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Hindlimb Unloading is Associated with Increased Nitric Oxide Production and Reduced Sympathetic Activity.  D. Eatman1, L. Coats, L. Ambe, M. Walton1, A. Ganafa1, N. Emmett2, M. A. Bayorh1.  1Depts of Pharmacology/Toxicology and 2Physiology.  Morehouse School of Medicine, Atlanta, GA.

   Orthostatic intolerance is one of the best documented symptoms experienced by astronauts postflight. The 30o Head-Down Tilt (HDT) model in the rat induces physiological changes similar to responses observed in astronauts.  Therefore, we tested the hypothesis that reduction in blood pressure post-suspension may be due, in part, to decreased sympathetic activity and increased vasodilator endothelium-derived factors.  Using the HDT model, we investigated the effect of long-term suspension on mean arterial pressure (MAP), vascular remodeling and on plasma constituents (i.e. prostacyclin (PGI2), nitric oxide (NO), catecholamines) in male Sprague-Dawley rats.  MAP and heart rate were measured prior to suspension, and immediately upon release from suspension. Upon release from suspension, blood pressure decreased significantly when compared to non-suspended rats.  Plasma norepinephrine (NE) was significantly lower in the post-suspended when compared to non-suspended group.  Plasma NO levels were elevated during post-suspension, while TXA2 levels were lower when compared to the non-suspended group.  Furthermore, the suspended rats tended to have a greater vasodilator response to acetylcholine when compared to non-suspended rats.  The vascular wall thickness to lumen diameter ratio was lower in suspended than in non-suspended group.  Taken together, the post-suspension hypotension observed in these studies implicates at least two mechanisms: reduced activity of the sympathetic system and increased endogenous vasodilatory and diminished vasoconstrictor endothelium-derived factors.  Also, long-term exposure to simulated microgravity appears to impair vascular structure and function.

(Supported by NASA grant NCC 9-112 and the NSBRI Grant NCC 9-58-24)