Heat stress attenuates skeletal muscle atrophy in hindlimb-unweighted rats

This study tested the hypothesis that elevation of heat stress proteins by whole body hyperthermia is associated with a decrease in skeletal muscle at rophy induced by reduced contractile activity (i.e., hindlimb unweighting). Female adult rats (6 mo old) were assigned to one of four experimental gro ups in = 10/group): I) sedentary control (Con), 2) heat stress (Heat), 3) h indlimb unweighting (HLU), or 4) heat stress before hindlimb unweighting (H eat + HLU). Animals in the Heat and Heat+HLU groups were exposed to 60 min of hyperthermia (colonic temperature similar to 41.6 degrees C). Six hours after heat stress, both the HLU and Heat+HLU groups were subjected to hindl imb unweighting for 8 days. After hindlimb unweighting, the animals were an esthetized, and the soleus muscles were removed, weighed, and analyzed for protein content and the relative levels of heat shock protein 72 (HSP72). C ompared with control and HLU animals, the relative content of HSP72 in the soleus muscle was significantly elevated (P < 0.05) in both the Heat and He at+HLU animals. Although hindlimb unweighting resulted in muscle atrophy in both the HLU and Heat+HLU animals, the loss of muscle weight and protein c ontent was significantly less (P < 0.05) in the Heat+HLU animals. These dat a demonstrate that heat stress before hindlimb unweighting can reduce the r ate of disuse muscle atrophy. We postulate that HSP70 and/or other stress p roteins play a role in the control of muscle atrophy induced by reduced con tractile activity.