ATRIAL-NATRIURETIC-PEPTIDE INHIBITS CYCLOSPORINE-A-INDUCED ENDOTHELINPRODUCTION AND CALCIUM ACCUMULATION IN RAT VASCULAR SMOOTH-MUSCLE CELLS

1. As we have previously shown, cyclosporin A enhances the vasoconstri ctor-induced rise in intracellular free calcium in vascular smooth mus cle cells. This effect may contribute to important side-effects in cyc losporin therapy, such as hypertension and nephrotoxicity. Atrial natr iuretic peptide has been shown to inhibit this effect as well as the c yclosporin-stimulated transmembrane calcium influx in smooth muscle ce lls. 2. The present study, therefore, was designed to examine the effe ct of cyclosporin and atrial natriuretic peptide on total cellular cal cium content in the rat. Furthermore, since cyclosporin was recently s hown to induce endothelin production in smooth muscle cells, we invest igated the effect of atrial natriuretic peptide on this potentially ad verse 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 cyc losporin (10 mu g/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 endotheli n 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 c ells is associated with enhanced production of enthothelin. Thus, it i s tempting to speculate that the cyclosporin-induced changes in calciu m kinetics may be mediated by endothelin. 5. In the presence of atrial natriuretic peptide (10(-8) mol/l), the cyclosporin-induced rise in t otal 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 p eptide (5 x 10(-9) mol/l) cyclosporin A did not affect the production of endothelin in smooth muscle cells (45.5+/-3.5 versus 50.1 +/- 2.8 f mol/mg of protein, not significant). 6. These results indicate that en dothelin may play a major role in cyclosporin-associated side-effects. By inhibiting the cyclosporin-stimulated production of endothelin, at rial natriuretic peptide may have a beneficial effect on cyclosporin-a ssociated hypertension and nephrotoxicity.