Muscle protein synthesis and mTORC1 signalling are concurrently stimulated following muscle contraction in humans. In an effort to determine whether mTORC1 signalling is essential for regulating muscle protein synthesis in humans, we treated subjects with a potent mTORC1 inhibitor (rapamycin) prior to performing a series of high-intensity muscle contractions. Here we show that rapamycin treatment blocks the early (1-2 h) acute contraction-induced increase (∼40%) in human muscle protein synthesis. In addition, several downstream components of the mTORC1 signalling pathway were also blunted or blocked by rapamycin. For instance, S6K1 phosphorylation (Thr421/Ser424) was increased post-exercise 6-fold in the control group while being unchanged with rapamycin treatment. Furthermore, eEF2 phosphorylation (Thr56) was reduced by ∼25% post-exercise in the control group but phosphorylation following rapamycin treatment was unaltered, indicating that translation elongation was inhibited. Rapamycin administration prior to exercise also reduced the ability of raptor to associate with mTORC1 during post-exercise recovery. Surprisingly, rapamycin treatment prior to resistance exercise completely blocked the contraction-induced increase in the phosphorylation of ERK1/2 (Thr202/Tyr204) and blunted the increase in MNK1 (Thr197/202) phosphorylation. However, the phosphorylation of a known target of MNK1, eIF4E (Ser208), was similar in both groups (P > 0.05) which is consistent with the notion that rapamycin does not directly inhibit MAPK signalling. We conclude that mTORC1 signalling is, in part, playing a key role in regulating the contraction-induced stimulation of muscle protein synthesis in humans, while dual activation of mTORC1 and ERK1/2 stimulation may be required for full stimulation of human skeletal muscle protein synthesis.

Skeletal muscle loss during aging leads to an increased risk of falls, fractures, and eventually loss of independence. Resistance exercise is a useful intervention to prevent sarcopenia; however, the muscle protein synthesis (MPS) response to resistance exercise is less in elderly compared with young subjects. On the other hand, essential amino acids (EAA) increase MPS equally in both young and old subjects when sufficient EAA is ingested. We hypothesized that EAA ingestion following a bout of resistance exercise would stimulate anabolic signaling and MPS similarly between young and old men. Each subject ingested 20 g of EAA 1 h following leg resistance exercise. Muscle biopsies were obtained before and 1, 3, and 6 h after exercise to measure the rate of MPS and signaling pathways that regulate translation initiation. MPS increased early in young (1-3 h postexercise) and later in old (3-6 h postexercise). At 1 h postexercise, ERK1/2 MNK1 phosphorylation increased and eIF2alpha phosphorylation decreased only in the young. mTOR signaling (mTOR, S6K1, 4E-BP1, eEF2) was similar between groups at all time points, but MNK1 phosphorylation was lower at 3 h and AMP-activated protein kinase-alpha (AMPKalpha) phosphorylation was higher in old 1-3 h postexercise. We conclude that the acute MPS response after resistance exercise and EAA ingestion is similar between young and old men; however, the response is delayed with aging. Unresponsive ERK1/2 signaling and AMPK activation in old muscle may be playing a role in the delayed activation of MPS. Notwithstanding, the combination of resistance exercise and EAA ingestion should be a useful strategy to combat sarcopenia.

We recently showed that resistance exercise and ingestion of essential amino acids with carbohydrate (EAA+CHO) can independently stimulate mammalian target of rapamycin (mTOR) signaling and muscle protein synthesis in humans. Providing an EAA+CHO solution postexercise can further increase muscle protein synthesis. Therefore, we hypothesized that enhanced mTOR signaling might be responsible for the greater muscle protein synthesis when leucine-enriched EAA+CHOs are ingested during postexercise recovery. Sixteen male subjects were randomized to one of two groups (control or EAA+CHO). The EAA+CHO group ingested the nutrient solution 1 h after resistance exercise. mTOR signaling was assessed by immunoblotting from repeated muscle biopsy samples. Mixed muscle fractional synthetic rate (FSR) was measured using stable isotope techniques. Muscle protein synthesis and 4E-BP1 phosphorylation during exercise were significantly reduced (P < 0.05). Postexercise FSR was elevated above baseline in both groups at 1 h but was even further elevated in the EAA+CHO group at 2 h postexercise (P < 0.05). Increased FSR was associated with enhanced phosphorylation of mTOR and S6K1 (P < 0.05). Akt phosphorylation was elevated at 1 h and returned to baseline by 2 h in the control group, but it remained elevated in the EAA+CHO group (P < 0.05). 4E-BP1 phosphorylation returned to baseline during recovery in control but became elevated when EAA+CHO was ingested (P < 0.05). eEF2 phosphorylation decreased at 1 and 2 h postexercise to a similar extent in both groups (P < 0.05). Our data suggest that enhanced activation of the mTOR signaling pathway is playing a role in the greater synthesis of muscle proteins when resistance exercise is followed by EAA+CHO ingestion.

The loss of skeletal muscle mass during aging, sarcopenia, increases the risk for falls and dependence. Resistance exercise (RE) is an effective rehabilitation technique that can improve muscle mass and strength; however, older individuals are resistant to the stimulation of muscle protein synthesis (MPS) with traditional high-intensity RE. Recently, a novel rehabilitation exercise method, low-intensity RE, combined with blood flow restriction (BFR), has been shown to stimulate mammalian target of rapamycin complex 1 (mTORC1) signaling and MPS in young men. We hypothesized that low-intensity RE with BFR would be able to activate mTORC1 signaling and stimulate MPS in older men. We measured MPS and mTORC1-associated signaling proteins in seven older men (age 70+/-2 yr) before and after exercise. Subjects were studied identically on two occasions: during BFR exercise [bilateral leg extension exercise at 20% of 1-repetition maximum (1-RM) with pressure cuff placed proximally on both thighs and inflated at 200 mmHg] and during exercise without the pressure cuff (Ctrl). MPS and phosphorylation of signaling proteins were determined on successive muscle biopsies by stable isotopic techniques and immunoblotting, respectively. MPS increased 56% from baseline after BFR exercise (P<0.05), while no change was observed in the Ctrl group (P>0.05). Downstream of mTORC1, ribosomal S6 kinase 1 (S6K1) phosphorylation and ribosomal protein S6 (rpS6) phosphorylation increased only in the BFR group after exercise (P<0.05). We conclude that low-intensity RE in combination with BFR enhances mTORC1 signaling and MPS in older men. BFR exercise is a novel intervention that may enhance muscle rehabilitation to counteract sarcopenia.