The control of Na+/K+ pump activity was studied in rat adrenal glomerulosa cells. Ninety percent of K+/86Rb accumulation was blocked by ouabain, and the dose-response curve of inhibition by ouabain was monophasic (IC50, ∽80 nM), suggesting the role of a single type of Na+/K+ pump (α-isoenzyme) in86Rb accumulation by rat glomerulosa cells.The basal activity of the Na+/K+ pump was much higher in glomerulosa cells than in adrenal fasciculate cells or hepatocytes, as judged by the ouabain-sensitive uptake of86Rb. In contrast to the two other cell types, increasing Na+ influx with the Na+ ionophore monensin failed to significantly affect ouabain-sensitive86Rb uptake in glomerulosa cells, suggesting that in glomerulosa cells even the resting intracellular Na+ concentration is sufficient for maximal activity of the Na+/K+ pump. Angiotensin-II (All) inhibited the ouabain-sensitive86Rb uptake by glomerulosa cells. The effect of All was abolished by the selective antagonist of the AT1 type of All receptors (DuP 753), while PD 123177, an AT2 antagonist was ineffective. AT1 receptors of glomerulosa cells coupled to phospholipase-C activation and, thus, to Ca2+ signal. The inhibitory effect of All was dependent on the extracellular Ca2+ concentration, but an elevation of cytoplasmic Ca2+ by Ca2+ ionophore ionomycin failed to mimic the effect of AH. These data suggest that Ca2+ is required for but does not mediate the inhibitory effect of All on the Na+/K+pump. Pharmacological activation of protein kinase-C by phorbol ester did not modify86Rb accumulation by the cells. Ouabain induced a nifedipine-sensitive elevation in the cytoplasmic Ca2+ concentration and exerted a stimulatory effect on aldosterone production, suggesting participation of the inhibition of the Na+/K+ pump in the aldosterone stimulatory action of All.
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