Myogenic protein expression before and after resistance loading in 26- and 64-yr-old men and women.

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 cyclin-dependent kinase 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 (O) women [females (F)] compared with men (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 O 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 days/wk for several weeks (3 sets of 80% one-repetition maximum for squat, leg press, and knee extension). Protein concentrations of MRFs (MyoD, myogenin, myf-6), cyclin D1, cyclin B1, alpha-actin, and the cyclin-dependent kinase inhibitor p27kip were determined by immunoblotting. Data were analyzed by using age x gender x load repeated-measures ANOVA. Myogenin expression was 44% higher (P <0.05) in O compared with Y, and myf-6 tended to be higher in OF compared with 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 alpha-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. Although the time point of postexercise 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.

Gender differences in resistance-training-induced myofiber hypertrophy among older adults.

We tested the hypothesis that older men (n = 9, 69 +/- 2 years) would experience greater resistance-training-induced myofiber hypertrophy than older women (n = 5, 66 +/- 1 years) following knee extensor training 3 days per week at 65-80% of one-repetition maximum for 26 weeks. Vastus lateralis biopsies were analyzed for myofiber areas, myosin heavy chain isoform distribution, and levels of mRNA for insulin-like growth factor 1 (IGF-1), IGFR1, and myogenin. Gender x Training interactions (p <.05) indicate greater myofiber hypertrophy for all three primary fiber types (I, IIa, IIx) and enhanced one-repetition maximum strength gain in men compared with women (p <.05). Covarying for serum IGF-1, dehydroepiandrosterone sulfate, or each muscle mRNA did not negate these interactions. In both genders, type IIx myofiber area distribution and myosin heavy chain type IIx distribution decreased with a concomitant increase in type IIa myofiber area distribution (p <.05). In summary, gender differences in load-induced myofiber hypertrophy among older adults cannot be explained by levels of circulating IGF-1 or dehydroepiandrosterone sulfate, or by expression of the myogenic transcripts examined.