Splanchnic extraction of phenylalanine in mature mares was not affected by threonine supplementation

Abstract

This study determined splanchnic extraction of phenylalanine at two intakes of threonine. Six Thoroughbred mares were supplemented with isonitrogenous amounts of either threonine or glutamate. Dietary threonine intakes were 119 (+Thr) and 58 (Basal) mg/kg/day, respectively. Each horse received each diet twice and each was studied once with an oral and once with an intravenous (IV) infusion of [1-(13)C]phenylalanine. A 2-h primed, constant IV infusion of [(13)C]sodium bicarbonate and a 4-h primed, constant infusion of [1-(13)C]phenylalanine, either orally or IV, were used to measure isotopic enrichments. Phenylalanine kinetics were not affected by diet (P > 0.05). Values for the splanchnic extraction of phenylalanine were 26 ± 5% and 27 ± 3% for the +Thr and Basal supplemented diets, respectively. These values will improve the accuracy of future equine indicator amino acid oxidation studies.

Using the indicator amino acid oxidation technique to study threonine requirements in horses receiving a predominantly forage diet

“…However, in this previous study, horses were fed less forage (1.2% BW) and a greater ratio of concentrate (0.8% BW) in their diets compared to the horses used in the present study (fed 1.6% BW of forage and 0.4% BW of concentrate). Therefore, more dietary threonine was expected to be digested precaecally in the previous study (Mastellar, Barnes et al., ), and we hypothesized that the horses in the current study would have a higher threonine requirement than horses in the previous study (Mastellar, Barnes et al., ). By bad luck, the results from the current study do not help to elucidate whether the threonine requirement is influenced by the level of forage intake in mature horses.…”

“…Breath samples were then collected every 30 min during the entirety of both isotope administration procedures and blood samples were collected every 30 min during the [1‐ 13 C]phenylalanine administration only. The prime to constant infusion rates for both isotopes (Urschel et al., ) and the oral administration of isotope phenylalanine (Mastellar, Barnes, Cybulak, & Urschel, ) were previously validated to be used in protein metabolism studies in horses.…”

Effects of dietary amino acid supplementation on measures of whole‐body and muscle protein metabolism in aged horses

“…On the morning of day 15, a jugular vein catheter was placed, and stable isotope administration procedures were conducted as previously described (Mastellar, Barnes, Cybulak, & Urschel, ; Tanner et al, ). Briefly, whole‐body phenylalanine kinetics were determined by a 2‐hr primed (5.3 μmol/kg) constant (4.4 μmol kg ‐1 hr −1 ) infusion of [ 13 C]sodium bicarbonate to measure total CO 2 production, immediately followed by 4‐hr primed (10.3 μmol/kg) constant (7.2 μmol kg −1 hr −1 ) oral administration of [1‐ 13 C]phenylalanine to measure phenylalanine oxidation to CO 2 and phenylalanine flux.…”

“…Prime and constant oral doses of [1‐ 13 C]phenylalanine administration were previously validated (Mastellar, Barnes, et al, ). The prime dose of [1‐ 13 C]phenylalanine was included with the first constant dose at the beginning of [1‐ 13 C]phenylalanine administration.…”

“…Plasma samples were analysed by Metabolic Solutions Inc. (Nashua, NH) for [ 13 C]phenylalanine enrichment, as previously described (Wagner, Urschel, Betancourt, Adams, & Horohov, ). Plasma (for phenylalanine administration) and breath (for phenylalanine and bicarbonate administration) isotopic enrichment plateaus were used to calculate the various phenylalanine kinetics parameters, as previously described (Mastellar, Barnes, et al, ; Tanner et al, ).…”