Dual inhibitory effects of dopamine on na+ homeostasis in rat aorta
smooth muscle cells.
Borin, Mikhail L.
Department of Physiology and the Center for Vascular Biology and
Hypertension, University of Maryland School of Medicine, Baltimore,
MD 21201, U.S.A.
APStracts 3:0312C, 1996.
Dopamine (DA) is an essential catecholamine, which acts not only as
neurotransmitter in sympathetic neurons but also exhibits
vasodilating and natriuretic effects in renal tubular cells, blood
vessels, etc. This study describes the effect of DA on Na+ influx,
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 using digital imaging of
cells loaded with the Na+-sensitive fluorescent dye, 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.
DA inhibited Na+ influx in dose-dependent manner with a maximal
inhibition, about 45%, achieved at 10-4 M. This effect of DA, as
suggested by several lines of evidence, was mediated by inhibition of
Na+/H+ exchange. Besides inhibition of Na+ efflux, DA 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 DA was due to the reduction in its Jmax, and not due to the
decrease in the Na+ sensitivity of the pump. Similarly to DA,
activation of protein kinase A by 8-Br cAMP caused inhibition of both
Na+ influx and Na+ pump-mediated Na+ efflux. In contrast, activation
of protein kinase C by the phorbol ester, PDBu, 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
DA or 8-Br cAMP on Na+ efflux, but did not affect the inhibitory
effects of these compounds on Na+ influx. DA either did not change
[Na+]i or evoked a slight, 2-3 mM, increase in [Na+]i. Taken together
these results demonstrate that, in rat aortic smooth muscle cells, i)
DA inhibits Na+/H+ exchange-mediated Na+ influx; ii) DA inhibits Na+
pump-mediated Na+ efflux; iii) These effects of DA are mediated by
increase in cellular cAMP and, at least in the case of inhibition of
the Na+ efflux, by the activation of protein kinase A; iv) DA causes
either small or no changes in [Na+]i due to almost equal inhibition
of Na+ influx and Na+ efflux .
Received 6 May 1996; accepted in final form 15 August 1996.
APS Manuscript Number C242-6.
Article publication pending Am. J. Physiol. (Cell Physiology).
ISSN 1080-4757 Copyright 1996 The American Physiological Society.
Published in APStracts on 5 November 1996