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Recruitment of Ca²⁺ release channels by calcium-induced Ca²⁺ release does not appear to occur in isolated Ca²⁺ release sites in frog skeletal muscle

Karine Fénelon and Paul C. Pape

Ca²⁺ release from the sarcoplasmic reticulum (SR) in skeletal muscle in response to small depolarisations (e.g. to -60 mV) should be the sum of release from many isolated Ca²⁺ release sites. Each site has one SR Ca²⁺ release channel activated by its associated T-tubular voltage sensor. The aim of this study was to evaluate whether it also includes neighbouring Ca²⁺ release channels activated by Ca-induced Ca²⁺ release (CICR). Ca²⁺ release in frog cut muscle fibres was estimated with the EGTA/phenol red method. The fraction of SR Ca content ([Ca_SR]) released by a 400 ms pulse to -60 mV (denoted f_Ca) provided a measure of the average Ca²⁺ permeability of the SR associated with the pulse. In control experiments, f_Ca was approximately constant when [Ca_SR] was 1500-3000 µM (plateau region) and then increased as [Ca_SR] decreased, reaching a peak when [Ca_SR] was 300-500 µM that was 4.8 times larger on average than the plateau value. With 8 mM of the fast Ca²⁺ buffer BAPTA in the internal solution, f_Ca was 5.0-5.3 times larger on average than the plateau value obtained before adding BAPTA when [Ca_SR] was 300-500 µM. In support of earlier results, 8 mM BAPTA did not affect Ca²⁺ release in the plateau region. At intermediate values of [Ca_SR], BAPTA resulted in a small, if any, increase in f_Ca, presumably by decreasing Ca inactivation of Ca²⁺ release. Since BAPTA never decreased f_Ca, the results indicate that neighbouring channels are not activated by CICR with small depolarisations when [Ca_SR] is 300-3000 µM.

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