Calcium channel inactivation in frog (Rana pipiens and Rana moctezuma) skeletal muscle fibres (original) (raw)

1. The decay of the Ca2+ current (ICa) during a maintained depolarization was studied in intact twitch skeletal muscle fibres of Rana pipiens and Rana moctezuma with the three-micro-electrode voltage-clamp technique. 2. ICa was recorded at 23 TC, after blocking K+ currents, in TEA methanesulphonate saline with 10 mM-Ca2+ made hypertonic by adding 350 mM-sucrose. 3. In two-pulse experiments, ICa during the test pulse was reduced to about 80 % (R. pipiens) or 50 % (R. moctezuma) of the control value, without any detectable inward ICa during 7 s conditioning pre-pulses. 4. The experimental points of the steady-state inactivation curve (h.) were fitted to hoo = (1 +exp ((Em-Vh)/kh))-, where Em is the membrane potential and with Vh =-33 + 3 mV and kh = 6 +1 mV for R. pipiens, and Vh =-44 + 3 mV and kh = 9-5 + ±10 mV for R. moctezuma. 5. The rate constant of decay for inactivated currents (range-8 to-47 mA cm3) and for control currents (range-23 to-62 mA cm-3), was independent of Ica amplitude. The average rate constant of decay at 0 mV was 118 + 002 s-1 (66). 6. These results indicate that in intact fibres under hypertonic solution Ica decay can be explained by a voltage-dependent inactivation process and not by depletion of tubular Ca2+. The absence of depletion could be due to a large fractional tubular volume or to the presence of a Ca2+ pump in the tubular system. INTRODUCTION Voltage-clamp experiments in frog skeletal muscle fibres have shown that Ca2+ current (Ica) spontaneously decays during a maintained depolarization (Beaty &