The α1-Na/K Pump does not Mediate the Involvement of Ouabain in the Development of Hypertension in Rats

Abstract

The Na/K pump of vascular smooth muscle cells (VSMC) and renal epithelial cells (REC) is viewed as a target of digitalis and endogenous ouabain (EO), leading to the development of hypertension. In this study, we compared the effect of ouabain on Na/K pump activity and the intracellular content of monovalent cations in VSMC and REC obtained from rats, humans and dogs. In VSMC from the rat aorta, ouabain inhibited maximal Na/K pump activity measured as the rate of 86 Rb influx in Na + -loaded cells, with an ID 50 of ~20-30 µM without any differences between two strains of normotensive rats (WKY and BN.lx) and three substrains of spontaneously hypertensive rats (SHR). Half-maximal inhibition of the Na/K pump in REC from the rat inner medullary collecting duct was observed at ~20 µM of ouabain. In contrast to rat cells, half-maximal inhibition of 86 Rb influx in VSMC from human coronary arteries and in REC from the Madin-Darby canine kidney was seen at ~0.03 and 0.1 µM ouabain, respectively. At concentrations lower than 100 µM, ouabain did not affect the intracellular content of exchangeable Na + and K + in rat VSMC, measured as the steady-state distribution of 22 Na and 86 Rb, whereas in human VSMC, it increased the intracellular Na + /K + ratio with an ID 50 of ~0.5 µM. Keeping in mind that the circulating level of administered digitalis and EO does not exceed 10 -9 M, our results strongly suggest that the involvement of these compounds in the pathogenesis of hypertension in rats is not mediated by inhibition of the f 1-isoform of the Na/K pump in VSMC and REC. Alternative mechanisms of the involvement of EO and ouabain-like factors in the development of hypertension are considered.