were determined using 2 h primed (7.1 μmol/kg BW), constant (6 μmol/kg BW/h)
infusions of [13C]sodium bicarbonate, to measure whole-body CO2 production (Coggan
et al., 1993; Moehn et al., 2004a), followed immediately by 4 h primed (13 μmol/kg
BW), constant infusions of [1-13C]phenylalanine (9 μmol/kg BW/h), to measure phenylalanine
oxidation to CO2 and phenylalanine flux. Validation was previously
performed in mature horses and resulted in a rapid rise to plateau isotopic enrichment
in the plasma phenylalanine (Urschel et al., 2012). The horses wore cordless
infusion pumps (J-1097 VetPro Infusion Pump, Jorgensen Laboratories) that were
attached to surcingles to infuse the isotope and allow the horses to move freely in
their stalls.