Nitric Oxide Opposes Myogenic Pressure Responses Predominantly in Large Arterioles In Vivo

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Abstract
—A myogenic vasoconstriction may amplify the effects of circulating vasoconstrictors. In cremaster arterioles, the contribution of a myogenic component to the constriction on intravenous infusion of norepinephrine (NE) or angiotensin II (Ang II) was studied. Second, the role of endothelium-derived nitric oxide (NO) in the control of these myogenic constrictions and its site of action in the resistance vascular bed was investigated. In 30 anesthetized (pentobarbital) hamsters, the cremaster was prepared for intravital microscopy, and a pneumatic vessel occluder was placed around the aorta to vary blood pressure in the hindquarter of the animal. Intravenous infusion of NE (0.5 nmol/min) increased the systemic blood pressure by 52±2 mm Hg. Simultaneously, constrictions of up to 33±6% were observed in the small arterioles (SAs; maximal inner diameter, 36 to 65 μm). The constrictions were not significantly altered by a local adrenergic blockade but were abolished when the pressure elevation in the cremaster arterioles was blocked by partial occlusion of the abdominal aorta. Diameters in large arterioles (LAs; maximal inner diameter, 65 to 127 μm), however, did not change significantly on NE infusion. Similar responses in the arterioles were observed when the local pressure was increased stepwise from 60 to 120 mm Hg by partial opening of the aortic occluder. However, after treatment of the cremaster tissue with the inhibitor of the NO synthase, N G -nitro- l -arginine (L-NNA, 30 μmol/L), a significant pressure-induced constriction of up to 16±3% occurred in LAs, whereas the magnitude of the constriction in SAs remained unchanged. L-NNA also abolished the increases in blood flow that were observed with increments in pressure in control animals. Similar results were obtained when Ang II was used to increase blood pressure. We conclude that a myogenic constriction of SAs contributes markedly to the overall response of cremaster arterioles to circulating vasoconstrictors. NO effectively opposes the myogenic response in LAs, thus preventing myogenic constrictions in a vascular region where constriction cannot be fully controlled by metabolic dilation. If this attenuating effect of NO on myogenic constriction also takes place in other organs, it might be a decisive mechanism in controlling changes of total peripheral vascular resistance elicited by vasoconstrictors.