Further studies of the potential‐dependence of the sodium‐induced membrane current in snail neurones.

Abstract

The potential-dependence of the membrane current induced by intracellular injections of Na+ was studied on giant neurons of the snail Helix pomatia. This current decreases with membrane hyperpolarization at room temperature and can be reversed at sufficiently negative holding potentials. The same injections at 7.degree. C, and injections of L+ or K+ do not induce membrane currents and do not increase membrane conductance. An increase in the amount of injected Na changes the potential-dependence of the induced membrane currents. Small injections (.apprx. 1 .mu.C) induce a current that does not depend upon the membrane potential. Further increase in the injection size increases the induced current and enchances its potential-dependence and often reveals the existence of a reversal potential. The latter reaches -60 to -65 mV with large Na injections. An increase in extracellular K concentration from 4-8 mM shifts the reversal potential 17 mV in the depolarizing direction, and a decrease from 4 to 2 mM shifts it 14 mV in the hyperpolarizing direction. Replacement of K by Rb or elimination of Na+ from the outside solution, does not affect the induced current or its potential-dependence. The coefficient of electrogenicity (the ratio between the amount of charge transferred by the Na-induced membrane current and the amount brought into the cell during the injection) increases with an increase in the injection size if the membrane potential is clamped near the resting potential level. This relation is reversed when the holding potential is -80 mV. The reversal takes place at holding potentials near -60 mV. The induced current and its potential-dependence are unaffected by 10 mM tetraethylammonium [TEA]. The potential-dependence of the Na-induced membrane current is probably a result of a specific increase in the membrane K conductance coupled with high activity of the Na pump.