Peripheral vascular responses to heat stress after hindlimb suspension.

PURPOSE:The purpose of this study was to determine whether hindlimb suspension (which simulates the effects of microgravity) results in impaired hemodynamic responses to heat stress or alterations in mesenteric small artery sympathetic nerve innervation. METHODS:Over 28 d, 16 male Sprague-Dawley rats were hindlimb-suspended, and 13 control rats were housed in the same type of cage. After the treatment, mean arterial pressure (MAP), colonic temperature (Tcol), and superior mesenteric and iliac artery resistances (using Doppler flowmetry) were measured during heat stress [exposure to 42 degrees C until the endpoint of 80 mm Hg blood pressure was reached (75 +/- 9 min); endpoint Tcore = 43.6 +/- 0.2] while rats were anesthetized (sodium pentobarbital, 50 mg x kg(-1) BW). RESULTS:Hindlimb-suspended and control rats exhibited similar increases in Tcol, MAP, and superior mesenteric artery resistance, and similar decreases in iliac resistance during heat stress (endpoint was a fall in MAP below 80 mm Hg). Tyrosine hydroxylase immunostaining indicated similar sympathetic nerve innervation in small mesenteric arteries from both groups. CONCLUSION:Hindlimb suspension does not alter the hemodynamic or thermoregulatory responses to heat stress in the anesthetized rat or mesenteric sympathetic nerve innervation, suggesting that this sympathetic pathway is intact.