Splanchnic Extraction of Phenylalanine in Adult Thoroughbred Mares Fed Two Different Levels of Threonine

Abstract Text: Previous studies in horses examining the effects of amino acid (AA) intake on whole-body protein synthesis have used intravenous phenylalanine isotope infusion and the resulting calculations required estimates of splanchnic phenylalanine extraction that were derived from other species. Threonine is believed to be the second limiting AA in some equine diets. The objectives of the study were to determine splanchnic extraction of phenylalanine, validate an oral infusion of [1-¹³C]phenylalanine, and test the effects of threonine supplementation on whole-body protein synthesis in horses. Six Thoroughbred mares were fed timothy hay and a low threonine concentrate supplemented with isonitrogenous amounts of either threonine (+THR) or glutamate (+GLU), which were top dressed on the concentrate portion of the diet. Threonine intakes were 119 (+THR) and 58 mg/kg/d (+GLU) and diets exceeded NRC recommendations for all nutrients, including threonine (33 mg/kg/d). Each horse received each diet twice for 7d; studied once with an oral infusion and once with an intravenous infusion of [1-¹³C]phenylalanine, for a total of 4 study periods per horse. On d6 of receiving each diet, blood samples were taken before and 90 min after the AA supplemented concentrate meal.  The next day, a 2h primed, constant intravenous infusion of [¹³C]sodium bicarbonate and a 4h primed, constant infusion of [1-¹³C]phenylalanine, either oral or intravenous, were used with blood and breath sampling to measure blood [¹³C]phenylalanine and breath ¹³CO₂ enrichment. Data were analyzed in the mixed procedure of SAS with threonine intake, route of infusion and the interaction as the fixed effects.  Baseline and post-feeding plasma concentrations of glutamate, serine, glycine, threonine, and methionine were affected by diet (P<0.05). Phenylalanine flux, intake, from protein breakdown, oxidation, and non-oxidative disposal were not affected by diet (P<0.05). Splanchnic extraction was 26±5% and 27±3% for the +THR and +GLU diets, respectively. This was the first study to use oral administration of [1-¹³C]phenylalanine in horses and this technique offers a less invasive alternative to the intravenous infusion method. Threonine does not appear to be a limiting AA in our diet, as phenylalanine kinetics were not affected by supplementation; however differences in plasma AA concentrations in response to threonine supplementation suggest that this AA can affect the metabolism of other AA. This project was supported by Agriculture and Food Research Initiative Competitive Grant no. 2012-67015-19448 from the USDA National Institute of Food and Agriculture.

Keywords: Splanchnic extraction, equine, amino acids