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J Appl Physiol (January 17, 2003). doi:10.1152/japplphysiol.00979.2002
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Submitted on October 24, 2002
Accepted on January 9, 2003

ENHANCED SYMPATHOINHIBITORY RESPONSE TO VOLUME EXPANSION IN CONSCIOUS HINDLIMB UNLOADED RATS

Patrick J Mueller1* and Eileen M Hasser1

1 Department of Biomedical Sciences and Dalton Cardiovascular Research Center, University of Missouri-Columbia, Columbia, MO, USA

* To whom correspondence should be addressed. E-mail: muellerp{at}missouri.edu.

Prolonged exposure to microgravity or bed rest produces cardiovascular deconditioning which is characterized by reductions in plasma volume, alterations in autonomic function, and a predisposition toward orthostatic intolerance. Although the precise mechanisms have not been fully elucidated, it is possible that augmented cardiopulmonary reflexes contribute to some of these effects. The purpose of the present study was to test the hypothesis that sympathoinhibitory responses to volume expansion are enhanced in the hindlimb unloaded (HU) rat, a model of cardiovascular deconditioning. Mean arterial pressure (MAP), heart rate (HR), and renal sympathetic nerve activity (RSNA) responses to isotonic volume expansion (0.9 % saline, i.v., 15% of plasma volume over 5 minutes) were examined in conscious HU (14 days) and control animals. Volume expansion produced decreases in RSNA in both groups, however, this effect was significantly greater in HU rats (-48 ± 2 % vs. -24 ± 4% in controls). Animals instrumented for central venous pressure (CVP) did not exhibit differences in CVP responses to volume expansion. These data suggest that enhanced cardiopulmonary reflexes may be involved in the maintenance of reduced plasma volume and contribute to attenuated baroreflex mediated sympathoexcitation following spaceflight or bedrest.




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