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Nutrient Metabolism

Whole-Body and Hindlimb Protein Breakdown Are Differentially Altered by Feeding in Neonatal Piglets¹

M. Carole Thivierge, Jill A. Bush^*, Agus Suryawan^*, Hanh V. Nguyen^*, Renan A. Orellana^*, Douglas G. Burrin^*, Farook Jahoor^* and Teresa A. Davis^*,2

^* U.S. Department of Agriculture/ARS Children’s Nutrition Research Center, Department of Pediatrics, Baylor College of Medicine, Houston, TX 77030 and Nutraceutical and Functional Food Institute/Département des sciences animales, Faculté des sciences de l’agriculture et de l’alimentation, Université Laval, Quebec, Canada, G1K 7P4

²To whom correspondence should be addressed. E-mail: tdavis{at}bcm.tmc.edu.

The high rate of muscle protein accretion in neonates is sustained by the marked increase in muscle protein synthesis in response to feeding. Little is known about the role of proteolysis in the regulation of protein accretion in response to feeding during the neonatal period. To determine the feeding-induced response of protein breakdown at the whole-body level and in the hindlimb of neonates, 10- and 28-d-old piglets that had been food deprived overnight were infused (7 h) with [1-¹³C]phenylalanine and [ring-²H₄]tyrosine during an initial food deprivation period (3 h), followed by a feeding period (4 h). During feeding, endogenous flux of phenylalanine decreased (P < 0.01) in both the whole body and the hindlimb. Feeding reduced (P < 0.01) whole-body proteolysis but increased hindlimb proteolysis (P = 0.04), suggesting that tissues other than the hindlimb are involved in the reduction in whole-body proteolysis during feeding. Overnight food deprivation resulted in a net mobilization of phenylalanine from whole-body proteins (P < 0.01) but not hindlimb proteins. These responses were unaffected by age. The results suggest that the hindlimb requires a continuous supply of free amino acids to sustain the high rate of muscle protein turnover in neonates and that adaptive mechanisms provide free amino acids to sustain skeletal muscle protein accretion in early postnatal life when the amino acid supply is limited.

KEY WORDS: • phenylalanine hydroxylation • phenylalanine kinetics • neonate • skeletal muscle • stable isotopes

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