A proteomic assessment of muscle contractile alterations during unloading and reloading

Unloading of skeletal muscle causes atrophy and altered contractility. To identify major muscle proteins responding significantly to the altered loading and to elucidate how the contractile alterations reflect potential proteomic modifications, we examined protein expression in the rat soleus muscle during 3-week hindlimb suspension and 2-week reloading. Compared with unsuspended controls, experimental animals had a 0.5- to 0.6-fold decrease in tension during unloading and early reloading, comparable to 0.2- to 0.6-fold decreases in the protein levels of myosin light chain 1 (MLC1), alpha-actin, tropomyosin beta-chain, and troponins T1 and T2. The observed 1.4- to 1.6-fold increase in shortening velocity appears to reflect 1.2- to 9.0-fold increases in the protein levels of fast-type MLC2, glycolytic enzymes, and creatine kinase, and 0.2- to 0.3-fold decreases in slow-type troponins T1 and T2. The levels of three heat shock proteins (p20, alpha crystallin B chain, and HSP90) decreased during unloading but returned to control levels during reloading. These results imply that proteomic responses to unloading change overall myofibrillar integrity and metabolic regulation, resulting in altered contractility.