Modulation of Na+-Ca2+ exchange in sarcolemmal vesicles by intravesicular Ca2+

When cardiac sarcolemmal vesicles were incubated at 37 degrees C in 160 mM NaCl containing 0.5 mM CaCl2 and subsequently assayed for Na-Ca exchange activity, they exhibited a threefold increase in the initial rate of 45Ca2+ uptake (at 20 microM 45Ca2+) compared with vesicles incubated without added CaCl2. Removal of endogenous Ca2+ by incubation of the vesicles with 0.1 mM ethylene-bis(beta-aminoethylether)N,N'-tetraacetic acid (EGTA) resulted in a 35% inhibition in exchange activity. The pretreatment with CaCl2 produced an acceleration of Na-Ca exchange activity rather than an increase in Ca2+ uptake due to Ca-Ca exchange. Pretreatment of the vesicles with CaCl2 lowered the apparent Km of the exchange system for Ca2+. The effects of the Ca treatment were reversed by subsequently incubating the vesicles with EGTA. In contrast to the effects of intravesicular Ca2+ on Nai-dependent Ca2+ uptake, external Ca2+ had no effect on Nao-dependent Ca2+ efflux. The results suggest that an understanding of the kinetics of the Na-Ca exchange system may be hampered by the autoacceleration of exchange activity that occurs during initial rate measurements as Ca2+ accumulates within the vesicles. This phenomenon may contribute to the variability that exists among different vesicle preparations in their apparent Km values for Ca2+.