Nitrogen metabolism of the ruminant liver

Abstract

This review examines both the quantitative flux of nitrogenous compounds from the portaldrained viscera (PDV) to the liver and the metabolic pathways within these tissues that facilitate interactions between these compounds. In order to estimate the flow of material between organs, it is necessary to be able to measure the rate of blood flow perfusing the organ under investigation. Methods of estimating blood flow are discussed. In general, splanchnic blood flow (c. 125 mL min-1kg-0.75 at maintenance feeding) is proportional to the intake of energy. Although the splanchnic bed only constitutes 7-13% of total body mass, it uses 4040% of total oxygen demand. Dietary protein (60-90%) and non-protein nitrogen (c. 100%) are both degraded to ammonia in the rumen, which is incorporated into microbial protein (50-70% derived from ammonia). The composition and amount of microbial protein is relatively constant and independent of the intake and composition of feed protein. Essential amino acids may be supplied to the animal via the undegraded feed protein fraction passing to the small intestine. Urea may be recycled to the rumen, in proportion to the plasma concentration, via saliva or directly by diffusion through the rumen wall in a ratio that lies between 2:l and 5:l. Any production of urea that raises its plasma concentration above 112 mg NL- may be taken up by the postruminal tract. A large amount of ammonia (0.04 to 0.25 g N day-1kg-0.75 proportional to nitrogen intake (from 0.45 to 2.65 g N day-1kg-0.75) and between 0.4 and 6.5 times higher than all the �-amino linked acids taken up by the PDV, may be absorbed from the rumen. As much as 1.8 to 2.3 g N day-1kg-0.75 for an intake of 1 g N day-1kg-0.75 may flow from the abomasum to the small intestine. Of this total nitrogen, between 0.60 and 1.36 g N day-1kg-0.75 represents �-linked amino acids. The increase above basal values largely represents unfermented feed protein. Amino acids and small peptides are absorbed mainly from mid to lower ileum. Most amino acids appear to be at least partially metabolized in passage through the gut wall (between about 56 and 68% in sheep fed a low or a high protein diet respectively). In particular, glutamine contributes at least as much as glucose to the energy metabolism of the splanchnic bed. Although relatively large amounts of-urea enter the small intestine (in proportion to blood urea concentration), a lack of significant urease activity suggests that most is reabsorbed. Between 0.6 and 0.9 g N day-1kg-0.75 (40 to 60% as amino acids, 15% as urea and 1 to 13% as ammonia) enters the caecum. Another 0.15 g N day-1kg-0.75 of urea may enter from the blood. Increasing the energy supply to hindgut fermentation will increase faecal excretion of nitrogen, while decreasing the concentration of urinary urea. Nitrogen is absorbed in net amounts, mainly as ammonia, from the hindgut (0.04 to 0.16 g N day-1kg-0.75) Net flux of ammonia, urea and the a-linked amino acids (0.47 to 0.59; -0.39 to -1.12 and 0.26 to 0.44 g N day-1kg-0.75 respectively) from this region appears to be proportional to nitrogen intake (0.74 to 2.90 g N day-1kg-0.75) The flux of amino acids in particular seemed to increase only at nitrogen intakes above 2 g N day-1kg-0.75 Glucogenic amino acids are taken up in net amounts from the PDV (e.g. 3 to 103 mg N day-1kg-0.75 for alanine), as are essential amino acids, albeit in smaller amounts (-2 to 62 mg N day-1kg-0.75 for leucine). Glutamine is utilized by the PDV in most studies (-45 to -230 mg N day-1kg-0.75 Net uptake of ammonia by the liver almost exactly balances the amount produced by the PDV. The proportion of the net flux of amino acids from the PDV that is taken up by the liver varies from about 20% to more than l00%, the amount retained being proportional to the nitrogen intake of the animal. Of the individual amino acids, large amounts of the glucogenic acids are removed (e.g. 50 to 200 mg N day-1kg-0.75 for alanine). Arginine may also be removed in similar amounts, at least some of which, like the glucogenic acids, must come from peripheral tissues. Conflicting results have been reported for hepatic flux of glutamine. The role of gluconeogenesis, transdeamination and the urea cycle is discussed in relation to the function of the liver as an organ integrating the nitrogen and energy metabolism of the animal. Possible mechanisms, other than the accepted hypothesis of urea recycling, are proposed to explain nitrogen recycling in ruminants. It is clear that animal scientists must take into account, together with the fermentative processes in the rumen and hindgut, the contribution of the host animal to the overall balance of nitrogen.