Effects of sodium and ouabain on endothelium-dependent responses in hypertension

Abstract

Ouabain is an inhibitor of sodium-potassium adenosine triphosphatase (Na+/ K+-ATPase, also known as Na+ pumps). It is released by high sodium chloride concentration from hypothalamus and adrenals. Upon released in the circulation, it increases arterial constriction and hence blood pressure. Since high circulating levels of sodium chloride and ouabain are likely present concurrently, the present study aimed to examine whether or not sodium affected the vascular effects of ouabain locally. Aortae were isolated from male Sprague Dawley rats (8-12 weeks old) and incubated in the organ chambers with modified Krebs-Henseleit solutions containing normal (140 mM) or high (160 mM) sodium, in the absence or presence of different contractions of ouabain (10-6, 10-4, 2 x 10-4, 5 x 10-4 M). They were contracted with phenylephrine (10-6.5 to 10-5 M) and relaxed with increasing concentrations of endothelium-dependent and -independent vasodilators, acetylcholine (10-9 to 10-5 M) and DETA NONOate (10-9 to 10-3.5 M). Ouabain, at 5 x 10-4 M, decreased the relaxation to acetylcholine in rat aorta with endothelium, and merely shifted the relaxation curve of DETA NONOate to the right in preparations without endothelium. Ouabain caused greater decrease in acetylcholine-induced relaxations under high sodium condition, and in aortae incubated in solutions containing mannitol (40 mM), which increased the osmolarity of the incubating solution to that similar to the increased sodium chloride level (from 140 to 160 mM). The increased sodium level or mannitol alone did not affect the relaxation to acetylcholine. The results, therefore, suggested that elevation of extracellular osmolarity [by high sodium concentration or mannitol] enhanced the inhibitory effect of ouabain. In order to elucidate the mechanism(s) through which ouabain affect relaxations, rat aortae were incubated without or with different concentrations of ouabain and relaxations to arachidonic acid (AA) were examned. Ouabain decreased AA-induced relaxation in aortae with endothelium, but not in those without endothelium. The inhibition by ouabain was not observed in the presence of indomethacin (cyclooxygenase inhibitor), L-NAME (nitric oxide synthase inhibitor) or ODQ (soluble guanylyl cyclase inhibitor). These findings suggested that ouabain likely inhibits endothelial cyclooxygenase or nitric oxide synthase to reduce relaxation.