Inactivation kinetics of the sodium channel in the egg and the isolated, neurally differentiated blastomere of the ascidian.

1. Inactivation kinetics of the sodium channel was compared between the egg-type channel in the egg cell and the differentiated-type channel in the cleavage-arrested, neurally differentiated blastomere of the ascidian. The techniques of the two-microelectrode voltage clamp and the cell-attached patch clamp were used. 2. In both types of channel, the time course of inactivation development obtained with a two-pulse protocol at potentials from -40 to -60 mV could be fitted with two exponentials with distinctive parameters. 3. The time course of recovery from inactivation at potentials more negative than -70 mV was compared between the two types of channel. At -80 to -120 mV, a delay of recovery was evident in the egg-type channel, whereas no delay was observed in the differentiated type. 4. In both types of channel, the two time constants of the inactivation of the macroscopic current, derived from the measurements of inward current, inactivation development and recovery from inactivation, had a bell-shaped voltage dependency. The fast time constants had a peak at -55 mV in the differentiated type and -70 mV in the egg type. The slow time constants had a peak around -60 mV in both types. 5. At the single-channel level, the averaged current from the differentiated-type channel showed both fast and slow decays. The frequency of late openings was higher in the differentiated-type channel than in the egg type. 6. The voltage dependence of the decay time constant and the carried charge in the summed current of the single-channel events was found to be shifted in the negative direction by 10-30 mV, compared with that of the macroscopic current. 7. The possibility that the higher frequency of late openings in the differentiated-type channel might be derived from delayed activation was excluded, since first-latency histograms of the single channel were not significantly different between the two types of channel.