Kinetic properties of calcium channels of twitch muscle fibres of the frog.

Calcium currents (ICa) were recorded in frog skeletal muscle fibres using the three‐micro‐electrode voltage‐clamp technique. The sartorius muscle was bathed in TEA methanesulphonate saline with 350 mM‐sucrose. 5 mM‐3,4‐diaminopyridine was added to the saline to minimize K+ currents. The I‐V relationship for peak Ca2+ currents showed that ICa was detected at ‐40 mV and reached a maximum value at ca. ‐10 mV. No net inward current was recorded at potentials positive to ca. +40 mV. Remaining K+ currents (IK) were recorded by replacing 10 mM‐Ca2+ with 5.5 mM‐Co2+. They were not noticeably time‐dependent up to +20 mV and would tend to diminish the amplitude of ICa without greatly affecting its time course. ICa tail currents could be separated from non‐linear capacity currents. Tail currents were measured 5 msec after repolarization and extrapolated to the end of the pulse. ICa tail‐current amplitudes at EK were measured with pulses of different durations. The envelope of tail‐current amplitudes declined with a time course similar or identical to that of inward current during a maintained depolarization. Consequently, the decline of inward current cannot be explained by an increase of outward IK with time. ICa inactivated with 9 sec prepulses which did not elicit detectable ICa. The fitted h infinity curve had a mid point of ‐33.0 mV and a steepness of 6.3 mV. ICa between ‐30 mV and +20 mV could be described adequately using the Hodgkin‐Huxley m3h relationship. The fitted m infinity curve had a mid point of ‐35.2 mV and a steepness of 9.9 mV. The limiting Ca2+ permeability PCa was 1.4 +/‐ 0.4 X 10(‐4) cm/sec.