Asymmetrical currents and sodium currents in Ranvier nodes exposed to DDT

Abstract

THE non-linear component of the capacity current (asymmetrical ‘displacement current’) of nerve membranes has been thought1–3 to result from the movement of intrinsic membrane charges gating the specific pathways for sodium ions (‘Na channels’). This interpretation is partly based on the observation that the voltage-dependent relaxation time constants of the sodium activating mechanism roughly agree with those of the displacement currents. According to this hypothesis, a similar correlation should be demonstrated when the kinetics characteristics of the Na system are modified under controlled experimental conditions. In this respect, the insecticide DDT is a useful tool as it slows down the turning-off of the Na-permeability of the nodal membrane4,5 without changing the rate of its turning-on. We have therefore compared the relaxation time constants of both the sodium current activation and the displacement current on DDT-treated Ranvier nodes. The results suggest that the intramembrane charge movement is not the sole transition involved in the gating process of the Na channels.