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 (V̇O₂max) 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°, horizontal suspension (HS) at 0°, or cage control (CC). Five separate exercise performance tests with measurements related to V̇O₂max, 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 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 > 0.05). When V̇O₂max 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. Apparent mechanical efficiency remained constant for the CC group, but this relationship did not prevail with the other two groups which exhibited reductions ranging from 22-35% during the experimental period. From these findings, we have concluded that it was unlikely that the reductions in V̇O₂max with 22-29 d of simulated microgravity were caused by a significant loss of whole body FFM and that other mechanisms should be considered.