L-TYPE CALCIUM CURRENT ACTIVATION IN CULTURED HUMAN MYOTUBES

The time course of activation of the skeletal muscle L-type calcium ch annel was studied in voltage-clamped myotubes derived from human satel lite cells. The slow L-type current was isolated by inactivating faste r calcium current components using appropriate prepulses or by subtrac ting the currents not blocked by 5 mu M nifedipine. The L-type current exhibited a single exponential activation and time constants which sh owed little voltage dependence in the range +10 to +50 mV. Currents bl ocked by nifedipine could be partially restored by W-light flash photo lysis. When a flash of light was applied during a depolarizing step, t he activation time course of the resulting inward current contained a rapid, almost instantaneous component followed by a slower component. The amplitude of the rapid component was different when the flash was applied at different times during the depolarizing step: depolarizatio n first increased and then decreased the fraction of channels which co uld rapidly be restored from the block by photolysis. Plotted versus t ime after the onset of the depolarization this fraction closely matche d the time course of the L-type current obtained before the block by n ifedipine. This indicates that the slow gating recations of the Ca2+ c hannel remain functional in the nifedipine-blocked state. Large condit ioning depolarizations which had been shown to enhance the speed of L- type current activation in frog muscle fibres showed no effect in huma n myotubes. Numerical simulations using a gating scheme proposed for f rog muscle demonstrate that such differences can be caused by changing just a single kinetic parameter.