http://www.loc.gov/mods/v3 http://www.loc.gov/standards/mods/v3/mods-3-2.xsd
effects of simulated microgravity on the regulation of cerebral perfusion
viii, 64 leaves : illustrations ; 28 cm.
The purpose of these studies was to test the hypothesis that cephalic fluid shifts induced by head-down tail-suspension (HDT) in the rat lead to alterations in cerebrovascular regulation of brain blood flow. Histological cross-sections made of basilar artery from control and 14-day HDT animals revealed that media cross-sectional area (CSA) and thickness were increased with HDT, intraluminal CSA decreased, and vessel outer perimeter and media nuclei number were unaltered. To determine whether these changes in morphology are due to mechanical factors, cerebral blood flow and vascular resistance and arterial pressure were measured in control rats during normal standing and following 10 min of HDT, and in two groups of rats following 7- and 28-day HDT. Mean arterial pressure was elevated at 10 min HDT relative to the other conditions. Total brain blood flow was reduced and total vascular resistance was greater following 10 min, 7- and 28-day HDT, respectively, relative to that during control standing. These findings suggest that HDT leads to enhanced cerebral vasoconstrictor responsiveness. To determine the mechanism and functional significance of a putative change in vasoconstrictor responsiveness, additional studies were performed on control and 14-day HDT rats. First, dose responses to pressure, luminal flow, potassium, and vasodilatory agents were determined in isolated middle cerebral arteries. These studies were performed with and without nitric oxide (NO) synthase blockade. In most cases, NO blockade attenuated response differences between groups. This demonstrates that basal release of nitric oxide is diminished by HDT, and this appears to account for enhanced myogenic, shear stress-induced and agonist-induced constriction. In the second study, blood pressure and blood flow to brain regions were determined during either normal standing and head-up tilt in both groups of rats. During head-up tilt, the HDT rats became hypotensive, yet cerebral vascular resistance was higher and perfusion lower than that of control animals in brain regions involved in equilibrium maintenance, hearing, and cardiorespiratory control. In conclusion, the combination of these vascular alterations serve to diminish cerebral perfusion during head-up tilt. If similar alterations occur in humans, these adaptations may underlie or contribute to the development of post-spaceflight orthostatic intolerance.
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Includes bibliographical references (leaves 52-63).