Activity-induced fiber regeneration in rat soleus muscle

In an attempt to understand why muscle recovery is limited following atrophy due to limb immobilization, satellite cell activity and muscle fiber regeneration were analyzed in rat soleus muscles. Adult rat hindlimbs were immobilized in plaster casts for a period of two to ten weeks. Soleus muscles were examined by electron microscopy for evidence of fiber degeneration or regeneration, and to quantify satellite cell nuclei. Immunocytochemical localization of embryonic myosin was used to identify regenerating myofibers. Soleus muscle wet weight to body weight ratios for the casted muscles significantly decreased over the 10-week immobilization period. The casted muscles displayed ultrastructural evidence of minor fiber damage, including myofibrillar atrophy, Z-disc disruption, and abnormal triadic junctions. No ultrastructural evidence of regeneration was seen in the casted animals. The number of satellite cells in the casted muscles significantly decreased from 6.4% to 3. 3% by eight to 10 weeks of immobilization. Approximately 1.0% of extrafusal fibers in the control soleus muscles appeared to be regenerating since they expressed embryonic myosin and were of a small diameter, while in casted muscles, only 0.1% of the fibers were embryonic myosin-positive. Following release from immobilization, a reappearance of embryonic myosin-positive fibers was noted within four days of renewed activity. In contrast to control muscles, embryonic myosin-positive fibers in the recovery muscles included both small and large diameter fibers. Subtle changes in functional activity influence muscle damage and subsequent myofiber regeneration. Reduced activity reduces muscle fiber regeneration, while increased activity, as seen by increased hindlimb weight bearing and return to normal activity following immobilization, increase regenerating fibers and also the expression of embryonic myosin in adult fibers.