INFLUENCE OF SIMULATED MICROGRAVITY ON THE VO2MAX OF NONTRAINED AND TRAINED RATS

Head-down suspension (HDS) of rats has evolved as a useful model for t he simulation of a microgravity environment. Previous HDS experiments with rats have shown an impaired capacity to perform aerobic exercise as demonstrated by reductions in maximum oxygen consumption (VO2max), treadmill run time (RT), and mechanical efficiency (ME) of treadmill r unning at submaximal conditions. To determine whether endurance traini ng (TR) before HDS would modify exercise performance, male Sprague-Daw ley rats were assigned to nontrained (NT) or TR groups for 6 wk and ex posed to HDS or cage control (CC) conditions for 29 days. The rats wer e tested for VO2max, RT, and ME before treatment and on days 7, 14, 21 , and 28. In addition, water and electrolyte excretion was measured on days 1 and 21 of the experimental period. Before HDS, the TR rats had significantly higher measures of VO2max (15%) and RT (22%) than the N T rats. On day 28, HDS was associated with significant reductions in a bsolute VO2max (ml/min) in TR (-30%) and NT (-14%) rats. Relative VO2m ax (ml.min-1.kg-1) was significantly reduced in TR (-15%) but not NT r ats. Similar reductions in RT occurred in TR (-37%) and NT (-35%) rats by day 28. ME was reduced 22% in both TR and NT rats after 28 days of suspension. HDS elicited diuresis, natriuresis, and kaliuresis in TR rats after 21 days but not after 24 h. In contrast, HDS-NT rats exhibi ted no diuretic, natriuretic, or kaliuretic responses. It was conclude d that rats endurance trained for 6 wk exhibited greater reductions in exercise capacity than NT rats exposed to HDS as manifested by greate r reductions in absolute and relative VO2max but not in RT or ME. In a ddition, it was concluded that TR rats exhibited greater diuretic, nat riuretic, and kaliuretic responses to HDS than NT rats.