Enns DL, Tiidus PM. Estrogen influences satellite cell activation and proliferation following downhill running in rats. To investigate the influence of estrogen on postexercise muscle repair processes, we examined the effects of estrogen supplementation (0.25-mg pellet) on numbers of myofibers positive for markers of total, activated, and proliferating satellite cells in rat skeletal muscles 72 h following downhill running. Ovariectomized female rats (n ϭ 44) were divided into four groups (n ϭ

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... 1 per group): sham (no estrogen) controls (SC); sham, exercised (SE); estrogen-supplemented controls (EC); and estrogen-supplemented, exercised (EE). After 8 days of estrogen exposure, animals were exposed to 90 min of treadmill running at 17 m/min (Ϫ13.5°). Seventy-two hours later, soleus and white vastus muscles were removed and immunostained for total [paired box homeotic gene 7 (Pax7)], [activated myogenic differentiation factor D (MyoD)], and proliferating [5-bromo-2Ј-deoxyuridine (BrdU)] satellite cells. ␤-Glucuronidase activity was increased (P Ͻ 0.05) in both muscles following exercise; however, the postexercise elevations in enzyme activity were attenuated in the EE group compared with the SE group in the soleus (P Ͻ 0.05). Immunohistochemical analysis revealed that exercised groups displayed increased numbers of myofibers containing total, activated, and proliferating satellite cells compared with control groups (P Ͻ 0.05). Furthermore, greater numbers of fibers positive for markers of total, activated, and proliferating satellite cells were observed postexercise in EE animals compared with SE animals for both muscles (P Ͻ 0.05). The results demonstrate that estrogen may potentially influence postdamage repair of skeletal muscle through activation of satellite cells.