The effect of cellular calcium on Na+/K+ cotransport in human red blood cells

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 microM) 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 microM 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 microM) 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 x 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.