Inactivation of the sodium channel. II. Gating current experiments.

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Abstract
Gating current (Ig) was studied in relation to inactivation of the Na channels [in Loligo pealei axons]. No component of Ig has the time course of inactivation; apparently little or no charge movement is associated with this step. Inactivation nonetheless affects Ig by immobilizing about 2/3 of gating charge. Immobilization can be followed by measuring on charge movement during a pulse and comparing it to off charge after the pulse. The off:on ratio is near 1 for a pulse so short that no inactivation occurs, and the ratio drops to about 1/3 with a time course that parallels inactivation. Other correlations between inactivation and immobilization are that they have the same voltage dependence; and charge movement recovers with the time course of recovery from inactivation. This is interpreted to mean that the immobilized charge returns slowly to off position with the time course of recovery from inactivation, and that the small current generated is lost in base-line noise. At -150 mV recovery is very rapid, and the immobilized charge forms a distinct slow component of current as it returns to off position. After destruction of inactivation by pronase, there is no immobilization of charge. A model is presented in which inactivation gains its voltage dependence by coupling to the activation gate.