Low-dose ouabain constricts small arteries from ouabain-hypertensive rats: implications for sustained elevation of vascular resistance

Abstract

Prolonged ouabain administration to normal rats causes sustained blood pressure (BP) elevation. This ouabain-induced hypertension (OH) has been attributed, in part, to the narrowing of third-order resistance arteries (∼320 μm internal diameter) as a result of collagen deposition in the artery media (see Ref. 6 ). Here we describe the structural and functional properties of fourth-order mesenteric small arteries from control and OH rats, including the effect of low-dose ouabain on myogenic tone in these arteries. Systolic BP in OH rats was 138 ± 3 versus 124 ± 4 mmHg in controls ( P < 0.01). Pressurized (70 mmHg) control and OH arteries, with only a single layer of myocytes, both had ∼165-μm internal diameters and ∼20-μm wall thicknesses. Even after fixation, despite vasoconstriction, the diameters and wall thicknesses did not differ between control and OH fourth-order arteries, whereas in third-order arteries, both parameters were significantly smaller in OH than in controls. Myogenic reactivity was significantly augmented in OH fourth-order arteries. Nevertheless, phenylephrine- (1 μM) and high K+-induced vasoconstrictions and acetylcholine-induced vasodilation were comparable in control and OH arteries. Vasoconstrictions induced by 5 μM phenylephrine and by 10 mM caffeine in Ca2+-free media indicated that releasable sarcoplasmic reticulum Ca2+ stores were normal in OH arteries. Importantly, 100 nM ouabain constricted both control and OH arteries by ∼26 μm, indicating that this response was not downregulated in OH rats. This maximal ouabain-induced constriction corresponds to a ∼90% increase in resistance to flow in these small arteries; thus ouabain at EC50 of ∼0.66 nM should raise resistance by ∼35%. We conclude that dynamic constriction in response to circulating nanomolar ouabain in small arteries likely makes a major contribution to the increased vascular tone and BP in OH rats.