KINETICS OF INTRAMEMBRANE CHARGE MOVEMENT AND CONDUCTANCE ACTIVATION OF BATRACHOTOXIN-MODIFIED SODIUM-CHANNELS IN FROG NODE OF RANVIER

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 prehyperpolarization. ON charge movement followed a single-exponential time course with time constants .tau.Q,ON slightly larger than .tau.Na,ON. For pulses between -70 and -50 mV, .tau.Q,ON/.tau.Na,ON = 1.14 .+-. 0.08. The OFF charge movement and OFF sodium current tails after a depolarizing pulse followed single-exponential time courses, with .tau.Q,OFF larger than .tau.Na,OFF. .tau.Q,OFF/.tau.Na,OFF increased with OFF voltage from 1 near -100 mV to 2 near -160 mV. At a set OFF potential (-120 mV), both .tau.Q,OFF and .tau.Na,OFF increased with ON pulse duration. The delay in INa activation and the effect of ON pulse duration on .tau.Q,OFF and .tau.Na,OFF are inconsistent with a simple two-state, single-transition model for the gating of batrachotoxin-modified sodium channels.