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Absorption, Plasma Flux and Oxidation of L-Leucine in Heifers at Two Levels of Intake¹

U.S. Department of Agriculture⁵ Beltsville, MD 20705 and Cornell University^,6, Ithaca, NY 14853

Abstract

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.

¹ Mention of companies or products is for the benefit of readers and does not constitute an endorsement or preferential treatment by the authors or the USDA.

² Present address: USDA, ARS, Subtropical Agr. Res. Sta., Brooksville, FL 34298.

³ Present address: USDA, ARS, Roman L. Hruska U.S. Meat Anim. Res. Center, Clay Center, NE 68933.

⁴ The authors thank Dennis Hucht, Maureen Pello, Banner Phillips and Ed Williams for technical assistance; the Energy Metabolism Unit and Animal Operations for help with care and feeding of experimental animals; and Linda Young for word processing.

⁵ ARS, Ruminant Nutr. Lab.

⁶ Dept. of Anim. Sci.