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1 Physiology and Biophysics, University of Alabama at Birmingham, Birmingham, Alabama, United States
2 Surgery, University of Alabama at Birmingham, Birmingham, Alabama, United States
* To whom correspondence should be addressed. E-mail: mbamman{at}physiology.uab.edu.
Skeletal muscle stem (satellite) cells supporting growth/regeneration are thought to be activated and incorporated into growing myofibers by both endocrine and locally expressed autocrine/paracrine growth factors, the latter being load-sensitive. We recently found myofiber hypertrophy with resistance training is superior in young men (YM) vs. young women and older adults (Kosek et al. J Appl Physiol 2006). We hypothesized the advanced myofiber hypertrophy in YM is facilitated by myonuclear addition in response to a milieu promoting stem cell activation. Twenty-six young (27.0±1 yr, 50% women) and 26 older (63.7±1 yr, 50% women) adults completed 16 wk of knee extensor resistance training. Vastus lateralis biopsies were obtained at baseline, 24 h after one bout, and after 16 wk. Muscle stem cells were identified immunohistochemically with anti-neural cell adhesion molecule (NCAM+). Muscle transcript levels of IGF-I and mechanogrowth factor (MGF) were determined by RT-PCR. Serum IGF-I, IGFBP-3, IGFBP-1, total and free testosterone, SHBG, and androstenedione were assessed by radioimmunoassay. Myofiber hypertrophy was 2-fold greater in YM vs. others and only YM increased NCAM+ cells/100 myofibers (49%) and myonuclei/fiber (19%) (P<0.05). IGF-IEa mRNA was higher in young and increased acutely (29%) with summation by 16 wk (96%) (P<0.05). MGF mRNA increased only in young after one bout (81%) and by 16 wk (85%) (P<0.001). Circulating IGF-I was 2-fold higher in young, while IGFBP-1 was lowest in YM (P<0.05). Among men, free testosterone was 59% higher in YM (P<0.01). Myonuclear addition was most effectively accomplished in YM, which likely drove the superior growth.
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