Protective Effects of Heat Stress on Skeletal Muscle in the Disuse Atrophy

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Skeletal muscle atrophy occurs as a consequence of disuse. Muscle atrophy is accompanied by a slow-to-fast transformation of the soleus muscle, characterized by a decrease in the expression of the slow myosin heavy chain (MHC) isoform. Previous reports indicate that whole body heat stress provides protection against skeletal muscle atrophy induced by hindlimb unweighting. This study examined whether a thermal preconditioning could prevent a slow to fast transformation of MHC isoform through the induction of heat shock protein (HSP) 72.I Rats were divided into three groups: hindlimb unloading was performed for 2 wks by the Morey method (HU); heat for preconditioning was applied for 60 min (Heat + HU); and untreated control (Cont).Mean fiber cross-sectional area was decreased 53% in HU and 56% in Heat+HU compared with Cont. The myofibrillar protein content per muscle of HU and Heat+HU muscles were significantly lower than the Cont. However, myofibrillar protein content normalized per gram of soleus muscle mass decreased 46% in HU and 4% in Heat + HU. A transformation of MHC isoform from slow to fast was inhibited after hindlimb unloading with thermal preconditioning. The expression level of HSP72 in the HU was 61.3% of that in the Cont, but it was 90 % higher in the Heat + HU than Cont. These results suggest that thermal treatment before hindlimb unloading attenuates the decrease of slow MHC in the soleus muscle. Therefore, thermal preconditioning provides a new approach to prevent disuse-induced type transformation of muscle fibers.