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Nutritional Methodology

Fractional Protein Synthesis Rates Measured by an Intraperitoneal Injection of a Flooding Dose of L-[ring-²H₅]Phenylalanine in Pigs^1,2

Kristjan Bregendahl^*,3, Lijuan Liu^*, John P. Cant^*, Henry S. Bayley, Brian W. McBride^*, Larry P. Milligan^*, Jong-Tseng Yen^** and Ming Z. Fan^*,4

Departments of ^* Animal and Poultry Science and Human Biology and Nutritional Sciences, University of Guelph, Guelph, ON, Canada N1G 2W1 and ^** U.S. Department of Agriculture/Agricultural Research Service, The United States Meat Animal Research Center, Clay Center, NE 68933

⁴To whom correspondence should be addressed. E-mail: mfan{at}uoguelph.ca.

Our objectives were to examine the effect of an i.p. injection of a flooding dose of L-phenylalanine (Phe) containing L-[ring-²H₅]Phe on time courses of physiologic responses, the tracer Phe enrichments, and fractional protein synthesis rates (FSR) in plasma, visceral organs, and muscles. In a randomized complete block design, 5 blocks of 5 littermate piglets were weaned at 16 d of age and injected i.p. with a flooding dose of L-Phe (1.5 mmol/kg body weight) on d 8 postweaning under fed conditions. Tissues were collected at 15, 30, 45, 60, and 75 min postinjection. Plasma glucose concentration increased (cubic effect, P < 0.05) from 4.8 preinjection to 5.8 mmol/L 15 min postinjection and returned to preinjection levels thereafter. Plasma insulin concentration did not change (P > 0.05) over time. Plasma Phe concentration increased logarithmically (P < 0.05) from 85 to 711 µmol/L and reached 95% of the maximum concentration 48 min postinjection, but no changes (P > 0.05) in tissue contents of other free amino acids were observed. The Phe free pools in plasma, visceral organs, and muscles were evenly enriched (32.3 ± 1.4 mol%) with L-[²H₅]Phe 15 min after the i.p. injection. The FSR in visceral organs did not change (P > 0.05), whereas plasma and muscle protein FSR decreased (P < 0.05) over time. We conclude that the i.p. injected tracer Phe rapidly distributed into plasma and intra- and extracellular spaces, and was effective for measuring FSR in visceral organs, but not in plasma and muscles of pigs.

KEY WORDS: • route of tracer delivery • stable isotopes • isotopic enrichment • piglets

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