Absorption, plasma flux and oxidation of L-leucine in heifers at two levels of intake.

Four Hereford X Angus heifers (379 +/- 11 kg) with indwelling hepatic portal vein, iliac artery and mesenteric vein catheters were used in a switchback design to measure net portal absorption of leucine at two levels of energy intake, 130 and 260 kcal metabolizable energy.(kg.75.d)-1. Nine hourly measurements were made through two consecutive 4-h feeding periods on each heifer at each intake level. In separate experiments, N balance was determined and 6-h continuous infusions of L-[1-14C]-leucine were used to measure total plasma flux and oxidation of leucine with two of these heifers, one at each level of intake, and one additional Hereford heifer at both levels of intake. Net portal absorption of leucine increased with increasing intake from 5.4 to 12.4 mmol/h, and was associated with average leucine flux and oxidation rates of 30.7 and 4.7 mmol/h at low intake and 49.9 and 8.8 mmol/h at high intake. Protein synthesis (flux - oxidation) and protein accretion (N balance X 6.25) averaged 1,367 and 39 g/d at low intake and 2,156 and 219 g/d at high intake, respectively. Percent contributions of absorption and oxidation to total leucine flux were 17.6 and 15.3 at low intake and were 24.9 and 17.6 at high intake, respectively.