Camp evokes a rise in intracellular na+ mediated by na+ pump
inhibition 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 2:0154C, 1995.
The effect of cAMP on intracellular Na+ concentration ([Na+]i) was
studied in primary cultured vascular smooth muscle cells from rat
aorta. [Na+]i was measured using digital imaging of cells loaded with
the Na+-sensitive fluorescent dye, SBFI. The cAMP level was raised
by: i) the membrane-permeable cAMP derivative, 8-Br cAMP, ii) the
combination of the adenylate cyclase activator, forskolin, and the
phosphodiesterase inhibitor, IBMX, and iii) the [beta]-adrenoceptor
agonist, isoproterenol. All three methods caused a dose-dependent
continuous rise in [Na+]i during 40-60 min of observations. A rise in
[Na+]i may be caused by stimulation of the Na+ influx and/or
inhibition of Na+ efflux; therefore, the involvement 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-induced
inhibition of Na+ efflux was not observed when the Na+ pump was
inhibited; therefore, cAMP inhibits the Na+ pump-mediated component
of Na+ efflux. Agents that raise the cAMP level also inhibited, in a
dose-dependent fashion, ouabain-sensitive 86Rb uptake in rat aorta
rings. The latter observation confirms that the cAMP-induced
inhibition of the Na+ pump occurs both in cultured cells and in the
native tissue. Changes in cellular cAMP levels evoked by the test
compounds were measured 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 aorta rings showed a similar dependence
on the net gain in cAMP. Taken together, the results suggest that, in
rat aorta smooth muscle cells, elevation of the cAMP level inhibits
the ouabain-sensitive Na+ pump and thus causes [Na+]i to rise.
Received 31 May 1994; accepted in final form 22 March 1995.
APS Manuscript Number C294-4.
Article publication pending Am. J. Physiol. (Cell Physiology).
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 4 April 1995.