals of the tannin-containing foods, particularly as the abundance of nutritious alternatives diminished.
Hutchings et al. (1999) have shown how lambs trade off high nutritive value of
herbage against the risk of being parasitized with Ostertagia; animals which had been
feed-restricted took greater risks of parasitism in order to obtain more nutritious grass
than unrestricted lambs.
It can be concluded, therefore, that diet selection is directed towards stabilizing
conditions in the rumen and the rest of the body, i.e. avoiding disease, metabolic
imbalance and upset. It is also presumably directed against over-stimulation of the
rumen by physical means, although an attempt to condition a flavour aversion by inflation of a balloon in the rumen of cows did not succeed (Klaiss and Forbes, 1999).
Visceral organs
From the mouth to the anus, food and digesta provide a continuously changing set of
stimuli to stretch and tactile receptors. The complexity of the information provided to
the CNS is formidable even though there is considerable convergence in the afferent
branches of the autonomic nervous system which relays the impulses (Forbes, 1996).
The seminal work of Leek and colleagues, summarized by Forbes and Barrio
(1992), showed how physical and chemical stimulation of receptors in the digestive
tract activates vagal afferent pathways to the gastric centre of the medulla oblongata. In
addition, the liver is sensitive to propionate, once again relaying its information to the
CNS via autonomic afferents (Anil and Forbes, 1987) and providing a comprehensive
assessment of the energy supply via the liver.
If a simple experimental procedure such as inflating a balloon in the rumen, or
infusing the salt of a volatile fatty acid at a constant rate, has complex effects then how
much more complex are the effects of the passage of a meal through the digestive tract,
with ever-changing physical and chemical properties? It seems unlikely that the CNS
could interpret individually the nature and degree of stimulation of each receptor as
there is both polymodal and polytopic integration of visceral signals with convergence
in hind, mid and fore brains (Forbes, 1996). Thus, severe stimulation of receptors in a
specific part of the digestive tract is felt as a dull, mid-line sensation by humans so presumably milder stimulation by physiological stimuli are also not ascribed by the CNS
to specific sites in the abdomen.
It is not necessary to be consciously aware of abdominal discomfort in order for
the CNS to be able to respond to events in gut and liver, i.e. to participate appropriately in the control of gut motility and secretion, and to remember the sensory properties of the food eaten shortly before the discomfort occurred (Provenza et al., 1994b;
Forbes, 1998). Abdominal pain is likely to be an extension of normal sensitivity to
stimuli such as distension of a viscus or irritation of a mucosal surface by chemicals so
that metabolic imbalance is relayed as a mild form of toxicity.
Body tissues
Some parts of the CNS, such as the nucleus of the solitary tract (NTS) and hypothalamus, are sensitive to a shortage of available energy (Grill, 1986). Starving cells of glucose by treatment with 2-deoxy-D-glucose, which blocks glucose uptake by cells, has
shown this. An excess of glucose in the blood does not influence the NTS activity, only
a shortage. For the rest of the body to fail to protect the CNS from starvation must be
seen as an unusual and serious situation and one that would be likely to be a potent
conditioning stimulus to eat more food in future.
At a broader level, adipose tissue has recently been shown to produce a hormone,
leptin or ob-protein, increasingly as adipocyte size increases. Leptin is taken up in the
CNS where it inhibits feeding via the neuropeptide Y system (Houseknecht et al.,
1998). It is reasonable to assume, for the time being, that any negative feedback due to
leptin is integrated with stimuli emanating from the viscera so that the normal meal-tomeal regulation is modulated to a slight extent, but one which, being persistent, results
in significant long-term reduction in intake in obese animals. We see no need to consider short-term and long-term controls of intake as being different in nature, just in
the rate of change of the signals involved.
No muscle hormone equivalent to leptin has so far been found and it seems likely
that protein deposition exerts its effect on feeding by taking amino acids out of the circulation and thus influencing liver metabolism.
10
over-stimulation of gut and liver receptors, and long-term excesses result in over-production of leptin, which generate a state of metabolic discomfort, otherwise known as
satiety. Conversely, under-stimulation of liver and hind-brain receptors due to undereating (relative to demands) generates a different type of metabolic discomfort, especially when coupled with low levels of leptin, usually known as hunger. We view the
range from extreme hunger to excessive satiety as a continuum. The animal directs its
behaviour to achieving the most comfortable situation, in which comfort is not only
induced by the appropriate supply of nutrients, but also by social pressures to eat or not
to eat, and by learned associations (see above).
The anatomical and physiological mechanisms underlying affective and cognitive
systems have been fairly well established (Provenza et al., 1998). Taste afferents converge with visceral afferents in the solitary nucleus of the brain stem and proceed to the
limbic system, where the hypothalamus and related structures maintain homeostasis in
the internal environment through the endocrine system, the autonomic nervous system, and the neural system concerned with motivation and drive (i.e. incentive modification). Higher cortical centres interact with the hypothalamus through the limbic
system, and regulate the internal environment primarily by indirect action on the external environment (i.e. behaviour modification). These alternative means of regulating
the internal environment generally function in parallel. For example, the taste of food
is adjusted according to that foods effect on the internal environment; on this basis,
animals use thalamic and cortical mechanisms to select foods that are nutritious and
avoid those that are toxic.
Given a choice of foods, and the ability to learn to eat that ratio between the foods
which balances nutrient supply with demand, we interpret the evidence presented
above to suggest that animals adjust their food choice to minimize metabolic discomfort. Thus, in meeting its requirements it is not fulfilling some long-term goal, but
rather reacting to hard- and soft-wired programmes, i.e. signals from body organs and
tissues and the associations previously established between these signals and the sensory
properties of the food (Provenza et al., 1998).
If nutrient requirements change then level of comfort changes and choice is adjusted
to regain a state of maximum comfort
If it is true that the animal eats to minimize discomfort then any change in the nutrient
uptake by tissues should lead to a change in the selection made between a choice of
foods.
Protein requirements are higher for pregnant than for non-pregnant ewes and
when Cooper et al. (1994) offered non-pregnant and late-pregnant ewes the choice
between high (HP) and low-protein (LP) foods, both of high energy content, they
found that the pregnant animals selected a significantly greater proportion of HP than
those not pregnant.
Lactating cows offered free access to grass silage and a restricted amount of concentrates, offered as a choice one with greater (HP) and another with less (LP) than the
estimated requirement for MP, ate a greater proportion of HP the higher their output
of milk protein (Lawson et al., 1999), i.e. they appeared to be directing their selection
between foods to supply their requirements for protein.
11
Growing lambs chose a diet well-matched to their requirements for growth (Hou
et al., 1991a) and worked to maintain this balanced diet when one of the foods
required up to 30 responses to obtain a reinforcement (Hou et al., 1991b).
Thus, diet selection is not only driven by the composition of the foods on offer,
but also by the requirements of the animal, which change in a systematic manner with
reproductive cycles.
What is true for choice between foods is, we propose, also true for a
single food
An animal experiments with different levels of intake of a single food until its
comfort levels are optimized
In much of the previous discussion we have concentrated on the control of dietary
choice, but we now propose that a logical extension of the proposal that dietary choice
is directed to minimizing metabolic discomfort, is that the intake of a single food is
also directed to achieving the same end. If our hypothesis is true then we should be able
to find evidence that animals given a single food experiment by increasing and
decreasing their intake in a cyclical manner in order to continually ascertain whether
their comfort is maximized.
12
A change in the composition of the food, or in the requirements for one or more
nutrients, results in a gradual re-learning of the amount to be eaten to re-establish
optimum comfort
Changes in food composition lead to changes in food intake. If the change is in the
nutritional value then the animals may well find that a higher or lower daily dry matter
(DM) intake gives it more comfort than simply continuing to eat the same daily weight
as before the change. On the other hand, if the sensory properties of the food change
without significant change in nutrient yield any change in intake level is usually shortlived. Conversely, a change in flavour causing a temporary drop in acceptability can be
masked by the inclusion of a familiar flavour (Frederick et al., 1988).
13
Change of diet
A particular situation in which it is necessary for animals to learn about food is when
the food to which they have been accustomed is suddenly changed to another with different sensory and nutritional properties. In most practical and experimental situations
sudden changes are avoided by a slow change of diet; in changeover experiments it is
usual to wait for animals to stabilize on a new diet before recommencing recording of
feeding behaviour. Mean daily intakes of 32 individual sheep accustomed to oaten
chaff and then suddenly changed to barley straw are shown in Fig. 1.1, together with
intakes of two individuals, selected at random (J. Hills, 1998, unpublished results). On
the first day after the change the sheep ate very little which suggests that they were not
familiar with the new food and were showing neophobia a wariness of new food in
which only small amounts are taken in order to assess whether there are unpleasant
consequences to eating it. A combination of increasing hunger and the realization that
the new food does not cause illness encouraged increased acceptance of the food and a
steadily increasing intake. On the 4th and 5th days there is a decline in intake, perhaps
due to rumen disturbance if the microflora has not adapted to the new food, followed
by a gradual climb to a plateau, stabilizing some 10 days after introduction of the barley straw. The two individuals shown on Fig. 1.1 conform quite well to the mean for all
32 animals, but with greater day-to-day fluctuations than the mean.
When the more stable data from the last 15 days were analysed, as for the cow data
above, there were again large day-to-day fluctuations in intake by individuals, in comparison with fluctuations in the mean for all animals. The mean standard deviation of
the residuals was 82, with a range from 31 to 162 g day21. Only in 15 out of the 32
individual sheep was daily intake in the last 15 days significantly (P < 0.05) correlated
with the daily mean intake of all 32 sheep. For the two individuals in Fig. 1.1, the one
represented by the dashed line was significantly correlated, but not the other. This
Fig. 1.1. Mean daily intakes for 32 sheep (solid line) and two individual sheep (lines
with symbols) selected at random; on 6 September the oaten chaff was replaced with
barley straw (J. Hills, unpublished results).
14
again is clear evidence for large variations in daily intake that are unrelated to external
factors such as food quality or climatic variables. Even if this day-to-day variation is not
purposeful, it still serves the animals intake control system by providing information
about whether a small increase or decrease in intake improves metabolic comfort.
15
excess. A third factor affecting intake in this model is neutral detergent fibre (NDF), as
an index of the bulk of the food and thus rumen fill. Fibre is included because an excess
inhibits food intake while the animals digestion suffers if there is a deficiency, in a
manner parallel to the way that energy and protein supply are proposed to generate discomfort when either greater or less than the rate at which they are being utilized by the
animal. Fig. 1.2 gives an example in which discomfort is plotted against intake of a typical forage. This illustrates how the sum of the discomforts due to several properties of
the food, in relation to the requirements of the animal, generates a total discomfort
signal which the animal attempts to minimize, according to the hypotheses presented
here. Iteration proceeds until the intake that produces the minimum metabolic discomfort is reached. Forbes (1999) describes the model in further detail.
When the ME, CP and NDF contents of the foods used by Kyriazakis and
Oldham (1993) are used as parameters in the model, together with the requirements of
their lambs, good agreement of daily intake was obtained for the high- and mediumprotein foods. Predicted intakes of the low-protein foods were much greater than
observed, however, suggesting that greater weighting be put on protein deficiency as a
contributor to metabolic discomfort and no doubt the weighting of the signals from
deviations in ME, CP and NDF will need refinement.
When two foods with different composition are available the model experiments
by changing the rates of intake of each until minimum discomfort is reached, thereby
predicting both daily food intake and the proportion of each of the two foods eaten.
The choices made by sheep when offered two foods with different protein contents in
the same experiment as that discussed in the previous paragraph (Kyriazakis and
Oldham, 1993) were modelled. Where possible the lambs chose foods in a ratio that
Fig. 1.2. Discomfort due to: l, metabolizable energy (ME); n, crude protein (CP);
, neutral detergent fibre (NDF); @, total; plotted against food intake. Discomfort is
the square of the weighted proportional deviation of the supply of each nutrient from
the animals requirement. In this example the requirements were those of a growing
lamb for 20 Mj ME, 250 g CP and 350 g NDF day21 and the forage food provided
10 Mj ME, 120 g CP and 600 g NDF kg21. Given the hypothesis that the animal eats
that amount of food at which total discomfort is minimized, the predicted intake in
this example is 1.2 kg day21.
16
gave approximately an optimum protein content in the diet while the model provides a
perfect dietary protein content and minimal metabolic discomfort with choice feeding. Choice-fed lambs were observed to eat more in total than similar sheep offered single foods while the model predicts daily intakes of single- and choice-fed animals to be
similar. Thus, the model behaves in a similar way to animals in some respects but not
in others.
A number of improvements are immediately apparent. The model could be refined
mathematically to avoid iteration in arriving at a solution; however, this iteration does,
according to our hypothesis, mimic what is going on in the animal. Many more qualities of the food (nutrients) should be included once there is a better understanding of
how they are involved in pathways that contribute to metabolic comfort/discomfort.
Feedbacks from body reserves (insulin, leptin) should be included as signals added to
those emanating from receptors in the digestive tract and liver to incorporate the
long-term aspects of intake control. We believe, however, that this novel approach to
simulating and predicting intake and choice by ruminants and other animals allows a
flexibility not previously possible in incorporating whatever level of detail in
metabolism is required by the user the form of the model presented here is the
simplest possible.
17
what individuals at the extremes which can be as much as half of the group (Provenza
et al., 1996) prefer and can tolerate. Diets that enable animals to select among foods
may better enable each individual to best meet its nutritional needs, and under some
circumstances, may lower daily feed costs.
Above all, we emphasize the need to acknowledge that metabolic factors, physical
factors and learning all have important roles to play in the complexities of the control
of food intake in ruminant animals. We need to recognize the true multifactorial nature
of the control of voluntary intake and diet selection if we are to advance understanding
and predictive ability.
Acknowledgements
The authors are grateful to EMBRAPA, Juiz de Fora, Brazil, and to James Hills,
University of New England, Australia, for access to their unpublished data of daily food
intakes of individual animals; also to Ilias Kyriazakis for critically reviewing the manuscript.
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Introduction
The daily rate of food intake is the single most important factor affecting animal performance and productive efficiency. Knowledge of food intake is necessary for diet formulation, for the prediction of animal performance, for the design and control of
production systems and for the assessment of animalresource interactions in grazing
ecosystems. Modelling the control of food intake and nutrient supply is a way of
furthering our understanding of mechanisms, of testing the consequences of our
assumptions, and of developing a mechanistic framework capable of accurate
prediction.
There are, broadly, two types of models: digesta kinetics and metabolic models.
The first type is concerned with the prediction of intake and digestion, on the assumption that rate of intake is limited by the rate of decrease in volume of rumen digesta by
digestion and passage. The second type is concerned with the production, absorption
and utilization of nutrients via microbial and animal metabolism. Whilst the former
type often relies on empirical estimates of parameters, and is mostly concerned with
prediction of intake and digestion, the latter type of model is rarely used to predict
intake, and indeed generally requires intake as a model input. Instead, it is most often
used in the pursuit of knowledge. We will exclude from this review statistical models
used for data analysis.
The distinction between these types of model goes back to the early 1970s, when
two models were published that had a profound influence on subsequent modelling:
Baldwin et al. (1970) and Waldo et al. (1972). These models are very different in complexity and represent divergent schools of thought. Baldwin et al. (1970) modelled the
rumen with chemically defined substrates and emphasized the stoichiometry of fermentation and the prediction of fermentation end-products. Digestion was a second-order
process affected by microbial mass. This model influenced many subsequent metabolic
models which describe the process of digestion as the appearance of fermentation endproducts, or are purely concerned with intermediary metabolism (Gill et al., 1984).
CAB International 2000. Ruminant Physiology: Digestion, Metabolism,
Growth and Reproduction (ed. P.B. Cronj)
21
22
The simple model of Waldo et al. (1972) was concerned with cellulose disappearance
rather than appearance of fermentation end-products, and partitioned cellulose into a
fraction that is potentially digestible by rumen microbes and an indigestible fraction.
The substrate was thus defined biologically rather than chemically, and the rates of
digestion and passage were assumed to be first order.
At present, the major uncertainty in the prediction of intake lies less in the cases
where intake is constrained by low digesta clearance rate than where voluntary intake is
limited by feedback from metabolic factors. Thus, variation in daily intake can more
clearly be related to the kinetics of digestion and passage, assessed on daily timescales,
than to shorter-term metabolic events. Voluntary intake is reduced by low diet
digestibility and passage rate, and it is therefore assumed that low rates of ruminal
digestion and passage lead to physical limitations on daily intake (Laredo and Minson,
1973). More highly digestible feeds can, potentially, be eaten in greater quantities
before the presumed physical constraints of gut clearance apply, and voluntary intake is
then more likely to be determined by metabolic constraints which are related to the
animals ability to utilize absorbed nutrients. The role of metabolic constraints in controlling intake of low-quality forages has been demonstrated (Egan, 1977), resulting in
reappraisal of whether physical constraints are the major determinant of intake of such
forages (Tolkamp and Ketelaars, 1992; Weston, 1996; Pitroff and Kothman, 1999).
Although the interaction between physical and metabolic factors is of evident interest
and importance for our understanding of the control food intake, there have been few
attempts to address it using modelling, perhaps because of the different time-scales and
data derivation methods adopted between the two modelling approaches outlined
above.
This chapter reviews the main themes and issues in modelling the processes governing food intake in ruminants. It addresses the two broad objectives of modelling:
making predictions and pursuing knowledge. An important application is to make
accurate predictions of intake and digestion for specified combinations of food and animal characteristics. Ruminant research is, in part, conducted in order that livestock
industries and society will benefit, and models are an effective means of consolidating
and applying diverse information from ruminant research programmes. However,
modelling is also an important investigative procedure in its own right. It allows us to
investigate mechanisms, test the consequences of our assumptions and hypotheses,
show where knowledge or data are lacking, and may be used to define hypotheses and
treatments in an experimental programme generating new data.
23
the systems behaviour with the smallest number of parameters required to reach the
prescribed level of accuracy. Far from attempting a thorough description of the underlying processes, the aim of this type of modelling is to achieve clarity by eliminating
unnecessary detail. On the other hand, mechanistic models attempt to represent the
underlying causal relationships, and are, in principle, capable of depicting the complexities of the system more faithfully. However, the depth of knowledge across all factors is
very uneven, and it should be remembered that increasing complexity is likely to
reduce accuracy and tractability. Certainly, any attempt at a complete representation of
every causal link is futile and has to be abandoned at some point. At that point, a
mechanistic model becomes phenomenological, and relies on empirical relationships.
For example, even the most detailed models of rumen function examined by Bannink
et al. (1997a) inevitably employ empirical descriptions at some point, for example to
describe particle dynamics and feed degradation characteristics. The point is often
made (Forbes and France, 1993) that a mechanistic model of some phenomenon
occurring at level i is one that may consist of empirical relationships at level i 2 1.
Despite what might be regarded as a weakness (Faichney, 1993), the use of empirical
descriptions in otherwise mechanistic models should be recognized as a strength: a representation of mechanisms is supported by a robust parameter description. The usual
proviso applies about only using empirical estimates within the range of the independent variables used to derive the estimate.
Rumen and intake modelling have been pursued by gradually increasing the
amount of detailed description of mechanisms, as they become elucidated. But the
important question of how much detail is necessary and justified in a model depends
on the models purpose. Accurate prediction requires robustness, and that requires simplicity and a sound basis for parameter estimation. Exploratory modelling justifies the
inclusion of more detail, and the use of parameters whose value cannot readily be estimated. Thus, the parameter values become the experimental variable, and the models
performance is then related to systematic variation in those values (Bannink et al.,
1997a).
Inadequate detail in models can lead to erroneous predictions. From consideration
only of the degradation rates of feed constituents it was predicted by Sinclair et al.
(1993) that asynchrony in the supply of fermentable nitrogen and carbohydrate would
cause rumen microbes to be limited by the supply of fermentable nitrogen for much of
the day, with negative consequences for the production of fermentation end-products.
Empirical results have failed to support this. Sinclair et al. (1993) did not, apparently,
consider endogenous contributions (urea recycling, sloughing of cells from rumen
epithelium and recycling of microbial matter in the rumen) to fermentable nitrogen.
Inclusion of the effect of these endogenous N-sources in a model suggested that ammonia and microbial recycling, and the contribution of hind gut fermentation, would
reduce the asynchrony in the balance of substrates available to microbes and of nutrients absorbed into the bloodstream (Illius and Jessop, 1996). The model predicted that
microbial production is mostly limited by the rate of supply of fermentable carbohydrate, the converse of that predicted by ignoring supply of endogenous N.
In principle, metabolic constraints on food intake result when there is a build-up
in concentration of one or more nutrients. This occurs when the rate of metabolism is
limited, either by an imbalance of the other metabolites required to synthesize protein
or fat, or because the rate is approaching the genetic potential of the animal. This will
24
vary with the animals stage of growth, reproductive status, physical and climatic environment, and ability to store or otherwise dispose of any surpluses. The balance of
nutrients required will depend on the mix of purposes to which they are put, such as
for protein or fat deposition, lactation, thermoregulation or locomotion. Some deviation from this ratio can be accepted, provided that body stores can be added to or
depleted to balance intake with requirements at maximal production. Surplus amino
acids or minerals may also be de-aminated or excreted. Further deviations from the
optimal diet, such as would cause a deficit of an essential amino acid relative to the
other nutrients absorbed, may cause the animal to compensate by eating more, provided it can dispose of the nutrients in surplus. More extreme dietary imbalances may
result in reduced intake. Based on this framework, Illius and Jessop (1996) developed a
conceptual model of metabolic constraints on intake of diets varying in energy and
protein yield of absorbed nutrients.
It is not at all clear how the principles of metabolic control over intake are actually
expressed by the animal, in terms of meal size and frequency, and daily intake, although
it is clear that cessation of eating a single meal is not controlled by post-absorptive signals. Throughout the animal kingdom, the satiation process appears to be under tight,
pre-absorptive sensory control (Smith and Gibbs, 1979). Most animals eat discrete
meals which end before absorption of all the ingested nutrients can take place, suggesting that the origin of satiety signals is the gastrointestinal tract (Houpt, 1982). Meals
are terminated in expectation of the post-absorptive consequences rather than being
solely the result of them, and thus satiety is a state partly specified by the stimulus conditions (Booth, 1985). For example, the sense of taste plays a role in the termination of
feeding (Swithers and Hall, 1994).
The complexity of these responses emphasizes why voluntary intake is so difficult
to predict from first principles: it is, ultimately, a psychological phenomenon. It
involves the neural integration of many signals, and is subject not only to the interplay
of positive and negative physical and metabolic signals, but also to psychological phenomena such as perceptual constraints and learning (Provenza, 1995). The elucidation
of how the integration of signals is affected by the animals physiological and mental
state remains an important challenge.
Attempting a truly mechanistic model of the regulation of food intake (combining
taste, pre-absorptive sensation, hormonal responses, conditioning) would present problems of overwhelming complexity, even if all the relevant parameters could be evaluated. A much more parsimonious approach would be to model the functional aspects
of the system by addressing the functions that short-term regulation of nutrient intake
has evolved to perform.
25
Fig. 2.1. Flow diagram of a compartment model sub-unit describing the kinetics of
passage and digestion of a substrate (S) as it progresses through a lag phase, undergoes
passage and fermentation in the rumen and hindgut. Fermentation causes the substrate
to be converted to microbial biomass (Micr) and fermentation products, absorbed from
the rumen, small intestine and hindgut. kp and kd are first-order rate constants for
passage and digestion.
Substrate fractionation
Most recent models divide foods into compartments for cell contents, digestible cell
wall and indigestible cell wall, in recognition of their different rates of digestion and
passage, and assign first-order rate constants to these processes. Compartments should
be defined as subsets of the whole that have homogeneous kinetic properties, and these
may not necessarily correspond with any physically or chemically definable compartment in the real system. Further disaggregation of cell wall by particle size is usually
thought necessary to account for selective retention in the rumen, and possibly different digestion rate, of large particles. The heterogeneity of potentially digestible cell wall
26
as a fermentation substrate was established by Van Milgen et al. (1993). They showed
differences in the potentially digestible fraction, discrete lag time and digestion rates
between cellulose and hemicellulose in both lucerne and wheat straw, with some additional differences in these parameters between particles of different size. Representation
of the carbohydrate, protein and lipid components of cell contents may be required to
interface with microbial sub-models specified in biochemical terms (Gordon and Illius,
1996). In essence, foods need to be described down to the level of detail that is consistent both with differences in digestion or passage rate or effects on metabolism of sufficient magnitude to affect the systems dynamics. However, the degree to which
information on food composition and component dynamics is available must also be
considered. Models that demand a high degree of specification of substrates are limited
in their application, due to a shortage of information about food composition and the
expense of routinely analysing foods for a wide range of components. Evaluation of
highly specified models is also hampered by the limited number of empirical observations conducted at a commensurate level of detail, i.e. of the supply of nutrients to the
animal.
Digestion kinetics
From a methodological standpoint, ruminal digestion may be regarded as two related
processes: the disappearance of feed constituents (as contributors to the dry matter) due
to solubilization and microbial fermentation, and the utilization of feed substrates by
the microbial population. Studies of digestion kinetics in situ and in vitro typically
measure substrate disappearance, which is the natural starting point for the digestion
component in digesta kinetics models.
In principle, a detailed assessment of digestion kinetic parameters for multiple substrates from in situ studies could be applied to the prediction of digestibility in an
appropriate model. In practice, methodological issues and the costs of estimating separate parameters for fractions of each feed present problems for this approach. The many
sources of error in estimating lag duration and the rate and potential extent of digestion
by in situ techniques have been summarized by Nocek (1988) and others. Artefacts
arise due to effects of, for example, pH in the bags, efflux of finely ground material and
unduly long lag times of unmasticated forage samples. Not accounting for and estimating the lag time and degradation constant simultaneously can reduce estimated degradation rates markedly, especially if the first bags are removed from the rumen during
the lag phase (Dhanoa, 1988). Methodological limitations such as these led Firkins et
al. (1998) to stress that more work needs to be done to improve accuracy of estimation
of kinetic parameters if models are to predict digestion properly. Until then, uncertainties remain about translating digestion kinetics studied in situ into events occurring in
vivo.
Animal factors
Most digesta kinetics models have addressed a particular animal type and have not
attempted to describe animal effects due to body size and physiological state. The size-
27
related effects of species differences can, broadly, be described by allometric relationships, due to the striking association between physiological processes and animal mass
(Taylor and Murray, 1987). The duration of physiological events is longer in large animals, and is expected to scale as M0.27. Examples given in Table 2.1 show that mean
particulate retention time and the time to comminute large particles scale with exponents not significantly different from 0.27, in the manner of other temporal variables
such as the time between successive heart beats or intestinal contractions (Clark, 1927).
It will be noted that, despite the high proportion of variance explained (r 2), there is still
appreciable prediction error, as indicated by residual coefficient of variation (cv) in the
range 1325% (Table 2.1).
Illius and Gordon (1991) showed that the scaling approach works well for interspecific differences. It has a less clear theoretical basis and is less likely to be successful
with intraspecific variation (within a group of dairy cows). Animal effects due to physiological state, age and environment are much harder to account for than effects due to
size. Chilibroste et al. (1997) adjusted animal effects for physiological state (pregnancy
and lactation) according to the Agricultural Research Council (1980). Physiological
effects are potentially a major source of error in model predictions, and are of considerable economic importance. Attention needs to be directed to isolating these sources of
between-animal variation and modelling the causal variables.
Passage kinetics
The values for passage rate that are used in models may be: derived from observation of
the food in question; generalized from a range of empirical observations (Sniffen et al.,
1992); derived from an allometric function of body mass (Illius and Gordon, 1991); or
be a set value for all animals and forages (Mertens and Ely, 1979). Empirical estimates
of passage rate are the norm in digesta kinetics models, because mechanistic modelling
of passage rate has seldom been attempted. Sauvant et al. (1996) linked particulate
outflow to a number of functions such as chewing during rumination and reticular
activity, but the approach is still at an investigative stage and the model has not yet
Table 2.1. Allometric relationships between physiological variables and body mass.
Range in
M (kg)
r2
Residual
cv (%)
Rumen digesta
load (kg DM)
Y=
0.01W1.15(0.039)
5.5725
21
0.98
25
Gordon and
Illius (1994)
Y = 16.7M0.23(0.026)
1.4907
48
0.89
13.3
Update of Illius
and Gordon
(1991)
Retention time of
large particles (h)
Y = 7.2N0.69(0.10)M0.22(0.058)
40
0.71a
25
Illius and
Gordon (1991)
Variable
Allometric
expression
Reference
W is body mass less digesta fresh weight; M is body mass; N is indigestible neutral detergent fibre (NDF)
concentration; DM is dry matter.
aAfter accounting for variation between experiments.
28
been tested on many forages. Better information on the pattern of outflow and its relation to chewing and gut motility is needed.
The assumption of first-order particle kinetics, when fluxes are a constant proportion of a homogeneous compartment, is increasingly under question. A high flow rate
of all nutrients would be expected in the duodenum immediately after feeding if homogeneous pools of specific nutrients are assumed to exist. However, food components
arriving in the rumen are not immediately mixed with their respective ruminal compartments and available for digestion and passage. This, together with the rumens sacculated structure, the stratification of particulate matter and the effects of particle
buoyancy suggest that any food component, such as small particulate matter, may
reside in a number of sub-compartments. Therefore, considerable sub-compartmentation
may be required to satisfy the requirement for homogeneity. Various mathematical
approaches have been used to differentiate between components which are immediately
available for passage and those which require hydration, rumination, digestion, etc.
Even if a homogeneous pool can be identified close to the reticulo-omasal orifice, constant fractional outflow is only likely to occur if rumen contractions (strength and frequency) are relatively constant throughout the day. Thiago et al. (1992) identified
increased numbers of contractions in the first 5 h after consumption of large meals by
steers, but total myoelectric activity peaked later in the feeding cycle. The consequence
of this later peak was apparent when fractional outflow rates of neutral detergent fibre
(NDF) were calculated from data obtained by emptying the rumen at different times
after feeding (Gill, 1990). Fractional outflow rate varied between 0.0140 h-1 (26 h)
and 0.0410 h-1 (2023 h after feeding). P.H. Robinson and M. Gill (unpublished)
went a step further and calculated the fractional outflow rates of a number of nutrients
during different time periods. The weighted mean fractional rate for NDF was the lowest (0.0242 h-1) and that for crude protein the highest (0.0825 h-1). Little diurnal variation was observed for NDF fractional rates, while that for crude protein declined
sharply from over 0.115 h-1 to less than 0.055 h-1 in immediate response to feeding a
protein meal.
The passage of heterogeneous digesta was modelled by Matis et al. (1989) as a
probabilistic process, using gamma-distributed residence times. They developed a
model of passage of heterogeneous digesta with, essentially, time-delay or mixing compartments followed by a final homogeneous compartment from which first-order outflow occurs. Models of this form agree closely with faecal marker flow patterns, and
suggest the existence of two particulate pools: a mixing pool that represents a time
delay of about 10 h in dairy cows followed by a homogeneous first-order outflow pool.
This view is supported by the much closer agreement between observed cell wall
digestibility and that predicted by using a two-compartment model with timedependence in the first compartment than by using a model with a single compartment
(Huhtanen and Vanhatalo, 1997).
Phenomena explaining such age dependency in particulate passage are comminution and buoyancy. Poppi et al. (1981) demonstrated retention times of large particles
of tropical forages of c. 11 h and 18 h in sheep and cattle, respectively. Studies with
plastic particles indicate that buoyancy also affects passage in both sheep and cattle.
Particles with high buoyancy have a lower fractional rate of passage than those with
lower buoyancy (Campling and Freer, 1962; Kaske and Engelhardt, 1990). Changes in
buoyancy over the time-course of particle digestion and comminution would result in
29
passage of particles deviating from first-order kinetics. Such deviation could possibly
explain why the model of Illius and Gordon (1991) predicted intake but underestimated digestibility of low-quality forages (Allen, 1996). To examine the possible effects
of particle buoyancy, Jessop and Illius (1999) modified the model of Illius and Gordon
(1991) to include separate sets of compartments for components of each meal, thereby
allowing passage rate to vary with particle age. The rate of passage of small particles, kp,
was a function of the proportion of indigestible neutral detergent fibre (INDF), Q,
remaining in each particle pool (Allen, 1996). The baseline passage rate, kp, was varied
over range R, with threshold INDF concentration Qd and rate s:
1
1
kp = kp 1 + R
-
s
(
Q
Q
)
d
2
1+ e
The parameters R and s were given values of 1.5 and 10 respectively, based on the
data of Campling and Freer (1962).
Predicted intakes and digestibilities were found to be only slightly sensitive to Qd
and rather insensitive to R and s. The optimal value of Qd, determined as the value
which gave the best agreement between observed and predicted intake and digestibility,
was found not to be constant across foods but to vary consistently with both forage
INDF and plant part (leaf or stem). This suggests that chemical and physical factors
determine the time-course of buoyancy in a more complex manner than suggested by
Allen (1996). The buoyancy model increased predicted digestibility and reduced intake
on a forage with low digestion rate (0.02 h-1) but increased both digestibility and
intake rate at a higher digestion rate (0.08 h-1). The predicted marker excretion pattern
was more realistic than from a model omitting the time-delay pool.
30
INDF (r 2 = 0.18). Chilibroste et al. (1997) used data on digesta and passage kinetics
from a variety of sources in the literature and reported close agreement between predicted and observed intake (r 2 = 0.92). Their model attempts to set digesta load as a
function of physiological state, to adjust passage for rumen fill, and to model associative effects on feed degradation.
Forage digestibility predicted by digesta kinetics models is generally within 15% of
observed values (Illius and Allen, 1994), with r 2 in the range 0.50.7. The bias in
digestibility estimates that frequently occur with these models is more likely to result
from poor parameter estimation of digestion kinetics of forages than from inaccurate
passage rate or from fundamental flaws in model structure (Illius and Gordon, 1991).
The sensitivity of model performance to variation in parameter values is seldom
reported, but is of interest as to how accurate models need to be. If model output is
sensitive to a particular parameter, then more attention needs to be paid to estimating
it accurately than to parameters making little difference to output. The model of Illius
and Gordon (1991) showed appreciable sensitivity of intake prediction only to digesta
load, particulate passage rate and the proportion of the cell wall that is digestible.
Parameters such as digestion rate, lag time, microbial growth efficiency and particle
comminution rate caused output to vary by only about 10% of a variation in parameter
value. Digestibility predictions were sensitive to the proportion of the cell wall that is
digestible, but to little else.
Causal role of digesta load and passage rate in digesta kinetics models
In modelling, as in any other form of scientific investigation, progress depends on correctly identifying causal factors. Digesta kinetics models of intake commonly assume
that intake is limited by the physical capacity of the rumen and determined by the
clearance rate of ruminal digesta, which is dependent on the processes of digestion, particle breakdown and passage rate. It is not necessary to assume that the rumen is ever
completely full, merely that some set point of digesta load, normally expressed as mass
of DM, serves to regulate intake. Provided that this set-point can be specified, and that
digesta turnover is accurately modelled, the amount of intake required to return to the
set-point can be predicted. Thus, Illius and Gordon (1991) used a model with a digesta
load set-point to achieve a 1:1 relationship (r 2 = 0.61, n = 25, residual cv = 14.5%)
between predicted and observed intake (expressed on a metabolic weight basis) in cattle
and sheep fed temperate and tropical forages ranging in NDF concentration from 622
to 875 g kg-1. They derived the digesta load set-point from an allometric analysis
(Table 2.1).
Despite the apparent success of digesta kinetics models at predicting intake, there
are doubts about the validity of the underlying assumptions. Mathison et al. (1995)
argued that digesta passage is not merely a property of foodstuffs but is also a function
of the propulsive activities of the forestomach, with the implication, at least, that the
animal may exert some control over passage. Doubts about the role of digesta load in
regulating intake of poor-quality forages have been raised by Weston (1996) and Pitroff
and Kothman (1999), arguing that digesta load is not normally at a fixed upper limit,
that it varies with the animals physiological state and environment, and increases with
energy deficit. Over a range of forage quality, Weston (1996) observed a negative cor-
31
relation between net energy (NE) intake and digesta load. However, the forages affording the lowest NE intake still allowed the lambs to meet their maintenance requirements for energy at a digesta load of 26% of live mass, suggesting that forages were
insufficiently poor to really test for a constant upper limit to digesta load. The best forages were associated with the digesta loads of 11% of live mass, and provided 2.25
times the maintenance energy intake when energy-based regulation would be
expected. Weston (1996) accepted that digesta load must have some upper limit that
constrains intake, but argued that metabolic factors such as energy status modify the
digesta load the animal will tolerate. Digesta kinetics models would need to incorporate
such flexibility in the digesta load constraint to account for variations in intake due to
the animals physiological status.
It is harder to address the question of whether passage rate is the driving variable
in digesta residence in the fermentation compartment, or whether the animals
demand for nutrients drives both intake rate and passage rate. Illius and Gordon
(1991) showed that retention time scales approximately as expected, with M0.27. But
since digesta load scales as about M1 and energy and protein requirements scale as
M0.73, it could equally be argued that retention time must scale as M1/M0.73, or M0.27
when animals are eating to requirement. The argument that other temporal variables
(see above) scale as M0.27 does not really resolve this debate about which is the fundamental causal variable because the reason why metabolic rates and time scale allometrically as they do is unclear. Given the frequently observed negative correlation
between NDF concentration and intake (Jung and Allen, 1995), which implies that
something other than energy demand constrains intake, we are still inclined to the
view that retention time is not largely under the control of the animal, and that constraint to passage rate induced by feed characteristics is a causal factor in the intake of
low-quality forages.
In summary, the current generation of digesta kinetics models can apparently predict intake of low-to-medium quality forages by animals with modest nutrient requirements, but deeper knowledge of the mechanisms underlying control of digesta load and
passage rate are needed to allow such models to predict how intake varies with physiological state and nutrient demand.
Metabolic models
Microbial metabolism
The objective of metabolic models that focus on substrate metabolism by microbes is
the prediction of nutrient supply to the animal (Black et al., 19801981; Baldwin et
al., 1987b; Dijkstra et al., 1992, 1998). In the earliest model, substrate supply to a single microbial pool followed first-order kinetics (Black et al., 19801981). The energy
(ATP) released from fermentation of substrate is used to meet the microbial maintenance requirement and in excess of this is used to drive microbial growth. Growth can
be limited by either ATP yield or fermentable nitrogen supply. In the models of
Baldwin et al. (1987b) and Dijkstra et al. (1992, 1998), supply of substrate is a secondorder process, being influenced by both substrate concentration and microbial mass following MichaelisMenten kinetics, but for which the derivation of appropriate parameter
32
values is problematic. A single pool of microbes is divided between large particles, small
particles and water in the model of Baldwin et al. (1987b), and therefore microbes flow
out of the rumen at a composite rate. The models of Dijkstra et al. (1992, 1998)
describe three microbial pools representing amylolytic and fibrolytic microorganisms
and protozoa. Microbial maintenance requirements vary between these pools, as do
substrate specificities. Amylolytic and protozoal pools had variable composition such
that starch levels could increase at high nutrient availability and provide a reserve of
carbohydrate at times of nutrient shortage. Protozoa engulf amylolytic and fibrolytic
microorganisms in relation to their relative pool sizes although engulfment rate was
reduced as protozoal starch content increased. The outflow rates of each pool differ,
amylolytic microorganisms being washed out of the rumen at the liquid passage rate,
fibrolytic ones at the solid passage rate and protozoa at a lower rate. None of the
models represent effects of low ruminal pH on cellulolytic activity although such
effects are being studied (Sauvant, 1998).
These models therefore place less emphasis on digesta kinetics. However, the shift
in focus from digesta disappearance to the metabolism of feed constituents does not
liberate such models from the exigencies of properly defining the digestion kinetics of
those constituents. These constituents are the substrates that are inputs to microbial
models, which are usually modelled at a given steady-state rate of intake. Bannink et al.
(1997a) compared three sophisticated models of rumen function (Baldwin et al.,
1987b; Danfr, 1990; Dijkstra et al., 1992) using inputs from seven diets with very
complete observations available of rumen dynamics, and found that the models gave
markedly different results. Much of the reason for this stems from the use of some concepts and parameter inputs, for instance concerning particle dynamics and the partitioning of carbohydrate fermentation, that can not be estimated from rumen
observations. Although the model of Dijkstra et al. (1992) uses digestion kinetics parameters derived from in situ studies, the other two do not, relying instead on the microbial utilization of substrate to drive digestion. Bannink et al. (1997a) concluded that,
without any input from in situ studies, microbial metabolism models are unlikely to be
able to replicate observed differences in feed degradation. Explanations of the divergence between model predictions of volatile fatty acids (VFA) production and values
measured in vivo, may relate both to inadequacies in modelling fermentation stoichiometry and absorption kinetics (Bannink et al., 1997b) and to artefacts in estimation of VFA production (Beever, 1993). In general, rumen models are highly sensitive
to changes in passage rates, because of their effects on the time for which substrates are
accessible by the microbes in the rumen. However, the study of passage rates has
received much less attention than that of digestion rates and thus it is difficult to assess
the degree of error introduced. For example, over a meal cycle it has been shown that
there is marked variation in the liquid outflow rate from the rumen (Warner and Stacy,
1968). Depending on the characteristics of the feed this can have quite different effects
on the pattern of nutrient absorption. There are few data describing the patterns of liquid and particulate outflow from the rumen during a meal cycle, i.e. in non-steadystate conditions. Nearly all experimental work has attempted to maintain steady-state
by constant, low-level feeding over the 24 h period where there is unlikely to be large
fluctuations in rumen dynamics.
Considering the large number of parameter values required as inputs by sophisticated models of rumen function, they have limited application for predictive purposes
33
at present, but are valuable research tools which need to be developed further (Bannink
et al., 1997a).
34
The main strengths of metabolic modelling are in testing where knowledge is inadequate, and in the coordination of experimental work. One example of synergistic links
between modelling and an experimental programme was research aimed at improving
understanding of voluntary intake by growing ruminants. Initially, a simplified model
of growth (Gill et al., 1984) represented all the protein in the body as one pool,
although it is known that rates differ between organs. A separate model was then developed (Gill et al., 1989) using experimental data on protein turnover rates generated by
work at the tissue level. This second model identified the importance of the contribution of protein metabolism in the liver and gut to overall energy maintenance, which
identified the need for research on the metabolism of the liver and gut. These initial
studies increased understanding of the differences in metabolism of these tissues in
steers fed forage and concentrate, and thus indirectly to an increased understanding of
factors controlling intake.
35
intake as a function of rumen distension and nutrient flow. The function contains a
double exponential term intended to relate the strength of each stimulus in relation to
the other (i.e. chemostasis being generally weaker than distension signals, especially at
high distension, but stronger at low distension). Investigation of the effect of varying
the parameter values demonstrated how the relative strength of the stimuli could be
altered, and values were found which gave good approximations between observed and
predicted intakes. This model requires the use of some imaginary parameters, and thus
can be said to be exploratory. The same applies to the model of Sauvant et al. (1996)
insofar as it represents feeding motivation in a feeding decision submodel which
includes energy status and food palatability. The model determines eating and rumination behaviour to predict intake and passage in an innovative manner, based around
more conventional rumen and microbial submodels. The model incorporates a function to relate feeding motivation to fill, rather than using an upper limit to fill as a fixed
constraint, but requires careful parameterization, particularly for assumed effects of
palatability.
Poppi et al. (1994) used the model of animal metabolism developed by Gill et al.
(1984) to examine the integration of intake regulation, the approach being to identify
pathways that could limit intake and to calculate the first limiting pathway or factor.
Both physical and metabolic pathways were examined: namely, instantaneous intake
rate, faecal output, rumen fill, genetic potential for protein deposition, heat dissipation
and ATP degradation. The ATP degrader required by their model to avoid a build-up
of ATP was used as an indicator of excessive energy intake or nutrient imbalance.
Modifications of the model by Illius and Jessop (1995) removed the requirement for an
ATP degrader, instead of which a build-up of acetate indicated excessive energy intake
or inadequate glycerol precursors for acetate clearance. Poppi et al. (1994) found energy
excess to be limiting, sometimes simultaneously with other pathways, on diets ranging
from poor-quality forages to cereals, and the authors concluded that this is indicative of
the wide range of dietary conditions under which nutrient balance is implicated in
intake regulation.
36
circumstances. The clear problem facing these very detailed models is the difficulty of
reaching satisfactory estimates of critical parameter values, especially when experimental work and modelling are not carried out in harmony (Baldwin and Sainz, 1995).
Even when they are (Gill and Beever, 1991), it is not clear that single, static values for
parameters is a satisfactory representation of dynamic and adaptive metabolic systems.
Metabolic modelling also needs to get away from steady-state conditions, in order to
accord with the reality of variable inputs and the many time-delaying components of
the system (Sauvant and Van Milgen, 1995).
Further work is required on the mechanistic modelling of passage, and on the significance of variable fractional outflow rates. Where daily nutrient supply is the output
of interest, the effect of variable outflow will only be important for nutrients with very
short mean retention times, which are also digestible, e.g. highly degradable proteins.
For models which include representation of hormonal effects dependent on nutrient
absorption, hour-by-hour changes in the flow of nutrient may be of considerable significance, but flows alone are inappropriate, since further delays or changes in profiles
may occur during absorption through the intestinal wall. The consequences of variable
flow rates on these different processes need to be examined.
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Introduction
Braingut interactions are critical in salivation, gastric motility and ingestive behaviour
in ruminants but the sites in the brain controlling these functions are largely unknown.
Better understanding in these areas may help to resolve some of the numerous problems associated with feeding high concentrate diets to grazing ruminants in intensive
production systems. Examples of the problems include acidosis, inappetence, vagal
indigestion and stasis. The basic mechanisms limiting roughage consumption may also
be addressed to enhance the productivity of ruminants when they are at pasture or
being fed preserved forages.
No papers exist on the sites in the ruminant brain which control salivation, except
for Grovum and Gonzalez (1994), whereas the central controls over motility of the
reticulorumen are limited to the brain stimulation work of Bell and Lawn (1955),
Dussardier (1960) and Andersson et al. (1958) and to the single unit studies of Beghelli
et al. (1963), Howard (1970) and Harding and Leek (1971). The bases for sequencing
and pacing reticuloruminal contractions are largely unknown. Although it is known
that the reticulo-omasal orifice and the reticular groove are under vagal control
(Titchen and Newhook, 1975), the origins of orifice constrictor and dilatory neurons
and of the groove closure neurons in the brainstem have never been reported. Most of
the brain stimulation work reported here has been published recently as short communications (Grovum and Gonzalez, 1994, 1998a,b,c; Grovum, 1998).
41
42
saliva secretion and/or contractions of the stomach and hence disclose their location in
a three-dimensional map relative to obex.
All experiments were acute, the sheep being anaesthetized with sodium pentobarbital. Fluids, consisting of normal saline with 5% glucose and 20 mEq KCl l-1 added,
were given at 1 drop s-1 (about 3 l per sheep). Normal body temperature was maintained by circulating warm water through a radiator under the operating table. Saliva,
dripping from cannulae inserted into the parotid and submandibular ducts via the
mouth, was measured with infrared drop counters and their outputs were recorded as
vertical deflections on a grass polygraph. Pressures, measured in anaesthetic breathing
bags inserted surgically into the reticulum (400 ml inflation) and the ventral sac of the
rumen (1400 ml) and in a catheter flushed with heparinized saline and inserted into
the saphenous artery, were also recorded. The sheeps head was secured in a stereotaxic
instrument and, after opening the skull and cutting the dura, the brainstem was
exposed by removing the cerebellum with suction and ligating the tentorium on the
right. The palate-bar was adjusted vertically to make the floor of the IV ventricle horizontal. This was not done when the forebrain was studied because the skull and dura
were opened only over the cerebrum. Current spread from the tip of the concentric
electrode was found to be less than 1 mm downward in studies on salivation.
43
44
to the base of the brain. From a dorsal perspective (see lower portions), the orientation
of most of the centres was essentially in the lateral plane except that the submandibular
centres angled slightly cranially from midline. The combined centres in four sheep were
located between cranial nerves VII and IX in two, adjacent to VII in one and adjacent
or slightly caudal to IX in the other. In the two sheep in which only the submandibular
centres were found (not illustrated), one was located between VII and IX and the other
was more adjacent to IX than between VII and IX. This variability with respect to the
cranial nerves was in addition to that for the stereotaxic coordinates of landmarks in the
sheep brain of up to 5 mm in the vertical and anteriorposterior (AP) planes observed
by McKenzie and Smith (1973). The practical significance of such variability for future
work is that the secretion sites will have to be localized with stimulation before singleunit recordings are made.
The finding of functionally separate sites controlling parotid and submandibular
secretion is in agreement with the fact that the parotid gland secretes during resting,
eating and rumination, whereas the submandibular gland produces saliva only during
eating (Denton, 1957; Carr, 1984). A functional separation of these two central systems existed, even though the neurons controlling these glands were anatomically
largely intermingled.
45
Fig. 3.2. Parotid secretion (PS) evoked in four sheep by stimulating the frontal cortex in the
sheep brain on average 1520 mm from midline and either on its apex or down to 15 mm on its
underside using 8 and 20 V. The vertical deflections in the traces indicate drops. Secretions by
the submandibular gland (SMS) were not affected. The marker line (middle trace) showed when
stimuli were applied (marker signal was not activated for sheep 15).
36bc) did not differ from that of glucose (32ab), PEG-200 (35ab), or sucrose (43c) (the
means with similar superscripts were not different (P > 0.05)). This effect of NaCl
could therefore be accounted for completely by its osmotic characteristics. The fact that
the inhibition by urea (29a) was 83% of that for the other solutes was also noteworthy.
In fact, the strong responses to both glucose and urea indicated that the inhibition of
parotid secretion was mediated by an osmosensitive system different from the osmoreceptors in the organum vasculosum of the lateral terminalis (OVLT) near the III ventricle
which evoke thirst and water conservation through antidiuretic hormone (ADH)
release. The osmoreceptor terminology is justified by a direct effect of solute on the
sensors due to the fact that the OVLT region is devoid of a bloodbrain barrier. The
reason for claiming that the system inhibiting salivation is different, is that thirst and
ADH release are affected either little or not at all by injections of glucose and urea
according to Fitzsimons (1989), because they can apparently diffuse into the osmoreceptors and fail to excite them as intracellular water is not withdrawn. On the other hand,
NaCl and sucrose remain outside such cells, withdraw water and evoke neuronal
responses, which lead to thirst and ADH release. An action of the solutes on salivation,
via the ventricular system, was also ruled out. There was no differential effect between
intracarotid NaCl, sucrose and urea on the one hand, which have been shown to withdraw water from the ventricles (and concentrate its Na+) and glucose on the other
hand, which did not withdraw water (McKinley et al., 1978).
Continuous infusions of NaCl into the circulations of the brain and the parotid
glands indicated that the inhibition of parotid secretion mediated centrally was
achieved well within physiological limits of hypertonicity (threshold was 5 mOsm kg-1)
whereas that mediated by the gland was not (threshold was > 42.4 mOsm kg-1). Since
46
Fig. 3.3. Inhibition of right parotid secretion (right ||| represents drops) by electrical
stimulation of the right ventro-medial amygdala (from 5 mm above the base of the brain at
earbars +26.7 mm, 13 mm to the right of midline) in sheep 272. Parotid secretion on the left
and blood pressure are also shown. The top trace shows when stimuli (15 V, 40 Hz, 1 ms, 8 s
every 30 s) were delivered through the concentric electrode and the numbers indicate the
distance of the electrode tip from the base of the brain.
47
48
by stimulating the central cut end of one vagus (Iggo, 1951). Dussardier and AlbeFessard (1954) were even able to maintain cyclic motility of the reticulorumen with
reticular distension and to stimulate it reflexly by vagal stimulation when the decerebration was made 5 mm below the junction of the pons and the medulla.
Brain stimulation
Contractions of the rumen were evoked by stimulating the interior of the brainstem
electrically from midline to the lateral edge and from 2 mm caudal to 6 mm rostral to
obex (Bell and Lawn, 1955). This site traversed many nuclei and fibre tracts and
involved much of the reticular formation. Later, Andersson et al. (1958) elicited contractions of the reticulorumen in conscious goats by stimulating next to midline considerably below the dorsal motor nucleus of the vagus from obex -1 mm to obex
+3 mm. They argued that this nucleus might contain the pace-maker for reticuloruminal motility even though stimulations close to it inhibited contractions. Curiously too,
most of the points stimulated by Bell and Lawn (1955) which evoked contractions
were outside this nucleus, and the two points within it were associated with
oesophageal contractions, not with contractions of the rumen. Dussardier (1960) made
the reticulum, but not the rumen, contract when he stimulated below the surface of the
medulla in medial planes from obex -2 mm to obex +1mm (the data can be seen in his
Table No. 5). He postulated that circuitry existed in the reticular formation which initiated contractions of the reticulum. The stimulation of efferent fibres did not appear
to account for many of these contractions as they were evoked from sites well below the
fibre tracts from the dorsal vagal motor nucleus to the vagus (see Figure 61 in
Dussardier, 1960). Dussardier (1960) further differentiated evoked contractions from
the ability of brainstem stimulations to excite the rate circuit and affect the frequency
of cyclic contractions. The loci which increased and decreased rate were distributed
from 5 mm caudal to 5 mm rostral to obex over about two-thirds of the brainstem
mass. The only clear groupings that may have constituted sites for decreasing rate were
located first near the surface surrounding the dorsal vagal motor nucleus 34 mm caudal to obex and secondly, perhaps dorsolateral to the nucleus between 2 and 3 mm rostral to obex. This agreed with Andersson et al. (1959) who had arrested cyclic motility
in conscious goats by stimulating the brainstem dorsolateral to this nucleus from 1 mm
caudal to 2 mm rostral to obex. Furthermore, Howard (1970) found an inhibitory area
in the interior of the medulla 2 mm from midline extending from the midpoint of the
dorsal vagal motor nucleus cranially and ventrally. The abundance of inhibitory effects
elicited by stimulation in and around the nucleus may explain why stimulating it
directly failed to evoke contractions of the reticulorumen in the work of Howard
(1970). The inhibition in some cases was short-lived as it was followed immediately by
a reticular contraction (Howard, 1970).
The dorsal vagal motor nucleus appears to be the origin of final efferent pathways
to the different structures in the reticulorumen. The gastric motor fibres appear to originate in this nucleus (Dussardier, 1960: Figure 61; Howard, 1970) and bilateral lesions
in these nuclei rostral to obex have abolished all motility in the reticulorumen (Beghelli
et al., 1964 as cited by Howard, 1970).
Single-unit recording
Beghelli et al. (1963) used rather large electrodes (30 m at the tip) to record bursts of
electrical activity from the dorsal nucleus of the vagus before and during reticular con-
49
tractions in young lambs (1218 kg). However, they concluded that the centres initiating contractions may be located in the reticular formation as postulated by Dussardier
(1960). This was because there was a delay of up to 10 s between the start of faradic
stimulation of the central cut end of the vagus and records of bursts of electrical activity
from the vagal nucleus, which preceded the evoked reticular contractions. Leek and
Harding (1975) localized the gastric centres in the dorsal vagal nucleus and up to
1 mm dorsal and lateral to it in the medulla from 2 mm caudal to 6 mm rostral to obex
and 1.52.5 mm lateral from midline. The gastric motor neurons were found
22.5 mm below the surface at points 1 and 2 mm rostral to obex. However, this site
was tiny compared with the sites where stimulations evoked contractions of the reticulorumen.
Surface sites from which reticular and ruminal contractions were evoked separately
Stimulating the surface of the brainstem evoked contractions of the reticulum at one
site (from 2 mm caudal to 2 mm rostral to obex and 1.85.6 mm lateral to midline)
and the rumen at another (6.08.0 mm rostral to obex and 1.53.5 mm lateral to midline). These effects are illustrated in Fig. 3.4 and were first reported by Grovum and
Gonzalez (1998c). Howard (1970) applied unipolar stimulation superficially to the
Fig. 3.4. Examples of stimulation on the surface of the brainstem, which evoked
contractions of the reticulum only or of the rumen only, in sheep. The coordinates of
the electrode tip are given relative to obex at 0 in the rostralcaudal or
anteriorposterior stereotaxic plane (AP positive numbers go in the rostral direction)
and 0 in the lateral plane. A concentric electrode was used (8 V, 1 ms, 40 Hz, 4 s).
These results show a reticular site lying 03 mm lateral and -2 to +1 mm rostral to
obex and a separate ruminal site at 13 mm lateral and +6 to +8 mm rostral to obex.
50
medulla but did not find such responses. The tentative interpretation is that the gastric
centres may receive inputs from these surface satellite sites to determine the amplitude
and form of one of three reticular and of one of two ruminal contraction types seen
during mixing, rumination or eructation.
Sites in the interior of the brainstem from which only ruminal contractions
were evoked
The data in Fig. 3.5 illustrate sites at considerable distances below the surface of the
brainstem where stimulation evoked only ruminal contractions in 11 sheep. Most sites
were between 4 and 8 mm rostral to obex (Grovum and Gonzalez, 1998c). There were
once again large differences between sheep. The data are evidence for a second satellite
site integrated with the gastric centres to determine ruminal contractions. However,
the extent of this ruminal site needs clarification. Stimulations nearby in the interior of
the medulla usually evoked ruminal and reticular contractions together. The stimulations evoking the responses in Fig. 3.5 may therefore have identified only the edges of
those ruminal sites which protruded beyond the overlapping reticular sites. The true
ruminal sites may then be much larger than shown in Fig. 3.5. According to this
Fig. 3.5. A lateral view of the sites in the brainstem of individual sheep where
stimulation with a concentric electrode evoked contractions of the rumen but not of
the reticulum. The coordinates of the electrode tip are given relative to obex at 0 in the
rostralcaudal or anteriorposterior stereotaxic plane (AP; positive numbers are rostral
to obex) and to the surface of the brain at 0 in the vertical plane. All points in the
lateral plane in each sheep have been collapsed into the sites depicted.
51
explanation, the neuronal circuits for the reticulum and the rumen were largely intermixed but functionally separate as seemed to be the case for the neighbouring salivary
centres.
Two interior brainstem sites from which only reticular contractions were evoked
One of these sites was usually oriented laterally close to obex (0.35.1 mm rostral to
obex and 2.77.5 mm lateral to midline) and was 1.38.0 mm below the surface
(Grovum and Gonzalez, 1998c). Stimulations alternating between this site and adjacent
regions evoked either just reticular or reticular and ruminal contractions together. This
is illustrated in one sheep in Fig. 3.6 even though the site was atypical in that it was oriented more in the cranial caudal direction than laterally as in most other sheep. The
points marked with an asterisk indicate loci where just the reticulum contracted.
Further, if one looks at the loci with rostralcaudal or anteriorposterior (AP) stereotaxic readings between -1 and +4, where the reticulum and the rumen contracted
together, it is clear that the reticular contractions were often at least five times stronger
than the ruminal contractions (e.g. 7 mm right, obex -1 and depths 57 mm). Whereas,
at obex +5 and +6, the difference was much smaller or even reversed (in second last
Fig. 3.6. Stimulation of the brainstem in sheep in the interior of the medulla (M) near obex
evoked contractions of the reticulum (Re) alone (*). The rumen (Ru) may have contracted by
itself also, albeit weakly (see 4th and 3rd last columns). A concentric electrode was used
(8 V, 1 ms, 40 Hz, 4 s). The coordinates of the electrode tip are given relative to obex at 0 mm
in the rostralcaudal or anteriorposterior (AP) and lateral (L) stereotaxic planes and to the
surface of the brain at 0 mm in depth (D). The top line indicates when stimuli were applied. The
last column on the right shows the effects of stimulating the vagal rootlets inside the cranium
with the electrode tip.
52
column on the right, the reticular strength was approximately one-third that of the
ruminal strength). This may indicate a transition from a reticular site near obex to a
ruminal site between 4 and 8 mm rostral to obex in accordance with Fig. 3.5. A careful
inspection of the third and fourth last columns of Fig. 3.6 shows that some stimulations between 5 and 7 mm rostral to obex evoked weak ruminal, but no reticular contractions. The independence of the reticular and ruminal contractions is also evident in
the middle of Fig. 3.6 at 5 mm lateral, AP +6 and 46 mm depth. At a depth of
4 mm, the reticular and ruminal contractions were of equal size whereas at 5 and 6 mm
depths, only reticular contractions were evoked. The contractions at 4 mm depth may
therefore have resulted from stimulating functionally separate but intermixed circuits
for the reticulum and the rumen. The last column on the right shows the effect of stimulating a point on the vagal rootlets within the cranium.
Reticular contractions were also elicited on their own in five sheep at another location cranially distant from obex (8.515.4 mm; Grovum and Gonzalez, 1998c),
beyond the most cranial sites from which contractions were evoked by Bell and Lawn
(1955) (6 mm); and Dussardier (1960) (5 mm). This site was 4.59.3 mm lateral from
midline and was 2.08.0 mm below the surface. Figure 3.7 illustrates representative
data from one sheep in which most of the reticular contractions were evoked 814 mm
rostral to obex. At the end of the experiment on this sheep both rumen and reticular
contractions of normal strength were produced by stimulating the medulla just rostral
to obex. The rumen was therefore capable of contracting but it did not respond at the
loci shown in Fig. 3.7.
53
Fig. 3.7. Stimulating the interior of the brainstem 814 mm rostral to obex evoked contractions
of the reticulum but not of the rumen (representative data from one of five sheep). Obex was at
0 mm in the rostralcaudal or anteriorposterior (AP) stereotaxic plane and at 0 mm lateral
(midline). The vertical plane was illustrated with 0 mm being on the base of the brain because
the surface was arbitrary due to the cerebellum having been removed by suction. Each of the
five sagittal sections shows the magnitude of the reticular contractions as vertical bars with
length being proportional to strength. Where the rumen contracted (7 mm right), the strength is
indicated by a horizontal bar.
Brainstem site from which rumination and hypermotility has been evoked
Rumination was evoked by stimulating the medulla in conscious sheep a few mm
below the surface just dorsolateral to the dorsal nucleus of the vagus (from obex +2 mm
to obex -1 mm in the rostralcaudal plane; Andersson et al., 1959). This may have
arisen from stimulation of epithelial receptor input pathways to the brainstem. Motility
of the reticulorumen was inhibited at the same time. A high frequency of cyclic motility (5 cycles min21) followed this inhibition. A similar result, except for the rumination, was obtained by Howard (1970) in halothane-anaesthetized sheep. Whether the
marked stimulation of frequency had anything to do with the rate circuit of Leek and
Harding (1975) is not known. Dussardier (1960) found that excitatory loci for cyclic
motility were scattered throughout the brainstem ventro-laterally from the vagal
nucleus.
Electrical stimulation of the medulla near obex (1.84.1 mm lateral and from
0.6 mm caudal to 2.0 mm cranial) in anaesthetized sheep caused gas to be eliminated
54
from the reticulorumen (Grovum and Gonzalez, 1998a). This site was thought to be an
eructation centre and was linked in later work (W.L. Grovum, unpublished data) to
relaxation of the upper oesophageal sphincter. Nevertheless, the loss of gas could also
have been simply associated with relaxation of this sphincter because the stimulation
activated circuits involved with rumination or emesis. An emetic area was located by
Andersson et al. (1959) near the surface of the medulla at obex +1 and obex +2 mm,
dorsal to their rumination area. Clearly, more work is required to understand the
function of the site reported by Grovum and Gonzalez (1998a).
A forebrain involvement in rumination is certain since it was evoked in conscious
sheep by stimulating the ventrolateral part of the anterior hypothalamus electrically
(Andersson, 1951; Larsson, 1954). Rumination could be stimulated with pentagastrin
injections into the ventricular fluid (Honde and Bueno, 1984); it occurred as a conditioned response to milking stimuli in the goat (Andersson et al., 1958); and lobotomy
and lesions in the ventral forebrain rostral to the optic chiasma increased activity to 24
h day-1 in one sheep (Clark, 1953). The latter may be the result of removing a brake on
the rumination centre, wherever it is, or of altering afferent input to that centre by
altering descending inhibition (Urabe et al., 1968). The present authors are not aware
of rumination being reported in decerebrate preparations but eructation can be
induced (Titchen and Reid, 1965).
55
Rumen contraction
sites
Reticulum contraction
sites
1st of 2
2nd of 2
(mixing)
(mixing)
S2
Delay
(Mixing)
S3
Synaptic
delay
Regurgitation
contraction
+
Eructation
centre
Rumen amplitude
circuit of
Leek and Harding (1975)
A
M
Reticulum amplitude
Rate circuit of
Leek and Harding (1975) circuit of Leek and Harding (1975)
C
S2
S1
Delay
Rumination
centre
+
S1
(Eructation)
Vagal fibre
Wall of
reticulum
Fig. 3.8. A putative system in the brainstem of ruminants for controlling contractions of the
reticulorumen during mixing, rumination and eructation (see the text for the details). There are
no experimental data linking the three reticular sites to any of the reticular contractions during
mixing or rumination. Similarly, there are no data linking the two ruminal sites to ruminal
contractions during mixing and eructation.
3. The two ruminal brainstem sites (on surface and in the interior) may act independently to trigger ruminal contractions during either mixing or eructation (see top right
of Fig. 3.8) but once again, there are no data to link a specific contraction to a specific
site. The contraction during eructation would have to be activated by an eructation
centre located elsewhere in the brainstem. Separate sites have not been identified for
the dorsal and ventral sacs of the rumen. Furthermore, there is no basis for the backward and forward moving contractions in the dorsal sac during mixing and eructation
respectively.
4. The early motor neurons of Harding and Leek (1971) may act like a final common
pathway to the reticulum as their discharges occurred in two phases, one of low frequency associated with the weaker first contraction in mixing and another of greater
frequency associated with the stronger second reticular contraction. Accordingly, the
scheme in Fig. 3.8 has the outputs from all reticular satellite sites routed through one
set of motor neurons for the reticulum.
5. The late motor neurons of Harding and Leek (1971) and Leek and Harding (1975)
for the rumen may also act as a final common pathway as they were active during both
mixing and eructation. This is accommodated in Fig. 3.8 by having the outputs from
two satellite sites for the rumen routed through the A interneurons and the motor neurons for the rumen.
6. The amplitude of the second reticular contraction during mixing was inhibited more
than the first as a result of stimulations starting from the midpoint of the dorsal
nucleus of the vagus (2 mm from midline in the region of obex) in a line going in a
56
cranialventral direction toward the nucleus ambiguous (Howard, 1970). This could
result if the satellite site for the second reticular contraction was inhibited rather than
the A type interneurons or the motor neurons in the gastric centres. The satellite site
for the first reticular contraction could then continue to operate normally.
7. Cyclic motility, which is generated in anaesthetized sheep, often starts with a single
rather than a bi-phasic reticular contraction. This could be accounted for if the anaesthetic selectively inhibited one of the two satellite sites on the top left in Fig. 3.8 associated with mixing in the reticulum.
8. The strength of secondary dorsal sac contractions and their associated Type A activity exceeded that of primary contractions considerably (Iggo and Leek, 1967; Harding
and Leek, 1971). Secondary contractions are more resistant than primary to drugs like
xylazine (Grovum, 1986). The possibility of separate satellite controls for ruminal contractions in mixing and eructation could account for such observations.
9. The fact that ruminal motility is more depressed by anaesthetics (Iggo and Leek,
1967; Harding and Leek, 1971) and various abdominal stimuli (Titchen, 1960) than
reticular motility, could be explained by a differential susceptibility of satellite sites.
10. The neural circuits in the brain which give rise to the synaptic delays, shown in Fig.
3.8, and produce a sequence of contractions in the reticulorumen have not been studied. The delays, however they are organized, must be variable as the mixing cycle is
completed much faster when the stomach is empty than when it is full.
11. Research is needed to link the rate circuit of Leek and Harding (1975) to stimulation sites that markedly increased the frequency of cyclic motility.
References
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brain stem in sheep and goats. Acta Physiologica Scandinavica 23, 823.
Andersson, B., Kitchell, R. and Persson, N. (1958) A study of rumination induced by milking in
the goat. Acta Physiologica Scandinavica 44, 92102.
Andersson, B., Kitchell, R.L. and Persson, N. (1959) A study of central regulation of rumination
and reticulo-ruminal motility. Acta Physiologica Scandinavica 46, 319338.
Beghelli, V., Borgatti, G. and Parmeggiani, P.L. (1963) On the role of the dorsal nucleus of the
vagus in the reflex activity of the reticulum. Archives Italiennes de Biologie 101, 365384.
Bell, F.R. and Lawn, A.M. (1955) Localization of regions in the medulla oblongata of sheep associated with rumination. Journal of Physiology London 128, 577592.
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Carr, D.H. (1984) The regulation of parotid and submandibular salivary secretion in sheep.
Quarterly Journal of Experimental Physiology 69, 589597.
Clark, C.H. (1953) The nerve control of rumination and reticulo-ruminal motility. American
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Dussardier, M. (1960) Recherches sur le contrle bulbaire de la motricit gastrique chez les ruminants. Doctoral thesis, Faculty of Science, University of Paris, France.
Dussardier, M. and Albe-Fessard, D. (1954) Quelques proprits du centre vagal contrlant lactivit rflexe de lestomac des ruminants. Journal de Physiologie 46, 354357.
Fitzsimons, J.T. (1989) Bengt Anderssons pioneering demonstration of the hypothalamic drinking area and the subsequent osmoreceptor/sodium receptor controversy. Acta Physiologica
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Grovum, W.L. (1998) A putative osmosensitive site in the forebrain inhibiting parotid secretion
in sheep. Journal of Physiology London 509, 8P.
Grovum, W.L. (1999) Stimulation of the brainstem evoked opening and closing of the reticuloomasal orifice and closure of the reticular/esophageal groove in sheep. South African Journal
of Animal Science 29(ISRP).
Grovum, W.L. and Bignell, W.W. (1989) Results refuting volatile fatty acids per se as signals of
satiety in ruminants. Proceedings of the Nutrition Society 48, 3A.
Grovum, W.L. and Gonzalez, J.S. (1994) Centres in the brain stem controlling the secretion of
parotid and mandibular saliva in sheep. Proceedings Society Nutritional Physiology 3, 76.
Grovum, W.L. and Gonzalez, J.S. (1998a) A putative eructation centre in the sheep brain.
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Grovum, W.L. and Gonzalez, J.S. (1998c) The reticulum and the rumen contracted separately to
brainstem stimulation in sheep. Journal of Physiology London, 511, 54P.
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with ruminant forestomach motility. Journal of Physiology London 219, 587610.
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sheep. Journal of Physiology London 206, 167180.
Iggo, A. (1951) Spontaneous and reflexly elicited contractions of the reticulum and rumen in
decerebrate sheep. Journal of Physiology London 115, 7475P.
Iggo, A. and Leek, B.F. (1967) An electrophysiological study of single vagal efferent units associated with gastric movements in sheep. Journal of Physiology London 191, 177204.
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58
Introduction
The earliest convincing evidence for life derives from 3.8 billion-year-old rock formations which have yielded microfossils including cellular filamentous forms and
sheathed colonies resembling bluegreen bacteria (Schopf, 1993). Also, all basic evolution in terms of biochemistry and types of energy metabolism must have occurred earlier than 3.5 billion years ago during periods for which we have no direct evidence.
Early evolution took place on earth in which most habitats (the atmosphere and
oceans) were anaerobic. Thus, bacteria are nearly as old as the planet earth and events
such as the oxygen forming atmosphere and even the age of dinosaurs are comparatively recent (Woese, 1994). Also, anaerobic habitats have existed continuously
throughout the history of the earth, the gastrointestinal tract being a contemporary
microniche (Fenchel and Finlay, 1995). The most obvious impact of fermentation in
the modern world is on human and animal nutrition. The annual cellular production
of prokaryotes based on population size (4.3 1024 cells) and turnover (once daily) in
the gut of domestic animals is 2 1027, which is 2% of that in soil and only 0.1% of
that in the oceans (Whitman et al., 1998).
All animals, including humans, are adapted to life in a microbial world. The complexity of animalmicrobe relationships varies tremendously, ranging from competition
to cooperation (Hungate, 1976, 1984). The animal alimentary tract has evolved as an
adaptation enabling the animal to secure food and limit consumption by other animals.
This allows the retention and digestion of ingested food, followed by absorption and
metabolism of digestion products, whilst feeding and other activities continue. Since
microorganisms grow rapidly under favourable conditions in the gut they could
become serious competitors for the animals food. This microbial challenge has modified the course of evolution in animals, resulting in selection for varied animalmicrobe
relationships. The evolutionary strategy in the first case has been to compete with the
resident microbes and in the second to cooperate with them. The third case incorporates a combination of the first two avoiding some of the disadvantages of the
CAB International 2000. Ruminant Physiology: Digestion, Metabolism,
Growth and Reproduction (ed. P.B. Cronj)
61
62
cooperation while still obtaining some benefit from fermentative digestion. These models of existing animalmicrobe relationship are useful when trying to determine the
evolution of mutualistic fermentative digestion in the gastrointestinal tract.
63
also support the theory that the development of foregut fermentation must have come
after an initial development of the hindgut and that all foregut fermenters should have
some fermentation in the hindgut (Hume and Warner, 1980; Langer, 1991).
The above mentioned adaptations, modifications and specializations in anatomy
and physiology of animals for herbivory would be ineffective in the absence of
endosymbiotic microbes to digest the plant cell wall. This raises the question of the
actual mode of acquisition of these endosymbionts by their hosts. Since it is unlikely
that the fossil record will ever provide direct clues for resolution of this issue, Hotton et
al. (1996) have hypothesized that microbes were picked up by detritivorous animals
foraging in plant litter and those that could survive in the gut environment have
assumed a role in the digestive processes of the host. However, it is equally plausible
that ingested insects, especially herbivores that harbour such bacteria in their own guts,
provided the original source for fermentative endosymbionts (Sues and Reisz, 1998).
This concept is compatible with contemporary studies on acquisition of gut microbiota
by neonates which document the importance of diet and environment in the early
development of microbial populations (Mackie et al., 1999). It is also consistent with
development of hindgut fermentation and omnivory that included consumption of
plant material prior to development of foregut fermentation. This consumption of
insect herbivores by mammalian hosts may be involved in coevolution, the continual
process of evolutionary change in the synthesis of secondary compounds by plants followed by comparable detoxification mechanisms in animals that consume them
(McSweeney and Mackie, 1997).
The opportunity to examine the intestinal contents of an extinct mammal is rare.
Recent excavations at two separate sites in the Great Lakes region of North America
revealed assemblages of plant material preserved in late-Pleistocene pond sediments
associated with skeletal remains of American mastodons (Mammut americanum).The
plant material from these assemblages varied in degree of comminution but differed in
texture and colour from surrounding sediment and resembled gut content from modern herbivores. The shape and size of the mass of plant material was consistent with the
intestinal dimensions of extant elephants (Lepper et al., 1991). The plant material was
dated to 11,500 years before present and the masses are thought to be remnants of the
small and large intestines. General and selective media were used to cultivate and identify bacteria from the intestinal contents, bone-associated sediments and sediments
located some distance from the remains (Rhodes et al., 1998). In all, 295 isolates were
cultivated and 38 individual taxa identified. Sequencing of 16S rDNA was used to confirm the taxonomic positions of selected enteric and obligately anaerobic bacteria.
Members of the family Enterobacteriaceae represented 41% of all isolates (Enterobacter
cloacae was the most commonly identified isolate) in the intestinal masses. However, no
Bacteroides spp. or expected intestinal anaerobes were recovered. In fact, the only obligate anaerobes recovered were clostridia and these were not from the intestinal masses. It
was concluded that microbiological evidence supported other macrobotanical data
indicating the intestinal origin of these plant masses but it was not possible to establish
whether these organisms are direct descendents of the original intestinal microbiota
(Rhodes et al., 1998). Although interesting, the results of this study are ambiguous and
a more direct method of studying ancient bacteria is required. It is apparent, using
rigorous DNA technology protocols, that a picture of the composition of the original
64
microbiota can be obtained at least under special circumstances which allow differentiation of ancient and modern DNA and that mammoths and similarly well-preserved
bodies are attractive candidates for future palaeomicrobiological investigations (Rollo
and Marota, 1999).
65
for nucleic acid probe design. Generally these can be divided into DNA-based methods
employing empirically characterized probes and rRNA-based methods based on comparative sequence analysis for design and interpretation of rational probes (Pace et al.,
1986; Stahl and Amann, 1991; Ward et al., 1992; Stahl, 1993b; Raskin et al., 1997).
66
67
labelled oligonucleotide probe targeting the 16S rRNA of the Archaea to demonstrate
that Entodinium species and Dasytricha ruminantium contained methanogenic
endosymbionts outside digestive vacuoles. Exosymbiotic methanogens had been well
documented previously, based on characteristic autofluorescence of these bacteria
(Vogels et al., 1980; Stumm et al., 1982; Krumholz et al., 1983). Recent research using
a small subunit (SSU) rRNA probe approach targeting ruminal methanogens revealed a
taxon-specific association between protozoal and methanogen populations both in the
rumen and a continuous culture fermentor system (Sharp et al., 1998). Methanobacteriaceae were the most abundant population in the rumen comprising 89.3% of
total Archaea and 99.2% in the protozoal fraction. This value decreased to 54% of
Archaeal signal after 48 h of fermentor operation and was correlated with the loss of
protozoa from the system. In contrast, the Methanomicrobiales, the most abundant
Archaeal population in this study, accounted for 12.1% of Archaeal signal in rumen
fluid and was not detected in the protozoal fraction suggesting a free-living lifestyle.
This group increased to 26.3% of Archaeal signal in fermentor content without protozoa. These studies suggest the importance, and perhaps specificity, of
Methanobacteriaceae as symbionts of rumen protozoa.
Initially, sequence analysis of the 18S rRNA genes from Polyplastron multivesiculatum (originally deposited as Entodinium simplex) and D. ruminantium was used to
phylogenetically position these protozoa within the hydrogenosome containing protozoa (Embley et al., 1995). More recently, seven other rumen ciliate protozoa, as well as
two additional isolates of P. multivesiculatum and D. ruminantium, have been analysed
and these studies indicate that the rumen ciliates are monophyletic and fall into three
distinct groups within the Class Litostomatea (Wright and Lynn, 1997a,b; Wright et
al., 1997). Signature probes for Entodinium caudatum, Epidinium caudatum and P.
multivesiculatum have been developed and are currently being analysed for use in the
study of rumen microbial ecology (Wright et al., 1997). Also, comparative sequence
analysis of the 18S rRNA genes was used to phylogenetically position Neocallimastix in
the Chytridiomycete class of fungi (Dor and Stahl, 1991). We have recently determined the 18S rRNA sequences for four additional rumen fungi and these data support the previous comparative analysis and suggest that the anaerobic rumen fungi are
monophyletic (Thill et al., 1997).
68
ration containing 26% lucerne, 30% maize silage and 35% concentrate. Sequences
which clustered with Prevotella ruminicola represented the majority of clones (101 of
133 total sequences) isolated in each of the PCR sets. However, many members of this
cluster represent phylogenetically distinct groups and are at least different species.
Relatively few 16S rRNA sequences similar to the commonly isolated B. fibrisolvens
were found in this study. Importantly, the majority of rDNA sequences analysed in this
study represented novel bacterial diversity which has not yet been cultivated or isolated.
A recent study (Tajima et al., 1999) described bacterial diversity by direct retrieval of
16S rDNA sequences in a culture-independent manner. Three SSU rDNA libraries
were constructed; the first from total DNA extracted from strained rumen fluid of representative samples obtained before feeding from cows fed a mixed ration (lucernetimothy hay and maize+barley based concentrate in a 4:1 ratio), the second from the
remaining feed particles from the first pooled sample, and the third from strained
rumen fluid of cows fed a high roughage (lucernetimothy hay) diet. The three libraries
containing almost full length SSU rDNA sequences (about 1.5 kb long) were completely sequenced and analysed (a total of 161 clones). Only 10 sequences (6.2%)
could be identified, six as B. fibrisolvens, two as P. ruminicola, one as S. ruminantium
and one as Succiniclasticum ruminis. For 34.6% of the sequences the similarity with
database sequences ranged from 90 to 98% while for the remaining 59.2% the similarity was less than 90%. Because of the large bacterial diversity, few operational taxonomic units (OTUs) represented a large percentage of the clones. Phylogenetic
placement of sequences from the mixed ration/rumen fluid library showed the following affiliations: low G+C Gram-positive bacteria (52.4%), Cytophaga
FlexibacterBacteroides (38.1%), Proteobacteria (4.7%) and Spirochaetes (2.4%). Values
for the analysis of sequences from the solids remaining (library two) were low G+C
Gram-positive bacteria (71.4%), CytophagaFlexibacterBacteroides (26.2%) and
Spirochaetes (2.4%). Corresponding sequences from the third library (high roughage
rumen fluid) were low G+C Gram-positive bacteria (44.2%), Cytophaga
FlexibacterBacteroides (50.6%) and Spirochaetes (3.9%). In addition, 418 randomly
isolated colonies from a number of non-selective media were characterized and SSU
rDNA sequences obtained. Results showed that 59.6% of strains were identified as P.
ruminicola, 10.8% as Prevotella spp. and 0.8% as Bacteroides. The remaining 27.0 and
1.8% of isolates were affiliated with low G+C and high G+C Gram-positive bacteria,
respectively. In contrast to the library-based analysis, the cultivation-based phylogenetic
approach revealed close clustering with strains that have already been isolated, characterized and sequenced. Similar results for the human colonic ecosystem in terms of
great diversity, large proportion of OTUs represented by single clones, and a large proportion of sequences distantly related to deposited sequences and so far uncultivated
(Wilson and Blitchington, 1996; Dor et al., 1998).
These approaches all provide useful and novel information but also have limitations which need to be recognized and resolved. The limitations relate to the extraction
of nucleic acids from environmental samples, biases, artifacts associated with enzymatic
amplification of nucleic acids and cloning of PCR products, and sensitivity and target
site accessibility in whole-cell hybridization techniques. These biases and limitations
have been well documented and reviewed (Wintzingerode et al., 1997; Head et al.,
1998; Muyzer and Smalla, 1998). However, of importance is interpretation of information derived from molecular ecology studies (Stackebrandt, 1997). For example the 16S
69
rRNA sequence variations due to interspecific and intraspecific rRNA operon heterogeneity make analysis of clone libraries or gel electrophoresis patterns derived from
environmental samples difficult to interpret (Hunter-Cevera, 1998). In addition, it is
hard to draw conclusions about physiological and biochemical properties and the ecological role of unknown (and known) microbes. However, considerable progress in
addressing and resolving these limitations is being made.
70
differed from the profile for F. succinogenes, and mixtures of PCR-amplified V3 regions
from the strains of F. succinogenes, R. albus, and R. flavefaciens were easily resolved in
DGGE gel. Further studies with different dilutions of input DNAs from these strains
showed that the DGGE technique is quantitative. These results show that DGGE can
be used to differentiate between closely related bacterial strains. Thus this sensitive
technique is highly suitable to the analysis of microbial diversity and population
dynamics for the major fibrolytic bacteria from the rumen.
We have also applied this procedure to rumen samples from steers fed different
diets in a preliminary study to determine the utility of these techniques for the analysis
of a complex microbial community (Kocherginskaya et al., 1997). Rumen samples were
collected from four steers fed a medium-quality grasslegume hay at maintenance
intake, and four steers fed a diet of 20% hay, 52% maize, 5% corn steep liquor, 3%
minerals and 20% of maize byproducts. Rumen samples were harvested approximately
one hour prior to feeding, passed through cheesecloth, and centrifuged. Total genomic
DNA was isolated from cell pellets and used for amplification of either the V3 or V9
region of the 16S rDNA gene. When the different PCR profiles obtained from amplification of the V3 region of the 16S rDNA were compared for samples from those animals fed the medium-quality grasslegume hay diet, the patterns were remarkably
similar. None the less, the DGGE profiles demonstrated at least 16 distinguishable
bands, with five of them being more predominant than the others. Banding profiles
obtained from rumen samples of animals fed the maize-based diet were different from
those obtained from the medium-quality grasslegume hay diet. Profiles from each of
the four maize-based-diet-fed animals also differed from each other. These results
demonstrate the utility of this technique in describing the genetic diversity and population structure of the rumen community, both in vitro and in vivo.
Bacterial diversity in human faeces was analysed using PCR-amplification of the
V6V8 regions of 16S rDNA by TGGE (Zoetendahl et al., 1998). Faecal samples from
two individuals showed remarkably stable profiles over a period of at least 6 months
and were unique for each individual. TGGE profiles derived from 16S rRNA (by
reverse transcriptase PCR) and rDNA amplicons showed similar banding patterns
although the intensities of bands with similar mobilities differed in some cases indicating a different contribution to the total active fraction of the predominant faecal bacteria.
These results confirm that TGGE analysis of 16S rDNA amplicons, combined with
cloning and sequencing of these amplicons, is a reliable approach to relative levels and
abundance in complex microbial communities such as faecal bacteria. Bacterial genetic
diversity in pig faecal samples was determined using DGGE (Simpson et al., 1999).
Optimization of the protocol resulted in a doubling of product bands visualized in the
gels. Unique and stable banding patterns were generated from faecal samples of pigs on
different diets and of different ages as well as from lumenal and mucosal samples
obtained from each gut segment between the stomach and colon. Analysis of the
DGGE banding profiles using PHYLIP showed that the patterns grouped according to
gut location and that lumenal and mucosal samples from each compartment had the
highest similarity to each other (Simpson et al., 1999). DGGE can be applied effectively to monitor changes in bacterial populations and for evaluation of bacterial diversity. However, many of the specific gut populations are minor constituents and there
are detection limits for populations comprising less than 1% of total template DNA
and PCR amplification with semi-conserved primer sets (Muyzer and Smalla, 1998).
71
Whole-cell hybridization
Whole-cell in situ hybridization with fluorescently-labelled oligonucleotide probes for
studies in microbial ecology was first developed 10 years ago (DeLong et al., 1989;
Amann et al., 1990). This technique was used successfully to analyse a wide variety of
ecosystems (Amann et al., 1995, 1996). However, applications to analysis of the rumen
ecosystem are limited. In short, the procedure involves cell fixation to permeabilize cells
while maintaining their morphological integrity. Hybridization with a fluorescent
probe to bind with complementary rRNA sequence is carried out on bacterial suspensions or after attachment to coated microscope slides. Following hybridization, the
sample is washed to remove unbound probe and the sample viewed by epifluorescence
microscopy (Amann et al., 1995). Recent developments that have improved our ability
to address structure and function of microbial communities in situ have been reviewed
(Amann and Kuhl, 1998).
Localization of microorganisms on mucosal or cell surfaces is usually performed by
classical histological and immunohistological techniques, which restrict identity of the
bacteria involved to morphological features. Even when specific antibodies are available
for in situ studies, the thick mucous layer above epithelial cells can block penetration of
antibodies and extensive washing can remove the mucus layer displacing organisms.
Poulsen et al. (1994) applied in situ 16S rRNA hybridization to investigate the microbiota of the large intestine of streptomycin-treated mice and were able to determine the
spatial distribution of Escherichia coli in thin sections of intestinal tissue. This allows
rapid detection of bacteria which may be difficult to cultivate and their relationship to
other cells either host or bacterial. In situ growth rates determined by single-cell analysis
of intracellular concentrations of DNA and RNA revealed that adherent and mucosal
bacteria were growing with generation times of 3080 min while those in the lumen
were static (Poulsen et al., 1995).
An advantage of the fluorescent in situ hybridization approach is the ability to
both identify and enumerate single cells within a complex ecosystem with specific 16S
rRNA-based oligonucleotide probes. Thus, six 16S rRNA-targeted oligonucleotide
probes were designed, validated and used to quantify predominant groups of anaerobic
bacteria in human faecal samples (Franks et al., 1998). The combination of the two
Bacteroides probes for the B. fragilis and B. distasonis groups detected a mean of 5.4
1010 cells g-1 (dry weight) of faeces. The Clostridium coccoidesEubacterium rectale
group-specific probe detected a mean of 7.2 1010 cells g-1 (dry weight) of faeces.
StreptococcusLactobacillus group-specific probes detected cells ranging in number from
1.7 107 to 7 108. The future aim of this work is to have a set of about ten probes
that can detect more than 90% of the colonic biota in large phylogenetic groups. This
approach which combines the power of molecular techniques with modern image
analysis at the single cell level will provide much insight into structurefunction relationships within gastrointestinal microbial ecosystems.
Conclusions
The use of molecular ecology techniques based on nucleic acid probes is likely to revolutionize our approach to microbial ecology in the gastrointestinal tract and will
72
provide, not simply a refinement or increased understanding but a complete description of gastrointestinal community for the first time. Modern molecular ecology techniques based on sequence comparisons of nucleic acids (DNA or RNA) can be used to
provide molecular characterization while at the same time providing a classification
scheme which predicts natural evolutionary relationships. In principle, nucleic acid
probes can be designed to hybridize with a complementary target sequence and thus
provide a complete description independent of the growth conditions and media used.
An example of the power of these modern molecular ecology techniques is provided by
the analysis of SSU rRNA sequences. The highly conserved regions of the SSU rRNA
molecule can serve as primer binding sites for in vitro amplification by PCR. The more
conserved regions are also useful, serving as targets for universal probes that react with
all living organisms or for discriminating between broad phylogenetic groups such as
the domains Archaea, Bacteria and Eucarya. The more variable sequence regions are
more appropriate for genus, species and sometimes even strain-specific hybridization
probes. Thus nucleic acid probes serve to evaluate the presence of specific sequences in
the environment and provide a link between knowledge obtained from pure cultures
and the microbial populations they represent in the gastrointestinal tract. Furthermore,
whole-cell hybridization using in situ PCR is a powerful technique which can be used
to describe an organisms expression of key enzymes. Thus development of these procedures and techniques will result in greater insights into community structure and activity of gut microbial communities in relation to functional interactions between
different bacteria, spatial and temporal relationships between different microorganisms
and between microorganisms and feed particles. The successful development and application of these methods promises to provide the first opportunity to link distribution
and identity of gastrointestinal microbes in their natural environment with their
genetic potential and in situ activities.
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79
80
81
Adhesion to cellulose
CBD
Catalytic domain
Linker
82
Gene
product
CinA
CinB
Epi3
CedA
CelG
EGB
EGC
EGD
EGE
EGF
CelA
XYN3
BnaA
BnaB
BnaC
CelA
CelB
XynA
XynB
CelA
CelB 2
CelB 29
AxeA
CelA
CelB
CelC
CelE
Organism
Neocallimastix patriciarum
N. patriciarum
N. patriciarum
N. patriciarum
N. patriciarum
N. patriciarum
N. patriciarum
Orpinomyces joyonii 26
O. joyonii SG4
O. joyonii SG4
Orpinomyces PC-2
Orpinomyces PC-2
Orpinomyces PC-2
Orpinomyces PC-2
Orpinomyces PC-2
Xylanase
Endoglucanase
Cellulase
Cellulase
AXE
Endoglucanase
Endoglucanase
Endoglucanase
Endoglucanase
AXE
AXE
AXE
Cellobiohydrolase
Glucanase
Xylanase
Descriptiona
CD
CD
CD5
CD5
CD 5A/SL/BTD
?/CD9
CD9/BTD
CD9/SL/BTD
CD9/BTD
CBD?/CD?
CD5/EL/TR23836
CD11/SL/CD11/TL/TR2
240,40
CD/TL/TR237,39
?/CD
CD/TR
CBD/NL/CD6
CD5/STL/TR236,37
CD11/SL/CD11/TL/TR2
240,40
CD10/L/TR128
CD5/TR2164,156
CD5/XL/Tdoc138
CD5/XL/TR236,37
CD
TR226,34/PL/CD6
CD5/L/TR236,37
TR226,29/TL/CD6
CD10/L/TR23537
Modular arrangementb
860
910
585
513
313
459
471
449
477
393
392
389
428
473
607
246
285
341
357
519
555
620
668
467
1053
?
607
S71569
U59432
AF015249
AF015248
AF001178
U63837
U57818
U63838
U97153
(Continued)
U66251
U66252
U66253
U29872
Z31364
X65526
U44893
U44893
AB011273
U07419
U33887
L14436
L48039
U05897
U05897
U39070
U38843
X82266
Gen Bank
#
14.5*
2/9.7*
0.9/785
0.24/2/5,980
2/57*
2/11.5*
2/16.5*
2/76*
Amino acid
residues Km/Vmaxc
Table 5.1. Recently characterized glycoside hydrolases and esterases from ruminal bacteria, fungi and protozoa.
83
LICA
XynA
CdxA
XynA
XynB
XynC
D31d
MANA
MANB
MANC
XYLA
PolyX
XYNA
XynA
EGIV
XynB
XynD
EndA
CelD
Orpinomyces PC-2
Orpinomyces PC-2
Prevotella ruminicola B14
P. ruminicola B14
P. ruminicola B14
P. ruminicola B14
P. ruminicola 23
Piromyces spp.
Piromyces spp.
Piromyces spp.
Piromyces spp.
Polyplastron multivesiculatum
P. multivesiculatum
Ruminococcus albus 7
R. albus F-40
Ruminococcus flavefaciens 17
R. flavefaciens 17
R. flavefaciens 17
R. flavafaciens FD-1
Streptococcus bovis JB1
CD16
CD11/L/TR240,40
CD3
CD10
CD ?
CD10/?/CD10
CD10/TR/CD10
CD26/TR336,36,36
CD26/NL/TR236,37
CD26/NL/TR236,37
CD11/TR239,36/CD11
CD11
CD11
CD
CD5/TR232,31
CD11/TSD/CDAXE/TL/
TR 232,33
CD11/TSD/TR232,33/TL/CD16
CD5/?/TL/TL/TR232
CD9
CD16
Modular arrangementb
759
405
237
802
623
175
218
680
312
781
606
245
362
789
369
319
560
2.8/338
5.0/3.5
0.91/5,320
Amino acid
residues
Km/Vmaxc
Z83304
L05368
Z92911 1
S61204
U63813
U57819
U35425
Z49241
Z49241
Z79595
U53926
X91858
X97408
X97520
X91857
AB011274
AJ009828
U43089
D16315
Z35226
Gen Bank
#
Xylanase/
b glucanase
Endoglucanase
Endoglucanase
b-(1,31,4) glucanase
Lichenase
Xylanase
b-Glucosidase
Xylanase
Glycosidase
Xylanase
Xylanase
Mannanase
Mannanase
Mannanase
Xylanase
Xylanase
Xylanase
Xylanase
Endoglucanase
Xylanase/AXE
Descriptiona
84
Gene
product
Organism
85
proteins of 120 kDa and lower had endoglucanase activity. Two CBPs of 120 and 225
kDa were purified from F. succinogenes S85, and the gene coding for the 120 kDa protein was cloned in Escherichia coli and sequenced (Mitsumori et al., 1996). This protein
was later shown to be an endoglucanase bearing a CBD (Malburg et al., 1997). In parallel, Gong et al. (1996) recognized two other CBPs, of 180 and 240 kDa. The 180
kDa protein is a glycosylated xylanase (E.E. Egbosimba and C.W. Forsberg, unpublished data) that shares common epitopes with numerous proteins from the outermembrane. Furthermore, polyclonal antibodies raised against this CBP decreased the
adhesion of F. succinogenes to cellulose by 60%, suggesting that it and related proteins
have an important role in adhesion.
To identify proteins important in adhesion of F. intestinalis to cellulose, polyclonal
antibodies prepared against whole cells were adsorbed with cells of the non-adherent
mutant (Miron and Forsberg, 1998, Section 2.4) to remove all antibodies reacting with
surface epitopes not involved in the adhesion process (Miron and Forsberg, 1999).
These adsorbed antibodies reacted strongly with six of the major CBPs of strain DR7,
but reacted very weakly with a non-adherent mutant. The antiserum was shown to
react with glycosyl residues rather than protein. Since the non-adherent mutant was
not missing any of the CBPs, glycosylation appears to have an important role in the
adhesion process. Monosaccharide analysis of the CBPs showed that they contained
mainly galactosamine, glucosamine, galacturonic acid and glucuronic acid. Of these
compounds, glucosamine and galacturonic acid, each at 10%, blocked binding. These
data were interpreted to indicate that the compounds interfered with binding by
occupying adhesion sites on the cellulose substrate; however, the precise role of the glycosyl components of the CBPs involved in adhesion is not known.
Recently, two low-molecular mass cellulose-binding polypeptides (16 and 21 kDa)
were isolated from R. albus 8 (Pegden et al., 1998). One of them (CbpC) possessed
structural motifs typical of the Pil protein family, which is comprised mostly of type 4
fimbrial proteins produced by Gram-negative pathogenic bacteria. Inclusion of either
ruminal fluid or phenyl propionic acid in the growth medium, a treatment that was
shown to increase R. albus adherence to cellulose, also increased the concentration of
CbpC transcripts (Pegden et al., 1998). DNA sequences homologous to the CbpC
gene were found in other strains of R. albus. The authors suggest that fimbrial-type
adhesion proteins may represent a novel strategy (in addition to the cellulosome?) for
the adhesion of Gram-positive bacteria to cellulose.
86
displayed lower cellulose-degrading activities than the wild type, and these activities
were located in the extracellular fluid, while they were mostly cell-bound in the wild
type. This may suggest that the mutant is impaired in assembling multi-enzyme complexes involved in both adhesion and hydrolysis of cellulose. Four mutants isolated
from the strain S85 of F. succinogenes displayed different phenotypes regarding their
capability to utilize cellulose (Gong and Forsberg, 1989), suggesting that the adhesion
mechanism may require several factors. An adhesion-defective derivative isolated from
R. albus SY3 was also impaired in its capacity to grow on cellulose (Miron et al., 1998).
Two adhesion-defective mutants were also isolated from R. albus 20; they showed both
a decreased rate and reduced extent of cellulose degradation (Mosoni, 1999). In addition to these studies with mutant strains, a great heterogeneity in adhesion and cellulolytic performances was observed among different wild strains of R. albus (Morris and
Cole, 1987). Altogether, these results suggest that adhesion of R. albus to cellulose is a
complex process that may be mediated by several factors.
In conclusion, there is evidence that the rumen fungi and the ruminococci produce multi-enzyme complexes that may reorganize to form cellulosomes. These complexes are likely to be involved in both adhesion of the cell to cellulose and degradation
of this substrate. Indeed, for all the fibrolytic microorganisms, adhesion and cellulolytic
activity appear closely, although not always strictly, related. However, there is no evidence at this time for a cellulosome-type organization for F. succinogenes. Complete
understanding of the molecular basis of adhesion of the main rumen fibrolytic species
will rely on further work, such as isolation of genes encoding the putative scaffoldins,
and the molecular characterization of the isolated mutants.
87
88
89
90
measure of the spacing between the individual polymers contributing to the wall structure and is remarkably similar in all crop vegetation used for feed purposes. Direct measurement by a variety of probe-based methods has shown that most pores have a
diameter between 2 and 4 nm (Chesson et al., 1997; Gardner et al., 1999). These
dimensions are not sufficient to allow free diffusion into the wall by simple globular
enzymes with masses greater than ~20 kDa. The porosity of the wall (Gardner et al.,
1999) and its composition (Chesson et al., 1986) changes little during the course of
degradation even when ~70% of dry matter has been eroded. The data are consistent
with a pattern of degradation based on surface erosion imposed on the rumen bacteria
by the impervious mixed polymer nature of the cell wall.
Recent work showed that enzyme (cellulase and xylanase) supplementation of the
diet can increase ruminal digestibility and milk production in cattle (Yang et al., 1999).
This result is surprising considering the extensive array of potent endogenous fibrolytic
enzymes produced by the rumen microflora. The most likely explanation for the beneficial effect is that addition of enriched extracellular polysaccharidases results in an
immediate attack on freshly ingested plant material thereby providing additional available carbohydrate that encourages more rapid microbial growth, shortening the lag
time required for microbial colonization. The net effect may be equivalent to a longer
retention time within the rumen. However, one cannot preclude the possibility that the
exogenous cellulases have more efficacious binding and catalytic properties.
Evolution has not produced a solution that overcomes this limitation. Instead,
most of the rumen bacteria and fungi have optimized a system of cell-wall degrading
activities in the form of the cellulosome which recognizes the diffusion limitations
imposed by the plant cell wall and which is highly adapted to a superficial mode of
action. This makes it unlikely that the introduction of genes coding for single activities,
as has been done in the past (see Forsberg et al., 1997), will contribute significantly to
the degradation process. There remains the option of engineering the cellulosome itself
(Bayer et al., 1998). However, more needs to be known about its structure and, in particular, the structurefunction relationships of its component parts in the organism of
choice, before this is likely to be effective. One possible option is the introduction of
CBDs that have a greater avidity for cellulose, which disrupt the crystalline structure of
the surface cellulose as shown by Din et al. (1991). Another is the introduction into the
cellulosome structure of CDs with higher catalytic efficiency and, depending upon the
organism of choice, ensuring that essential hemicellulases, for example, feruloly and
coumaroyl esterase, arabinofuranosidase and acetylxylan esterase, are included as illustrated in Fig. 5.2. There may be more efficacious CBDs and CDs in non-ruminal
organisms or those organisms, for example the ruminal fungi, with highly efficient cellulases, but which usually are a minor component of the population.
With surface erosion as the predominant mechanism of bacterial (and rumen fungal?) cell wall breakdown, two factors are particularly important. Firstly, the amount of
surface area available for colonization and, secondly, the chemistry of the available surface. Surface is created by feed processing and then by mastication/rumination, each
opening plant cells and exposing the inner surface to colonization. Subsequent removal
of polysaccharide from that surface can, in more lignified cell types, lead to the development of an inert surface in which any remaining polysaccharide is protected from
attack by the presence of phenolic compounds. Consequently, available surface area
reaches a maximum and then diminishes with time resulting in the slowing of the rate
EG
91
CBD
EGII
XynB
CBH
EGII
CBH
EGI
CBD
Man
XynA AXE
CBD
XynB
CBH
EGII
CBH
EGI
CBH
EGII
CBH
EGI
CBD
1
Man
XynB
XynA AXE
Ar
CBD
CBD
1
Ar
Man
XynA AXE
Ar
CBD
3
CBD
Catalytic domain
Linker
Fig. 5.2. Schematic representation of a cellulosome with high catalytic activity on plant cell
walls. Symbols: Man, Mannanase; Xyn, xylanase; CBH, cellobiohydrolase; EG, endoglucanase;
AXE, acetyl xylan esterase; Ar, arabinofuranosidase.
92
More permanent solutions through selective breeding and the use of recombinant
DNA technology are under active investigation.
Enzymes involved in the biosynthesis of lignin and tannin precursors have been
targeted for genetic modification, and crop plants have been produced in which one or
more of these enzymes have been down-regulated. One general observation made is
that phenolic polymers appear much more plastic than was originally thought (Ralph,
1997). Modifying the nature of the precursors was found rarely to decrease the total
amount of lignin formed but did have significant effects on its composition and properties (Boudet, 1998). Blocking a biochemical pathway at one point can also lead to a
redirection of the flux of precursor molecules. Down-regulation of cinnamyl CoA
reductase, an enzyme which catalyses the reduction of phenolic acids to the corresponding aldehyde, led to greater amounts of free ferulic acid present in the cell and
increased significantly the number of diferulate cross-links within the wall (Piquemal et
al., 1998). Because of the plasticity of lignin, targeting the extent of cross-linking
between polymers within the wall may be a more effective way of altering cell wall
degradation characteristics (Grabber et al., 1998).
There is also an increasing appreciation of the importance to nutritional value of
the spatial distribution of cell wall material within the plant. Unfortunately, the ability
to apply this new understanding to the breeding of new crop varieties was, until
recently, hampered by the lack of quantitative tools for the routine measurement of
anatomy. Analysis of images from microscope sections now has revolutionized
approaches for the measurement of anatomical features enabling the automated recognition of cell types and tissues to be used as selection criteria (Travis et al., 1996).
However, the lack of knowledge of the genetic basis to anatomy has meant that development of improved crop plants using anatomical features as selection criteria is
restricted to conventional breeding at present.
Concluding remarks
As we look to the future there would appear to be no short-term opportunities to
develop genetically modified ruminal organisms that will radically improve the rate and
extent of plant cell wall digestion. However, furthering our understanding of the structure of the cellulosome and related cellulase systems of ruminal organisms and comparative studies on the CBDs and CDs of ruminal and other organisms may reveal new
opportunities to improve the catalytic properties of the ruminal cellulases. This
strategy, in conjunction with continued exploration of genetic methods to reduce the
lignin content of fibrous plants used as ruminant feeds, should eventually give rise to
improved digestion of plant components that at present are considered to be largely
recalcitrant to ruminal digestion.
Acknowledgements
The authors thank E.A. Bayer, P. Beguin, J.-P. Belaich, J. Ha, P. Mosoni and J.H.D.
Wu for communicating data prior to publication, and L.B. Selinger for providing journal articles not available at the authors institutions.
93
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Introduction
The relationships between the ruminant animal and its resident microflora, and the
ensuing impacts on protein nutrition, have been examined in detail for more than 40
years. The foundational studies such as those by Hogan and co-workers demonstrated
the interface between host and microbial metabolism of non-protein nitrogen sources;
and the practical ramifications of this interface were clearly demonstrated in the widely
referenced study by Virtanen (1966). Egan (1965) helped establish another precedent
in our understanding of ruminant protein nutrition. By showing that positive responses
in animal physiology coincided with the intra-abomasal infusion of casein, their studies
serve as the underpinning of the by-pass protein concept. To this day, much of the
practical interest in ruminal nitrogen metabolism relates to the optimization of
microbial growth with different nitrogen sources, as well as the optimization of postruminal supply of microbial and feed proteins. Additionally, as the relative amount
and/or biological value of feed protein increases, there is a shift in research interest,
away from the biosynthetic processes of ruminal microbes, and more towards their
degradative processes. For these reasons, the ruminal microflora continues to provide a
unique and varied set of challenges for nutritionists and microbiologists intent on
improving the protein nutrition of animals, in both extensive and intensive production
scenarios.
Hungate (1960) considered that a meaningful analysis of any microbial habitat
requires an understanding of: (i) the types of kinds of microorganisms present (ecology); (ii) the activities possessed by these microorganisms (enzymology); and (iii) factors affecting the expression of these activities (regulation). There is no shortage of
recent reviews that provide a detailed accounting of our current understanding of
these aspects, from an organismal and biochemical perspective (Morrison and Mackie,
1996; Cotta and Russell, 1997; Wallace et al., 1998 ). This review will attempt to take a
different approach to the topic, by focusing on recent and pending advances contributed via genetics and molecular biology. In addition to providing new information
CAB International 2000. Ruminant Physiology: Digestion, Metabolism,
Growth and Reproduction (ed. P.B. Cronj)
99
M. Morrison
100
relating to the ecology and enzymology of ruminal nitrogen metabolism, these techniques are likely to help elucidate how nitrogen metabolism is coordinated, and what
other physiological process(es) ensue. Ultimately, a more precise identification of the
rate-limiting parameter(s) of ruminal nitrogen metabolism, from an ecological, enzymatic, or regulatory perspective, should expedite advances in ruminant protein
nutrition.
101
properties have been isolated from ruminants in other parts of the world. Attwood et
al. (1998) recently isolated from New Zealand ruminants a variety of bacteria with
high specific rates of ammonia production. Five of these strains are phenotypically similar to the isolates identified by Chen and Russell (1988, 1989), at least in terms of sensitivity to monensin and their growth being restricted primarily to small peptides or
amino acids. Phylogenetic (16S rRNA) analyses placed two of the isolates in the genus
Peptostreptococcus, one in the genus Eubacterium, and two more in the family
Bacteroideacea (presumably, the first Gram-negative bacteria of this type). Wallace et al.
(1999) have also described the biochemical and phylogenetic characteristics of a Grampositive rod named Eubacterium pyruvovorans sp. nov., which is capable of growth with
either peptides, amino acids, or organic acids such as pyruvate or oxaloacetate. In addition to acetate, this bacterium also produced long- and branched-chain organic acids
such as valerate and caproate during growth. It is now widely accepted that bacteria
with high specific rates of ammonia production are not uncommon in ruminant animals and, in some instances, might comprise a significant percentage of the culturable
bacteria in the rumen.
Despite the breakthroughs in our understanding of ruminal ammonium production arising from these cultivation-dependent studies, there are some inherent difficulties with such an approach. The culturable number of these bacteria can be relatively
low (~107 per gram of ruminal contents; Chen and Russell, 1989) and the lack of completely selective media can make it difficult to accurately quantify their numbers
(Attwood et al., 1998). To overcome these difficulties, molecular techniques have been
developed and utilized effectively for population analysis, and with a variety of production/nutritional scenarios. Krause and Russell (1996) were the first to use a combination of in vivo sampling and a continuous culture system inoculated with predominant
ruminal bacteria (PRB), to examine how monensin influences the persistence and
abundance of the obligate amino-acid-fermenting bacteria. Despite all three strains
being monensin sensitive in pure culture under batch conditions, C. aminophilum was
found to persist and increase in relative abundance, both in vivo and within in vitro
continuous cultures containing PRB, in the presence of monensin. Although other reasons have been forwarded to explain the protein sparing effect of monensin, the conclusions reached by Krause and Russell (1996) also seem valid: monensin does not have
a greater impact on ammonia production kinetics because of the persistence of C.
aminophilum. It will be interesting to see if similar patterns of persistence exist for any
of the other types of obligate amino-acid-fermenting bacteria isolated elsewhere.
Studies such as these also highlight that while much can be learned from pure culture
studies, microorganisms can behave quite differently when cultivated as part of a more
complex consortia.
Another molecular-based technology which circumvents cultivation-dependent
analysis of ruminal microbiology is competitive PCR (cPCR). The technique has been
used to enumerate the abundance of the proteolytic bacterium Clostridium proteoclasticum
in New Zealand ruminants (Reilly and Attwood, 1998). Under laboratory conditions,
this bacterium produces extremely high levels of proteinase activity, and 16S rRNA
analysis showed it is very similar to several bacterial strains currently classified within
the genus Butyrivibrio fibrisolvens. The cPCR technique is dependent on the development
of a competitor DNA template, which is usually generated by removal of a restriction
enzyme fragment within a clone of a 16S rRNA gene isolated from the bacterium
M. Morrison
102
being studied. Primers specific only for the intact gene (target DNA) and the competitor template are then used in PCR reactions, the products are resolved by agarose gel
electrophoresis and the log ratio of band intensities for the target and competitor PCR
products are determined. A standard curve is first generated by combining serial dilutions of cells containing the target DNA with fixed amounts of competitor DNA, and
the log ratio of PCR band intensities is plotted relative to cell numbers. DNA from
environmental samples are then spiked with a known quantity of the competitor template and subjected to PCR. In this manner, the quantity of target DNA in an environmental sample can be quantified, and in the case of C. proteoclasticum, as few as 2500
cells g21 of ruminal contents can be detected. Reilly and Attwood (1998) showed the
numbers of C. proteoclasticum and closely related species did not vary greatly among
animals consuming rations with varying levels of carbohydrate and/or protein. These
type of studies have now been expanded to include a greater range of proteolytic bacteria, including Streptococcus, Butyrivibrio and Eubacterium spp. (Attwood and Klieve, 2000).
Interestingly, although the numbers of Streptococcus spp. were unchanged in response to
diet, the numbers of Butyrivibrio were stimulated, and the numbers of Eubacterium
spp. suppressed, by carbohydrate supplementation. Similar studies are now underway
for populations of obligate amino-acid-fermenting bacteria (G.T. Attwood, personal
communication), and it will be interesting to see whether the fluctuations in proteolytic
bacteria in response to plane of nutrition extend also to this group of ruminal bacteria.
103
protozoal lysis contributes to intraruminal N-recycling, and whether the elevated level
of ammonium present in faunated animals is attributable more to protozoal predation
of bacteria, or protozoal lysis as suggested by Wells and Russell (1996).
More recently, the relationship(s) between intraruminal N-recycling and bacteriophage populations has received some attention, but unfortunately, efforts to understand this component of ruminal ecology are not widespread. Largely due to the efforts
of Klieve and co-workers, molecular technologies have provided and may continue to
provide relevant breakthroughs in our understanding of bacteriophage biology and
population dynamics. Bacteriophages capable of infecting some of the better-known
ruminal bacteria have been isolated and partially characterized (Klieve et al., 1989) and
both lytic and temperate bacteriophages have been identified (Lockington et al., 1988;
Kleive et al., 1989, 1991). All known ruminal bacteriophages possess a single copy of a
double-stranded DNA molecule, which has facilitated the development of techniques
to examine genetic diversity and bacteriophage abundance in response to diet and feeding behaviour. Klieve and Swain (1993) developed a method of harvesting bacteriophages from ruminal fluid coupled with the lysis of the bacteriophage capsid and
separation of bacteriophage DNA by pulsed-field gel electrophoresis (PFGE). Genome
size can be estimated by the migration distance of the DNA molecules in PFGE gels,
relative to a set of DNA standards of known size. Additionally, changes in the abundance of DNA within a particular region of the gel can be quantified by laser densitometry, and the diurnal variation in bacteriophage numbers can be estimated from these
changes in DNA staining intensity. Using these methods, Klieve and Swain (1993)
found bacteriophage DNA could be subdivided into two main components. The bulk
of the DNA, postulated to have been derived from temperate bacteriophages, was present as a broad band ranging from 30 to 200 kb. Distinct bands of DNA, both small
(~10 kb) and very large (~850 kb) in size, were also apparent and postulated to represent blooms of lytic bacteriophage. Using these same methods, Swain et al. (1996) also
showed there is a marked diurnal fluctuation in total bacteriophage numbers, the lowest concentrations occurring within two hours of feeding, and maximal concentrations
occuring 810 h later. In addition to this daily fluctuation in numbers, Klieve et al.
(1998) found that animals consuming green pasture tend to have the highest concentration of bacteriophages, and the concentrations were twofold and tenfold lower in
animals consuming dry forages, and grain-fed animals, respectively. However, whether
these differences are the result of sampling time relative to feeding, or other factors such
as bacterial resistance to bacteriophage infection, are not clear.
With the development of these techniques the next logical step appears to be to
superimpose measurements of the intraruminal turnover of nitrogen among ammonium and non-ammonium pools upon quantitative measurements of fluctuations in
protozoal, bacterial and bacteriophage populations. Accordingly, the relative contributions from each of these biotic components of the ruminal habitat to intraruminal
nitrogen recycling, and the physicochemical factor(s) underpinning the phenomenon,
should be elucidated. Such information is most likely necessary for the development of
new, productive methods of curbing intraruminal nitrogen recycling.
In summation, the molecular methods outlined above are providing the means
necessary to examine rumen microbial diversity and phylogeny in more detail. The
transition of these technologies from providing information of fundamental microbiological significance to the realm of providing information of a more pragmatic nature is
M. Morrison
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gradually occurring. These technologies also provide the opportunity for detailed,
quantitative examination of microorganisms in situ and their response to fluctuations
in nutrient supply.
105
active against neutral oligopeptides such as penta-alanine, as well as the synthetic substrate L-glycyl-L-argininyl-methylnapthylamide (Gly-Arg-MNA; Wallace and McKain,
1989; Wallace, 1994). Using pure cultures of ruminal bacteria, Wallace et al. (1990)
and Wallace and McKain (1991) showed that P. ruminicola and B. fibrisolvens both
exhibited a dipeptidyl aminopeptidase-like activity against penta-alanine, but P.
ruminicola isolates were the only bacteria with measurable Gly-Arg-MNAse (hereafter
referred to as PrtA) activity. McKain et al. (1992) extended these studies by isolating
bacteria from ruminal contents and screening for PrtA activity and of the twelve positive isolates obtained, ten were consistent with the subgroupings of Prevotella spp. outlined by Avgustin et al. (1994).
Wallaces group has since attempted to use chromatographic procedures to enrich
and purify the peptidases produced by Prevotella spp. Main peaks of activity following
size exclusion chromatography of P. bryantii and Prevotella brevis extracts were estimated to contain proteins in excess of 100 kDa (Wallace et al., 1995); ion-exchange
chromatography of Prevotella albensis cell extracts separated peptidase activity into four
distinct peaks with different substrate specificities (Wallace et al., 1997). Madeira et al.
(1997) found the inhibition profile of the PrtA was strikingly similar to that of gingipain, an extracellular, trypsin-like enzyme isolated from Porphyromonas (Bacteroides)
gingivalis, that requires cysteine for activation and calcium for stabilization (Chen et al.,
1992). Despite these efforts, none of these peptidases appear to have been purified to
homogeneity and the relative contribution of individual enzymes to proteolysis and
ammonium production by ruminal bacteria were not quantified. Within this context,
Madeira et al. (1997) chose to generate mutants of P. bryantii strain B14 defective in
PrtA activity for three primary reasons: (i) to assess the physiological role of this
enzyme activity on growth of P. bryantii; (ii) to quantify the contribution from this
enzyme to ruminal proteolysis, and ammonium production by the obligate aminoacid- and peptide-fermenting bacteria isolated by Russell and co-workers; and (iii) to
facilitate comparative examination of wild type and mutant strains to identify polypeptide(s) responsible for this activity. Two independently derived mutants were isolated by
Madeira et al. (1997) and in addition to the virtual elimination of measurable PrtA
activity in both mutants, activity towards Arg-Arg-MNA was also lost. Interestingly, a
second cysteine proteinase from P. gingivalis, termed argingipain, which presented a
narrow specificity for synthetic substrates containing Arg in the P1 site and hydrophobic amino acids in the P2 and P3 sites, has also been described (Kadowaki et al., 1994)
and subsequent studies showed gingipain and argingipain to be the same enzyme
(Okamoto et al., 1995). Therefore, the P. bryantii mutant strains may lack Arg-ArgMNAse activity because the same enzyme is responsible for both activities, rather than
the mutation(s) giving rise to polar or pleiotropic effects on the expression of multiple
genes. That the mutation(s) in the mutant strains does not result in pleiotropic effects
is further supported by the proteome profiles of the wild type and mutant strains,
which are virtually indistinguishable from each other. The similarity of the results
obtained with P. gingivalis, and P. bryantii, also suggests that this family of cysteine proteinases, although not identified in other eubacterial lines of descent, is widespread
amongst bacteria belonging to the family Bacteroidacea.
Although the physiological role of Gly-Arg-MNAse enzyme activity in P. bryantii
is still not clear, an ecological role for this enzyme activity was clearly demonstrated by
co-culture experiments with the obligate amino-acid-fermenting bacteria isolated by
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M. Morrison
Russells group. Gelatin hydrolysate was chosen as the substrate for these experiments,
to ensure that the results were directly comparable with earlier studies that established
the prominent roles of C. aminophilum and P. anaerobius in controlling ruminal ammonia production (Chen and Russell, 1989). Both the rate and extent of ammonia production was decreased approximately 25% when the P. bryantii mutants replaced the
wild type strain in co-cultures. Future strategies which specifically inhibit PrtA activity
should therefore result in productive alterations in ruminal ammonia production. We
also have isolated mutant strains of P. bryantii defective in their growth with peptides as
sole N-source (Peng and Morrison, 1995) and Wallaces group also have a set of mutant
P. albensis strains which appear to possess reduced levels of all measurable peptidases.
Further characterization of these mutants is currently underway and will probably provide further insights into the genetics and molecular biology controlling rate-limiting
steps in proteolysis and peptide uptake in this numerically dominant group of ruminal
bacteria. With such information, new productive strategies to control ruminal proteolysis should be forthcoming.
We have also used these mutagenesis procedures to examine ammonia assimilation
by P. bryantii. A number of Prevotella isolates have been found to lose cell viability following growth in a defined medium containing a relatively high concentration of glucose and a relatively low concentration of ammonia; this glucose toxicity was correlated
with methylglyoxal production and its accumulation in the growth medium (Russell,
1992). More detailed studies with P. bryantii showed that, unlike enteric bacteria,
methylglyoxal production occurred independently of phosphate limitation, perhaps
instead related to aspects of carbon and/or nitrogen metabolism (Russell, 1993). Given
that glutamate dehydrogenase (GDH) serves as a key enzyme linking carbon and nitrogen metabolism, we were interested in determining the role of this enzyme under different growth conditions. A mutant lacking GDH activity was isolated following EMS
mutagenesis and ampicillin enrichment procedures (Z. Wen and M. Morrison, unpublished data) and nucleotide sequence analysis of the gdhA gene from the mutant strain
confirmed a missense mutation had been created within one of the highly conserved
motifs of the GdhA protein subunit. A clone of the mutant gdhA gene also did not
complement the E. coli glutamate auxotroph used to successfully isolate clones of the
wild type gdhA gene from P. bryantii. A number of growth experiments have been conducted with this mutant strain, and the only measurable phenotype associated with the
GDH mutant strain was a slower growth rate when ammonium was used as the sole
nitrogen source (108 min as compared with 75 min for the mutant and wild type
strains, respectively). The mutant strain was also still capable of growth with both limiting (1 mM) and excess (10 mM) ammonium. The mutant strain was also incapable of
growth in the presence of the glutamine synthetase inhibitor methiosulphoxamine,
unless trypticase or a supplement of glutamine was provided. Although GDH activity
appears relevant for optimal growth of P. bryantii, these results confirm the bacterium
possesses more than one route of ammonium assimilation and glutamate biosynthesis,
which also appears to be functionally independent of the prevailing ammonium concentration in the growth medium.
In summation, mutagenesis strategies offer a valuable and productive means of
examining bacterial physiology, and to evaluate the implications on ruminal function
that may arise from the inhibition or elimination of specific enzyme activities.
Although the strategy outlined here has been productive, other methods of mutational
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M. Morrison
108
comparative model with other cellulolytic and/or Gram-positive bacteria. We are currently adapting these procedures to examine differential gene expression by P. bryantii in
response to nitrogen source and availability, and similar efforts are already underway in
other labs with Ruminococcus flavefaciens. The challenge will be to ensure that other genetic
technologies are also advanced, to ensure that these efforts extend beyond an exercise in
gene identification and influence our understanding of rumen physiological ecology.
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M. Morrison
110
putative cis-acting sequences, and a motif similar to the consensus NtcA (nitrogen control protein A) binding motif, the global nitrogen regulatory system of cyanobacteria
was identified. A role for this element in the regulation of gdhA expression in response
to nitrogen source was confirmed by PCR-mediated deletion of the motif, and using
the reporter gene vector described above. Despite the predicted changes in GdhA
enzyme activity in response to nitrogen source, xylosidase activity remained low in
those transconjugants containing the reporter gene fused with the mutagenized gdhA
promoter. From these results, we conclude the NtcA-like motif is actually a cis-acting
element, and that it facilitates a positive regulation of gdhA expression in response to
ammonium. In cyanobacteria, the NtcA protein is required for the transcriptional activation of glnN, the structural gene encoding the glutamine synthetase (GSIII) enzyme
subunit, which to date, has only been identified in the cyanobacteria, Bacteroides,
Prevotella, Butyrivibrio and Ruminococcus . The NtcA protein has also recently been
shown to repress gene expression in Anabaena PCC 7120, notably, the rbcLS operon,
which encodes the large and small subunits of ribulose-1,5-bisphosphate carboxylase
(Rubisco; Jiang et al., 1997). It appears that in cyanobacteria, the NtcA protein has a
role to play in balancing rates of nitrogen and carbon assimilation. Although putative
NtcA binding motifs can be identified upstream of various nitrogen assimilation genes
from the ruminal bacteria mentioned above, further studies are required to confirm
conservation of this regulatory protein, in addition to the glnN gene.
111
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Introduction
As we enter the new millennium, we can reflect on recent efforts and know that an
important body of literature, primarily from biomedical research, has accumulated over
the last 10 years that is helping to define the physiological relevance of peptide absorption. No longer are we limited to just suggesting that peptide absorption occurs, now
we are aware of the existence of special proteins that are responsible for transmembrane
movement of peptides. The mRNA for some of these proteins has been cloned and the
structural and functional characteristics of the encoded proteins are being determined.
As the basis for understanding peptide absorption has grown, so also has the interest of
animal scientists. Our laboratory has been conducting investigations in this area for
over 20 years and now several others join us. In contrast with the biomedical community, our interest as animal scientists is to explain the appropriate dietary foundation of
nutrient absorption that will result in the desired growth, development, and production in animals. Thus, we are interested in peptide absorption from the gastrointestinal
tract, but our interests go beyond this. We are interested in knowing what, if any, role
peptides may have as sources of amino acids for protein synthesis in different tissues
and how they might be involved in controlling this process. These and other issues will
be addressed in this paper. The primary focus will be on the ruminant, but some attention will be given to the human and laboratory species, simply because of the greater
availability of information regarding these species.
117
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K.E. Webb, Jr
119
0.40 mM previously observed in our laboratory (Matthews et al., 1996b). The Kts for
Met-Met, Lys-Lys and Glu-Glu uptake at pH 5.5 were 27.5, 20.4 and 28.7 mM,
respectively. Affinity constants ranging from 81 mM for Gly-Leu to 2.5 mM for GlyGly were reported by Fei et al. (1994).
Peptide absorption across ruminal and omasal epithelia has been demonstrated in
vitro as has mRNA in sheep omasal epithelial cells that encodes for a peptide transporter(s). Substrate specificity of the transporter(s) indicates that many but not all di-,
tri- and tetrapeptides can be transported. Initial kinetic evaluation of ovine PepT1 indicates a range in affinity constants for substrates.
While results from our initial research indicated that absorption may be largely by
non-mediated processes, it is obvious that mechanisms for mediated transport of peptides in the ruminant stomach are present. Omasal epithelium was collected from sheep
and mounted in parabiotic chambers to measure the uptake of Gly-Sar in the presence
of other peptides (McCollum and Webb, 1998). If multiple substrates are transported
on a given transporter, as peptides are believed to be, then they compete with one
another if more than one is present. To our surprise, co-incubating high levels of potentially competing peptide substrates with Gly-Sar resulted in a stimulation and not a
depression in Gly-Sar uptake. Even though our previous results clearly indicate that
mRNA for a peptide transporter is present in omasal epithelium, these results are certainly not consistent with mediated transport being the only mechanism involved in
peptide transport across omasal epithelium. Madara and Pappenheimer (1987) discussed paracellular transport through intestinal epithelia and showed that a prerequisite
for this process is the mediated uptake of the substrate. It may be that paracellular
transport is the mechanism that was responsible for what appeared to be a nonmediated uptake of peptides by epithelial tissues mounted in parabiotic chambers
(Matthews and Webb, 1995).
From the evidence that is accumulating, it seems reasonable to suggest that multiple
mechanisms may be involved in the transport of peptides across gastrointestinal
epithelia. Validation of the presence of and the clarification of the relative importance
of each of these mechanisms will increase our understanding of peptide absorption and
the contribution this may make to the overall nutritional status of the animal.
K.E. Webb, Jr
120
the five sheep and three cows tested, the pattern of distribution appeared to be consistent within the same species, although the relative abundance varied between animals.
Among these tissues, the abundance of PepT1 was higher in the jejunum and ileum
in comparison with omasum and duodenum, while abundance was lowest in the
rumen. PepT1 was not detectable in the abomasum, liver, kidney, caecum, colon,
longissimus and semitendinosus muscles in either sheep or cows, or in the mammary
gland in cows.
121
dicted structural model shows a long hydrophilic loop on the extracellular side containing several N-linked glycosylation sites. With all the 12 a-helices accommodated
within the membrane, both the amino and carboxyl termini are located on the cytoplasmic side. These PepT1 proteins are all highly homologous. The amino acid
sequence of the rat PepT1 is 77% and 83% identical with the rabbit and human
PepT1, respectively (Saito et al., 1995). The rabbit and human PepT1 are 81% identical (Liang et al., 1995). The amino acid sequences of these transporters do not show
strong homology with other known classes of transport proteins. It is worth mentioning that, while all of the transmembrane domains are highly conserved, the extracellular loops are much less so. Structural differences among these transport proteins
indicate that there are different numbers and locations of sites for protein kinase A and
C phosphorylation. The extent to which these differences may affect the function of
these transporters is still not clear.
Another peptide transporter (PepT2) that is structurally and functionally different
from PepT1 has been identified (Liu et al., 1995; Boll et al., 1996; Saito et al., 1996).
The human full-length PepT2 cDNA is 2.7 kb with an open reading frame of 2.2 kb
(Liu et al., 1995). The rat PepT2 cDNA is 3.9 kb (Saito et al., 1996). Although the
sizes of these PepT2 cDNAs are different, they both encode a 729 amino acid protein
(Boll et al., 1996). The shared features include the putative 12 transmembrane domains,
the large extracellular loop, and several sites for protein kinase dependent phosphorylation.
These structural features are observed in both the PepT1 and PepT2 transporter
groups. Even with these similarities, these appear to belong to two distinct transporter
groups. When PepT1 and PepT2 are compared from the same species, they display
lower identity than do the members in the same group. For example, human PepT2
has 83% amino acid identity with rat PepT2 whereas only 50% with human PepT1.
PepT2 proteins have a greater molecular mass than PepT1.
Transport activities of these cloned peptide transporters have been characterized
mainly by in vitro expression of the cDNA in X. laevis oocytes or other cell lines.
Transport studies have shown that the cloned peptide transporters are capable of taking
up a broad range of di- and tripeptides, regardless of whether they contain acidic, basic
or hydrophobic amino acids. The peptides transported may be in either the charged or
neutral form under different conditions. Results from studies designed to examine the
effects of the net charge of a substrate on peptide transport suggest that neutral substrates are preferred by the peptide transporter when compared with charged peptides
under physiological pH conditions (Lister et al., 1997). Amasheh et al. (1997) suggest
that, under physiological conditions, the affinity of PepT1 for zwitterionic or anionic
substrates is greater than for cationic substrates. For transport of a cationic substrate,
the pH must be higher.
One of the features of peptide transporters is the necessity for the proteins to cotransport a proton along with the peptide substrate. Brandsch et al. (1997) studied the
effect of protons on the affinity and Vmax of Gly-Sar uptake by Caco-2 (PepT1) and
SKPT cells (PepT2). Uptake of Gly-Sar in both cells was measured over a concentration range at an extracellular pH of either 6.0 or 7.0. In Caco-2 cells, the Kt was
1.0 mM at pH 6.0 and 1.2 mM at pH 7.0. However, the Vmax was 13.7 0.3 nmol 10
min21 mg21 of protein at pH 6.0 and 5.8 0.3 nmol 10min21 mg21 of protein at pH
7.0. Similar results were obtained with SKPT cells. In both cell types, protons affected
only Vmax but did not affect the affinity of the transporters for the substrates. Steel et al.
122
K.E. Webb, Jr
(1997) suggested that there is a protonpeptide coupling ratio of 1:1, 2:1 and 1:1 for
neutral, acidic and basic dipeptides, respectively. They also showed that, at a pH of
5.56.0, PepT1 favoured substrates in neutral and acidic forms.
Given the role of these proteins as peptide transporters, factors that influence the
protein structure, its localization on the membrane, and the specific position for substrate binding may all have an effect on the transport activity. It was reported that,
among histidyl residues present, His-57 in PepT1 and His-87 in PepT2 are the most
critical histidyl residues necessary for the uptake function and probably represent critical binding sites (Fei et al., 1997).
It may be that protein kinases C and A are involved in the regulation of peptide
transport. Muller et al. (1996) showed that increased intracellular levels of cAMP in
Caco-2 cells that expressed PepT1 resulted in a 50% reduction of Gly-Sar uptake. Zinc
is an essential trace element that plays a fundamental role in the structure and function
of many proteins, e.g. stabilizing the structure of an enzyme, being an essential component of the active site of an enzyme, and as a regulatory factor. Daniel and Adibi (1995)
concluded that zinc had a selective effect on peptide transport. They incubated brushborder membrane vesicles (BBMV) with zinc sulphate and observed an increased
uptake of Gly-Gln and Leu-Tyr, without changing the diffusion rate of the substrates.
Zinc had no effect on the uptake of either Gln or glucose by BBMV.
Information on the distribution of the peptide transporters among various tissues
comes largely from the search for the messenger RNA that encodes for the protein.
Along with the cloning of PepT1 and PepT2 from various species, distribution of their
mRNA has been studied. The basic technique is Northern blot analysis using a specific
radiolabelled DNA probe, where for most studies reported, full-length cDNA was
used. A 2.9 kb mRNA was found in the small intestine of the rabbit, while much lower
levels were observed in the liver and kidney and only trace amounts were found in the
brain (Fei et al., 1994). No mRNA was detected in the colon, skeletal muscle, heart,
spleen or lung. The mRNA for PepT1 was 2.93.0 kb in the rat, the major location of
which was in the small intestine (Saito et al., 1995; Miyamoto et al., 1996). Trace
amounts of mRNA were found in kidney cortex, but none was observed in liver. Liang
et al. (1995) reported the size of human PepT1 mRNA to be 3.3 kb, based on
Northern blot analysis. They observed a major presence of this PepT1 mRNA in the
small intestine as well as in an intestinal epithelium derived cell line (Caco-2). They
also observed PepT1 mRNA in kidney, placenta, liver and pancreas. The mRNA transcript was absent in muscle, brain and heart.
A ~4 kb mRNA for PepT2 was found mainly in the kidney medulla and at lower
abundance in the kidney cortex (Saito et al., 1996). The mRNA was also detected in
brain, lung and spleen, but was undetectable in the heart, liver and small intestine. In
the rabbit, a 4.8 kb mRNA was found in the kidney cortex as well as in brain, lung,
liver and heart (Boll et al., 1996). PepT2 mRNA was detected in human kidney and
small intestine (Liu et al., 1995).
PepT1 mRNA was expressed all along the small intestine and to a much lesser
extent in the colon (Freeman et al., 1995). PepT1 mRNA was not detected in the stomach, sacculus rotundus or caecum. Expression was restricted to the epithelial surface of the
small intestine and there was no detectable expression in deeper tissues such as the
lamina propria, muscularis mucosa or serosa (Fei et al., 1994). Along the cryptvillus
axis, the mRNA was detected at or above the cryptvillus junction with the maximal
123
expression occurring at about 100200 mm above the junction. The mRNA was absent
in the lower-to-mid crypt throughout the entire small intestine.
Ogihara et al. (1996) were the first to investigate the localization of the transporter
protein itself instead of its mRNA. They developed an anti-PepT1 antibody and used it
(by means of immunoblotting) to look for PepT1 protein in the rat. Their results confirmed the exclusive expression of PepT1 throughout the length of the small intestine
and the absence of PepT1 in the crypt. They also found that PepT1 was specific to the
differentiated absorptive epithelial cells and was located mainly on the brush-border
membrane of the cell.
From these results, we can conclude that PepT1 and PepT2 are distributed differently between the tissues. PepT1 is mainly an intestinal peptide transporter whereas
PepT2 is mainly a renal peptide transporter. Interestingly, PepT1 is also detectable in
the kidney. The existence of PepT1 in the small intestine of animals suggests its nutritional importance. The PepT2 in the kidney may play a significant role in conserving
peptide forms of amino nitrogen by means of reabsorption (Daniel and Herget, 1997).
Peptide absorption
In vitro characterization of gastrointestinal absorption of peptides
Casein, soybean meal and distillers dried grains were incubated in a buffered ruminal
fluid inoculum for 8 h (Jayawardena, 2000). Following incubation, cell-free supernatants were obtained by centrifugation and these were used as the mucosal fluids in
parabiotic chambers containing either ruminal or omasal epithelium. Initially, free
amino acid concentrations ranged from 7.2 to 60 mg l21 and peptide amino acid concentrations ranged from 100 to 270 mg l21 in these mucosal buffers. Serosal appearance of free and peptide amino acids was measured after 240 min. For ruminal
epithelium, serosal appearance of free amino acids from casein, soybean meal, and distillers dried grains was 242, 220 and 234 mg l21 mg21 dry tissue, respectively.
Corresponding figures for peptide amino acids were 493, 329 and 453 mg l21 mg21
dry tissue. For omasal epithelium, serosal appearance of free amino acids from casein,
soybean meal and distillers dried grains was 438, 323 and 340 mg l21 mg21 dry tissue.
Corresponding figures for peptide amino acids were 2249, 807 and 1191 mg l21 mg21
dry tissue. Serosal appearance of peptide amino acids was greater than serosal appearance of free amino acids in both tissues, probably reflecting the concentration effect of
the substrates in the mucosal buffer. Movement through omasal epithelium was much
greater than through ruminal epithelium, especially for peptides.
Uptake of Gly-Sar by sheep jejunal and ileal BBMV in a study we conducted
showed that these membranes have the capability of translocating this dipeptide
(Bowers, 1997). Uptake was greater in BBMV from jejunal tissue than from ileal tissue.
Uptake of 0.3 mM Gly-Sar was not stimulated by an inwardly directed H+ gradient
(pH 6.4 outside, pH 7.5 inside) in either jejunal or ileal BBMV.
Uptake of 10 mM Gly-Pro by BBMV prepared from sheep duodenal epithelium
was reported by Backwell et al. (1995), who found that uptake was dependent on a H+
gradient (based on results obtained from an intravesicular pH of 8.4 and extravesicular
pHs of 6.0 and 8.4). At first glance, the results of these studies appear to be in conflict.
K.E. Webb, Jr
124
A careful examination of the protocol reported by Backwell et al. (1995) reveals that
the extravesicular pH in their study was probably closer to 7.0 than to the pH 6.0
reported. Furthermore, an extravesicular pH of 8.4 would cause peptide substrates to
be more negatively charged than would occur physiologically. Because peptide molecules need to be in zwitterionic form in order to be transported (Ganapathy and
Leibach, 1985), the results reported by Backwell et al. (1995) may be due to a reduction in uptake because of the charged nature of the peptide substrate at pH 8.4 rather
than an enhancement of uptake in the presence of a H+ gradient.
BBMV prepared from the proximal jejunum of dairy cows was used to examine
Gly-Sar uptake (Wolffram et al., 1998). Results from this study clearly indicate that an
inwardly directed H+ gradient stimulated a greater rate of uptake of Gly-Sar. In contrast to the two previous studies (Backwell et al., 1995; Bowers, 1997), this transport
was uphill. There is no clear explanation for this concentrative uptake of a peptide. The
authors attributed their ability to measure concentrative uptake to the fact that they
used a low substrate concentration (25 mM) while others used much higher concentrations (0.10.8 mM). Even though Backwell et al. (1995) used a concentration of 10
mM and did not observe concentrative uptake, there may be other problems with the
design of this study, as previously discussed, that preclude observing a concentrative
uptake. Wolffram et al. (1998) argue that if a transport mechanism with a low substrate
affinity or transport capacity or both is responsible for peptide transport, then, under
conditions of a high substrate concentration, the driving forces for uphill transport
might be exhausted well before significant amounts of the substrate accumulates inside
the vesicle.
125
ance of free amino acids in blood was greater than peptide amino acids across MDV
than across PDV. Conversely, net appearance of peptide amino acids in blood was
greater than free amino acids across PDV than across MDV. This is consistent with the
ruminant stomach contributing significantly to the appearance of peptides in blood.
Gastrointestinal absorption of peptides was examined in sheep fed a forage-based
diet (Backwell et al., 1997). They failed to observe any net flux of peptide amino acids
across either MDV or PDV. They also observed lower peptide concentrations in blood
plasma than reported previously by Koeln et al. (1993) and Seal and Parker (1996).
They attributed this absence of a net flux of peptide amino acids across MDV and
PDV as being due to a matter of technique, more specifically a more efficient deproteinization of blood plasma, thus not creating artificially high peptide concentrations.
Lactating dairy cows were fed diets based on either dry-rolled or steam-flaked
sorghum and portal fluxes of free and peptide amino acids were quantified (H. Tagari
et al., personal communication). As would be expected, there was a positive portal flux
for all amino acids. The quantity of peptide amino acids appearing in the portal vein
was lower than free amino acids, but appearance was positive for 70% of the amino
acids. This included fluxes differing (P < 0.059) from zero for Ala, Arg, Asp, Glu, Gly,
His, Leu, Lys, Met, Ser, Thr and Val. The deproteinization procedure used in this study
involved protein precipitation with methanol followed by ultrafiltration through
3000 mol. wt filters. Residual peptides would, therefore, be small.
Evidence suggests a net appearance of peptide amino acids in portal and possibly
mesenteric blood plasma. Methodological differences are likely contributors to variations observed in the magnitude of portal appearance of peptide amino acids. The procedure employed by H. Tagari et al. (personal communication) appears to be the
soundest yet employed. Continued efforts in this area will provide further clarification
regarding the absolute magnitude of the contribution of peptide amino acids to amino
acid flux across MDV and PDV.
K.E. Webb, Jr
126
serve as amino acid sources for protein accretion in sheep skeletal muscle. In some
cases, the molecular arrangement of the dipeptides with the same amino acid composition influenced the relative ability of the dipeptides to serve as methionine sources. For
all peptides studied, however, only Ala-Met was utilized to support protein accretion as
well as free methionine.
127
teins promoted by Gly-His-Lys was about 17% greater than that promoted by lysine.
The other peptides were not different from lysine in promoting protein synthesis which
ranged from 91 to 108% of the synthesis promoted by lysine. Within each of the three
peptide pairs, Asp-Lys and Lys-Asp, Gly-Lys and Lys-Gly, and Val-Lys and Lys-Val,
location of the lysyl residue at either the N- or C-terminal position did not affect protein synthesis.
These results are consistent with those previously discussed for cultured MAC-T
mammary epithelial cells. Together, these studies indicate that a wide range of peptidebound methionine and lysine substrates can support the synthesis of milk proteins by
mammalian epithelial cells, at least as well as free methionine and lysine.
K.E. Webb, Jr
128
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Influence of Gastrointestinal
Metabolism on Substrate Supply
to the Liver
C.J. SEAL AND D.S. PARKER
Department of Biological and Nutritional Sciences, Faculty of Agriculture and
Biological Science, University of Newcastle upon Tyne, Newcastle upon Tyne,
UK
Introduction
Tissues of the splanchnic bed include the gastrointestinal tract (GIT), liver, spleen,
pancreas and mesenteric fat depots. Cumulatively these organs, together with the associated connective tissue and blood vessels, contribute approximately 1520% of total
body mass. During pregnancy and lactation, hypertrophy of the GIT can increase this
still further. Collectively the splanchnic bed plays a central role in moderating nutrient
supply to peripheral tissues for maintenance and productive processes such as muscle
deposition, wool growth and milk production. Liver output is affected by the pattern
and quantity of nutrients delivered to the portal vein from the GIT and this process is
discussed separately in Chapter 9. Intestinal tissues maintain a high rate of metabolic
activity which is sustained by both the luminal supply of nutrients available during the
process of absorption and also from metabolites derived from the arterial blood supply.
This chapter will review the effects of GIT metabolism on the supply of energy-yielding substrates to the liver, and will examine the effects of changing diet on the supply of
nutrients such as short-chain volatile fatty acids (SCVFA), glucose and amino acids to
the liver.
131
132
active tissues. For the GIT, the additional effects of compartmentalization of the
digestive process along the tract cannot be identified with reference to portal blood
alone.
Technically, the biggest advance in understanding splanchnic metabolism has been
the development of in vivo, multi-catheterization procedures to sample portal blood
and blood draining other areas of the GIT. Elegantly described by Huntington et al.
(1989) for cattle, this has provided the benchmark for much of the work in this field
over the past decade, following the early work of Bergmans group with sheep (Katz and
Bergman, 1969). Anatomically, however, there are significant differences between cattle
and sheep, which has made direct comparison of catheterization techniques difficult.
The common portal vein of cattle, for example, is much shorter than in sheep making
the implantation of transonic flow probes complex (see later). The proximity of the gastro-splenic vein also makes location of the catheter tip difficult (Huntington, 1982).
Additionally, the venous drainage of sheep is not associated with discrete gut components as observed with the closed loop draining into a mesenteric vein in cattle (Seal
and Reynolds, 1993). In an attempt to resolve this problem in sheep, Neutze et al.
(1994) have described a complex procedure in which they have measured a-aminoN
uptake in the caudal mesenteric vein upstream and downstream of the ileocaecocolic
vein junction. The surgery involved in this procedure is technically demanding, requiring the precise location of catheters in small blood vessels. Their results, however, were
very comparable with similar data from mesenteric vein samples obtained from cattle,
suggesting that this may be a reasonable approach for future studies.
133
mination of blood flow, regardless of the dye used, is dependent on complete mixing of
the dye with the blood before it reaches the tip of the sampling catheter. This is
usually achieved by implanting the infusion catheter several centimetres distal to the
sampling tip. However, streaming of blood through vessels may still be a major factor
in causing variability in this data and can be reduced by placing the catheter tip in the
lateral lobe of the liver beyond the turbulent flow of the porta hepatis (Seal and
Reynolds, 1993).
Direct measurement of portal and mesenteric vein blood flow is possible through
the use of transonic flow probes. This procedure has been in use for more than 30 years
(Carr and Jacobson, 1968), but has been limited by technical difficulties during surgical implantation and in calibration. This is a particular problem for large cattle, where
the size and accessibility of the portal vein severely restricts the positioning of the probe
(Huntington et al., 1990). The size of the probe necessary to encase the portal vein in
these animals may also influence the transmission of the Doppler signal across the vein,
giving false measurements of blood flow. The procedure has been used with greater success in sheep (Neutze et al., 1994; Remond et al., 1998) and in small growing steers
(Kim et al., 1998). In some of these studies it has been reported that blood flow determined by ultrasonic transit-time flow meters underestimates blood flow compared with
indicator dilution techniques (Huntington et al., 1990; Kristensen et al., 1996;
Remond et al., 1998), but this observation is not consistent, and may be improved
with the use of newer types of probe (Remond et al., 1998). This underestimate is presumed to be due to turbulence in the blood flow through the probe. The principal
advantage of the transonic probes is the ability to measure blood flow continuously
over long time periods using automated data-handling systems. This reduces the frequency with which blood samples are taken, and gives an integrated blood flow profile
which can be used to describe and model diurnal variations. Measurements of this type
in steers show that blood flow in twice-daily-fed animals varies in a sinusoidal manner
(Fig. 8.1; Kim et al., 1998) which is similar to data obtained using frequent blood sampling and complex non-parametric curve-fitting methods (Whitt et al., 1996). Future
developments in transonic probe technology will further increase the value of this
methodology.
134
Fig. 8.1. A typical circadian pattern for portal vein blood flow in a 150 kg
HolsteinFriesian steer (feed offered at 60 and 780 min). Reproduced with permission
from Kim et al. (1998).
for example acetate, net fluxes are positive across ruminal tissues, but may be negative
across mesenteric-drained tissues (Seal et al., 1992; Seal and Parker, 1994). Hind-gut
fermentation may also result in positive fluxes of SCVFA into the caecal vein
(DeGregorio et al., 1984).
Isotopic labelling of nutrients in combination with classic VA techniques provides the opportunity to estimate true rates of absorption from the gut and can be used
to determine rates of utilization of nutrients within gut tissues as well as sequestration
of nutrients from the blood supplying the gut. Combinations of luminal and vascular
infusions of labelled substrates can be used to quantify whole-body irreversible loss
rates for individual nutrients, inter-conversion of metabolites and metabolism across
tissues in single experiments, thus maximizing the data produced from a multi-faceted
approach. If these studies are further combined with surgical cannulation of the gut tissues, direct measurements of intestinal disappearance of nutrients can be compared
with simultaneous measurements of nutrient flux into the venous drainage.
Improvements in analytical procedures and developments in stable isotope technologies
will undoubtedly further increase the potential for this type of study.
135
approximately 20% for the GIT alone (Cant et al., 1996). These tissues account for
about 1013% of total body mass, and thus, on a unit mass basis, their metabolic
activity is considerable. Quantitatively, the major components of this energy expenditure are Na+, K+-ATPase-linked ion transport systems (approximately 3060% of the
expenditure) and protein turnover (2023% for protein synthesis and 4% for protein
degradation; McBride and Kelly, 1990). Factors which influence energy expenditure
include meal consumption (Christopherson and Brockman, 1989; Kelly et al., 1989)
and energy intake expressed as either metabolizable energy (see Seal and Reynolds,
1993) or digestible energy (Goetsch and Patil, 1997; Goetsch, 1998). Physiological
state also has a major impact on intestinal energy expenditure. For example, pregnancy
and lactation are associated with increased intestinal tissue mass (Fell et al., 1972;
Hammond and Diamond, 1994). This latter response, observed in both ruminant and
non-ruminant species, results in increased mucosal mass and concomitant increases in
protein synthetic rates in GIT tissues. This increased intestinal transport capacity also
results in the synchronous upregulation of all intestinal transporters (Bird et al., 1996).
The relationships between digestible energy intake, fibre digestion and urea-N flux
have been described in some detail by Goetsch (1998) in a series of regression equations based on 11 separate experiments with sheep. These data show that, in forage-fed
animals, there is a complex interaction between portal-drained viscera energy use and
digestible energy intake and neutral detergent fibre (NDF) digestion; these factors and
hepatic urea net flux also impact on total splanchnic energy use and increase with
increased forage consumed. The combined effects of these variables, especially the
increased energy required in hepatic ureagenesis to dispose of ruminal ammonia
(Lobley et al., 1996b), may account for the observed inefficiency of growth observed in
animals fed forage-based diets. The processes involved in changing GIT energy expenditure, and the consequences on animal performance, have been the focus of several
recent reviews (Lobley, 1994; Cant et al., 1996; Goetsch, 1998).
136
Absorption of SCVFA
SCVFA may be absorbed across the rumen epithelium in both undissociated and dissociated forms. Undissociated SCVFAs are lipid soluble and cross the epithelial membrane more easily than the dissociated form; absorption rates of the undissociated acids
are thus affected by the pH of ruminal fluid although decreasing pH only results in a
small increase in SCVFA clearance (Dijkstra et al., 1993). In contrast to non-ruminant
colonic and caecal tissue (Holtug et al., 1992), Na+/H+ exchange is not involved in
SCVFA absorption (Lpez et al., 1996). Dissociated SCVFAs are absorbed via anion
exchange with bicarbonate, in a mechanism which appears similar for non-ruminant
large intestinal and ruminal tissues and involves an additional electroneutral anion
exchange between bicarbonate and chloride.
137
Table 8.1. Net portal flux of acetate, propionate and butyrate in sheep and cattle expressed as a
proportion of measured ruminal production rate. (Updated from Seal and Reynolds, 1993.)
Proportion of rumen production rate
appearing in portal vein
Animal
Diet
Lambs
Intragastric infusion
Lambs
Intragastric infusion
Sheep
Intragastric infusion
Sheep
Sheep
Forage
High fibre
Low fibre
Concentrate
Concentrate
Forage
Concentrate
Forage
+ 0.5 mol propionate day21
+ 1 mol propionate day21
Concentrate
+ butyrate
Cattle
Cattle
Steers
Steers
Steers
0.69
0.62
0.57
0.69
0.43
0.48
0.52
0.96
0.74
0.76
0.77
0.52
0.78
0.66
0.66
0.67
0.57
0.59
0.49
0.51
0.58
0.49
0.40
0.30
0.30
0.52
0.56
0.58
0.25
0.14
0.15
0.28
0.28
0.26
Referencea
a
c
0.08
0.08
0.66
d
e
e
f
g
h
i
0.47
0.31
0.34
0.32
0.30
0.29
aData
from: a, Gross et al., 1990b; b, Gross et al., 1990a; c, Weekes and Webster, 1975; d, Bergman, 1990;
e, Calculated from Linington et al., 1998a, and Linington et al., 1998b; f, Harmon et al., 1988; g, Calculated
from Huntington and Reynolds, 1983 and Sharp et al., 1982); h, Seal et al., 1992; i, Seal and Parker, 1994;
j, Krehbiel et al., 1992.
Glucose metabolism
Glucose metabolism by gut tissues
Glucose metabolism in ruminant species is dominated by the requirement for suitable
precursors for gluconeogenesis, reflecting the lack of glucose absorbed from the digestive tract of forage-fed animals. Glucose requirements for tissue metabolism are similar
to those of other species (Weekes, 1991), although glucose sparing from cellular activity such as fatty acid synthesis limits carbon flux through key metabolic pathways.
Experiments which measure overall glucose utilization by the animal and the contribution of different tissues to that flux, show that the gut plays a major role in these
processes. Glucose has been shown to be an energy substrate for the intestinal mucosa
138
(Stangassinger and Giesecke, 1986; Britton and Krehbiel, 1993; Okine et al., 1993)
and calculations of net glucose utilization by ruminants fed a range of diets (Parker,
1990) demonstrated that, in most dietary situations, the gut is a net consumer of glucose (negative VA difference across the tissue). In studies in pregnant and lactating
sheep (Van der Walt et al., 1983; Perry et al., 1994), glucose utilization by both the
mesenteric-drained viscera (MDV) and portal-drained viscera (PDV) increased in animals post partum, with a concomitant rise in lactate production. In studies with sheep
and steers where glucose irreversible loss (GIL) has been manipulated by either increasing glucose or propionate supply, a similar relationship between portal glucose utilization and GIL has been shown (Seal and Parker, 1994; Balcells et al., 1995; Piccioli
Cappelli et al., 1997); the results of these studies are summarized in Fig. 8.2. These
data clearly show that the use of glucose by gut tissues increases in line with whole body
turnover rate. Further work in our laboratory has also shown that glucose utilization by
gut tissues is responsive to changes in both vascular and luminal glucose supply
(Piccioli Cappelli et al., 1997), underlining the ability of the tissue to capitalize on both
sources of the nutrient.
Fig. 8.2. Relationship between whole body glucose irreversible loss rate (mmol min21)
and portal glucose utilization rate (mmol min21) in sheep and steers (data from
Balcells et al., 1995, and C.J. Seal, D.S. Parker, J.C. MacRae and G.E. Lobley,
unpublished data; y = 0.25x + 0.007, r = 0.642).
139
Fig. 8.3. Relationship between starch intake (g day21) and digestibility (expressed as a
percentage of intake) in the rumen (; y = 24.24x + 95.83, r = 0.793), postrumen (v;
y = 3.61x + 1.35, r = 0.859) and total tract (; y = 20.64x + 97.66, r = 0.404) of beef
and dairy cattle. Data from Huntington (1997).
140
and that in many cases the capacity of the ruminant small intestine to absorb glucose is
in excess of supply. These data also suggest that any reduction in starch digestibility is a
result of insufficient amylase activity and that stimulation of amylase synthesis/ release
would overcome this limitation.
Recent experiments investigating the expression of SGLT1 Na+/glucose co-transporter gene in gut tissues from lactating cows (Zhao et al., 1998) confirm that the
epithelial cells throughout the digestive tract, including the rumen and omasum, have
the capacity to actively transport glucose. The kinetics of the transport process show
that the system has a high affinity for D-glucose with a Km of about 0.1 mM confirming
earlier studies by Bauer et al. (1995). In these experiments, ruminants unadapted to
digesting starch were shown to have an ability to transport glucose which was blocked
by the presence of phlorizin. In contrast to earlier work with sheep (Shirazi-Beechey et
al., 1991), these later experiments demonstrated that both cattle and sheep maintained
a capacity for glucose uptake by the small intestine in the absence of a flow of dietary
starch to the small intestine, an observation confirmed in work in our own laboratory
(Balcells et al., 1995; Piccioli Cappelli et al., 1997). The relationship between transporter number at the apical and basolateral membranes and gut capacity to transport
glucose has been reviewed (Ferraris et al., 1992; Croom et al., 1998) and endocrine factors involved in transport regulation identified (Bird et al., 1996). Almost all these
studies have been carried out on monogastric species, apart from one in which sheep
treated with bST showed up-regulation of Na+-dependent glucose transport in the duodenum (Bird et al., 1994). This effect was not apparent in jejunal or ileal tissue indicating a site-specific response, possibly promoted by release of factor(s) such as epidermal
growth factor.
141
of digestion monitored across the gut. Energy release by the PDV per unit of digestible
energy intake was the same, whichever site was used for starch infusion. Net release of
glucose by the PDV, however, was higher during simultaneous infusion of starch and
casein into the abomasum, when compared with abomasal starch infusion and intraruminal casein infusion. These data support an earlier hypothesis that increased protein
flow to the duodenum stimulated pancreatic enzyme release, resulting in increased
digestibility of starch escaping fermentation in the rumen. Analysis by Huntington
(1997) on the impact of maize inclusion in the diet of feedlot steers and high-yielding
dairy cows on glucose kinetics and the use of glucose by the gut tissues identifies the
contribution of post-ruminal starch digestion to splanchnic tissue metabolism.
Although this is significant in both production situations, it is apparent that for the
growing steer increased fermentation of the available starch in the rumen, resulting in
energy substrate release and an increase in microbial protein flow might improve the
overall efficiency of nutrient use for growth.
142
sequestered in the gut tissue arise from the arterial supply to the gut. These data are
shown in Fig. 8.4, which includes data for the proportion of whole body flux of
selected amino acids which is utilized by the gut. These are important observations
because they indicate that, far from capitalizing on the availability of dietary amino
acids, the GIT is effectively competing with the other tissues of the body for arterially
supplied amino acids, similar to the situation with glucose (see above). Recent work
from Lobley et al. (1998) in which increased levels of a mixture of amino acids were
infused into the mesenteric vein tends to support this hypothesis. During these infusions, net appearance of amino acids across the PDV was below the theoretical level,
suggesting that either there was an inhibition of amino acid uptake from the gut or
there was increased uptake of amino acids from the systemic circulation. Sequestration
of amino acids in gut proteins represents a significant element in overall amino acid
flux, whether it is derived from arterial or luminal supply. The extent to which this
process can be manipulated in ruminant species is unclear. Studies in which steers were
treated with recombinant somatotrophin during infusion of casein into the abomasum
(Bruckental et al., 1997) demonstrate that hormone treatment can alter the partition of
amino acids into gut tissues resulting, in this case, in reduced PDV flux. Similarly, Yu et
60
(0.81)
(0.85)
50
(0.84)
(0.83)
mmol day21
40
(0.78)
30
(0.48)
20
(0.77)
10
0
Leu
Val
Lys
Thr
Iso
Phe
His
0.43
0.62
0.56
0.50
0.52
0.51
0.32
Fig. 8.4. Rates of amino acid sequestration (mmol day21) in gut tissues from arterial
(open portion of bars) and luminal (hatched portion of bars) precursors in sheep fed
800 g day21 lucerne pellets (reproduced with permission from MacRae et al., 1997).
143
144
will provide a framework for improving the efficiency of protein utilization by ruminant species.
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148
Introduction
The liver is positioned at the anatomic and metabolic crossroads of the body and, as
such, controls the amount and nature of nutrients available to the peripheral tissues
from digestive tract absorption. This role in nutrient partitioning encompasses all of
the major macronutrients and may be influenced by the involvement of the liver in the
production (insulin-like growth factor-I, IGF-I) and removal (insulin, glucagon) of key
hormones. For the sake of brevity, this review will focus on the metabolic products of
protein-N, particularly ammonia and amino acids. Hepatic detoxification of the former, to prevent deleterious peripheral hyperammonaemia, is probably a more vital
function in ruminants than non-ruminants, particularly for grazing animals. Because
peripheral hyperaminoacidaemia may also produce adverse effects, it is evident that the
regulation of systemic plasma amino acid concentrations is also important. This is
achieved by modifying the quantity of individual absorbed amino acids removed by the
liver and, in particular, controlling the amount oxidized versus the proportion converted to other metabolites, such as the plasma export proteins. Although the biochemical pathways involved in these partitions are well documented and a number of recent
reviews have described the quantitative events (e.g. van der Walt, 1993; Reynolds,
1995; Lobley and Milano, 1997), the regulatory aspects still remain obscure.
Furthermore, the limited data available to provide a conceptual framework for metabolic control mechanisms are often obtained from non-ruminants, or in vitro studies,
and may not apply directly to ruminants. The shortcomings of these approaches should
always be kept in mind.
Liver architecture
The liver is the most richly vascularized organ in the body, with blood comprising
approximately 25% of its mass. Despite constituting less than 2% of body weight in
CAB International 2000. Ruminant Physiology: Digestion, Metabolism,
Growth and Reproduction (ed. P.B. Cronj)
149
150
adult ruminants, the liver also receives approximately 25% of cardiac output. Total
blood flow through the liver is sensitive to nutrition, plus other regulatory factors, and
can reach values of 3 l min21 kg21 liver weight (or 60 ml min21 kg21 body weight).
Blood supply to the liver arises from two sources, the hepatic portal vein and the
hepatic artery, although the latter makes only a small contribution to total hepatic
blood flow in cattle and sheep (812%; Reynolds, 1995; Isserty et al., 1998).
Regulation of hepatic blood flow is unique. The main blood supply, through the
hepatic portal vein, is not controlled by the liver; even if vascular resistance is increased
to a maximum (for example, by stimulation of the hepatic sympathetic nerves), the flow
remains unaltered, although the pressure rises. Furthermore, the well-known autoregulation of arterial blood flow through most other organs (driven by the level of tissue oxygenation) is absent from the liver. Instead, the arterial flow appears to be inversely linked
to the hepatic portal venous flow, providing the liver with a constant total flow (Lautt,
1996). The mechanism may involve steady release of adenosine into the Space of Mall,
followed by removal through both arterial and hepatic portal inflows. If the portal supply is compromised, the local concentration of adenosine increases, stimulating arterial
vasodilation and flow to remove the metabolite. During periods of maximal hepatic portal flow, e.g. at peak absorption from the rumen, the inverse should occur.
While most information on spatial organization within the mammalian liver is
derived from rodents, ruminants may differ somewhat. The liver is comprised of four
cell types: hepatocytes (by far the largest group); macrophages (Kupffer cells); endothelial cells lining the Space of Disse; and fat storage (Ito) cells (see Gebhardt, 1992;
Jungermann and Kietzmann, 1996). The hepatic acinus, accepted as the smallest functional unit, comprises parenchymal cells in berry-like clusters, approximately 2 mm in
radius, on a vascular stalk (portal triad) containing the finest branches of the hepatic
artery, hepatic portal vein and the bile duct. This arrangement means that blood flows
past both sides of not more than 16 hepatocytes, before exiting via the central drainage
system (Lautt, 1996). The hepatocytes exhibit heterogeneity in both spatial and biochemical characteristics and may be divided simplistically into two populations, periportal and perivenous (also known as pericentral), based on their ability to synthesize
urea or glutamine (Fig. 9.1). Although other metabolic activities of these cells show a
smooth transition along the acinus from periportal to perivenous (Jungermann and
Kietzmann, 1996), the enzymes of the ornithine cycle and glutamine synthesis are
sharply separated, at least in the rodent.
Periportal cells are the first to receive blood from the afferent hepatic portal vein
and contain characteristically all five enzymes of the ornithine (urea) cycle, plus glutaminase in the mitochondria. Furthermore, mRNA for a variety of export proteins,
including albumin, are also present (Gebhardt, 1992; Jungermann and Kietzmann,
1996), as are transporters for most of the amino acids, with the notable exceptions of
aspartate and glutamate. The perivenous cells are clustered around the efferent hepatic
vein and lack the ornithine cycle enzymes, albumin mRNA and mitochondrial glutaminase. Instead, they possess cytosolic glutamine synthetase, the Xag transporter for glutamate and aspartate entry (Hussinger and Gerok, 1983), plus ornithine
aminotransferase (necessary for the catabolism of arginine; Kuo et al., 1991). A similar
perivenous localization of glutamine synthetase in sheep is suggested by the data
obtained from antegrade and retrograde perfusion of isolated liver, with and without
inhibition of glutamine synthesis by sulphoximine (Rossouw et al., 1997).
151
Blood flow
Periportal
Perivenous
Tyrosine aminotransferase
Serine dehydratase
Ornithine aminotransferase
Fig. 9.1. Metabolic zonation and pattern of enzyme distribution between periportal
and perivenous hepatocytes in non-ruminants (data from Jungermann and Kietzmann,
1996; OSullivan et al., 1998). The magnitude of the symbols indicates the relative
contribution within each zone, e.g. serine dehydratase activity declines between
periportal and perivenous hepatocytes, but still occurs in the latter, while glutamine
synthetase is restricted to the perivenous region.
152
(McEvoy et al., 1997), lowered appetite and, in extreme cases, coma and death
(Summerskill and Wolpert, 1970).
153
with intra-abomasal infusions of either ammonium bicarbonate or amino acids, coupled with intravascular infusion of 5-15N (amido) glutamine, between 28 and 61% of
the [15N]urea produced across the liver was derived directly from glutamine (R.M.
Nieto, T. Obitsu, A. Fernandez and G.E. Lobley, unpublished results).
Net hepatic removal of glutamine in pregnant, dry cows was proportional to the
amounts of an amino acid mixture infused into the mesenteric vein (Wray-Cahen et al.,
1997). In contrast, complementary studies involving amino acid infusions into sheep
(Lobley et al., 1998) failed to alter liver glutamine uptake. These observed differences
probably relate to the proportions of ammonia and amino acids extracted by the liver
(Lobley et al., 1998). Amino acid-N may also transfer to ammonia more directly,
because ovine hepatocytes, incubated with unlabelled ammonia and [15N]alanine, also
produced [15N15N]urea, although this was smaller than the amount of [14N15N]urea
synthesized (Mutsvangwa et al., 1997; Table 9.1).
Whether these various mechanisms operate effectively enough may have important
implications for ruminant productivity. Before the paradigm of the ornithine/glutamine
cycles was proposed, it was thought that, under conditions of ammonia excess, either
peripheral hyperammonaemia would occur or amino acid catabolism would be stimulated to provide additional aspartate-N. Indeed, from retrospective analysis of transhepatic ruminant data, a scenario was proposed that removal of ammonia-N by the liver
was accompanied by a similar input from other N sources, i.e. amino acid catabolism
would match ammonia detoxification (Parker et al., 1995). This hypothesis would
Table 9.1. Effect of propionate on urea production in ovine hepatocytes and sources of
ornithine cycle-N.
nmol mg21 wet cells h21
Propionate*
(mM)
0
0.2
0.4
0.8
1.6
Propionate
(mM)
0
0.31
0.63
1.25
NH3 uptake
17a
58b
54b
43c
36c
Urea
production
Glucose
production
Urea
([15N15N]:[14N15N])
28
63b
62b
54c
43c
6a
16b
22b
29c
31c
1.0
1.1
1.1
1.2
1.1
Urea
([15N15N]:[14N15N])
0.29
0.31
0.26
0.27
* Medium also contained 0.67 mM 15NH4Cl, plus a physiological mixture of amino acids; data from
M.A. Lomax and G.E. Lobley (unpublished data).
Within study different superscripts in a column indicate P < 0.05 or better.
Medium also contained 0.63 mM NH Cl, plus 1.25 mM [15N]alanine; data from Mutsvangwa et al. (1997).
4
154
apparently account for the poorer utilization of dietary amino acids by ruminants, particularly those fed fresh or conserved forages. In the event, controlled perturbation of
ammonia supply to the liver, by intra-mesenteric vein infusion of ammonium bicarbonate, showed that the extra urea produced exceeded ammonia uptake by between 13 and
17% (Lobley et al., 1996b; Lobley and Milano 1997). This is substantially less than the
doubling predicted by the hypothesis but, none the less, does represent increased amino
acid catabolism thereby losing the equivalent of 0.61.3 g amino acid-N for daily
anabolism. With ovine hepatic preparations in vitro, addition of ammonia led to similar
amounts of N released as urea plus glutamine (Luo et al., 1995; Rossouw et al., 1997,
1999), i.e. the removal of ammonia did not appear to stimulate amino acid catabolism.
Overall, therefore, the importance of ammonia detoxification may well have resulted in
the process being effectively uncoupled from amino acid oxidation.
155
ureagenic capacity of the other. This is despite the observations with ovine hepatocytes
in vitro that ureagenesis can proceed with ( 15N) ammonia as the sole exogenous N substrate and with [ 15N15N]urea as the primary product (Luo et al., 1995); a finding confirmed with the perfused sheep liver (Rossouw et al., 1997, 1999).
Control of ureagenesis
A major aim of any high-production system is to increase the amount of anabolic
metabolites (e.g. amino acids) available to the peripheral tissues. Clearly, any treatment
that changes anabolism without changing intake or absorption of nutrients must alter
ureagenesis (e.g. bovine-somatotropin; Bruckentahl et al., 1997). Despite many years of
research, it is still unclear whether ureagenesis is actively or passively controlled. Simple
consideration of the quantities and maximal activities of the ornithine cycle enzymes
would suggest that arginosuccinate synthetase (ASS) is limiting (Rattenbury et al.,
1980). In practice, however, CPS1 is a more probable candidate because this is positively regulated by the mitochondrial concentration of N-acetylglutamate (N-AG; see
Meijer et al., 1990). The activity of N-AG synthetase is regulated in vitro by several factors, including insulin (an inhibitor) and glucagon (an activator). This would fit with
the competing roles of these hormones; glucagon leads to poorer N retention (thus
more urea synthesis), while insulin diverts amino acids towards peripheral tissue
anabolism. Direct, or immediate, actions of insulin on the ornithine cycle, however, are
probably doubtful in vivo. For example, although the liver of cattle, which were acutely
(3 h) hyper-insulinaemic, extracted substantial amounts of insulin, urea production
remained unchanged (Eisemann and Huntington, 1994). In other bovine studies,
chronic administration of growth hormone (or its releasing factor) decreased hepatic
amino acid extraction and ureagenesis (Bruckentahl et al., 1997), although this has not
been a universal finding (McLeod et al., 1997). The long-term nature of these latter
studies does not allow resolution of direct hormonal effects on the ornithine cycle enzymes.
Another putative regulator of ureagenesis is propionate, which has been shown at
supra-physiological concentrations to be a potent in vitro inhibitor of ureagenesis (e.g.
Stewart and Walser, 1980). Such regulation through propionate would provide an
attractive explanation for the superior performance of animals on concentrate versus
forage diets. This role has been reinvestigated recently with ovine hepatocytes and,
while a minimal amount of propionate (or other 3-carbon unit) is needed to activate
the ornithine cycle, ureagenesis is suppressed at propionate concentrations above
0.63 mM (Mutsvangwa et al., 1997; see Table 9.1). At these higher propionate concentrations, gluconeogenesis continues to be stimulated, thus breaking the normal link
between glucose synthesis and urea production. Inhibition of ureagenesis by propionate
probably does not affect the relative inflows into the ornithine cycle of ammonia and
amino acids, as shown by incubating hepatocytes in the presence of either [15N]ammonia or [15N]alanine. Indeed, the relative ratios of [15N15N]:[14N15N] urea produced
were uninfluenced by the presence of propionate (Table 9.1). While these effects are
only apparent towards the upper end of the physiological range in vitro, the elevated
arterial ammonia concentrations, which result from episodic infusion of propionate
into the rumen (Choung and Chamberlain, 1995), may indicate that the ornithine
cycle may also be inhibited in vivo.
156
Fig. 9.2. Re-utilization of hepatic urea from the digestive tract quantified by use of
infusion of [15N15N]urea. Urea-N is either lost in faecal-N, converted to microbial
protein (then digested and absorbed as amino acids), or reabsorbed as 15NH3 when it
can either be reconverted to urea (as 14N15N) or used to aminate (transaminate) amino
acids.
157
ureagenesis and urea-N entry to the GIT doubled as intake of grass pellets was
increased from 0.6 to 1.8 3 maintenance, the proportion recycled to the ornithine
cycle remained constant and represented 2230% of urea production (Sarraseca et al.,
1998). Similar values (3031%) were obtained with rations of either a mixed forageconcentrate or chopped hay plus grass pellets (unpublished results). In practice, this
means that, under these dietary conditions (Lobley et al., 1996b, 1998), as much as
50% of the ammonia absorbed from the GIT and detoxified by the liver arises from
urea synthesized earlier and thus represents part of N recycling in the ruminant.
158
unidirectional, i.e. into the liver cells. Furthermore, because liver intracellular enrichments usually exceed those in the red blood cell (Lobley et al., 1996a; Connell et al.,
1997), then any outward transfer from hepatocytes to erythrocytes should increase
enrichment in the latter. In practice, enrichment within the red blood cells was either
unchanged or decreased slightly (0 to 26%). These data suggest strongly there is little
net transfer of amino acids from the liver via the erythrocytes. This issue needs to be
resolved, because of the marked consequences for both net and gross (isotopic) transfers.
0.41
0.25
0.67
0.52
0.50
0.39
0.01
0.20
0.14
0.29
0.64
0.23
20.04
0.58
2.66
1.18
0.80
1.16
20.13
20.01
0.23
0.53
1.12
1.11
0.56
0.32
1.17
1.00
1.10
0.90
0.57
0.45
0.45
0.71
1.00
0.84
0.50
Control
0.75
0.82
0.85
0.80
0.50
20.08
0.13
0.41
0.82
0.48
0.17
+Ala
0.97
0.20
1.26
0.73
0.57
0.41
0.01
0.16
0.70
0.67
0.72
0.12
Pre
0.54
0.70
0.73
0.37
0.50
0.49
0.30
0.31
0.83
0.87
0.49
0.25
+AA
Wray-Cahen et al.
(1997)c
0.70
0.42
0.70
1.12
0.57
0.61
0.40
0.35
0.42
0.68
0.94
1.79
0.45
Control
0.74
0.80
0.60
0.46
0.78
0.78
0.32
0.28
0.45
0.73
0.84
1.32
0.41
+Casein
Bruckentahl et al.
(1997)d
1.30
225.5
1.29
0.49
0.89
20.85
20.23
20.14
21.94
20.36
0.49
0.43
20.27
3.18
7.29
2.01
1.65
1.46
2.01
0.50
0.42
0.44
1.37
1.39
1.04
0.63
AL
Burrin et al.
(1991)e
0.92
0.38
2.43
0.61
0.89
1.64
0.28
0.43
0.68
0.83
1.03
0.24
0.25
0.66
0.39
0.88
0.44
0.83
0.02
0.08
0.38
0.61
0.63
0.37
20.08
+AA
Lobleyf
Control
Sheep
b Fed
calves.
75% maize:soybean meal, with (+ala) or without (control) infusion of L-alanine into the mesenteric vein.
c Pregnant dry cows (plasma data), basal ration (pre) plus intra-mesenteric vein infusion of an amino acid mixture (+AA).
d Fed a high concentrate diet without (control) or with (+casein) 300 g day21 casein infused into the abomasum.
e Sheep at maintenance (M), or fed ad libitum (AL).
f Sheep fed at 1.2 3 maintenance with (+AA) or without (control) intra-mesenteric vein infusion of amino acid mixture (equivalent to 4 3 basal intake); unpublished
data.
a Fed
Alanine
Arginine
Glycine
Serine
Tyrosine
Histidine
Isoleucine
Leucine
Lysine
Methionine
Phenylalanine
Threonine
Valine
Lapierre
Koeln et al.
et al. (2000)
(1993)a
Reynolds et al.
(1994)b
Cattle
160
Table 9.3. Per cent hepatic extraction of total amino acid inflows to the livera.
Cattle
Alanine
Arginine
Glycine
Serine
Tyrosine
Histidine
Isoleucine
Leucine
Lysine
Methionine
Phenylalanine
Threonine
Valine
Sheep
Controlb
+alab
Controlc
Controld
+AAd
19.7
8.8
16.7
11.2
3.9
3.1
3.3
6.3
13.3
9.1
0.7
19.6
6.4
15.1
12.4
2.5
0.8
1.5
5.2
13.6
7.4
1.0
6.7
1.8
7.2
9.4
8.2
3.0
1.7
0.1
1.7
8.8
5.9
7.0
0.5
9.7
6.4
3.4
9.3
7.5
5.7
2.3
2.4
4.3
9.9
11.5
1.2
1.2
13.0
6.0
8.7
18.3
11.0
1.4
1.4
5.4
14.4
12.3
7.3
1.0
a Based
required by peripheral tissues. In sheep, for example, this may result in a relative underprovision of histidine and phenylalanine, with a lesser deficit of methionine. Their
availability may limit peripheral tissue growth, but leave the other essential amino acids
in excess and subject to catabolism.
If, however, plasma export proteins can provide amino acids for peripheral tissue
needs, then, based on known rates of synthesis (Connell et al., 1997), the limitation
changes to methionine and, to a lesser extent, phenylalanine. This fits with the generally perceived view that methionine is often the first-limiting amino acid for many
ruminant diets. Extra-hepatic use of hepatic export proteins has been shown in rodents,
by adding serum proteins tagged with residualizing labels (Thorpe et al., 1993), where
as much as 20% of albumin is degraded in each of skeletal muscle and skin (Maxwell et
al., 1990). Interestingly, hepatic removal and degradation of albumin appears to be
dominated by endothelial cells (Eskild et al., 1989), adding another aspect to liver
metabolic zonation.
Because the balance between the synthesis of metabolites and export protein synthesis will vary with physiological state, then clearly neither the magnitude of hepatic
fractional extractions nor the pattern of amino acids available to the periphery will be
constant. In sheep, protein intake and parasitic infestation (Abbott et al., 1985)
markedly influence both albumin pool size and biological half-life (degradation and
clearance). Ovine albumin synthesis is sustained even under fasting conditions (Koeln
et al., 1993) and is sensitive to intake (Connell et al., 1997), at least up to a certain
limit of amino acid supply (Lobley et al., 1998).
161
Glutamine
Aside from the major metabolic role of glutamine, it is now used as a therapeutic aid in
certain clinical situations, particularly those where cells proliferate, such as in the GIT
epithelia and the immune system (Heitmann and Bergman 1978; Calder 1995).
Besides a key role in maintaining ureagenic flux and ammonia detoxification, glutamine may also impact on hepatic mechanisms related to acidbase balance and regulation of protein turnover (see next section).
Maintenance of acidbase balance is a major role for the liver in non-ruminants.
In rodents, chronic metabolic acidosis, induced by either HCl or ammonium chloride,
results in suppression of ureagenesis and urinary urea elimination, but with concomitant stimulation of amino acid oxidation, net glutamine synthesis and urinary NH4+
release (Welbourne et al., 1986; May et al., 1992). Production of urea yields a neutral
product (involving utilization of both NH4+ and HCO32), whereas synthesis of glutamine spares bicarbonate. This bicarbonate is then available to neutralize protons, and,
on transfer to the kidney, the amido group is released into urine as ammonium (NH4+)
ion (Meijer et al., 1990). In sheep, acidosis induced by ammonium chloride had little
effect on whole body and liver flux of [U-14C]glutamine (Heitmann and Bergman,
1978), but the complication of acidosis and the additional supply of free ammonia
confounded interpretation. Certainly, mild acidosis, induced by [15N]ammonium chloride infusion into sheep, resulted in elevated enrichments of hepatic [15N]glutamine,
comparable to those observed for urea (Lobley et al., 1995). Net movements of glutamine across the liver are little affected by ammonia but, under both neutral and acidotic conditions, there is a large change in rates of both utilization and synthesis (Table
9.4), i.e. transfer of ammonia through the glutamine amido group is stimulated.
For sheep infused with HCl, urea production and urinary elimination were elevated, with a smaller increase in hepatic glutamine synthesis (Milano, 1997). The latter
Table 9.4. Effect of ammonia supply and acidosis on ovine hepatic glutamine kinetics.
Glutamine kinetics (mmol min21)
Net utilization
Hepatic synthesis
Hepatic utilization
Study 1
Control
+NH4Cl
230a
211b
47a
87b
277a
298b
Study 2
Control
+HCl
243
237
26
42
270
279
Study 3
Control
+NH4HCO3
221
233
37a
51b
258
284
Data from: Study 1, [U-14C]glutamine, acidosis, Heitmann and Bergman (1980); Study 2, [5-15N]glutamine,
acidosis, Milano (1997); Study 3, [5-15N]glutamine, ammonia overload (R.M. Nieto, T. Obitsu, A.
Fernandez, G.E. Lobley, unpublished results).
For each study, values within a column but with different superscripts are significantly different by P < 0.05
or lower.
162
accounted for a minor fraction of the extra NH4+ appearing in the urine, with most
arising from extra-hepatic sources. This suggests that the liver is probably less important in regulation of acidbase balance in sheep, compared with the rat (Fig. 9.3). For
example, the activity of the ovine hepatic glutamine synthetase (50100 mmol min21
kg21 liver) is only 15% that of the rat (Hussinger, 1990). Thus, the maximal rate of
urea production in sheep exceeds hepatic glutamine synthesis by 510-fold (Rossouw
et al., 1999; Table 9.5), compared with equal activities in the rodent. Furthermore,
both the ornithine cycle and glutamine synthesis are sensitive to pH in the rat but not
in the sheep, as shown by perfusion of the ovine dorsal lobe with buffers varying in pH
from 7.2 to 7.6 (Rossouw et al., 1999; Table 9.5). There are other metabolic differences
in acidbase responses between species. Notably, net changes in protein-N retention
appear to be regulated by decreased protein synthesis in sheep (Milano, 1997) but by
elevated protein degradation in rodents (Manier et al., 1994; Fig. 9.5).
Protein synthesis
Protein synthesis
Protein degradation -
WHOLE BODY
Protein degredation =
Protein oxidation
Protein oxidation
Urea synthesis
Urea synthesis
Net glutamine
Net glutamine
=(-)
Urea
Urea
NH4+
NH4+
Fig. 9.3. Responses in whole body protein metabolism, plus liver and kidney urea and
glutamine kinetics to acidosis in rodents and sheep (from Welbourne et al., 1986; May et al.,
1992; Manier et al., 1994; Milano, 1997).
163
Table 9.5. Effect of ammonia supply and acidosis on ovine hepatic glutamine kinetics in vitro.
Condition
NH3 uptake
Urea production
Glutamine production
Antegrade
+NH4Cl (0.3 mM), pH 7.2
+NH4Cl (0.3 mM), pH 7.4
+NH4Cl (0.3 mM), pH 7.6
2NH4Cl, pH 7.4
827 134
770 118
791 96
224 40a
596 66
590 71
594 60
34 10a
176 27
172 40
184 34
40 10a
Retrograde
+NH4Cl (0.3 mM), pH 7.2
+NH4Cl (0.3 mM), pH 7.4
+NH4Cl (0.3 mM), pH 7.6
2NH4Cl, pH 7.2
606 83
707 108
730 83
219 20a
530 52b
590 58
556 60
38 40a
192 19
222 26c
186 28
20 4a
Values are expressed as nmol N g21 wet weight liver min21 and are the means of seven perfusions
(H. Rossouw, J.G. van der Walt, unpublished results).
All analyses conducted within direction of flow.
a Differs from all values obtained with NH Cl, at all pH values (P < 0.05).
4
b Differs from the value obtained with NH Cl, at pH 7.4 (P < 0.10).
4
c Differs from the value obtained with NH Cl, at pH 7.6 (P < 0.10).
4
30 g kg21 weight day21 to either ad libitum or just sufficient to maintain body weight.
Under these conditions, fractional rates of protein gain or loss were, at maximum, 5
and 21.5% day21 (Burrin et al., 1992).
In contrast, fractional rates of hepatic protein synthesis are much greater, but are
also very variable (from 3 to 98% day21; see Lescoat et al., 1997). This wide disparity
makes interpretation of metabolic regulation difficult, with the problem exacerbated by
technical difficulties. These include the under-estimation of synthesis of export proteins, as well as those constitutive proteins with high rates of turnover, using the continuous infusions of labelled amino acids. This problem is further complicated by the
presence of different precursor pools within the liver. Mixed intracellular sources provide for the synthesis of constitutive proteins, while polypeptides destined for secretion
are derived from amino acids that have newly entered the cell (see Connell et al., 1997).
Notwithstanding these technical concerns, the large difference between fractional
rates of protein synthesis and gain indicates that constitutive proteins are extensively
degraded within the ruminant liver. If these liberated amino acids were to be channeled
into a pool for catabolism, it would significantly affect the overall N economy of the
animal. There is also strong evidence that the regulation of constitutive protein production differs from that of proteins destined for export. For example, in the transition
from fasting to feeding, synthesis of albumin is markedly stimulated, in both absolute
and fractional terms (Connell et al., 1997). Regulation through synthesis probably also
largely accounts for changes in plasma protein concentration and content commonly
observed under conditions of stress (e.g. Abbott et al., 1985) and under-nutrition (e.g.
Liu et al., 1995). In contrast, liver constitutive protein synthesis is less susceptible to
acute changes in amino acid supply (Lobley et al., 1998) or general intake (Connell et
al., 1997). Consequently, changes in ovine hepatic protein mass probably occur through
alteration in protein catabolism, possibly involving up- and down-regulation of lysosomal
activity as proposed for the perfused rodent liver (Mortimore et al., 1989).
164
How then is such control exerted? Recent studies with rodent hepatocytes and perfused livers have indicated that the mechanism probably involves changes in cell volume (Hussinger et al., 1994). Cellular uptake of a number of amino acids, mediated
by Na+-dependent transporters (e.g. the A system for glycine and alanine, or the Nsystem in the case of glutamine), is accompanied by concomitant inflow of Na+. This is
then expelled by exchange with K+ ion, the energy being provided by the membrane
Na+-K+ ATPase. This flow of ions leads to increased uptake of intracellular water and,
therefore, cell swelling. As cell volume expands, protein degradation is reduced, probably related to a decrease in the rate of formation and acidification of lysosomes (Luiken
et al., 1996). Under the converse conditions, i.e. efflux of these amino acids, the hepatocytes shrink, degradation is enhanced and protein synthesis inhibited (Hussinger et
al., 1994). Net production of export protein appears to be regulated primarily by
changes in synthesis, whereas constitutive protein mass is modulated more by alterations in the rate of proteolysis (Connell et al., 1997). These contrasting mechanisms
may have a common step involving intracellular targetting of the mRNAs for the
export proteins to the endoplasmic reticulum (Hesketh et al., 1998) and phosphorylation of ribosomal protein S6 (Blommart et al., 1995). The binding there of additional
ribosomes will augment synthesis of export proteins and, at the same time, reduce the
release of reticulum membrane needed for formation of the autophagosomes. Thus,
proteolysis would be inhibited. If targetting and binding to the endoplasmic reticulum
is inhibited (as at low intakes) then export protein synthesis decreases while lysosome
formation and activity is enhanced.
Interestingly, two major protein anabolic hormones, insulin and IGF-I, also lead
to increased cell hydration, while the catabolic hormone, glucagon, causes shrinkage.
Both insulin and glucagon are extracted by the ruminant liver (Lapierre et al., 1992)
and play competing roles in determining the fates of amino acids. Cell volume
increases also lead to enhancements of amino acid catabolism (Hussinger et al.,
1992b), ureagenesis, possibly through increased ASS activity (Quillard et al., 1996),
and glutaminase activity (Hussinger et al., 1990). Change in cell hydration, therefore,
provides a link for the responses often seen with improved nutrition, i.e. a co-stimulation of protein anabolism and amino acid catabolism.
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10
Introduction
Adipose tissue is a much maligned tissue! The growing prevalence of obesity in the
USA and Western Europe in particular has led to a search for means to reduce the
amount of adiposity both in ourselves and also in the animals we eat. A number of
agents (e.g. growth hormone, b-agonists) which can markedly reduce adiposity have
been produced and immunological approaches to destroying adipocytes, and hence
decreasing adiposity, have been developed. However, in this rush to reduce adiposity, it
is worth remembering that adipose tissue has played a key role in mammalian evolution
and still has important physiological functions today, both as an energy reserve and as a
source of paracrine and endocrine factors.
171
172
and is reflected in the much more complex array of adipose tissue depots in mammals
and birds than in poikilotherms (Pond, 1992). Detailed anatomical studies by Pond
(1986) on a wide variety of mammals has shown that there are some 16 depots, some
in the abdominal cavity, some within the musculature and some under the skin, which
are present in essentially all mammals including marsupials. The relative size of different depots may vary between species, and in fat animals, depots may effectively merge;
such changes are considered to be adaptations to meet the needs of that particular
species (Pond, 1992). While adipocytes from different depots appear superficially to be
identical, there are subtle differences in their properties, and there is some evidence to
suggest that the small depots of the musculature are more metabolically active than the
larger abdominal and subcutaneous depots which are usually studied (Pond, 1992).
Evolution of adipose tissue has allowed mammals to inhabit some remarkably
inhospitable environments; it also facilitates migration, allowing animals to exploit seasonally available food in different locations. Furthermore, adipose tissue has permitted
development of a variety of reproductive strategies (Vernon and Pond, 1997).
Pregnancy increases the energy requirements of mammals, which are further increased
by lactation. Additional demands are mostly met by increasing food intake, but in
addition there is often an accumulation of adipose tissue lipid during early pregnancy
for use during the later stages of pregnancy, at parturition and during lactation (Vernon
and Pond, 1997). Indeed, some species of seal, whale and also boreal bears actually fast
during lactation, so deriving all the nutrients required for milk production from tissue
reserves (Oftedahl, 1992). Dairy cows, especially those producing large amounts of
milk, usually draw heavily on adipose tissue reserves during early lactation, when for
reasons as yet unknown, milk output increases more rapidly than appetite. As a result,
for a period, mobilization of adipose tissue lipid may account for more than 50% of
milk fat production (Vernon and Flint, 1984).
Thus adipose tissue has played a central role in mammalian evolution (indeed our
evolution would have been impossible without it), and continues to play a key role in
mammalian physiology in both wild and domestic species. However, there is a potential cost, at least for wild species, in that too much adipose tissue can reduce mobility
and turn an animal into a succulent meal for a predator. Thus the amount of adipose
tissue in an animal will usually reflect the relative threats to survival of predation and
starvation. For example, African gazelles with a plentiful supply of food have relatively
little adipose tissue; in contrast, reindeer in the Arctic have a layer of subcutaneous adipose tissue about 5 cm thick at the beginning of the winter (Larsen et al., 1985).
Predators also have this problem as obesity can decrease their chance of obtaining a
meal. European foxes are normally thin, but in the Arctic they are markedly fatter
(Lindstrm, 1983). Barnacle geese before migrating from the Arctic to Scotland for the
winter, accumulate just enough adipose tissue lipid to meet the needs of the flight
(Butler et al., 1998). Animals can thus regulate their levels of adiposity very closely to
meet their needs; this means that there must be signals which indicate the size of the
body reserves of lipid. Indeed, it is now apparent that adipocytes secrete a variety of
substances (Box 10.1) which have autocrine, paracrine and endocrine functions.
173
Complement system
Factors B, C3 and D (adipsin)
Binding proteins
IGF-binding proteins
Retinol binding protein
Cholesterol ester transfer protein
Hormones
Leptin
Oestrone, oestradiol
Testosterone
Cytokines
Tumour necrosis factor a (TNF-a)
Interleukin-6
Other
Plasminogen activator inhibitor-1
Acrp30/Adipo Q
174
175
176
177
nants, and the mass of the depot does not correlate with the total mass of adipose tissue
in the animal (Marchington et al., 1989). Futhermore, the epicardial adipose tissue
abuts directly onto the myocardium without any intervening fascia as is found in skeletal muscle (Marchington et al., 1989). Although circumstantial, these various observations suggest a role for epicardial adipose tissue in the development and functioning of
the heart. The former could again relate to the ability of adipose tissue to produce
mitogenic factors, while it is possible that cardiac adipocytes provide a direct supply of
fatty acids for use by cardiocytes. There is as yet no direct evidence for either possibility
and for the latter, there is always a question of whether the rate of supply of fatty acids
(presumably transported by diffusion, for there is no obvious portal system in the tissue) would be sufficient for lipolysis in cardiac adipocytes to be a quantitatively significant direct source of fatty acids for heart tissue.
The possibilities that inter- and intramuscular adipocytes may influence the development of surrounding skeletal muscle (Hossner et al., 1997), and that they may provide a direct source of fatty acids for adjacent skeletal muscle have also been proposed,
but the presence of a fascia between intermuscular adipose tissue and surrounding muscles (Marchington et al., 1989) would suggest that diffusion of fatty acids and other
factors is likely to be restricted.
The liver receives a direct supply of fatty acids from an adipose tissue depot, for
venous drainage from mesenteric adipose tissues leads into the hepatic portal vein. The
physiological importance of this is uncertain, but it may have pathological consequences (Barzilai et al., 1999).
178
(e.g. TNF-a) which accentuate lipolysis (Feingold et al., 1992; Green et al., 1994),
while the fatty acids released from adipocytes stimulate lymphocyte proliferation
(Pond, 1996). Stimulation of the immune system by injection of an endotoxin
(lipopolysaccharide) results in enhanced lipolysis and the appearance of TNF-a type I
receptors on adipocytes close to the lymph node but not on adipocytes remote from the
lymph node (MacQueen and Pond, 1998; Pond and Mattacks, 1998). However, studies with ovine popliteal adipose tissue failed to detect any variation in fatty acid composition with distance from the lymph node (Vernon and Pond, 1997), and TNFa had
no effect on basal or catecholamine-stimulated lipolysis in adipose tissue near to (5 mm
or less) or distant from the lymph node (S.E. Melrose, M.D. Houslay and R.G.
Vernon, unpublished observations). However, preliminary studies suggest that the ability of the adenosine analogue, N6-phenylisopropyladenosine, to inhibit lipolysis
appears to be diminished in adipocytes close to the lymph nodes, and culture with
growth hormone diminished the ability of PIA to inhibit lipolysis in adipocytes distant
from, but not in those close to, the lymph node (S.E. Melrose, M.D. Houslay and R.G.
Vernon, unpublished observations). Thus there may be differences in the lipolytic system in different parts of popliteal adipose tissue in sheep, but, if so, they appear to be
much smaller than in guinea pigs.
Leptin also influences the immune system. Impaired T-lymphocyte immunity is
found in ob/ob mice which produce a defective form of leptin (Chandra, 1980), and
very recent studies show that leptin enhances proliferation of T-lymphocytes and their
subsequent ability to respond to antigens (Lord et al., 1998). In addition, preadipocytes
may also contribute to the bodys defence mechanisms as there is evidence for their having macrophage-like properties (Cousin et al., 1999); this may explain the production
of macrophage colony-stimulatory factor by adipose tissue and its promotion of adipose tissue growth (Levine et al., 1998). Thus there are several ways in which adipose
tissue can modulate functioning of the immune system, but their relative importance is
uncertain. That such interactions are physiologically relevant is indicated by the fact
that anorexia, which results in greatly diminished adiposity, causes diminished functioning of the immune system (Chandra, 1991). High-yielding dairy cows tend to be
more susceptible to disease (e.g. mastitis, laminitis) and this is exacerbated by poor
nutrition (Sinclair et al., 1999); adiposity is likely to markedly diminish in such animals. From an individuals point of view, for starvation, and other conditions which
lead to a loss of adipose tissue reserves, to result in a failure of the immune system
seems singly unfortunate. However, when considered from the perspective of the
species as a whole, it does make sense, for when food is in short supply it increases the
likelihood that the weakest animals, with the least reserves, die leaving more of the limited food available for the stronger. A case of the survival of the fattest!
Leptin
The discovery of leptin provided the long-postulated link between adipose tissue stores
and brain centres which regulate food intake and energy homeostasis. Since this seminal discovery, our understanding of the role of leptin has expanded from that of a
sensor of body fat mass to include participation in, and regulation of, multiple physiological systems including reproduction, inflammation and cell-mediated immunity, as
179
well as the coordination of whole body energy homeostasis (Friedman and Halaas,
1998; Houseknecht and Portocarrero, 1998; Houseknecht et al., 1998a).
The leptin (ob) gene has been cloned in several species including the mouse, pig,
chicken and human (Houseknecht and Portocarrero, 1998) and also the cow (Ji et al.,
1998); sequence homology among species is high. Leptin is primarily expressed in
white adipocytes; however, in rodents and humans, leptin expression has also been
reported in the placenta (Hoggard et al., 1998) and stomach (Bado et al., 1998). In cattle and swine, leptin is reported to be expressed exclusively in adipose tissue
(Houseknecht and Portocarrero, 1998). Furthermore, Ji et al. (1998) reported no difference in the level of leptin gene expression among adipose tissue depots (subcutaneous, renal and omental depots) in cattle, which differs from reports in humans
(Montague et al., 1998; Russell et al., 1998; Vanharmelen et al., 1998).
The leptin receptor is a member of the cytokine family of receptors and is
expressed as either a long form, or as multiple short forms due to alternative splicing
(Tartaglia, 1997). The long form of the leptin receptor predominates in the hypothalamus and is credited with the central regulation of appetite; the short isoforms, and to a
limited extent, the long form, are expressed in multiple peripheral tissues as well and
may be involved in the regulation of tissue metabolism (Tartaglia, 1997). Soluble forms
of the leptin receptor have been reported (Houseknecht et al., 1996); their role is
uncertain, but leptin binding to proteins in blood complicates the accurate assay of circulating leptin in many species. Dyer et al. (1997a) cloned a partial ovine long-form
leptin receptor cDNA and reported that mRNA expression for the long-form receptor
is present in the hypothalamus, anterior pituitary and adipose tissue of sheep.
Furthermore, they reported that expression was highest in the arcuate nucleus and the
ventromedial hypothalamus, and that hypothalamic expression was up-regulated by
undernutrition as found in rodents (Houseknecht and Portocarrero, 1998).
Leptin is a powerful regulator of food intake in rodents; effects are most dramatic
in ob/ob mice which lack leptin (Friedman and Halaas, 1998; Houseknecht et al.,
1998a; Houseknecht and Portocarrero, 1998). Intracerebro ventricular administration
of recombinant ovine leptin to sheep also caused a reduction in food intake (Morrison
et al., 1998). Large-scale preparation of recombinant ruminant leptin is now underway
(Gertler et al., 1998) which will facilitate further studies in ruminants. Studies on the
mechanism whereby leptin decreases appetite and increases whole-body energy expenditure have, in the past, focused mostly on the neuropeptide Y (NPY) system of the
hypothalamus, but other neuropeptides are most probably involved. Leptin acts centrally to prevent NPY-induced appetite stimulation and suppression of thermogenesis
by inhibiting its synthesis in the arcuate nucleus of the hypothalamus (Friedman and
Halaas, 1998; Houseknecht et al., 1998a). Dyer et al. (1997b) showed that peripheral
NPY administration up-regulated the expression of leptin and NPY receptor subtype 1
in ovine adipose tissue. These data are indicative of a feedback loop between leptin and
NPY expression which has been shown for rodents by others (Houseknecht et al.,
1998a; Houseknecht and Portocarrero, 1998).
Leptin expression is under complex control by both hormones (Friedman and
Halaas, 1998; Houseknecht et al., 1998a) and the sympathetic nervous system
(Trayhurn et al., 1998); modulatory hormones include insulin, glucocorticoids, thyroid
hormones and oestrogen. Effects appear to be chronic and due to changes in leptin
gene expression rather than to acute stimulation of leptin secretion from intracellular
180
storage pools. In vitro studies from cattle (Houseknecht et al., 1998b) show that leptin
mRNA abundance in bovine subcutaneous adipose tissue is increased by insulin and
glucocorticoids. In vivo, growth hormone increased leptin gene expression in bovine
(Houseknecht et al., 1998b) and ovine adipose tissue (Raymond et al., 1997).
However, effects of GH may be indirect, as leptin expression in bovine adipose tissue
was strongly, positively correlated with IGF-I gene expression while GH had no effect
on leptin expression in vitro (Houseknecht et al., 1998b).
Serum leptin concentration in the fed state is highly, positively correlated with
body fat mass in rodents, pigs and humans (Friedman and Halaas, 1998;
Houseknecht et al., 1998a) and in cattle (Ji et al., 1997; Chilliard et al., 1998; Minton
et al., 1998) and sheep (Kumar et al., 1998). This led to the hypothesis that leptin
allows the body to sense the size of the bodys energy storage pool and adjust appetite
and whole-body energy metabolism accordingly. Fasting causes a rapid, profound
down-regulation of leptin gene expression in rodents and humans (Friedman and
Halaas, 1998). Fasting (2 days) also down-regulates leptin gene expression in subcutaneous adipose tissue of cattle (Tsuchiya et al., 1998) and swine (Spurlock et al.,
1998a). However, the effects of fasting on leptin expression in pigs are modest compared with rodents, and there appears to be no effect of maintenance intake on leptin
gene expression (Spurlock et al., 1998a), suggesting that this species may be less sensitive to this form of regulation of the leptin gene. Fasting also results in changes in the
secretion of pituitary hormones, including thyroid stimulating hormone (TSH), GH
and the gonadotropins, and these changes are partly prevented by administration of
leptin (Friedman and Halaas, 1998). These various observations indicate that leptin
plays a central role in the adaptations to fasting and suggest that the primary evolutionary role for leptin is to prevent death by starvation rather than to prevent obesity
(Spiegelman and Flier, 1996).
Leptin is an important regulator of reproduction in rodents and humans, providing a link between energy reserves and an energetically demanding function (Hoggard
et al., 1998; Houseknecht et al., 1998a; Macut et al., 1998). There is potential for leptin to act both centrally and peripherally in both males and females, as the leptin receptor is expressed not only in the hypothalamus and pituitary but in the ovary and testis
as well (Houseknecht et al., 1998a). Leptin stimulates gonadotrophin secretion in
ob/ob mice and undernourished animals, and advances the onset of puberty
(Houseknecht et al., 1998a). These data have obvious, important implications for productive efficiency of livestock.
Links between leptin and the immune system have been discussed in a previous
section. In addition, leptin expression is up-regulated by inflammatory cytokines such
as TNF-a and interleukins, and an increase in leptin secretion may cause the anorexia
following administration of endotoxin (Houseknecht et al., 1998a). However, Spurlock
et al. (1998b) found no effect of acute endotoxin treatment on leptin expression in
fasted pigs, despite physiological indications of a robust inflammatory response.
Additionally, Leininger et al. (1998) found that endotoxin treatment down-regulated
leptin gene expression in fully fed pigs, and that the endotoxin-induced fall in leptin
was highly correlated with changes in blood insulin, IGF-I, glucose and free fatty acid
concentrations. These data (Leininger et al., 1998) suggest that changes in energy
metabolism and associated hormones can overcome, at least in the pig, the positive
effects of inflammatory cytokines on leptin gene expression. Obviously, further work is
181
necessary to elucidate the role of leptin (if any) in the anorexia and altered energy
metabolism associated with acute and chronic inflammation in livestock.
Conclusions
Ideas on adipose tissue have progressed a long way from the days when it was perceived
as a relatively inert tissue. Not only do we now know that it is very active metabolically,
it is also a source of a plethora of factors which influence events within adipose tissue,
within adjacent tissues or indeed with endocrine effects on distant parts of the body a
veritable node on the information superhighway (Flier, 1995). The importance of adipose tissue in vertebrate evolution, and mammalian evolution in particular, is now
apparent, and its central role in mammalian physiology is beginning to be appreciated.
This is emphasized by two very recent studies with transgenic mice essentially devoid of
adipose tissue; many mice died before reaching adulthood and those that survived
showed symptoms of acute diabetes (McKnight 1998); the reason for the diabetes is
uncertain but may be due to an ability to remove fatty acids from the blood. Curiously,
both too little and too much adipose tissue can lead to the same pathological condition!
Animals have affected means of adjusting their amount of adipose tissue to meet their
needs, although the mechanisms may be attenuated by domestication. Some of the
substances produced by adipocytes and their associated stromal-vascular cells must act
as signals to help achieve this. Leptin, initially perceived as the factor which could solve
the problem of obesity, may, in fact, be more important for signalling inadequate or
declining adipose tissue reserves; leptin clearly does not prevent the development of
Insulin
()
Adipsin
ASP
()
TNF-a
(+)
TRIACYLGLYCEROL
Precursor supply
Leptin
()
Appetite
(+)
Energy expenditure
182
obesity, but it may retard it. Leptin also has an important role in the adaptations to
negative energy balance. As adipocytes enlarge, they increase production of other substances (e.g. TNF-a) as well as leptin, and decrease production of others (e.g. adipsin,
acylation-stimulation protein) which may act to limit further expansion. Adipose tissue
mass thus appears to be modulated both directly by autocrine factors and indirectly by
endocrine factors such as leptin (Fig. 10.1). When one chooses to ignore these signals,
obesity follows and thence undesirable pathological conditions (e.g. diabetes).
The implication of this for domestic ruminants (and indeed other food animals) is
that we need to know more precisely the amounts of adipose tissue required for optimal
function, for while too much adipose tissue is undesirable both for the animal and as a
food for us, equally too little is deleterious for the animals well-being.
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11
Introduction
This chapter will review the endocrine regulation of growth and metabolism in ruminants and focus on recent insights into the central role of the somatotropic axis.
Growth and metabolism during postnatal life are regulated by multiple hormones and
growth factors acting in an endocrine (systemic) and an autocrine or paracrine (local)
manner. The somatotropic axis is a multi-level hormonal system, primarily consisting
of growth hormone (GH; somatotropin) and insulin-like growth factor-I (IGF-I;
somatomedin), their associated carrier proteins and receptors. There are a number of
other hormonal axes that are involved in the regulation of intermediary metabolism
and the growth process. Among these are insulin, thyroid hormones, glucocorticoids,
sex steroids, the melanocortin-leptin axis and a number of locally produced growth
factors.
Circulating GH is a major regulator of metabolism and growth during postnatal
life. GH deficiency is associated with a reduction in growth rate and in final adult size.
During postnatal development and during adult life circulating GH regulates key
metabolic pathways of intermediary metabolism (Breier et al., 1991). Treatment with
GH is lipolytic, elevating circulating concentrations of free fatty acids (FFA), increasing
the bodys ability to respond to lipolytic stimuli and reducing lipogenesis. GH treatment also creates a state of positive nitrogen balance by increasing nitrogen retention
and decreasing protein catabolism. Furthermore, GH administration elevates plasma
insulin and glucose levels, which in the long term creates a state of insulin resistance.
IGF-I was originally described as an endocrine factor secreted from the liver in
response to GH stimulation which mediated the effects of GH in peripheral tissues.
However, while IGF-I treatment increases growth in normal and in GH-deficient animals, growth rates rarely equal those obtained with GH treatment. GH stimulation of
IGF-I mRNA expression is not limited to the liver, suggesting that some of the effects
of GH treatment on tissue growth are derived from the direct stimulation of autocrine
or paracrine IGF-I production. Treatment with IGF-I stimulates protein metabolism
CAB International 2000. Ruminant Physiology: Digestion, Metabolism,
Growth and Reproduction (ed. P.B. Cronj)
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and reduces plasma glucose, as a result of both increased glucose uptake and reduced
glucose production (Douglas et al., 1991). IGF-I is important for the function of a
variety of organs. For example, elevated circulating IGF-I levels increase glomerular filtration rate (Hammerman and Miller, 1993) and renal growth, while local concentrations
of IGF-I determine the destiny of maturing follicles in the ovary (Giudice et al., 1995).
IGF-I is also a potent inotropic agent in the heart and regulates immune cell numbers
as well as other aspects of the immune response. While IGF-I is often considered a proliferation or differentiation factor, it is also a potent cell survival factor through its role
as an inhibitor of programmed cell death. This review will elucidate the complexity of
the regulation of growth and metabolism during postnatal life by the somatotropic axis
using four main examples: developmental changes, effects of nutrition, GH and IGF-I
treatment, and altered GH action in conditions of metabolic imbalance.
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has two binding sites for the extracellular domain of the GHR and forms a 1:2
GH:GHR complex by sequential homodimerization. Ilondo et al. (1994) demonstrated
in adipocytes that the receptor dimerization process is critical for GH-stimulated glucose
incorporation into lipid. GH binding to the GHR activates a number of intracellular
pathways. Transcription of the IGF-I gene is rapidly increased soon after GH administration, suggesting that it is an immediate consequence of GHR signalling. The GHR
may also interact with cell membrane associated G proteins to regulate the metabolic
effects of GH (Roupas and Herington, 1994).
The multiple physiological and metabolic actions of GH could partly be explained
by heterogeneity of the GHR. Heterogeneity of the GHR has been demonstrated by
ligand binding studies, by affinity cross-linking, by epitope mapping with monoclonal
antibodies and by molecular studies. The soluble growth hormone binding protein
(GHBP) can also be viewed as a variant form of the GHR. The GHBP represents the
extracellular domain of the GHR, although its derivation appears to be species-specific.
In rodents, a separate alternatively spliced mRNA exists for GHBP, conferring on the
protein a unique hydrophilic 17-residue. In some species plasma GHBP arises from
proteolysis of the full-length GHR protein (Baumann, 1994), while in humans, sheep
and chickens the GHBP is thought to be mainly produced by proteolytic cleavage of
the GHR extracellular domain (Bingham et al., 1994). The GHBP in ruminant plasma
has been investigated by Davis et al. (1992) using a charcoal separation assay, demonstrating the presence of GHBP in cattle and sheep. Devolder et al. (1993) provided further evidence for the existence of GHBP in cattle using size-exclusion chromatography
and ligand and Western blotting techniques. Three major specific bands that bind GH
were demonstrated; two of these bands (190 and 58 kDa) were recognized by a monoclonal antibody directed against a part of the extracellular domain of the bovine growth
hormone receptor (bGHR). The function of GHBP is not clear at present, however, it
is thought to prolong the half-life of GH in circulation and may play a role in tissuespecific delivery. Since the GHBP is regulated by a number of physiological variables, it
is possible that the GHBP may represent an additional mechanism that regulates GH
action.
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GH treatment on tissue growth are derived from the direct stimulation of autocrine or
paracrine IGF-I production. As will be discussed later, the endocrine and the paracrine
or autocrine mode of action are interrelated and contribute to a coordinated adaptive
process.
An interesting concept in the regulation of IGF-I action relates to the IGF-I gene
itself. The IGF-I gene consists of at least six exons dispersed over a region of more than
80 kb (Rotwein, 1991). Exons 3 and 4 encode the mature IGF-I peptide. The synthesis
of IGF-I is a complex process involving two promoters, multiple sites of transcription
initiation, alternate splicing and multiple polyadenylation sites (Adamo et al., 1993).
The observation that exons 1 and 2 are differentially spliced to exon 3, producing alternate class 1 and class 2 transcripts, has stimulated discussion about endocrine versus
paracrine or autocrine functions of the IGF-I peptide. Changes in abundance of
hepatic class 2 transcripts have been associated with concomitant changes in circulating
(endocrine) IGF-I. For example, Pell et al. (1993) suggest that in sheep, hepatic IGF-I
output may be regulated by nutrition and GH, primarily through class 2 transcripts.
Thus, in conditions of optimal growth, the increase in circulating IGF-I correlates with
an increased abundance of exon 2 transcripts in liver IGF-I mRNA. However, in
peripheral tissues like muscle, exon 1 IGF-I transcripts predominate and do not
respond markedly to a range of metabolic and endocrine stimuli (Gilmour, 1994). The
switching of class 1 and class 2 transcripts in different tissues, depending on the
endocrine and metabolic conditions of the animal, could represent an adaptive process
to accommodate the changing needs of different tissues.
While the growth promoting properties of IGF-I are well established, the function of
IGF-II in ruminants is not clear, although there is some evidence that IGF-II can
impair the anabolic effects of IGF-I in sheep (Koea et al., 1992a). It is known that at
the cellular level, IGF-II has proliferative actions, enhancing proliferation of ovarian
granulosa cells and stimulating wound repair processes such as keratinocyte proliferation.
A proliferative role for IGF-II is also evident in many cancerous tissues (Isaksson et al.,
1991).
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IGFBPs may alter the distribution of circulating IGFs within target tissues by modifying receptorIGF interactions, thus enhancing or reducing biological activity.
IGFBP-1
The circulating IGFBP-1 peptide is approximately 26 kDa in size and has a similar
affinity for IGF-I and IGF-II. IGFBP-1 is under strong metabolic control and is produced mainly by hepatocytes, although some expression is also found in the kidney and
uterine decidua. Serum levels of IGFBP-1 are high in the fetus but decline postnatally.
Plasma levels of IGFBP-1 are inversely proportional to those of insulin (Lee et al.,
1993) and rapidly increase in response to treatment with stress-related hormones such
as glucocorticoids, glucagon and catecholamines (Hooper et al., 1994). The regulation
of IGF-I availability by IGFBP-1 plays an acute glucose counter-regulatory function,
restricting the insulin-like actions of the IGFs during states of a relative insulin deficiency
(Baxter, 1995).
IGFBP-2
The IGFBP-2 protein is approximately 32 kDa in size and has two distinct but overlapping IGF binding sites, of which the IGF-II site exhibits a fivefold greater affinity for
its ligand than the IGF-I site. Circulating IGFBP-2 is mainly produced by hepatocytes
(Scharf et al., 1995) but it is also present at high concentrations in milk and seminal
plasma. Plasma levels of IGFBP-2 in the postnatal circulation vary in response to many
factors including ontogeny, nutritional status and GH status. Despite a striking inverse
relationship between plasma concentrations of GH and IGFBP-2, the regulation of
IGFBP-2 is unlikely to occur directly via GH action (Gallaher et al., 1995). The role of
plasma IGFBP-2 in the functional regulation of circulating IGFs is unclear. IGFBP-2
levels are high in catabolic states where IGFBP-3 is suppressed and it may restrict IGFI availability in such situations. However, since the sensitivity of IGFBP-2 gene transcription to insulin is much lower compared with that of IGFBP-1, IGFBP-2 is more
likely be a chronic, as opposed to an acute, regulator of IGF action.
IGFBP-3
IGFBP-3 is expressed in a wide range of tissues and undergoes extensive post-translational modifications. IGFBP-3 is secreted in a phosphorylated state but the effect on
IGF affinity is minimal. However, since charged regions on IGFBP-3 are important in
ternary complex formation in plasma and cell surface binding, phosphorylation has
been implicated in serum half-life or tissue distribution (Coverly and Baxter, 1995).
The predominant IGF carrier in the postnatal circulation is a 150 kDa ternary complex
consisting of IGF-I or IGF-II, IGFBP-3 and an acid labile subunit (ALS). Circulating
IGFBP-3 is predominantly derived from the non-parenchymal cells of the liver (Chin
et al., 1994) while ALS and endocrine IGF-I are produced by adjacent hepatocytes.
The ternary complex is formed firstly by the association of IGF-I or -II with IGFBP-3,
followed by the binding of ALS to the IGFIGFBP-3 dimer. When IGFs are incorporated in the ternary complex they are considerably more stable than the free peptide.
The binary complex IGFBP-3/IGF-I has a much shorter circulating half-life and rapidly
crosses into the extravascular compartment where it is targeted to specific tissues (Arany
et al., 1993). Plasma IGFBP-3 correlates with GH and IGF-I, and is a chronic indicator of GH-dependent growth status. However, this relationship is indirect and reflects
the GH dependency of ALS and IGF-I production.
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IGFBP-4, -5 and -6
The mature IGFBP-4 peptide is 24 kDa in size and is n-glycosylated to create a 28 kDa
form. The function of IGFBP-4 glycosylation is unclear although one role may be in
the regulation of vascular endothelial transport. Unlike other IGFBPs, IGFBP-4 persistently inhibits IGF action. For example, IGFBP-4 is elevated and IGF-I activity is suppressed in atretic ovarian follicles but the opposite is observed in dominant follicles
(Giudice et al., 1995). The IGFBP-5 gene is expressed in a tissue-specific fashion and
the mature peptide is approximately 29 kDa in size, with phosphorylation and glycosylation variants (Jones and Clemmons, 1995). IGFBP-5 generally enhances the effects
of IGFs on cell proliferation. IGFBP-6 mRNA is widely expressed and yields a mature
peptide that is approximately 22 kDa in size. It has a strong preference for binding
IGF-II over IGF-I. IGFBP-6 has been found in plasma, amniotic fluid, follicular fluid
and cerebrospinal fluid (CSF) in sheep and humans. Levels in serum decline slightly
from mid to late gestation but are stable after birth (Lewitt et al., 1995).
IGF receptors
The classical IGF-I receptor (IGF-IR) is structurally homologous to the insulin receptor, forming a heterotetramer. The a subunit, 90 kDa in size, is involved in ligand
binding and forms the extracellular portion of the receptor. The affinity of the IGF-IR
is highest for IGF-I, twofold lower for IGF-II and 100-fold reduced for insulin.
Following the binding of IGF-I to the IGF-IR, there is an activation of the receptor
tyrosine kinase activity which results in the phosphorylation of insulin receptor substrate I (IRS-I). The mitogenic actions of IGF-I appear to be exclusively mediated
through the IGF-IR.
In cells where both IGF-I and insulin receptor subunits are both synthesized,
hybrid receptors (IGF-HR) can form by the association of an (ab) subunit from each
of the IGF-IR and insulin receptors to create a heterotetramer. IGF-I is bound to these
receptors with an affinity similar to that of the IGF-IR, and IGF-II binds with fourfold
lower affinity. Insulin binds with a very low affinity to the IGF-HR relative to IGF-I
and also relative to insulin binding to its own receptor (Soos et al., 1994). While the
physiological significance of the IGF-HR is unclear, the sequestration of IR subunits
into less insulin-responsive hybrids might be a mechanism to modulate insulin sensitivity
in tissues.
The IGF-II receptor (IGF-II/M6PR) exclusively binds IGF-II with high affinity
and is identical to the cation-independent mannose-6-phosphate receptor, consisting of
a short intracellular/membrane spanning domain and a large extracellular domain
made up of multiple structural repeats (Kiess et al., 1994). In rat tissues, IGF-II/M6PR
immunoreactivity and mRNA abundance is high in the fetus but very low after birth,
suggesting a minimal role in postnatal development and an active role in fetal growth
and/or development. Also deduced from IGF gene knockout experiments (Baker et al.,
1993) is the existence of a further IGF-II receptor (IGF-XR) which is expected to
mediate a number of the effects of IGF-II on fetal and placental development.
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and the stage of gestation. Fetal growth in late gestation is limited by maternal constraint. The primary elements ensuring adequate fetal growth are the ability of the
mother to provide adequate nutritional support to the placenta and the fetus, and the
ability of the fetus to manage the maternal resources appropriately. One of the key roles
of endocrine factors during fetal growth and development therefore is provided by hormones which influence the partitioning of nutrients between mother, placenta and
fetus, and which regulate the ability of the fetus to utilize the available substrate.
Postnatal growth is predominantly determined by genotype and by nutrition. Animals
of different genotype grow within predictable boundaries and the growth process is
under central endocrine control of pituitary GH.
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ruminants. Given that IGFBP-1 is under strong metabolic control and that plasma levels of IGFBP-1 are inversely proportional to those of insulin, there is considerable
scope for further research in this area. IGFBP-2 is the most abundant IGFBP prior to
birth and, although plasma concentrations fall after birth, it is the second most abundant carrier protein in adult sheep plasma (Gallaher et al., 1995). Recent work in sheep
confirms data from other species which shows that plasma concentrations IGFBP-2 are
high during fetal life, they fall gradually after birth and plateau after one year of age.
Levels of IGFBP-3 observed in fetal sheep plasma are significantly lower than those in
the adult (Gallaher et al., 1998) and during postnatal life there is a distinct ontogenic
increase in plasma concentrations of IGFBP-3 in sheep, similar to the ontogenic pattern observed for IGF-I (Gatford et al., 1998). The marked ontogenic changes in IGFIR concentrations serve as additional evidence for a key role of IGF-I during fetal
development. There is a rapid decline of IGF-IR levels in skeletal muscle; the concentrations in fetal muscle are about tenfold higher than in postnatal muscle (Boge et al.,
1995).
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organs are protected during catabolic states. Conversely, IGF-II is slightly increased following undernutrition in adult sheep (Hua et al., 1995). This observation together
with the demonstration that co-administration of IGF-II blocks the reduction in protein catabolism induced by IGF-I treatment in the fasted lamb (Koea et al., 1992b)
invites the hypothesis that the increase in the plasma IGF-II/IGF-I ratio during undernutrition may exacerbate the development of a catabolic state.
The effect of undernutrition on the IGFBPs is varied. Consistent with its glucose
counter-regulatory function, IGFBP-1 gene transcription and serum concentrations
are significantly and rapidly elevated by reduced food intake. Nutritional restriction
generally results in an increase in plasma IGFBP-2 which occurs via an increase in
transcription rate. This increase has been specifically linked to a reduction in protein
intake (Smith et al., 1995). However, there is still some controversy about the regulation of IGFBP-1 and IGFBP-2 in farm animals. Fasting of newborn pigs results in an
increase in plasma levels of IGFBP-1 but a decrease in IGFBP-2 levels (McCusker et
al., 1991). In response to 72 h starvation in adult sheep, IGFBP-1 is not significantly
changed, while IGFBP-2 increases (Gallaher et al., 1992). A clear inverse relationship
has been demonstrated recently in sheep between plasma insulin and glucose concentrations and plasma concentrations of IGFBP-1 and IGFBP-2, suggesting that both
are inversely regulated by plasma insulin (Gallaher et al., 1995). While the changes in
plasma IGFBP-1 and -2 following undernutrition appear to restrict the insulin-like
activities of the IGFs during catabolic states, the changes in IGFBP-3 and -4 may
reflect an attempt to maximize the availability of the remaining IGFs to the tissues.
Circulating levels of IGFBP-3 and the ternary complex (IGF storage capacity) are
reduced (Gallaher et al., 1992) while specific IGFBP-3 protease activity to reduce IGF
affinity for IGFBP-3 is enhanced. In conditions of reduced nutritional intake, IGFBP4, an inhibitor of IGF action, is decreased both at the serum and transcriptional level
(Holt et al., 1996).
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a rise in its plasma concentrations. Since plasma IGFBP-3 is increased while IGFBP-2
is decreased with GH treatment, this change in relative concentrations of IGFBPs may
facilitate an increased availability of IGF-I at target tissues. Treatment with bGH of
lambs significantly reduces plasma concentrations of IGFBP-2 while nutritional restriction increases plasma concentrations of IGFBP-2. Since these changes observed in
plasma concentrations of IGFBP-2 show a negative correlation with changes in plasma
IGF-I, it has been suggested that IGF-I itself may be an important regulating factor for
the availability of IGFs (Gallaher et al., 1995).
There is increasing evidence for ontogenic changes in responsiveness to GH treatment at identical doses. The treatment with GH (bGH at 0.3 mg kg21 day21) of wellfed healthy adult ewes leads to a marked reduction in protein breakdown and elevated
circulating concentrations of IGF-I. In addition, a marked diabetogenic effect on
plasma glucose and insulin is observed, and a major rise in plasma FFA levels shows significant lipolytic action in the healthy adult state (Hennies et al., 1998). While the
lipolytic action of GH is well recognized, it is minimal in young animals or in conditions of enhanced metabolic demand such as lactation, where high levels of energy
expenditure and utilization prevent a major rise in plasma FFA. There is no increase in
plasma FFA in young pre-pubertal lambs with GH (bGH at 0.3 mg kg21 day21). In
addition, these lambs showed a comparatively modest increase in plasma IGF-I and a
very mild diabetogenic response with only slightly elevated plasma glucose and insulin
levels during the well-fed phase of the study (Ogawa et al., 1996). This comparison
suggests that the same dose of GH leads to somewhat different actions depending on
the age of the animal.
Consistent with its growth promoting activity, IGF-I administration stimulates
protein metabolism and it reduces protein catabolism in the lamb, while passive immunization against IGF-I in lambs elevates net protein catabolism (Koea et al., 1992b).
Treatment with IGF-I also has insulin-like effects on glucose metabolism. IGF-I
reduces plasma glucose as a result of both increased glucose uptake and reduced glucose
production. However, in the sheep such data reflect a pharmacological response since at
physiological doses, IGF-I suppresses plasma insulin but increases glucose concentrations. Only as the dose increases does IGF-I saturate the IGFBPs, free IGF-I levels are
increased and a direct effect on glucose metabolism is observed (Douglas et al., 1991).
There is general agreement that elevation of plasma concentrations of IGF-I by treatment with high doses of IGF-I shows characteristic insulin-like effects inducing hypoglycaemia. This effect depends on saturation of the IGFBPs in circulation leading to a
marked increase in free IGF-I which binds to the insulin receptor.
Two different experimental paradigms were developed to investigate the metabolic
effects of short-term IGF-I infusion (5 h infusion) in the 48-h fasted lamb and longterm effects of 8-hourly IGF-I injections for 8 weeks in well-fed lambs. In short-term
treatment studies, young lambs were infused with IGF-I for 5 h after a 48-h period of
starvation (Douglas et al., 1991). Two doses of IGF-I were used: a low dose which
caused no elevation of free IGF-I in the circulation, and a higher dose which caused a
sevenfold elevation of free IGF-I. The lower dose did not alter plasma concentrations
of glucose, or the rate of glucose production or clearance, while the higher dose caused
mild hypoglycaemia. When this higher dose of IGF-I and a dose of insulin of equivalent hypoglycaemic potential were compared, only IGF-I increased protein synthesis in
skeletal muscle, heart and liver. Both IGF-I infusions at the low and at the high dose
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markedly reduced protein degradation and this effect may be mediated via the IGF-I
receptor (Douglas et al., 1991). Thus, short-term IGF-I infusion in the lamb can both
reduce protein degradation and increase protein synthesis in a number of different
tissues.
In long-term studies, well-fed yearling sheep were treated for 8 weeks with IGF-I,
which increased plasma concentrations of IGF-I by approximately 4060% (Cottam et
al., 1992). There was no increase in body weight or food intake. The weight of the
spleen increased by 40%, but there was no effect of IGF-I on carcass weight, composition and dimensions, or on long bone length, although the weight per unit length of
the tibia was increased. The somatogenic effect of long-term IGF-I treatment was minimal, suggesting that in well-fed animals with an intact somatotropic axis, IGF-I treatment at doses which increase plasma IGF-I within the physiological range do not
enhance somatic growth performance (Cottam et al., 1992). Exogenous IGF-I may
suppress tissue IGF-I production by interactive feedback between the systemic and the
local IGF-I system. Of particular note in the long-term IGF-I treatment study were the
effects on plasma glucose and insulin (Cottam et al., 1992). There was a marked and
rapid decline in circulating levels of insulin. Decreased plasma insulin was associated
with, and may have been responsible for, a significant increase in blood glucose levels in
IGF-I-treated sheep. It is possible that insulinopenia reduces the growth response by
restricting substrate uptake. Thus, the insulinopenic effect of long-term IGF-I treatment contrasts with the effect of GH, which induces hyperinsulinaemia. The lack of a
somatogenic effect with long-term IGF-I treatment may also be related to changes in
IGFBPs. While bGH treatment of sheep significantly increases plasma IGFBP-3 and
the ternary 150 kDa complex, IGF-I treatment of sheep was unable to induce such an
increase. Since plasma IGFBP-1 was increased and IGFBP-2 levels were suppressed
with IGF-I treatment, the change in the relative concentration of the different IGFBPs
may have affected the availability of injected IGF-I to target tissues. An independent
but related study by Min et al. (1996) showed that long-term administration of IGF-I
in young energy-restricted sheep lead to negative feed-back of circulating IGF-I on GH
secretion and the GHR number in the liver. Activation of such negative feed-back
loops by IGF-I treatment explains the lack of effects on growth rate, metabolic parameters or body composition in endocrinologically normal animals.
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of GH to ameliorate the effects of severe catabolism is much reduced due to the induction of GH resistance in these patients (Roth et al., 1995). It has been argued that in
cases such as elective surgery GH pre-treatment prior to the onset of GH resistance
could minimize post-operative catabolic stress. Such an approach has been used successfully in the fasted sheep (Ogawa et al., 1996). However, the interactions between
the intermediary metabolism, the immune system and the somatotropic axis during
infections and septic shock are even more complex. A number of studies have
employed endotoxin-induced septic shock to investigate the endocrine interactions
under pathophysiological conditions similar to microbial infections.
Endotoxin is a complex lipopolysaccharide molecule situated within the outer
membrane of Gram-negative bacteria; it is the main mediator of septic shock
(Morrison and Ryan, 1987). Endotoxin administration stimulates the immune system
and modulates the secretion of several mediators and hormones. Endotoxin induces a
rapid increase in plasma concentrations of tumour necrosis factor, interleukin-1, and
interleukin-6. Its action is mediated, at least in part, by an endotoxin receptor, which
has been identified in brain, pituitary, monocytes and plasma (Holst et al., 1996).
Changes in several hormonal axes have also been observed after endotoxin injection;
there is an increase in activity of the hypothalamicpituitaryadrenal axis (Dadoun et
al., 1998) and a decrease in thyroid-stimulating hormone and alterations in the
gonadotropic axis (Battaglia et al., 1997). Interactions between the immune and
endocrine systems are multi-directional and the hormonal changes induced by endotoxin or cytokines can act on the immune system. The pathophysiological responses to
endotoxin injection include increased heart rate, increased respiratory rate, changes in
lung permeability, activation of the complement cascade, hypoglycaemia, insulinaemia,
reduction in plasma concentrations of FFA and hypocalcaemia (Kinsbergen et al.,
1994). Septic shock is also associated with metabolic abnormalities, cardiovascular dysfunction, and multiple organ failure mainly related to ischaemia in part due to alterations in regional microcirculatory blood flow (Kinsbergen et al., 1994).
Endotoxin administration induces species-specific effects on GH secretion; it
increases GH secretion in sheep but decreases it in rats and cattle (Coleman et al.,
1993). In a recent study, Hennies et al. (1998) investigated the effects of GH therapy in
well-nourished sheep exposed to endotoxin-induced septic shock. Endotoxin injections
alone resulted in a modest fall in plasma glucose and no change in plasma insulin concentrations. However, the metabolic responses to bGH (0.3 mg kg21 day21) were
markedly altered during endotoxin-induced metabolic stress, leading to a dissociation
of the known effects of GH on intermediary metabolic pathways. The anticatabolic
effect of GH treatment was abolished despite continuation of considerably elevated
levels of plasma IGF-I. Furthermore, the GH-treated sheep showed a decline in plasma
insulin and glucose levels within 48 h after endotoxin challenge. This suggests that,
after observing an initial phase of diabetogenic effects of GH, endotoxin-induced septic
shock altered glucose homeostasis, despite continuation of the marked lipolytic effect
of GH. Studies in rats have shown that GH-treatment amplifies the adverse effects of
endotoxin-induced septic shock. Rats primed with a GH infusion developed more
severe hypoglycaemia, hyperlipidaemia and renal and hepatic dysfunction than
endotoxin-treated control animals (Liao et al., 1996). These effects were not observed
when IGF-I was administered to rats given an endotoxin challenge, implying that the
enhanced toxicity of endotoxin was a direct effect of GH administration. Whether the
200
Acknowledgements
The authors acknowledge support from the Health Research Council of New Zealand.
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Introduction
Grazing ruminants have to cope with large seasonal changes in the amount and quality
of herbage consumed. Their ability to survive is improved by seasonality of reproduction, which confines productive activities, namely late gestation, lactation and growth,
to spring and summer when the climate is favourable and food of good quality is abundant. Furthermore, the chances of survival are increased by depositing fat reserves
which are then used during periods of food shortage. Domesticated livestock, which are
descended from ancestral types that were probably strongly seasonal, may have conserved some of these peculiarities. However, genetic selection pressure, applied under
standardized conditions and aimed at increasing year-round production, may have had
the effect of smoothing this seasonality. Sheep, however, are not able to reproduce during all seasons. Nevertheless, there is great variability between sheep breeds, the most
sensitive to seasonal changes being located in high latitudes (Soay breed of the
Hebrides), while tropical breeds exhibit few responses to seasonal changes. Among seasonal cues, photoperiod (i.e. daily light duration) has been clearly identified to be an
effective signal which controls the period of reproduction in sheep (Ortavant et al.,
1988; Lincoln and Richardson, 1998). This raises the possibility that photoperiod,
which varies during the year, may also have effects during subsequent physiological
stages, i.e. reinforcing other physiological adaptive mechanisms.
205
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with high-quality diets (+11% with fescue hay and +35 % with dehydrated grass;
Milne et al., 1978). There are, however, differences between rams of Soay (+71%) vs.
Suffolk (+52%) breeds, and gonad-intact vs. castrated animals, the former being more
sensitive (Kay, 1985). Although it is generally accepted that minimum food intake
occurs during winter and maximal food intake during summer, few experiments have
been conducted to analyse the exact timing of the effect of photoperiod on food intake.
When photoperiod variations are cyclic (semestrial cycle), maximum food intake
occurs between 6 and 13 weeks after the maximum duration of light is reached (Kay,
1985). However, in the case of abrupt changes of photoperiod, the food intake of Soay
rams begins to change within 2 weeks (Lincoln and Richardson, 1998).
In sheep energy balance studies, neither the digestibility (Walker et al., 1991) nor
the metabolizability (Blaxter and Boyne, 1982) of the diet were affected by daylength.
Voluntary food intake is generally positively correlated with fasting heat production,
and this could suggest that intake oscillations may be driven by seasonal changes in
metabolic rate. To test this hypothesis, measurements of heat production were made at
a constant level of food intake (near maintenance) in intact ewes subjected to the following photoperiod treatments: constant long-day, natural, simulated natural or
reverse-natural (Walker et al., 1991). Oscillations of metabolic heat production
observed in ewes subjected to constant illumination provide evidence of a long-term
(more than one year) endogenous rhythm in energy metabolism. In all other sinusoidal
photoperiodic treatments, whole animal energy metabolism was driven by the pattern
of changes in daylight, even when the light cycle was reversed. The peak of metabolic
heat production did not occur in phase with the longest or the shortest daylength periods, but 15 3 weeks following the start of the day-length increment (i.e. 11 weeks
before the maximum duration of light). The amplitude of changes in fasting heat production (+21%) is in general agreement with previous results obtained in mature
wethers (+29%) kept under natural daylength (Blaxter and Boyne, 1982). It is noteworthy that the relative annual amplitude of variation in metabolic rate is far lower
than that of food intake variations. Furthermore, maximum heat production under
annual photoperiod conditions occurs before maximum daylength and long before
maximum food intake would occur.
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ing the winter period (Larsen et al., 1985a,b). However, a decrease in in vitro adrenaline-stimulated lipolytic capacity was observed during winter. This could prevent excessive breakdown of fat stores.
In the domestic sheep in Scotland, the rate of adipose tissue fatty acid synthesis
and the activity of the lipoprotein lipase, a key enzyme for tissue uptake of plasma
triglycerides, increased between October and May (Vernon et al., 1986). These
increases were, however, probably related to increased food intake, since plasma concentrations of insulin, glucose and acetate were increased also.
In the animal models discussed previously, changes in adipose tissue metabolism
paralleled changes in voluntary food intake and/or changes in plasma insulin and
energy metabolites that were available for lipogenesis. For this reason it is not possible
to distinguish whether the effects were due to a direct effect of photoperiod (or other
seasonal factors) on adipose tissue metabolism or due to indirect effects related to
changes in food intake. In order to avoid these problems Bocquier et al. (1998) used
dry non-pregnant adult ewes which were subjected during winter to either long
(16L:8D) or short (8L:16D) daylength for 46 weeks, and given fixed amounts of food
within two feeding treatments: (i) restricted to 22% of energy requirements for 1 week;
or (ii) subsequently re-fed to 190% of requirements for 2 weeks (pair-feeding across
photoperiod treatments). Ewes were also ovariectomized in order to avoid any putative
indirect effect of photoperiod via changes in the secretion of reproductive hormones, as
these are potential modulators of tissue metabolism (Chilliard, 1987). Under these
conditions, long days increased lipoprotein lipase activity in Longissimus thoracis
muscle and, in overfed ewes, the activities of lipoprotein lipase and malic enzyme (an
enzyme involved in NADPH generation for fatty acid synthesis) in subcutaneous adipose tissue, as well as malic enzyme activity in perirenal adipose tissue. Furthermore,
long days increased the amount of lipoprotein lipase mRNA in cardiac muscle and
perirenal adipose tissue (Table 12.1). There was also a non-significant trend for long
days to slightly increase the activity of three other lipogenic enzymes in adipose tissues
of re-fed ewes (Bocquier et al., 1998; Faulconnier et al., 1999). Simultaneously, long
days increased plasma leptin concentrations and the amount of adipose tissue leptin
mRNA independent of feeding level, and decreased plasma non-esterified fatty acids
concentrations in underfed ewes (Fig. 12.1).
It was previously reported that long photoperiod also increased adipose tissue leptin mRNA in Djungarian hamsters (Klingenspor et al., 1996). However, food consumption was not controlled in this trial. Furthermore, adipose tissue mass and
gonadal activity, which have been shown to modulate leptin production in rats and
humans, were sharply increased along with leptin mRNA (Klingenspor et al., 1996).
The study of Bocquier et al. (1998) in sheep shows, for the first time, an effect of photoperiod on leptin that is independent of food intake, adiposity or gonadal activity.
Moreover, the photoperiod-driven changes in leptin, lipogenic enzyme activities and
plasma non-esterified fatty acids were not related to changes in plasma insulin, glucose,
acetate, lactate, 3-hydroxybutyrate, triglycerides or urea concentrations, since these
parameters were not affected by photoperiod. These results suggest the existence of
direct effects of photoperiod on sheep adipose tissue lipogenic potential, leptin secretion and lipomobilization, as well as on muscle lipoprotein lipase activity, which are not
induced by nutrient supply.
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Table 12.1. Effects of nutritional status and daylength on malic enzyme activity in adipose
tissue and lipoprotein lipase activity and mRNA in adipose tissue and muscles (from Bocquier
et al., 1998; Faulconnier et al., 1999).
Underfed
Re-fed
Short
Long
Short
Long
Statistical effectsa
41
34
51
35
117
264
158
415
F, p
F, P, F3P
74
158
22
87
184
32
147
589
111
208
604
291
F, P, f3p
F
F, p
Cardiac muscle
Lipoprotein lipase activitye
Lipoprotein lipase mRNAd
133
71
129
118
187
158
160
252
F
F, p
23
30
33
43
F,p
a5
ewes in each group (N = 20). P, p: effect of photoperiod (P : P < 0.05; p: P < 0.10). F, f: effect of feeding
level (F: P < 0.05; f: P < 0.10). f3p: = interaction (P < 0.10).
b nmol NADPH min21 1026 adipocytes.
c nmol fatty acids min21 1026 adipocytes.
d Arbitrary units.
e nmol fatty acids min21 g21 DNA.
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Fig. 12.1. Effects of daylength (shaded bars: short days (8 h day21) and open bars: long
days (16 h day21)) and nutritional status (underfed (22%) or overfed (190% of energy
requirements)) on (a) plasma leptin, (b) adipose tissue leptin mRNA (in arbitrary units),
(c) plasma NEFA and (d) plasma prolactin (5 ewes per group) (from Bocquier et al.,
1998).
light duration from those of changes in light duration, light treatments were abruptly
switched from constant long (15.30L:08.30D), or constant short (08.30L:15.30D) to a
common treatment of equal light and dark phases (12.00L:12.00D) (Bocquier et al.,
1997). The shortening (3.30 h) of light exposure induced a dramatic decline in milk
yield (38% 42 days later), while lengthened light exposure (+3.30 h) limited the normal decline of milk yield to 8% during the same period. Hence sheep milk production
is affected both by changes in daylength and by the duration of light exposure itself, via
multiple effects either on mammogenesis, lactogenesis and/or galactopoiesis.
Milk fat and protein concentrations are lower during long than during short days.
During early lactation, the maximal difference between short and long days was
observed (Bocquier, 1985; Bocquier et al., 1997) 3040 days after lambing for both fat
(Prealpes: 10 g l21; Sardinian: 14 g l21) and protein content (Prealpes: 3 g l21;
Sardinian: 11 g l21). After the progressive switch of light treatments during late lactation (Fig. 12.2), milk concentration of ewes exposed to long days was even more lowered (19 g l21 for fat and 13 g l21 for protein) when compared with ewes exposed to
short days. After the abrupt change in daylength to an equal dark/light exposure treatment (Bocquier et al., 1997), it took 23 days until the maximal difference in milk concentration occurred. During this period, fat and protein content of milk from ewes
subjected to an increase in daylength (+3.30 h) decreased by 8 g l21 and 6 g l21 respectively compared with that from ewes subjected to a decrease (3.30 h) in daylength.
210
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Fig. 12.2. Milk production of ewes subjected to either long (j) or short ()
photoperiod; in trial A (January lambing; n = 24 per group) and trial B (September
lambing; n = 22 per group). Light treatments were established 42 days before lambing,
and were reversed from the 9th week of lactation, by progressive changes within a
fortnight (
) (Bocquier, 1985).
This suggests that a single change from long or short days towards a 12L:12D constant
photoperiod is sufficient to produce responses in milk yield and composition that
could be as high as 50% of the response attained after an exposure of several weeks to a
constant difference of 8 h in daily light exposure.
The negative relationship between the level of milk yield, and milk fat and protein
concentrations may be due to a dilution effect since the increase in milk yield by long
daylength (+20% in early lactation, i.e. between day 1 and day 30, and +67% in late
lactation, i.e. between day 90 and day 110; Table 12.2) is accompanied by a decline in
both fat and protein content of milk (5% in early lactation and 19% in late lactation). This decline is, however, lower than the positive effect on milk yield, because
both fat and protein yields are increased by long daylength in early (+14%) and late
(+36%) lactation (Table 12.2). Surprisingly, the effects of photoperiod on milk lactose
and mineral content have not been studied, despite the well-known contribution of
these constituents to the osmotic regulation of water secretion in milk.
In ad libitum-fed ewes, voluntary intake was identical between groups during the
first month of lactation and became higher (+16% after 150 days of lactation) in longday exposed ewes (Bocquier et al., 1997). This resulted in a lower calculated energy balance (11% of energy requirements) during the first month of lactation for long-day
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Table 12.2. Photoperiod and milk secretion by dairy ewes (Bocquier, 1985).
Early lactationa
Late lactationb
Shortc
Longd
Long % Short
Shortc
Longd
Long % Short
980
1178
120
328
550
167
Content (g kg21)
Fat
Protein
73
50
69
47
95
94
99
71
80
57
81
80
Yield (g day21)
Fat
Protein
71
49
82
56
114
114
32
22
43
31
135
137
Milk yield (g
day21)
a From
exposed ewes which produced more milk energy, than for short-day exposed ewes
(5% of requirements), while the reverse was observed between days 30 and 150 of lactation (+22% and +16% of requirements, for long- and short-day exposed ewes, respectively). In order to isolate a direct effect of photoperiod on milk secretion Bocquier et
al. (1986) fed ewes a limited and identical amount of food in both photoperiod treatments. Under these conditions, there was an increase in the apparent efficiency of milk
yield at the expense of body energy deposition in ewes exposed to long days (Fig. 12.3).
This partitioning of energy towards milk output occurred in ewes that were either in
positive or equilibrated energy balance. Changes in body energy content were mainly
due to changes in body lipids, suggesting that, in sheep, photoperiod acts on the partitioning of nutrients between mammary gland and adipose tissue independently of
effects on food intake.
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Fig. 12.3. Effect of photoperiod (long versus short) on energy partitioning between
milk (open bars) and body reserves (shaded bars) in lactating (pair-fed) ewes (Bocquier
et al., 1986). Trials A and B are described in Fig. 12.2. Body energy was measured by
D20 dilution technique (Bocquier et al., 1999).
Peripheral hormones
Prolactin is second only to melatonin in terms of responsiveness to photoperiod, and
responds to an abrupt change of photoperiod within a week (Lincoln et al., 1978).
Prolactin is elevated both in intact (Kann, 1997) and ovariectomized (Bocquier et al.,
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1998) ewes exposed to long daylength. Long photoperiod is also effective in the pregnant ewe (Perier et al., 1986) and induces a 40% higher and earlier (20 h) pre-partum
surge of plasma PRL (Bocquier, 1985). In lactating ewes exposed to long photoperiod,
mean plasma PRL concentration was increased by 311% (Bocquier et al., 1990).
However, photoperiod-induced increments in serum concentration of PRL began to
wane when sheep were maintained on 16L:8D for more than 16 weeks (Almeida and
Lincoln, 1984). This indicates that PRL secretion becomes refractory to a prolonged
constant photoperiodic stimulus.
Plasma growth hormone (GH) has been shown to be slightly elevated by long
daylength in rams (Lincoln and Richardson, 1998) and in the pregnant ewe (Perier et
al., 1986), but this has never been observed in the lactating ewe (Bocquier, 1985;
Bocquier et al., 1990), nor in the dry non-pregnant ovariectomized ewe (Chilliard et
al., 1998b). It has been suggested recently that the effect of long photoperiod on milk
yield in the cow could be mediated by circulating insulin-like growth factor-I (IGF-I).
Cows exposed to long photoperiod produced more milk (+6%) without increased feed
intake, and had an elevated concentration of IGF-I (+77%), without altered concentrations of GH (as in lactating sheep) or IGFBP-2 and -3 (Dahl et al., 1997).
Other hormones were either unaffected by daylength (insulin; Bocquier et al.,
1998) or gave conflicting results (thyroid hormones and corticosteroids; Forbes 1982;
Vernon et al., 1986; Petitclerc and Zinn, 1991; Lincoln and Richardson, 1998). In ad
libitum-fed sheep, there was an increase in plasma b-endorphin concentration at the
beginning of a short-day period that could have stimulated insulin secretion and adipose tissue lipogenesis (Lincoln and Richardson, 1998). However, as this followed a
period of high food intake at the end of a long-day period, it is not clear whether the
changes in b-endorphin concentrations were due to photoperiod per se, or to the previous change in food intake.
Adipose tissue
Peripheral effects of melatonin on adipose tissue are plausible, since specific binding
sites in Siberian hamster brown adipose tissue have been described (Le Gouic et al.,
1997). However, the functionality of these receptors remains to be shown. Although
plasma PRL concentration increases in response to increasing both daylength and feeding level (Bocquier et al., 1998), this hormone is probably not directly involved in adipose tissue response to daylength because sheep adipose tissue appears to lack PRL
receptors (Emane et al., 1986; Knight and Flint, 1995). Furthermore, addition of PRL
in vitro did not change lipolysis in bovine or ovine adipose tissue (Houseknecht et al.,
1996), nor was lipogenesis changed in ovine (Vernon and Finley, 1988) or bovine adipose tissue (Etherton et al., 1987).
There could be, however, indirect effects of PRL in vivo. In the rat, PRL infusion
was shown to stimulate the secretion of a mammotrophic and lactogenic hepatic factor
(synlactin) (English et al., 1990), the effects of which remain to be unravelled for adipose tissue. PRL injections increased mammary acetyl-CoA carboxylase activity and
mRNA abundance in the lactating rat, and decreased it in adipose tissue (Barber et al.,
1992). The adipose tissue lipolytic response was increased by PRL injections in female
rats during recovery from lactation (Vernon and Finley, 1986). Although these results
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suggest that the effects of PRL in rodents are antilipogenic and/or lipolytic (indirect),
there was no in vivo effect of PRL on plasma non-esterified fatty acids in adult sheep
(Luthman and Johnson, 1972), lactating dairy cows (Plaut et al., 1987) or lactating
goats (Jacquemet and Prigge, 1991). Furthermore, administration of bromocriptine (a
dopamine agonist which inhibits PRL secretion) did not change adipose tissue
lipogenic rate in post-partum beef cow (Mills et al., 1989), and PRL administration did
not affect adipose tissue lipogenic or lipolytic rates, although the weight of perirenal
adipose tissue was increased in growing sheep (Eisemann et al., 1984). Results from
other studies with bromocriptine suggest that PRL has lipogenic (in synergy with
insulin) rather than antilipogenic effects on the liver, and promotes body fat deposition
in several rodent species and pig (Cincotta and Meier, 1989a; Cincotta et al., 1989).
Interestingly, the decrease in body fat after bromocriptine administration was observed
without (or with very few) changes in food consumption, energy expenditure and lean
body mass (Cincotta et al., 1993a). However, the antilipogenic effects of bromocriptine
could be due to dopaminergic effects in the hypothalamus (Luo et al., 1997), rather
than simply due to a decrease in PRL secretion.
In rodents, it appears that the lipogenic effects of photoperiod result from a synergy between insulin, corticosteroids and PRL which is dependent on the time of day,
both in vivo (Cincotta et al., 1993b) and in vitro (Cincotta and Meier, 1989b). These
observations are related to a paradigm which was proposed more than 20 years ago by
Meier and Burns (1976) from studies in several vertebrate species (including fishes,
birds and mammals) that exhibit marked seasonal cycles in body fat stores:
These cycles are controlled by the circadian rhythms of corticosteroids and prolactin.
Prolactin stimulates increases or decreases in fat storage depending on whether it is present
in larger quantities during daily intervals of lipogenic or lipolytic sensitivities. The intervals
of sensitivity are entrained by the daily photoperiod and mediated by the adrenal
corticosteroids. Thus the temporal synergism of the circadian rhythms of corticosteroid and
prolactin hormones involves a relation between sensitivity rhythms of cells involved in lipid
metabolism and rhythms of the stimulatory hormone, prolactin. The cumulative effects of
various temporal hormonal patterns account for the seasonal changes in fat stores.
This results in annual cycles of metabolic activities that are the result of an interaction
of the daily photoperiod with an endogenous seasonal timing mechanism, termed
scotosensitivity. Alteration in PRL rhythm could reflect an altered phase of a central
circadian pacemaker that entrains other circadian responses (Meier and Cincotta,
1996).
Alternating insulinepinephrine infusions in the rat resulted in a complete inversion of the normal circadian distribution of sleeping and feeding patterns. This led to
the hypothesis that the circadian light cycle might act on sleeping and feeding by influencing the tonus of secretion of metabolic hormones mediating the lipogenic vs. lipolytic ratio (Danguir and Nicolaidis, 1980). However, plasma insulin and GH are not
changed by photoperiod in the non-pregnant ewe.
Meiers hypothesis can also be compared with the fact that the in vitro synergism
between insulin and corticosteroids for lipogenic effects was more important in sheep
than in cattle adipose tissue explants (Chilliard and Faulconnier, 1995; Faulconnier et
al., 1996), bearing in mind that the latter species is less photoperiod-sensitive.
Nevertheless, the lack of a clear effect of season or photoperiod on plasma insulin and
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corticosteroid concentrations in sheep does not argue for a simple extrapolation to the
ovine species of the PRLglucocorticoidinsulin hypothesis, although it appears to be
applicable to other species. There is a need for information on circadian variations of
these three hormones in sheep, the sensitivity of adipose tissue to their effects, and on
whether their secretion is altered by variations in daylength.
The mechanisms by which photoperiod influences leptin gene expression in adipose tissue remain unknown. It has been shown recently that intravenous injection of
neuropeptide Y (NPY) increased leptin mRNA level in ovine adipose tissue (Dyer et al.,
1997), but it is not known whether daylength affects NPY or related peptides. It has
also been shown that glucocorticoids can modulate leptin in monogastric and ruminant species (Houseknecht et al., 1998; Chilliard et al., 1999).
In the Siberian male hamster, exposure to short days activated the sympathetic
nervous system and increased norepinephrine turnover (McElroy et al., 1986). A similar catecholaminergic effect of short days, which remains to be demonstrated in sheep,
could account for the increase in plasma non-esterified fatty acid concentrations, as
well as for decreases in adipose tissue leptin and lipogenic activities (Fig. 12.1 and Table
12.1). Catecholamines are lipolytic and antilipogenic in sheep and it was recently
shown that they lowered leptin concentrations in rodents (Trayhurn et al., 1995) and
cattle (Chilliard et al., 1998a). Differences in catecholaminergic innervation between
anatomical sites of adipose tissues could explain why short days deplete more internal
than subcutaneous adipose tissue, a phenomenon which could serve to enhance insulation from the cold during winter (Youngstrom and Bartness, 1995). However, the
effect of short days on the pattern of adipose tissue depletion, which was observed in
males, was not apparent in female Siberian hamsters (Bartness, 1995). In ovariectomized ewes, short days decreased lipoprotein lipase activity to a greater extent in subcutaneous than in perirenal adipose tissue, and in skeletal than in cardiac muscle (Table
12.1). The noradrenergic sensitivity of adipocytes was not changed in Djungarian hamsters exposed to short days, suggesting that their rapid fat mobilization is determined
centrally by the chronic activation of the sympathetic nervous systems, without desensitization of adipocyte b-adrenergic lipolysis (Mercer et al., 1995).
Mammary gland
The effects of photoperiod on lactation can be analysed either at the level of whole
body metabolism and nutrient partitioning or at the level of mammogenesis (i.e. the
formation of a functional mammary gland) and galactopoiesis (milk production).
The mammogenic effect of long photoperiod observed in ewes can be explained
by the elevated and prolonged pre-partum PRL surge (Bocquier, 1985; Perier et al.,
1986); prolactin is necessary for the complete structural differentiation of epithelial
cells and for the onset of copious milk production (Knight, 1993; Kann, 1997). A role
for placental lactogen in photoperiod-induced differences in mammogenesis is, however, doubtful, since circulating concentrations were unchanged by daylength (Perier et
al., 1986). Artificial induction of lactation is an interesting model since it avoids the
interference with numerous hormones that are prevalent during pregnancy. Prealpes
maiden ewes, receiving an oestroprogestative treatment before being submitted to the
machine-milking stimulus, produced more milk (+27%) under long-day treatment
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than under short-day treatment (Kann, 1997). In addition to the classical increment of
circulating PRL, there was an increase in plasma concentrations of GH (+136%) and
IGF-I (+23%), concurrently with an enhancement of GH hepatic receptor number.
Although galactopoeitic effects of GH have been clearly demonstrated in cows
(Bauman and Vernon, 1993), its mechanism of action remains controversial. GH may
not act directly on the mammary gland but via IGF-I. Although the plasma concentration of GH is generally not altered by photoperiod in the lactating sheep, IGF-I was
elevated moderately in the ewe (Kann, 1997) and substantially in cattle (Dahl et al.,
1997) subjected to long daylength. These observations can be related to an increase in
the number of GH receptors in the liver (Kann, 1997). It seems likely then that
photoperiod-induced changes in IGF-I concentration may be partly responsible for
effects on milk production.
Although systematically elevated by long daylength, the exact role of PRL on lactation is not clear, even if this hormone exerts some important galactopoietic effects in
the ewe, a smaller effect in the goat (Knight and Flint, 1995) and is without effect in
cattle (Plaut et al., 1987). In the ewe it was demonstrated (Hooley et al., 1978; Kann et
al., 1978; Gow et al., 1983) that a pharmacological PRL depression, using bromocriptine, decreased milk yield by 2030%. Furthermore, the negative effects of bromocriptine on milk yield were reversed by concurrent infusion of PRL (Hooley et al., 1978).
However, since milk production was not totally suppressed by the almost complete
(< 1 ng ml21) depression of PRL, this hormone may not be absolutely necessary in the
galactopoeitic hormonal complex of the ewe (Gow et al., 1983). It is worth noting,
however, that sheep, which have been less subjected to selection for milk production
than dairy cattle, are also more sensitive to the effect of PRL and to the effects of photoperiod, while dairy cattle may have become exquisitely sensitive to small amounts of
circulating PRL (Knight, 1993), and hence less dependent on photoperiod because
PRL concentration would never be limiting. Separate consideration of the roles of GH
or PRL is probably an oversimplification, since it has been shown in goats that GH
response is higher when given together with prolactin than when given alone (Knight
and Flint, 1995).
Although ewes exposed to long daylength had a higher milk-solid yield, their milk
was more diluted than that of short-day exposed ewes. One hypothesis is that such a
dilution effect may be attributed to high levels of PRL observed in ewes exposed to
long daylength. In the case of dairy cattle, injection of exogenous PRL had no effect on
milk concentration of fat, protein or lactose and had a small positive effect on -lactalbumin (Plaut et al., 1987). There is, however, evidence that PRL acts on water and
solute transport across mammary epithelia, although no direct effect has been isolated
(Sheenan, 1994).
The stimulatory effect of photoperiod on milk production is accompagnied by a
readjustment of whole body energy metabolism, with a decrease in body fat (Fig. 12.3)
before an increase in food intake. The effect of photoperiod may override and exacerbate the homeorhetic (from Greek meaning uniform flow; Bauman and Currie, 1980)
or teleophoretic (from Greek meaning a transport that is oriented towards a goal;
Chilliard, 1986, 1999) mechanisms whereby an animal re-adjusts its homeostatic setpoints to new physiological conditions. Besides the complex interactions linked to the
circadian and seasonal rhythms of hormonal secretions and tissue sensitivity to hormones, the effects of PRL on adipose tissue are also dependent on the presence or the
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217
Conclusion
Wild animals have developed numerous non-exclusive adaptive strategies to survive in
their environmental conditions. In non-equatorial regions, photoperiod is the most
noise-free signal of the annual variations in food resources and general climatic conditions. Logically, the reproductive process is strongly linked to light duration, since it
induces the succession of pregnancy and lactation both of which lead to high nutritional requirements. As the feed resources available to meet this demand are generally
maximal in spring and/or summer, the reproduction of the different species occurs at
different periods of the year according to gestation length. Because of the wide variation in the length of pregnancy, there must be a great diversity of mechanisms whereby
animal species translate the photoperiodic signals into biological adaptations, although
there appears to be a strong communality of central neuroendocrine and peripheral signalling systems (melatonin and PRL responses) between species (Morgan and Mercer,
1994).
Although domesticated, sheep have not lost the ability to respond to photoperiod.
It is of interest to note the differences in time-scale of the maximal response of physiological functions to changes in daylength, namely: milk composition (less than a week),
milk production (a few weeks), adipose tissue metabolism (several weeks) and basal
metabolic rate and food intake (several months). In parallel with this, hormonal
changes are very rapid for prolactin (a few days), rapid for leptin and IGF-I (less than a
month), or uncertain (GH, T3, T4, corticosteroids). These results together confirm the
hypothesis formulated by Ortavant et al. (1988) that photoperiod may also be involved
in metabolic adaptations that accompany the high nutritional requirements for pregnancy and lactation in the sheep.
Our proposed interpretation of the effect of photoperiod on metabolic regulation
in sheep is illustrated in Fig. 12.4. Homeostasis in the dry non-productive ewe (Fig.
12.4a) reflects regulatory processes that ensure the constancy of internal conditions for
vital processes (according to given set-points) in the face of changes in environmental
conditions. We speculate that the set-point for body lipids is diminished by short days.
Reproduction, which is favoured by short days, may be impaired if ewes are underfed.
Hence, the increase in non-esterified fatty acids and decrease in leptin by short days can
be seen as facilitating the general adaptation of ewes to winter undernutrition, and
increasing the sensitivity of reproduction blockade in these adverse conditions.
When the reproductive process is successful, the organism is then subjected to
teleophoresis, i.e. the orchestrated control of body tissue metabolism necessary to support physiological functions such as pregnancy or lactation. In the case of the ewe, lactation occurs naturally during long days due to the length of gestation. Here again, the
physiological regulation is probably changed because milk yield is increased by
daylength at the expense of body reserves (Fig. 12.4b), and later on food intake is also
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Feeding level
High
Lactation
function
Teleophoresis
Body fat
set-point
a
Equilibrated
Homeostasis
Homeostasis
Homeostasis
Body fat
set-point
(+)
(Remnant
Reproduction
effects)*
(+/)
Insufficient
Reproduction
(Yes)
()
()
(+)
(Effects of short days)
Dry ewe
Dry ewe
Fig. 12.4. Diagrammatic representation of the effects of photoperiod (g) on energy metabolism
regulation and body fat set-point during the reproductive cycle in the ewe (* remnant effects
due to changes in body fatness during lactation).
increased. After drying off, ewes are generally still naturally subjected to long days (Fig.
12.4c), and body fat is restored towards a set-point that is higher than during the previous short-day season. This is in apparent contradiction with elevated leptin levels,
which would decrease food intake and adipose tissue lipogenesis (Houseknecht et al.,
1998; Chilliard et al., 1999). As this does not occur, central and peripheral resistance to
leptin probably occurs. The sheep subjected to long days could thus furnish a new
model to study leptin resistance, a key issue in obesity studies.
The biological significance of this ability to restore body reserves during long days
is that it occurs at a period when long photoperiod postpones the occurrence of the
next reproductive cycle and when an anticipatory body reserve replenishment, when
food resource is abundant, is necessary to support future reproduction. Similarly, the
increase in muscle lipoprotein lipase activity could reflect an adaptation for sparing
body protein despite increasing physical activity and foraging, since the enzyme controls, in part, the entry of energy fuel, as fatty acids, into muscle cells. These observations can be viewed as annual rhythms regulating the expression of the thrifty
genotype/phenotype, or fattening physiology, that enabled animals and human ancestors to adapt efficiently to large seasonal changes in food availability and in the nutrient
content of food (Meier and Cincotta, 1996).
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Acknowledgements
We thank Yannick Faulconnier, Muriel Bonnet, Guy Kann and Daniel Sauvant for
helpful discussions during the preparation of the manuscript, and Pascale Braud for
the artwork and secretarial assistance. This research was supported by the INRA
Lipogenesis in farm animals and Leptin and reproduction in farm animals grants.
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Introduction
Growth of a whole animal, or its organs, or tissues, is clearly under a coordinated, integrated control system, of which there is limited understanding (Conlon and Raff,
1999). The size of an organ/tissue depends on the number and size of its cells and
extracellular components. Therefore, the growth of a specific tissue, such as muscle, is
dependent on the number of myogenic progenitor cells, proliferation of these committed myogenic cells (myoblasts) and the subsequent hypertrophic growth of these muscle fibres. An increase in total muscle mass is the result of a balance between cell
proliferation, programmed cell death and cell/fibre growth. Growth of an individual
muscle and the overall coordination of muscle growth throughout the body requires
the integration of both local and systemic control systems. Research to date has largely
focused on local control systems and how they interact with systemic systems, such as
the growth hormone (GH) axis. Little work on the coordination of whole-body muscle
growth has been undertaken, yet, in numerous selection trials, weight gains in cattle
and sheep have shown that coordinated overall growth and associated muscle growth is
highly heritable and therefore under genetic control (Notter, 1999).
This chapter will briefly review the stages and known control systems of muscle
development and then will focus on some of the specific genes which have been shown
to regulate muscle growth in ruminants. The continuous development of muscle can
be divided into three basic phases for convenience: determination of progenitor cells,
proliferation of myoblasts, differentiation and growth of muscle fibres (Fig. 13.1).
Determination
Mesodermal cells are the source of all vertebrate skeletal muscle (Cossu et al., 1996)
and these originate from progenitor cells in the embryonic epiblast. The mesoderm,
which initially consists of lateral plates, divides into somatic mesoderm, splanchnic
CAB International 2000. Ruminant Physiology: Digestion, Metabolism,
Growth and Reproduction (ed. P.B. Cronj)
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Determination
Myogenic
progenic cells
Proliferation
Proliferation
differentiation
Myostatin
Myogenin
Fusion
IGF-II
Myotubes
Maturation
MRF4
Mature
muscle
fibres
Hypertrophy
IGF-I
IGF-I
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Medial
myf5
Dermomyotome
Pax 3
Neural
tube
Lateral c-met
Myotome
Notochord
Sclerotome
Pax1
myogenic cells (Cossu et al., 1996; Capdevila et al., 1998) (Fig. 13.1). Examples of
inducing factors are: Pax 3, which has been shown to be essential for the determination
of myogenic progenitor cells and their migration into limb muscles (Cossu et al., 1996)
and sonic hedgehog, which has been identified as a candidate for inducing the onset of
myogenesis (Marcelle et al., 1997).
The myogenic regulatory factors (MRFs), members of the helix-loop-helix group
of transcription factors, are expressed in the medial dermomyotome and may have a
role in determination of myogenic cells as well as late stages of muscle fibre development. Both Myf5 and MyoD are believed to be essential for the early development of
muscle, as mice deficient in both lack normal myoblasts and skeletal muscle
(Edmondson and Olson, 1993).
An understanding of the mechanisms that control the formation of myogenic
progenitor cells will allow the future identification of critical gene pathways for ruminants as well as other species. At the moment, no observed variations in these early
myogenic pathways have been associated with changes in muscle mass of ruminants.
Proliferation
The second stage of muscle development is proliferation. During this stage, committed, mononucleated myogenic cells (myoblasts) first proliferate and then differentiate
into fusion-competent myoblasts. These fusion-competent myoblasts divide and eventually fuse end-to-end, forming long, multinucleated myotubes. Myoblasts can be
divided into three types on developmental age and morphology in culture (Franzini et
al., 1994). These are the embryonic or early myoblasts, isolated from embryonic muscles prior to and during primary myotube formation; the fetal or late myoblasts, isolated from fetal muscles throughout secondary fibre formation; and the satellite cells,
230
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isolated from adult muscles. The number of myoblast divisions may be important in
regulating the skeletal musculature in the growing animal. Myoblasts have limited
capacity to proliferate (Grounds and Yablonka-Reuveni, 1993), compared with other
cells. Whether a myoblast continues dividing or differentiates appears to be dependent
on extracellular cues, such as growth factors, extracellular matrix and cell-to-cell interactions. A large number of in vitro studies have indicated that myoblast proliferation
can be regulated by growth factors (Florini and Magri, 1989).
To date, the major growth factors which affect myoblast proliferation are fibroblast
growth factor, insulin, insulin-like growth factors (IGF), platelet-derived growth factor
and transforming growth factor-b (TGF-b). Other mitogens such as epidermal growth
factor and leukaemia inhibiting factor have also been shown to affect myoblast proliferation (Hauschka, 1994). Growth factors often have different biological effects which
appear to depend on their concentration and the developmental stage of the myoblasts.
This divergence of function is seen with IGF-I that can stimulate proliferation as well
as maintain differentiation (reviewed by Florini et al., 1996) and in TGF-b that inhibits
both proliferation and differentiation of myogenesis (Massagu et al., 1986).
An example of how a change in the control system of myoblast proliferation can
affect muscling of cattle is to be found in the double-muscled Belgian Blue breed. In
certain breeds, intense selection over many generations has resulted in extreme muscle
hypertrophy known variously as double-muscling, culard, etc. (Boccard, 1981). This
type of muscle hypertrophy is mainly associated with an increase in the number of
muscle fibres, especially secondary muscle fibres. It has been postulated that the
increased fibre number results from increased numbers of late myoblasts, either because
of increased myoblast proliferation or because of delayed differentiation into myotubes.
The inheritance of double-muscling in Belgian Blue cattle has been identified as a
monogenic autosomal segregation pattern (Hanset and Michaux, 1985; Charlier et al.,
1995). The muscular hypertrophy (mh) locus has been termed partially recessive,
because, while a single copy of the allele may have some effect, both are required for
full expression of the double-muscled phenotype. Gene mapping (Charlier et al., 1995)
of the Belgian Blue cattle localized the mh gene to the centromeric end of the bovine
chromosome 2 (BTA2) linkage group. The mh locus, in more refined studies in the
Piedmontese breed, localized the gene to a 35-cm interval near the centromere of
BTA2 close to the position of the a-collagen type III (COL3A1) locus (Casas et al.,
1999).
Recently, growth differentiation factor-8 (GDF-8) (McPherron et al., 1997), a
member of the TGF-b superfamily, was disrupted in mice. These GDF-8 null mice
were significantly larger than their wild type counterparts, and showed an increase in
body weight resulting from a two- to threefold increase in muscle mass. This increase in
muscle was due to an increase in the number of muscle fibres. As GDF-8 seemed to
function as an inhibitor of muscle growth, it was renamed myostatin. The myostatin
gene has since been mapped to the same interval as the mh locus in cattle by genetic
linkage (Smith et al., 1997). The finding that the myostatin null mice have a similar
phenotype to double-muscled cattle and that the gene maps to a similar site, suggests
that the myostatin and the muscular hypertrophy genes are one and the same. This
hypothesis was further supported by finding an 11 bp mutation in the myostatin gene
in Belgian Blue cattle and a point mutation in Piedmontese cattle, both doublemuscled breeds (Kambadur et al., 1997).
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More recently, Grobet et al. (1997) have identified seven DNA sequence polymorphisms, of which five were predicted to disrupt the function of myostatin. These studies clearly demonstrate that the double-muscling phenotype in cattle is genetically
heterogeneous and involves several mutations in the myostatin gene. The 11 bp deletion in the Belgian Blue cattle results in the loss of three amino acids, that causes a
frame-shift after amino acid 273. This frame-shift leads to a stop codon after amino
acid 286 that is predicted to produce a truncated, biologically inactive protein. In the
Piedmontese breed, a mutation at position 941 bp results in the loss of a cysteine at
amino acid 314 (Kambadur et al., 1997). This cysteine has been shown to be essential
in other TGF-b family members in order to form a cysteine knot, which stabilizes the
TGF-b dimer. In general, these studies suggest that myostatin is probably the mh locus
and that it acts as an inhibitor of muscle development by limiting muscle fibre number,
and to some extent, muscle fibre size.
When the development of proliferating and fusing myoblasts was followed, it was
found that myostatin mRNA was elevated during primary fibre formation through to
the early stages of secondary fibre formation. This suggests that myostatin may participate in the proliferation and terminal differentiation of late myoblasts (Oldham et al.,
1998). This study also suggested that a link exists between MyoD and myostatin, as
MyoD expression was increased during the fibre formation in muscle from doublemuscled cattle, which lack myostatin. Interestingly, the different myostatin mutations
found in cattle do not give similar increases in muscle mass. This suggests that the
myostatin proprotein and the mutant proteins inhibit muscle growth differently. The
possibility also exists that associated muscle-controlling genes have been differentially
selected in the different breeds.
A further level of control of muscle development by myostatin has been identified
in the compact hypermuscular mouse (Szabo et al., 1998). The mutation that causes an
increase in muscle mass in the compact mouse is a deletion in the pro-peptide region,
which precedes the proteolytic processing site of myostatin. The pro-peptide region, by
analogy with TGF-b (Miyazono et al., 1991), may be involved in the folding and secretion of myostatin. Such changes possibly decrease the biological activity of myostatin,
but do not completely remove it. Myostatin appears to be a tissue-specific inhibitor of
myoblast proliferation that can have a direct effect on myoblast proliferation and so
also on muscle fibre number.
Examples of factors that stimulate rather than inhibit myoblast proliferation
include IGF-I and -II, as both have been shown to enhance myoblast proliferation in
culture (Florini et al., 1996). Whether both IGFs are local growth factors that are critical in controlling myoblast proliferation in vivo is unclear, because local expression of
IGF-II increases at the end of secondary myofibre formation, whereas IGF-I does not
show marked changes associated with stages of fetal muscle formation in cattle
(Oldham et al., 1998). In most species, serum concentrations of IGF-I, but not IGF-II,
correlate positively with fetal weight (Gluckman and Brinsmead, 1976; Humbel, 1990).
The IGFs have been related to fetal growth in general. When gene targeting is used to
disrupt IGF-II, the offspring are only 30% of the weight of the wild type (Baker et al.,
1993), while IGF-I null mutants are growth retarded, depending on the magnitude of
the reduction in IGF-I concentration (Liu et al., 1998). While IGF-I appears to act
locally on developing muscle (Tollefsen et al., 1989), it dramatically increases IGF-II
mRNA in regenerating adult muscle when satellite cells from healthy muscle divide and
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migrate into damaged areas to initiate regeneration (Kou and Rotwein, 1993; Kirk et al.,
1996). This indicates that IGF-II is important in myoblast proliferation and differentiation.
GH, which is the hormone controlling the expression of IGF-I, does not seem to
be directly involved in myoblast proliferation, as mice overexpressing GH do not necessarily show increased muscle growth or increased IGF-I expression at birth (Liu et al.,
1998). However, the treatment of pregnant sows with GH early in gestation increases
both the number of muscle fibres and the liveweight at birth of their offspring
(Rehfeldt et al., 1993). Such an effect of GH may be indirect via maternal changes
rather than acting directly on the fetus.
Differentiation
When myoblasts leave the cell cycle and become terminally differentiated, they fuse
with one another to form myotubes (Fig. 13.1). Myotubes are immature muscle fibres
which form in distinct phases. Primary myotubes are thought to arise from end-to-end
fusion of early myoblasts. Late myoblasts then accumulate on the surface of the preexisting primary myotubes and fuse to form secondary myofibres. In all muscles, there
are many more secondary myotubes than primary myotubes. In large animals such as
sheep and cattle, there is possibly a third phase of myogenesis, where the tertiary myofibres use the secondary fibres as a scaffolding, similar to the way developing secondary
fibres use primary fibres for support. These tertiary fibres have a different myosin
expression and are mainly distributed along the border of muscle fascicles (Franzini et
al., 1994). Any change in the number of myofibres which form could have a profound
effect on the total muscle mass of the mature animal.
A number of the factors that regulate expression of muscle-specific genes following
commitment to terminal differentiation have been established. The MRFs are the best
characterized of these factors (Edmondson and Olson, 1993) and their sequential activation commits cells to induce genes required to establish terminally differentiated
muscle cells.
Of the MRFs, MyoD and Myf-5 are involved in the regulation of myoblasts and
satellite cell proliferation, whereas myogenin induces differentiation (Olsen and Klein,
1994). MRF4 is expressed late in muscle differentiation and may share some of the
functions of myogenin (Buckingham, 1994). MRFs, however, do not function alone,
they require transcriptional co-activators such as the myocyte enhancer factor-2
(MEF2) before they can regulate myoblast and satellite cell proliferation (Olson et al.,
1995).
MRFs are also expressed in adult muscle while MyoD protein accumulates in
satellite cells of regenerating myotubes and skeletal muscle fibres. This appears to be
neurally regulated (Koishi et al., 1995).
There are now indications that polymorphisms in the MRFs could be used as
selection markers for improved growth rates in pigs. Polymorphic sites at the myogenin
and Myf-5 gene loci have been associated with increases in birth weight, postnatal
growth and weight of lean meat in pigs (Te Pas et al., 1998). However, Myf-5 polymorphisms were not related to increased growth rates. Furthermore, expression levels of
MyoD showed no relation to selection for liveweight gain (Te Pas et al., 1998). Similar
associations have not so far been reported for ruminants.
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Hypertrophy
The bulk of muscle growth results from satellite cell division and incorporation of
daughter cells into the multinucleated fibre. This enables the nuclei/cytoplasmic ratios
of the muscle fibre to be maintained, even though the nuclei of the fibre are unable to
undergo division. During normal growth, and in response to stretch, muscle fibres
lengthen by the addition of new sarcomeres at the ends of these fibres (Williams and
Goldspink, 1973). Under conditions of normal growth, the longitudinal and radial
growth of muscle fibres is highly correlated. However, in sheep heavier than 45 kg,
there is little further increase in radial growth of muscle fibres, while in cattle, radial
fibre growth is rapid at birth. This growth, as in sheep, does level off in older animals
(Swatland, 1984).
Specific muscle hypertrophy can be work-induced, which results in the increased
local expression of IGF-I and an increase in insulin sensitivity in association with
increased glucose metabolism, amino acid transport, protein synthesis and protein
degradation. More recently, the expression of the muscle growth inhibitor, myostatin,
has been shown to decrease immediately after stretch-induced hypertrophy is initiated
(J. Martyn et al., personal communication). However, while muscle fibre hypertrophy is
found in the myostatin knockout mouse (McPherron et al., 1997), it has not been identified in double-muscled cattle that have a mutated, non-active myostatin (Boccard, 1981).
There are a number of animal models that show an abnormal increase in postnatal
muscle size. The callipyge sheep was derived from a ram with extreme muscling, especially in the hindquarter (Cockell et al., 1994). The fine mapping of the CLPE locus to
ovine chromosome 18 has so far failed to identify a candidate gene for this trait. The
compact mouse (Szabo et al., 1998) also shows a postnatal increase in fibre size, resulting in enlarged hindquarter muscles. This muscle hypertrophy has been associated with
a mutation in the myostatin pro-peptide region, which precedes the proteolytic processing site (Szabo et al., 1998). This mutation may, as in the TGF-b proteins
(Miyazono et al., 1991), affect the folding of myostatin and hence its biological activity.
The lack of information concerning muscle hypertrophy compared with the number of publications on myoblast proliferation and differentiation results from a lack of
suitable in vitro models, in which satellite cell division may be observed in association
with increased muscle fibre growth. In future, the clever use of transgenic models and
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Cre-Lox knockouts (Sauer, 1998), which may be both tissue- and time-based, will
allow the identification of genes which may be controlling postnatal growth of muscle
fibres, the period during which muscle size increases the most.
Acknowledgements
Research into growth physiology by AgResearch is funded by the New Zealand
Foundation of Research Science and Technology. We thank the organizers for inviting
this review and the colleagues who have contributed.
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14
237
238
IGF
Insulin
GH
-
FGF
-
-
EGF/ TGF-a
HGF
T3
-
-
Test b-agonist
ND
LIF
ND
IL-6
ND
-
RA
ND
-
LA
ND
-
CGRP
IGF, insulin-like growth factors -I and -II; GH, growth hormone; FGF, fibroblast growth factor; EGF/TGFa, epidermal growth factor/transforming growth factor-a (same
receptor); HGF, hepatocyte growth factor; TGF-b, transforming growth factor-b; PDGF, platelet-derived growth factor; Dex, Dexamethasone; T3, tri-iodothyronine; Test,
testosterone; b-agonist, clenbuterol, ractopamine or isoproterenol; LIF, leukaemia inhibitory factor; IL-6, interleukin-6; RA, retinoic acid; LA, linoleic acid; CGRP, calcitonin gene related peptide; -, stimulates; , inhibits; , no effect; ND, no data found.
Proliferation
Differentiation
Factor
Table 14.1. Effects of various growth factors and hormones on muscle precursor cell proliferation and differentiation (compiled from Allen et al.,
1985; Arnold et al., 1992; Brameld et al., 1998; Gal-Levi et al., 1998).
240
241
242
both primary and secondary fibre size and in the secondary:primary fibre ratio is seen
in muscles from large sheep fetuses obtained from in vitro embryo culture (Maxfield et
al., 1998). In vitro embryo culture in serum-supplemented media appears to significantly stimulate growth in some fetuses and this growth advantage is retained throughout gestation (Sinclair et al., 1998). McCoard et al. (1997) demonstrated similar
reductions in muscle fibre number in constrained ovine fetuses at 140 days gestation,
with the constraints being the presence of twin or single fetuses, or fetuses due to be
born in the spring or autumn. Both these constraints result in reduced fetal growth,
presumably due to reduced availability of nutrients per fetus. The previously described
relationships between muscle fibre number and both postnatal growth rates and
gain:feed ratios (Dwyer et al., 1993) demonstrate the importance of maternal nutrition
during pregnancy on subsequent growth potential.
In species such as rats, a period of in utero undernutrition in early gestation may
not result in permanent reduction in muscle fibre numbers if adequate nutritional realimentation occurs in the remaining gestation and early postnatal life (Hegarty and
Allen, 1978; Wilson et al., 1988); secondary fibre proliferation continues into early
postnatal life in these species (see Dwyer et al., 1995).
Administration of growth hormone (GH) to pregnant sows results in increased
secondary muscle fibre number in the piglets at birth, but only in early (1024 days)
gestation when muscle cell proliferation is maximal (Rehfeldt et al., 1993).
Administration of GH in mid-gestation results in delayed muscle maturation, whilst
administration in late-gestation results in an increased muscle fibre diameter, with no
effect on muscle fibre number (i.e. hypertrophic effects see later).
All of these increases in muscle fibre number are via increases in the ratio of secondary-to-primary fibres, due to increased secondary fibres. The number of primary
fibres is thought to be determined by genotype, but the number of secondary fibres can
be influenced by environmental factors. The mechanism for these effects of environmental factors on muscle fibre number is not known. However, a likely candidate is the
fetal GHIGF axis, which has been shown to be regulated by nutrition in exactly the
same way as that of the mother (Bauer et al., 1995). Our recent studies (J.M.
Brameld et al., unpublished data) indicate that nutrient restriction of pregnant ewes
between 28 and 80 days gestation, followed by feeding to control levels results in
decreased expression of IGF-I in fetal liver at 80 days, but increased hepatic expression
of IGF-II. The effects of nutritional and hormonal manipulations on postnatal growth,
and in particular the GHIGF axis, has recently been reviewed (e.g. Brameld,
1997). We have demonstrated local expression of the IGF-I gene in both skeletal muscle and adipose tissue in the pig, as well as in liver, the major site of production of circulating IGF-I. IGF-I gene expression is affected by diet in a tissue-specific manner
(Weller et al., 1994; Brameld et al., 1996b). Evidence from studies in whole animals
and cultured hepatocytes suggest that both the protein and energy constituents of a
diet have direct effects on hepatic expression of growth-regulatory genes. In pigs, glucose directly increases hepatic expression of the GHR gene and therefore enhances the
effects of GH on IGF-I expression, with protein (amino acids) controlling GH-stimulated expression of the IGF-I gene (Brameld et al., 1999a). Interestingly, neither dietary
energy nor protein alter IGF-I expression in skeletal muscle, despite having effects on
muscle GHR expression opposite to those seen in liver (see Brameld, 1997). This is
corroborated by our recent studies which show no effect of nutrient restriction on
243
IGF-I expression in fetal sheep skeletal muscle, although expression was higher at 80
days than at 140 days gestation (J.M. Brameld et al., unpublished data). We hypothesized that a possible mechanism for nutrient restriction decreasing muscle fibre number
may be via: (i) reduced fetal hepatic IGF-I expression, and therefore reduced mitogenic
activity in the blood; and (ii) the reduced proliferation may induce an early peak of
IGF-II expression in skeletal muscle, resulting in an early induction of differentiation
and therefore reduced muscle fibre number. Indeed, our recent studies of nutrientrestricted ewes indicate an earlier peak of IGF-II expression in skeletal muscle of nutrient-restricted fetal sheep, although only two time points were investigated (J.M.
Brameld et al., unpublished data).
In some species, including man (Draeger et al., 1987), sheep (Wilson et al., 1992)
and pigs (Mascarello et al., 1992), a third generation of muscle fibres have been identified. These tertiary fibres occur during late fetal and very early neonatal development,
after the completion of primary and secondary fibre production. Their origin is
unclear, but it has been suggested that they are derived from satellite cells (Mascarello
et al., 1992) and their development may be affected by nutritional supply (see Dauncey
and Gilmour, 1996), since they are present in maternal-fed piglets, but not in piglets
fed sow-milk substitute. Whether this is due to differences in the composition of the
milk or to the amount of food eaten is still unknown, since maternal-fed animals are
likely to have ingested more than the sow-milk substitute-fed animals, which were only
fed at 6-hourly intervals (M.J. Dauncey, Cambridge, 1999, personal communication).
This suggests that nutrition in late pregnancy and very early neonatal life is important
to maximize muscle fibre formation.
IGF
Insulin
GH
ND
FGF
EGF/ TGF-a
TGF-b
-
ND
PDGF
-
-
Dex
T3
Test
b-agonist
ND
-
TNF-a
PGE2
PGF2a
IGF, insulin-like growth factors -I and -II; GH, growth hormone; FGF, fibroblast growth factor; EGF/TGF-a, epidermal growth factor/transforming
growth factor-a (same receptor); TGF-b, transforming growth factor-b; PDGF, platelet-derived growth factor; Dex, Dexamethasone; T3, tri-iodothyronine; Test, testosterone; b-agonist, clenbuterol, ractopamine or isoproterenol; TNF-a, tumour necrosis factor-a; PGE2, prostaglandin E2; PGF2a,
prostaglandin F2a; -, stimulates; , inhibits; , no effect; ND, no data found.
Protein synthesis
Protein breakdown
Factor
Table 14.2. Effects of various growth factors and hormones on differentiated muscle cell protein synthesis and breakdown (see Brameld et al.,1998).
244
245
which have been used to increase muscle mass in farm livestock. The anabolic growth
promotors, although no longer available for commercial use in the UK, have been
shown to effectively increase muscle mass through hypertrophy. The mode of action of
these agents differs, depending on the compound. Longissimus DNA concentration is
higher in bulls than in steers (Morgan et al., 1993) and in animals treated with testosterone (Grigsby et al., 1976), indicating an effect of testosterone on satellite cell proliferation. It has been suggested that trenbolone acetate, a testosterone analogue, increases
skeletal muscle hypertrophy by enhancing the sensitivity of satellite cells to FGF and
IGF-I (Thompson et al., 1989). Testosterone increases both protein synthesis and
degradation rates, with a greater effect on the former (Martinez et al., 1984), while
trenbolone acetate increases muscle mass predominantly by reducing protein degradation, with a lesser effect on protein synthesis (Vernon and Buttery, 1976). The mode of
action of oestradiol is believed to involve enhancement of endogenous GH secretion
(Gopinath and Kitts, 1984; Breier et al., 1988), although measurable increases in protein synthetic rate are not always detected in treated animals (Dawson et al., 1991).
The response to anabolic agents, however, is clearly dependent on nutritional status
(Gill et al., 1987; Bauman et al., 1994).
Treating animals with exogenous GH increases the fractional rates of protein synthesis and degradation in skeletal muscle, with the increase in synthesis exceeding the
increase in degradation. This results in protein accretion (Pell and Bates, 1987;
Eisemann et al., 1989) and therefore hypertrophy. Total muscle RNA concentration is
increased in GH-treated muscles suggesting increased protein synthetic capacity rather
than increased protein synthesis efficiency (Pell and Bates, 1987). The mechanism for
these effects is presumably via endocrine or autocrine/paracrine GH-stimulated IGF-I,
since GH has no direct effect on protein metabolism in cultured muscle cells (see
above). Although growth hormone administration to pigs increases fibre diameter
(Beerman et al., 1990; Rehfeldt and Ender, 1993), its effect on meat quality is variable.
Shear force values for samples taken at slaughter were similar for control- and GHtreated pigs, but were higher in GH-treated samples from carcasses hung for 5 days in a
cool room (Solomon et al., 1994). This suggests that the post-mortem biochemical
processes may be affected by GH, as has been demonstrated in animals treated with bagonist (see later). The ambiguity as to whether GH affects meat quality may relate to
the magnitude of the effect on proteolysis, since the rate of protein degradation before
death affects the rate of protein degradation post mortem, and therefore the tenderization process.
Treatment of growing animals with other exogenous agents, such as b-adrenergic
agonists, also results in increased muscle mass via muscle hypertrophy, with no proliferation of satellite cells. Indeed, muscle DNA concentration (mg g21 protein) is sometimes less in treated muscles than in control muscles (Kim et al., 1987). The
predominant mechanism of action of these agents is believed to be a reduction in protein degradation, although protein synthesis has also been shown to be stimulated in
some studies (Dawson et al., 1991). This is supported by the observation that translational efficiency (i.e. the amount of protein synthesized per unit RNA) is increased by
b-agonists (Maltin et al., 1992). Studies at Nottingham and in the USA have shown
that, in sheep and cattle, b-adrenergic agonist administration over several weeks has a
remarkable effect on the Ca2+-dependent cysteine proteinase (calpain) system (Higgins
et al., 1988; Wang and Beermann, 1988; Parr et al., 1992; Speck et al., 1993). The calpain
246
system comprises at least three components, two of which have proteolytic activity in
vitro at micromolar and millimolar concentrations of calcium, namely m-calpain and
m-calpain respectively, with the third important factor being a specific endogenous
inhibitor known as calpastatin. b-agonist treatment for several weeks results in significant increases in the level of activity of calpastatin and calpastatin mRNA in LD muscle
(Higgins et al., 1988; Wang and Beermann, 1988; Parr et al., 1992; Speck et al., 1993).
However, not all muscles respond equally to treatment with b-agonists (Dawson et al.,
1991), probably related to differences in the content of fibre types of different muscles.
The cross-sectional area of type II (fast contracting, mixed glycolytic-oxidative) fibres
tends to be increased more consistently than type I (slow contracting, oxidative) fibres
(see Yang and McElligott, 1989). There also appears to be a change in fibre type proportions in muscles of treated animals following long-term administration of these
agents, with a shift towards fibres with increased anaerobic metabolism (Maltin et al.,
1986; Zeman et al., 1988). These changes in fibre diameter and metabolism, along
with the reduced protein degradation, are believed to contribute to the reduced tenderness of meat treated with b-agonists (Aalhus et al., 1992).
247
several weeks after birth. This suggests that the mechanisms that give rise to these conditions in cattle differ from those in sheep. The double-muscle syndrome is characterized by increased muscle fibre number, resulting from faster and more prolonged
hyperplasia during prenatal growth and development (see above). The callipyge condition is associated with increased muscle DNA content, suggesting greater satellite cell
proliferation and increased RNA content, which points to a greater capacity for protein
synthesis (Koohmaraie et al., 1995). This condition is thus due to increased hypertrophy rather than hyperplasia. Protein degradation is significantly reduced in these animals, contributing to the increased muscle mass but also reducing meat tenderness, and
is due to extremely high levels of calpastatin (Koohmaraie et al., 1995). Fibre type
changes are also apparent, with an increase in both proportion and area of the type II
fibres and a reduction (or no change) in the size of the red, type I fibres. Sheep with the
callipyge gene therefore have 4862% greater total muscle fibre areas compared with
normal lambs (Koohmaraie et al., 1995). These effects on muscle fibre area and fibre
type proportions are similar to those often seen in b-agonist-treated muscles (Yang and
McElligott, 1989), as are the changes in protein synthesis and degradation and meat
tenderness.
Meat quality
The final eating quality of meat depends on a number of organoleptic properties,
including appearance (comprising colour and fat content), taste, texture and tenderness. Whilst colour and fat content are important in influencing meat purchase, consumer studies indicate that it is the degree to which muscle tenderizes after slaughter
that is the most important factor contributing to overall meat quality in cattle, sheep
and pigs (Koohmaraie, 1994). After slaughter, the loss of oxygen supply to tissues initiates anaerobic metabolism, resulting in the utilization of primary energy stores. In the
case of skeletal muscle, this means that muscle glycogen is depleted, producing an
increase in lactic acid in the muscle, thereby reducing muscle pH. The rate of postmortem glycogenolysis can be altered in ruminants by circumstances which produce or
mimic stress. For example, adrenaline infusion in cattle reduces the rate of pH decrease
post mortem by depleting glycogen stores prior to slaughter, giving rise to dark, firm,
dry beef (Geesink et al., 1992), with highly fragmented fibres and poor taste qualities.
Similar observations have been made on pork carcasses following long-term stress
(Warriss et al., 1989), whilst short-term stress increases the rate of pH fall, giving rise to
pale, soft, exudative meat. Stress also affects the biochemical processes involved in the
tenderization process (see later).
The relationship between muscle fibre structure and meat-eating quality is complex, because a number of factors are involved. Attempts to manipulate growth by
selective breeding, experimental diets or exogenous growth promoters may change
muscle fibre type, mean fibre diameter and the physiological status of the muscle at the
time of slaughter. Separating the effects of these factors on tenderness can be difficult,
since, for example, glycolytic (white) fibres tend to have larger diameters than oxidative
(red) ones. Some studies suggest that the toughness of uncooked meat, as judged from
its shear value, may increase with overall fibre diameter. In a trial involving 120 crossbred steers, shear force was positively correlated with fibre diameter, and negatively
248
correlated with the percentage of oxidative fibres (Seideman et al., 1987). Pigs with
more muscle fibres also tend to have less fat (Stickland and Goldspink, 1975) while
other indices of quality (e.g. toughness) may have also improved. It would therefore
seem that high fibre number, which correlates with smaller fibres, may be a relevant
parameter that correlates with meat quality. Studies into the effects of dietary protein
on tenderness have yielded variable results (see Soloman et al., 1994). EssnGustavsson et al. (1994) found an increase in the toughness of the LD muscle in barrows and gilts fed a high protein diet. These changes were associated with a decrease in
intramuscular lipid and an increase in muscle fibre diameter.
Among the most important reactions influencing toughness are those catalysed by
certain endogenous proteolytic enzymes that act in a highly selective manner on a small
number of key intracellular muscle proteins to initiate the tenderization process.
Several candidate enzyme systems have been proposed over the last 10 years, including
cathepsins and the multicatalytic protease (proteasome). However, in recent years a
consensus has emerged that the most important proteolytic enzymes affecting tenderization belong to the calpain system (see above), with the most important substrates
being myofibrillar, Z-line and costamere proteins in muscle fibres (Koohmaraie, 1994;
Taylor et al., 1995). The calpain system is highly sensitive to fluctuating levels of calcium ion, pH and temperature, all of which change rapidly in the immediate postmortem period (Suzuki et al., 1995). Unlike other proteolytic systems which may be
active during the post-mortem period, the calpains are the only proteolytic enzymes in
skeletal muscle which do not degrade the major myofibrillar proteins actin and myosin,
both of which remain intact during the tenderization process (Goll et al., 1991, 1992).
Further evidence has shown that CaCl2 infusion into the carcass increases the rate of
tenderization in beef whilst infusion of ZnCl2, a calpain inhibitor, reduces the rate of
tenderization in both beef and lamb (Koohmaraie, 1990; Geesink et al., 1994). Longterm b-agonist treatment is known to produce tough meat in cattle and sheep, presumably due to suppression of post-mortem proteolysis by the elevated activity of
calpastatin (Kretchmar et al., 1990; Wheeler and Koohmaraie, 1992). The rate of tenderization also varies between the principal meat-producing species, where (in order of
greater tenderization) pork > lamb > beef. Significantly, calpastatin levels at slaughter
are greatest in beef and lowest in pork, adding further evidence to the involvement of
the calpain system in the tenderization process (Koohmaraie et al., 1991). Callipyge
lambs, characterized by their enhanced muscle growth and excessively tough meat, have
extremely high levels of calpastatin (Koohmaraie et al., 1995). Whilst the meat toughness is not further enhanced by b-agonist treatment, it can be reduced by infusion of
CaCl2 (Koohmaraie et al., 1996; Clare et al., 1997). Thus, a genetic condition has been
shown to alter the calpain system in a manner consistent with its effects on tenderization. We have shown that the same system is involved in the conversion of pig muscle
to pork and that pre-slaughter stress can alter the post-mortem responsiveness of the
calpain system (Sensky et al., 1996, 1999; Parr et al., 1999a, b).
Acknowledgements
The authors gratefully acknowledge the financial support of the BBSRC, Meat and
Livestock Commission and the MAFF for much of their research.
249
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254
15
Introduction
Despite numerous studies on wool growth, there is not yet an integrated biological
understanding of how wool growth rate is controlled. Genetic and nutritional effects
on wool growth have been studied intensively, but the biological mechanisms underlying the substantial interactions between these factors remain ill-defined (Jackson and
Roberts, 1970; Woolaston, 1987). This chapter aims to develop an integrative framework that will facilitate collaboration between nutritionists, physiologists, and cell and
molecular biologists to develop practical systems to improve wool growth. Such a
framework is needed, for example, to assess the likely value of individual genes for wool
growth, as new information becomes available. Subsequent work is planned to use the
framework to develop more quantitative applied models relating wool growth rate to
staple strength, meat production and reproduction.
We have approached this task by considering follicles as scattered units of a single
organ that produces wool, and treating the organ as a whole (much as milk production
is normally considered as a function of the mammary gland, rather than of individual
mammary alveoli). This contrasts with the more traditional approach of considering
wool growth as an aggregation of the characteristics of individual follicles, and describing wool growth by detailed analysis of follicle biology. Although our approach glosses
over much of the detailed information about function within the follicle, it enables the
integration of follicle function with the overall protein synthetic activity in the skin,
and in turn, in the body. This approach is possible because wool growth represents a
relatively constant proportion of protein synthesis in skin (Table 15.1).
Figure 15.1 presents the framework developed in this chapter. Wool growth rate is
considered as a function of the mass of follicular tissue, the rate at which this tissue
synthesizes protein, and the proportion of that protein extruded as wool. In this
schema, genotype may affect the capacity for wool growth at a number of sites, including skin mass, the fraction of skin made up of follicular tissue, the capacity of follicles
to use amino acids, follicle efficiency, uptake of amino acids by other tissues,
CAB International 2000. Ruminant Physiology: Digestion, Metabolism,
Growth and Reproduction (ed. P.B. Cronj)
255
256
Table 15.1. The proportion of skin protein synthesis deposited into wool (skin
efficiency) in sheep with different fractional synthesis rates (FSR) of protein in the skin.
Dietary
level (M)
FSR
(% day21)
Romney ewes
1.4
0.6
15.3
5.6
0.15
0.14
Harris et al.
(1994b)
Merino lambs
1
0.6
17.3
15.3
0.18
0.20
Liu et al.
(1998)
Merino wethers,
Fleece+
Fleece2
1
1
18.2
14.4
0.19
0.16
1
1
1
11.7
10.1
11.9
0.160.21a
0.10a
0.16a
Sheep
Skin efficiency
for wool
Source
S. Liu et al.
(unpublished)
a Ewes fed to maintain empty body weight. Skin efficiency was calculated from wool growth
(Masters and Mata, 1996) and estimated skin protein content.
Wool growth
Efficiency
(wool/skin)
(G, H)
Follicle
mass
Skin protein
synthesis rate
Sensitivity and
responsiveness to
nutrients (G)
Fraction of skin
as follicles
(G, H)
Skin mass
(G)
Supply of
amino acids to
skin
Use of
amino acids by
other tissues
(G, H, N)
Previous
nutrition (N)
Supply of
amino acids
(N)
Fig. 15.1. Diagrammatic model of the control of wool growth, indicating points at which
genetic (G), hormonal (H) and nutritional (N) control may be exerted.
257
responsiveness of the body to nutrient supply, and feed intake. Nutrition may affect
wool growth in one of two ways. Firstly, it may increase total mass of follicular tissue.
As with other body tissues (e.g. skin or muscle mass), such a change will depend
primarily on the level of feed intake. Secondly, the rate of protein synthesis in follicles
depends primarily on the supply of amino acids. Hormonal and physiological state may
modulate and mediate these effects at several sites.
Follicle models
There has been considerable work relating cell dynamics in the wool follicle to wool
growth. The pioneering model in this area used the supply of amino acids to the follicle
to drive the rate of mitosis in the follicle bulb, which had a genetic limit to its rate of
response (Black and Reis, 1979). This concept has been developed further (e.g. Scobie
and Woods, 1992; Bowman et al., 1993).
Subsequent work showed that enhanced nutrient supply not only increased
mitotic rate (or decreased cell turnover time), but also increased the number of cells in
the bulb germinal epithelium, and increased the volume of fibre cortical cells (Table
15.2). The relative importance of these three components of wool growth varied, presumably depending on the diet and duration or rate of feeding, but this cannot be
determined from these studies. Although the role of cell apoptosis in determining the
availability of cells to the fibre is unknown, the total number of cells generated (i.e. the
product of bulb volume and mitotic rate) correlated well with changes in wool fibre
production, while cortical cell size played a less important role (Table 15.2).
Genetic differences in fleece weight appear to be independent of cell turnover time
(Williams and Winston, 1987; Hynd, 1989), but are also reflected in the amount of
germinal tissue in follicle bulbs, which in turn is a product of bulb volume and follicle
density. Seasonal changes in wool growth in Romney rams were also independent of
changes in cell turnover time (Holle et al., 1994). Thus, despite the conclusion of
Wilson and Short (1979), cell turnover time alone does not provide a reliable means for
modelling wool growth.
Total cell production rate by the bulb correlates closely with wool growth rate, but
this does not prove causation. As with many other tissues, the rate of cell production by
the follicle is inversely related to the rate of cell differentiation, so that cells labelled
with [3H]thymidine take longer to differentiate and mature as the cell production rate
increases (Chapman et al., 1980). When wool production rate was doubled as a result
of increased feed intake, the time cells took to pass through the keratinization zone of
258
Table 15.2. Percentage changes in fibre production and follicle characteristics associated with
increased feeding in Merino sheep.
Duration of
study (days)
Increase in cell
turnover rate
Increase in
bulb volume
Increase in
cortical cell
volume
250
28
47
12
19
164
365
20
22
33
130
106
133
186
56
49
6
25
30
94
5
76
Increase in
wool growth
Reference
Short et al.
(1968)
Wilson and
Short (1979)
Williams and
Winston (1987)
Hynd (1989)
Hynd (1994b)
the follicle increased from 19 to 25 h, whilst the time taken for incorporation of
[35S]cystine into insoluble keratin doubled from 7 to 14.5 h (Chapman et al., 1980). It
is therefore equally sustainable to hypothesize that wool growth rate is limited by the
capacity for cell maturation in the developing fibre shaft.
Robust and complex control mechanisms ensure close coordination between cell
production, cell differentiation and cell maturation in the wool follicle. This makes it
impossible to nominate any one of these functions as the ultimate controller of wool
growth, because they all correlate closely with each other and also with wool growth.
Nevertheless, the simplest hypothesis is that nutritional control of wool growth is
exerted by affecting the capacity of the follicle to support protein synthesis for cell maturation, rather than by direct control of cell production in the bulb.
Nutritional models
The earliest nutritional models were simply equations derived to show a linear relationship between feed intake and wool growth. These were later adjusted to linear relationships between wool growth and digestible dry matter intake, to take account of
variation in feed quality between experiments (Allden, 1979). However, wool growth is
even more closely related to the amount of protein absorbed from the small intestine
(Black et al., 1973). The amount of protein arriving in the small intestine is a function
of the fermentable energy available for microbial protein synthesis, the protein content
of the diet and the extent to which that protein is degraded in the rumen. The most
successful model to date (GrazFeed; Freer et al., 1997) estimates Digestible Protein
Leaving the Stomach from a range of feed and intake variables, including those
described above.
This model achieves accurate prediction in many circumstances and has been
applied widely in practical animal husbandry to calculate appropriate levels of pasture
and supplement for sheep. However, differences between genotypes are dealt with by
259
classifying sheep broadly by age, sex and breed type, and therefore the model does not
directly account for interactions between genotype and nutrient supply. In addition,
GrazFeed predicts wool growth as a constant proportion of protein absorbed, regardless
of the composition of that protein. However, the amino acid composition of the
absorbed protein can have a profound effect on the efficiency of conversion of protein
to wool. For example, the low level of sulphur amino acids in microbial protein usually
limits wool growth, and provision of additional methionine may increase efficiency of
conversion of absorbed protein to wool by 30% (Mata and Masters, 1999). It is likely
that nutritional models will develop further to include absorption and use of specific
amino acids for wool.
The rate of blood flow to the skin regulates the nutrient supply to the follicle, but
it is almost certainly not the primary control mechanism (Hales and Fawcett, 1993).
This does not mean it is not important. Changes in wool growth rate are usually
accompanied by changes in blood flow to the skin, and Harris et al. (1994b) suggested
that changes in blood flow were as important in determining the rate of protein synthesis in the skin as they are in mammary gland.
Protein synthesis
Most nutritional models focus on protein deposition, which is the difference between
protein synthesis rate and protein degradation rate. Thus, they model mass balance,
rather than rate constants. However, wool protein is excreted by the wool follicle so it
does not undergo turnover. Wool growth therefore depends only on protein synthesis
rate. Models of protein turnover rates are in their infancy (Sainz and Wolff, 1990;
Knap and Schrama, 1996), but offer a basis for specialized wool models of the future.
If we consider wool growth as part of the function of skin as whole, a simple
model to relate wool growth to protein synthesis in skin can be described as follows:
Wool growth = Protein content (Pc) 3 Synthesis rate (Ps) 3 Skin efficiency (r)
= [Pc0 + Pc0 3 (Ps 2 Pd)] 3 Ps 3 r
Pc0 may be defined as Pc at a maintenance state, Pd is protein degradation rate in
skin and r is the proportion of skin protein synthesis committed into wool. We can
replace Pc with [Pc0 + Pc0 3 (Ps Pd)] to allow for the change in skin protein mass,
which is determined by both synthesis and degradation rates. Based on this model,
wool growth will reflect three main components: skin (protein) mass and its change,
synthesis rate and skin efficiency. The following discussion explores each of these components in greater detail, as a basis for developing such a model.
260
(MacRae et al., 1993, recalculated to exclude wool N). However, skin mass can be
altered differentially by the level of nutrition.
Nutrition
As shown in Table 15.2, changes in the amount of germinal epithelium in skin contribute to the effect of nutrition on wool growth. With increased levels of feed intake,
both the weight of skin per unit area and the amount of non-collagenous protein per
unit mass increase (Williams and Morley, 1994). However, there are no reliable data on
the effects of nutrition on the proportion of skin mass made up by follicles.
While skin and muscle mass generally move in the same direction when feed
intake changes, the overall rates and amplitudes of change can be quite different. For
example, Murray and Slezacek (1994) reported a 35% reduction in skin weight, compared with a 13% reduction in muscle and a 53% reduction in alimentary tract, associated with an 18% body weight loss in young sheep. Similarly, when Merino hoggets
were fed at 0.44 M, 1.15 M or 1.8 M, the carcass weight increased by 29% from the
lowest to highest intake, while the skin weight increased by 65% (N.R. Adams, unpublished observations). The protein concentration in skin did not change with intake, so
the weight change indicated similar changes in protein mass in the skin.
Although changes in skin mass are relatively large, the rate of change is slower than
changes in fractional protein synthesis rate (Liu et al., 1998) or in the mitotic rate of
bulb cells (A.C. Schlink, unpublished observations). The slow rate of change in mass
would contribute to the delayed response of wool growth to changes in nutrition summarized by Nagorka (1977). In addition to slow changes in follicle mass, the substantial delays in wool growth response to re-feeding in sheep that have been subjected to
restricted nutrition (Butler-Hogg, 1984) are likely to be affected also by diversion of
amino acids to tissues with a greater demand (see p. 264).
Genetics
Genetic factors affect the mass of follicles mainly through effects on the proportion of
skin mass made up of follicles. Jackson et al. (1975) calculated that 83% of genetic difference between sheep in fleece weight was correlated with four descriptors of follicle
morphology: follicle length, follicle curvature, the number of follicles per unit area of
skin, and the ratio of primary to secondary follicles. Subsequent work supports these
observations. Hocking Edwards and Hynd (1992) found that the relative volume of
germinative tissue in skin of sheep is correlated with their genetic capacity for wool
growth, and Williams and Winston (1987) found that the major difference between
sheep that differed genetically for fleece weight was the length of follicles, rather than
their diameter.
Genetic effects on wool production through total skin mass are more limited. Low
and variable genetic correlations have been reported between skin wrinkles, skin thickness, body size and wool production, summarized by Williams (1987). Gregory (1982)
reported a positive genetic correlation (0.39) between skin thickness and clean fleece
weight, but Hynd et al. (1996) observed the opposite, with thinner-skinned sheep pro-
261
ducing more wool than thick-skinned animals. Williams and Morley (1994) reported
that sheep that were genetically low wool producers had a higher skin collagen content
per unit skin mass than high wool producers.
Hormonal control
Hormones affect the mass of active follicular tissue in two different ways. Firstly, hormones influence the follicular cycle of anagen (development), telogen (constant secretion) and catagen (degeneration), which occurs as part of an annual moulting cycle.
Secondly, hormones are involved in follicular shut-down from nutritional or other
stresses. Such shut-down can be distinguished from normal follicle cycles (Schlink and
Dollin, 1995).
Follicle cycles
The impact of annual follicular cycles, coordinated by photoperiod, varies among
breeds. The Wiltshire Horn breed may shed the entire fleece, the Romney undergoes
seasonal fluctuations of up to 40% in wool growth, while changes in the Merino are
relatively limited. Seasonal cycles also appear to affect follicle efficiency (see p. 267).
Prolactin mediates photoperiodic effects by synchronizing endogenous follicular cycles,
but it does not drive the follicular events. Thus, pharmacological manipulation of prolactin does not affect wool growth (Wallace, 1979), although prolactin receptors are
widely distributed in dermal papilla, inner and outer root sheath, germinal matrix, and
the sebaceous and sweat glands (Choy et al., 1997). An increased concentration of
receptors for insulin-like growth factor-I (IGF-I) has been associated with catagen
(Nixon et al., 1997). The effects of the follicular cycle on protein synthesis in skin have
not been measured.
Follicle shut-down
Increased cortisol causes loss of bulb and root sheath cells, resulting in fibre shedding
(Chapman and Bassett, 1970). Prolonged treatment with cortisol causes the skin to
become thinner, with loss of collagen from the dermis, reduction in the size of sebaceous glands and regression of follicles. Epidermal growth factor can also cause follicle
shut-down. Other parts of the glucocorticoid axis, including corticotrophin releasing
factor, pro-opiomelanocorticotropin and adrenocorticotropic hormone (ACTH), are
found in skin (Slominski et al., 1998), but their involvement in fibre production is
unknown. However, local treatment with ACTH can stimulate growth of fibre during
the anagen stage in mink (Rose, 1998).
262
wool fibre (Table 15.1), but the proportion of skin protein synthesis accounted for by
the follicle must be greater than this. The fibre represents only 1020% of bulb matrix
cells (see p. 266), although these undergo a 13-fold increase in protein mass during keratinization (Short et al., 1968). Additional protein synthesis is associated with the generation, maintenance and maturation of the remaining 8090% of cells generated in
the bulb. Even if these only double in total protein synthesized during maturation and
function as inner root sheath, their number is such that they would account for more
protein synthesis than that involved in fibre formation. Further protein synthesis occurs
in the generation, maturation and maintenance of cells of the outer root sheath, not
associated with the bulb (Chapman et al., 1980). Thus, up to half the protein synthesis
in sheep skin is likely to be associated with the wool follicle. This value would explain
the close relationship between protein synthesis in skin and wool fibre production
recorded in Table 15.1.
263
Table 15.3. Fractional synthesis rate (FSR, % day21) of protein in midside skin and skeletal
muscle of sheep in response to different diets.
Diet or
treatment
Muscle
FSR
Skin FSR
0.44 M
1.15 M
1.8 M
1.41
2.27
2.93
12.8
18.2
20.6
Merino lambs
2426 kg, fed
0.83 M
Canola meal
Lupin seeds
2.33
2.21
17.3
15.8
D.G. Masters
(unpublished
observations)a
Merino lambs
2533 kg
0.6 M
1M
1.64
2.28
15.4
16.9
Liu et al.
1998
2 Cimaterol
+ Cimaterol
1.45
2.763.01
11.6
6.310.9
0.6 M
1.2 M
1.8 M
1.16
1.67
3.30
Sheep
Merino wethers
3547 kg
Suffolk cross
wether, fed 1.7 M
3541 kg
Suffolk cross
wether
2635 kg
Romney sheep
4244 kg
Suffolk cross
wether 2635 kg
1.5 M
+ 2 g cysteine
0.6 M
1.8 M
1.91
2.84
Wholebody FSR
2.20
2.52
Source
N.R. Adams
(unpublished
observations)
Nash et al.
(1994)
2.83
3.76
4.73
Harris et al.
(1992)b
12.8
22.8
Harris et al.
(1994b)c
5.6
9.5
Lobley et al.
(1992)
aSource
264
265
Table 15.4. Summary of increases (-), decreases () or no significant change () in whole body
protein metabolism and in wool growth, reported to result from treatment with hormones.
Hormone
Protein
synthesis
Protein
degradation
Specific follicle
effects
Wool effect
Thyroxine
Androgens
b-adrenergic agonist
Growth hormone
Insulin
IGF-I
Cortisol
Prolactin
-?
, -?
Follicle efficiency
Nil
Nil
Nil
Nil
Nil
Follicle shut-down
Follicle efficiency
-or
- or
or
or
organs. In addition to these effects, specific effects of hormones on the mass or efficiency of wool follicles are dealt with on pp. 261 and 264.
Most of the effects of hormones on wool growth are consistent with their effects
on general protein metabolism. For example, the small increase in wool growth
reported in wethers treated with androgens (Southcott and Royal, 1971; Hynd and
James, 1987) probably resulted from an increase in feed intake. Insulin and cortisol,
which affect protein degradation rather than synthesis, have no effect on wool growth
under normal physiological concentrations. High concentrations of cortisol may have
specific effects on follicular function (p. 261). Growth hormone may increase
(Johnsson et al., 1985) or decrease (Wynn et al., 1988) wool growth, depending on the
relative balance between an increase in overall body protein synthesis resulting from
increased feed intake, and the diversion of nutrients to tissues more responsive to
growth hormone, such as gut and muscle. Diversion of nutrients is even more marked
with the adrenergic b-agonist cimaterol, which decreased wool growth by 16% and
shorn skin weight by 9%, while increasing carcass protein and decreasing lipid deposition (Fennessy et al., 1990). Nash et al. (1994) found that the reduction in wool
growth was directly counter-balanced by the increase in protein deposition in muscle of
treated sheep.
Recent work has explored the effects of IGF-I on wool growth. Expression of IGFI with a keratin promoter in transgenic sheep increased wool growth during spring and
summer, but not in winter, so that staple strength was decreased (Damak et al., 1996).
More extensive studies (Su et al., 1998) on the subsequent generation of these transgenic animals were unable to detect a significant increase in wool growth or in plasma
IGF-I. Sheep treated with IGF-I had a transient increase in skin protein synthesis
which disappeared by 24 h (Lobley et al., 1998). The extensive buffering of IGF-I
through specific binding proteins makes assessment of its role difficult. It mediates the
stimulation of protein synthesis by growth hormone (Bell et al., 1998) and may affect
the interaction between cortisol and low nutritional status (Chapman and Bassett,
1970), because treatment with IGF-I can prevent the deleterious effects of glucocorticoids on nitrogen metabolism (Tomas, 1998).
266
Fig. 15.2. Fractional synthesis rate (FSR) of protein in skin (a) and muscle (b) from ewes selected
for low (v) and high (j) staple strength. Significant (P < 0.05) interaction between genotype and
feed intake for both tissues.
267
the phenomenon has been studied as the proportion of cells from the bulb matrix that
enter the fibre (as opposed to the root sheath). Normally, between 9 and 40% of cells
formed in the bulb enter the fibre, the remaining cells becoming root sheath and subsequently lost (Wilson and Short, 1979; Hynd, 1989). This factor has been called the
follicle efficiency. Its exact relationship to protein synthesis is not defined.
Hormones
In addition to its role in mediating the effect of photoperiod on the follicular cycle
described on p. 261, prolactin may also mediate seasonal changes in the efficiency of
wool growth. Holle et al. (1994) reported that seasonal changes in wool growth were
accompanied by changes to the relative distribution of cells between the fibre and inner
root sheath. This was estimated by the production ratio, the ratio of area of fibre to
root sheath in horizontal sections of skin (Butler and Wilkinson, 1979). However,
Hynd (1989) found a poor correlation between the production ratio and follicle efficiency. Measures of seasonal changes in the ratio of protein synthesis to wool production might offer valuable insights.
Hynd (1994b) showed that abnormally low thyroxine concentrations reduced follicle efficiency by 40%, and also reduced the production ratio. Cell division was only
reduced by 16%, although the proportion of inactive follicles was increased. Thus, it
appears that thyroxine affects follicle efficiency more than bulb activity. However, it is
likely that such low levels of thyroxine do not occur in normal husbandry.
Conclusions
The impact of genotype, nutrition and physiological state on wool growth can be integrated with other bodily functions by considering it as part of the overall protein synthesis in the body. Indeed, the rate of protein synthesis for wool growth broadly reflects
that in other tissues, so wool growth offers a simple index of changes in protein synthesis rate in the body. Furthermore, protein synthesis and protein degradation rates
together provide a sounder basis for understanding of nutrient partitioning between
wool and other tissues than do concepts based on mass flow.
The rate of wool growth is affected both by specific follicle mechanisms and by
factors controlling protein metabolism in the whole body, but their relative importance
268
Acknowledgements
Research by CSIRO Animal Production and the Co-operative Research Centre for
Premium Quality Wool is supported by Australian wool growers through the
Australian Wool Research and Promotion Organization.
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16
Regulation of Macronutrient
Partitioning between Maternal
and Conceptus Tissues in the
Pregnant Ruminant
Introduction
The concept that pregnant animals partition available nutrients to favour their developing offspring was formally postulated by Hammond (1947) who considered that tissues compete for circulating nutrients on the basis of their relative metabolic rates. This
idea was reinforced by early demonstrations of high metabolic rates in tissues of the
conceptus relative to those of the dam (Meschia et al., 1980). However, more recent
thinking on the extracellular regulation of nutrient partitioning has focused on
endocrine coordination rather than tissue competition as a general explanatory mechanism, building on the concept of homeorhesis elaborated by Bauman and Currie
(1980). This concept more satisfactorily accommodates the coordinated metabolic
compromises implicit in the constraint of fetal growth during late pregnancy, to levels
that optimize opportunity for neonatal survival and postnatal development, yet
minimize excessive depletion of maternal energy and protein reserves during late
pregnancy.
This chapter will describe patterns of partitioning of macronutrients between
maternal and conceptus tissues, as a background to more detailed consideration of their
mechanistic explanations involving placental functions and adaptive responses of
maternal (non-uterine) tissues to pregnancy.
275
276
of oxygen, nutrients and metabolites (see Battaglia and Meschia, 1988; Bell, 1993).
These and similar data from pregnant cows (Comline and Silver, 1976; Reynolds et al.,
1986; Ferrell, 1991) have been used to construct metabolic balance sheets for fetal
sheep (Bell, 1993) and cattle (Bell, 1995). In both species during late gestation,
3540% of fetal energy is supplied as glucose and its fetal-placental metabolite, lactate,
and a further 55% is taken up as amino acids. Most of the residual 510% is contributed by acetate, placental transfer of which is meagre relative to its abundance and
energetic importance in the maternal system. Placental capacity for maternalfetal
transport of long-chain, non-esterified fatty acids (NEFAs) and ketoacids is even more
limited (see Bell, 1993), denying fetal access to substrates derived from maternal fat
mobilization and thus constraining fetal ability to grow at the direct expense of maternal energy reserves. Almost all of the nitrogen acquired by the fetus is in the form of
amino acids, although a small net umbilical uptake of ammonia, derived from placental
deamination of amino acids, is detectable in fetal sheep during mid (Bell et al., 1989a)
and late gestation (Holzman et al., 1977).
Patterns of utilization of glucose and amino acids for oxidative metabolism and
growth in fetal ruminants are reviewed elsewhere (Battaglia and Meschia, 1988; Bell,
1993, 1995; Hay, 1996, 1998).
Partitioning of glucose and amino acids between conceptus and maternal tissues
In well-fed, monotocous ewes during late pregnancy, uterine uptake of glucose consumes 3050% of maternal glucose supply (Prior and Christenson, 1978; Hay et al.,
1983b; Oddy et al., 1985; Leury et al., 1990), which accounts for all of the pregnancyinduced increment in whole-body production and utilization of glucose (Petterson et
al., 1993; Freetly and Ferrell, 1998). About 10% of this glucose carbon is returned to
the maternal circulation as lactate (Meschia et al., 1980; Faichney et al., 1981), presum-
277
ably resulting from glycolysis in uteroplacental tissues of maternal origin, and contributing to the increased cycling between glucose and lactate that is characteristic of
ruminants during late pregnancy (van der Walt et al., 1983).
No studies have directly addressed the partitioning of amino acids between the
gravid uterus and maternal non-uterine tissues. However, we have used traditional
nitrogen balance and comparative slaughter techniques to estimate the partition of
apparently digested crude protein in ditocous ewes that were fed to predicted energy
and protein requirements between days 110 and 140 of pregnancy (McNeill et al.,
1997). Calculations suggest that as much as 80% of apparently digested crude protein
was partitioned to the gravid uterus, the remainder being used to support increased
metabolism and net deposition of amino acids in the developing mammary glands and
visceral organs (Bell and Ehrhardt, 1998). However, it is notable that even though the
ewes were fed to predicted requirement for dietary protein (> 90 g day21 digestible
crude protein), the available pool of circulating amino acids was augmented by the net
mobilization of protein from maternal carcass tissues (mostly skeletal muscle), amounting to almost 10% of the digestible crude protein intake.
278
requirement (120 g kg21 crude protein) (McNeill et al., 1997). This occurred despite a
substantial net loss of protein from maternal carcass tissues and reduced net accretion
of protein in maternal visceral and mammary tissues, indicating a major shift in partitioning of maternal amino acids to partially offset the effects of dietary deficiency on
conceptus growth (Fig. 16.1).
Fig. 16.1. Protein deposition between days 110 and 140 of pregnancy in maternal
tissue components of ditocous ewes fed diets containing different levels of dietary
crude protein. Histograms are means for eight ewes. Pooled standard errors were 214 g
for carcass, 84 g for organs, and 44 g for mammary gland. Within tissue components,
means with different letters are significantly different (P < 0.05). Adapted from the data
of McNeill et al. (1997).
279
In distinct contrast, recent novel studies have shown that overfeeding and rapid
growth of primiparous ewes during earlymid pregnancy causes profound reductions in
placental growth, followed by severe fetal growth retardation with all the hallmarks of
placental insufficiency (Wallace et al., 1996, 1999). Intriguingly, the negative effects of
overfeeding before day 50 of pregnancy were ameliorated by feed restriction after day
50; conversely, overfeeding after day 50 partially induced placental and fetal growth
retardation in ewes fed at a moderate level up to that time (Table 16.1; Wallace et al.,
1999).
Maternal body condition may also affect the partitioning of nutrients during late
pregnancy by mechanisms not directly related to placental size or function. For example, McNeill et al. (1998) observed that when given ad libitum access to feed during
late pregnancy, lean ewes ate more than fatter ewes but partitioned the extra nutrients
to maternal rather than conceptus tissues. Also, fatter ewes were better able than lean
ewes to sustain conceptus growth during moderate undernutrition in late pregnancy
(McNeill et al., 1999). Placental size did not differ appreciably between lean and fatter
ewes in either of these studies, and was not considered to be an important determinant
of differences in nutrient partitioning between maternal and conceptus tissues.
MM
457a
103a
4.94a
HH
258b
79b
3.03b
HM
MH
PSE2
381ac
96a
4.45a
312bc
109a
3.11b
44
6.3
0.39
moderate intake, growth rate 57 g day21, days 050; -M, moderate intake, days 50100; H-,
high intake, growth rate 280 g day21, days 050; -H, high intake, days 50100. All ewes were fed
to maintain body condition from day 100 to term.
2 PSE, pooled standard error.
Row means with different letters are significantly different (P < 0.05).
1M-,
280
281
maternal supply is not a limiting factor for uterine uptake of amino acids (Chung et al.,
1998). Negligible uterine uptake of glycine, which is most abundant in maternal blood,
and glutamate suggest that the sodium-dependent transport systems for these acids,
observed in other tissues, are not expressed in the ovine placenta.
282
released back into the umbilical bloodstream (Chung et al., 1998). Some of this glutamine is converted back to glutamate by the fetal liver, which produces most of the glutamate consumed by the placenta (Vaughn et al., 1995). This establishes a
glutamateglutamine shuttle which promotes placental oxidation of glutamate and
fetal hepatic utilization of the amide group of glutamine.
Similarly, the placenta almost quantitatively converts serine, mostly taken up from
maternal blood, to glycine (Chung et al., 1998), reconciling the discrepancy between
the negligible net uptake of glycine by the uterus and substantial net release of this
amino acid into the umbilical circulation (see Hay, 1998).
The complexity of interrelations among placental uptake, metabolism, and transport of amino acids was further illustrated by a recent study of alanine metabolism in
ewes during late pregnancy (Timmerman et al., 1998). Application of tracer methodology showed that negligible net placental consumption or production of alanine masks
an appreciable metabolism of maternal alanine entering the placenta which exchanges
with endogenously produced alanine. Thus, most of the alanine delivered to the fetus is
of placental origin, derived from placental protein turnover and transamination.
283
Table 16.2. Maternal weight change, fetal weight, and indices of placental glucose
transport at day 135 of pregnancy in ditocous ewes fed 100% or 60% of predicted
energy requirements for the preceding 14 days (R.A. Ehrhardt and A.W. Bell,
unpublished observations).
Energy intake,
% requirement a
Variable
D maternal weight (kg)
Fetal weight (kg)
Plasma glucose (mM)
Maternal
Fetal
D maternalfetal
Placental 3MGe
clearance (ml min21 kg21
placenta)
CBf sites (pmol mg21 protein)
GLUT-3 protein (arbitrary unitsg)
60
PSEb
Pc
5.3
3.58
22.7
3.46
1.0
0.16
< 0.001
NSd
3.72
0.57
3.15
117
2.84
0.49
2.33
176
0.09
0.03
0.03
7
< 0.001
< 0.05
< 0.001
< 0.001
105
1.00
126
1.19
3
0.04
< 0.01
< 0.05
100
a Values
284
Glucose metabolism
Hepatic gluconeogenesis increases in ewes during late pregnancy even when feed intake
is not increased above non-pregnant levels (Freetly and Ferrell, 1998), consistent with
earlier observations of pregnancy-specific effects on whole-body glucose kinetics (see
Bell, 1993). This was concomitant with increased hepatic uptake of lactate (Freetly and
Ferrell, 1998), apparently derived from uteroplacental metabolism (Meschia et al.,
1980; Faichney et al., 1981) and increased glycolysis in maternal peripheral tissues
(Hough et al., 1985). Part of the moderate net mobilization of amino acids from carcass tissues of late-pregnant ewes fed to predicted nutrient requirements (Fig. 16.1;
McNeill et al., 1997) may also be used to sustain increased hepatic gluconeogenesis as
term approaches.
Glucose uptake by maternal peripheral tissues such as hindlimb muscle and adipose tissue tends to decline during late pregnancy, although the evidence for some of
these responses in well-fed animals may have been confounded by variations in voluntary feed intake (see Bell and Bauman, 1997).
Amino acid metabolism
Effects of pregnancy on the quantitative metabolism of amino acids have not been systematically studied in ruminants. However, in agreement with data from rats (Ling et
al., 1987), the fractional rate of hepatic protein synthesis increases 45% during late
pregnancy in dairy cows (Bell, 1995). This is consistent with the moderate increase in
hepatic protein accretion (Campbell and Fell, 1970), and an apparent decrease in
hepatic amino acid catabolism (Freetly and Ferrell, 1998) in late-pregnant ewes. These
changes in hepatic metabolism occurred despite a decreased (dairy cows) or unchanged
(ewes) protein intake and hepatic uptake of amino acids, implying endogenous regulation of hepatic disposal of amino acids as in late-pregnant rats (Casado et al., 1987).
285
Protein
As discussed earlier, protein deprivation of ditocous ewes during late pregnancy
increased the net loss of protein from carcass tissues and abolished the normal, pregnancy-related increase in protein accretion of visceral organs. Conversely, feeding protein at levels above predicted requirements (160 g kg21 versus 120 g kg21 dry matter),
caused an appreciable reversal of the net flux of nitrogen from maternal carcass tissues
and allowed significant net accretion of tissue protein during late pregnancy (Fig. 16.1;
McNeill et al., 1997).
Fig. 16.2. Relative abundance of GLUT-4 protein in perirenal (PR) adipose tissue and
semitendinosus (ST) muscle at day 135 of pregnancy in ditocous ewes that had been
fed at 100% or 60% of predicted energy requirements for the preceding 14 days.
Histograms are means for five ewes, expressed relative to control values (100% energy
intake) for each tissue. Pooled standard errors were 0.07 units for PR adipose, and 0.06
units for ST muscle. Within tissues, means with different letters are significantly
different (P < 0.05). From the data of Ehrhardt et al. (1998).
286
Homeorhetic regulation
General concept
The concept of homeorhesis as applied to regulation of nutrient partitioning (Bauman
and Currie, 1980) is discussed elsewhere in this volume (Chapter 18). Key postulates of
this concept include its simultaneous influence on multiple tissues and functional systems, implying extracellular mediation, and its operation through altered tissue
responses to homeostatic effectors such as insulin, at various levels of extracellular and
intracellular signalling.
Altered tissue responses to insulin
Application of the euglycaemic, hyperinsulinaemic clamp technique has demonstrated
that in sheep, as in humans and laboratory animals, late pregnancy is associated with
moderate insulin resistance, and that this condition is exacerbated by undernutrition.
This was manifested as diminished sensitivity to insulin of several variables of wholebody glucose utilization (Petterson et al., 1993) and decreased insulin responsiveness of
lipolysis and NEFA mobilization (Petterson et al., 1994). Tissue specificity of these
whole-body phenomena was not assessed in these studies, but it is likely that adaptations in the major insulin-responsive peripheral tissues, skeletal muscle and adipose tissue, were mostly responsible (see Bell and Bauman, 1997).
As pregnancy advances, ovine adipose tissue in vitro becomes refractory to the
stimulation of lipogenesis by insulin (Vernon et al., 1985; Guesnet et al., 1991).
Although glucose is not an important carbon precursor for lipogenesis in ruminants, it
is required for synthesis of glyceride glycerol, and for part of the NADPH necessary for
de novo fatty acid synthesis (see Bauman and Davis, 1975). This is consistent with our
recent observation of progressive development of insulin resistance in terms of wholebody glucose disposal during pregnancy in fat-tailed Karakul ewes (Slepetis et al.,
1999).
The role of skeletal muscle in the evolution of insulin resistance during pregnancy
has not been addressed in ruminants. However, it seems likely that the ability of insulin
to promote glucose uptake by muscle is diminished in late pregnancy as in early lactation (Vernon et al., 1990) because of the similar degrees of whole-body insulin resistance observed in ewes in these two states (Slepetis et al., 1999). The reduction in
muscle abundance of the insulin-responsive GLUT-4 protein in underfed versus wellfed pregnant ewes (Fig. 16.2; Ehrhardt et al., 1998) provides a mechanistic clue.
Possible homeorhetic agents
Various hormones, including progesterone, oestradiol and placental lactogen (PL), may
act as homeorhetic regulators of observed changes in tissue responses to insulin and
associated metabolic adaptations to the state of pregnancy in ruminants (Bell and
Bauman, 1994, 1997). The argument for PL is especially hard to dismiss, despite a
continuing lack of direct experimental evidence. This uniquely placental peptide crossreacts with both growth hormone (GH) and prolactin receptors in ruminant tissues
(Byatt et al., 1992), and its specific binding in adipose tissue increases with advancing
pregnancy in sheep (NGuema et al., 1986). However, it remains unclear whether highaffinity binding of PL in adipose tissue or liver is mediated through structurally unique
receptors (Anthony et al., 1995). Cross-reactivity with the GH receptor would be con-
287
sistent with the development of insulin resistance in adipose tissue during late pregnancy because GH is a potent homeorhetic effector of this response in ruminant adipose tissue (see Bell and Bauman, 1997; Chapter 18). Other indirect evidence for a
homeorhetic role for PL includes enhanced placental gene expression and secretion of
ovine placental lactogen in moderately undernourished, late-pregnant ewes, coincident
with decreased expression of GLUT-4 in maternal insulin-responsive tissues (Fig. 16.2;
Ehrhardt et al., 1998) and exaggeration of indices of whole-body insulin resistance discussed earlier in this section.
It is also possible that leptin, the ob gene product expressed almost exclusively in
adipose tissue (see Houseknecht et al., 1998), plays a role in the homeorhetic coordination of conceptus nutrient demands and metabolic adaptations in maternal tissues of
pregnant ruminants. In addition to its commonly postulated role as a signal of peripheral energy status to the central nervous system, leptin may have pleiotropic effects on
peripheral tissues, including mediation of insulin resistance in adipocytes (Muller et al.,
1997). Coincident with the evolution of insulin resistance, we have recently observed a
consistent, threefold increase in expression of leptin mRNA in tail adipose tissue from
fat-tailed Karakul ewes during mid (5060 days) and late (125135 days) pregnancy
compared with levels in the same animals when non-pregnant (Fig. 16.3). A similar,
pregnancy-induced increase in leptin mRNA expression in white adipose tissue was
associated with a marked increase in serum concentrations of leptin in mice
(Tomimatsu et al., 1997).
Leptin may also act on the placenta, which, in sheep, strongly expresses the leptin
receptor splice variant OB-Rb (Ehrhardt et al., 1999) that is considered essential for
intracellular signal transduction after binding leptin. This raises the possibility that, in
Fig. 16.3. Relative abundance of leptin mRNA in tail adipose tissue of Karakul ewes
(n = 8) sequentially sampled while non-pregnant and non-lactating (NP), in mid
pregnancy (MP, day 5060), in late pregnancy (LP, day 125135), and in early lactation
(L, day 1522 post-partum). Pooled standard error was 0.46 units. Means with different
letters are significantly different (P < 0.05). From the unpublished data of R.A.
Ehrhardt, R.M. Slepetis, Y.R. Boisclair and A.W. Bell.
288
addition to its putative influences on adipose and other maternal tissues, leptin may
directly or indirectly mediate observed adaptations in placental capacity for glucose
transport during advancing gestation and moderate maternal undernutrition.
Conclusions
We propose that the placenta plays a pivotal, multi-faceted role in the regulation of
nutrient partitioning between maternal tissues and the conceptus, especially during late
pregnancy when most of fetal growth occurs. First, it is a highly regulated nutritional
and excretory conduit that provides a means of coordinating fetal nutrient demands
with maternal capacity to supply these nutrients, especially glucose and amino acids.
Under optimal conditions, this will result in appropriate constraint of late-gestation
fetal growth such that birth weight will be sufficient to permit a good chance of neonatal survival, but insufficient to cause dystocia or an inappropriate depletion of maternal
nutrient reserves prior to the onset of lactation. The mechanistic bases for this general
role of the placenta are beginning to be understood, although much of the details about
amino acid transport and its regulation have yet to be elucidated.
Second, through its capacity to synthesize and secrete a plethora of bioactive molecules, including proteins, steroids, and eicosanoids, into maternal and fetal circulations,
the placenta offers unique opportunities for direct and indirect communication
between the conceptus and its maternal host. Although lacking direct evidence, the
hypothesis that PL and/or other placental hormones are responsible for effecting
insulin resistance and other metabolic adaptations in maternal tissues remains attractive. We also speculate that communication of maternal energy status to the placenta
may be mediated through the leptin system, adding a further layer of complexity to
homeorhetic coordination of maternal nutrition and energy stores with nutrient
requirements of the conceptus.
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Introduction
We have come to know much about the thermal physiology of the ruminant fetus not
because the thermal physiology of ruminants is peculiar, but because so much of the
research on the physiology of fetal mammals has been carried out on sheep. That
includes our own research. The experimental evidence, however, leads us to believe that
much of what is known about the thermal physiology of the sheep fetus also applies to
other ruminant species, and some non-ruminant species, including our own. It would
be a reasonable expectation that the thermal relationship the fetal lamb has with the
pregnant ewe is representative of the fetalmaternal relationship in other ruminants,
where fetuses typically number one or two per pregnancy, and where total fetal mass
constitutes approximately 10% maternal body mass at term.
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H. Laburn et al.
296
and model predictions of the fetomaternal thermal relationship, Gilbert et al. (1985)
estimated that approximately 85% of fetal heat is lost via the utero-placental circulation
(Fig. 17.1). That heat is delivered to the placenta by the umbilical vessels, and leaves
the placenta in the uterine circulation, so increases or decreases in umbilical and/or
uterine blood flow will lead to increased or decreased heat transfer respectively, between
the fetus and the mother animal (Schrder et al., 1988). The remainder of the heat produced is dissipated via convection and conduction through the amniotic fluid and subsequently the uterine wall. Ultimately, the heat is lost to the environment, via typical
heat loss strategies employed by the pregnant animal, such as vasodilation in peripheral
blood vessels, and panting.
Fig. 17.1. Diagrammatic representation of the routes of heat loss from the fetal lamb.
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297
and humans (Wood and Beard, 1964; Adamsons and Towell, 1965). Our measurements reveal typical values for body temperatures of the sheep fetus to be 39.6C, and
of the pregnant ewe to be 39.2C. We recently have measured these temperatures in
pregnant nanny goats and their fetuses, and have found almost identical values to those
in sheep for fetal and maternal body temperatures in this ruminant species too. Why
the equilibrium feto-maternal temperature difference is the same in species of different
mass remains unknown.
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H. Laburn et al.
Fig. 17.2. Fetal lamb (F), and pregnant ewe (M) body temperatures as measured by
radiotelemetry in a group of seven animals over the last 35 days of pregnancy (lambing
at day 0). Lowest panel shows the feto-maternal (FM) body temperature gradient. All
points are means standard error of a mean. Reprinted with permission from the
American Physiological Society, from Laburn et al. (1992).
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Fig. 17.3. Top panel shows circadian variations in fetal lamb and pregnant ewe body
temperatures over a 24-h period, and lower panels show changes in the feto-maternal
(F-M) temperature gradient, and feto-amniotic (F-A) temperature gradient. The F-A
gradient is approximately midway between the F-M gradient, confirming the amnion
as a conduit of heat between fetus and maternal tissues. Other details as in Fig. 17.1.
Reprinted with permission from the American Physiological Society, from Laburn et al.
(1992).
Fetal hyperthermia
A potentially serious consequence of the fetus having a higher body temperature than
that of its mother, and of being thermally clamped to its mother, is that the fetus experiences greater hyperthermia than the mother does, when maternal body temperature
rises. Heat is damaging, especially to developing tissues, and the developing central nervous system is particularly susceptible. A rise in temperature of 1.5C arrests fetal brain
cell division, and a 3C rise kills dividing cells (Gericke et al., 1989). As a result, maternal hyperthermia in early pregnancy leads to congenital abnormalities (Edwards et al.,
1995; Chambers et al., 1998). In later pregnancy, raised intrauterine temperature is
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Fig. 17.4. Body temperatures of a pregnant ewe (M) and her fetal lamb (F).
Measurements were made while the ewe was housed in laboratory conditions (a), and
in field conditions (see text, b). Data for a 3-day (a) or a 5-day period (b) are shown.
associated with fetal growth retardation, such that the lambs of animals kept at abnormally high ambient temperatures during their pregnancy have significantly lower body
mass than their counterparts whose mothers were kept in more moderate conditions
(Alexander and Williams, 1971; Dreiling et al., 1991).
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302
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Fig. 17.5. Summary of changes in fetal lamb (F) and pregnant ewe (M) body
temperatures and effects on the feto-maternal (F-M) gradient of exposure of the ewes to
various conditions of thermal stress. (a) exposure at rest to 40C and 60% relative
humidity. (b) Treadmill exercise (2 km h21, 5 gradient) for 30 min at room
temperature. (c) Experimental fever following intravenous injection of bacterial
lipopolysaccharide into the ewe at time zero. (d) Exposure to 4C. Reprinted with
permission from the American Physiological Society, from Laburn (1996).
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303
304
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Fig. 17.6. Changes in body temperatures of fetus or lamb, and mother animal, and the
feto-maternal (F-M) temperature gradient, measured for 2 h before and 4 h after
lambing (at time zero). Each point is the mean standard error of a mean (SEM) of
seven ewes and eight fetuses/lambs. Reprinted with permission from Birkhuser Verlag
AG, from Laburn et al. (1994).
the lower the risk of ischaemic brain damage, should neonatal circulation or respiration
be compromised in the birth process (Kuroiwa et al., 1990).
The third phase of the thermal response to parturition is that more dramatic
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305
phase, for the newborn ruminant animal at least, immediately after birth. The measurements shown in Fig. 17.6 were taken in a group of ewes giving birth in indoor conditions. The neonatal lamb, delivered soaked in amniotic fluid, is confronted by an
environment which, compared with the intrauterine environment, is cold and dry.
Evaporation of fluid from the lambs skin, and its predisposition to heat loss by virtue
of its high surface area to mass ratio, results in a precipitous fall in body temperature,
by about 1.5C within minutes of birth. Lambs in the field are delivered into much
colder conditions (Barlow et al., 1987) so one should expect that the fall in neonatal
body temperature would be more precipitous than it was in our experiments.
Nevertheless, within an hour, in the indoor environment at least, thermoregulatory
effector mechanisms come into play, not only to halt the plummeting body temperature, but to reverse it. By 3 h post-partum, lamb and ewe body temperatures are not
significantly different (Fig. 17.6).
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Fig. 17.7. Data from experiments performed by Gunn et al. (1991) showing the
activation of non-shivering thermogenesis (NST) in a group of fetal lambs during 5 h of
simulated post-birth conditions. Top curve, changes in plasma glycerol concentrations.
Bottom curve, increase in brown adipose tissue (BAT) temperature above that of the
lambs core body temperature. Lambs were ventilated, and primed with a b-stimulant.
NST was stimulated by cooling the fetus in utero, but only after the umbilical cord was
snared. Reprinted with permission from the American Physiological Society, from
Laburn (1996).
and Gluckman, 1983). Normally, however, fetal lambs probably are peripherally
vasodilated (Schrder et al., 1988) and presumably shivering, like NST, is suppressed
in utero. Other thermoregulatory mechanisms, including the ability to pant in response
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to high ambient conditions, also probably are mature in the late-gestation fetal lamb,
but presumably are suppressed.
Fig. 17.8. Changes in fetal and lamb (upper panel) and ewe (bottom panel) body
temperatures measured for 10 days prior to lambing and for 7 weeks thereafter. Each
point is the mean standard error of a mean for seven ewes and eight fetuses/lambs.
Lambing is indicated by a vertical dashed line.
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higher even than that towards the end of pregnancy, at least for 3 weeks. What the
mechanism is of the sustained rise in body temperature is unknown, but it may be a
defended elevation in temperature; lactating ewes have attenuated fever responses in
response to administration of a Gram-negative pyrogen (S. Glassom et al., South
Africa, unpublished observations).
Acknowledgements
The work reported in this article was supported by the Council of the University of the
Witwatersrand, Johannesburg, and the South African Medical Research Council.
Table 17.1. Abortion following pyrogen injection. Percentage of abortions, and time to
abortion (in days, mean standard error of a mean (SEM), after either Gram-positive or
Gram-negative pyrogens were injected directly into the circulation of pregnant ewes
(n = 12), or fetal animals (n = 14), or into the amniotic fluid (n = 4).
Pyrogen to ewe
Pyrogen to fetus
Pyrogen to amnion
Abortion (%)
41
64
75
10.5 2.6
4.6 0.9
1.6 0.7
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18
Regulation of Nutrient
Partitioning During Lactation:
Homeostasis and Homeorhesis
Revisited
D.E. BAUMAN
Department of Animal Science, Cornell University, Ithaca, New York, USA
Introduction
Nature has accorded a high priority to lactation, and the ability of mammals to synthesize milk is essential for survival of the newborn. The mammary glands have a high
metabolic rate during lactation, yet this organ is unique because its biosynthetic products represent no direct benefit to the mother. Rather, the extensive rate of nutrient use
to make milk imposes a substantial demand on the mother and mandates that the
metabolism of the mammary glands and other maternal tissues be coordinated. Thus,
during lactation many physiological changes occur and there are profound alterations
in the metabolism of many tissues. The extensive physiological adaptations which
occur during lactation have the overall effect of providing the proper quantity and pattern of nutrients for milk synthesis. Thus, lactation provides an excellent opportunity
to elucidate the broad concepts of metabolic regulation and identify specific mechanisms involved in the partitioning of nutrients. The regulation of nutrient use is not only
important for a successful lactation, but it also represents the physiological basis for differences in productive efficiency, and this is of special importance in agricultural
species. Furthermore, if the coordination of nutrient use is inadequate then animal
well-being is compromised which may result in stress, subclinical conditions and metabolic disorders.
In 1980, Currie and I reviewed the regulation of nutrient partitioning during
pregnancy and lactation. In discussing the concepts of regulation, we crystallized the
concept of homeorhesis, proposed mechanisms, and addressed the interrelations
between homeostasis and homeorhesis (Bauman and Currie, 1980). This review will
elaborate and trace the development of the concepts of homeostasis and homeorhesis,
focus on the regulation of nutrient use during lactation, and review current understanding of the mechanisms and applications of these concepts. Because of its breadth,
reviews will be cited frequently for various aspects of the biology.
311
D.E. Bauman
312
Concepts of regulation
At 12 minutes past eight on 3 February 1783, Lavoisier initiated experiments with a
guinea pig in what is now recognized as the first animal calorimeter (Blaxter, 1989;
Welch, 1991). In collaboration with Laplace, he carried out two experiments in a 24 h
period, which for the first time firmly linked the evolution of heat by animals with the
consumption of oxygen and the formation of carbon dioxide. These simple, but elegant, experiments established that life is a chemical process and represented an initial
important step in the search to understand bioenergetics and the regulation of metabolism. It was almost 100 years later that another milestone occurred when Bernard, a
physiologist, recognized the ability of living organisms to maintain their own constancy. He cogently observed that all the vital mechanisms, however varied they may
be, have only one object, that of preserving constant the conditions of life in the internal environment (Bernard, 1878).
Using Bernards concept of a stable milieu interieur as the cornerstone, Cannon
concluded that the coordinated physiological reactions which maintain most of the
steady states in a living organism were complex, involving many different tissues and
organs all working cooperatively (Cannon, 1932). He recognized that higher organisms had more elaborate and effective systems to maintain steady-state conditions, and
suggested a special designation for these states homeostasis. Cannon (1929) chose the
term homeostasis with a great deal of thought and foresight. He concluded that the
prefix homeo was preferred because it meant like or similar, thereby indicating some
variation. He rejected the prefix homo because it meant same and implied a rigid
constancy. Likewise Cannon (1929) chose stasis because it represented a condition.
By choice of terminology and examples to elaborate the concept, Cannon (1929,
1932) emphasized that dynamic regulation and coordination were key features of
homeostasis.
Today the concept of homeostasis is well known to biologists, and there are many
systems where the positive and negative feedback controls to preserve steady state are
well established. Glucose was an example used by Bernard (1878) in developing the
concept of milieu interieur and by Cannon (1932) in crystallizing the concept of homeostasis. The homeostatic controls to maintain steady-state conditions for glucose are
also of special significance during lactation. Glucose is critical during lactation because
its uptake by the mammary gland is essential for the synthesis of milk lactose, the
major osmotic regulator of milk volume. The pancreatic hormones, insulin and
glucagon, are key controls of glucose homeostasis. Thus, acute regulation of plasma
glucose concentration by the reciprocal actions of insulin and glucagon ensure the
proper balance in glucose supply and utilization by body tissues and organs during
lactation.
While the concept of homeostasis is universally accepted today, it resulted in substantial debate over several decades following its introduction (Waddington, 1942,
1953; Lerner, 1954; Lewontin, 1956). In part, this debate focused on the fact that the
concept was not adequate to address regulation over a wide range of physiological and
developmental situations. Waddington (1957) reasoned that there are at least three
types of temporal change in biological adaptations, all occurring simultaneously. The
first, on the longest time scale, is evolution. The second, of medium time scale relates
to development throughout the life cycle and the third, on the shortest time scale,
313
D.E. Bauman
314
the level that is defended. Likewise, Kuenzel et al. (1999) introduced the term poikilostasis (poikilo = various) to describe the dynamic shifts in homeostatic regulation of
metabolism and food intake that occur in birds over different physiological states such
as growth, migration and reproduction.
In recognition of the original work of Waddington (1957) and the earlier use by
Kennedy (1967), we chose to use the term homeorhesis (Bauman and Currie, 1980).
Nevertheless, in all cases the concept relates to the ability of the animal to adjust biological processes in a manner to support a dominant physiological state for animal wellbeing and survival of the species. As illustrated in Table 18.1, the general concept
represented by homeorhesis has been extended to an impressive range of biological situations encompassing many different physiological, nutritional and even pathological
states. In particular, Mrosovsky (1990) provided an extensive list of examples of this
regulation in terms of both physiological states and biological processes. A key physiological state in the survival of mammals is lactation, and the remainder of this review
will relate the concepts of regulation to lactation.
Adaptations to lactation
Lactation represents an impressive example of homeorhesis, and at peak lactation the
proportion of nutrients used by the mammary gland can be extraordinary. For example,
in high-producing dairy cows nutrient utilization by the mammary glands exceeds that
of the rest of the body causing Brown (1969) to suggest that the cow should be envisioned as an appendage to the udder rather than vice versa. Nutrient supply and use
during lactation are similar for all species, although there are qualitative and quantitative differences (Linzell, 1967; Williamson et al., 1995). In all species, the extent of
nutrient use for milk synthesis requires integrated regulation of the metabolism of the
mammary glands and other body tissues.
Using the dairy cow as an example, a partial list of the physiological adaptations
which occur during lactation is presented in Table 18.2. The importance accorded to
lactation in mammals is demonstrated by the fact that the physiological adaptations
involve many, perhaps most, of the body tissues and relate to the metabolism of all
nutrient classes. The net effect is that the increase in mammary gland metabolic rate
and nutrient use which occurs during lactation coincides with alterations in the metab-
Table 18.1. Partial list of physiological situations where the general concept
representing homeorhetic regulation has been applied.a
Lactation
Pregnancy
Growth
Puberty
Ageing
Chronic undernutrition
Chronic illness
a References
Hibernation
Premigration/migration
Egg laying
Incubation anorexia
Seasonal cycles
Exercise
include Kennedy (1967), Bauman and Currie (1980), Dilman (1982), Mrosovsky
(1990), Wade and Schneider (1992), Vernon (1998), and Kuenzel et al. (1999).
315
Table 18.2. Partial list of physiological adaptations which occur in lactating dairy
cows.
Process or tissue
Response
Mammary tissue
Food intake
Increased quantity
Digestive tract
Increased size
Increased absorptive capacity
Increased rates of nutrient absorption
Liver
Increased size
Increased rates of gluconeogenesis
Increased glycogen mobilization
Increased protein synthesis
Adipose tissue
Skeletal muscle
Bone
Heart
Plasma hormones
Decreased insulin
Increased somatotropin
Increased prolactin
Increased glucocorticoids
Decreased thyroid hormones
Decreased IGF-I
olism of other body tissues so that an adequate quantity and pattern of nutrients to
support milk synthesis is ensured. Several reviews have detailed the physiological adaptations which occur during lactation and readers are referred to these for quantitative
information (Williamson, 1980; Bauman and Elliot, 1983; Chilliard, 1986, 1987;
Vernon, 1989; Williamson and Lund, 1994; McNamara, 1995). Nevertheless, some of
these adaptations merit mention as examples which illustrate the concepts of homeostasis and homeorhesis.
During lactation, food intake is increased in many species. Corresponding adaptations also occur in the size and absorptive capacity of the gastrointestinal tract, thereby
allowing for an increased absorption of nutrients (Bauman and Elliot, 1983; Vernon,
1989). In rodents where the milk demand of the nursing pups gradually increases, the
increase in food intake is also gradual and can amount to a 300400% increase
316
D.E. Bauman
compared to the non-lactating state (Cripps and Williams, 1975). Furthermore, the
magnitude of the increase in intake is related to the number of nursing pups (Millar,
1979), providing convincing evidence of the coordinated nature of these changes.
Increases in feed intake and digestive tract size also occur in dairy cows, with the magnitude of the increase in voluntary intake being related to milk yield (Bauman and
Elliot, 1983). However, for this species the increase in feed intake occurs over a longer
interval so that high-producing dairy cows may not achieve a positive energy balance
until 812 weeks post-partum (Bauman and Currie, 1980).
Despite adjustments in food intake, many species rely on body reserves during
early lactation. Vernon (1998) pointed out that this results in many body tissues having
metabolic characteristics that are typical of the adaptations which occur with chronic
undernutrition. However, the extent to which reserves are needed in early lactation to
meet nutrient requirements, especially energy requirements, varies among species. In
some species, the use of energy reserves is minimal to modest because either the
amount of milk required by the nursing young is limited (e.g. human and guinea pig),
or the increase in voluntary intake is nearly adequate to meet requirements (e.g. rat and
mouse). In other species such as the cow, goat and pig, the use of body reserves is more
extensive during the lactation cycle. For example, lactational yields in dairy cows are
related to the magnitude of body reserve utilization (Bauman et al., 1985), and in highproducing cows the mobilization of body fat during the first month of lactation can be
energetically equivalent to over one-third of the milk produced (Bauman and Currie,
1980).
The use of body reserves is extraordinary in several species and some examples of
these merit special mention because they dramatically illustrate the concepts of metabolic regulation. Seals make spectacular use of body reserves to meet their nutrient
requirements during lactation (Riedman, 1990; Oftedal, 1993). This is illustrated by
the pregnant elephant seal which gives birth to a single pup and sustains a 28 day lactation. The nursing pup averages a rate of body weight gain of approximately 10%
day21. In contrast, the mother loses weight, as she neither eats nor drinks throughout
the 4 week lactation. Maternal use of body reserves over the lactation interval results in
a 42% loss of body weight, and this represents a 58% reduction in body fat content
and a 14% reduction in body lean weight (Costa et al., 1986). Whales, especially
baleen whales, also make extraordinary use of body reserves during lactation and the
blue whale provides an example (Lockyer, 1981; Oftedal, 1993). Female blue whales
have a body weight of about 80,000 kg and during pregnancy their 40,00055,000 kg
gain in body weight primarily represents the accretion of body reserves. The blue whale
calf weighs about 2500 kg at birth and gains at the rate of 80100 kg day21 during the
7 month lactation. During this interval the blue whale mother produces about 90 kg of
milk day21. Even more impressive, she relies almost exclusively on body reserves to
support the nutrient needs for milk synthesis and her own sustenance, as she eats little,
if at all, throughout the 7 month lactation.
In seals and whales, as well as other lactating mammals, the physiological adaptations to support lactation are extensive. Based on work with laboratory and farm
animals, we can envision how body fat reserves may serve to meet the energy
requirements of the lactating elephant seal and blue whale. However, body reserves in
these species must also account for the protein, carbohydrate, mineral and vitamin
components of milk. To a large extent the qualitative and quantitative regulation of
317
Mechanisms
The metabolic adaptations occurring with the onset of lactation are undoubtedly
related to the plethora of hormonal changes occurring throughout this period. Some of
the hormones undergoing major changes are listed in Table 18.2. Obviously, there
must be synergisms and redundancy in the hormonal signalling systems, but our
approaches to date to investigate these have been relatively simple. Nevertheless, somatotropin, prolactin and glucocorticoids have the clearest identified effects which are
consistent with homeorhetic controls (Bauman and Elliot, 1983; Bell and Bauman,
1997; Vernon, 1998; Chilliard, 1999).
The overall mechanisms for shifting nutrient partitioning and metabolism involve
alterations in the set-points for physiological responses to homeostatic controls. These
alterations can be reflected by changes in the sensitivity or the magnitude of the biological response. The former is reflected by a change in the effective dose to obtain a
50% response (ED50) for the homeostatic signal whereas the latter is reflected by a
change in the maximum response (Rmax) to the homeostatic signal (Kahn, 1978).
Specific mechanisms for the alterations of the response to homeostatic signals can
include alterations in tissue receptors and binding kinetics, changes in the intracellular
signal transduction systems and effects on the expression and activity of key enzymes in
the biochemical pathways. The net effect is that homeorhetic controls have tissue-specific effects on both the amounts and activity of critical metabolic enzymes and the signalling proteins that regulate them. Therefore, homeorhetic adaptations allow for
chronic alterations or even redirection of physiological processes while still allowing
homeostatic systems to preserve constant conditions. These same general mechanisms
involving alterations in the set-points for responses to homeostatic controls have been
demonstrated for many biological processes in the different physiological situations
listed in Table 18.1.
Several of the tissues and processes in which alterations to homeostatic controls
occur during lactation are listed in Table 18.3. Feed intake provides a general example,
and a number of homeostatic controls for regulating feed intake have been identified
(Forbes, 1996; Langhans, 1999). These homeostatic controls are obviously functioning
in non-lactating and lactating animals, but in species such as the rat and cow the set
points are altered during lactation. Thus, homeostatic controls of feed intake still occur
but the altered set-points allow for a greater voluntary intake so that nutrient supply
more adequately meets the nutrient requirement.
Insulin is an especially powerful mediator of many different physiological effects,
most of which serve to acutely maintain metabolic equilibrium in the face of short-term
D.E. Bauman
318
variations in nutrient supply and demand. Thus, this acute regulatory signal is a pivotal
target for chronic metabolic adaptations. As illustrated by examples in Table 18.3,
many tissues have specific responses to insulin which are attenuated with the onset of
lactation. This includes liver (gluconeogenesis inhibition), adipose tissue (fat synthesis),
skeletal muscle (glucose uptake) and whole body (glucose oxidation) (Vernon and
Sasaki, 1991; Williamson and Lund, 1994; Bell and Bauman, 1997; Vernon, 1998).
These adaptations during early lactation are frequently referred to as reflecting an
insulin resistance. However, judicious use of this term is needed. An attenuated
response is not observed for all acute regulatory functions of insulin. For example, the
antilipolytic effect of insulin is greater in lactating sheep compared with non-lactating
sheep (Vernon et al., 1990), and the inhibition of whole-body rates of protein degradation is enhanced during early lactation (Tesseraud et al., 1993). Thus, the changes in
response to insulin are specific for certain tissues and certain biochemical processes
within those tissues, rather than representing any generalized phenomenon. Overall,
the physiological adaptations in the response of various processes to insulin have the
net effect of enhancing hepatic production of glucose, and sparing glucose use by nonmammary tissues, consistent with the increased glucose requirement of the mammary
gland.
An example of the attenuated response to insulin which occurs during lactation is
presented in Fig. 18.1. In this example of glucose uptake by the hindlimb, the responsiveness to insulin is substantially reduced during lactation but the sensitivity to insulin
is relatively unaltered. Similar studies on glucose and acetate uptake by adipose tissue
indicates the response to insulin is virtually abolished in early lactation (Burnol et al.,
1986; Vernon, 1989; Vernon and Sasaki, 1991; Chilliard, 1999). A second example is
Table 18.3. A partial list of adaptations in metabolic regulation which occur during
lactogenesis and early lactation in ruminants.
Tissue/processes
Homeostatic control
Feed intake
Multiple controls
Adipose tissue
Insulin
Lipogenesis
Uptake of preformed fatty
acids
Stimulation of lipolysis
Inhibition of lipolysis
Catecholamines
Adenosine
Skeletal muscle
Insulin
Insulin(?)
Glucose uptake
Protein synthesis
Amino acid uptake
Protein degradation
Liver
Insulin
Gluconeogenesis
Pancreas
Insulinotropic agents
Insulin release
Whole animal
Insulin
Glucose oxidation
Glucose utilization by nonmammary tissues
319
Fig. 18.1. Effect of insulin on the arterio-venous difference for glucose across the
hindlimb of non-lactating (j) and lactating () sheep. Results represent mean
standard error of the mean. From Vernon (1986).
the effect of b-adrenergics on adipose tissue rates of lipolysis (Bauman and Elliot,
1983; Vernon and Sasaki, 1991; Vernon, 1996; Chilliard, 1999). As illustrated by the
elegant studies of Guesnet et al. (1987), the ability of isoproterenol, a specific breceptor agonist, to stimulate lipolysis is markedly altered by stage of pregnancy and
early lactation (Fig. 18.2).
Somatotropin (ST) is the homeorhetic control for which mechanisms have been
most extensively investigated. Much of these data come from investigations of the
response to exogenous ST in lactating or growing farm animals (Bauman and Vernon,
1993; Burton et al., 1994; Etherton and Bauman, 1998). In the case of dairy cows, the
studies convincingly demonstrate that exogenous bovine somatotropin (bST) results in
an increase in milk yield of the treated animal and a series of coordinated adaptations
in body tissues to support the greater use of nutrients for milk synthesis. Many of the
coordinated adaptations are manifested by alterations in tissue responses to homeostatic
signals. In fact, the changes which occur with bST treatment are similar to those illustrated in Figs 18.1 and 18.2 for the onset of lactation. While our understanding of the
signal transduction systems regulating enzyme activity and gene expression is still
incomplete, the specific effects have been reviewed recently for both the onset of lactation (Vernon, 1998; Chilliard, 1999) and treatment with exogenous ST (Bauman and
Vernon, 1993; Etherton and Bauman, 1998).
Overall, the changes which occur with the onset of lactation or the initiation of
bST treatment allow for a chronic alteration of nutrient utilization. This is illustrated
by the above examples. When a meal is consumed and circulatory insulin increases, less
nutrients are directed to body fat reserves and other non-mammary tissues because of
their altered response to insulin, and more nutrients are taken up by the mammary
gland consistent with the increased milk synthesis. Likewise, if nutrient supply is
D.E. Bauman
320
321
322
D.E. Bauman
The fact that biological regulation involves a series of orchestrated responses is frequently overlooked in articles relating to animal welfare. At regular intervals over the
last 50 years some have expressed concern that practices to improve the productive efficiency of dairy cows may be pushing them too far, thereby compromising animal
health and shortening the lifespan. Hammond (1952) and Bauman et al. (1985)
reviewed the physiological limits to production, and they did not share that concern.
Indeed, milk yield and productive efficiency have continued to increase throughout
this century and today we have herds with annual milk yields over 15,000 kg per cow
and individual cows that have produced over 27,000 kg year21 (Bauman, 1999).
Nevertheless, some continue to assume that genetic selection and improved management practices which lead to more efficient, high-producing animals are at variance
with the physiological controls for animal well-being (Rauw et al., 1998; Broom,
1999). For example, a recent review on welfare of dairy cattle suggested that it may
well be necessary to stop using genetic selection and some feeding methods to increase
milk yield because these practices have resulted in stressed cows in which their normal
biological functioning controls are overtaxed (Broom, 1999). This viewpoint clearly
fails to appreciate that genetic selection and management improvements are successful
because they have altered the biological controls in a coordinated manner. Rather than
the biological controls being at discord with increased performance, it is the improvements in the biological control systems which are responsible for the increases in milk
yield and the gains in productive efficiency.
The role of coordinated responses in metabolic regulation is also important in the
development of dynamic models of metabolism. Recent studies on the regulation of
milk protein synthesis provide an example of this coordination. Most investigations of
milk protein production in dairy cows have examined adequacy of amino acid (AA)
supply. In general, results indicate that supplemental amino acids increase milk protein
content and yield when the AA supply from microbial and bypass protein is inadequate, but little or no response occurs in well-fed cows (Sutton, 1989; Rulquin et al.,
1995). We were interested in the endocrine regulation of milk protein synthesis, and
focused on the chronic effects of insulin because of data summaries showing a high correlation between dietary energy intake and milk protein content (Sporndly, 1989). By
using the hyperinsulinaemic-euglycaemic clamp technique we were able to examine the
role of insulin without the confounding effects of hypoglycaemia. This technique
involves intravenous infusion of insulin to achieve a constant elevated concentration
and simultaneous infusion of sufficient glucose to maintain normal blood concentrations. Results indicated that a 4-day insulin clamp resulted in a dramatic increase in
milk protein content and yield; in the well-fed cow milk protein yield increased by over
25% (Griinari et al., 1997a; Mackle et al., 1999). Plasma urea nitrogen concentrations
were markedly reduced during the insulin clamp, demonstrating that one component
of the coordinated response was a reduction in whole-body oxidation of AA (McGuire
et al., 1995; Griinari et al., 1997a; Mackle et al., 1999). The means by which the mammary gland obtained sufficient AA to support the increased output of milk protein was
of special interest because plasma concentrations of essential AA were reduced by
3050% during the insulin clamp. We found that during the insulin clamp both blood
flow (ml ml21 of milk) and extraction of essential AA (per cent) were increased (Mackle
et al., 2000). Thus, the mammary gland component of the coordinated responses
included alterations in the local control of blood flow and transport mechanisms for
323
D.E. Bauman
324
Fig. 18.3. Temporal pattern of milk fat content during abomasal infusion of conjugated
linoleic acid (CLA) isomers. Infusions represented 10 g day21 of cis-9, trans-11 CLA or
trans-10, cis-12 CLA. From Baumgard et al. (2000).
Conclusions
As our knowledge of biology increases we appreciate even more the remarkable system
for the regulation of metabolic processes which occurs in different physiological states.
This review has re-examined the concepts of homeostasis and homeorhesis in the regulation of metabolism. Lactation provides an impressive example of homeostasis and
homeorhesis in action, and examples were used to provide an overview of the integrated mechanisms. Overall, the homeostatic and homeorhetic mechanisms provide a
coordinated regulation of the metabolism of different organs and tissues to ensure the
proper nutrient supply to the mammary gland.
In crystallizing the concept of homeorhesis and the interrelationships between
homeostasis and homeorhesis, we originally concluded with a quotation (Bauman and
Currie, 1980). This quotation from Duclaux, an eminent French scientist, also provides an appropriate perspective for this review. Duclaux (1920) reviewed the scientific
contributions of Louis Pasteur and noted that many of his ideas were incorrect.
However, he pointed out we see clearly how much a matter of indifference it is
whether a theory or a doctrine is right, provided, it incites to work, and results in the
discovery of new facts.
325
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19
C.R. BAUMRUCKER
Department of Dairy and Animal Science, Penn State University,
Pennsylvania, USA
IGF receptors
The biological effects of the IGFs are mediated through specific cell surface protein
receptors. These include the IGF-I receptor (IGF-IR), the insulin receptor, and the
IGF-II (IGF-IIR)/mannose-6-phosphate receptor (m-6-pR). The IGF-IR is structurally similar to the insulin receptor. Both couple tyrosine kinase activity to a series of
intracellular signalling pathways (LeRoith et al., 1993). The similarity between these
ligands and receptors explains much of the ambiguity of research findings when high
levels of either ligand are applied to an experimental system. Under such conditions,
IGF-I and insulin are known to cross-react with the other species receptor and trigger
intracellular signal cascades associated with both receptors. In addition, both IGF ligands
CAB International 2000. Ruminant Physiology: Digestion, Metabolism,
Growth and Reproduction (ed. P.B. Cronj)
329
C.R. Baumrucker
330
Protease
IGFBP-3
IGFBP-1
IGFBP-2
IGFBP-5
IGFBP-4
IGFBP-6
Ligand
Receptor
IGF-II
Type 2 IGF-R
IGF-I
Insulin
Type 1 IGF-R
IGFBP-3
Insulin-R
BP-3BP
Function
Unknown
Growth
Metabolism
Apoptosis
Fig. 19.1. Figure showing the components of the insulin-like growth factor (IGF) system. Arrows
indicate the binding and interactions that occur between the components.
bind with high affinity with the IGF-IR and IGF-I has a reduced affinity with the
IGF-IIR.
The IGF-IIR is not linked to tyrosine kinase activity, but rather appears to be
coupled to a G protein (Nishimoto, 1993). The receptor expression is developmentally
regulated, with high expression in fetal and neonatal tissues (Nissley et al., 1993). The
IGF-IIR is also the cation-independent m-6-pR, although the ligand binding sites are
separate. The latter is known to function as a lysosomal enzyme targeting protein, the
activation of transforming growth factor-b (TGF-b) (Dennis and Rifkin, 1991), and
the degradation of IGF-II (Oka et al., 1985). Most recently, it has been reported that
the m-6-p/IGF-IIR is also a receptor for retinoic acid (Kang et al., 1997).
IGFBPs
The majority of IGF ligands are bound to IGFBPs in vivo. Unlike the transmembrane
IGF receptors, the IGFBPs are secreted. They are present in blood serum, all biological
fluids, and conditioned media from all in vitro cell cultures. While IGF-I and IGF-II
bind to all IGFBPs with high affinity (~10210 M), insulin does not bind to IGFBPs.
Six distinct human IGFBPs have been identified and have been termed IGFBP-1
through to IGFBP-6 (Anonymous, 1992). Additionally, four IGFBP-related proteins
that exhibit reduced binding affinity (four to tenfold less) for IGFs have been recently
identified (Baxter et al., 1998). All of these proteins exhibit the presence of the specific
IGFBP motif (GCGCCXXC) (Rosenfeld, 1998). Table 19.1 shows the proposed
nomenclature for the superfamily of IGFBPs.
331
IGF affinity
Other/previous names
IGFBP-1
IGFBP-2
IGFBP-3
IGFBP-4
IGFBP-5
IGFBP-6
High
High
High
High
High
High
PP12, BP-28
Low
Low
Low
Low
BP-53
The role of IGFBPs in the regulation of the IGF system has been the source of
many reviews (Jones and Clemmons, 1995; Kelley et al., 1996; Murphy, 1998) and
concepts relating to cancer have emerged (Werner and LeRoith, 1996). Generally,
IGFBP bio-action is thought to be: (i) modulation action of IGF-I or IGF-II by competition with the IGF-IR or IGF-IIR; (ii) alteration of the action of IGF-I or IGF-II;
(iii) transport and/or extended half-life of IGFs; (iv) interactions with other growth
factor systems or (v) IGF-independent action via (a) nuclear localization sequence, (b)
IGFBP binding proteins, or (c) yet unknown mechanisms (Murphy, 1998). The most
important insight into these suggested bio actions is that they mainly emerge from in
vitro cell culture experiments.
The IGFBP related proteins (IGFBPrP14) are a family of proteins (CCN family;
connective tissue growth factor; cef10/cry61 and nov) (Bork, 1998) that have been
recently identified as low-affinity IGFBPs. These proteins are expressed in cells within
minutes (called immediate-early genes) of cellular stimulation by growth factors or
transforming oncogenes (Williams et al., 1992). Because of the high level of sequence
similarity among the members of the CCN family, they probably have common molecular functions. Because of their low IGFBP affinity and rather new entry into the IGF
arena, these proteins will not be considered in the remainder of this chapter.
The differential nature of responses to IGF ligands in various experiments in different tissues has amplified the interest of the IGFBPs in these interactions. Because
IGFBP bind to IGFs with high affinity that effectively competes with the IGF-IR
(Clemmons, 1997), they may be viewed as agents that prevent IGFs action. Apparently
to counteract this blocking action, proteases have been shown to attack IGFBPs and
release free IGF resulting from the altered IGFBP affinity (Fowlkes, 1997). For example, IGFBP-5 that is released by cells in culture is rapidly cleaved into 23 and 16 kDa
fragments by a cation-dependent serine protease (Imai et al., 1997). It is not known if
this protease is present in mammary cell cultures. Other means of altering IGFBP
action with IGFs is by protein phosphorylation (IGFBP-1) (Jones et al., 1991) and cell
surface binding that lowers IGFBP-1 and IGFBP-3 affinity (Mccusker et al., 1990).
C.R. Baumrucker
332
IGFBP-3
Because IGFBP-3 is the major IGFBP and a reliable recombinant source became available some time ago, innumerable studies have focused upon its biological action.
Although the binding and inhibition of IGF action can explain the inhibition of cellular growth by the application of exogenous IGFBP-3, several lines of evidence have
emerged to suggest that other mechanisms may also be involved in the actions of the
IGF system. First, IGFBP-3 has been shown to negatively regulate cell proliferation
through an IGF receptor-independent pathway. Stable transfection of BALB/c 3T3
mouse fibroblasts with human IGFBP-3 cDNA decreased the rate of cell proliferation
(Lamson et al., 1993). Studies using a fibroblast cell line developed from mice with targeted disruption of the IGF-I receptor established that the growth inhibitory effects of
IGFBP-3 do not involve IGF binding to the IGF-IR (Valentinis et al., 1996). IGF
receptor-independent actions of IGFBP-3 have also been observed in breast cancer cells
(Oh et al., 1993b) and are believed to be responsible for the predisposition of breast
cancer cells to apoptosis (Gill et al., 1997). Secondly, both IGF-I and IGFBP-3 have
been shown to be present in the cell nucleus (Radulescu, 1994; Li et al., 1998; Peralta
Soler et al., 1990; Radulescu and Wendtner, 1993) including that of breast cancer cells
(Schedlich et al., 1998) and primary cultures of mammary cells (Baumrucker et al.,
1999). In one of these reports (Li et al., 1998), IGF-I and IGFBP-3 have been observed
to be co-localized in the nucleus of opossum kidney cells. Based on the presence of a
putative nuclear localization sequence in the structure of IGFBP-3, it was proposed
that IGFBP-3 might act as a carrier for IGF-I (Li et al., 1998).
A recent report indicates that IGFBP-3 competes for binding with the TGF-b
receptor (Leal et al., 1997) and the retinoic acid receptor (Kang et al., 1997). Utilizing
a yeast two-hybrid system, Murphy (1998) suggests that a cDNA encoding latent
TGF-b-binding protein 1 interacts with IGFBP-3. In support of the IGFBP-3 role
associated with negative growth regulation action, the expression of IGFBP-3 has also
been shown to be regulated by other growth-inhibitory (and apoptosis-inducing)
agents such as TGF-b (Gucev et al., 1996; Huynh et al., 1996; Rajah et al., 1997) antioestrogens (Buckbinder et al., 1995) and tumour necrosis factor-a (TNF-a) (Yateman
et al., 1993). In normal human mammary epithelial cells, growth inhibition by atRA is
independent of p53 expression (Seewaldt et al., 1999).
333
IGFBP proteases
Degradation of IGFBP-3 by specific proteases has been reported in the milk and serum
of humans and rats (Lamson et al., 1991). Generally, protease modified binding proteins have decreased affinity for IGFs. Plasmin, a protease found in bovine milk (Politis
et al., 1988) that has been projected to be involved in mammary involution, is an
IGFBP protease (Politis et al., 1995). Since the initial discovery of the IGFBP-3 protease, other proteases have been described that cleave IGFBP-2, -4, -5 and -6 (CollettSolberg and Cohen, 1996). These proteases are known as prostate-specific antigen
(Cohen et al., 1994), cathepsins (Conover and De Leon, 1994), matrix metalloproteinases and others (Rajah et al., 1995). While the proteases have been suggested as a
local tissue capacity to free bound IGF from the IGFBP and thereby provide free IGF
to the IGF-IR, independent actions of IGFBP fragments remain a possibility since
Yamanaka et al. (1997) demonstrated that an IGFBP-3 fragment derived from protelolytic attack binds to insulin.
334
Fig. 19.2. Comparison of the changes in insulin-like growth factor binding proteins (IGFBP) in (a) milk colostrum, (b) milk whey, and (c) blood
serum during lactation. Cows were sampled for blood and milk at 2-week intervals. Western blots utilizing [125I]IGF-II as a ligand. Data was
standardized by the use of an internal blood serum standard. Lines are regressions for each IGFBP. n = 33 cows.
C.R. Baumrucker
335
IGFBP-2 change in both blood and milk during the course of lactation, their patterns
of change are significantly different (Fig. 19.2b and c). While milk is approximately
tenfold less in concentration of IGFBPs than that of blood (Fig. 19.2b versus 2c), the
pattern clearly shows that blood IGFBP changes (Fig. 19.2c) are not reflected in milk
changes (Fig. 19.2b). Milk IGFBPs all drop precipitously after the colostrum phase and
the onset of copious milk secretion and IGFBP-3 shows an increase towards the end of
the milk production period. The inverse relationship with milk production is evident.
Milk IGFBP-2 is relatively lower in concentration in both fluids and may exhibit
some changes, but the variation between animals precludes interpretation. Milk and
blood IGFBP-4 and -5 change little during the course of lactation. While nutrition has
an impact upon the IGF axis (Thissen et al., 1994), feed restriction experiments show
little influence over IGFBP-3 serum concentrations (McGuire et al., 1995). Because
IGF-I is low when serum bovine growth hormone concentrations are high, the growth
homone/IGF axis is said to be uncoupled. The 33 animals used for the data shown in
Fig. 19.2 were fed adequately relative to NRC requirements (National Research
Council, 1988).
Statistical analysis of the milk changes for all of the IGFBPs showed that there
were two main effects upon IGFBP changes. The first was time of lactation and the second was an effect of pregnancy. The repetitive sample study with 33 cows from the
Pennsylvania State University dairy herd, shown in Fig. 19.2, had eight cows that did
not become pregnant. Figure 19.3ac shows that there are large differences between the
appearance of IGFBPs in milk of cows that do not become pregnant (Fig. 19.3a) compared with those that become pregnant (Fig. 19.3b). Most notable is that the cows that
do not become pregnant show high concentrations and higher variances of IGFBP-3
and -2 in their milk. This is not reflected in the IGFBP concentrations occurring in
blood (Fig. 19.3c and d). These findings support three major concepts. First, some
event related to pregnancy is communicating to the mammary gland to alter milk
IGFBP profiles; second, that milk changes occur independent of blood changes clearly
suggests that the mammary gland regulates IGFBPs occurring in milk; and lastly, the
blood changes, being different from that of milk, indicate that blood IGFBPs must be
reflections of more than mammary gland tissue and therefore changes in blood IGFBP
components will not reflect impacts upon the mammary gland.
Finally, many investigations with humans and rodents have shown that IGFBPs
may be affected by the presence of proteases (Giudice, 1995). These enzymes have been
shown to specifically modify IGFBPs so that they exhibit lowered binding of IGFs such
that Western blotting may fail to detect them. Since our recently published characterization of IGFBPs in blood and milk utilized Western blotting, we needed to know if
the milk IGFBP patterns were influenced by the presence of proteases. Extensive investigations for the presence of the pregnancy-related IGFBP-3 protease reported for the
rodent and human (Staley, 1998) has been negative for bovine blood and milk. Thus,
the changes observed in milk IGFBP-3 are not due to protease activity. The presence of
other IGFBP proteases is currently unknown.
336
C.R. Baumrucker
Fig. 19.3. Comparison of insulin-like growth factor binding proteins (IGFBP) in milk whey and
blood serum of pregnant and non-pregnant cow changes during lactation. (a) Cow milk whey
from non-pregnant cows and (c) serum (n = 8). (b) Cow milk whey from pregnant cows and (d)
serum (n = 25). Samples are the same as those shown in Fig. 19.2. Data was standardized by the
use of a internal blood serum standard. Lines are regressions for each IGFBP.
337
IGF receptors
High-affinity receptors for IGF-I, IGF-II and insulin have been demonstrated for
bovine mammary tissue and mammary epithelial cells (Oscar et al., 1986; Hadsell et
al., 1990). Studies have shown that while type-1 IGF-IR binding increases with lactogenesis (Dehoff et al., 1988), this is an apparent increase that results from a decrease that
occurs during the pre-partum period (Hadsell et al., 1990). Although the type-2 IGF-II
receptor shows approximately fivefold greater binding (number of receptors), no microsomal binding changes were observed during lactation (Hadsell et al., 1990). Since the
mammary type-2 receptor does not change during the course of lactation, its capacity
to invoke changes in the mammary gland becomes less significant, yet it probably contributes to the disappearance of IGFs from the cellular environment. Perhaps this is the
mechanism of IGF appearance in milk (Prosser and Fleet, 1992; Donovan et al., 1995).
The significance of the type-1 receptor change (1.6-fold increase) is perhaps debatable.
This is apparent when considering insulin receptor action. Characterization of the
insulin receptor has shown that less than 5% of receptor binding leads to maximal
insulin stimulated events (White and Kahn, 1994; Taylor et al., 1996) leading to the
concept of spare receptors. Is this also true for the type-1 receptor? Studies by
Neuenschwander et al. (1995) with breast cancer cells (MCF-7) indicated that there
appears to be no spare receptors for the type-1 receptors, at least in MCF-7 cells, since
the antisense suppression of the expression of the type-1 receptor translated directly
into a linear decrease in cellular growth in the presence of a constant supply of IGF-I.
Thus, the decline observed for the type-1 receptor in mammary tissue during the late
pre-partum period may be a signal for less growth and perhaps provide more opportunity for cellular differentiation.
This raises the question, what is the IGF signal to the mammary gland: growth,
differentiation, or apoptosis? Three transgenic studies have shed some light upon the
explanation of this question. When milk protein promoters were utilized to express
excess IGF-I (and analogues) during lactation, no significant difference was observed in
the mammary gland milk production (pup weight gain) or structural morphology
(Brem et al., 1994; Benito et al., 1996; Hadsell et al., 1996) Some reports of smaller
alveoli were reported. The main effect observed is the delay in cellular decline during
C.R. Baumrucker
338
involution providing strong support for a role of IGF-I in the resistance of apoptotic
mechanisms, at least during involution. In light of the low IGF-I in milk and blood
during early lactation, perhaps the increase in IGF-I induced by bovine somatotropin
(bST) treatments are decreasing lactational apoptosis and thereby increasing milk production.
In the cow, although circulating levels of IGF are increased by bovine growth hormone, the overall pattern of IGF-I concentration is inversely related to levels of milk
production (Ronge et al., 1988). This correlative data may indicate that the IGFs are
low during peak lactation in order to allow maximal differentiation and continued milk
production from those cells established into lactation. However, systemic circulation of
IGFs is not the only source of IGF for mammary tissue. Mammary stromal fibroblasts
are a source of IGF-I while epithelial cells have not been shown to have this capacity
(Glimm et al., 1988; Yee et al., 1989). This suggests that although IGF-I is synthesized
in mammary tissue, the level of synthesis is low and discourages the concept that local
IGF-I production accounts for the galactopoietic potential during lactation. On the
other hand, we have demonstrated that IGF-II is synthesized and regulated in mouse
mammary epithelial cells (COMMA-D1) (Campana and Baumrucker, 1994) and primary cultures of bovine mammary epithelial cells (Fig. 19.4).
25,000
20,000
15,000
10,000
5,000
SF
M
co
nt
ro
l
C
or
(5 tis
00 o
ng l/pr
/2 ola
In
0 ct
ng in
(5 su
0 lin
)
ng /c
/5 ort
00 is
ng ol/p
/2 ro
0 la
In
ng ct
(5 sul
) in
i
0 n/
ng co
/5 rti
00 so
ng l/p
/5 rol
00 ac
ng tin
)
Fig. 19.4. Secretion of insulin-like growth factor II (IGF-II) into conditioned media (CM)
from primary cultures of bovine mammary epithelial cells. CM was changed each day
and was analysed for IGF-II by RIA after acetic acid column separation. Values are the
mean standard error of a mean (n = 3).
339
How can receptor numbers and ligand changes be reconciled with changes in milk
production? We believe that changes in IGFs and their receptors need to be considered
in the whole IGF system (Fig. 19.1). It has been demonstrated that the application of
IGFs and other endocrine factors to most tissues and mammary cells (Skaar and
Baumrucker, 1993; Cohick, 1998) stimulates a change in the appearance of IGFBPs.
C.R. Baumrucker
340
Fig. 19.5. Primary cultures of bovine mammary epithelial cells secrete insulin-like
growth factor binding proteins (IGFBP) into conditioned media. Cells were plated on
plastic in serum-free media and media changed every 2 days. IGFBPs were detected by
Western blots utilizing [125I]IGF-II as a ligand. All trans retinoic acid (1 mM; atRA)
stimulates the appearance of IGFBP-3 in the conditioned media after 12 days of
treatment.
341
not all binding was due to the introduced RGD sequence that delineates the integrin
binding site (Ruoslahti and Pierschbacher, 1987).
It has been speculated that the mechanism of action of IGFBP-3 may be attributed
to its ability to bind the serum and autocrine IGF-II and prevent action at the type-1
IGF receptor as shown with a mouse mammary cell line (Campana et al., 1994) and
the primary bovine cells shown in Fig. 19.5. However, the presence of a type-1 receptor-independent action of IGFBP-3 cannot be excluded and we now have descriptive
evidence for such a mechanism as reported in preliminary data described below.
C.R. Baumrucker
342
Fig. 19.6. Specific binding of [125I] recombinant human insulin-like growth factor
binding protein 3 (rhIGFBP-3) to proteins solubilized from cellular microsomes. Data
is relative specific binding that is corrected for non-specific binding with excess
unlabelled rhIGFBP-3 to Western blots from microsomes isolated from mammary
epithelial cells from bovine (MEBo) and COMMA-D1 cells.
binding. Reverse experiments with [125I]b-Lf (provided by F. Schanbacher) show specific binding to IGFBP-3, but not to any other IGFBP found in bovine milk or serum
(IGFBP-2, -4, -5). Additional studies have documented that not only does Lf bind to
IGFBP-3, but that if IGF is bound to IGFBP-3, Lf competes for binding with a K a of
1 mM and will displace bound IGF with an apparent K d of 10 mM (Baumrucker et al.,
1999).
Lactoferrin
As true for IGFBP-3, nuclear appearance of Lf has also been documented (Garre et al.,
1992). Lf is a highly positively-charged ~80 kDa iron-binding glycoprotein that
exhibits a high degree of homology with transferrin (Baker et al., 1998). Although the
protein has been sequenced and characterized by X-ray crystallography (Lonnerdal and
Iyer, 1995), its biological function(s) remain largely elusive. Milks of different species
vary in the relative content of Lf and transferrin. Human milk has the highest level of
Lf (16 mg ml21) with little transferrin present, while bovine milks have low levels of
Lf (0.1 mg ml21) and detectable amounts of transferrin.
Lf content of mammary secretions varies with developmental stage. The message is
highly expressed and the protein is present in high concentration during the final prepartum stages (human, 10 mg ml21; bovine, 12 mg ml21) and at higher concentra-
343
tions (2050 mg ml21) in both human and bovine mammary gland during involution.
Lf mRNA and protein concentration is low during lactation (Nuijens et al., 1996). It is
thought that mammary cell Lf secretion occurs via the normal apical secretory pathway
since Lf has been shown to be co-localized with casein micelles in secretory vesicles
(Neville et al., 1998), but these studies were conducted during lactation, and prepartum and involution mechanisms may be different.
C.R. Baumrucker
344
Fig. 19.7. Presence of a nuclear localization sequence (NLS) in bovine insulin-like growth
factor binding protein 3 (IGFBP-3) amino acid sequence. Italic underlined sequences are the
NLS. Bold residues are the specific residues that comprise the NLS recognition signal:
(i) two adjacent basic amino acids; (ii) a spacer of ten residues; (iii) at least three basic amino
acids within the next five residues following the space. Data from GenBank.
Conclusions
When many researchers entered the somatomedin C arena in the early 1980s, they perhaps thought that this was yet another hormonal ligand that would clearly answer the
questions posed by the bST application to lactating dairy cows. The new discoveries
since that time that have expanded the IGF system components and discoveries of biological actions have expanded our thinking about the regulation of bovine mammary
tissue in growth, differentiation, and involution. The IGF system, as perhaps a model
for many endocrine, paracrine and autocrine factors, demonstrates that complexity and
the potential for levels of regulation that exist in the mammary gland. Each level of
complexity provides more opportunities for control and perhaps selection as we seek to
understand efficient milk production.
345
(a)
(b)
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20
J.P. MCNAMARA
Department of Animal Sciences, Washington State University, Pullman,
Washington, USA
Introduction
Mammals have evolved to store several basic nutrients in order to minimize the effects
of variation in environmental supply (Pond, 1984). Body fat is the largest storage
organ, often containing several months worth of maintenance energy requirements.
Body protein is also stored, in the form of blood and organ proteins, but the largest
mass is muscle protein. Reserves of body protein can often be more important to the
function of the animal than body fat. During periods of deficit of even a few amino
acids, body proteins must be broken down to supply the proper balance of amino acids
for synthesis of critical regulatory, enzymatic or structural proteins. In addition,
because of the strict requirement for glucose by several organs, especially the central
nervous system, body protein serves as a reservoir of glucose. Deficiencies of glucose
supply longer than roughly 1 day can only be supplied by gluconeogenesis from
amino acids released from visceral or muscle protein, and muscle is by far the largest
source.
In animal agriculture, management of body reserves has become most important
in the feeding and care of dairy and beef cattle. Dairy animals of even average milk producing ability undergo a period of deficit of both energy and amino acids in early lactation. Research has discovered basic principles and some parameters describing storage
and use of body fat and protein during lactation. However, a quantitative description
of genetic, endocrine and neural regulation of the ability to store and utilize body
reserves is lacking. From a practical feeding management standpoint, we routinely use
subjective measures of body fat and protein storage (usually termed body condition)
in our feeding programmes. It would be advantageous to have a more precise description of body fat and protein use.
To do this we need a more complete mechanistic description of storage and use of
body fat and protein in lactating animals. Various empirical equations have been developed to describe these interactions in lactating cows. However, the most efficient
utilization of natural resources requires a better understanding of the genetic and
CAB International 2000. Ruminant Physiology: Digestion, Metabolism,
Growth and Reproduction (ed. P.B. Cronj)
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J.P. McNamara
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355
One focus in a study of BCS was to investigate the relationship between BCS (fat
storage) at calving and feed intake and milk production during lactation. In dairy
animals, there has been a negative effect of too much fat at calving, known as fat cow
syndrome, such that overly fat animals display an increased incidence of a variety of
post-partum metabolic and reproductive disorders (Garnsworthy, 1988; Jones and
Garnsworthy, 1989; McNamara, 1994); this phenomenon is also present in lactating
swine (McNamara and Boyd, 1999). The hypothesis was that the extra body fat is
rapidly mobilized due to the initialization of the lactation hormone complex and
enhanced by the negative energy balance in early lactation. Due to the high amounts of
circulating free fatty acids, neural pathways regulating feed intake depress feed intake
from a point that it may have otherwise reached.
I have no argument with this theory or phenomenon, and it has proved its utility.
As producers have paid more attention to proper pre-partum nutrition, the incidence
of fat-cow related health problems has diminished. However, through the years the
concept was applied to all dairy animals as gospel, that is a BCS of 3.5 or more should
be avoided as it will lead to fat-cow syndrome and increased problems. However, many
observations in university and field situations in the US belied the generality of this. In
fact, for many animals it was more of a challenge to achieve sufficient body fatness
prior to the subsequent lactation, and milk production was compromised. This was
thought by some, including me, to be due to the fact the genetic ability of these
Holstein dairy cows to produce milk had outstripped their ability to recover body fat
in late lactation. Also, several large herd survey studies demonstrated that, although
there was a positive association with fatness at calving and disease, the actual incidence
of fat cows was quite small (less than 10%; Jaquette et al., 1988; Gearhart et al., 1990;
Waltner et al., 1993; Heuer et al., 1999). This is likely because of the increased milk
production ability and the improved management compared to the industry 1020
years earlier. Thus, the biological principle of too much fat is bad was not to be discarded, but rather we sought a more specific definition of what too much really was
for various populations.
We measured the changes in BCS of cows in one herd (over 200 lactation records)
to develop equations relating BCS to milk production and days in milk. In addition,
we asked the question mathematically of What is the relationship of BCS at calving to
subsequent milk production? For animals in this herd, which averaged 9541 kg of
3.5% fat corrected milk in 305 days (range 882610818) during this study, in order to
maximize milk production, BCS needed to be between approximately 3.5 and 4.0 (Fig.
20.1). Below that and milk production was lower. However, for older cows more body
fat did not decrease milk production. The BCS range of these cows only went up to
4.5, and there were very few cows at this level. It is also interesting to note that a BCS
at calving of 4.5 was associated with the same milk production as that at a BCS of 3.0.
This herd-level study simply suggests that we should move beyond any one target BCS
and rather look at body fat storage and use as a continuum. The genetic potential, body
size and available feed all need to be considered when interpreting BCS or making
recommendations about desired BCS.
Another endeavour in the last 20 years of studying body reserves has been to
develop useful and reasonable techniques to measure reserves in live animals. Several
studies used dilution of body water with deuterium oxide to derive equations describing body fat and protein. There was a tremendous amount of work on this technique to
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J.P. McNamara
Fig. 20.1. Relationship of body condition score (BCS) at calving with production of
fat-corrected milk (FCM) (from Waltner et. al., 1993). Note that milk is maximized at
a score of 3.54 for first lactation animals, while for second and higher lactation,
increasing body fat does not have a negative effect on milk production.
measure body water as a way to estimate body fat (see Andrews et al., 1995; Komagiri
and Erdman, 1997; Komagiri et al., 1998 and references therein). The literature is too
large to expand upon here, but in summary, this technique has been useful, but has as
drawbacks the need for more sophisticated equipment and intense labour.
Another technique now in use is the measurement of fat cell size. A small biopsy of
subcutaneous adipose tissue is taken and cells are isolated and the average diameter is
measured. Because body lipid mass in sub-adult and adult ruminants is primarily a
function of adipose hypertrophy, the relationship of fat cell size to body lipid mass is
quite strong in lactating dairy cattle (Robelin et al., 1989; Waltner et al., 1994). This
technique has the benefits of much reduced cost, and a characteristic of the adipose tissue is being directly measured. Thus it is more likely to have a greater utility than the
body water technique, especially in early lactation. In addition to being useful to estimate total body fat, it teaches us something about the cellular changes going on as well.
An important point from the discussion above is that regardless of the metabolic or
molecular techniques which may be used to define phenotypic or genotypic attributes,
we have to estimate total body fat at some point. Otherwise, we do not have a complete
picture of what the mechanistic elements mean in relation to total body energy use.
My colleagues and I were able to conduct several investigations covering from
1983 to the present on the rates of biochemical pathways in adipose tissue of dairy
cattle varying in genetic background, parity and nutritional environment. We defined a
357
time-course of adaptations in lipogenesis, lipolysis and esterification throughout lactation (McNamara and Hillers, 1989; McNamara, 1994). Lipogenesis decreased 90% in
all cows at day 15 of lactation, but even by day 30 was starting to recover (Fig. 20.2).
Cows of high milk-producing ability had lower rates than did cows of average ability.
Lipogenesis in all cows stayed markedly elevated throughout lactation. Low energy
intake diminished lipogenesis in cows of average milking ability, but not in superior
ones. Lipolysis increases during lactation (Fig. 20.3), and as lactation progresses, rates
of fatty acid release in adipose tissue stay elevated (McNamara and Hillers, 1989;
McNamara, 1994). There was a faster rate of lipolysis in high merit versus low merit
cows, and little effect of dietary energy intake on lipolysis rates. These genetic differences in lipogenesis have been described elsewhere as changes in substrate sensitivities
and maximal velocities (McNamara et al., 1991a; McNamara and Baldwin, 2000). This
was the first demonstration that genetic selection for milk production altered enzymatic expression in adipose tissue of dairy cattle.
This continued maintenance of high rates of lipolysis, even in positive energy balance, demonstrates the adaptations of endocrine and kinetic systems to maintain a high
rate of milk-fat output. Because these processes are so essential to survival, there is a
very strict and redundant series of controls exerted on these reactions. It is likely that
the sympathetic nervous system helps maintain a higher rate of lipolysis, driven by the
need for fat by the mammary gland (McNamara and Murray, 1994). Such control of
Fig. 20.2. Lipogenesis in adipose tissue of dairy cattle varying in genetic merit and
dietary intake. Note logarithmic scale. Animals of high merit (HGHE) had lower rates
of lipogenesis than lower merit animals (LGHE) when fed normally. Lower energy
intake decreased lipogenesis in LG animals (LGLE) but not in HG cows. Note the
classic metabolic overshoot of lipogenesis by day 60 and sustained high rates during
all of lactation.
J.P. McNamara
358
6000
5500
HG
5000
LG
4500
4000
3500
3000
2500
2000
30
15
15
30
60
180
349
Fig. 20.3. Catecholamine stimulated lipolysis in adipose tissue of lactating dairy cattle
varying in genetic merit. Note that high genetic merit animals (HG) had higher rates
than low merit (LG) animals, even through late lactation. Also note that lipolysis
remained elevated during late lactation, even in positive energy balance. This is likely
to supply the continued demand for milk fat precursors, and results in a sustained
increase in lipid turnover.
adipose tissue accumulation by the sympathetic nervous system has been demonstrated
during growth and obesity (Knehans and Romsos, 1983; Dulloo and Miller, 1985).
Differential control of tissue metabolism by the sympathetic nervous system during
pregnancy and lactation also occurs; for example, there is less activity by the brown fat
in rodents, probably to conserve energy (Trayhurn and Richard, 1985). Although this
regulation is obviously genetically dictated, the heritability of nervous regulation is not
known, and would be difficult to determine for lactating cattle. Rodent models of
growth and lactation may have utility for study of this question, as may various potential genetic knockout constructs.
In order to understand this regulation of body reserve use, we have run studies to
define how the integration of lipogenesis, esterification and lipolysis occurs under various situations of dietary energy intake, genetic ability of the animal, environmental
stress, hormonal management, and stage of lactation. This was done with the purpose
of challenging present understanding as explicit in the research model of metabolism in
dairy cattle introduced above. The following equations from that model provide a
quantitative framework for our discussion of genetic and environmental regulation of
body reserve use.
359
reactions within a given time period (for example, 1 day of a 305-day lactation). The
notation FaTs indicates the esterification of preformed fatty acids into triglycerides,
AcTs is the integrated total of acetate formation into fatty acids and then triglycerides,
and TsFa is the rate of triglyceride hydrolysis to free fatty acids.
The amount of ITs can represent a function of genetic makeup of the animal,
measured in such ways as composition of gain to a given day of age for first-lactation
animals. The amount of ITs can be measured either directly by slaughter analysis, or
more usefully by indirect means, such as determining body water or fat cell size (Brown
et al., 1989; Robelin et al., 1989; Waltner et al., 1994; Andrews et al., 1995). Using
equations we derived from slaughter trials, the amount of fat can be related to the body
weight and condition score or vice versa (Waltner et al., 1994). Incorporation of body
condition score into various lactational models is already underway, but accurate
descriptions will require estimates of body fat. In most models, body condition score is
calculated from body fat predicted from basic energetic use, and not the other way
around.
Most equations in the model are either substrate saturation (MichaelisMenten) or
mass action. Thus, the maximal velocity of a reaction for a given animal may be seen as
a phenotypic trait, for example the Vmax for lipogenesis in an aggregate biochemical
model can represent such mechanisms as the amount of enzyme complex, total number
of fat cells, and activity of the enzyme. The substrate sensitivity constant (Ks) can initially be a phenotypic trait, representing again the sensitivity of the enzyme complexes
for a given reaction. In addition, the Ks can be variable, and affected by nutritional
environment. Biologically, for example, we know that a decreased amount of glucose
(even in ruminants) will decrease the carbon flux through lipogenic pathways. Due to
decreased insulin or increased glucagon or catecholamines, the enzyme will have
decreased activity (increase in Ks) and eventually enzyme synthesis will decrease
(decreased Vmax). Thus, substrate concentration is a direct effect of nutritional programme, and over time, may alter parameter values describing phenotypic variables as
well.
The equation for lipogenesis from acetate is:
AcTs = VAcTs / (1.0 + KAcTs / cAc + K1AcTs / (Ahor2 3 cGl )).
In this equation, VAcTs is the maximal velocity of acetate conversion to triglyceride,
KAcTs is the substrate sensitivity constant for Ac, K1AcTs is the sensitivity constant for
glucose (cGl), Ahor2 (anabolic hormone) represents the effect of insulin on acetate
conversion to lipid, and cGl is the concentration of glucose. Additionally, Ahor2 is a
function of concentration of glucose compared with the reference concentration
([cgl]/[rcgl]), reflective of glucose supply and/or energy balance. Thus, most of our
basic principles concerning lipogenesis are explicit in this equation. Readers interested
in a more detailed treatment of adipose biochemistry should see Chapters 12 and 16 in
Baldwin (1995).
The equation describing esterification of fatty acids to triacylglycerols is :
FaTs = VFaTs 3 (EBW**0.75) / (1.0 + KFaTs / cFa + K1FaTs / (Ahor 3 cGl)).
This represents esterification from fatty acids (direct from diet or from re-circulation)
as a function of maximal velocity (VFaTs), metabolic body size (EBW**0.75; ** indicates raising to an exponent), sensitivity to circulating fatty acids (KFaTs), and sensitivity
J.P. McNamara
360
to glucose (K1FaTs), which can be altered by anabolic hormone. Note that the Ahor
in this equation is not the same Ahor2 in the lipogenesis equations, recognizing the
potential differential responsiveness of lipogenesis and esterification to insulin or circulating glucose. This initial hypothesis was based on work done in other species or in
preliminary trials. This differential control of lipogenesis and esterification was confirmed with studies of adipose tissue metabolism (McNamara and Hillers, 1989;
McNamara et al., 1991a). Thus the model explicitly describes hormonal control of
major pathways and has the mathematical as well as biochemical attributes to test
hypotheses concerning actions of various hormones. When more data become available, other genetically controlled hormonal actions can be included.
The reaction describing hydrolysis of triacylglycerols to fatty acids and glycerol is:
TsFa = VTsFa 3 (EBW**0.75) 3 CHOR1 3 T3 / (1.0 + (cFa / K1TsFa ) **
EXP10 + (KTsFa /cTs )** THETA1) ;
in which lipolysis (TsFa) is a function of metabolic body size, maximal velocity, sensitivity to catabolic hormone (CHOR1, norepinephrine), injected thyroid hormone
(T3), and feedback from circulating fatty acids (K1TsFa; note this is set to be an inhibition on lipolysis as Fa concentration increases). The exponents EXP10 and THETA1
alter the sensitivity of the reaction to the substrate under the exponent. It is likely that
this parameter also represents genetically controlled differences in total enzyme present
or pool size of substrate. Note that there is a protected, very low amount of body fat
(KTsFa/cTs) which, if approached, will rapidly diminish rates of lipolysis. The amount
of protected body fat is set very low, representing basic membrane lipid, so the animal
does not remove all lipid from the body. Note also that lipolysis is controlled by the
catabolic hormone and not the anabolic hormone directly.
Thus, the current model recognizes and allows the testing of hypotheses on the
differential nature of metabolic control. It is not explicit that the effect of circulating
glucose, for example, must have an equally opposite effect on lipogenesis and lipolysis;
this captures the concept of homeorhetic control of metabolism quite well (see
Chapter 18). The effect can have differential sensitivity based on stage of lactation,
rate of milk production, or genetic differences in adipose phenotype. Also note that
with use of various exponents (the value of which can be changed from simulation to
simulation), the control of sub-components of this equation can alter the sensitivity of
response to various substrates as necessary. Studies may be designed to measure
these parameters in breeds of cattle widely varying in milk production or growth
ability.
361
in the form of either visceral or muscle protein, is available for mobilization to supply
essential amino acids and glucose during periods of deficit.
There were hurdles that diminished research interest in this area. One was the
inconsistent response in milk protein output in experiments designed to alter protein
reserves pre-partum. This was partly due to the lower production level of the animals
at that time, such that the amount of body protein at calving was probably not a limiting factor in the average cow (Botts et al., 1979). Experiments were often done with
the idea that a greater amount of protein reserves would increase milk protein output
regardless of the potential production rates of the cow (i.e. it was assumed that protein
reserves were always limiting). It is probably more exact to test the hypothesis that
maximal lactational protein output may be limited in situations in early lactation if
protein intake is not adequate, and in those situations a greater amount of body protein
at calving may help allow the maximal milk protein output. In addition, the glucose
balance of the animals in these early studies was not often controlled for, and the role of
body protein for gluconeogenic precursor supply was ignored, adding to variation in
experimental results. Another hurdle was technological, in that measurement of body
protein content and rates of body protein synthesis and degradation were quite variable, difficult and costly to make. Progress has been made on this front, but technical
precision and cost is still a major block to a better quantitative understanding in this
area.
Muscle protein accretion is the sum of the integration of protein synthesis and
degradation (Waterlow, 1995). Amino acid storage and release from the muscle is also
important in metabolic health, as acidosis results from excess oxidation of ketogenic
amino acid carbon as well as from oxidation of long-chain fatty acids. The high-producing lactating dairy cow in early lactation can lose significant amounts of body protein (Andrews et al., 1995; Meijer et al., 1995; Komagiri and Erdman, 1997), and not
all of these amino acids appear in milk protein. Most present models treat these pathways at a very aggregate level, with admittedly inadequate equation forms and parameter values. Again, this is due to a lack of good data on these processes, not to a lack of
appreciation of their importance. Equations below are critical ones for which we need
information.
362
J.P. McNamara
363
rates (Danfaer, 1990; Baldwin, 1995; Waterlow, 1995; Overton et al., 1999). There are
wide ranges in the estimates of gluconeogenesis coming from amino acids in lactating
ruminants, from 2 to 40% (see Overton et al., 1999, and several references therein). It
is likely that a significant part of this variation is due to differences in muscle protein
turnover and availability of gluconeogenic amino acids. Causes of these differences
could include parity, stage of lactation, genetic makeup, as well as the amino acid and
glucose status of the animals.
This muscle is regenerated during middle to late lactation (even more so in first
and second parity animals). At 20 kg of protein to be regained and assuming 150 days
to accumulate it, then to the 1.6 kg day21 of turnover we add 133 g (20,000 g per 150
days) of accumulation. To put this in perspective, this amount is more than the average
observed increase in milk protein due to increasing dietary protein by 2.8 percentage
points (Santos et al., 1998; NRC 1989). Thus we propose that variation in muscle protein turnover during lactation, among parities and across diets, can significantly affect
our ability to predict milk protein output. Also, simulation analyses performed with
several model systems, including the National Research Council (1989), the Cornell
Net Carbohydrate and Protein System (Kohn et al., 1998) and the mechanistic model
Molly (Baldwin, 1995), demonstrate inadequacies in describing body muscle and fat
use during lactation.
In a recent comparison, it was shown that in order for the Cornell system to predict outputs from inputs, the average daily gain function was severely underestimated
(5 kg day21; Kohn et al., 1998). This system has been well designed and evaluated, and
is useful for describing nutrient use for milk production in dairy cattle. Daily gain (or
loss) in lactating animals can be both fat and protein. This lack of knowledge of muscle
metabolism limits our ability to accurately predict animal nutrient use.
364
J.P. McNamara
Fig. 20.4. Simulated and observed body fat in lactating dairy cattle fed control (2.5%
lipid) or high-fat (6.0% lipid) diets. Dietary carbohydrate was approximately 3
percentage units less on high-fat diet and intakes were approximately 1 kg day21less
on high-fat diet from day 17 to day 100, then they were similar. Simulated control diets
are dotted lines and diamonds, observed control data are triangles; simulated high-fat
diets are solid line and boxes, observed high-fat diet data are circles.
quite well (Table 20.2). This demonstrates the utility of mechanistic models for
describing genetic differences in body reserve use.
However, the second trial was more extensive and allowed a more severe challenge
of our knowledge as described in the model. When feeding cows greater amounts of
fat, body fat accumulation was over-simulated at a fast rate (Table 20.2 and Fig. 20.4;
McNamara and Baldwin, 2000). Body fat was simulated adequately on control diets
but over-simulated on high-energy diets, compared with observed data (open squares
and solid circles, Fig. 20.4). Thus the model can balance for inputoutput, but a severe
challenge shows it is inadequate in describing protein and lipid metabolism over longer
periods of time. It is telling that the Cornell model system, which was constructed
independently, with a different objective than the research Molly model, also has a
bias in the average daily gain function in order to balance inputs and outputs in milk
(Kohn et al., 1998).
The lack of precision in simulating the fat reserve led us to examine the use of
body protein as one of the sources of inadequacy. This is primarily because protein
turnover accounts for a large (~20%), variable and little-defined cost of basal metabolism (Baldwin, 1995; Waterlow, 1995). Figure 20.5 shows the use of body protein in
the simulations described above. The 1524 kg of body protein simulated to be lost by
day 77 (Fig. 20.5) is remarkably close to the 20 kg reported in one of the few direct
observations of protein loss in lactating dairy cattle (Komagiri and Erdman, 1998). It is
also telling to note that the model described a greater loss of body protein on the animals
fed the high-fat diet. In the experiment, dietary carbohydrate percentages were 34%
365
Table 20.1. Observed and simulated data from dairy cattle fed rations varying in fat content.
Controla
WCS
WCSFA
Variable
Obsb
Sim
Obs
Sim
Obs
Weeks 3 to 17
DM intake (kg day21)
Milk production (kg day21)
Milk fat (kg day21)
Milk protein (kg day21)
23.0
43.3
1.39
1.32
23.0
43.9
1.48
1.37
23.9
43.1
1.44
1.29
23.9
44.1
1.53
1.39
21.3
41.1
1.53
1.21
21.3
41.6
1.48
1.33
Weeks 18 to 35
DM intake (kg day21)
Milk production (kg day21)
Milk fat (kg day21)
Milk protein (kg day21)
21.6
33.4
1.16
1.11
21.6
33.6
1.09
1.04
22.8
33.8
1.17
1.14
22.8
33.5
1.12
1.04
22.7
35.4
1.22
1.12
22.7
32.8
1.15
1.01
Sim
From McNamara and Baldwin (2000). Dairy cattle were fed lucerne hay (23% of dry matter (DM)), grass
silage (23% of DM) and 54% concentrate diets (control); same diet with 12.7% of whole cottonseeds in the
concentrate, replacing maize (WCS); and WCS plus an additional 1.89% of ruminally protected long-chain
fatty acids (WCSFA) from 17 to 305 days in milk (DIM). Simulations were run on the mechanistic model of
Baldwin (1995) from 1 to 245 DIM. Intakes of nutrients were simulated explicitly. Initial body weight and fat,
and pre-treatment milk yields were used to set initial parameter values.
b Obs = observed values; Sim = simulated values.
Table 20.2. Observed and simulated data on body fat in dairy cattle.
Controla
Body
lipidsc
Time
Week 2
Week 9
Week 17
Week 35
a
(kg)
WCS
WCSFA
Obsb
Sim
Obs
Sim
Obs
Sim
101
90
98
125
69
55
65
101
109
104
105
134
84
85
110
186
98
81
100
135
71
64
93
205
From McNamara and Baldwin (2000). Dairy cattle were fed lucerne hay (23% of dry matter (DM)), grass
silage (23% of DM) and 54% concentrate diets (control); same diet with 12.7% of whole cottonseeds in the
concentrate, replacing maize (WCS); and WCS plus an additional 1.89% of ruminally protected long-chain
fatty acids (WCSFA) from 17 to 305 days in milk (DIM). Simulations were run on mechanistic model of
Baldwin (1995) from 1 to 245 DIM. Intakes of nutrients were simulated explicitly. Initial body weight and fat,
and pre-treatment milk yields were used to set initial parameter values.
b Obs = observed values; Sim = simulated values.
c Body fat was biopsied at times indicated. Fat cell size was determined microscopically after cell fixation
and dispersion. Equations to predict body fat from body weight and fat cells size, validated using cows from
this herd, were used (Waltner et al., 1994). The standard deviation of these measurements is 15 kg of body
fat.
J.P. McNamara
366
Fig. 20.5. Simulated body protein in lactating dairy cattle fed control (2.5% lipid) or
high-fat (6.0% lipid) diets. Dietary carbohydrate was approximately 3 percentage units
less on high-fat diet and intakes were approximately 1 kg day-1 less on high-fat diet
from day 17 to day 100, then they were similar. Note model describes a greater use of
body protein on the high-fat diet, probably in response to the smaller intake of glucose
and amino acids.
less on the high-fat diet, and voluntary feed intake was depressed for several weeks. The
model would suggest, even when total energy was not limiting, that the apparent glucose deficit (and potential nitrogen deficit) dictated a larger mobilization of body protein. So there is confidence that the model is behaving adequately, but the precision is
not yet adequate. Experiments designed to measure protein use will be essential if we
are to describe genetic differences in body reserves.
This combination of animal experiments and model testing allows us to define the
parameters and fluxes we need to know if we are to describe metabolism in the cow.
These experiments and model simulations lead to the hypotheses proposed herein on
muscle metabolism in lactation. A more precise description of the regulation of muscle
metabolism will improve our ability to predict responses to nutritional, genetic and
pharmaceutical management of milk production.
367
responses, feed intake ability? We need identification of genotypes, by DNA fingerprinting or other methods, which relate to most efficient use of nutrients, including
potential storage of body fat and protein. How do we identify the critical pathways on
which to focus our research efforts? We know most of the biochemical pathways
involved. We need to continue to focus on the control systems of lipogenesis, lipolysis,
protein synthesis and proteolysis. We need better methods to assess total body protein
synthesis and breakdown. Indirect methods described above will be helpful but
naturally limited in precision. A few well-designed experiments using stable isotopes of
amino acids in dairy cattle to define equations describing these pathways will be very
helpful. Less costly and invasive techniques can then be used in more practical situations to define the effects of breeding and nutritional strategies.
We must do research in the context of improving integrative models which explicitly include the complexity of biochemical and endocrine interactions among tissues
and dynamically over time. Demand for glucose by the mammary gland affects several
pathways in adipose and muscle tissue, from kinetic thermodynamics to altering
endocrine regulation. These controlling functions are, in turn affected by genotype,
stage of lactation and nutritional environment, often in subtle ways; various systems
may become limiting to milk production as lactation progresses. Using examples given
above, research on how insulin or catecholamines alter rates of muscle proteolysis and
adipose lipolysis will be very useful. Research needs to identify how critical variables
change during lactation, and estimate the parameters which define the biochemical and
endocrine systems. A description of experimental designs and data needed have been
presented previously (McNamara et al., 1991b).
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21
Genetic Manipulation of
Ruminant Biochemistry and
Physiology for Improved
Productivity: Current Status and
Future Potential
K.A. WARD
CSIRO Animal Production, LB1, Delivery Centre, Blacktown, Australia
Introduction
The provision of an adequate supply of food during the next 50 years is one of the
major challenges that face mankind. We have two related problems, namely, maintaining the current food supply to ensure an adequate diet for our present population of
about 6 billion people and then increasing production levels to provide for the
increased population to AD 2050, which may reach about 89 billion people. While
improvements in production, storage and distribution procedures could have a major
impact on current food availability, it is highly likely that increased productivity will
also be required over the next 5 decades. Over the past 100 years, total world food productivity has increased at just under 3% per annum, due to the implementation of
farm mechanization, irrigation, the widespread use of fertilizers, treatments for plant
and animal parasites and infectious diseases, and greatly improved methods of food
storage and distribution. We might expect these past innovations to help maintain current levels of production and to provide further increases in productivity when applied
to farms that are not yet using advanced farming technology. However, there are growing signs that these approaches are reaching their current limits because of environmental factors such as water availability, soil degradation, increasing disease resistance and
more subtle deleterious climatic changes. They may therefore not be capable of fully
supporting the increases in productivity that will be required by AD 2050.
A powerful method for the long-term improvement in plant and animal productivity stems from the use of genetic selection, in which a desirable genotype is chosen as
the source of parental stock for the next generation. This approach has been shown to
be capable of increasing the productivity of specific traits in plants and animals by
24% per annum, with the rate of increase sustainable in most cases for many years
Italics have been used when referring to natural genes, but not when referring to recombinant molecules.
CAB International 2000. Ruminant Physiology: Digestion, Metabolism,
Growth and Reproduction (ed. P.B. Cronj)
373
K.A. Ward
374
(Lindsay, 1998; Tribe, 1998) and it is clearly one of the most important tools currently
available for addressing the requirement for more food over the next 50 years. Recently,
its potential has been greatly enhanced by the new techniques of genetic engineering
which allow the direct manipulation of the genotype. This is achieved by enabling purified genes, isolated and characterized by recombinant DNA technology, to be added as
naked DNA directly to the genomes of most domestic animals and plants. Using these
techniques, it is now possible to consider introducing to a plant or animal a single gene
of major effect on the phenotype. In animals, the techniques stem from the pioneering
work of several laboratories (for review, see Palmiter and Brinster, 1986) which demonstrated that it was possible to introduce to the mouse genome small pieces of recombinant DNA. This was followed shortly thereafter by the pioneering experiments which
demonstrated that it was possible to alter the phenotype of an animal by this technique
(Palmiter et al., 1982, 1983). When the technology was subsequently shown to be
applicable to domestic animals (Hammer et al., 1985), the potential for practical
manipulation of livestock genomes became a reality.
This chapter will attempt to address the current status of genetic engineering
research directed towards the modification of the biochemistry and physiology of ruminant livestock for improved farm productivity. The potential targets for such modifications are the endocrine system, intermediary metabolism and the animals defences
against disease. The genes for transfer are currently selected on the basis of known function and predicted effect on phenotype, although the identification and isolation of
genes by genome mapping is now becoming a genuine practical possibility. It is worth
noting that no transgenic animal with altered physiology or modified biochemistry has
yet been introduced to the farmyard on a commercial basis, despite the fact that
research in these areas has been very active for more than 10 years. This highlights the
difficulties inherent in introducing to animals novel genetic properties that disrupt the
homeostasis of a carefully balanced physiology derived from existing gene combinations that have been optimized by many years of selective breeding.
Transgenic technology
The application of genetic engineering to ruminants has largely been achieved by the
use of pronuclear injection of single-cell embryos, although it appears likely that the
rapidly emerging technique of nuclear transfer may soon replace it. An excellent review
of microinjection technology and its history of development is provided in Palmiter
and Brinster (1986) and hence its details will only be briefly summarized here. The
technique, pioneered in laboratory mice, involves the introduction of a small quantity
of recombinant DNA into one of the pronuclei of single-cell embryos by microinjection, achieved by the insertion of a fine needle into the embryo pronucleus and the
injection into this organelle of about 2 picolitres of solution. The embryos are then
briefly cultured and survivors transferred to recipient mothers with reproductive cycles
suitably synchronized to accept the embryos. In a small number of these embryos, the
injected DNA becomes integrated into the embryo genome and an animal born from
such an embryo contains the new piece of DNA in all its cells. Such an animal is called
transgenic. The process is not very efficient and in the case of ruminants, about 0.9%
of injected embryos produce transgenic animals (Wall, 1996). This is due in part to the
375
presence of dense cytoplasmic granules in ruminant embryos which tend to obscure the
pronuclei during the microinjection procedure. While the problem is less apparent in
sheep and goats than in cattle and can be partially overcome by centrifugation of the
embryos prior to microinjection, it presents difficulties in all species of larger animals.
Recently, a novel technique known as nuclear transfer has been developed and may
soon prove to be the method of choice for the introduction of transgenes to ruminants.
It involves the introduction of a transgene to cells maintained in cell culture and the
use of one of these cells as a source of genetic information to programme the development of an enucleated oocyte. Based on the pioneering work of Willadsen (1986), the
recent technology has been developed by Campbell et al. (1996) and Wilmut et al.
(1997) who have discovered that the donor cells must be in the G0 stage of the cell
cycle for maximum efficiency of the process. The nuclei of these cells are apparently reprogrammed by the cytoplasm of metaphase II enucleated oocytes, allowing them to
undergo the full developmental programme normally reserved for zygotes derived by
sperm-mediated fertilization. A crucial aspect of the recent discoveries is the finding
that somatic cell lines derived from adult tissue can serve as nuclear donors in this
process (Wilmut et al., 1997); a remarkable finding recently confirmed in laboratory
mice (Wakayama et al., 1998). The technique has also now been extended to the production of transgenic cattle (Cibelli et al., 1998), indicating its probable wide applicability to many domestic animal species. This latter paper also questions the need for
donor cells to be in the G0 phase of the cell cycle; a crucial finding if confirmed since it
has significant implications for the intellectual property rights associated with the widespread commercial use of the technology.
There are many clear advantages to the use of nuclear transfer compared with
microinjection as a method of introducing transgenes to domestic animals. Since the
transgene is first introduced to cells in culture, this overcomes the highly inefficient and
time-consuming step of using fertilized embryos as the target for gene integration.
Cultured cells can be obtained in large numbers and recombinant DNA is readily
introduced to these cells by a variety of well-proven techniques. Selected cells can be
made into clonal lines and evaluated for site and stability of integration, and in some
cases can even be tested for expression, all under conditions of cell culture prior to
using them for the production of transgenic animals. In addition, for cells in culture,
techniques are available to carry out homologous recombination between the recombinant DNA and the equivalent homologous gene in the target cell, thus allowing specific genes to be inactivated or replaced by variant alleles. This adds a powerful new
capability to gene manipulation of domestic animals, since the potential is for genes to
be added, inactivated or replaced in specific animal lines. At its current stage of development, nuclear transfer is still in an experimental phase but it appears well-placed
eventually to replace microinjection as the method of choice in introducing transgenes
to domestic animals.
The techniques of microinjection and nuclear transfer are at present the two preferred methods for the production of transgenic ruminants. In the future, it is possible
that transposons, which are a modification of retroviral technology, may provide a third
capability, but other methods such as cytoplasmic injection, ballistic guns and spermmediated transfer do not seem relevant to ruminants.
K.A. Ward
376
377
Transgenics
expressing
mMT-hGH
mMT-bGH
mMT-hGRF
mTF-bGH
11
mAL-hGRF
oMT-oGH9
oMT-oGH10
35
12
Gene
Totals
Reference
K.A. Ward
378
oMTSGH10 gene, but when considered together with the results obtained in transgenic mice, the research looks very encouraging.
A recent approach that has been carried out in swine may also prove to be useful
for ruminants. In this case (Pursel et al., 1996, 1998), the IGF-I gene is controlled by
the avian skeletal a-actin promoter. Using this transgene, expression of IGF-I is
primarily confined to the muscle cells and has its effect directly on this tissue. In consequence, average daily weight gain and feed efficiency do not differ from control animals, but the transgenic animals had 11.4% and 3.3% less fat and 5.1% and 2.2%
more protein in the eviscerated carcasses of males and females, respectively (Pursel et
al., 1998). The IGF-I concentrations of plasma were between 9% and 10% higher than
in control pigs and the general health of the animals was good. The avian skeletal aactin promoter has also been used in another approach recently described in mice. In
this case, the promoter is joined to the gene sequence encoding GRH and the fusion
gene inserted into mice by injecting the DNA directly into regenerating quadriceps
muscle tissue. This results in the uptake of some of the DNA into the muscle cells,
which then produce and secrete GRH in levels sufficient to elevate GH and produce
increased body growth (Draghia-Akli et al., 1997).
The above results together indicate that the concept of increasing body growth and
feed utilization efficiency while simultaneously improving the carcass composition of
domestic animals to make it more suitable for human consumption remains a worthwhile and achievable goal. The difficulty that must still be overcome is the adverse
effect on health that results in animals that produce GH at high levels. However, optimization of GH-encoding genes is progressing to the stage where transgenic animals
with genuine commercial application are close to reality and might be expected to be in
field trials within the next few years.
379
venously into sheep, wool growth could be increased substantially (Reis, 1979). The
simple procedure of feeding additional cysteine to deficient animals is not effective
because most ingested nutrients are rapidly metabolized by the ruminal microflora in
sheep and the sheep is lacking the genes necessary for the enzymes of cysteine biosynthesis. The relevant biosynthetic pathway in Escherichia coli is shown in Fig. 21.1. It
can be broadly divided into a pathway for the reduction of sulphur to an active form of
reduced sulphide and a carbon pathway in which, firstly, the amino acid serine is converted to O-acetylserine in the presence of acetyl-CoA and the enzyme serine
transacetylase (SAT) and then the O-acetylserine is converted to cysteine in the presence of sulphide and the enzyme O-acetylserine sulphydrylase (OAS). The sulphide
concentration in the sheep rumen has been reported to be in the range of 0.6 mg ml21
to 288 mg ml21 (Bray and Till, 1975), suggesting that a functional biosynthetic pathway might be possible in sheep if the enzymes SAT and OAS could be provided in the
ruminal epithelial cells.
In E. coli, the cysE gene encodes the enzyme SAT and the cysK or cysM genes
encode variants of the enzyme OAS. Accordingly, the cysE and cysK (Denk and Bock,
1987; Byrne et al., 1988) genes were isolated and modified for expression in eukaryotic
organisms (Leish et al., 1993). The structure of the gene, named MTCEK1, is shown
in Fig. 21.2 and consists of the cysE and cysK coding sequences, each regulated independently by a sheep metallothionein-Ia (MT-Ia) promoter sequence and each containing
exon 5 of the sheep growth hormone gene spliced downstream of the bacterial coding
sequences (Leish et al., 1993).
When tested in eukaryotic cells in tissue culture, this gene could be transcribed
and translated into the relevant bacterial enzymes and these could be synthesized at
high levels (Ward and Nancarrow, 1991; Leish et al., 1993). The gene was then introduced into transgenic mice where it was shown to be expressed at high levels in several
tissues including the small intestine (Ward et al., 1994). When this intestinal tissue was
incubated with sulphide, the synthesis of cysteine was clearly demonstrated. The most
convincing demonstration of the functionality of the cysteine pathway in transgenic
mice, however, was obtained by a dietary study in which transgenic mice and appropriate control mice were placed on a synthetic diet which was supplemented with Na2S
but in which the sulphur amino acids cysteine and methionine were reduced in concentration to low levels. After 7 days on this diet, substantial hair loss and weight loss
was experienced by the control animals but the transgenic animals continued to grow
normally and did not lose any hair (Ward et al., 1994).
Fig. 21.1. The carbon pathway portion of the cysteine biosynthetic pathway in
Escherichia coli.
380
K.A. Ward
Fig. 21.2. The gene MTCEK1 which encodes the cysteine biosynthetic pathway shown in
Fig. 21.1.
While the pathway for cysteine biosynthesis appears to operate effectively in transgenic mice, it has not yet been shown to function in ruminants, despite being introduced into 28 different primary transgenic sheep. A summary of the results obtained in
our own laboratory and that of Rogers et al. (Sivaprasad et al., 1992; Bawden et al.,
1995) is provided in Ward et al. (1998). Briefly, these show that 28 transgenic sheep
with the genes encoding cysteine biosynthesis have been produced. These animals contain the bacterial coding sequences for the enzymes SAT and OAS, isolated either from
E. coli or from Salmonella typhimurium (Sivaprasad et al., 1992) and regulated by three
different eukaryotic promoters. While some low-level expression has been detected
(Bawden et al., 1995; Ward et al., 1998), no useful expression of the genes has been
obtained in any of these animals. In addition, the number of transgenic animals produced as a percentage of embryos microinjected is low compared with the efficiency
obtained for other genes (Ward et al., 1998). A possible explanation for these results is
that high levels of expression of a cysteine biosynthesis pathway in sheep embryos are
lethal so that the only transgenic animals obtained are those in which the genes have
been inserted in a region of the genome that prevents their expression or at best allows
only low levels of expression. Two suggestions for the way this might occur are that the
cellular level of acetyl-CoA becomes unacceptably depleted during early embryogenesis
(Ward et al., 1998) or that the level of cysteine rises to toxic levels (Bawden et al.,
1995). At present there is no convincing evidence to support either hypothesis and resolution of the alternatives will require further research. However, regardless of which
explanation is correct for the difficulties experienced in duplicating in sheep the functionality of the pathway that was obtained in transgenic mice, a promoter needs to be
found that regulates expression of the genes only to the rumen epithelium and which
381
also prevents such expression until after the birth of the animal. In this way, the pathway should be able to function effectively in sheep, assuming that it is not totally
incompatible with the intermediary metabolism of adult ruminants.
The research directed towards the introduction of a cysteine biosynthetic pathway
is a well-advanced example of the use of genetic engineering to modify the biochemical
capacity of an animal, but several other projects are in earlier stages of development. It
has been proposed to introduce to non-ruminants the pathways for the biosynthesis of
the amino acids threonine and lysine (Rees et al., 1990), since these need to be added
to cereal protein for maximal utilization. An interesting and valuable aspect of this
paper was the use of computer simulation to predict the flux of biosynthetic products
produced by the introduction of a novel pathway to an animal. The general concepts of
the cysteine biosynthesis project described above are equally applicable to threonine
and lysine, but the pathways are more complex and hence the number of genes that
need to be isolated, modified and transferred to the target animal increases substantially. However, it is now possible to construct genes encoding proteins with multiple
active sites, thus reducing the number of separate genes that need to be introduced to
the target animal (Robinson and Sauer, 1998). It is also now possible to utilize from the
encephalomyocarditis virus a DNA sequence called an internal ribosomal entry site
(IRES). This sequence, when interspersed between two coding sequences of a single
mRNA, allows the initiation of translation to occur both at the start of the mRNA and
also internally within the mRNA, resulting in the production of two separate polypeptides (Kim et al., 1992).
Another project which is in its early stages of development involves introducing to
ruminants a functional glyoxylate cycle (Byrne, 1990; Ward and Nancarrow, 1991)
which would allow such animals to synthesize glucose from the abundant supplies of
acetate they receive from the rumen. Ruminants are substantially less efficient in their
utilization of feedstuff compared with monogastric animals and one of the major
reasons for this is thought to be the lack of glucose available to them for direct absorption from the gut (Bergman, 1975; Van Soest, 1982). There are several tissues in ruminants that could conceivably benefit from the ability to synthesize glucose directly from
acetate. These include the mammary epithelium, because of the high carbohydrate content of milk, and the sheep wool follicle, which has an extremely active pentose phosphate pathway thought to be involved in the maintenance of the cellular redox
potential during keratin protein biosynthesis (Chapman and Ward, 1979).
A functional glyoxylate cycle requires the presence of two enzymes, isocitrate lyase
which cleaves isocitrate to succinate and glyoxylate, and malate synthase, which catalyses the fusion of glyoxylate and acetyl-CoA to form malate (Ward and Nancarrow,
1991). The aceA and aceB genes of E. coli encode these two enzymes and both genes
have been isolated, characterized and assembled into a fusion gene construct
MTAceAB1 (Byrne, 1990) which is similar in structure to that of the gene MTCEK1
encoding cysteine biosynthesis (Leish et al., 1993) (Fig. 21.2), except that the cysE and
cysK coding sequences have been replaced by the aceA and aceB coding sequences. This
DNA has been introduced into mammalian cells in culture and shown to be actively
transcribed and translated into the appropriate enzymes (Byrne, 1990; Ward and
Nancarrow, 1991), a valuable piece of information that demonstrates that the glyoxylate cycle can apparently co-exist with existing biochemistry in a mammalian cell without sequestering the new enzymes in specific organelles. The same gene has also been
K.A. Ward
382
inserted into transgenic mice and has been shown to be expressed in a variety of tissues
including the liver and small intestine (Saini et al., 1996). The level of expression in
these animals was not as high as that found for the gene MTCEK1 and the reason for
this is not yet known. It may be that more animals have to be produced in order to
generate one with a high level of expression. However, it is interesting that attempts so
far in our laboratory to insert the gene into sheep have been unsuccessful. It is conceivable that high expression of a glyoxylate cycle in an animal may not be well tolerated, in
which case the gene will need to be constructed with promoters that can be regulated
and which can direct expression to specific tissues such as the mammary epithelium,
the ruminal epithelium and the skin, where the increased production of glucose might
be advantageous. This research will benefit from the nuclear transfer technology as it
matures into a useful method for transgenic animal production, because the pathway
can be introduced and examined for functional integration within its new genome
prior to its transfer to animals.
383
areas, but no commercially useful animals have yet been produced. Thus, it has been
shown that it is possible to isolate portions of functionally-rearranged immunoglobulin
genes encoding the antibody recognition to a specific antigen and to introduce this
DNA to transgenic mice (Brinster et al., 1983; Ritchie et al., 1984; Storb et al., 1986).
These mice then produce antibodies to the bacterial antigen without ever being
exposed to it. The results to date have been encouraging but have yet to be extended to
domestic animals. One of the problems encountered in mice was that the expression of
the transgene had a suppressive effect on the production of other immunoglobulins,
thus potentially reducing the overall disease resistance of the transgenic animals.
Nevertheless, it is clearly an approach which needs further examination as a practical
method for altering disease resistance.
A quite different approach currently in progress within CSIRO in Australia is the
introduction to animals of a gene encoding a protein which has inherent diseaseresistance properties. This approach is being pursued in an attempt to increase the resistance of sheep to attack by blowfly larvae. In this project, the protein under
investigation is the tobacco chitinase, which has been shown to have significant larvicidal activity against blowfly larvae. It is the aim of this research to provide sheep with
the ability to produce the chitinase protein in their sweat gland secretions in quantities
sufficient to inhibit the larval attack when blowfly eggs hatch on the skin surface. At
present, various forms of the chitinase are being evaluated for larvicidal activity to identify the most active form needed to insert into transgenic animals. However, to date the
results obtained are not encouraging (Dr A.G. Brownlee, CSIRO Animal Production,
personal communication), since they suggest that animals will need to produce very
high levels of the chitinase in order to be protected from blowfly larvae. Nevertheless,
the overall concept underlying this research is sound and should be pursued further,
particularly since similar approaches have proven to be highly effective in the protection of plants from insect predators.
Conclusions
The modification of ruminant physiology and biochemistry by the use of transgenic
techniques offers a wide range of possibilities. While the research is proving difficult to
bring from laboratory to the practical farming environment, nevertheless, several projects appear to be making significant progress towards this goal. The most advanced are
those that involve the modification of growth and carcass and it is possible that genetically-modified pigs and sheep could be in field trial within the next few years. The
introduction to the farm environment of animals with modified biochemical properties
is still in the research phase and likely to remain at this point for some time yet. While
it is apparent that new pathways interacting with existing intermediary metabolism can
be introduced to animals, species-specific difficulties are clearly a factor in slowing the
progression of the work from the laboratory to the field environment. At present, the
introduction of the cysteine biosynthetic pathway to sheep appears to be the project
closest to application, but there remain several crucial areas of research yet to be completed before this can take place. The introduction of the glyoxylate cycle to ruminants
is much further from completion but holds the promise of a greater long-term impact
on animal agriculture.
K.A. Ward
384
Nuclear transfer and animal cloning are going to provide a major stimulus to the
application of transgenic techniques to domestic animals, because these techniques
allow the introduction and preliminary testing of DNA constructions in cells in culture
and the subsequent use of such cells to produce viable animals containing the recombinant DNA. It has been apparent for some years now that the ability to produce whole
organisms from cultured cells has been one of the major advantages held by those scientists working with plants compared with those working with animals, allowing the
genetic modification of plants to proceed very quickly in recent years.
Acknowledgements
I am grateful to my colleagues Drs Jim Murray, Colin Nancarrow, Alan Brownlee,
Zdenka Leish and Mr Bruce Brown for their help in the original research described in
this paper. I am also indebted to the expert technical assistance of Mrs Nola Rigby, Mrs
Cathy Pruss, Miss Tania Radziewic, Mr Alan Fawcett, Mr John OGrady, Mrs Jenny
Gordon, Mr Peter Mitchell and Mr Jim Marshall.
Some of the research described in this paper was supported by the International
Wool Secretariat.
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Page 389
Genetics of Rumen
Microorganisms: Gene Transfer,
Genetic Analysis and Strain
Manipulation
Introduction
Interest in the genetics of rumen microorganisms was first sparked by the prospect of
creating manipulated strains that might be used to improve rumen function (Smith
and Hespell, 1983). While some progress has been achieved towards this end, it is now
more widely recognized that molecular genetics has a vital role to play in understanding
the dynamics and diversity of rumen microbial communities, in understanding the
functioning of enzyme systems and in unravelling the evolution of rumen microorganisms. In addition natural horizontal gene transfer is a potentially important, but little
studied, factor in the adaptation and evolution of the rumen community and might
also be involved in disseminating antibiotic resistance genes, or possibly even transgenes derived from modified feed plants or microbial additives, to different gut
microorganisms. Furthermore, since the ruminant harbours human pathogens such as
Escherichia coli, the rumen and hindgut are potential sites for exchange of pathogenicity
determinants including toxin genes.
Because this chapter concentrates on transfer genetics it deals almost exclusively
with the bacterial flora of the rumen. Despite the absence of work on gene transfer in
rumen eukaryotes, there has been considerable effort in the isolation of genes from
anaerobic rumen fungi and more recently from anaerobic protozoa which is leading to
information on gene expression signals and codon usage in these organisms as well as
casting light upon their evolutionary origins. These areas are beyond the scope of the
present review, but have been discussed in several recent reviews and articles (Trinci et
al., 1994; Hespell et al., 1996; Orpin and Joblin, 1997).
389
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Page 390
390
Table 22.1. Distribution and location of tetracycline resistance determinants among TcR strains
of ruminal obligate anaerobes.
TetQ
Prevotella ruminicola
Butyrivibrio fibrisolvens
Selenomonas ruminantium
Mitsuokella multiacidus
a Pl,
TetO
TetW
Reference
Chr
Pl, Chr
Chr
Pl, Chra
Chr
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Page 391
391
100
0.1
99
100
100
95
92
47
Fig. 22.1. Unrooted phylogenetic tree showing the evolutionary relationships between
ribosome protection-type TcR proteins. The scale bar refers to amino acid substitutions
per position.
There are few reports of gene transfer events between rumen bacteria that involve
traits other than antibiotic resistances. Lactate utilization was apparently transferred
between S. ruminantium strains, although the mechanism of transfer has not been
established (Gilmour et al., 1996). It has also been suggested that amylolytic activity is
transferred between strains of rumen Lactobacillus spp. (Kmet et al., 1989).
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Phage can mediate gene transfer events through a variety of mechanisms including
specialized and generalized transduction and transfection. Potentially the most significant transfer events are those involving toxin genes, and the verocytotoxin genes found
in E. coli strains including serotype O157 are known to have been acquired through
bacteriophage-mediated transfer (Saunders et al., 1999). To what extent the diversity of
verotoxigenic coliform bacteria is enhanced by transfers occurring in the ruminant gut
is an unanswered, but important, question.
Plasmids
The incidence of plasmids varies greatly between different rumen species (Ogata et al.,
1996). Few strains of rumen Prevotella/Bacteroides and of Ruminococcus spp. appear to
harbour plasmids, and there are no reports of plasmids from Fibrobacter succinogenes
strains. However, a high proportion of Selenomonas and Butyrivibrio strains carry plasmids (e.g. Zhang et al., 1991). B. fibrisolvens 2221 carries a very large plasmid of 300
kb (Teather, 1982; Scott et al., 1997) and as many as four plasmids ranging up to 40 kb
in size are present in some Selenomonas strains (Fig. 22.2, Fliegorova et al., 1998). Traits
encoded on these larger plasmids might have a significant role in the ability of the
rumen population to adapt to environmental and dietary change. Among the smaller
plasmids, which are cryptic and of interest mainly as the basis for vectors for genetic
work, some appear likely to replicate via a single-stranded intermediate (e.g. pOM1;
Fig. 22.2. Electron micrograph of plasmids purified from the sheep isolate
Selenomonas ruminantium 5521cl. Three plasmids of 30 kb, 2.4 kb and 1.4 kb are
visible. (Courtesy of K. Fliegerova.)
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Hefford et al., 1997) while others show theta-mode replication (e.g. pRJF2; Kobayashi
et al., 1995).
Transposable elements
Chromosomal elements are important agents of antibiotic resistance transfer in Grampositive bacteria and in Bacteroides from gut environments other than the rumen (for
review see Salyers and Shoemaker, 1995). The mobile TcR chromosomal element
TnBut1230 identified recently in B. fibrisolvens 1.230 (Scott et al., 1997) is inferred to
be 4050 kb in size, with a preferred insertion site in the chromosome of B. fibrisolvens
2221. Since no evidence was found of hybridization with Tn916 or Tn5253 this
appears to be a novel chromosomal element. Tn916 can also transfer to B. fibrisolvens,
but appears unable to transfer out of this species (Hespell and Whitehead, 1991a,b)
while Tn1545 can transfer to Eubacterium cellulosolvens (Anderson et al., 1998).
Free DNA and natural transformation
Turnover of free DNA in the rumen is assumed to be very rapid, but it is not ruled out
that some fraction of released DNA is protected from degradation, e.g. by feed or soil
components, and brief survival may be sufficient to yield transformants (Mercer et al.,
1999). There is currently very little information on the natural transformability of
rumen bacteria, but the ability to take up and incorporate foreign DNA (competence)
is common in other groups of bacteria (Lorenz and Wackernagel, 1994). Recent work
shows that S. bovis JB1 is transformable using protocols developed for natural transformation of Streptococcus gordonii (Mercer et al, 1999). Transformation of rumen
microorganisms by DNA of non-microbial (e.g. plant) origin is expected to require
either the generation of a plasmid capable of replication in the bacterial host, or some
DNA homology with the bacterial chromosome, and would be predicted to occur only
under very special circumstances.
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Vector
Selectable
marker a
EmR
pBHerm
(ApR)
pRJF1
(Butyrivibrio
fibrisolvens)
OB156)/pUC118
pOM1
EmR (ApR)
(B. fibrisolvens
Bu49)/pUC19
pRJF2 (B.
EmR (ApR)
fibrisolvens)/
pUC18
pRRI12
EmR (ApR)
(Prevotella
ruminicola)/
pHG165
pRRI2
TcR (ApR)
(P. ruminicola)/
pBluescript
pSMerm1
pYK4
pRRI207
pRH3
pTCCOW
pB851
(Bacteroides)/
pBR 328
pVA838
pTRW10
pVA838
a Resistances
TcR
CmR(ApR)
EmR
(ApR,CmR)
EmR (ApR)
Hosts
Size (kb)
Other
characteristics
Reference
B. fibrisolvens,
Escherichia
coli
9.3
Beard et al.
(1995)
B. fibrisolvens,
E. coli
7.8
Hefford et al.
(1997)
B. fibrisolvens,
E. coli
7.9
Kobayashi et al.
(1995)
Bacteroides,
E. coli
11
Bacteroides,
E. coli
8.8
Bacteroides,
Prevotella
bryantii B14,
E. coli
G + bacteria,
E. coli
G + bacteria,
E. coli
13.3
9.2
7.1
Mobilizable by
pRK2013
Thomson et al.
(1992)
Nonmobilizable.
Multiple
cloning site
Mobilizable in
Bacteroides
Daniel et al.
(1995)
Mobilizable by
pVA797
Multiple
cloning site,
Mobilizable by
pVA797
Gardner et al.
(1996)
Macrina et al.
(1982)
Wykoff and
Whitehead
(1997)
shown in parenthesis allow selection in E. coli but not in the alternative host.
strains (Beard et al., 1995) perhaps in part because of differences in the production of
extracellular polysaccharides. B. fibrisolvens shuttle vectors have been created by fusing
the native B. fibrisolvens plasmids, pRJF1 and pRJF2, with E. coli plasmids (Ware et al.,
1992; Hefford et al., 1993; Beard et al., 1995; Kobayashi et al., 1995). A fluoroacetate
dehalogenase gene was introduced into such a shuttle vector (pBHerm; Fig. 22.3)
downstream of an erythromycin-resistance promoter and was electroporated into B.
fibrisolvens OB156. The resulting cultures were able to degrade the toxin fluoroacetate
(Gregg et al., 1994a). The modified strains were found to persist for at least 5 months
following introduction into the rumen of a sheep. When trial sheep were inoculated
with four modified B. fibrisolvens strains the sheep did not exhibit strong symptoms of
fluoroacetate poisoning provided the modified bacteria were allowed to establish for 5
weeks before challenging with fluorocetate (Gregg et al., 1998). It is significant that
these modified bacteria were able to establish and be maintained in the rumen in competition with the resident flora.
The same shuttle vector was also used to introduce a xylanase gene from
Neocallimastix patriciarum into B. fibrisolvens OB156 (Xue et al., 1997). In this case the
396
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Page 396
Fig.22. 3. Selected shuttle vectors available for the manipulation of rumen bacteria. pBHerm
Butyrivibrio fibrisolvens (Beard et al., 1995); pTC-COW BacteroidesPrevotella (Gardner et
al., 1996); pTRW10 Streptococcus bovis (Wykoff and Whitehead, 1997); pRH3
BacteroidesPrevotella (Daniel et al., 1995).
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The potential of the native B. fibrisolvens transposable elements (Scott et al., 1997),
which are capable of transfer between distantly related strains, is clearly worth exploring.
Ruminococcus spp.
Two plasmid vectors developed for use in other Gram-positive bacteria have been introduced into four different R. albus strains by electroporation (Cocconcelli et al., 1992)
and an efficiency of 3 3 105 transformants mg21 was achieved with one of the plasmids, pSC22. A low frequency of transfer of the broad host range plasmid pAMb1 was
also achieved into R. albus by conjugation from Bacillus thuringiensis BT351 (Aminov
et al., 1994). Despite these encouraging developments there have been no reports of
introduction of vectors into R. flavefaciens strains and no studies on gene inactivation
or the expression of genes introduced into Ruminococcus spp.
Prevotella/Bacteroides
No discussion of this group can ignore the extraordinary genetic diversity among
rumen strains revealed by 16S rDNA sequence analysis (Avgustin et al., 1997;
Whitford et al., 1998; Wood et al., 1998). Cultured rumen Prevotella isolates have been
reclassified into four species (Avgustin et al., 1997) and further subdivision may well be
justified. Thus it may not be easy to obtain vector contructs that are successful in all
strains. Electrotransformation of Prevotella bryantii B14 was demonstrated with the
native plasmid pRRI4, which carries a tet(Q) marker, using plasmid DNA extracted
from the same strain background (Thomson and Flint, 1989). Transformation of the
same strain by vectors that carry the tet(Q) marker, using DNA derived from E. coli
has, however, been unsuccessful (Shoemaker et al., 1991; Thomson et al., 1992) probably due to restriction barriers. Introduction of vector constructs into P. bryantii B14 has
been achieved by a conjugation procedure that relies on the ability of certain conjugative transposons present in Bacteroides spp. to mobilize Bacteroides plasmids. By this
means the vector pRDB5, which carries the Bacteroides plasmid replicon pB851, has
been transferred into P. bryantii from Bacteroides uniformis (Shoemaker et al., 1991),
and another vector pTC-COW is now also available that carries a second selectable
marker, CmR, in addition to tet(Q) (Fig. 22.3; Gardner et al., 1996). pTC-COW has
been used to transfer a hybrid P. ruminicola CMCase gene into P. bryantii B14, but
expression was not observed, possibly because the P. ruminicola promoter used failed to
express in P. bryantii B14 (Gardner et al., 1996).
Naturally occurring plasmids have been reported in a few Prevotella strains (Flint
and Stewart, 1987; Ogata et al., 1996). pRRI2, a 3.4 kb plasmid from P. ruminicola
223/M2/7, has been used as the basis for the vectors pRRI207 (EmR marker) and
pRH3 (tet(Q) marker; Fig. 22.3) which replicate in a range of Bacteroides strains and in
the Prevotella/Bacteroides strain 2202 (Thomson et al., 1992; Daniel et al., 1995).
pRH3 has been used to express a cellulase/xylanase gene from P. ruminicola 23 and two
xylanase genes (xynA and xynB) from P. bryantii B14 in Bacteroides vulgatus and in
Bacteroides/Prevotella strain 2202. Interestingly, the P. bryantii xylanase genes were not
expressed in Bacteroides hosts when introduced on the plasmid pTC-COW (T.R.
Whitehead and H.J. Flint, unpublished observations). This may indicate that a fortuitous plasmid promoter is driving their expression in the pRH3 construct.
Some work has also been done on the exploitation of Prevotella bacteriophage
genes in construction of vectors for chromosomal integration. Following identification
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of the excisionase and integrase genes from the P. ruminicola bacteriophage AR29
(Gregg et al., 1994b), these genes were cloned into a P. ruminicola shuttle vector which
was electroporated into a different P. ruminicola host strain, AR20, possibly resulting in
integration into the chromosome (reviewed in Vercoe and White, 1996).
F. succinogenes
There are no reports of indigenous plasmids or of successful attempts at gene transfer
into this important cellulolytic species.
S. bovis
S. bovis offers the advantages of being aerotolerant and of being able to support replication of many plasmids used in related Gram-positive bacteria. Hespell and Whitehead
(1991b) were able to transfer the transposon Tn916 and the plasmid pAMb1 into S.
bovis JB1 by conjugation from E. faecalis. The vector plasmid pVA838 (9.2 kb) has
been used to express a cellulase (endA) and a bifunctional xylanase/ b(1,31,4) glucanase (xynD) from R. flavefaciens following electroporation into S. bovis (Whitehead,
1992; Whitehead and Flint, 1995; Ekinci, 1997). An improved version of pVA838
(pTRW10; Fig. 22.3) (Wykoff and Whitehead, 1997) was also used to express a S.
bovis b(1,31,4)-glucanase (Ekinci et al., 1997) and the green fluorescent protein from
the jellyfish Aequoria victoria (Scott et al., 1998) in Gram-positive bacteria including S.
bovis. Another strategy is to fuse pIL253, a high copy number vector derived from
pAMb1, with pUC18 constructs to create shuttle constructs capable of replication in
E. coli and S. bovis (Ekinci et al., 1997). S. bovis JB1 produces a native secreted
b(1,31,4)-glucanase whose gene has been isolated and sequenced (Ekinci et al., 1997)
allowing the construction of translational fusions in which the N terminal regions of
the secreted enzyme drive expression of foreign gene products in S. bovis (M.S. Ekinci
et al., unpublished results).
Suicide constructs have also been used to inactivate an intracellular a-amylase in S.
bovis helping to elucidate the role of this enzyme (Brooker et al., 1995; Brooker and
McCarthy, 1997).
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appears to be a problem in some species, notably Ruminococcus spp. where the recovery
of full-length cellulase genes may have been limited (Kirby et al., 1997). Among rumen
eukaryotes, both the protozoa and the fungi show exceptionally low G+C contents and
there are early indications that genes from ruminal protozoa show highly biased codon
usage (Eschenlauer et al., 1998). Such differences in codon usage and promoter recognition can complicate heterologous expression of cloned genes.
Genomic analysis
The uncertain recovery of genes by activity screening, and the relative inefficiency of
piecemeal sequencing, argue that genome sequencing of some of the major ruminal
bacteria will be worthwhile. The first to be considered are probably cellulolytic bacteria,
where it is likely that only a small proportion of the significant genes required for fibre
breakdown have so far been identified. F. succinogenes belongs to a little-studied group
of bacteria and has a relatively small genome size (3.5 mega base pairs) making it an
attractive proposition (Aminov, 1998). Genome sequencing would ultimately provide
the most definitive information on gene flow between rumen organisms.
Biotechnology
Exploitation of genes isolated from ruminal microorganisms
The most obvious category of exploitable genes from rumen organisms are those
responsible for the rapid breakdown of plant cell wall material, which have potential in
animal feed pretreatment, paper pulp treatment, food processing and textile manufacture. The rumen fungi in particular have yielded cloned xylanases whose specific activities are higher than for any other xylanase available (reviewed by Selinger et al., 1996).
Enzymes of comparable activity have not so far been reported from rumen bacteria, but
relatively few gene products have been purified to date. Although their catalytic
domains generally show close similarities with those of non-rumen microorganisms,
some rumen polysaccharidases show unique organization (e.g. Flint et al., 1997). In the
case of cellulases, recent evidence points to the importance of enzyme complexes both
in rumen fungi and in Ruminococcus spp. (Fanutti et al., 1995; Flint, 1997; Kirby et al.,
1997) and the organization of multiple enzyme subunits is likely to be critical for maximum activity. Nevertheless it has been shown that the activity of an endoglucanase
from P. bryantii against crystalline cellulose can be enhanced by fusion to a cellulosebinding domain from the non-rumen bacterium Thermomonospora fusca (Maglione et
al., 1992). Exploitation of ruminal polysaccharidase genes has resulted in their expression in an increasingly wide range of bacterial, fungal and plant hosts, and in cultured
mammalian cells as illustrated in Table 22.3.
There are clearly potential applications for many other genes from rumen organisms ranging from proteinases and lipases to restriction endonucleases and methylases
(Selinger et al., 1996). Bacteriocins have also attracted recent interest. These small proteinaceous antibiotics may play an important role in competition between bacteria for
particular niches, and may have potential for exploitation as inhibitors of undesirable
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Table 22.3. Exploitation of polysaccharidase genes from rumen microorganisms heterologous expression
systems.
Expression
host
Gene product
Species of origin
Reference
Escherichia coli
Streptococcus
bovis
Endoglucanase
Ruminococcus
flavefaciens
Bacteroides
vulgatus
Xylanase
Endoglucanase/xylanase
R. flavefaciens
Prevotella ruminicola
Ekinci (1997)
Daniel et al. (1995)
Butyrivibrio
fibrisolvens
Xylanase
Xylanase
Neocallimastix patriciarum
Eubacterium ruminantium
Lactococcus
lactis
b(1,31,4)glucanase
Streptococcus bovis
Enterococcus
faecalis
b(1,31,4)glucanase
S. bovis
Saccharomyces
cerevisiae
Endoglucanase
B. fibrisolvens
Cellodextrinase
R. flavefaciens
Tobacco
Xylanase
b(13,14)glucanase
Xylanase
Cellulase
R. flavefaciens
R. flavefaciens
Ruminococcus albus
R. albus
Brassica
Xylanase
N. patriciarum
Chinese hamster
ovary cells
Xylanase
Fibrobacter
succinogenes S85
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acquire transgenes from the diet, perhaps leading to further transfer of these genes to
transient members of the rumen flora that can inhabit the digestive tracts of other animals and of man. More basic information on ruminal gene transfer will help to estimate the probability (or improbability) of such events.
Conclusions
The past decade has seen the isolation of more than 100 genes, mainly encoding polysaccharidases, from ruminal bacteria and fungi. By contrast, progress in studies on gene
transfer has been relatively slow, although significant progress has been made with the
development of vectors and the identification of plasmids, transposable elements, and
bacteriophage. Gene transfer and insertional inactivation remain the key to analysing
gene function and regulation in important ruminal microorganisms, without which the
rewards of developments in rapid sequencing, including genome sequencing, will not
be realized.
Acknowledgement
The authors wish to acknowledge the support of the Scottish Executive Rural Affairs
Department and the Ministry of Agriculture, Fisheries and Food (UK).
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23
NutrientGene Interactions:
Future Potential and Applications
P.B. CRONJ
Department of Animal and Wildlife Sciences, University of Pretoria,
Pretoria, South Africa
Introduction
It is commonly accepted that the aim of the discipline of animal science is to increase
the efficiency of human edible protein production. As a result, past research has been
directed at either improving input efficiencies (i.e. feed characterization) or output efficiencies (i.e. breed characterization and genetic selection). Despite these efforts, the
accuracy with which current systems can predict individual and breed-related responses
to differing nutritional inputs is still not adequate for practical purposes. The reason for
this inability to predict nutritiongenotype interactions becomes evident when research
priorities are considered within the context of the perceived primary limiting factors in
different farming systems.
409
410
P.B. Cronj
NutrientGene Interactions
411
P.B. Cronj
412
Increased
sensitivity
Fig. 23.1. Sigmoidal response curve described by the equation: Production response =
Vmax / (1 + (Km / Nutrient supply)z). The dotted line indicates the effect of a decrease in
the value assigned to Km.
Genotypenutrition interactions
In the wild, evolutionary genetic changes in animals are profoundly influenced by the
constancy of nutritive resources. In environments where there is relatively little seasonal
change in nutrient availability, there is little need for expression of mechanisms that
facilitate adaptive flexibility (adaptability) to changes in the plane of nutrition. Under
these circumstances selection pressure tends to promote the development of specialized
mechanisms that result in maximum utilization of a specific resource (adaptation). The
fact that many of the worlds endangered species are found in environments where
nutrient supply is relatively constant (e.g. tropical rain forests) suggests that specialization (adaptation) has occurred at the expense of the capacity to adapt (adaptability). In
terms of the nature of response functions as discussed above, this would suggest an
increase in responsiveness and in sensitivity of regulatory mechanisms to a specific level
NutrientGene Interactions
413
Increased
responsiveness
Fig. 23.2. Sigmoidal response curve described by the equation: Production response =
Vmax / (1 +(Km / Nutrient supply)z). The dotted line indicates the effect of an increase in
the value assigned to Vmax.
of nutrition (Fig. 23.3). On the other hand, animals subject to large seasonal and periodic (drought-induced) fluctuations in the availability of nutrients are far more likely
to evolve towards optimum utilization of the long-term mean nutrient supply, and
selection pressure would favour adaptability at the expense of specialization.
Adaptation to the long-term mean nutrient supply would increase nutrient partitioning
to endogenous reserves as a precautionary measure against potential future decreases in
exogenous nutrient supply, and result in a decrease in sensitivity and responsiveness to
plane of nutrition (Fig. 23.3) for non-essential productive functions. Although these
singular and integrated genotypenutrition interactions are evident throughout biology, it is conspicuous that little information exists as to whether genetic selection for
high production in domestic species has changed sensitivity to level of nutrition.
P.B. Cronj
414
Increased
responsiveness
and
increased
sensitivity
The Angora goat is a good model of a breed that has been subject to intense singletrait genetic selection for fibre production. Although the rate of mohair production in
this breed by far surpasses that of sheep selected for high wool-growth rates, it exhibits
a high rate of abortions and a low survival rate under conditions of cold-stress.
Comparisons between Angora goats and goats selected for meat production (Cronj,
1992a,b) and between Angora phenotypes that differ in respect of fibre production
rates (Cronj, 1995) have established that the primary cause of the high incidence of
abortions and cold-stress fatalities in animals selected for high fibre-growth rates is an
inability to maintain blood glucose concentrations when the plane of nutrition is
decreased. It would appear that Angora goats selected for high levels of mohair production preferentially deposit a greater proportion of ingested nitrogen as fibre, resulting in
insufficient labile protein reserves and decreased substrate availability for gluconeogenesis when the plane of nutrition is decreased (Cronj, 1998). This evidence suggests that
in this instance, the effects of genetic selection have been to decrease the ability of the
animal to adapt to changes in plane of nutrition, and indicates that this was probably
mediated through increased sensitivity as well as increased responsiveness of fibre production to plane of nutrition (Fig. 23.3). Evidence exists that a similar situation prevails in Merino sheep selected for high wool-production rates (Cronj and Smuts,
1994; Herselman et al., 1998).
Phenotypic and genetic correlations between milk yield and gross feed efficiency
are often high (Veerkamp and Emmans, 1995), suggesting that selection for milk yield
automatically increases the partial efficiency of conversion of nutrients to milk.
However, several reviews have shown that there is little evidence to indicate that genetic
variation exists with respect to the partial efficiency of conversion of absorbed nutrients
NutrientGene Interactions
415
to milk (Blake and Custodio, 1984; Bauman et al., 1985; Veerkamp and Emmans,
1995). Blake and Custodio (1984) concluded that feed efficiency is an artefact of the
selection response to milk yield. Although large genetic differences have been observed
for gross efficiency, it would appear that high genetic merit animals are more efficient
because they partition the available energy differently from low genetic merit cows, and
not because the processes used to transform consumed feed into product have become
more efficient(Veerkamp and Emmans, 1995). Because total energy expenditure during early lactation exceeds intake capacity in high-producing dairy cows, these genetic
differences in nutrient partitioning are manifest as increased subsidy of nutrient
requirements for milk production by tissue reserves (Blake and Custodio, 1984). More
specifically, selection for milk yield is considered to increase the cows ability to mobilize adipose tissue reserves in early lactation and to replace them in late lactation
(Bauman et al., 1985). The fact that there is evidence to suggest that continued selection pressure for milk yield has caused a decrease in reproduction rates in dairy breeds
(Muller et al., 1999) indicates that genetic selection has had the effect of increasing sensitivity as well as increasing responsiveness of milk production to plane of nutrition.
The fact that reproduction rates of high-producing genotypes typically decrease to
levels lower than that of existing low-producing indigenous animals when introduced
into developing areas (Zarate, 1996) corroborates this.
In summary, there is evidence to indicate that genetic selection for production
rates under conditions where nutrition is not a limiting factor has increased the sensitivity of production response mechanisms and resulted in a decreased ability to adapt to
variations in the plane of nutrition. As lactation is the single most energetically
demanding physiological phase in the female reproductive cycle, misdirection of
resources between milk and endogenous reserves will typically impact negatively on
reproductive rates. The principal pathway sustaining responses to selection for milk
yield appears to be via an endocrine-controlled accession of body tissue reserves (Blake
and Custodio, 1984). It is evident that a better understanding of the genetic and nutritional basis of hormonal regulation of nutrient partitioning will be a key factor if the
accuracy of prediction of feed efficiency is to be improved.
416
P.B. Cronj
this way, the availability of circulating glucose for uptake by tissues such as the mammary gland in which glucose uptake is determined by the concentration gradient (and
not by insulin) is increased. Because of the effect of insulin on intracellular enzyme
activity, decreased insulin sensitivity also decreases the rate of triglyceride synthesis
from glucose metabolism and accelerates the rate of fat breakdown in adipose cells.
Recent work in our laboratory (P.B. Cronj, E. Vlok and M. de Jager, unpublished
results) suggests that the insulin sensitivity may play a key role in regulating genotyperelated differences in nutrient partitioning. Responses to plane of nutrition in lactating
Indigenous goats were compared with those in SaanenIndigenous cross-breed goats.
The Saanen cross-breed produced more milk than the Indigenous genotype at the same
plane of nutrition. The consequence of this difference in nutrient partitioning is
reflected in the fact that the Saanen cross-breed lost 19% of initial body mass by week
10 of lactation, while the Indigenous goat regained initial body mass by week 6 of lactation (Fig. 23.4). Plasma glucose concentrations were depressed to a lesser extent by
insulin in the Saanen cross-breed than in the Indigenous goat (Fig. 23.5). This indicates that skeletal muscle and adipose tissues in the Saanen cross-breed genotype were
less sensitive to insulin than in the Indigenous genotype and represents a possible
mechanism whereby genotype-related differences in nutrient partitioning are mediated.
Differences in insulin receptor concentrations have recently been observed in different
breeds of sheep (Wylie et al., 1998), and it has been suggested that variations in
GLUT4 gene expression may be the major determinant of insulin sensitivity in
humans (Charron et al., 1999). This evidence suggests that genetic increases in feed
efficiency or milk yield may, in part, be mediated by decreased insulin sensitivity.
Growth hormone or bovine somatotropin (ST) has been shown to change nutrient
partitioning in dairy cattle (Bauman and Vernon, 1993) and thus also represents a
likely candidate for regulation of genotype-related differences in nutrient partitioning.
Fig. 23.4. Mean liveweight of Indigenous () and Saanen cross-breed (j) goats during
lactation.
NutrientGene Interactions
417
The metabolic effects of ST administration in dairy cattle include reduced insulin sensitivity (Rose et al., 1996), reduced expression of GLUT4 mRNA in adipose and muscle tissue (Zhao et al., 1996) and increased mobilization of adipose tissue reserves
(Etherton and Bauman, 1998). It also probable that many of the metabolic effects of
ST are as a result of ST-induced secretion of insulin-like growth factor-I (IGF-I) from
the liver. IGF-I is mitogenic, and has been implicated in preventing programmed cell
death in mammary cells (apoptosis) and may therefore play an important role in lactation persistency (Cohick, 1998). Genetic increases in milk yield may be mediated by
differences in the STIGF hormonal axis, as Gallo et al. (1997) have shown that
Holstein cows of low estimated breeding value respond differently to ST than those of
high breeding value. There is also evidence that ST concentration and ST responses to
challenges are associated with genetic merit for milk yield in male and juvenile cattle
(Woolliams and Lovendahl, 1991).
Leptin is a hormone that is secreted by adipocytes, and is thought to function as a
sensor and regulator of body energy stores. Leptin is known to regulate insulin secretion and affect insulin sensitivity, and it has been suggested that leptin functions to
orchestrate the complex array of signals which regulate nutrient partitioning
(Houseknecht and Portocarrero, 1998). Furthermore, leptin is known to influence
appetite and also modulate ST secretion, and could represent a link between adipose
tissue reserves, voluntary intake and milk yield. This may explain why selection for
milk yield and milk components has been reported to influence voluntary intake at various stages of lactation in dairy cows (Veerkamp et al., 1994; Akerlind et al., 1999). In
humans, leptin concentrations have been found to have a heritability (h2) of 0.39
P.B. Cronj
418
(Rotimi et al., 1997). Although data on leptin in domestic animals are only now starting to emerge, the fact that plasma leptin concentrations in sheep are highly variable
and repeatable (Chilliard et al., 1998) suggests that leptin expression may be related to
genetic differences in nutrient partitioning.
The above discussion has emphasized some of the most important mechanisms
regulating nutrient partitioning during lactation. Although differences in concentrations of hormones and their receptors may explain why differences in responsiveness
exist between different genotypes at the same plane of nutrition, the postulated existence of differences in sensitivity implies that differences exist with respect to the way
in which endocrine factors respond to changes in nutritional stimuli. Genetic differences in milk yield may be a function not only of differences in genes coding for these
endocrine factors but also differences in factors which regulate gene expression.
Nutrientgene interactions
Genes determine the maximum possible rates of formation of gene products but, in
most if not all situations, these maximum rates will not be attained. Observed
responses to genetic selection are therefore most probably not a function of the animals
genes per se, but of genetic variation in the extent to which genes are expressed. There is
increasing evidence to show that post-transcriptional regulation of RNA expression by
nutrients in the cytoplasm of the cell exerts a major influence over the expression of
many genes (Hesketh et al., 1998). The total DNA in the genome of cattle consists of
approximately 3,000,000,000 base pairs. Of this, less than 5% consists of functional
genes. In early studies, it was thought that the nucleotide sequences in these and the
untranslated regions (UTR) which flank functional RNA sequences were redundant
and of no importance, but it is now becoming evident that UTR are highly important
for nutrient-mediated regulation of gene expression. Hesketh et al. (1998) have proposed that an understanding of the interaction between nutrients and gene expression
will provide a basis for determining nutritional requirements of humans on an individual basis. In the present context, this may represent the key to understanding why
nutrient partitioning varies between individual animals and genotypes, and so enable
the accuracy of prediction of inputoutput responses to be improved.
One of the best known physiological responses to selection for milk yield is known
to be a complex genetic mechanism for maximizing the amount and availability of adipose tissue during early lactation (Bauman et al., 1985). Several studies have also established that the activity of lipogenic enzymes varies between genotypes (see Chapter 20).
Differences in the sensitivity of adipose tissue enzyme responses to nutritional supply
could be mediated via differences in endocrine responses to nutrients or by differences
in the direct effects of nutrients on enzyme gene-expression.
Polymorphisms (variations in DNA sequences) exist for most of the genes coding
for endocrine factors involved with lactation. These differences are functionally significant, as polymorphisms for the genes coding for growth hormone releasing hormone,
growth hormone, the growth hormone receptor and Pit-1, a pituitary specific transcription factor controlling the expression of ST, have been associated with milk traits in
dairy cattle (for review see Parmentier et al., 1999). Although it is not yet known
exactly how these small differences in the DNA sequence of genes coding for the same
NutrientGene Interactions
419
hormone cause differences in milk yield between individuals and breeds, it is possible
that they may differ with respect to sensitivity to transcription factors such as nutrients.
Differential sensitivities of polymorphisms to nutrition have been demonstrated for
IGF-I and ST (Duan, 1998; Gootwine et al., 1998), and may represent a possible
explanation of why genotypes differ in their sensitivity to variations in nutritional
supply.
Differences in sensitivity to nutrition could also be mediated by nutrientgene
interactions at a more direct level. Recent studies (Clarke and Abraham, 1992; Towle,
1995; Girard et al., 1997) have shown that a metabolite of glucose, probably glucose-6phosphate, directly regulates the expression of the rate-limiting enzymes for lipogenesis,
acetyl-CoA carboxylase and fatty acid synthase, as well as other key enzymes such as
phosphofructokinase and pyruvate kinase. Expression of fatty acid synthase is also regulated by poly-unsaturated fatty acids (Clarke and Abraham, 1992), and long-chain fatty
acids have been shown to regulate gene expression of carnitine palmitoyl transferase
and hydroxy-methyl-glutaryl-CoA synthase, both of which are critical for lipid metabolism. In addition to absorbed nutrients, other dietary components such as the fat-soluble vitamins may play a role in the direct regulation of adipose tissue metabolism, as
retinoic acid has been shown to regulate the expression of glycerol-P-dehydrogenase
(Clarke and Abraham, 1992). Furthermore, the predominant control of GLUT-4,
which is critical for the intracellular supply of glucose for metabolism by these
enzymes, is now thought to be linked to the intracellular metabolism of glucose
(Charron et al., 1999).
Whereas past research dealing with homeostasis has been concentrated on
endocrine regulation of metabolism, the significance of these recently discovered nutrientgene interactions is such that Girard et al. (1997) have suggested that insulin only
has a potentiating role in adipose tissue, and Charron et al. (1999) have suggested that
GLUT-4 gene expression is consistent with metabolic rather then hormonal regulation.
Clearly, this invokes the question of whether a paradigm shift is not called for in the
field of domestic animal physiology research: a shift in emphasis towards individual
nutrient concentrations and metabolism would certainly be complementary to the
individual nutrient approach which has been recommended for future ruminant nutrition research by the BBSRC (1998). In the context of nutritiongenotype interactions,
it is not unlikely that the differences in inputoutput response sensitivity which appear
to be induced by genetic selection could be directly related to the sensitivity of different
polymorphisms to regulation of gene-expression by individual nutrients. Although the
exact location and associations of UTR with functional genes is still unclear, the finding that polymorphisms in the UTR of mRNA influence susceptibility to hyperlipidaemia in humans (see Hesketh et al., 1998) indicates that markers for sensitivity,
or variations in nutrientgene interaction, can be developed.
Conclusions
In the past, research programmes aimed at increasing the efficiency of animal production systems have tended to concentrate on either breed improvement or feed evaluation. Unfortunately, neither of these approaches has proved to be of much value for
predicting animal responses to changes in nutrition. It is evident that more information
P.B. Cronj
420
on the nature and extent of interactions between genotype and nutrition is required in
order to enable accurate prediction of animal responses to be made. There is a substantial body of evidence indicating that different genotypes respond differently to changes
in nutrition. One of the most promising recent developments in this regard is increasing evidence of regulation of gene expression by individual nutrients, and it is proposed
that differences in the sensitivity of gene-regulatory mechanisms to nutrients may
underlie variation of responses between individuals and genotypes. As gene expression
is, in many instances, a function of nutrient concentrations, an integrated approach
encompassing the disciplines of physiology, molecular genetics and nutrition is called
for if we are to increase the accuracy of response prediction in dairy cattle.
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24
Host Resistance to
Gastrointestinal Parasites
of Sheep
Introduction
Sustainable parasite control requires an understanding of the mechanisms involved in
the immunological resistance, and of how such mechanisms can be induced.
We will concentrate here on those nematodes whose parasitic stage is solely
restricted to the gastrointestinal tract. Relatively isolated from the systemic immune
system, this presents a particular set of problems to the host immune system and to
livestock managers. Some of the aspects of the acquisition of immunity and mechanisms of resistance to these species may also operate in infections with parasites such as
lungworm (Dictyocaulus) and tapeworms (Taenia) which have a gastrointestinal stage to
their life cycle.
The most common pathogenic nematode species of the ruminant gastrointestinal
tract are Teladorsagia (Ostertagia), Haemonchus (H) (abomasum), Trichostrongylus (T)
(abomasum and small intestine), Nematodirus, Cooperia (small intestine) and
Oesophagostomum (large intestine). Some occur and are equally pathogenic in more
than one ruminant species, while others are host specific. These parasites have a direct
life cycle consisting of free-living stages on pasture (egg to infective larvae, L3) and,
after ingestion, parasitic stages (L4 to adult) in the host gastrointestinal tract. They do
not have a tissue migratory phase. There are, however, some differences in their relationship with the host gut tissue, and therefore with the host immune system.
Haemonchus attaches to the mucosa of the abomasum and sucks blood. Ostertagia and
Oesophagostomum larvae penetrate into the abomasal glands or colonic mucosa before
emerging and residing on the mucosal surface. Trichostrongylus live in mucus-covered
tunnels eroded on the surface of intestinal villi or abomasal folds.
425
426
427
Cellular responses
Observations of infections with various worm species suggest that in immune sheep
which have not mounted a rapid rejection of incoming larvae, worm challenge
induces a rapid increase (35 days after challenge) in lymphoid cell availability to the
gut (especially those lymphocytes expressing the gamma delta T-cell receptor, granulocytes and antigen-presenting dendritic cells) and rapid sequestration and activation of
memory cells in lamina propria (Dawkins et al., 1989; Buddle et al., 1992; McClure et
al., 1992; Bendixsen et al., 1995; Pfeffer et al., 1996; S.J. McClure, unpublished observations). This appears to be followed by increased activation of lymphocytes in the
draining node and return of memory cells, initially activated and later (> 7 days) resting, to the blood (Adams and Cripps, 1977; Haig et al., 1989; Emery et al., 1991; S.J.
McClure, unpublished observations).
There are also changes in the local nerves. The autonomic innervation of the gastrointestinal tract is complex and extensive, consisting of both extrinsic and intrinsic
nerves. The density of intrinsic nerve cell bodies within the gut is very high, with numbers comparable to the total number of neurons within the spinal cord. Sheep immune
to Trichostrongylus colubriformis showed increases in the number and metabolic activity
of enteric nerve fibres following challenge infection, suggesting that the nervous system
can be primed by exposure to nematodes in a similar manner to the priming of the
immune system (Stewart et al., 1995b). In addition, immune and nervous systems can
synthesize and respond to shared chemical mediators. Thus it is probable that the local
and central nervous systems have a role in integrating the anamnestic immunological,
muscular and physiological changes that follow worm challenge into a coordinated and
flexible protective response.
It is not yet clear which of the many associated cellular responses are protective,
and some have been depleted in vivo in an attempt to further define the protective
mechanism. Depletion of CD4+ helper T-cells during challenge of sheep or goats
immunized by viable infection or non-viable vaccines impaired the rejection of
Haemonchus contortus, suggesting a role for CD4+ cells in protection (Gill et al., 1993;
Karanu et al., 1997). Depletion of CD8+ or WC1/Tcrgd+ T-cells during induction of
immunity to T. colubriformis both resulted in enhanced rejection of worms, suggesting
that these cells may be involved in the slowness of normal induction (McClure et al.,
1995). The conventional method for reducing resistance to worms is administration of
glucocorticoids, but a range of cellular responses is affected, and a specific mechanism
has not been identified.
Humoral responses
Systemic
Protective immunity in sheep against abomasal and intestinal worm species is associated with early increases in worm-specific antibody and IgA concentrations in local
efferent lymph (Smith et al., 1984, 1985; S.J. McClure, unpublished observations),
and with elevated levels of all isotypes of antibody in serum (McClure et al., 1992;
Pfeffer et al., 1996; Shaw et al., 1998). However, with the possible exception of IgA
antibody in Ostertagia infection (Stear et al., 1996), serum antibody level in sheep
428
Local
Immune sheep after challenge have increased concentrations of worm-specific antibody
in gut tissue and mucus, with all isotypes represented in the increase, and an increase in
the number of cells with surface-bound IgE (McClure et al., 1992; Pfeffer et al., 1996).
There are suggestions that IgG1 and IgE correlate best with protection against
Trichostrongylus species, but more studies are required to confirm this.
Neuropeptides
The role of neuropeptides in immunity to worms has not been investigated in vivo.
However, the neuropeptides employed by the enteric nervous system (Substance P,
somatostatin, vaso-intestinal peptide and b-endorphin) rendered mucosal mast cells
more sensitive to limiting concentrations of worm antigens in vitro. These peptides also
enhanced the in vitro proliferation to worm antigen of lymphocytes from mesenteric
lymph node and prefemoral efferent lymph of immune sheep but did not affect specific
antibody production by these cells (Stewart et al., 1995a, 1996).
Inflammatory mediators
Local tissue concentrations and secretion into the intestine of mediators such as leucotrienes, 5-hydroxytryptamine and histamine increase during immune rejection of T.
colubriformis (Steel et al., 1990; Jones et al., 1990). The administration of glucocorticoids immediately before challenge abrogates immunity to H. contortus and T. colubriformis; however, injection of more specific antagonists of leucotrienes, histamine,
platelet-activating factor or phosphodiesterases failed to affect worm rejection (Adams,
1988; Jackson et al., 1988; Emery and McClure, 1995).
Non-specific mechanisms
A number of local mechanisms important in the rejection of gut parasites are non-specific in effect but immunologically specific in induction. They are thus adaptive
responses, developing after exposure to the antigens, and possibly only after exposure to
the viable parasite. They may partly explain the observation that, to date, non-viable
vaccines are less protective against browsing worms than are viable infection or irradiated larval vaccines.
429
Mucus
Mucus per se has been proposed to have a protective role in trapping incoming worms
and preventing their establishment (reviewed by Rothwell, 1989). Protective immunity
to T. colubriformis in Merinos is associated with increased numbers of goblet cells and
increased quantities of mucus in the jejunum (R.G. Windon, personal communication). The quality of the mucus also changes in sheep immune to gastrointestinal
nematodes, with altered composition of the muco-polysaccharides and -proteins, the
leucotriene content (Douch et al., 1983; Jones et al., 1990, 1994), and the content of
various as yet unidentified molecules with inhibitory effects on nematodes. Mucus also
stabilizes and prolongs the biological activity of inflammatory mediators (W.O. Jones,
unpublished observations). The induction of these mechanisms is not well understood.
Peristalsis
Immunity in sheep to gut worms appears to be associated with increased enteric nerve
fibre number and metabolic activity (Stewart et al., 1995b), and with hypertrophy and
hyper-contractility of local smooth muscle (Tremain and Emery, 1994; S.J. McClure,
unpublished observations). In rodents, gut smooth muscle function is subject to modulation by the immune system, with T lymphocytes, particularly those expressing CD4,
implicated in the alteration of smooth muscle contractility seen by 6 days after infection with Trichinella spiralis (Vallance and Collins, 1998). These adaptive changes in
gastrointestinal motility persist for some time after exposure to worms, and in addition
to hustling incoming larvae, may have wider and more long-term repercussions in the
physiology and function of the gastrointestinal tract.
Epithelial sloughing and proliferation
If immediate rejection of incoming larvae does not occur, expulsion of T. colubriformis
from immune sheep is associated with transient loss of the jejunal epithelium at 45
days after intra-duodenal infection, effectively dislodging the larvae, which do not
embed in the sub-epithelial mucosa (McClure et al., 1992). The epithelium is repaired
within approximately 24 h.
Fluid and electrolyte movement into lumen
Mediators such as mast cell protease disrupt cell junctions, increasing mucosal permeability (Miller, 1996). This both increases the fluid content of the lumen and allows
access of potentially protective host molecules to the worm.
Thus the host mounts a wide range of immune and inflammatory responses to a challenge infection with worms, and probably no one response is essential or sufficient on
its own for protection.
430
431
432
30,000
Day 22
Day 15
Worm count
25,000
L3
Ad
L3
Ad
20,000
15,000
10,000
5,000
0
Fig. 24.1. A total of 32 sheep were fed a normal ration (2, 16 sheep) or a ration in
which navy beans (10% wt/wt) provided 50% of dietary protein (+, 16 sheep). Eight
sheep from each group were infected with 20,000 Trichostrongylus colubriformis L3
21 days prior to initiation of feeding, to examine effects on established adult worms
(Ad), and the remaining sheep were infected with 20,000 T. colubriformis L3
coincidentally with the diet. Four sheep from each of the four treatment groups were
killed for worm counts after 15 and 22 days of the two rations.
Although the protective effects of these relatively diverse treatments could act
directly or indirectly on the gut immune responses of the host, the exact mechanisms
are not resolved at present.
Conclusions
Unlike some internal parasites, GIN of ruminants have a simple life-cycle, and during
their parasitic phase reside only in the gastrointestinal tract of the host. They therefore
present a difficult immunological problem for the host, requiring protective immunity
from an organ normally restrained from reacting to locally-presented molecules, and
one with a good barrier between local immune system and pathogen. The gut immune
responses associated with rejection of these parasites are physiologically complex and
appear to contain much redundancy in that there is no evidence that any one mechanism is essential for protection. Protective immune responses against GIN parasites
which are allergic in nature are also highly regulated within the local mucosal environment to avoid systemic sequelae. Mucosal immune responses come at considerable cost
to the host in terms of cell and protein loss, reduced nutrient absorption and increased
metabolic demand for nutrients; a cost exacerbated by the damage to gastrointestinal
function due directly to the worm.
It should be remembered that these mechanisms are subject to a number of influences. These include physiological factors such as age, liveweight and sex, and external
factors such as nutrition and weaning stress. The nutritional requirements for optimal
mucosal immunity appear to be greater than currently recognized, and additional to
433
those required to meet the demands of growth and production. The efficacy of the
mucosal immune responses can also be enhanced by altering the local immunological
environment, for example the degree of inflammation. Thus gut immunity is not an
isolated component of ruminant physiology, but is highly integrated with the nervous,
digestive and endocrine systems, and underlaid by a common cell biology regulating
signalling, activation, metabolism, replication and differentiation. In such a situation,
intervention which is directed towards addressing any single factor contributing to susceptibility will predictably have limited prospects of success.
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25
Introduction
Interest in resistance of sheep and cattle to external parasites arose soon after emergence
of blowfly strike and cattle tick as diseases of major economic importance in Australia.
It is most likely that blowflies were introduced to Australia on several occasions: Lucilia
sericata from Britain during the early days of European settlement; and Lucilia cuprina
from South Africa or India during the latter stages of the 19th century (Gilruth et al.,
1933). Blowfly strike was sporadic until 1903 when it became widespread in New
South Wales and Victoria, extending throughout the range of sheep over the next few
years (Gilruth et al., 1933). Emergence of the disease coincided with introductions of
American Merinos from Vermont, which had a high grease content in the fleece and
pronounced skin wrinkle (Belschner, 1966). With the realization that body conformation was a predisposing factor for susceptibility to blowfly strike (Seddon et al., 1931a)
and that these predisposing traits were heritable (Seddon et al., 1931b), interest turned
to breeding sheep resistant to the disease and to surgical removal of skin folds in the
breech (mulesing). Wetting of fleece was found to induce bacterial growth (Stuart,
1894), which in turn induced green or red discoloration of wool (Seddon and
McGrath, 1929) and dermatitis (Bull, 1931). These studies led to the conclusion that
predisposing conditions for blowfly strike are wool characteristics and body conformations that favour, principally through susceptibility to wetting and resultant dermatitis,
attraction of flies, oviposition and subsequent nutrition of larvae (Gilruth et al., 1933).
Thus fleece rot and blowfly strike occur as a disease complex, with the strongest interdependence when blowfly strike occurs over the shoulders, back and flanks (body
strike).
This early work provided the basis for the next 70 years of research into resistance
to blowfly strike. Major areas of research have been genetic studies on direct and indirect selection for resistance to blowfly strike, mechanisms of resistance in selected
flocks, innate immunity, acquired immunity and vaccines. Progress in these areas will
be considered in turn.
CAB International 2000. Ruminant Physiology: Digestion, Metabolism,
Growth and Reproduction (ed. P.B. Cronj)
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438
439
ences might lie at the level of receptor number or function controlling vascular permeability rather than in supply of mediator. Contrary to this finding is the observation
that when excretory and secretory products from larvae were injected intradermally,
there was a greater wheal response in resistant animals (OMeara et al., 1992). The
wheal response to excretory and secretory products has also been correlated with resistance to fleece rot and blowfly strike in unselected sheep (Broadmeadow, 1988) and has
been proposed as a selection marker for resistance to the disease complex (Raadsma et
al., 1992).
Cellular components of the inflammatory response in skin and resident leucocytes
in skin have also been examined in the resistant and susceptible lines. Intense neutrophil accumulation is a feature of the dermatitis that accompanies fleece rot and fly
strike (Bull, 1931; Burrell et al., 1982; Bowles et al., 1992) and there is extensive production of inflammatory cytokines interleukin (IL)-1a, IL-1b, IL-6 and IL-8 (Elhay et
al., 1994) at the wound site. Neutrophil accumulation in response to intradermal injection of ovine IL-1b, human IL-8, human IL-1a, human tumour necrosis factor
(TNF)-a activated complement, leukotriene B4 and endotoxin from Pseudomonas
aeruginosa did not differ between lines (Colditz et al., 1994). Leucocyte populations in
untreated skin and in skin sites collected 6 h following intradermal injection of TNF-a
were examined by immunohistology. WC1+ lymphocytes and eosinophils were more
prevalent in skin of susceptible sheep and IgE+ cells (probably mast cells) were more
prevalent in skin of resistant animals (Colditz et al., 1994). In an independent study
(Nesa, 1994) mast cells were also found to be more prevalent in skin from animals of
the resistant genotype. This difference in the prevalence of mast cells may underlie the
greater wheal response to intradermal injection of larval antigens in resistant sheep
noted above. Furthermore, in an examination of the IgE gene, Engwerda et al. (1998)
found a restriction fragment length polymorphism between resistant and susceptible
lines. Together these findings suggest that the role of mast cells in resistance to blowfly
strike deserves further attention.
The response of resistant and susceptible sheep to artificial infestation with larvae
has been examined in two experiments. OMeara et al. (1995) found greater exudation
of serum proteins onto the skin surface during the first 12 h of infestation in resistant
animals, which is in accord with greater skin wheal responses seen in these animals. In
the second experiment, the effects of lymphocyte and interferon (IFN)-g depletion on
larval establishment and growth have been studied by treatment of sheep with monoclonal antibodies. In contrast to results seen with the internal parasite, Trichostrongylus
colubriformis, where depletion of CD8+, WC1+ lymphocytes or IFN-g enhanced resistance to infection (McClure et al., 1996), depletion of CD4+, CD8+, WC1+ lymphocytes or IFN-g had no effect on establishment or growth of blowfly larvae (Colditz et
al., 1996). Importantly in both artificial infestations, there were no differences between
the resistant and susceptible lines in the establishment or growth of larvae.
Lymphocyte subsets in blood do not differ between resistant and susceptible lines
(Colditz et al., 1996; McColl et al., 1997). Following intravenous challenge with endotoxin from P. aeruginosa there are higher neutrophil counts and monocyte counts in the
resistant line (I. Colditz, unpublished findings). Antibody responses to intradermal
injection of P. aeruginosa antigens are generally greater in resistant sheep (Chin and
Watts, 1991; Gogolewski et al., 1996); however, no significant differences between the
lines were found in the antibody response to L. cuprina antigens (OMeara et al.,
440
1997). Sheep from the resistant line develop higher titres of antibody to P. aeruginosa
antigens during simulated fleece rot conditions when live cultures of P. aeruginosa are
applied epicutaneously to wetted skin (Chin and Watts, 1991). This may result from
differences in skin characteristics between the lines resulting in greater uptake of antigens through skin in the resistant animals, or alternatively to differences between lines
in immunological recognition and response to P. aeruginosa antigens. Systemically
administered P. aeruginosa vaccines can confer resistance to fleece rot (Burrell, 1985) so
the differences in antibody titres noted by Chin and Watts (1991) and Gogolewski et
al. (1996) may well contribute to the differences in prevalence and severity of fleece rot
seen between the selection lines. Taken together, these extensive studies on the Trangie
resistant and susceptible lines suggest that reactivity of skin may contribute to resistance but may play a subsidiary role to wool characteristics that predispose to fleece rot.
441
infective larvae, which coincided with the treatment of all sheep with the organophosphate Diazinon to terminate infestations. Plasma cortisol levels were elevated in
infested sheep from day 2 to day 6 of infestation. The cytokines IL-1b, IL-6, IL-8 and
TNF-a were assayed in plasma. IL-6 was elevated from day 2 to day 6 of infestation,
whereas no significant differences were observed between treatment groups for concentrations of the other cytokines. Longitudinal growth of wool fibres did not differ
between groups, though there was a trend towards less growth in struck and pair-fed
sheep than in controls. Staple strength was significantly lower in struck sheep than in
control and pair-fed sheep, but did not differ between sites near to and distant from the
strike lesion. Taken together these findings suggest that the host response to infestation
has systemic consequences that lead to reduced fibre strength throughout the fleece,
and that the reduced feed intake accompanying blowfly strike is not primarily responsible for reduced fibre strength. Cortisol and IL-6 are implicated as systemic mediators
that contribute to reduced fibre strength; however, the interdependence, pleiotropism
and redundancy of mediators associated with stress and inflammatory responses make
it unlikely that a single mediator will be identified as accountable for the effect of
blowfly strike on fibre strength.
442
the normal functions of the gut in a manner deleterious to the larvae. The strategy
relies on the ingestion of sufficient quantities of biologically active antibodies. It was
found that larvae feeding on sheep ingest substantial quantities of functional antibodies
(Eisemann et al., 1993). High concentrations of ingested antibody persist throughout
the larval foregut and the anterior midgut. The quantity of antibody decreases sharply
posterior to this region as a consequence of proteolysis. In contrast, only very small
quantities of ingested antibody penetrate the wall of the midgut to reach the
haemolymph and thence internal tissues. The most promising concealed targets for
immunological attack therefore would appear to lie in the anterior midgut. This region
is exposed to ingested antibodies and is not protected by an impermeable layer of cuticle as is present in the foregut and hindgut. However, the midgut region is protected by
a semi-permeable matrix, the peritrophic membrane, which is thought to have a central
role in the facilitation of the digestive process in the gut and protection of the insect
from invasion by bacteria.
East et al. (1993) used this knowledge in testing crude extracts of peritrophic
membrane in sheep vaccination trials. Sera from vaccinated sheep strongly inhibited
larval growth in an in vitro feeding bioassay (East et al., 1993). Moreover, significant
although weaker effects on larval growth were observed using bioassays directly on the
backs of sheep. Vaccination trials in sheep were then used as an assay to guide the
purification of specific peritrophic membrane antigens. This process, which involved
many steps, led to the identification of several potential vaccine antigens but particularly the glycoprotein, peritrophin-95 (Casu et al., 1997). Vaccination of sheep with
this antigen led to greater than 50% reduction in larval weights as measured by an in
vitro larval feeding bioassay. The cDNA coding for this protein has been sequenced and
various recombinant proteins produced. These were tested in vaccination trials which
showed significant, albeit weak, anti-larval effects. Further studies of both the structure
of the recombinant antigens and the nature of the antibody response to the native protein demonstrated that the nature of the oligosaccharides attached to this glycoprotein
is an important determinant of the efficacy of the antilarval immune response induced
by this antigen. Further studies are under way to ensure the appropriateness of the
oligosaccharides attached to the recombinant proteins.
Isolation of specific anti-peritrophin-95 antibodies and their re-constitution into
control sera transferred the antilarval growth activity when measured in in vitro feeding
bioassays. Moreover, higher concentrations of antibody resulted in more severe effects
on larval growth. There was also a direct correlation between antibody titre from a
number of sheep and the degree of larval growth inhibition measured from each serum
in an in vitro feeding bioassay (R.L. Tellam and C.H. Eisemann, unpublished results).
These experiments and others proved that the antilarval effect was mediated by antibody and that complement was not required. Examination of the peritrophic membrane from larvae feeding on sera from sheep vaccinated with peritrophin-95 showed
that the luminal side of the peritrophic membrane was lined with a new layer of material of undefined composition. This layer was impermeable to 6 nm colloidal gold particles that normally freely diffused across the peritrophic membrane. Presumably, the
layer inhibited the movement of nutrients into the digestive epithelia and resulted in
the starvation of the larvae (R.L. Tellam and C.H. Eisemann, unpublished results). A
vaccine based on the peritrophin-95 antigen is being developed but will require greater
efficacy to be effective in the field. Combinations of this antigen with antigens possibly
443
involved in larval establishment on the host epidermis (Bowles et al., 1996) may result
in a more efficacious vaccine.
Vaccine delivery
With the realization that antibody held promise as a defence mechanism for protection
against blowfly larvae, we turned our attention to investigating immunization protocols
for elevating antibody concentrations in skin. Colleagues in our laboratory developed
an apparatus for inducing transudation of interstitial fluid by applying a vacuum of
225 kPa for 90 min to the skin surface (Watson et al., 1992). Up to 40 ml of interstitial fluid collects on the skin surface during this procedure (Colditz et al., 1992b). The
ratio of 125I-IgG1 to 111In-transferrin in blood was compared with the ratio in normal
skin and at skin sites receiving the vascular permeability mediators histamine,
bradykinin, activated complement, platelet-activating factor or serotonin to determine
whether there is preferential transfer of IgG1 to these extravascular sites. No evidence
was obtained for selective transport of IgG1 into the dermis or onto the skin surface
(Colditz et al., 1992b). Immunoglobulins can be detected on the surface of sheep skin
in low concentrations (Lloyd et al., 1979) and our results suggest that for IgG1 this
occurs by filtration rather than by selective transport. It is noteworthy, however, that
IgA is selectively transported across sebaceous glands and sweat glands in man (Gebhart
and Metze, 1990) and can be detected in skin washings from sheep following infection
with Dermatophilus congolensis (Sutherland et al., 1987).
The potential to induce local antibody responses in sheep by topical application of
antigens was examined in a number of experiments (Colditz and Watson, 1993).
Antigen delivery via a Panjet vaccination gun was more effective than topical application of antigen onto skin. Adjuvation of the experimental antigen ovalbumin with
immunostimulating complexes (ISCOMs) resulted in further elevation of the antibody
response. Antibody titres were higher at locally immunized skin sites than at nonimmunized skin sites of the same animals. Isotype analysis of antibody present in vacuum transudates indicated that immunoglobulin (Ig)M and IgG1 were elevated at the
locally immunized skin sites, IgG2 antibody did not differ between sites and IgA antibody was not detected. Numbers of CD4+, CD8+ and WC1+ lymphocytes were elevated in the dermis at sites of local immunization (I.G. Colditz, D.L. Watson and S.J.
McClure 1999, unpublished observations). Similar results have been obtained with
recombinant peritrophic membrane antigens PM44, PM48 and PM95 from L. cuprina
(I.G. Colditz, C.H. Eisemann and R.L. Tellam, unpublished observations). Together
these findings demonstrate the potential for elevating the concentration of antibody in
skin by local application of antigen.
It has recently been shown that antigens combined with cholera toxin can induce
high titres of systemic antibody when applied to unbroken skin in mice (Glenn et al.,
1998a,b). No local gross or histological reactions accompanied this transcutaneous
immunization, which protected mice from a lethal intranasal challenge with cholera
toxin. We have recently observed a systemic antibody response in sheep following topical application of 100 mg of cholera toxin to skin (R.B. Cope and I.G. Colditz, unpublished observations). Interestingly, during fleece rot and dermatophilosis, sheep develop
systemic antibody responses to these non-invasive infectious organisms, although
444
Conclusions
A centurys research on blowfly strike has yielded a great deal of information on the
pathogenesis of infestation and the nature of the hostparasite interaction. Despite
these advances, the two major goals for research in this field, development (or selection)
of resistant animals and the induction of resistance by vaccination, remain unattained.
In view of the sporadic occurrence of conditions that permit direct selection for resistance, a major shortcoming of research to date is the absence of a reliable trait for indirect selection for resistance. As discussed by K.A. Ward in Chapter 21, transgenesis may
yet yield novel solutions to the quest for resistant animals. For vaccines, solutions to the
problem of invoking protective immunity may come not from harnessing components
of naturally acquired resistance but by broaching the adapted host parasite interface by
vaccination with evolutionarily nave, concealed antigens.
Acknowledgements
The collaboration of S. Walkden-Brown, B. Crook, B. Daley and C. Eisemann in studies of the effect of blowfly strike on fibre strength is gratefully acknowledged. This work
was supported in part by Australian woolgrowers through the Australian Wool
Research and Promotion Organisation and the L.W. Bett trust.
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448
26
K. PERSSON WALLER
Swedish University of Agricultural Sciences, Faculty of Veterinary Medicine,
Department of Obstetrics and Gynaecology, Uppsala, Sweden
Introduction
Udder diseases, mainly mastitis induced by bacterial infections, are major problems for
the dairy industry world wide, and the yearly costs are substantial (Philpot, 1984;
Nilsson and Holmberg, 1996). Current programmes for control of bovine mastitis have
improved the situation substantially, but the incidence of clinical and subclinical mastitis is still high. This indicates problems in the application of the control measures, or
limitations in their effects. A need for new and innovative approaches for mastitis control especially concerning preventive measures is obvious. Lately, emphasis has been put
on working towards a better understanding of the host resistance against udder infections, in order to find ways to increase the natural ability of the cow to resist, or defend
herself, against udder infections.
This paper will consist of a review of the latest knowledge in defence mechanisms
of the mammary gland of ruminants. Special emphasis will be put on the distribution
and function of various leucocyte populations, and the influence of different inflammatory mediators, such as cytokines, during the lactation cycle. Genetic influence on
immunological functions will also be highlighted. Moreover, the influence of nutrition
and stress on immunological functions and the defence against udder infections will be
discussed, especially in relation to periods of immunosuppression, e.g. around calving,
when the risk for mastitis is increased.
449
K. Persson Waller
450
451
Waller and Colditz, 1998). Memory cells of both B- and T-types are usually produced
after an infection.
Leucocyte functions
Phagocytosis and intracellular killing of bacteria are crucial functions of the neutrophils, which is illustrated by a higher prevalence of clinical mastitis when the cellular
capacity for phagocytosis is low (Grommers et al., 1989; Hill, 1994). Their antimicrobial effects can be enhanced by opsonins, i.e. antibodies and complement (Craven and
Williams, 1985; Paape et al., 1991). The functions of neutrophils may often be reduced
in milk as they are affected by local conditions. However, newly recruited cells from the
bloodstream are considered to have more efficient antibacterial properties (Herbelin et
al., 1997), thus the importance of migration of sufficient numbers of neutrophils.
Changes in adhesion molecules, shedding of L-selectin and up-regulation of CD18,
occur in ovine neutrophils due to migration from blood into the udder (Persson Waller
and Colditz, 1998).
Macrophages are also phagocytic cells but their bactericidal capacity is lower than
for neutrophils. The most important function of macrophages is probably their role as
initiators of inflammatory processes (Adams and Hamilton, 1988). They recognize
antigen, which they phagocytose, process and present to lymphocytes, which, in turn,
become activated. Stimulated macrophages release inflammatory mediators important
for inflammation and attraction of neutrophils (Adams and Hamilton, 1988). Also
bovine mammary macrophages are capable of inducing proliferation of both blood and
mammary lymphocytes (Concha and Holmberg, 1990).
As mentioned earlier, lymphocytes can be divided into T-cells, involved in cellmediated immune response, and B-cells, which are precursors of antibody-producing
plasma cells. Recent research has shown the importance of different lymphocyte subpopulations for the mammary immune response. CD8+ T-cells seem to have varying
functions, being either of suppressor or cytotoxic type, depending on the stage of lactation (Shafer-Weaver and Sordillo, 1997). T-helper cells (CD4+) play an important role
in the immune response by activation of other cells, like lymphocytes and macrophages
(Tizard, 1996). The proportion of CD8+ lymphocytes in udder secretion is often larger
than the proportion of CD4+ cells, which might have implications on the immune
response of the mammary gland (Taylor et al., 1994; Guiguen et al., 1996; Persson
Waller and Colditz, 1998). CD8+ cells have been shown to suppress immune function
during udder infections (Park et al., 1993). The functions of gd T-cells in the udder are
not certain but these cells are thought to be associated with protection of epithelial surfaces (Allison and Havran, 1991). Shafer-Weaver et al. (1996) found that the percentage of gd T lymphocytes decreased significantly in mammary parenchyma during times
of increased susceptibility to infections.
Antibody production is the primary role of B-cells. Antigen activation of cellular
receptors results in a proliferation into antibody-secreting plasma cells which produce
immunoglobulins directed against invading pathogens (Tizard, 1996). They also present antigen to T-helper lymphocytes, which produce cytokines inducing further proliferation and differentiation of B-cells.
452
K. Persson Waller
453
in sera appears to relate to the severity of clinical symptoms during coliform mastitis
(Nakajima et al., 1997).
K. Persson Waller
454
cells were higher. Significant differences in sire progeny groups for various neutrophil
assays have also been observed (Kehrli et al., 1991), and results of Fitzpatrick et al.
(1998) indicated that immunological assays may be useful in identifying bulls whose
progeny would be associated with a higher resistance to intramammary infections with
S. aureus.
A selection programme for animals resistant to immunosuppression and disease
around parturition would be especially desirable. Genetic variability in certain immune
functions, i.e. total numbers of neutrophils, neutrophil chemokinesis, assays of neutrophil respiratory burst associated with phagocytosis, and in serum concentrations of
immunoglobulins and hemolytic complement activity was observed in periparturient
dairy cows (Detilleux et al., 1994). Moreover, Mallard et al. (1998) showed that cows at
peripartum may be categorized as high or low responders to immunizations and that
the heritability for specific antibody responses was moderate to high. These findings
may be used to identify animals with high resistance to disease.
The importance of adhesion molecules, essential for leucocyte migration, has
recently been highlighted. Genetic leucocyte adhesion deficiencies leading to chronic,
or even fatal, infections have been observed in several animal species including cattle
(bovine leucocyte adhesion deficiency) (Kehrli et al., 1992). All species affected show
signs of chronic and recurrent infections due to a deficiency in the chemotactic and
phagocytic properties of leucocytes, particularly neutrophils.
Periods of immunosuppression
As a result of depressed immune functions, susceptibility to infectious diseases, such as
mastitis, is associated with the peripartum period (Sordillo et al., 1997). High blood
levels of glucocorticoids are present during this time. Examples of stress factors during
this period are parturition, onset of lactation, and changes in feeding and management
455
K. Persson Waller
456
Micronutrients
A balanced supply of micronutrients (vitamins and trace elements) is essential during
periods of immune suppression. Deficiencies in selenium (Se), vitamin E, vitamin A,
copper (Cu) and zinc (Zn) have been associated with a negative influence on the
immune response in association with mastitis (Reddy and Frey, 1990; Harmon and
Torre, 1994; Smith et al., 1997).
Se deficiency is a problem in areas where Se concentrations in soils and pastures
are low. Se is an important component of the enzyme glutathione peroxidase, which is
essential for the protection of cells and tissues from auto-oxidative damage from production of oxygen radicals (Reddy and Frey, 1990). Se deficiency results in reduced
neutrophil migration into the udder and impaired intracellular killing of bacteria
(Erskine, 1993; Hogan et al., 1993). Supplementation of Se may improve udder health
by reducing the severity and duration of cases of clinical mastitis (Erskine, 1993).
Vitamin E is important for both cellular and humoral immune functions. This
may be elicited through its effect on cell membrane stability and regulatory role in
biosynthesis of various inflammatory mediators (Reddy and Frey, 1990; Smith et al.,
1997). Dietary supplementation of vitamin E is of value, especially as the concentration of this vitamin in fodder decreases with age and length of storage. Moreover, the
serum concentration of vitamin E drops around calving, a period of increased suscept-
457
ibility to disease (Smith et al., 1997). Supplementation with vitamin E increased intracellular killing of bacteria by neutrophils, and reduced the numbers of new intramammary infections (Smith et al., 1997). Dietary supplementation with vitamin E should
be considered as a preventive measure in the control of mastitis during the peripartal
period.
Vitamin A and the precursor b-carotene are also important for mucosal integrity
and stability. Both substances have stimulatory effects on immune cell populations and
have been correlated with an increased resistance to disease (Sordillo et al., 1997).
Deficiencies were related with severity of mastitis, and there is a negative correlation
between plasma vitamin A and somatic cell counts in milk (Johnston and Chew,
1984). Plasma vitamin A and b-carotene concentrations decrease during the periparturient period contributing towards increased susceptibility to new udder infections
(Johnston and Chew, 1984).
Limited information is available on the importance of Cu and Zn in minimizing
the risk for mastitis in dairy cows. However, Cu and Zn have important biological
functions, such as being parts of enzyme systems with antioxidative properties. These
systems protect cells and tissues from detrimental effects of oxidative substances
released at phagocytosis and killing of bacteria by white blood cells (Reddy and Frey,
1990). A Cu deficiency has also been reported to decrease the antibacterial effects of
the immune defence (Reddy and Frey, 1990; Harmon and Torre, 1994). The addition
of Cu in the feed gave a considerable decrease in the numbers of infected udder quarters at calving compared with untreated controls (Harmon and Torre, 1994). Moreover,
the cell count in milk tended to be lower in the group receiving Cu supplementation.
Zn is important for the integrity of the skin, the first barrier against infections, and has
also importance for immune functions. Zn deficiency can cause degeneration of lymphoid tissues and have a negative influence on immune cells (Reddy and Frey, 1990;
Harmon and Torre, 1994). Few studies have been made to study the relationship
between Zn and mastitis. However, deficiency in Zn can predispose cows to secondary
infections, which can be reversed by supplementation of Zn (Reddy and Frey, 1990;
Harmon and Torre, 1994). Problems associated with Zn deficiency can be exacerbated
by having a feeding regime containing high amounts of calcium during early lactation.
Plasma Zn concentrations decrease markedly at parturition which may be connected
with the immune suppression observed during this period (Goff and Stabel, 1990).
Conclusions
Proper immune functions are essential for the defence against udder infections.
Detailed knowledge about the immune response and important defence factors are
essential in order to find new ways for the prevention and treatment of udder infections
leading to mastitis. Work should be concentrated on ways of minimizing the negative
influence on immune functions and/or ways of stimulating these functions, especially
during periods of immune suppression such as around calving. Possibilities of stimulating the non-specific immune response, for example through the use of cytokines and
other immunomodulatory substances, should be comprehensively investigated.
As mentioned above, it is important to identify risk factors, which negatively influence the defence mechanisms of the udder. The importance of management and
K. Persson Waller
458
adequate nutrition are some important factors to consider. Provision of suitable dietary
supplies of vitamins and trace elements is one important step to ensure a good mammary defence and prevent mastitis. The possibility to find markers for genetic selection
of individuals with a well-developed immune system should also be further evaluated.
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Index
Index
464
amino-acid-fermenting 100101
cellulolytic 79, 82
culturable number 101
cyanobacteria 110
Gram-negative 85
Gram-positive 68, 85, 100
mucosal 71
non-cellulolytic 82
obligate peptide 100101
phylogenetic placement 68
proteolytic 101102
ruminal 87, 108110
bacterial glycoside hydrolases 8384, 8687
bacteriophages 103, 391392
Bacteroideacea 101
Bacteroides distasonis 71
Bacteroides fragilis 71
barley 7
barley straw 13
Barnacle geese 172
biotechnology 399402, 400
blackbrush 8
blood flow 132133, 134
hepatic 150
blood pressure 46
blood transport, amino acids 157158
blowfly strike 437
immunity to 440441
resistance 438440
vaccines 441444
body condition score (BCS) 354356, 356
body heat 295297, 296
body temperature
birth-related changes 303305, 304,
307308
elevated 47
feto-maternal relationship 297299,
298, 299, 300, 302
homeostasis 295297, 296
body tissues 9
body weight 198, 206
bone 188
brain 45, 211212
photoperiodic changes 211212
brainstem
control of gastric motility 4748
brain stimulation 48, 49
interior sites 5051, 50, 53
surface sites 4950
hypermotility 5354
reticuloruminal motility 5456, 55
rumination 5354
control of salivation 4244, 43
amygdala see amygdala
frontal lobe 44
branch-chain amino acids (BCAA) 158
buoyancy 2829
Butyrivibrio fibrisolvens 65, 68, 101, 102,
394397, 396
Index
465
conditioning stimuli 811
copper (Cu) 457
Cretaceous period 62
cyanobacteria 110
cyclical intake 1112
cysteine proteinase 105
cytokines 177178, 179, 188189, 452
DNA
free 393
gut ecology 6364, 65
PCR amplification 69
dockerins 82
dorsal vagal motor nucleus 4849
dry matter (DM) 12
duodenum 28
dye-dilution procedures 132133
Index
466
fetus
abortion 308
growth 192193
thermal physiology
birth-related changes 303305,
304, 307308
feto-maternal relationship
body temperatures 297299,
298, 299, 300, 302
fetal hyperthermia 299300
maternal hyperthermia
301303
maternal hypothermia 303
homeostasis 295297, 296
thermoregulatory mechanisms
305307, 306
fibre 7, 15, 86, 104
Fibrobacter 65, 66
Fibrobacter succinogenes 66, 6970, 79, 82,
89, 108, 398
fibrolytic enzymes 90
fibrolytic microbes 32
flavours 45
flow rates
microbes 32
nutrients 28
rumen liquid 32
fluorescent probes 71
follicles
wool 257, 258
cycles 261
mass 259261
protein synthesis 261266
shutdown 261
food
changes in composition 1214
choice of 1114
adaption 4
timescales 67
flavour 45
sensation and metabolism 38
single 1112
food aversion 4, 5
food intake
conditioning stimuli 811
cyclical 1112
daily 1314, 13
modelling 21, 22
learned associations 38
mathematical models 2124, 3536
prediction of intake 2431
metabolic control 24
photoperiodic effects 205206
voluntary 22, 24
forage 7, 12, 30
forebrain 54
fractional outflow rate 28
frontal cortex 45
frugivores 62
fungal glycoside hydrolases 8384, 8687
fungi 64, 79, 86, 87
gastric motility
control sites 4142
brainstem 4748
brain stimulation 48
interior 5052
mechanism 5456, 55
single unit recording 4849
surface sites 4950, 49, 53
forebrain 54
gastrointestinal metabolism 131144
gastrointestinal parasites 425
defence mechanisms 426429
effect on host 429430
gastrointestinal tract
anatomy and vasculature 131132
energy metabolism 134135
measurement of blood flow 132133
microbial community 6465
urea 156157, 156
use of labelled substrates 133134
gazelles 172
GDH (glutamate dehydrogenases) activity
109
gene transfer 87, 390394
barriers to 393394
evolution of rumen microorganisms 394
genetic effects
hyperplasia 240
hypertrophy 246247
wool growth 260261
genetic fingerprinting 69
genetic manipulation 373375
digestion 382
disease resistance 382383
endocrine system 376378
intermediary metabolism 378382
genetic selection 413415, 414
genomic analysis 399
genotypenutrition interactions 412413
Index
467
glucanases 86
glucocorticoids 454
glucose
blocking uptake 9
metabolism
absorption 139140
availability 140141
gut tissues 137138, 138
release 139140
glucose co-transporter gene 140
glucose irreversible loss (GIL) 138, 138
glutamate dehydrogenase (GDH) 152
glutamine 143, 153, 161, 161162, 455456
Gly-Arg-MNAse 105106
glycanase genes 87
glycoside hydrolases 8384, 8687
glycosyl hydrolases 8889
goats 4, 7, 48
grazers 62
Great Lakes, North America 63
growth hormone
circulating 188, 195
effect on metabolism 196198
galactopoeitic effects 216
GH somatotropin 187
hyperplasia 238
receptors (GHR) 188189
growth hormone binding protein (GHBP)
189
growth regulation see postnatal development
gut ecology
DNA-based studies 6364, 65
electrophoresis
DGGE 6970
TGGE 6970
RNA-based studies 6567
gut tissues
amino acid absorption 143
amino acid requirements 141143
IGF
binding proteins 190192
receptors 192
IGF (insulin-like growth factor) 329, 330
IGFBP proteases 333
IGFBP-3 332
IGFBPs 330332, 331
lactoferrin 342343
mammary physiology
blood versus milk changes
333336, 334, 336
IGF receptors 337339
IGFBP 196, 339340, 340
IGFBP-3
binding proteins 340, 342, 342
and IGF-I 341
and illness 198199
nuclear appearance 343344
nuclear localization sequence
(NLS) 343
local IGF ligands 337
receptors 329330
Index
468
IGF-I 164, 187, 196198
endocrine 189
gene 190
plasma levels 194
IGF-II 194
illness 198199
immune system
adipose tissue interactions 177178
genetic influences 453454
influence of nutrition 455457
influence of stress 454455
leptin 180181
immunity
non-specific enhancement 431432
protective 430431
immunological functions 453454
immunosuppression 454455
inductive phenomena 426
inflammatory mediators 428
inflammatory process 451, 452453
ingestion 67
insulin 164, 175, 177, 198, 319
insulin-like growth factors 189190
interferons (IFN) 452
interleukin-6 173
interleukins (IL) 452
interspecific differences 27
intramuscular adipocytes 177
intraruminal infusion 4
intraruminal nitrogen recycling 102104
isotopic labelling 134
isotopomer analysis 152
keratin 450
kinetics, digestion 2431
Lachnospira multiparus 66
lactation 264
adaptations to 314317, 315, 318
mechanisms 317320
dairy cattle see dairy cattle
nutrient partitioning 311
coordination 320324
endocrine control 415418
regulation 312314
photoperiodic effects 215217
Lactobacillus 71
lactoferrin 342343
lambing 298, 299
lambs 4, 49
dietary choice 56, 78, 11
gastric motility 49
larkspur 11
learned associations
food flavours 45
food intake 1214
food and metabolism 38
leptin 9, 10, 174, 175, 178181, 182
Leucaena leucocephala 66
leucine 143
leucocytes 449
distribution 450451
function 451
proportions 453, 454, 456
LGMs (large ground mammals) 62
LiCl aversion studies 4, 7
lignin 9192
limbic system 10
lipase 207
lipogenesis 357
lipolysis 175176, 178, 196197
milkfat output 348, 357358
lipoprotein lipase 174
liver
amino acid extraction 159, 160
amino acid metabolism see amino acids
ammonia metabolism see ammonia
metabolism
architecture 149151, 151
fatty acids 177
hepatic protein turnover 162164, 162,
163
link to CNS 8
substrate supply 131144
ureagenesis see ureagenesis
lumen 139, 429
lymph nodes 177178
lymphocytes 450451
lysing 177
Index
469
tissue explants 126127
photoperiodic changes 215217
Mammut americanum 63
mandibular secretions 4244, 43
mastitis
genetic influences 453454
host resistance 449450
inflammatory process 452453
leucocyte functions 451
leucocyte populations 450451
maternal tissues 284288
mathematical models 2123
dairy cattle metabolism 363366
digesta kinetic 2431
inadequate detail 23
intake and digestion 2431, 3536
metabolic 3136
ME (metabolizable energy) 6, 14, 33
meal patterning 34
meat quality 247248
mechanistic models 2223
medulla oblongata 9
melatonin 211212, 213
mesenteric drained viscera (MDV) 138
mesodermal cells 227229
metabolic constraints 2324, 3435
metabolic discomfort 910
metabolic imbalance 56, 198200
metabolic models 21
animal response to nutrients 3334
constraints on intake 3435
microbial metabolism 3133
metabolic pathways 33
metabolic signals 3
metabolic stress 455456
metabolic zonation 151, 151
metabolism
amino acid see amino acids
ammonia see ammonia metabolism
energy 196198
gastrointestinal tract 134135
genetic manipulation 378382
glucose 137141
and learning see learned associations
microbial 3133
muscle 206208, 208
nitrogen see nitrogen
postnatal development see postnatal
development
protein 162, 196198
short-chain volatile fatty acids 135137
Index
470
neuropeptides 428
neutral detergent fibre (NDF) 15, 28, 135
neutrophils 450, 451, 452, 455
New Zealand 101
nitrogen control 108110
nitrogen metabolism
liver see liver
microbial ecology 99104
microbial physiology
genetic examination 104108
nitrogen recycling 102104
non-adherent mutants 8586
non-shivering thermogenesis (NST) 306
NTS (nucleus of the solitary tract) 9
nuclear localization sequence (NLS) 343
nucleases 393
nucleic acid probes 6465
nutrient flux, glucose availability 132,
140141
nutrient partitioning
lactation see lactation
pregnancy see pregnancy
sheep 208211, 209, 210, 211
nutrient ratio 56
nutrient requirements 1011, 1214
nutrientgene interactions 409410,
418419
genotypenutrition interactions
412413
prediction of growth responses
411412, 412
nutrients
animal response, models of 3334
choice of food 4, 56, 1011
control of hyperplasia 238
deficiency 45
essential 45
excess 5, 910
flow rates 28
receptors 910
sufficiency 5
nutrition
and immunity 455457
postnatal 246
prenatal 241243
somatotropic axis 195196
wool growth see wool growth
nutritional wisdom 5
oaten chaff 13
obesity 175
obligate anaerobes 63
obligate peptide bacteria 100101
oestrogens 173174
oligonucleotide probes 71, 107
omasum 140
operational taxonomic units (OTUs) 68
opsonins 451
ornithine cycle 152154, 153
substrate priority 154155
osmosensitive system 4447
Ostertagia 8
ovine peptide transporters
functional characteristics 117119
structure 120
tissue distribution of mRNA 119120
OVLT (organum vasculosum, lateral
terminalis) 45, 46
oxygen consumption 134135
Palocene period 62
paracrine effects 174177
parasitism 8
parotid nerve 46
parotid secretions 4244, 43
inhibiting 4447, 46
stimulating 45
particle kinetics 28
particulate outflow 2728
particulate pools 28
parturition 14, 454
passage
kinetics 27
rate 3031
PCR 101102
PCR-amplified DNA 69
peptidases 105
peptides
absorption 123125
transport 117123
utilization 125127
Peptostreptococcus 101
Peptostreptococcus anaerobius 100
perinatal transition 193
periportal cells 150
periportal hepatocytes 152
peristalsis 429
perivenous cells 150
PFGE (pulsed-field gel electrophoresis)
103
Index
471
pH
peptide transport 118119
ruminal fluid 136
phagocytosis 451
phenolics 78
phenomenological models 2223
phlorizin 140
photoperiodic effects 218
adipose tissue 206208, 208
energy expenditure 206
food intake 205206
hormonal mediation
adipose tissue 213215
brain 211212
mammary gland 215217
peripheral hormones 212213
milk secretion 208210, 209, 210, 211
nutrient partitioning 210211, 212
sheep 205219, 218
phylogenetic analyses 101
pigs 70
pituitary hormones 180
placental lactogen (PL) 188
placental substrate 193
plant cell walls
microbial degradation 8992, 91
microbial interactions 7986
plasma
amino acids 157158
catecholamines 206207
GH concentrations 193, 195
IGF-I levels 194, 195
IGFBP-1 194195
insulin 198
plasma export proteins 160
plasmids 392393, 392
polyclonal antibodies 85
polyethylene fibre 7
polyethylene glycol (PEG) 8
Polyplastron multivesiculatum 67
population analysis 101
porta hepatis plexus 131132
portal-drained viscers (PDV) 138
portal flux
peptides 123125
SCVFA 136137, 137
portal vein blood flow 134
positive feedback 34
post-ingestive feedback 6
postnatal development 187188
growth regulation
somatotropic axis
circulating growth hormone
188
developmental changes
192193, 194195
growth hormone receptors
188189
IGF binding proteins 190192
IGF receptors 192
influence of nutrition 195196
insulin-like growth factors
189190
role in metabolic imbalance
198200
metabolism
effects of GH and IGF-I 196198
postprandial hypertonicity 47
predominant ruminal bacteria (PRB) 101
pregnancy 264
macronutrient partitioning
amino acids 276277
fetal nutrient requirements
275276
glucose 276277
maternal nutrition 279, 282283
direct effects 277278
indirect effects 278279
maternalfetal transfer 281282
non-fetal conceptus tissues 276
maternal tissues
homeorhetic regulation 286288
metabolic adaptations
amino acid metabolism 284
glucose metabolism 284
modulation by nutrition
energy 284285
protein 285
placenta
metabolism
amino-acid 281282
glucose 281, 283
nutrient transport
amino acids 280281
glucose 280
size 277279, 282
preterm labour 308
Prevotella bryantii 65, 104107
Prevotella ruminicola 68
Prevotella/Bacteroides 397398
probabilistic models 28
prolactin (PRL) 188
Index
472
propionate 5, 8, 136, 153, 155
prostaglandin E2 175
protein
binding 174, 190192
crude (CP) 14, 28
metabolism 162, 196198
requirements 1011
supply 6
synthesis
hepatic 163
wool see wool growth
proteolytic bacteria 101102
proteolytic enzymes 108
protozoa 32, 6667, 79, 102
protozoal lysis 102103
protozoan glycoside hydrolases 8384, 8687
PrtA activity 106
genetics 389
exploitation of genes
399400, 400
gene expression 140,
394400
gene transfer 390394
genomic analysis 399
regulation 398399
vector systems 394398, 395
models of 23, 25
rumen inoculants 401
rumen microbial diversity 6769
rumen osmolality 7
rumination 5354
Ruminococcus albus 66, 6970, 79, 82, 107
Ruminococcus flavefaciens 66, 6970, 79, 82
Ruminococcus spp. 397
rabbits 120123
ratio of nutrients 56
rats 4, 47, 120123
rDNA sequencing 63
RDP (rumen degradable protein) 6
receptors, nutrients 910
regression model 22
reindeer 172, 206
reiterated sequences 82
resistance see host resistance
restriction modification systems 393
retention timescales 31
reticular contractions 4950, 49, 5152, 52,
53
reticular/oesophageal groove 52
reticulo-omasal orifice 52
reticuloruminal motility 5456, 55
retinol binding protein 174
RNA-based studies, gut ecology 6567
rumen function
contractions 28, 4951, 50, 51
fermentation 56
fungi 64, 6667, 86, 87
maintenance of 7
microorganisms
adherence to plant cell walls 7986
cellulose-binding proteins
82, 85
cellulosome equivalent 8182
non-adherent mutants 83, 86
diversity 6465, 6769
effect of diet 70
Index
473
parotid secretion 45, 46
photoperiodic effects see photoperiodic
effects
portal vein 132
salivation 4147, 43
septic shock 199200
wool growth see wool growth
short-chain volatile fatty acids 135
absorption 136
metabolism by gut tissues 136, 137
signature probes 67
single unit recording 4849
small intestine 25
sodium bicarbonate 7
somatic cell count (SCC) 453, 456
somatostatin (SRIF) 188
somatotropic axis 187, 198200
developmental changes 192193
perinatal transition 193
postnatal ontogeny 194195
influence of nutrition 195196
metabolic imbalance 198200
regulation of postnatal growth 188192
splanchnic bed 131134, 158
Staphylococcus aureus 452
starch 45
starch digestion 139140, 139
statistical models 2223
Streptococcus 71, 102
Streptococcus bovis 65, 398
stress 454415
submandibular gland 45
submandibular secretions 4244, 43
substrate fractionation (food) 2526
substrate supply, liver 131144
substrates, labelled 133134
sulphur 4
surface erosion (cell walls) 9092, 91
Synergistes jonesii 66
vaccines 441444
venousarterial (VA) differences 132134
villus 143
visceral afferents 10
visceral organs 89
vitamin A 457
vitamin E 456457
VLDLs (very low density lipoproteins) 174
volatile fatty acids (VFA) 32
Index
474
wool growth continued
skin and follicles continued
protein synthesis continued
amino acids
dietary supply 262263,
263
growth 264
hormonal effects 264265
pregnancy and lactation
264