Hepatic detoxification of ammonia in the ovine liver: possible consequences for amino acid catabolism

Abstract

The effects of either low (25 pnollmin) or high (235 pnol/min) infusion of NH&l into the mesenteric vein for 5 d were determined on 0, consumption plus urea and amino acid transfers across the portaldrained viscera (PDV) and liver of young sheep. Kinetic transfers were followed by use of l5NH4C1 for 10 h on the fifth day with simultaneous infusion of [l-'3C]leucine to monitor amino acid oxidation.Neither PDV nor liver blood flow were affected by the additional NH, loading, although at the higher rate there was a trend for increased liver 0, consumption. N&-N extraction by the liver accounted for 64-70 % of urea-N synthesis and at the lower infusion rate the additional N required could be more than accounted for by hepatic removal of free amino acids. At the higher rate of NH, administration additional sources of N were apparently required to account fully for urea synthesis. Protein synthesis rates in the PDV and liver were unaffected by NH3 infusion but both whole-body (P < 0.05) and splanchnic tissue leucine oxidation were elevated at the higher rate of administration. Substantial synthesis of [15wglutamine occurred across the liver, particularly with the greater NH, supply, and enrichments exceeded considerably those of glutamate. The [15N]urea synthesized was predominantly as the single labelled, i.e. [14N15N], species. These various kinetic data are compatible with the action of ovine hepatic glutamate dehydrogenase (EC1.4.1.2) in periportal hepatocytes in the direction favouring glutamate deamhation. Glutamate synthesis and uptake is probably confined to the perivenous cells which do not synthesize urea. The implications of NH, detoxification to the energy and N metabolism of the ruminant are discussed.

Responses of hepatic blood flows to changes in intake in sheep: a meta-analysis

“…Isserty et al (1998) showed that the choice of the laboratory technique for PAH determination significantly influenced the calculated hepatic arterial/venous blood flows. In sheep, much lower hepatic arterial/venous blood flows (2% to 6%) were measured by Barnes et al (1986) using microspheres and by Lobley et al (1995) with PAH dilution (with the inclusion of a deacetylation step in the analysis). In the available dataset, different techniques were used for determination of PAH between Below Maintenance (where heating and acid deproteinisation was mostly used) and Above Maintenance (mainly dialysis method).…”

The partial replacement of soyabean meal and rapeseed meal with feed grade urea or a slow-release urea and its effect on the performance, metabolism and digestibility in dairy cows

“…Ammonia produced in the rumen that is in excess of microbial requirement may be absorbed across the ruminal epithelium and converted to urea in the liver (Lobley et al, 1995), with the liver having the ability to remove ammonia added to the portal blood up to a maximum of 182 mg/min before peripheral blood concentrations increase (Symonds et al, 1981). It can therefore be concluded that in all three dietary treatments evaluated in this study, ammonia uptake by the portal drained viscera was well within the ability of the liver to metabolise it to urea.…”

Colostrum production in ewes: a review of regulation mechanisms and of energy supply

“…When sheep are fed large quantities of a diet with a high percentage of RDP, as is the case for large lupin supplements, the concentration of ammonia in the circulation can reach toxic levels (Hungerford, 1990), especially if the intake of digestible carbohydrates is low (Hibbit, 1988). Although, lupins can supply large quantities of ME, unfortunately much of this energy is used for hepatic detoxification of the ammonia liberated in and absorbed from the rumen, particularly when large quantities of lupins are consumed each day (Lobley et al, 1995;Greaney et al, 1996).…”