Four 40 kg wethers were used in a crossover design to quantify, by arterio -venous procedures, the mass transfer of NH 3 , urea and amino acids (AAs) across the portal-drained viscera and the liver during a 31 min infusion of either 0 (C0) or 1100 (C1100) mmol NH 4 HCO 3 /min into the mesenteric vein. In C1100, hepatic NH 3 extraction remained stable at 1214 mmol/min (1 : 90 mmol/min per g wet liver weight), the capacity for hepatic NH 3 removal was exceeded by 654 mmol/min ðP , 0 : 05Þ and the

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... incremental (C1100-C0) urea-N release: NH 3 -N removal ratio increased progressively, from 0 : 52 to 0 : 90. The NH 4 HCO 3 infusion reduced total branchedchain AA ðP , 0 : 05Þ transfer across the portal-drained viscera and total AA-N ðP ¼ 0 : 09Þ and lysine ðP ¼ 0 : 02Þ extraction by the liver. Hepatic release of glutamate was augmented ðP ¼ 0 : 03Þ; ornithine switched from net release to net removal ðP , 0 : 001Þ and net splanchnic release of free essential AA (44 mmol/min (SED 9 : 2), P ¼ 0 : 04) and branched-chain AA (33 mmol/min (SED 2 : 0), P ¼ 0 : 001) were reduced to 0 : 58 of their basal rate. The study showed that conversion of excess NH 3 to urea during a short-term hepatic NH 3 overload required no additional contribution of AA-N to ureagenesis; essential AA and branched-chain AA supply to non-splanchnic tissues was, however, temporarily decreased.