Sodium pump stoichiometry in Aplysia neurones from simultaneous current and tracer measurements

Abstract

THERE is growing evidence for the involvement in neuronal performance of hyperpolarising current resulting from the activities of the membrane ‘sodium pump’¹. Studies of the stoichiometry of metabolically fuelled ionic movements^1–6 have consistently shown that more Na is extruded than K accumulated. But while the electrical effect of such an imbalance is of immediate significance to the ongoing activity of a neurone, we are aware of only one study in which the electrogenicity of sodium pump activity in a neurone has been estimated⁷. Here we make such an estimate by comparing simultaneous measurements from a single neurone somata of tracer Na efflux and the transmembrane current attributable to that efflux. The results show that at least half of the actively extruded Na is electrogenic.