A possible action of nicardipine on the cardiac sarcolemmal Na+-Ca2+ exchange

The effects of nicardipine on sodium-calcium exchange activity of cardiac sarcolemma-enriched vesicles isolated from the rat heart were examined. Sodium-loaded, sarcolemma-enriched vesicles, when exposed to a medium containing 40 microM CaCl2, exhibited about 5 nmoles Ca2+/mg protein of the maximal calcium uptake; the initial rate was 21 nmoles Ca2+/mg protein/min. The calcium uptake was dependent on the extravesicular concentration of calcium ion. Nicardipine at concentrations of 0.1 to 10 microM depressed the rate of calcium uptake activity by 60-90%. The isolated membrane vesicles preloaded with Ca2+ showed a calcium efflux activity, when exposed to a medium containing sodium ion. The rate of calcium efflux was 2.5 nmoles Ca2+/mg protein/min, when measured in a medium containing 6.5 mM NaCl. The efflux rate was facilitated with increased concentrations of sodium ion in the medium. About 75% of the preloaded calcium in the vesicles was released within 3 min of incubation. The rate of calcium efflux was stimulated in the presence of 0.1 to 10 microM nicardipine (2.5- to 4-fold increase). The present results suggest a possible action of nicardipine on the sodium-calcium exchange mechanism at cardiac sarcolemmal sites.