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 Ca2+ (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 mum in the internal solution altered the time course of P-SR. The maximal peak-to-steady-level ratio st as 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.