Muscle regeneration during hindlimb unloading results in a reduction in muscle size after reloading

The hindlimb-unloading model was used to study the ability of muscle injure d in a weightless environment to recover after reloading. Satellite cell mi totic activity and DNA unit size were determined in injured and intact sole us muscles from hindlimb-unloaded and age-matched weight-bearing rats at th e conclusion of 28 days of hindlimb unloading, 2 wk after reloading, and 9 wk after reloading. The body weights of hindlimb-unloaded rats were signifi cantly (P < 0.05) less than those of weight-bearing rats at the conclusion of hindlimb unloading, but they were the same (P > 0.05) as those of weight -bearing rats 2 and 9 wk after reloading. The soleus muscle weight, soleus muscle weight-to-body weight ratio, myofiber diameter, number of nuclei per millimeter, and DNA unit size were significantly (P < 0.05) smaller for th e injured soleus muscles from hindlimb-unloaded rats than for the soleus mu scles from weight-bearing rats at each recovery time. Satellite cell mitoti c activity was significantly (P < 0.05) higher in the injured soleus muscle s from hindlimb-unloaded rats than from weight-bearing rats 2 wk after relo ading, but it was the same (P > 0.05) as in the injured soleus muscles from weight-bearing rats 9 wk after reloading. The injured soleus muscles from hindlimb-unloaded rats failed to achieve weight-bearing muscle size 9 wk af ter reloading, because incomplete compensation for the decrease in myonucle ar accretion and DNA unit size expansion occurred during the unloading peri od.