Abstract
Resistance exercise training (RT) is the most effective method for increasing skeletal muscle mass in older adults; however, the amount of RT-induced muscle growth is highly variable between individuals. Recent evidence from our laboratory and others suggests ribosome biogenesis may be an important factor regulating RT-induced hypertrophy, and we hypothesized that the extent of hypertrophy is at least partly regulated by the amount of RT-induced ribosome biogenesis. To examine this, 42 older adults underwent 4 wk of RT aimed at inducing hypertrophy of the knee extensors (e.g., 2 sets of squat, leg press, and knee extension, 10-12 repetition maximums, 3 days/wk), and vastus lateralis muscle biopsies were performed pre- and post-RT. Post hoc K-means cluster analysis revealed distinct differences in type II myofiber hypertrophy among subjects. The percent change in type II myofiber size in nonresponders (Non; n = 17) was -7%, moderate responders (Mod; n = 19) +22%, and extreme responders (Xtr; n = 6) +83%. Total muscle RNA increased only in Mod (+9%, P < 0.08) and Xtr (+26%, P < 0.01), and only Xtr increased rRNA content (+40%, P < 0.05) and myonuclei/type II fiber (+32%, P < 0.01). Additionally, Mod and Xtr had a greater increase in c-Myc protein levels compared with Non (e.g., approximately +350 and +250% vs. +50%, respectively, P < 0.05). In vitro studies showed that growth factor-induced human myotube hypertrophy is abolished when rRNA synthesis is knocked down using the Pol I-specific inhibitor CX-5461. Overall, these data implicate ribosome biogenesis as a key process regulating the extent of RT-induced myofiber hypertrophy in older adults.