Kinetics of intramembrane charge movement and conductance activation of batrachotoxin-modified sodium channels in frog node of ranvier

Abstract

Sodium current and intramembrane gating charge movement (Q) were monitored in voltage-clamped frog node of Ranvier after modification of all sodium channels by batrachotoxin (BTX). Sodium current activation followed a single-exponential time course, provided a delay was interposed between the onset of the step ON depolarization and that of the current change. The delay decreased with increased ON depolarization and, for a constant ON depolarization, increased with prehyper polarization. ON charge movement followed a single-exponential time course with time constants τQON slightly larger than τNa. For pulses between -70 and -50 mV, τQON/τNa = ON = 1.14 ± 0.08. The OFF charge movement and OFF sodium current tails after a depolarizing pulse followed single-exponential time courses, with τQOFF larger than τNaOFF, τQOFF/τNaOFF increased with OFF voltage from 1 near -100 mV to 2 near -160 mV. At a set OFF potential (-120 mV), both τQOFF and τNaOFF increased with ON pulse duration. The delay in IN. activation and the effect of ON pulse duration on τQOFF and τNaOFF are inconsistent with a simple two-state, singletransition model for the gating of batrachotoxin-modified sodium channe. © 1985, Rockefeller University Press., All rights reserved.