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

1. Inactivation and recovery kinetics of L-type calcium currents were measu red in myotubes derived from satellite cells of human skeletal muscle using the whole cell patch clamp technique. 2. The time course of inactivation at potentials above the activation thres hold was obtained from the decay of the current during 15 s depolarizing pu lses. At subthreshold potentials, prepulses of different durations, followe d 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. 3. Restoration from inactivation caused by 15 s depolarization to +20 mV wa s slowed by depolarization in the restoration interval. The time constant i ncreased from 1.11 +/- 0.17 s at -90 mV to 7.57 +/- 2.54 s at -10 mV. 4. Restoration showed different kinetics depending on the duration of the c onditioning depolarization. While the time constant was similar at restorat ion potentials of -90 and -50 mV after a 1 s conditioning prepulse, it incr eased with increasing prepulse duration at -50 mV and decreased at -90 mV. 5. The experiments showed that the rates of inactivation and restoration of the L-type calcium current in human myotubes were not identical when obser ved at the same potential. The results indicate the presence of more than o ne inactivated state and point to different voltage-dependent pathways for inactivation and restoration.