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The time course and extent of satellite cell activation were studied in the soleus (m-SOL) and extensor digitorum longus (m-EDL) muscles of untrained growing and mature rats after a single bout of prolonged eccentric treadmill running. At 24, 48, 72, and 120 h postexercise, satellite cell mitotic activity was quantitated in autoradiographs of whole-fiber segments after injection of 3Hthymidine. Fiber damage and localization of labeled cells were also examined in muscle cross sections. Labeling in growing muscles progressively increased to peak levels (approximately 250% of control) at 72 h postexercise, whereas mature muscles exhibited an earlier peak (approximately 250% of control) at 24 (m-SOL) and 48 (m-EDL) h, followed by a more rapid decline to control levels by 120 h postexercise. In all exercised muscles the calculated satellite cell activation was far greater than required to repair the small number (less than 3.0%) of necrotic fibers identified at the light-microscopic level. These results suggest that satellite cells were activated not only on fibers exhibiting overt necrosis but also on those with lesions not discernible with light microscopy.
Darr et al. (Sun,) studied this question.