The hypothesis of a Ca2+-induced Ca2+ release (CICR) from the sarcoplasmic reticulum (SR) is supported by experiments done in skinned cardiac cells (sarcolemma removed by microdissection). According to this hypothesis, the transsarcolemmal Ca2+ influx does not activate the myofilaments directly but through the induction of a Ca2+ release from the SR. The stimulus gating CICR is not a small change in free Ca2+ concentration (delta[free Ca2+]) outside the SR but a function of the rate of this change (delta[free Ca2+/delta t]). The initial relatively fast component of the transsarcolemmal Ca2+ current would trigger Ca2+ release; the subsequent slow component, perhaps corresponding to noninactivating Ca2+ channels, would load the SR with an amount of Ca2+ available for release during subsequent beats. Inactivation of CICR is caused by the large increase of [free Ca2+] outside the SR resulting from Ca2+ release, which inhibits further release. This negative feedback helps to explain that CICR is not all or none. During relaxation the Ca2+ reaccumulation in the SR is backed up by the Ca2+ efflux across the sarcolemma through Na+-Ca2+ exchange and the sarcolemmal Ca2+ pump. Computations of the Ca2+ buffering in the mammalian ventricular cell and of the systolic transsarcolemmal Ca2+ influx do not support the alternative hypothesis that this influx of Ca2+ is large enough to activate the myofilaments directly. Yet the hypothesis of a CICR can be challenged because of many problems and uncertainties related to the preparations and methods used for skinned cardiac cell experiments.

SUMMARY1. The effects of decreasing pH from 7 40 to 6-20 on the tension developed by direct activation of the myofilaments and by Ca2+ release from the sarcoplasmic reticulum were studied comparatively in segments of single cells of skeletal muscle (frog semitendinosus) and cardiac muscle (rat ventricle) from which the sarcolemma had been removed by micro-dissection (skinned muscle cells).2. The concentration of free Ca2+ in the solutions was buffered with ethylene glycol-bis (/1-aminoethylether N,N'-tetraacetic acid (EGTA). 4. The pH optimum for loading the sarcoplasmic reticulum of skinned fibres from skeletal muscle decreased when the pCa (-log [free Ca2+]) in the loading solution decreased. The optimum was pH 7 40-700 for a loading at pCa 7 75, pH 7-00-6'60 at pCa 7*00 and pH 6-60-6-20 at pCa 6-00.5. The pH optimum for loading the sarcoplasmic reticulum of skinned cardiac cells with a solution at pCa 7-75 was about pH 7 40 as in skeletal muscle fibres. But the cardiac sarcoplasmic reticulum could not be loaded with a [free Ca2+] much higher than pCa 7*75 because a higher [free Ca2+] triggered a Ca2+-induced release of Ca2+ from the sarcoplasmic reticulum.6. The pH optimum of about 7*40 for the loading of the cardiac sarcoplasmic reticulum was also optimum for the Ca2+-induced release of Ca2+ from it. 7. It was concluded that the effects of acidosis on the cardiac sarcoplasmic reticulum accentuate the depressive action of decreasing pH on the myofilaments. This may explain the pronounced depression of contractility observed during acidosis in cardiac muscle. In contrast, a moderate acidosis causes an effect on skeletal muscle sarcoplasmic reticulum that could compensate for the depressive action on the myofilaments, which is, in addition, less pronounced than in cardiac muscle.

Microprocessor-controlled changes of [free Ca 2 '] at the outer surface of the Sarcoplasmic reticulum (SR) wrapped around individual myofibrils of a skinned canine cardiac Purkinje cell and aequorin bioluminescence recording were used to study the mechanism of Ca 2 '-induced release of Ca is through a channel across the SR membrane with time-and Ca 2 '-dependent activation and inactivation . The inactivating binding site would have a higher affinity for Ca2' but a lower rate constant than the activating site . Inactivation appeared to be a first-order kinetic reaction of Ca 2' binding to a single site at the outer face of the SR with a QIo of 1 .68. The removal of inactivation was the slowest step of the cycle, responsible for a highly temperature-dependent .00) refractory period .

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Ca2+ to a critical level at which it released a fraction of the Ca2+ it contained. Each contraction was followed by a re-sequestration of Ca2+, the kinetics of which conditioned the duration of the cycles.6.