Symmetry properties of the Na+-Ca2+ exchange mechanism in cardiac sarcolemmal vesicles

Abstract

The Na+-Ca2+ exchange mechanism in cardiac sarcolemmal vesicles can catalyze the exchange of Ca2+ on either side of the sarcolemmal membrane for Na+ on the opposing side. Little is known regarding the relative affinities of Na+ and Ca2+ for exchanger binding sites on the intra- and extracellular membrane surfaces. We have previously reported (Philipson, K.D. and Nishimoto, A.Y. (1982) J. Biol. Chem. 257, 5111-5117) a method for measuring the Na+-Ca2+ exchange of only the inside-out vesicles in a mixed population of sarcolemmal vesicles (predominantly right-side-out). We concluded that the apparent Km(Ca2+) for Na+i-dependent Ca2+ uptake was similar for inside-out and right-side-out vesicles. In the present study, we examine in detail Na+o-dependent Ca2+ efflux from both the inside-out and the total population of vesicles. To load vesicles with Ca2+ prior to measurement of Ca2+ efflux, four methods are used: 1, Na+-Ca2+ exchange; 2, passive Ca2+ diffusion; 3, ATP-dependent Ca2+ uptake; 4, exchange of Ca2+ for Na+ which has been actively transported into vesicles by the Na+ pump. The first two methods load all sarcolemmal vesicles with Ca2+, while the latter two methods selectively load inside-out vesicles with Ca2+. We are able to conclude that the dependence of Ca2+ efflux on the external Na+ concentration is similar in inside-out and right-side-out vesicles. Thus the apparent Km(Na+) values (approximately equal to 30 mM) of the Na+-Ca2+ exchanger are similar on the two surfaces of the sarcolemmal membrane. In other experiments, external Na+ inhibited the Na+i-dependent Ca2+ uptake of the total population of vesicles much more potently than that of the inside-out vesicles. Apparently Na+ can compete for the Ca2+ binding site more effectively on the external surface of right-side-out than on the external surface of inside-out vesicles. Thus, although affinities for Na+ or Ca2+ (in the absence of the other ion) appear symmetrical, the interactions between Na+ and Ca2+ at the two sides of the exchanger are not the same. The Na+-Ca2+ exchanger is not a completely symmetrical transport protein.