Simulated microgravity increases myogenic tone in rat cerebral arteries.

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Abstract
Adaptation of the cerebral circulation to microgravity was investigated in rat middle cerebral arteries after 20 days of hindlimb unweighting (HU). Myogenic responses were measured in isolated, pressurized arteries from HU and control animals. Maximal passive lumen diameters, obtained in the absence of extracellular Ca2+ plus EDTA, were not significantly different between groups (249 vs. 258 μm). In physiological salt solution, arteries from both HU and control animals maintained a constant lumen diameter when subjected to incremental increases in transmural pressure (20–80 mmHg). However, the diameter of arteries from HU animals was significantly smaller than that of arteries from control animals at all pressures; this difference could be eliminated by exposure to the nitric oxide synthase inhibitor N G-nitro-l-arginine methyl ester. After HU treatment, transient distensibility of the artery wall in response to pressure was also significantly decreased, whereas the frequency and amplitude of vasomotion were increased. The latter changes were not affected by N G-nitro-l-arginine methyl ester. Thus simulated microgravity increases cerebral artery myogenic tone through both nitric oxide synthase-dependent and -independent mechanisms.