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Am J Physiol Endocrinol Metab (January 11, 2005). doi:10.1152/ajpendo.00510.2004
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Submitted on October 26, 2004
Accepted on December 30, 2004

A Physiological Rise in Plasma Leucine Stimulates Muscle Protein Synthesis in Neonatal Pigs by Enhancing Translation Initiation Factor Activation

Jeffery Escobar1, Jason W. Frank1, Agus Suryawan1, Hanh V. Nguyen1, Scot R. Kimball2, Leonard S. Jefferson2, and Teresa A. Davis1*

1 Department of Pediatrics, USDA/ARS Children's Nutrition Research Center, Baylor College of Medicine, Houston, TX, USA
2 Department of Cellular and Molecular Physiology, Pennsylvania State University College of Medicine, Hershey, PA, USA

* To whom correspondence should be addressed. E-mail: tdavis{at}bcm.tmc.edu.

Protein synthesis in skeletal muscle of adult rats increases in response to oral gavage of supraphysiological doses of leucine. However, the effect on protein synthesis of a physiological rise in plasma leucine has not been investigated in neonates, an anabolic population highly sensitive to amino acids and insulin. Therefore, in the current study, fasted pigs were infused intra-arterially with leucine (0, 200 or 400µmol.kg-1.h-1) and protein synthesis was measured after 60 or 120 min. Protein synthesis was increased in muscle, but not in liver, at 60 min. At 120 min, however, protein synthesis returned to baseline levels in muscle but was reduced below baseline values in liver. The increase in protein synthesis in muscle was associated with increased plasma leucine of 1.5- to 3-fold and no change in plasma insulin. Leucine infusion for 120 min reduced plasma essential amino acids levels. Phosphorylation of eukaryotic initiation factor (eIF) 4E binding protein-1 (4E-BP1), ribosomal protein (rp) S6 kinase, and rpS6 were increased, and the amount of eIF4E associated with its repressor, 4E-BP1, was reduced after 60 and 120 min of leucine infusion. No change in these biomarkers of mRNA translation was observed in liver. Thus, a physiological increase in plasma leucine stimulates protein synthesis in skeletal muscle of neonatal pigs in association with increased eIF4E availability for eIF4F assembly. This response appears to be insulin-independent, substrate-dependent, and tissue-specific. The results suggest that the branched-chain amino acid, leucine, can act as a nutrient signal to stimulate protein synthesis in skeletal muscle of neonates.




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