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HIGHLIGHTED TOPIC
Regulation of the Cerebral Circulation

Endothelium-dependent vasodilation of cerebral arteries is altered with simulated microgravity through nitric oxide synthase and EDHF mechanisms

¹Division of Exercise Physiology and ²Department of Physiology and Pharmacology, and Center for Interdisciplinary Research in Cardiovascular Sciences, West Virginia University School of Medicine, Morgantown, West Virginia; ³Department of Pharmacology, University of Vermont College of Medicine, Burlington, Vermont; ⁴Department of Anesthesiology, Baylor College of Medicine, Houston, Texas; and ⁵Department of Medical Physiology and the Cardiovascular Research Institute, Texas A&M University Health Science Center, College Station, Texas

Submitted 1 August 2005 ; accepted in final form 25 January 2006

Cephalic elevations in arterial pressure associated with microgravity and prolonged bed rest alter cerebrovascular autoregulation in humans. Using the head-down tail-suspended (HDT) rat to chronically induce headward fluid shifts and elevate cerebral artery pressure, previous work has likewise shown cerebral perfusion to be diminished. The purpose of this study was to test the hypothesis that 2 wk of HDT reduces cerebral artery vasodilation. To test this hypothesis, dose-response relations for endothelium-dependent (2-methylthioadenosine triphosphate and bradykinin) and endothelium-independent (nitroprusside) vasodilation were determined in vitro in middle cerebral arteries (MCAs) from HDT and control rats. All in vitro measurements were done in the presence and absence of the nitric oxide synthase inhibitor N^G-nitro-L-arginine methyl ester (10^–5 M) and cyclooxygenase inhibitor indomethacin (10^–5 M). MCA caveolin-1 protein content was measured by immunoblot analysis. Endothelium-dependent vasodilation to 2-methylthioadenosine triphosphate and bradykinin were both lower in MCAs from HDT rats. These lower vasodilator responses were abolished with N^G-nitro-L-arginine methyl ester but were unaffected by indomethacin. In addition, HDT was associated with lower levels of MCA caveolin-1 protein. Endothelium-independent vasodilation was not altered by HDT. These results indicate that chronic cephalic fluid shifts diminish endothelium-dependent vasodilation through alterations in the endothelial nitric oxide synthase signaling mechanism. Such decrements in endothelium-dependent vasodilation of cerebral arteries could contribute to the elevations in cerebral vascular resistance and reductions in cerebral perfusion that occur after conditions of simulated microgravity in HDT rats.

orthostatic intolerance; cerebral blood flow; hindlimb unloading; middle cerebral artery; endothelium-dependent hyperpolarizing factor