Thermal preconditioning prevents fiber type transformation of the unloading induced-atrophied muscle in rats.

Muscle atrophy is accompanied by a slow-to-fast transformation of the slow muscle, e.g., the soleus muscle, which is characterized by a decrease in the expression of the slow myosin heavy chain (MyHC) isoform. Heat stress before hindlimb unloading, i.e., thermal preconditioning, has been shown to reduce the rate of disuse-induced muscle atrophy. The present study examined whether thermal preconditioning could prevent a slow-to-fast transformation of the MyHC isoform through the induction of heat-shock protein (HSP) 72. Thermally preconditioned rats (Heat + HU) were individually placed in an environmentally controlled heat chamber for 1 h before hindlimb unloading for 2 weeks (HU). Although the mean fiber cross-sectional areas of the soleus muscle decreased in the HU and Heat + HU group, the loss of myofibrillar protein was attenuated in the Heat + HU group. Furthermore, a slow-to-fast transformation of MyHC isoform was inhibited in the Heat + HU group with the overexpression of HSP72. These results indicate that thermal preconditioning before hindlimb unloading attenuates the decrease of the slow MyHC isoform in the soleus muscle. Therefore, thermal preconditioning provides a new approach to prevent disuse-induced fiber type transformation of skeletal muscle.