Modulation of Na+-Ca2+ exchange in cardiac sarcolemmal vesicles by Ca2+ antagonists

The purpose of this study was to examine the effect of three classes of Ca2+ antagonists, diltiazem, verapamil and nifedipine on Na+-Ca2+ exchange mechanism in the sarcolemmal vesicles isolated from canine heart. Na+-Ca2+ exchange and Ca2+ pump (ATP-dependent Ca2+ uptake) activities were assessed using the Millipore filtration technique. Sarcolemmal vesicles used in this study are estimated to consist of several subpopulations wherein 23% are inside-out and 55% are right side-out sealed vesicles in orientation. The affect of each Ca2+ antagonist on the Na+ dependent Ca2+ uptake was studied in the total population of sarcolemmal vesicles, in which none of the agents depressed the initial rate of Ca2+ uptake until concentrations of 10 microM were incubated in the incubation medium. However, when sarcolemmal vesicles were preloaded with Ca2+ via ATP-dependent Ca2+ uptake, cellular Ca2+ influx was depressed only by verapamil (28%) at 1 microM in the efflux medium with 8 mM Na+. Furthermore, inhibition of Ca2+ efflux by verapamil was more pronounced in the presence of 16 mM Na+ in the efflux medium. The order of inhibition was verapamil greater than diltiazem greater than nifedipine. These results indicate that same forms of Ca2+-antagonist drugs may affect the Na+-Ca2+ exchange mechanism in the cardiac sarcolemmal vesicles and therefore we suggest this site of action may contribute to their effects on the myocardium.