Early Changes in Hsp25 Expression and Phosphorylation in Functionally Overloaded Rat Muscles

Abstract

Functional overload (FO, removal of major synergists) induces high levels of oxidative and mechanical stress in muscles. Hsp25 may protect against these stressors as we have shown that its expression and phosphorylation (pHsp25) are regulated by chronic changes in muscle activation/loading. The highest oxidative and mechanical stressors imposed on the muscle fibers likely occur early after FO since the muscle has not fully adapted to the increased activation/loading. Our purpose was to test the hypothesis that Hsp25 and pHsp25 are increased during the early stages of FO. Protein levels of Hsp25 and pHsp25 were quantified with Western analysis in the soleus and plantaris (Plt) after 12h, 1d, 2d, or 3d of FO. In the Plt, pHsp25 was increased 5- to 6-fold over control, 12h, 1d, and 2d after FO and 4-fold after 3d. In the soleus, pHsp25 was increased ~2-fold over control at all time points. Increases in Hsp25 were not observed until 2–3d of FO in either muscle (~130 and 250% of control for 3d soleus and Plt, respectively). These results indicate that FO is associated with immediate increases in pHsp25, suggestive of a role in the protection against oxidative and mechanical insults induced by FO. The greater response in the Plt than soleus may be indicative of a higher relative stress imposed on the Plt since the soleus is a highly oxidative antigravity muscle normally adapted to tonic activation. Funded in part by NIH NS16333 & AR049855.