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Simulated Microgravity in Wistar Rats Markedly Increases Vasoconstrictor Capacity in Endothelium Denuded Thoracic Aorta.     S.M. Summers, A.S. Ulloa, and R.E. Purdy,    Dept. of Pharmacology, University of California at Irvine. Irvine, CA.

   Exposure to microgravity causes cardiovascular deconditioning.  As a consequence, microgravity-adapted astronauts returning to the gravity of Earth can experience both orthostatic intolerance and decreased exercise capacity. Using a rat model of simulated microgravity, Delp et al (J Applied Physiol 75:2620, 1993) found that a potential contributor is decreased vasoconstrictor responsiveness. They found a decreased maximal response to norepinephrine (NE) in isolated thoracic and abdominal aorta.  Using the same model, hind limb unweighting (HU), we sought to characterize the contractile deficit in the thoracic aorta in male Wistar rats. In vitro isometric tension in thoracic aorta was measured after 20 days HU.

   There was no HU effect on the contractile response to NE in Wistar thoracic aorta.  Moreover, in endothelium-denuded thoracic aorta, the contraction to NE was markedly enhanced in HU compared to control tissues. These results in thoracic aorta stand in sharp contrast to our previous finding that HU causes a marked, endothelium-independent reduction in vasoconstrictor capacity in abdominal aorta (Sangha et al. J Applied Physiol 88:507, 2000).  The present results may also reflect a strain difference between Wistar and Sprague-Dawley (SD) rats.  Thus, in preliminary experiments, we have confirmed the previously reported HU-induced contractile deficit in thoracic aorta from SD rats (Delp et al., 1993).  In the present experiments, denuding the endothelium eliminated this HU effect.  It is hypothesized that the increased contractile capacity in denuded HU Wistar thoracic aorta and the enhanced contribution of the endothelium in HU thoracic aorta from both rat strains reflect the hemodynamic consequences of HU treatment. 

(Supported by NASA: NN04CK29G)