METABOLIC RESPONSES TO HEAD-DOWN SUSPENSION IN HYPOPHYSECTOMIZED RATS

Rats exposed to head-down suspension (HDS) exhibit reductions in maxim al O-2 consumption (Vo(2max)) and atrophy of select hindlimb muscles. This study tested the hypothesis that an endocrine-deficient rat expos ed to HDS would not exhibit reductions in Vo(2max) or hindlimb muscle mass. Hypophysectomized (HYPX) and sham-operated (SHAM) rats were test ed for Vo(2max) before and after 28 days of HDS or cage control (CC) c onditions. No significant reductions in Vo(2max) were observed in HYPX rats. In contrast, SHAM-HDS rats exhibited a significant reduction in absolute (-16%) and relative (-29%) measures of aerobic capacity. Tim e course experiments revealed a reduction in Vo(2max) in SHAM-HDS rats within 7 days, suggesting that cardiovascular adjustments to HDS occu rred in the 1st wk. HDS was associated with atrophy of the soleus (-42 %) in SHAM rats, whereas HYPX rats exhibited atrophy of the soleus (-3 6%) and plantaris (-13%). SHAM-HDS rats had significantly lower (-38%) soleus citrate synthase activities per gram muscle mass than SHAM-CC, but no significant differences existed between HYPX-HDS and -CC rats. HDS rats had an impaired ability to thermoregulate, as indicated by s ignificantly greater temperature increases per unit run time, compared with their CC counterparts. Pretreatment plasma epinephrine levels we re significantly lower in HYPX than in SHAM rats. Norepinephrine conce ntration was similar for all groups except HYPX-HDS, in which it was s ignificantly higher. HDS had no significant effect on thyroxine or tri iodothyronine. SHAM-HDS rats had significantly lower concentrations of testosterone and growth hormone. We concluded that HYPX rats could ma intain aerobic capacity but not hindlimb muscle mass when exposed to H DS.