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

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 cmH(2)O. The resistance arteries were then relaxed with 10(-4) M nitroprusside, fixed, and cut into transverse cross sections (5 microm 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 microm(2); C 12,904 +/- 1,433 microm(2)) and thickness (HU 33.9 +/- 4.1 microm; C 22.3 +/- 3.2 microm) were increased with hindlimb unloading (P < 0.05), intraluminal CSA decreased (HU 7,816 +/- 3,045 microm(2); C 13,469 +/- 5,500 microm(2)) (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.

Impaired cerebrovascular autoregulation and reduced CO2 reactivity after long duration spaceflight

“…Reduced cerebrovascular dynamic autoregulation after long-duration spaceflight was consistent with some (38) but not all (29) findings after HDBR and with astronauts who had orthostatic intolerance after short-duration spaceflights (8). Altered postflight cerebrovascular control might reflect the structural and functional changes observed in the animal models of spaceflight (24,34,36,37).…”

“…The human HDBR studies reported variable results with increases, decreases, and no change in CBFV. Animal models have revealed reduced flow (34,35) accompanied by significant structural changes with increased thickness of vessel walls (24,34,36,37) and reduced internal dimensions (36). The animal models also suggested an involvement of renin-angiotensin system might in structural modifications with hind limb suspension (6,15).…”

“…Cerebral arteries from suspended rats have demonstrated vascular smooth muscle hypertrophy (24,34,36,37) with a smaller luminal cross-sectional area (36) and increased basal myogenic tone (16,24,35). Further work has also shown that these changes are associated with reduced cerebral blood flow (34,35) and greater vasoconstrictor responses possibly acting through nitric oxidedependent (24,30,35) or renin-angiotensin systemdependent mechanisms (6).…”

Contractile Responses of the Basilar Artery Isolated from Rabbits Exposed to 8-Day Head-Down Tilt.

“…The thickness of medial layer of basilar arteries was increased by HDT (Table 3), which is consistent with the finding in rat basilar artery [10]. Our results suggest that the vascular reactivity to KCl and 5-HT, but not to NE, was enhanced by exposure to HDT in proportion to the medial hypertrophy.…”

Simulated microgravity alters rat mesenteric artery vasoconstrictor dynamics through an intracellular Ca2+release mechanism

“…The animals were randomly assigned to either cage control (n ϭ 43) or 14 day HU (n ϭ 44) groups. The hindlimbs of the HU group were elevated to an approximate spinal angle of 40 -45°via orthopedic tape adhered to the tail (5,15,56), which is a modification of techniques previously described (58). Control animals were individually housed and maintained in a normal cage environment, while HU rats were kept in the head-down position for 14 days.…”