Centrifugation studies have enabled researchers to distinguish effects of acute versus chronic hypergravity exposure. Short-term centrifugation even at high forces of 4.1 g for 15 min had no detectable effect on the T levels of male rats. However, centrifugation at 4 g for 4 h resulted in reduced T levels, suggesting that duration of exposure is important. The hypergravity experienced at liftoff and re-entry is short term; therefore, it is unlikely that the reduced T in spaceflight rats was due to acute hypergravity.
Chronic centrifugation at 2 g had no effect on testes weight, T, or spermatogenesis compared with control rats, but 2.3 g reduced T during the initial 3 days, and 4.1 g resulted in continuous suppression of T throughout 52 days of centrifugation (Table 1). These results suggest that over time, rats are capable of adapting to gravitational forces of >2.3 g but not to 4.1 g. Using centrifugation to validate the spaceflight findings has been difficult because of the pulsatile secretion of LH and elevated LH in response to disturbing stimuli such as the stopping of the centrifuge. Ortiz et al., avoiding the bias of daily periodicity by measuring T in 24-h urine samples, found a transient increase in urinary T in male rats centrifuged at 2 g. This finding contrasts with those of other reports of reduced T in response to centrifugation. Stress rather than hypergravity may have been responsible for elevated T, because elevated T has been reported as a response to acute stress. Clearly more studies are needed, but the results of these studies suggest that hypergravity altered male reproductive hormone levels and that the response of the HPG axis depends on duration and g force.