CYCLOSPORINE-A ENHANCES TOTAL CELL CALCIUM-INDEPENDENT OF NA-K-ATPASEIN VASCULAR SMOOTH-MUSCLE CELLS

The effect of cyclosporine A in enhancing vasconstrictor-induced calci um (Ca2+) mobilization in vascular smooth muscle cells may contribute to important side effects in cyclosporine therapy such as hypertension and nephrotoxicity. As we have previously shown, cyclosporine A stimu lates transmembrane Ca2+ influx. Since Ca2+ efflux was not affected by cyclosporine A, we concluded that cyclosporine augments angiotensin I I induced Ca2+ mobilization in vascular smooth muscle cells by an incr eased amount of Ca2+ in angiotensin II sensitive intracellular Ca2+ st ores. The present study was therefore designed to examine the effect o f cyclosporine A. on cellular calcium content and on membrane calcium transport mechanisms. An important mechanism of Ca2+ extrusion from th e cell is the Na-Ca exchanger. Its activity is closely related with th at of the Na-K-ATPase. By increasing cellular sodium concentration the blockade of Na-K-ATPase would in turn activate cellular calcium uptak e bx the Na-Ca exchanger. Therefore, we hypothesized that cyclosporine A might exert its effects in the same manner as a circulating Na-K-AT Pase inhibitor. Total cell calcium was measured by atomic absorption a nd activity of Na-K-ATPase was estimated by an assay measuring phospha te production. Preincubation of the cells with cyclosporine (10 mu g/m l) for 15 min increased total cell calcium from 31.4+/-5.0 to 46.5+/-5 .3 nmol/mg protein (P < 0.05). Activity of Na-K-ATPase was not affecte d by cyclosporine A (3.9+/-0.2 vs. 4.3+/-0.2 mu mol P-i h(-1) mg(-1) p rotein). Therefore, cyclosporine A induced Ca2+ influx is not mediated by an inhibition of the Na-K-ATPase. Cyclosporine-stimulated accumula tion of cellular calcium may be mediated, for example, by opening of c alcium channels in the plasma membrane. Increased Ca2+ mobilization in the presence of cyclosporine A may be due to an increased amount of C a2+ available from intracellular Ca2+ stores. These results are of sub stantial significance for understanding the pathophysiological mechani sms of cyclosporine A induced vasoconstriction.