The Na+/Ca2+ Antiporter in Aortic Smooth Muscle Cells. Characterization and Demonstration of an Activation by Phorbol Esters

The Na+/Ca2+ antiporter is present in aortic smooth muscle cells of the A7r5 cell line. Imposing an outward Na+ gradient to the cells promoted a 45Ca2+ uptake component which was sensitive to amiloride derivatives and insensitive to blockers of the voltage-dependent Ca2+ channel. The Ca2+ uptake system was dependent on intracellular Na+ concentration; it was inactive when Li+ replaced intracellular Na+ and it was electrogenic. Flow cytometric analysis of cells that had been loaded with the Ca2+ indicator indo-1 showed that all conditions that promoted Ca2+ influx led to corresponding increases in the free cytoplasmic Ca2+ concentration. Treatment of the A7r5 cells with phorbol myristate acetate, a known activator of protein kinase C (Ca2+/phospholipid-dependent enzyme), led to a two-fold activation of the system and to larger intracellular Ca2+ transients when cells were shifted to Na+-free solutions. Activation was observed at all intracellular Na+ concentrations. Changing the activity of the Na+/Ca2+ system did not affect the size and duration of intracellular Ca2+ transients elicited by the Ca2+ mobilizing hormone vasopressin. It is concluded that the Na+/Ca2+ antiporter in smooth muscle cells is a target for protein kinase C but that the system is not involved in the regulation of Ca2+ transients induced by vasopressin.