Sodium and gating current time shifts resulting from changes in initial conditions.

Abstract

The Na and gating currents of the squid [Loligo pealei] giant axon elicited by a depolarizing pulse are delayed, with little change in shape, as a result of a hyperpolarizing prepulse. The delays are almost completely saturated, at .apprx. 45 .mu.s, for prepulses to -140 mV. At 8.degree. C they develop with time constants of between 60-180 .mu.g for prepulses in the -130- to -50-mV range. There is a correlation between the extra charge moved during the gating current and the increase in the time delay of the Na current as the magnitude of the hyperpolarizing prepulse is increased. These results strengthen the conclusion that the gating current is indeed closely associated with the process of Na channel opening and provide information concerning the kinetics of the early steps, which are hidden in ionic current measurements. The main features of the gating and Na current time shifts and the correlation between charge movement and time shifts are duplicated by a sequential 6-state model for Na activation.