cAMP signaling inhibits dihydropyridine-sensitive Ca2+ influx in vascular smooth muscle cells

This study examines the role of the cAMP signal pathway in the regulation of Ca-45 influx in cultured vascular smooth muscle cells from the rat aorta. K-o(+)-induced depolarization of smooth muscle cells increased the rate of Ca-45 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 Ca-45 uptake by 50% to 70%. Neither the dihydropyridines nor the cAMP activators affected basal Ca-45 influx. Direct addition of the protein kinase inhibitor H-89 to the incubation medium in the 1- to 10-mu mol/L range did not alter basal Ca-45 uptake but completely abolished voltage-dependent Ca2+ transport. Preincubation of cells for 1 hour with 10 mu mol/L H-89 did not modify basal and depolarization-induced Ca-45 uptake in H-89-free medium but prevented forskolin-induced inhibition of voltage-dependent Ca2+ influx. The addition of cytoskeleton-active compounds reduced voltage-dependent Ca2+ 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 Ca-45 uptake. Thus, data obtained in this study show that in contrast to cardiac and skeletal myocytes, in vascular smooth muscle cells, Ca-45 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.