Slow sodium inactivation in Myxicola axons. Evidence for a second inactive state

Sodium inactivation and reactivation have been examined in voltage-clamped Myxicola axons after long-lasting membrane depolarizations produced either directly by changes in holding potential or indirectly by elevation of external K+ concentration. The results suggest the existence of a second inactivated state of the sodium channel with associated voltage-dependent rate constants at least two orders of magnitude lower than those of the fast inactivation process commonly examined. No specific influence of external [K+] on slow Na+ inactivation could be detected.