Equilibrium calcium-calcium exchange in cardiac sarcolemmal vesicles

Ca-Ca isotopic exchange activity in bovine cardiac sarcolemmal vesicles was measured under equilibrium conditions, i.e. where intra- and extravesicular media have identical compositions. The results demonstrate that the monovalent cations K+, Li+, and Rb+ stimulate the process of Ca-Ca exchange. The presence of monovalent cations increases the Vmax for this process but has little effect on the Km for Ca2+. Maximal stimulation of exchange activity occurs when the monovalent cations are present on both sides of the vesicle membrane; a lesser degree of stimulation is observed when the cations are present at only one membrane surface. Na+ also stimulates Ca-Ca exchange at low concentrations (0.5-8 mM) but at high concentrations causes a marked inhibition. The latter effect is due to a competition between Na+ and Ca2+ for a site on the exchange carrier. The presence of monovalent cations alters the pH profile for Ca-Ca exchange. In the absence of stimulating cations, Ca-Ca exchange activity exhibits a maximum at pH 7.0 and then drops to approximately half the peak value at pH 7.5 or higher. In contrast, the rate of Ca-Ca exchange in the presence of monovalent cations continues to increase as the pH is raised from pH 6 to pH 9. Measurements of 86Rb+ fluxes demonstrate that Rb-stimulated Ca-Ca exchange does not involve the co-transport of Rb+. Monovalent cations produce no stimulation of Na-dependent Ca2+ fluxes (Na-Ca exchange) when present on the same side of the membrane as Ca2+. The results are discussed in terms of an extension of the model for the Na-Ca exchanger that was described in the accompanying paper (Reeves, J.P., and Sutko, J.L. (1983) J. Biol. Chem. 258, 3178-3182).