Kinetics of inactivation and restoration from inactivation of the L‐type calcium current in human myotubes

1Inactivation and recovery kinetics of L‐type calcium currents were measured in myotubes derived from satellite cells of human skeletal muscle using the whole cell patch clamp technique.2The time course of inactivation at potentials above the activation threshold was obtained from the decay of the current during 15 s depolarizing pulses. At subthreshold potentials, prepulses of different durations, followed by +20 mV test pulses, were used. The time course could be well described by single exponential functions of time. The time constant decreased from 17.8 ± 7.5 s at ‐30 mV to 1.78 ± 0.15 s at +50 mV.3Restoration from inactivation caused by 15 s depolarization to +20 mV was slowed by depolarization in the restoration interval. The time constant increased from 1.11 ± 0.17 s at ‐90 mV to 7.57 ± 2.54 s at ‐10 mV.4Restoration showed different kinetics depending on the duration of the conditioning depolarization. While the time constant was similar at restoration potentials of ‐90 and ‐50 mV after a 1 s conditioning prepulse, it increased with increasing prepulse duration at ‐50 mV and decreased at ‐90 mV.5The experiments showed that the rates of inactivation and restoration of the L‐type calcium current in human myotubes were not identical when observed at the same potential. The results indicate the presence of more than one inactivated state and point to different voltage‐dependent pathways for inactivation and restoration.