Abstract

Hypertension is a serious side-effect of the clinical use of Cyclosporin A (CsA). One notion is that alterations of vascular reactivity contribute to this hypertension. In this study we used the isolated rat mesenteric vascular bed to test the specific hypothesis that CsA modifies vascular calcium regulation to potentiate vascular contractility. Mesenteric vessels from CsA-treated rats (10 mg/kg/day i.m. for 7 days) exhibited significantly greater vasoconstrictor responses to exogenous norepinephrine (NE) and potassium-induced depolarization than those of vehicle-treated animals. Similarly, in vitro CsA (8.3 X 10(-8) to 8.3 X 10(-6) M) augmented in a concentration-dependent manner both the sensitivity and the maximum response to NE and potassium. This effect of CsA on vasoconstriction was critically dependent on the presence of external calcium [( Ca++]o). The degree of vasoconstriction potentiation correlated significantly with the [Ca++]o which was used during exposure of the mesenteric bed to CsA. Reapplication of external calcium in the presence of CsA to "calcium-depleted" preparations increased significantly the amplitude of subsequent NE responses in a calcium-free medium. Thus, CsA-potentiated NE responses, once established, were not reversed by removing external calcium; however, attenuation occurred with the intracellular calcium antagonist dantrolene. Verapamil and nifedipine blocked potassium-elicited responses, but failed to prevent the CsA effect on NE-induced vasoconstriction. We conclude that CsA potentiation of vasoconstriction depends on extracellular calcium, and results from an enhanced transmembrane calcium transport. We speculate that CsA also increases the filling of intracellular stores of releasable calcium. These effects lead to greater calcium influx and greater intracellular calcium release upon stimulation.