L-type calcium current activation in cultured human myotubes.

The time course of activation of the skeletal muscle L-type calcium channel was studied in voltage-clamped myotubes derived from human satellite cells. The slow L-type current was isolated by inactivating faster calcium current components using appropriate prepulses or by subtracting the currents not blocked by 5 microM nifedipine. The L-type current exhibited a single exponential activation and time constants which showed little voltage dependence in the range +10 to +50mV. Currents blocked by nifedipine could be partially restored by UV-light flash photolysis. When a flash of light was applied during a depolarizing step, the 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: depolarization first increased and then decreased the fraction of channels which could rapidly be restored from the block by photolysis. Plotted versus time after the onset of the depolarization this fraction closely matched the time course of the L-type current obtained before the block by nifedipine. This indicates that the slow gating recations of the Ca2+ channel remain functional in the nifedipine-blocked state. Large conditioning depolarizations which had been shown to enhance the speed of L-type current activation in frog muscle fibres showed no effect in human myotubes. Numerical simulations using a gating scheme proposed for frog muscle demonstrate that such differences can be caused by changing just a single kinetic parameter.