CHARGE MOVEMENT DURING NA+ TRANSLOCATION BY NATIVE AND CLONED CARDIAC NA+/CA2+ EXCHANGER

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+ stoichiometry 3, 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+ exchanger 11. 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)) and Na+-induced charge movements. Charge movements indicate about 400 exchangers per mu-m2 in guinea-pig sarcolemma. Fully activated I(NaCa) densities (20-30-mu-A cm-2) indicate maximum turnover rates of 5,000 s-1. As has been predicted for consecutive exchange models 4-7, the apparent ion affinities of steady state I(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.