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

Hindlimb unloading (HU) of rats induces a cephalic shift in body fluids. We hypothesized that the putative increase in cranial fluid pressure and decr ease in peripheral fluid pressure would alter the morphology of resistance arteries from 2-wk HU male Sprague-Dawley rats. To test this hypothesis, th e cerebral basilar, mesenteric, and splenic arteries were removed from cont rol (C) and HU animals. The vessels were cannulated, and luminal pressure w as set to 60 cmH(2)O. The resistance arteries were then relaxed with 10(-4) M nitroprusside, fixed, and cut into transverse cross sections (5 mu m thi ck). Media cross-sectional area (CSA), intraluminal CSA, media layer thickn ess, vessel outer perimeter, and media nuclei number were determined. In th e basilar artery, both media CSA (HU 17,893 +/- 2,539 mu m(2); C 12,904 +/- 1,433 mu m(2)) and thickness (HU 33.9 +/- 4.1 mu m; C 22.3 +/- 3.2 mu m) w ere increased with hindlimb unloading (P < 0.05), intraluminal CSA decrease d (HU 7,816 +/- 3,045 mu m(2); C 13,469 +/- 5,500 mu m(2)) (P < 0.05), and vessel outer perimeter and media nuclei number were unaltered. There were n o differences in mesenteric or splenic resistance artery morphology between HU and C rats. These findings suggest that hindlimb unloading-induced incr eases in cephalic arterial pressure and, correspondingly, increases in circ umferential wall stress result in the hypertrophy of basilar artery smooth muscle cells.