High Levels of Myogenic Tone Antagonize the Dilator Response to Flow of Small Rabbit Cerebral Arteries

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Abstract
Background and Purpose —Pressure and shear stress exerted by flowing blood are two mechanical forces that play a major role in the regulation of vascular tone. We sought to evaluate the interaction between pressure and flow in isolated rabbit cerebral arteries. Methods —Responses to intraluminal flow of isolated pressurized rabbit posterior cerebral arteries were investigated at low, medium, and high levels of myogenic tone by setting the luminal pressure at 40, 60, and 80 mm Hg, respectively. Results —At both low and medium levels of myogenic tone, flow induced dilation. The response was significantly larger at 40 than at 60 mm Hg. At the high level of myogenic tone, the response to flow consisted of a combination of an initial transient dilation followed by sustained constriction. Flow-induced dilation but not flow-induced constriction response was endothelium dependent. Removal of the endothelium inhibited the dilator response by ≈80%. Flow-induced dilation was inhibited (≈40%) by N ω -nitro- l -arginine (100 μmol/L) but not by indomethacin (10 μmol/L). Endothelium removal not only decreased the amplitude of flow-induced dilation but also promoted the appearance of flow-induced constriction at low and medium levels of myogenic tone. Conclusions —The intraluminal pressure and in consequence the level of myogenic tone at which flow is applied determine the nature of the response of the smooth muscle cells of the blood vessel wall.