Block of Na current and intramembrane charge movement in myelinated nerve fibres poisoned with a vegetable toxin

Abstract

In nerve membrane, the non-linear capacity current (displacement current) is assumed to reflect the movement of intrinsic membrane charges which control the opening of specific pathways for sodium ions (Na channels). However, various discrepancies have been reported between the effects of pharmacological agents on sodium and displacement currents (for a review see ref. 1). It is generally supposed that the opening and closing of Na channels constitutes one step of a multi-step system in which each configuration change may or not give rise to a measurable charge movement². New drugs affecting either sodium or displacement currents may elucidate the relationship between charge movement and the control of sodium conductance. We therefore now report a comparison of the effects of a vegetable toxin (oenanthotoxin or OETX)³ on both sodium current (I_Na) and intra-membrane charge movement (Q) in Ranvier nodes. We show that OETX reversibly blocks both sodium and displacement current. Studies of I_Na and Q during partial suppression by the toxin reveal differences in the effects of OETX on the remaining I_Na and Q. The findings are discussed in relation to recent models for the Na-channel gating process and for Na-channel block by local anaesthetics.