Immunosuppressive therapy with cyclosporine A (CsA) may be associated with severe side-effects such as nephrotoxicity and arterial hypertension. The partial reversability of these effects suggests that they are at least in part functional. We examined the effects of CsA on cellular signaling in cultured vascular smooth muscle cells from rat aorta. Intracellular free calcium concentrations ([Ca2+]i) were measured using fura-2. Total cell calcium was measured by atomic absorption and cellular endothelin production was estimated by radioimmunoassay. In the presence of CsA the calcium mobilizing effect of angiotensin (Ang) II was significantly enhanced. While the ETA receptor antagonist BQ 123 did not affect Ang II-induced calcium mobilization, the potentiating effect of CsA on [Ca2+]i was blocked by BQ 123. Preincubation of the cells with cyclosporine (10 micrograms/ml) for 30 minutes increased total cell calcium from 2.6 +/- 0.5 to 6.9 +/- 0.3 nmol/mg protein (P < 0.01). Within 24 hours endothelin production was significantly enhanced in the presence of cyclosporine (52.2 +/- 2.5 vs. 65.9 +/- 2.7 fmol/mg protein, P < 0.05). Therefore, the cyclosporine-induced rise of total cell calcium in smooth muscle cells is associated with an enhanced production of endothelin. We speculate that cyclosporine induced changes of Ca(2+)-kinetics may be mediated by endothelin. These results indicate that endothelin may play a major role in cyclosporine-associated side-effects.