Effects of exercise during growth on the expresslon of myostatin in muscle

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Exercise stimulus during growth is likely important not only for the development of various physiological functions but also for maintaining these functions throughout the later stages of life. Myostatin, a growth factor that strongly suppresses the proliferation of muscle satellite cells, decreases during growth and also as the muscle is mechanically loaded, so that it may play a part in normal development of muscle. The present study investigated the effects of either exercise or mechanical unloading during early stage of growth on the content of myostatin and the number of satellite cells within muscles during the later stages of growth. Mice (3-wk age) were assigned into control, uphill-run exercise, downhill-run exercise and unloaded groups, and subjected to either treadmill running or hindlimb suspension for 2 wk, thereafter returned to the normal in-cage activity. In all groups, the content of myostatin in gastrocnemius (GA) and soleus (SOL) muscles decreased at 5-8 wk without significant difference between groups. However, the number of satellite cells per muscle fiber was significantly smaller in unloaded group and larger in exercise groups than in control group. In unloaded group, wet weight of both muscles markedly decreased after the hindlimb suspension. Although the muscle weight was recovered at 12 wk, the number of muscle fibers was significantly smaller than in other three groups. Thus in the second part of study, the mice were hindlimb-suspended at 3-5 wk, returned to normal activity until 12 wk, and then trained with uphill running for 2wk. In both GA and SOL, however, no significant differences in muscle weight were seen between hindlimb-suspended and control groups. The results suggest that the mechanical unloading during the early stage of growth persistently impairs the normal development of muscle, but does not strongly affect the adaptability of muscle to relatively low-intensity exercise in adult.