We tested the hypothesis that increasing blood amino acid (AA) availability would counter the physical inactivity-induced reduction in muscle protein synthesis. We determined how 14 days of unilateral knee immobilization affected quadriceps myofibrillar protein synthesis (MPS) in young healthy subjects (10 men, 2 women, 21 ± 1 years; 80.2 ± 4.0 kg, mean ± s.e.m.) in the post-absorptive state and after infusing AA (10% Primene) at low or high doses (43 and 261 mg kg −1 h −1 ). Muscle cross-sectional area (MRI) and peak isometric torque declined in the immobilized leg (−5.0 ± 1.2% and −25 ± 3%, respectively, both P < 0.005), but were unchanged (all P > 0.6) in the non-immobilized leg. Immobilization induced a 27% decline in the rate of post-absorptive MPS (immobilized, 0.027 ± 0.003: non-immobilized, 0.037 ± 0.003% h −1 ; P < 0.001). Regardless of dose, AA infusion stimulated a greater rise in MPS in the non-immobilized legs; at 4 h MPS was greater by +54 ± 12% with low dose and +68 ± 17% with high dose AA infusion (both P < 0.001). There was some evidence of delayed responsiveness of phosphorylation of Akt to high doses of AA and p70S6k at both doses but no marked differences in that of mTOR, GSK3β or eEF2. Phosphorylation of focal adhesion kinase (Tyr 576/577 ) was reduced (P < 0.05) with immobilization. We observed no change in polyubiquitinated protein content after immobilization. We confirm that 14 days of immobilization reduces MPS in the post-absorptive state and this diminution is reduced but not abolished by increased provision of AA, even at high rates. The immobilization-induced decline in post-absorptive MPS with the 'anabolic resistance' to amino acids can account for much of immobilization-induced muscle atrophy.

We investigated the effect of resistance exercise and feeding on the activation of signaling proteins involved in translation initiation. Nine young men (23.7+/-0.41 yr; BMI=25.5+/-1.0 kg/m2; means+/-SE) were tested twice after they performed a strenuous bout of unilateral resistance exercise, such that their contralateral leg acted as a nonexercised comparator, in either the fasted and fed [1,000 kJ, each 90 min (3 doses): 10 g protein, 41 g carbohydrate, 4 g fat] states. Muscle biopsies were obtained 6 h postexercise from both legs, resulting in four experimental conditions: rest-fasted, rest-fed, exercise-fasted, and exercise-fed. Feeding increased PKB/Akt (Ser473) phosphorylation (P<0.05), while exercise increased the phosphorylation of Akt and the downstream 70 kDa S6 protein kinase (p70S6K1, Thr389) and ribosomal protein S6 (rpS6, Ser235/236, Ser240/244; all P<0.05). The combination of resistance exercise and feeding increased the phosphorylation of p70S6K1 (Thr389) and rpS6 (Ser240/244) above exercise alone (P<0.05). Exercise also reduced phosphorylation of the catalytic epsilon subunit of eukaryotic initiation factor 2B (eIF2Bepsilon, Ser540; P<0.05). Mammalian target of rapamycin (mTOR, Ser2448), glycogen synthase kinase-3beta (GSK-3beta, Ser9), and focal adhesion kinase (FAK, Tyr576/577) phosphorylation were unaffected by either feeding or resistance exercise (all P>0.14). In summary, feeding resulted in phosphorylation of Akt, while resistance exercise stimulated phosphorylation of Akt, p70S6K1, rpS6, and dephosphorylation eIF2Bepsilon with a synergistic effect of feeding and exercise on p70(S6K1) and its downstream target rpS6. We conclude that resistance exercise potentiates the effect of feeding on the phosphorylation and presumably activation of critical proteins involved in the regulation of muscle protein synthesis in young men.

We determined the effectiveness of low-volume resistance exercise (EX) for the attenuation of loss of muscle mass and strength during leg immobilization. Men (N = 5) and women (N = 12, age 24 ± 5 years, body mass index 25.4 ± 3.6 kg/m(2)) were divided into two groups: exercise (EX; n = 12) and control (CON; n = 5). Subjects wore a knee brace on one leg that prevented weight bearing for 14 days. Resistance exercise (EX; 80% of maximal) was performed by the immobilized limb every other day. Immobilization induced a significant reduction (P < 0.05) in muscle fiber and thigh cross-sectional area (CSA), isometric knee extensor, and plantarflexor strength in the CON (P < 0.01) but not in the EX group. There were significant losses in triceps surae CSA in the CON and EX groups (P < 0.05), but the losses were greater in CON subjects (P < 0.01). A minimal volume (140 contractions in 14 days) of resistive exercise is an effective countermeasure against immobilization-induced atrophy of the quadriceps femoris but is only partially effective for the triceps surae.

We aimed to determine the effect of muscle disuse on activation by mixed amino acid (AA) feeding of three anabolic signaling proteins: mTOR, eIF2Bε and p70s6k. 10 men and 2 women wore an immobilizing knee brace for 14d, reducing muscle by CSA (5±1%, P<0.001). Subjects then received infusions of mixed AA at either 44 mg kg−1h−1 or 261 mg kg−1h−1. Quadriceps biopsies were taken from both legs when fasted and fed (1, 2, 4 h). After 1 h AA infusion phosphorylation of mTOR (Ser2448), eIF2Bε (Ser 539) and p70s6k (Thr389) in the non‐immobilized leg increased by 35–56% (P<0.05), with a return to baseline by 2 h (p70s6k) or 4 h (mTOR, eIF2Bε). In the immobilized leg mTOR phosphorylation did not significantly increase at all after AA infusion whereas phosphorylation of eIF2Bε increased by only 22% at 2 h (P<0.05); phosphorylation of p70s6k increased by only 32 % at 1 h (P<0.05), but remained elevated until 4 h. p70s6k phosphorylation was 22 % higher (P=0.05) in the immobilized leg in the fasted state. These findings, together with an observed depressed AA‐induced muscle protein synthetic response, suggest a dampened response of the underlying translational control mechanisms with disuse atrophy in humans. We propose that a reduction in the feeding‐induced stimulation of muscle protein synthesis is of major importance in causing muscle loss with disuse in humans. Support: NSERC, CIHR, UK BBSRC, and EC EXEGENESIS

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