TRANSFERS OF N-METABOLITES ACROSS THE OVINE LIVER IN RESPONSE TO SHORT-TERM INFUSIONS OF AN AMINO-ACID MIXTURE INTO THE MESENTERIC VEIN

The effect of acute (4.5 h) infusions into the mesenteric vein of an a mino acid (AA) mixture, which simulated the composition of rumen micro bial protein, on net transfers of NH3, urea and total AA across the po rtal-drained viscera (PDV) and liver in the ovine has been examined. F our wether sheep were surgically prepared with vascular catheters acro ss the PDV and liver (Lobley et al. 1995) and were offered a basal die t of 1000 g grass pellets/d (approximately 1.4 x energy maintenance). Each animal was infused at weekly intervals with one of four dilutions of the AA mixture. These dilutions provided 0.44, 0.88, 1.32 and 1.84 mmol AA-N/min infused, the lowest of which approximately doubled the net absorption of AA-N from the basal diet. Animals were treated with heparin to allow continuous collection of blood by peristaltic pump fo r 2 h preceding, and between 0.5-2.5 and 2.5-4.5 h after, the start of the AA infusions. Blood flow in the hepatic artery increased (100 v. 208 g/min; P = 0.002) in response to AA infusion, while hepatic portal venous flow decreased (2090 v. 1854 g/min; P = 0.006). The AA infusio n also stimulated O-2 uptake by the PDV (P < 0.001) and liver (P = 0.0 16). Absorption across the PDV and hepatic removal of NH3 were unchang ed between basal and amino acid infusion conditions. Urea-N removal ac ross the PDV was unaltered, but hepatic production increased (P < 0.00 1) with level of AA infusion. During infusions, net appearance of AA a cross the PDV was below the theoretical level. This may have been due to inhibition of AA uptake from the small intestine, and/or increased removal by the digestive tract of AA from the systemic circulation ass ociated with greater arterial concentrations. Hepatic extraction of AA increased with level of infusion, both for total AA and those include d in the infusate. Total hepatic urea-N production tended towards a ma ximum (estimated as 2 mu mol N/g liver wet weight per min). The AA rem oved by the liver and not used for ureagenesis remained similar (170 m u mol AA-N/min) between basal and AA infusions. This was presumed avai lable for anabolic purposes (mainly synthesis of export proteins). The proportion of net AA-N appearance (absorption plus infused) across th e PDV removed by the liver declined from 0.71 to 0.53 between basal an d AA infusions. In contrast to findings from cattle (Wray-Cahen et al. 1997), increased AA infusion did not alter the net removal of glutami ne across the liver. This may reflect differences between the studies in NH3:AA-N absorbed. Further differences between the cattle study and the current findings may relate to the different physiological state (pregnancy v. growth), which may alter the partition of AA between ana bolic and catabolic fates.