CAMP EVOKES A RISE IN INTRACELLULAR NA-MUSCLE CELLS( MEDIATED BY NA+ PUMP INHIBITION IN RAT AORTIC SMOOTH)

The effect of adenosine 3',5'-cyclic monophosphate (cAMP) on intracell ular Na+ concentration ([Na+](i)) was studied in primary cultured vasc ular smooth muscle cells from rat aorta [Na+](i) was measured using di gital imaging of cells loaded with the Na+-sensitive fluorescent dye s odium-bonding benzofuran isophthalate. The cAMP level was raised by I) the membrane-permeable cAMP derivative 8-bromoadenosine 3',5'-cyclic monophosphate, 2) the combination of the adenylate cyclase activator f orskolin and the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthi ne, and 3) the beta-adrenoceptor agonist isoproterenol. All three meth ods caused a dose-dependent continuous rise in [Na+](i) during 40-60 m in of observations. A rise in [Na+](i) may be caused by stimulation of the Naf influx and/or inhibition of Na+ efflux; therefore, the involv ement of both mechanisms was studied. Elevation of the cAMP level had no effect on Na+ influx, measured as the rate of rise of [Na+](i) when Na+ efflux was inhibited with 1 mM ouabain. In contrast, elevation of the cAMP level attenuated Na+ efflux, measured as the rate of decline of [Na+](i) in Na+-loaded cells exposed to Na+-free medium. cAMP-indu ced inhibition of Na+ efflux was not observed when the Na+ pump was in hibited; therefore, cAMP inhibits the Na+ pump-mediated component of N a+ efflux. Agents that raise the cAMP level also inhibited, in a dose- dependent fashion, ouabain-sensitive Rb-86 uptake in rat aortic rings. The latter observation confirms that the cAMP-induced inhibition of t he Na+ pump occurs both in cultured cells and in the native tissue. Ch anges in cellular cAMP levels evoked by the test compounds were measur ed and correlated with changes in [Na+](i) and Na+ pump activity: both the increase in [Na+](i) in cells and the inhibition of the Na+ pump in aortic rings showed a similar dependence on the net gain in cAMP. T aken together, the results suggest that, in rat aortic smooth muscle c ells, elevation of the cAMP level inhibits the ouabain-sensitive Na+ p ump and thus causes [Na+](i) to rise.