Recovery from charge immobilization in sodium channels of the frog node of Ranvier

Abstract

The gating current off-response of the frog node of Ranvier shows a fast and a slow phase, reflecting presumably charges moving back into the resting position and charges returning from immobilization. The paper describes measurements of the time constant of the slow component, υ_off2, at different potentials at 20 or 17°C. The time constant υ_off2 decreased markedly when the potential to which the fibre was repolarized at the end of the test pulses was decreased from −100 to −145 mV. υ_off2 was compared with υ_{off2 gat} and υ_{off2 Na}, the time constants of the recovery of gating current and peak sodium current from the effect of a depolarizing conditioning pulse. The three time constants were equal at −145 mV, but somewhat different at −100 to −130 mV, the sequence being516-6. The inequality υ_{off2 gat} < υ_off2 was small and statistically not significant. It does not seem to be due to contamination of the charge movement with ionic currents because a) theQ _off/Q _on ratio was near 1.0 at −100 to −120 mV, b) partial replacement of internal CsCl by KCl did not significantly alter υ_off2 orQ _off/Q _on, c) υ_off2 was independent of pulse size. The small incquality υ_{re gat} < υ_off2 suggests that charges which have returned from immobilization are not immediately available for displacement. The inequality υ_{re gat} < υ_{re Na} was larger and statistically significant at −100 and −110 mV. It confirms previous measurements of Nonner (1980). Although υ_{re gat} < υ_{re Na}, recovery of the gating current preceded recovery of the sodium current, because it started from a higher level.