Charge movement during Na+ translocation by native and cloned cardiac Na+/Ca2+ exchanger

Abstract

Na+/Ca2+ exchange is electrogenic and moves one net positive charge per cycle. Although the cardiac exchanger has a three-to-one Na+/Ca2+ stoichiometry, details of the reaction cycle are not well defined. Here we associate Na+ translocation by the cardiac exchanger with positive charge movement in giant membrane patches from cardiac myocytes and oocytes expressing the cloned cardiac Na+/Ca2+ exchanger. The charge movements are initiated by step increments of the cytoplasmic Na+ concentration in the absence of Ca2+. Giant patches from control oocytes lack both steady-state Na+/Ca2+ exchange current (INaCa) and Na(+)-induced charge movements. Charge movements indicate about 400 exchangers per micron 2 in guinea-pig sarcolemma. Fully activated INaCa densities (20-30 microA cm-2) indicate maximum turnover rates of 5,000 s-1. As has been predicted for consecutive exchange models, the apparent ion affinities of steady state INaCa increase as the counterion concentrations are decreased. Consistent with an electroneutral Ca2+ translocation, we find that voltage dependence of INaCa in both directions is lost as Ca2+ concentration is decreased. The principal electrogenic step seems to be at the extracellular end of the Na+ translocation pathway.