Recruitment of Ca 2+ release channels by calcium‐induced Ca 2+ release does not appear to occur in isolated Ca 2+ release sites in frog skeletal muscle

Ca 2+ 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 Ca 2+ release sites. Each site has one SR Ca 2+ release channel activated by its associated T‐tubular voltage sensor. The aim of this study was to evaluate whether it also includes neighbouring Ca 2+ release channels activated by Ca‐induced Ca 2+ release (CICR). Ca 2+ 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 Ca 2+ permeability of the SR associated with the pulse. In control experiments, f Ca was approximately constant when [Ca SR ] was 1500‐3000 μ m (plateau region) and then increased as [Ca SR ] decreased, reaching a peak when [Ca SR ] was 300‐500 μ m that was 4.8 times larger on average than the plateau value. With 8 m m of the fast Ca 2+ 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 μ m . In support of earlier results, 8 m m BAPTA did not affect Ca 2+ 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 Ca 2+ 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 μ m .