Immobilization induces anabolic resistance in human myofibrillar protein synthesis with low and high dose amino acid infusion

Abstract

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, GSK3beta 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.

Figure 1. Schematic representation of the infusion protocol

Figure 2. Quadriceps femoris cross-sectional area

Values are means +s.e.m.*Significantly different from pre-immobilization in immobilized (P < 0.005) and non-immobilized (P = 0.47).

Figure 3. Summed concentration of essential amino acids (EAA; A) and plasma insulin (B)

Values are means +s.e.m. Values associated with different letters are significantly different from each other (P < 0.05). *Significantly different from low dose condition (P < 0.01).

Figure 4. Pooled (from both low and high infused groups) resting fasted myofibrillar protein fractional synthetic rate (FSR)

*Significantly different from immobilized (P < 0.001).

Figure 5. Myofibrillar fractional synthetic rate (FSR) and aggregate response of FSR indicated by area under the curve (AUC)

Fasted and fed state (2 h and 4 h) myofibrillar fractional synthetic rate (FSR) at low (A) and high (C) dose infusions. *Significantly different from non-immobilized (P < 0.001). Aggregate response of FSR indicated by area under the curve (AUC) for the low (B) and high (D) dose infusions. Means with different letters are significantly different from each other (P < 0.05). *Significantly different from immobilized (P < 0.001).

Figure 6. Phosphorylation (Tyr^576/577) of focal adhesion kinase (FAK) pooled from rested fasted immobilized (N = 12) and non-immobilized (N = 12) legs

*Significantly different (P < 0.05) from immobilized.

Figure 7. Phosphorylation (phos) of Akt, mTOR and p70S6k (normalized to α-actin) in immobilized and non-immobilized legs at a low dose (43 mg kg⁻¹ h⁻¹; panels on left) and a high dose (261 mg kg⁻¹ h⁻¹; panels on right)

*Significantly different from the opposite condition at the same time point (P < 0.05), means with different letters are significantly different across time (P < 0.05). Values are means + s.e.m.