cAMP Signaling Inhibits Dihydropyridine-Sensitive Ca 2+ Influx in Vascular Smooth Muscle Cells

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Abstract
This study examines the role of the cAMP signaling pathway in the regulation of 45 Ca influx in cultured vascular smooth muscle cells from the rat aorta. K + o -induced depolarization of smooth muscle cells increased the rate of 45 Ca uptake by twofold to threefold. This effect was completely abolished by the dihydropyridine derivatives nifedipine and nicardipine, with a K i of 3 and 10 nmol/L, respectively. Activators of cAMP signaling (isoproterenol, forskolin, cholera toxin) increased cAMP content by 50- to 100-fold and decreased voltage-dependent 45 Ca uptake by 50% to 70%. Neither the dihydropyridines nor the cAMP activators affected basal 45 Ca influx. Direct addition of the protein kinase inhibitor H-89 to the incubation medium in the 1- to 10-μmol/L range did not alter basal 45 Ca uptake but completely abolished voltage-dependent Ca 2+ transport. Preincubation of cells for 1 hour with 10 μmol/L H-89 did not modify basal and depolarization-induced 45 Ca uptake in H-89–free medium but prevented forskolin-induced inhibition of voltage-dependent Ca 2+ influx. The addition of cytoskeleton-active compounds reduced voltage-dependent Ca 2+ transport and completely abolished its regulation by cAMP. Activation of cAMP signaling decreased the volume of smooth muscle cells by 12% to 15%. The same cell volume diminution in hyperosmotic medium did not alter voltage-dependent 45 Ca uptake. Thus, data obtained in this study show that in contrast to cardiac and skeletal myocytes, in vascular smooth muscle cells, 45 Ca influx, putatively due to L-type channels, is inhibited by cAMP. This regulatory pathway appears to be mediated via protein kinase A activation and cytoskeleton reorganization.