Evidence for voltage-dependent inactivation of slow currents through calcium channels in frog muscle membrane.

Two mechanisms of inactivation of slow inward current through calcium channels in frog skeletal muscle have been described: voltage dependent inactivation similar to that of classical sodium channels (Cota et al. 1982, 1984; Stanfield 1977); and calcium ions depletion inside the T-tubules as a result of inward current flowing through the T-membrane. In the latter case the inactivation is current dependent (Aimers et al. 1981). The present communication describes an additional potential-dependent inactivation mechanism of calcium channels in skeletal muscle. The currents were studied in isolated twitch muscle fibre segments from Rana temporaria (m. ileofibularis) by means of the vaseline-gap voltage clamp method (Zachar et al. 1982) at 18—20 °C. The current through calcium channels was elicited by more permeable Ba 2 + ions. The external solutions containing different Ba 2 ~ concentrations used were (in mmol/1): 1. of all solutions was 7.5. Barium inward currents, which are known to pass through calcium channels in T-membrane, were recorded in 14 experiments. Values of 100—340 /JA 'cm 2 (mean + S.E.M. 193 ± 39/iA/cm :) in 10mmol/1 Ba 2+ solution and 280— 720/iA/cnr (460 ± 42/yA/cm 2) in 76 mmol/1 Ba 2+ solution were measured. Fig. 1 shows families of voltage clamp Ba 2+ currents recorded in 10 and 76 mmol/1 Ba 2 + and corresponding current-voltage relations. The records show some distortion suggesting that the voltage clamp was not entirely tight. The localization of calcium channels on the T-tubular membrane apparently is the cause of this complication (Cota et al. 1983). / Ba current-voltage curve in 10 mmol/1 Ba 2+ was close to that obtained by Cota and Stefani (1984), and it was shifted by + 15mV in 76 mmol/1 Ba 2+ in agreement with the effect of the surface charge on the voltage dependent calcium channel activation. The correct shape