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^1,2. Although the cardiac exchanger has a three-to-one Na⁺/Ca²⁺ stoichiometry³, details of the reaction cycle are not well defined^2,4–8. Here we associate Na⁺ translocation by the cardiac exchanger with positive charge movement in giant membrane patches from cardiac myocytes^9,10 and oocytes expressing the cloned cardiac Na⁺/Ca²⁺ exchanger¹¹. 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 (I_NaCa) Na⁺-induced charge movements. Charge movements indicate about 400 exchangers per (μm² in guinea-pig sarcolemma. Fully activated I_NaCa densities (20–30 μA cm^–2) indicate maximum turnover rates of 5,000s⁻¹. As has been predicted for consecutive exchange models^4–7, the apparent ion affinities of steady stateI_NaCa increase as the counterion concentrations are decreased. Consistent with an electroneutral Ca²⁺ translocation, we find that voltage dependence of I_NaCa 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.