Unloading of juvenile muscle results in a reduced muscle size 9 wk after reloading

The role of satellite cells and DNA unit size in determining muscle size wa s examined by inhibiting postnatal skeletal muscle development by using hin dlimb suspension. Satellite cell mitotic activity and DNA unit size were de termined in the soleus muscles from hindlimb-suspended and age-matched weig ht-bearing rats before the initiation of hindlimb suspension, at the conclu sion of a 28-day hindlimb-suspension period, 2 wk after reloading, and 9 wk after reloading. The body weights of hindlimb-suspended rats were signific antly (P < 0.05) less than those of weight-bearing rats at the conclusion o f hindlimb suspension, but they were the same (P > 0.05) as those of weight -bearing rats 9 wb after reloading. The soleus muscle weight, soleus muscle weight-to-body weight ratio, myofiber diameter, nuclei per millimeter, and DNA unit size for the hindlimb-suspended rats were significantly (P < 0.05 ) smaller than for the weight-bearing rats at all recovery times. Satellite cell mitotic activity was significantly (P < 0.05) higher in the soleus mu scles from hindlimb-suspended rats 2 wk after reloading, but it was the sam e (P > 0.05) as in weight-bearing rats 9 wk after reloading. Juvenile soleu s muscles failed to achieve normal muscle size 9 wk after reloading because there was incomplete compensation for the hindlimb-suspension-induced inte rruptions in myonuclear accretion and DNA unit size expansion.