Modulation of Na/sup +/-Ca/sup 2 +/ exchange in sarcolemmal vesicles by intravesicular Ca/sup 2 +/

When cardiac sarcolemmal vesicles were incubated at 37/sup 0/C in 160 mM NaCl containing 0.5 mM CaCl/sub 2/ and subsequently assayed for Na-Ca exchange activity, they exhibited a threefold increase in the initial rate of /sup 45/Ca/sup 2 +/ uptake compared with vesicles incubated without added CaCl/sub 2/. Removal of endogenous Ca/sup 2 +/ 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 CaCl/sub 2/ produced an acceleration of Na-Ca exchange activity rather than an increase in Ca/sup 2 +/ uptake due to Ca-Ca exchange. Pretreatment of the vesicles with CaCl/sub 2/ lowered the apparent K/sub m/ of the exchange system for Ca/sup 2 +/. The effects of the Ca treatment were reversed by subsequently incubating the vesicles with EGTA. In contrast to the effects of intravesicular Ca/sup 2 +/ on Na/sub i/-dependent Ca/sup 2 +/ uptake, external Ca/sup 2 +/ had no effect on Na/sub 0/-dependent Ca/sup 2 +/ 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 Ca/sup 2 +/ accumulates within themore » vesicles. This phenomenon may contribute to the variability that exists among different vesicle preparations in their apparent K/sub m/ values for Ca/sup 2 +/.« less