Contraction-mediated mTOR, p70^S6k, and ERK1/2 phosphorylation in aged skeletal muscle

With age, skeletal muscle experiences substantial atrophy and weakness. Although resistance training can increase muscle size and strength, the myogenic response to exercise and the capacity for muscle hypertrophy in older humans and animals is limited. In the present study, we assessed the ability of muscle contractile activity to activate cellular pathways involved in muscle cell growth and myogenesis in adult (Y; 6 mo old) and aged (O; 30 mo old) Fischer 344 X Brown Norway rats. A single bout of rat hindlimb muscle contractile activity was elicited by high-frequency electrical stimulation (HFES) of the sciatic nerve. Plantaris (Pla) and tibialis anterior (TA) muscles were assayed for mammalian target of rapamycin (mTOR), 70-kDa ribosomal protein S6 kinase (p70^S6K), and extracellular signal-regulated kinase (ERK) 1/2 phosphorylation and total protein either at baseline, immediately after, or 6 h after HFES. mTOR phosphorylation was elevated in Pla (1.3 ± 0.3-fold, P < 0.05) immediately after HFES and to a lesser extent 6 h after HFES (0.6 ± 0.1-fold, P < 0.05) in O rats. Post-HFES, p70^S6K phosphorylation increased 1.2 ± 0.3-fold in TA (P < 0.05) and remained elevated 6 h later (0.6 ± 0.2-fold, P < 0.05) in O rats. ERK phosphorylation was lower in O rats immediately after exercise in both TA (11.1 ± 2.9 vs. 2.1 ± 0.5-fold, P < 0.05) and Pla (6.5 ± 1.5 vs. 1.8 ± 0.5-fold, P < 0.05) and returned to baseline by 6 h in both Y and O rats. Phosphorylation of mTOR, p70^S6K, and ERK1/2 are increased in skeletal muscle after a single bout of in situ muscle contractile activity in aged animals, and the response is less than that observed in adult animals. These observations suggest that the anabolic response to a single bout of contraction is attenuated with aging and may help explain the reduced capacity for hypertrophy in aged animals.