Effect of 29 days of simulated microgravity on maximal oxygen consumption and fat-free mass of rats.

To determine the time course of changes in maximal oxygen consumption (VO2max) and its relationships with body composition, a 29-d simulated microgravity study was initiated with 30 adult female rats (230-280 g) randomly assigned to one of three experimental groups; namely, head-down suspension (HDS) at 45 degrees, horizontal suspension (HS) at 0 degree, or cage control (CC). Five separate exercise performance tests with measurements related to VO2max, treadmill run time, and mechanical efficiency were conducted before suspension and on days 7, 14, 21, and 28. The animals were sacrificed and chemically analyzed for fat and fat-free free mass (FFM) between days 22 and 29. When oxygen consumption results were expressed on an absolute basis (ml.min-1), both suspension groups had significant decreases at day 7. Thereafter, the HDS rats remained significantly decreased, while the HS rats returned to values not significantly different than the presuspension values by day 28. When oxygen consumption was expressed on a relative basis (ml.kg-1.min-1), all groups exhibited significant reductions between 6-8%. Both suspended groups had significantly less fat than the CC rats but had similar values for FFM (p greater than 0.05). When VO2max was evaluated on a FFM basis, the HDS rats had a group mean that was 6% lower than the other two groups, but the difference was not statistically significant. Treadmill run time was significantly reduced in all groups after 28 d with the reduction from pre-suspension values being more pronounced with HDS animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Influences of chemical sympathectomy and simulated weightlessness on male and female rats.

Maximum oxygen consumption (VO2max) has been shown to be reduced after periods of simulated weightlessness. To assess the role of the sympathetic nervous system in these reductions, Sprague-Dawley rats were either chemically sympathectomized (SYMX) or injected with saline (SHAM) and assigned to head-down suspension (HDS), horizontal restraint with the hindlimbs weight bearing (HWB), or cage-control (CC) conditions. VO2max, run time (RT), and mechanical efficiency (ME) were measured before suspension and on days 7 and 14. Male and female SHAM HDS groups exhibited reduced measures of VO2max (12-13%) after 7 and 14 days, and this decrease was attenuated in the SYMX and HWB rats. HDS resulted in a significant reduction in RT (9-15%) in both the male and female rats, and ME was significantly reduced after HDS in male and female SYMX and male SHAM rats (23-33%) but not in the female SHAM rats. Lesser reductions in ME were observed in the HWB rats. HDS and HWB were associated with lower body, fat-free, and fat masses, which were similar in male and female rats as well as for the SHAM and SYMX conditions. In a related HDS experiment with normal rats, plasma norepinephrine and epinephrine were increased by 53 and 42% after 7 days, but only epinephrine returned to baseline after 14 days. It was concluded that chemical sympathectomy and/or a weight-bearing stimulus will attenuate the loss in VO2max associated with simulated weightlessness in rats despite similar changes in body mass and composition. The mechanism(s) remains unclear at this time.