Our previous findings have provided strong evidence that mechanical stress is an effective countermeasure against unloading-induced atrophy in antigravity skeletal muscles. The generations of heat shock proteins (HSP) are induced in response to mechanical and metabolic stress in the cytosol and play important roles as molecular chaperones for protein synthesis and degradation. HSP70 is down-regulated by mechanical unloading but is upregulated by mechanical overloading. In the present study, we investigated whether HSP70 acts as a factor for maintaining muscle mass in the skeletal muscle. Female F344 rats (10 months old) were assigned to either control or hindlimb unloaded (HU) group. HU rats had their hindlimbs suspended for 3 weeks with or without isometric resistance exercise (IRE). IRE was done for 30 min a day, 6 days a week, with an additional load of 30% body mass to the tail during HU period. HU decreased maximum force by 76% and myofibrillar protein by 80% in the soleus muscle compared to the control. IRE suppressed the decreases in maximum force and myofibrillar protein (by 25% and 19%, respectively, compared to the control). HU reduced HSP70 expression in the soleus muscle by 53% but IRE prevented this reduction of HSP70 by 16% compared to the control. These findings suggest that HSP70 expression corresponds with changes in muscle mass, and that HSP70 plays an important role in maintaining the muscle mass.

DESCENTE SPORTS SCIENCE Vol.25/THE DESCENTE AND ISHIMOTO MEMORIAL FOUNDATION FOR THE PROMOTION SPORTS SCIENCE