Inhibitory action of norepinephrine on sodium transport in vascular smooth muscle cells in culture

Cultured vascular smooth muscle cells from porcine aortas incubated in Na+-free medium rapidly release their intracellular Na+ contents (Nai) (23±4% of baseline after 60 min incubation, mean ± SEM of 18 experiments). Total Nai release was inhibited by 35–40% after addition of ouabain and by 60–70% after addition of ouabain + bumetanide. Norepinephrine inhibited ouabain and bumetanide-sensitives Na+ efflux with an IC50 of about 10−9–10−8 M. Addition of the alpha-adrenergic agonist phenylephrine (10 μM) to the cells mimicked the inhibitory action of norepinephrine on Nai release. Conversely, the beta-adrenergic agonist isoproterenol was without effect on Nai release. Simultaneous addition of 10 μM norepinephrine and the alpha-adrenergic antagonist phentolamine prevented any effect of norepinephrine on the rate of Nai decline. In A-10 cultured vascular smooth muscle cells, the alpha-adrenergic agonist phenylephrine (10 μM) inhibited 40.0±8.1% of ouabain-sensitive Rb+ influx and 70.7±6.9% of bumetanide-sensitive Rb+ influx (mean ± SEM of three experiments). 50% inhibition of bumetanide-sensitive Rb+ influx was obtained with about 5×10−7 M of phenylephrine. Our results show that in vascular smooth muscle cells a [Na+, K+, Cl−]-cotransport system is able to catalyze outward Na+ movements (in Na+-free media) of a similar order of magnitude to those of the Na+, K+ pump and that alpha-adrenergic stimulation markedly inhibits Na+ efflux (and Rb+ influx) through these two transport systems.