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1 Department of Pediatrics, United States Department of Agriculture/Agricultural Research Service, Children's Nutrition Research Center and Section of Neonatology, Baylor College of Medicine, Houston, Texas, USA; Department of Cellular and Molecular Physiology, Pennsylvania State University College of Medicine, Hershey, Pennsylvania, USA
* To whom correspondence should be addressed. E-mail: tdavis{at}bcm.tmc.edu.
The high efficiency of protein deposition during the neonatal period is driven by high rates of protein synthesis, which are maximally stimulated after feeding. Infusion of amino acids, but not insulin, reproduces the feeding-induced stimulation of liver protein synthesis. To determine whether amino acid-stimulated liver protein synthesis is independent of insulin in neonates, and to examine the role of amino acids and insulin in the regulation of translation initiation in neonatal liver, we performed pancreatic-glucose-amino acid clamps in overnight fasted 7-day-old pigs. Pigs (n=9-12/group) were infused with insulin at 0, 10, 22, and 110 ng.kg-0.66.min-1 to achieve 0, 2, 6, and 30 µU/ml insulin, respectively. At each insulin dose, amino acids were maintained at fasting or fed levels, or in conjunction with the highest insulin dose, allowed to fall to below fasting levels. Insulin had no effect on the fractional rate of protein synthesis in liver. Amino acids increased fractional protein synthesis rates in liver at each dose of insulin, including the 0 µU/ml dose. There was a dose response effect of amino acids on liver protein synthesis. Amino acids and insulin increased S6K1 and 4E-BP1 phosphorylation, however, only amino acids decreased formation of the inactive 4E-BPI.eIF4E complex. The results suggest that amino acids regulate liver protein synthesis in the neonate by modulating the availability of eIF4E for 48S ribosomal complex formation and this response does not require insulin.
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