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.

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(Huntington, 1984).No variation in intake, milk yield and composition was observed during the infusion of propionate. It resulted in an increase (P < 0.05) in arterial concentration of proprionate (+ 25%) and an astonishing slight decrease (P < 0.1 ) in glucose production (-11%) by the splanchnic tissues. This loss in carbon source for the peripheral tissues is partially filled (55%) by the greater release of lactate, alanine and other sources to these tissues. Part of the decrease in urea production could be explained by the lesser amino acid desamination (hepatic alanine/urea = -60%). Glucose and urea metabolisms are energy users; the decrease of these activities in the liver could account for the overall decrease in splanchnic oxygen consumption.