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.
D015761:4-Aminopyridine / Q000031:analogs & derivatives*; D000077770:Amifampridine; D000631:Aminopyridines / Q000494:pharmacology; D000818:Animals; D002118:Calcium / Q000378:metabolism* / Q000502:physiology; D066298:In Vitro Techniques; D007473:Ion Channels / Q000187:drug effects / Q000502:physiology*; D007700:Kinetics; D008564:Membrane Potentials; D009132:Muscles / Q000378:metabolism / Q000502:physiology*; D011188:Potassium / Q000502:physiology; D011896:Rana temporaria; D019789:Tetraethylammonium; D013757:Tetraethylammonium Compounds / Q000494:pharmacology
D000631:Aminopyridines; D007473:Ion Channels; D013757:Tetraethylammonium Compounds; D019789:Tetraethylammonium; D015761:4-Aminopyridine; D000077770:Amifampridine; D011188:Potassium; D002118:Calcium