Dual inhibitory effects of dopamine on Na+ homeostasis in rat aorta smooth muscle cells.

Dopamine is an essential catecholamine, which acts not only as a neurotransmitter in sympathetic neurons but also exhibits vasodilating and natriuretic effects in renal tubular cells, blood vessels, etc. This study describes the effect of dopamine on Na+ influx and Na+ efflux and the resulting changes in intracellular Na+ concentration ([Na+]i). [Na+]i was measured in primary cultured vascular smooth muscle cells from rat aorta with digital imaging of cells loaded with the Na+-sensitive fluorescent indicator, SBFI. Na+ influx and Na+ efflux were measured as changes in [Na+]i under the conditions of inhibition of the Na+ flux in the opposite direction. Dopamine inhibited Na+ influx in a dose-dependent manner with a maximal inhibition, approximately 45%, achieved at 10(-4) M. This effect of dopamine, as suggested by several lines of evidence, was mediated by inhibition of Na+/H+ exchange. Besides inhibition of Na+ efflux, dopamine also, with a similar potency, inhibited Na+ efflux. The latter effect was due to inhibition of the Na+ pump-mediated component of Na+ efflux, since it was not observed when Na+ pump was inhibited. Inhibition of the Na+ pump by dopamine was due to the reduction in its maximal flux and not due to the decrease in the Na+ sensitivity of the pump. Similar to dopamine, activation of protein kinase A by 8-bromoadenosine 3',5'-cyclic monophosphate (8-BrcAMP) caused inhibition of both Na+ influx and Na+ pump-mediated Na+ efflux. In contrast, activation of protein kinase C by the phorbol ester, phorbol 12,13-dibutyrate, caused activation of both Na+ influx and Na+ pump-mediated Na+ efflux. H-7, a nonspecific protein kinase inhibitor, abolished the inhibitory effects of either dopamine or 8-BrcAMP on Na+ efflux but did not affect the inhibitory effects of these compounds on Na+ influx. Dopamine either did not change [Na+]i or evoked a slight, 2-3 mM, increase in [Na+]i. Together, these results demonstrate that, in rat aortic smooth muscle cells, 1) dopamine inhibits Na+/H+ exchange-mediated Na+ influx, 2) dopamine inhibits Na+ pump-mediated Na+ efflux, 3) these effects of dopamine are mediated by an increase in cellular cAMP and, at least in the case of inhibition of the Na+ efflux, by the activation of protein kinase A, and 4) dopamine causes either small or no changes in [Na+]i, due to almost equal inhibition of Na+ influx and Na+ efflux.