Abstract
During orthostasis, approximately 20% of the elevation in peripheral vascular resistance occurs through constriction of the splanchnic vasculature. Because rodents experience orthostatic hypotension following hindlimb unloading (HU), a model to simulate microgravity, the purpose of this study was to determine if 14 days of HU alters vasoconstrictor responsiveness of mesenteric and splenic resistance vessels. Mesenteric (control 237.4 ± 16.3μm; HU 253.9 ± 6.9μm) and splenic (control 364.8 ± 16.4μm; HU 349.3 ± 12.8μm) resistance arteries from control and HU rats were dissected free, cannulated on glass micropipettes, pressurized, and studied in vitro. Myogenic properties of these vessels were characterized by establishing pressure-diameter relationships. Vasoconstrictor properties of mesenteric resistance arteries were characterized by establishing concentration-response curves for norepinephrine and potassium chloride, and of splenic resistance arteries with norepinephrine, potassium chloride, and arginine vasopressin. Myogenic properties were unaltered by HU in both splenic and mesenteric arteries. Vasoconstrictor responsiveness of splenic resistance arteries was also unaltered by HU. However, significant reductions in mesenteric vasoconstrictor responsiveness to KCl (P = 0.005) and NE (P = 0.006) were found following HU. These data suggest that HU-induced orthostatic intolerance may be due in part to altered vasoconstrictor properties of mesenteric resistance arteries. Supported by NASA grant NAG2-1430
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