The effect of Li+ on Na-Ca exchange in cardiac sarcolemmal vesicles.

The effects of Li+ on Na-Ca exchange in bovine cardiac sarcolemmal vesicles were examined. The initial rate of Na(+)-dependent Ca2+ uptake and efflux was inhibited by Li+ in a dose dependent manner. The initial rate of Na(+)-dependent Ca2+ uptake was inhibited 49.8 +/- 2.9% (S.E.) (n = 6) in the presence of Li+ compared to activity in external K+ or choline+. Kinetic analysis indicated that Li+ increased the Km for Ca2+ (96.3 microM) compared to K+ and choline+ (25.5 and 22.9 microM respectively) while Vmax (1.4, 1.2 and 1.1 nmol Ca2+/mg protein/sec respectively) remained unchanged. Li+ did not alter the experimentally derived stoichiometry of the exchange reaction of 3 Na+ for 1 Ca2+.