1. As we have previously shown, cyclosporin A enhances the vasoconstrictor-induced rise in intracellular free calcium in vascular smooth muscle cells. This effect may contribute to important side-effects in cyclosporin therapy, such as hypertension and nephrotoxicity. Atrial natriuretic peptide has been shown to inhibit this effect as well as the cyclosporin-stimulated transmembrane calcium influx in smooth muscle cells.

2. The present study, therefore, was designed to examine the effect of cyclosporin and atrial natriuretic peptide on total cellular calcium content in the rat. Furthermore, since cyclosporin was recently shown to induce endothelin production in smooth muscle cells, we investigated the effect of atrial natriuretic peptide on this potentially adverse cellular effect of cyclosporin therapy.

3. Total cell calcium was measured by atomic absorption, and cellular endothelin production was estimated by radioimmunoassay.

4. Preincubation of the cells with cyclosporin (10 μg/ml) for 30 min increased total cell calcium from 2.6 ± 0.5 to 6.9 ± 0.3 nmol/mg of protein (P < 0.01). Within 24 h endothelin production was significantly enhanced in the presence of cyclosporin (52.2 ± 2.5 versus 65.9 ± 2.7 fmol/mg of protein, P < 0.05). Therefore, the cyclosporin-induced rise in total cell calcium in smooth muscle cells is associated with enhanced production of enthothelin. Thus, it is tempting to speculate that the cyclosporin-induced changes in calcium kinetics may be mediated by endothelin.

5. In the presence of atrial natriuretic peptide (10-8 mol/l), the cyclosporin-induced rise in total cell calcium was significantly reduced (6.9 ± 0.3 versus 5.1 ± 0.2 nmol/mg of protein, P < 0.05). In the presence of atrial natriuretic peptide (5 × 10−9 mol/l) cyclosporin A did not affect the production of endothelin in smooth muscle cells (453 ± 3.5 versus 50.1 ± 2.8 fmol/mg of protein, not significant).

6. These results indicate that endothelin may play a major role in cyclosporin-associated side-effects. By inhibiting the cyclosporin-stimulated production of endothelin, atrial natriuretic peptide may have a beneficial effect on cyclosporin-associated hypertension and nephrotoxicity.

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