Deterministic inactivation of calcium release channels in mammalian skeletal muscle

Abstract

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(re1)) 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 μM 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.