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

We have investigated the potential dependence of the electrogenic sodium pump in Aplysia neurons by recording the potential and current induced by sudden change of the artificial sea water from one containing K⁺ at various concentrations to K⁺-free sea water in the presence or absence of ouabain. Both K⁺-free sea water and ouabain block sodium transport and result in a significant 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 inward current induced by changing external K⁺ concentration from zero to some value between 1 and 20 mM, and this current is greater with a greater K⁺ concentration gradient. The current induced by change from zero to 1 mM K⁺ does not show any potential dependence, although those currents induced by higher K⁺ concentrations are potential dependent. We conclude that the activity of the electrogenic sodium pump is not potential dependent, but that the potential independence is obscured if higher concentrations of K⁺ are used to activate the electrogenic sodium pump.