The electrogenic sodium pump activity in Aplysia neurons is not potential dependent

We have investigated the potential dependence of the electrogenic sodium pu mp in Aplysia neurons by recording the potential and current induced by sud den change of the artificial sea water from one containing K+ at various co ncentrations to K+-free sea water in the presence or absence of ouabain. Bo th K+-free sea water and ouabain block sodium transport and result in a sig nificant depolarization due to removal of a maintained outward current that is a result of transport of more Na+ out of the cell than K+ into the cell during pump operation. In the presence of ouabain there is, however, an in ward current induced by changing external K+ concentration from zero to som e value between 1 and 20 mM, and this current is greater with a greater Kconcentration gradient. The current induced by change from zero to 1 mM Kdoes not show any potential dependence, although those currents induced by higher K+ concentrations are potential dependent. We conclude that the acti vity of the electrogenic sodium pump is not potential dependent, but that t he potential independence is obscured if higher concentrations of K+ are us ed to activate the electrogenic sodium pump.