The voltage dependence of depolarization-induced calcium release in isolated skeletal muscle triads

We demonstrate for the first time in this study that triadic vesicles derived from skeletal muscle display a voltage dependence of depolarization-induced calcium release similar to that found in intact muscle. We confirm previous studies by Dunn (1989) which demonstrated that changes in extravesicular potassium induced membrane potential changes in isolated transverse tubules with the voltage sensitive dye DiSC(3)-5. Depolarization-induced calcium release was studied in isolated triadic vesicles through similar changes in extravesicular [K] while clamping extravesicular Ca++ to submicromolar concentrations. The amplitude of fast phase of calcium release, identified as depolarization-induced calcium release, varied with the percentage of transverse tubules in the preparation (determined through 3H-PN200-110 specific activity) and different levels of depolarization. Threshold activation of calcium release was obtained with a 40.5 mV potential change; maximal calcium release was obtained with a 75 to 81 mV potential change. Boltzmann fits to the normalized depolarization induced calcium release plotted against the membrane potential change yielded a voltage dependence (k = 4.5 mV per e-fold change) very similar to that found in intact muscle (k = 3-4 mV per e-fold change; Baylor, Chandler & Marshall 1978, 1983; Miledi et al., 1981). Substitution of methanesulfonate for propionate as the impermeant ion or addition of valinomycin in the depolarizing solutions had little effect on the voltage dependence of calcium release.