ISOPROTERENOL REDUCES ANGIOTENSIN II(AII) -INDUCED CA2-MG-ATPASE VIA CAMP( MOBILIZATION IN VASCULAR SMOOTH MUSCLE CELLS VIA ACTIVATION OF CA)

cAMP is known to mediate the vasorelaxant effect of P-adrenergic agent s. We examined the effect of isoproterenol on ALI-induced signalling i n cultured rat vascular smooth muscle cells. Isoproterenol (10(-9) M, 10 minutes) stimulated cAMP synthesis (6.5 +/- 0.3 vs. 43.8 +/- 1.9 pm ol/mg protein, p < 0.001). Isoproterenol dose-dependently lowered basa l [Ca2+](i) after 10 minutes preincubation (80 +/- 6 vs. 45 +/- 4 nM, isoproterenol 10(-5) M, p < 0.01 vs, control) as measured with fura-2. In the presence of isoproterenol, All-induced peak [Ca2+](i), was sig nificantly reduced (742 +/- 50 vs. 342 +/- 21 nM, p < 0.01). The effec ts of isoproterenol could be mimicked by cholera toxin (20 mu g/ml, 18 hours preincubation), an activator of the adenylate cyclase which red uced AIT-stimulated peak [Ca2+](i), to 320 +/- 37 nM (p < 0.02 vs. All alone). Thapsigargin (10(-7) M), an inhibitor of Ca-Mg-ATPase, induce d an increase in [Ca2+](i) within 12 minutes from 80 +/- 6 to 210 +/- 27 nM (p < 0.001). In the presence of thapsigargin the effects of isop roterenol to lower basal [Ca2+](i), as well as AII-stimulated peak [Ca 2+](i), were completely blocked. These results suggest that the vasore laxant action of cAMP is at least in part mediated by its effects on C a2+ kinetics in vascular smooth muscle cells. Activation of the sarcop lasmic Ca-Mg-ATPase increases Ca2+ sequestration into intracellular st ores and may contribute to the reduction of the basal and the vasopres sor-induced Ca2+ signal.