Effects of partial sarcoplasmic reticulum calcium depletion on calcium
release in frog cut muscle fibers equilibrated with 20 mM EGTA.
J. Gen. Physiol., 112 (3):
263--295 (September 1998)Abstract
Resting sarcoplasmic reticulum (SR) Ca content (CaSRR) was varied
in cut fibers equilibrated with an internal solution that contained
20 mM EGTA and 0-1.76 mM Ca. SR Ca release and CaSRR were measured
with the EGTA-phenol red method (. J. Gen. Physiol. 106:259-336).
After an action potential, the fractional amount of Ca released from
the SR increased from 0.17 to 0.50 when CaSRR was reduced from
1, 200 to 140 microM. This increase was associated with a prolongation
of release (final time constant, from 1-2 to 10-15 ms) and of the
action potential (by 1-2 ms). Similar changes in release were observed
with brief stimulations to -20 mV in voltage-clamped fibers, in which
charge movement (Qcm) could be measured. The peak values of Qcm and
the fractional rate of SR Ca release, as well as their ON time
courses, were little affected by reducing CaSRR from 1,200 to 140
microM. After repolarization, however, the OFF time courses of
Qcm and the rate of SR Ca release were slowed by factors of 1.5-1.7
and 6.5, respectively. These and other results suggest that, after
action potential stimulation of fibers in normal physiological condition,
the increase in myoplasmic free Ca that accompanies SR Ca release
exerts three negative feedback effects that tend to reduce additional
release: (a) the action potential is shortened by current through
Ca-activated potassium channels in the surface and/or tubular membranes;
(b) the OFF kinetics of Qcm is accelerated; and (c) Ca inactivation
of Ca release is increased. Some of these effects of Ca on an SR
Ca channel or its voltage sensor appear to be regulated by the value
of Ca within 22 nm of the mouth of the channel.
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