Submitted to: Journal of Animal Science
Publication Type: Abstract Only
Publication Acceptance Date: 6/17/2006
Publication Date: 7/9/2006
Citation: Escobar, J., Frank, J.W., Suryawan, A., Nguyen, H.V., Davis, T.A. 2006. Stimulation of muscle protein synthesis by leucine is dependent on plasma amino acid availability [abstract]. Journal of Animal Science. 84(Suppl 1):284. Interpretive Summary:
Technical Abstract: We have reported that a physiological increase in plasma leucine increased translation initiation factor activity during 60- and 120-min leucine infusion. Muscle protein synthesis was stimulated at 60 min but not at 120 min, perhaps due to the decrease (-50%) in plasma essential amino acids (AA). Thus, the objective of the present study was to determine the effect of AA replacement during a 120-min leucine infusion on muscle protein synthesis. Piglets (5 d of age) were food-deprived overnight and infused intra-arterially with saline or 400 micromol.kg(-1).h(-1) of leucine for 120 min to raise plasma leucine within the physiological postprandial range. Leucine was infused in the presence or absence of an AA mixture (without leucine) to maintain baseline levels of plasma AA. At the end of the infusion, protein synthesis and the activation of translation initiation factors were determined in longissimus dorsi muscle and liver. Administration of AA prevented the leucine induced reduction (P < 0.05) in plasma essential AA. Leucine infusion, with and without AA replacement, increased (P < 0.05) the phosphorylation of eukaryotic initiation factor (eIF) 4E binding protein-1 (4E-BP1), decreased (P < 0.01) the inactive 4EBP1•eIF4E complex, and increased (P < 0.02) the active eIF4G•eIF4E complex in skeletal muscle. Protein synthesis was increased (P < 0.02) in muscle by the infusion of leucine replacement with AA, but not by leucine alone. In liver, translation initiation factor activation was not affected by leucine infusion with or without AA replacement. Hepatic protein synthesis was unaffected by leucine alone, but tended to increase (P < 0.06) with AA replacement. Our results indicate that the leucine induced stimulation of skeletal muscle protein synthesis in neonatal pigs during prolonged leucine infusion is dependent on plasma AA availability.