Effects of hindlimb unloading on rat cerebral, splenic, and mesenteric resistance artery morphology.

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Abstract
Hindlimb unloading (HU) of rats induces a cephalic shift in body fluids. We hypothesized that the putative increase in cranial fluid pressure and decrease in peripheral fluid pressure would alter the morphology of resistance arteries from 2-wk HU male Sprague-Dawley rats. To test this hypothesis, the cerebral basilar, mesenteric, and splenic arteries were removed from control (C) and HU animals. The vessels were cannulated, and luminal pressure was set to 60 cmH2O. The resistance arteries were then relaxed with 10−4 M nitroprusside, fixed, and cut into transverse cross sections (5 μm thick). Media cross-sectional area (CSA), intraluminal CSA, media layer thickness, vessel outer perimeter, and media nuclei number were determined. In the basilar artery, both media CSA (HU 17,893 ± 2,539 μm2; C 12,904 ± 1,433 μm2) and thickness (HU 33.9 ± 4.1 μm; C 22.3 ± 3.2 μm) were increased with hindlimb unloading ( P < 0.05), intraluminal CSA decreased (HU 7,816 ± 3,045 μm2; C 13,469 ± 5,500 μm2) ( P < 0.05), and vessel outer perimeter and media nuclei number were unaltered. There were no differences in mesenteric or splenic resistance artery morphology between HU and C rats. These findings suggest that hindlimb unloading-induced increases in cephalic arterial pressure and, correspondingly, increases in circumferential wall stress result in the hypertrophy of basilar artery smooth muscle cells.