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. Skeletal muscle protein synthesis in neonatal pigs is stimulated by A-ketoisocaproic acid, but not by norleucine [abstract]. Journal of Animal Science. 84(Suppl 1):258. Interpretive Summary:
Technical Abstract: In neonatal pigs, skeletal muscle protein synthesis is stimulated when plasma leucine is increased within the physiological postprandial range. We previously have shown that valine and isoleucine were not able to stimulate protein synthesis when their plasma concentrations were elevated within the physiological postprandial range. The objective of the present study was to determine the effect of an elevation in plasma levels of alpha-ketoisocaproic acid (KIC, the alpha-keto acid of leucine) and norleucine (an aliphatic leucine analogue that does not charge leucyl tRNA) on skeletal muscle protein synthesis and the activation of translation initiation factors. Piglets (5 d of age) were food-deprived overnight and infused intra-arterially with saline or 400 micromol.kg(-1).h(-1) of leucine, KIC or norleucine for 60 min. At the end of the infusion period, protein synthesis and the activation of translation initiation factors were determined in longissimus dorsi muscle and liver. Plasma concentration of leucine was reduced (P<0.02) by norleucine and increased (P<0.01) by KIC compared to saline controls. Infusion of leucine and KIC increased the phosphorylation of eukaryotic initiation factor (eIF)4E binding protein-1 (4E-BP1, P<0.01), decreased (P<0.04) the inactive 4EBP1.eIF4E complex, and numerically increased the active eIF4G.eIF4E complex in muscle. Both leucine and KIC increased (P<0.03) muscle protein synthesis. Norleucine had no effect on muscle translation initiation factor activation or protein synthesis. In the liver, the activation of translation initiation factors and protein synthesis were not affected by any treatment. Our results indicate that the ability of leucine to act as a nutrient signal to stimulate skeletal muscle protein synthesis is likely specific for leucine or its metabolites.