Interactions of monovalent cations with sodium channels in squid axon. I. Modification of physiological inactivation gating.

Abstract

Inactivation of Na channels was studied in voltage-clamped, internally perfused squid giant axons during changes in the ionic composition of the intracellular solution. Peak Na currents were reduced when tetramethyl-ammonium ions (TMA+) were substituted for Cs ions internally. The reduction reflected a rapid, voltage-dependent block of a site in the channel by TMA+. The estimated fractional electrical distance for the site was 10% of the channel length from the internal surface. Na tail currents were slowed by TMA+ and exhibited kinetics similar to those seen during certain drug treatments. Steady state INa was simultaneously increased by TMA+, resulting in a cross-over of current traces with those in Cs+ and in greatly diminished inactivation at positive membrane potentials. Despite the effect on steady state inactivation, the time constants for entry into and exist from the inactivated state were not significantly different in TMA+ and Cs+. Increasing intracellular Na also reduced steady state inactivation in a dose-dependent manner. Ratios of steady state INa to peak INa varied from .apprx. 0.14 in Cs+- or K+-perfused axons to .apprx. 0.4 in TMA+- or Na+-perfused axons. These results are consistent with a scheme in which TMA+ or Na+ can interact with a binding site near the inner channel surface that may also be a binding or coordinating site for a natural inactivation particle. A simple competition between the ions and an inactivation particle is, however, not sufficient to account for the increase in steady state INa and changes in the inactivation process itself must accompany the interaction of TMA+ and Na+ with the channel.