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1 Geriatric Research, Education, and Clinical Center, VA Medical Center, Birmingham, Alabama, USA; Department of Physiology and Biophysics, The University of Alabama at Birmingham, Birmingham, Alabama, USA
2 Department of Physiology and Biophysics, The University of Alabama at Birmingham, Birmingham, Alabama, USA; Geriatric Research, Education, and Clinical Center, VA Medical Center, Birmingham, Alabama, USA
3 Department of Surgery, The University of Alabama at Birmingham, Birmingham, Alabama, USA
4 Geriatric Research, Education, and Clinical Center, VA Medical Center, Birmingham, Alabama, USA; Division of Gerontology and Geriatric Medicine, The University of Alabama at Birmingham, Birmingham, Alabama, USA
* To whom correspondence should be addressed. E-mail: mbamman{at}uab.edu.
Based on the growing body of evidence implicating an important role for myogenic regulatory factors (MRFs) in the adaptive responses of skeletal muscle to mechanical load, we tested the hypothesis that protein concentrations of MRFs as well as cell cycle proteins (i.e. cyclins and cdk inhibitors) would be altered after heavy leg resistance exercise (RE). Because we and others, however, have shown a blunted adaptive response to long-term resistance training in older females (F) compared to males (M), we also tested the hypothesis that these myogenic responses to RE would be influenced by age and gender. Twenty-two younger (Y) adults (20-35 yr, 11 YF, 11 YM) and 20 older adults (60-75 yr, 9 OF, 11 OM) consented to vastus lateralis muscle biopsy before and 24 h after a bout of RE using a regimen known to induce myofiber hypertrophy when performed 2-3 d/wk for several weeks (3 sets of 80% 1RM for squat, leg press, and knee extension). Protein concentrations of MRFs (MyoD, myogenin, myf-6), cyclin D1, cyclin B1, 
-actin, and the cyclin-dependent kinase inhibitor p27kip were determined by immunoblotting. Data were analyzed using age x gender x load repeated measures ANOVA. Myogenin expression was 44% higher (P<0.05) in O compared to Y, and myf-6 tended to be higher in OF compared to YF (95%, P=0.059). A significant gender x load interaction indicated that, in F, RE led to a reduction in p27kip (20%) (P<0.05), which was driven mainly by a 27% drop in OF. Levels of cyclin D1, cyclin B1, MyoD, myf-6, and -actin were not influenced by age, gender, or loading. We report a novel finding in humans of markedly higher myogenin protein content in older sedentary muscle. The results do not, however, support the hypothesis that myogenic protein expression is altered 24 h after RE irrespective of age or gender. While the timepoint of post-exercise muscle biopsy could be viewed as too early to capture maximal effects for most of these proteins, the significant decline in p27kip concentration found in OF suggests that mechanical load may provide one means of overcoming the inhibitory influence of p27kip.
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