Stoichiometry of the sodium-calcium exchanger in nerve terminals.

The stoichiometry of the Na+-Ca2+ exchanger from synaptic plasma membranes was studied in both native and reconstituted preparations. In kinetic experiments performed with the native preparation, initial rates of Na+ gradient-dependent Ca2+ influx were compared to Ca2+-dependent Na+ efflux. These experiments showed that 4.82 Na+ ions are exchanged for each Ca2+ ion. A thermodynamic approach in which equilibrium measurements were made with the reconstituted preparation resulted in a similar (4.76) stoichiometry. The effects of membrane potential generated by valinomycin-induced K+ fluxes could be demonstrated in the reconstituted preparation. In addition, the direct contribution of the Na+-Ca2+ exchanger to the membrane potential across the reconstituted vesicle membrane could be demonstrated by using the lipophilic cation tetraphenylphosphonium.