Increase of sodium concentration near the inner surface of the nodal membrane

Abstract

Voltage-clamp experiments have been performed in order to investigate the after effects of Na-accumulation in myelinated nerve fibres from the frogRana esculenta. 1. The ionic current associated with a depolarizing test-pulse adjusted to the sodium equilibrium potential has been measured following conditioning volleys of depolarizing pulses. 2. A marked shift ofENa towards less positive potentials has been observed following volleys of more than 50 msec; the time course of recovery ofENa showed a rapid phase followed by a slow one. Both phases proved to be insensitive to suabain, DNP and K-free solution. The slow recovery phase was, however, very much prolonged when the conditioning volleys were applied to a node superfused with Li-Ri. 3. The shift ofENa was associated with a drastic reduction of the outward potassium current and with anomalous rectification; the reduction of the potassium outward current, however, was observed when both the sodium and potassium ions moved from inside to the external solution. 4. The results may be explained by an influx of sodium ions into the nerve fibre which are temporarily accumulated near the inner surface of the membrane. They compete with potassium ions at the inner opening of the potassium “channel” and thereby reduce the number of potassium ions which are available to move through the membrane under the influence of the electrochemical gradient.