Deterministic inactivation of calcium release channels in mammalian skeletal muscle

1 Enzymatically dissociated fibres from the extensor digitorum communis muscle of rats were mounted into a double Vaseline gap chamber. The rate of calcium release ( R rel ) from the sarcoplasmic reticulum (SR) and changes in SR permeability to Ca 2+ ( P SR ) were calculated from measured changes in intracellular calcium concentration. 2 Calcium release during a prepulse attenuated the inactivating component of P SR of the subsequent test pulse. The suppression was graded, larger release causing greater suppression, as expected from a calcium‐dependent inactivation process. However, if the dissociation constant of the putative inhibitory calcium binding site ( K d ) was estimated using different test pulses different affinities were obtained: a smaller test pulse yielded a smaller K d . 3 Comparing the suppression of the inactivatable component of P SR during the test pulse (suppression) with the inactivatable component during the prepulse (pre‐inactivation) revealed a linear relationship with a regression coefficient close to unity. 4 Lowering intracellular magnesium by decreasing its concentration to 25 μ m in the internal solution altered the time course of P SR . The maximal peak‐to‐steady‐level ratio was increased to 6·3 ± 0·4 ( n = 10 , mean ± s.e.m.) from a control value of 3·0 ± 0·2 ( n = 19 ). Despite the apparent change in steady‐state inactivation, suppression remained equal to that pre‐inactivation. 5 Our results support the view that a depolarizing pulse always recruits the same set of calcium release channels and a portion of these channels undergoes a deterministic inactivation process.