Amino Acid Supplementation Does Not Alter Whole-Body Phenylalanine Kinetics in Arabian Geldings 3

Abstract

Stable isotope infusion methods have not been extensively used in horses to study protein metabolism. The objectives were to develop infusion and sampling methodologies for [1-13C] phenylalanine and apply these methods to determine whether the addition of supplemental amino acids to a control diet affected whole-body phenylalanine kinetics in mature horses. Arabian geldings were studied using a 6-h primed (9 μmol/kg), constant (6 μmol · kg-1 · h-1) i.v. infusion of ʟ-[1-13C] phenylalanine, with blood and breath sampled every 30 min, to measure whole-body phenylalanine kinetics in response to receiving the control diet (n = 12) or the control diet supplemented with equimolar amounts of glutamate (+Glu; 55 mg · kg-1 · d-1; n = 5), leucine (+Leu; 49 mg · kg-1 · d-1; n = 5), lysine (+Lys; 55 mg · kg-1 · d-1; n = 5), or phenylalanine (+Phe; 62 mg · kg-1 · d-1; n = 6). The plasma concentrations of the supplemented amino acid in horses receiving the +Leu, +Lys, and +Phe diets were 58, 53, and 36% greater, respectively, than for the control treatment (P< 0.05). Isotopic plateau was attained in blood [1-13C] phenylalanine and breath 13CO2 enrichments by 60 and 270 min, respectively. Phenylalanine flux (+20%) and oxidation (+110%) were greater (P < 0.05) in horses receiving the +Phe treatment than in those fed the control diet. There was no effect of treatment diet on nonoxidative phenylalanine disposal or phenylalanine release from protein breakdown. The developed methods are a valuable way to study protein metabolism and assess dietary amino acid adequacy in horses and will provide a useful tool for studying amino acid requirements in the future.