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 thought^1–3 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 membrane^4,5 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.