CYCLIC-GMP STIMULATES NA+/CA2+ EXCHANGE IN VASCULAR SMOOTH-MUSCLE CELLS IN PRIMARY CULTURE

We examined the effect of cGMP on Na+/Ca2+ exchange in rat aortic smooth muscle cells (VSMCs) in primary culture. The intracellular Ca2+ concentration ([Ca 2+]i) was raised by adding ionomycin to VSMCs incubated at high extracellular pH (pH(o)) (pH(o) = 8.8) and high extracellular Mg2+ (Mg2+o) (Mg2+o = 20 mM), conditions that inhibit activity of the sarcolemmal Ca2+ pump. Ca-45(2+) efflux observed under these conditions was mostly extracellular Na+ (Na+o)-dependent and thus presumably catalyzed by the Na+/Ca2+ exchanger. Brief treatment of VSMCs with 8-bromo-cGMP or atrial natriuretic peptide increased this Na+(o)-dependent Ca-45(2+) efflux by about 50%. The 8-bromo-cGMP treatment did not significantly influence total cell Na+, membrane potential, and cell pH. Conversely, when VSMCs were loaded with Na+ and then exposed to a Na+o-free medium, the rate of Ca-45(2+) uptake into VSMCs increased as cell Na+ increased. Prior treatment of VSMCs with 8-bromo-cGMP accelerated Ca-45(2+) uptake by up to 60% without influencing Na+ loading itself. Treatment of VSMCs with 25-mu-M 2,5-di-(tert-butyl)-1,4-benzohydroquinone, an inhibitor of the sarcoplasmic reticulum Ca2+-ATPase, induced a transient elevation of [Ca2+]i. 8-Bromo-cGMP stimulated the rate of recovery phase of this Ca2+ transient measured in the high pH(o)/high Mg2+o medium. All these results indicate that cGMP stimulates Na+/Ca2+ exchange in VSMCs.