- Sodium-pump ATPase is known to be a cell membrane enzyme being responsible for vectorial transport of Na+ against electrochemical gradient across the plasma membranes of most cell types. In smooth muscles, this Na+-pump ATPase activity is notoriously low and its measurement is complicated by remarkably high level of the basal ATPase activities. In this communication, the properties of ouabain-sensitive, potassium-activated p-nitrophenyl phosphatase (pNPPase) activity which is a partial reaction of the Na+-pump are reported and compared in plasma membrane enriched fractions of mesenteric arteries from hypertensive and normotensive rats. The pNPPase activity of isolated plasma membrane vesicles from this vascular muscle is optimally stimulated by low concentrations of KCl (5-25 mM) at pH 7.8. Ouabain at 1.0 mM inhibits up to 75% of the K+-activated pNPPase activity under the optimal assay condition. Osmotic shock of the plasma membrane vesicles resulted in about 30% enhancement of the ouabain-sensitive K+-pNPPase activity compared to the control values. Although apparent difference was observed in this enzyme activity using similar preparations obtained from rats of different strains, the relative enhancement of such ouabain-sensitive K+-pNPPase activities by osmotic shock was not significantly different between vascular membrane fractions from the spontaneously hypertensive rats and the Wistar-Kyoto normotensive rats. Our results suggest that the intrinsic Na+-pump activity of small blood vessels in genetic hypertension in rats is not altered and the isolation procedure does not have differential effect on the orientation of plasma membrane vesicles in fractions obtained from hypertensive and normotensive rats.