| Popis: |
The objective of the present study was to quantify, in vivo, the effects of portal NH3 supply, feeding frequency and chronic metabolic acidosis on urea synthesis and amino acid metabolism in the ovine liver. The work involved a combination of arteriovenous measurements across the splanchnic tissues and the use of 13C and 15N labelled compounds as tracers for amino acid and NH3 metabolism. In a first series of studies under frequent feeding conditions, portal NH3 supply was increased from 400 (basal) to either 600 or 900 μmol min-1 by long-term (4 d) or 2000 μmol min-1 by short-term (31 min) continuous infusion of NH4HCO3 into the mesenteric vein. All additional NH3 removed was converted by urea by the liver but there were no significant alterations in hepatic amino acid utilisation for ureagenesis. This indicates that hepatic ammonia detoxification can be accomplished without compromising amino acid availability to non-splanchnic tissues. Pre and post-feeding hepatic N movements were assessed by reducing feeding frequency to once daily. Rates of hepatic NH3 removal and urea-N release were stimulated equally after feeding, with no apparent change in amino acid N contribution to ureagenesis. Individual post-feeding rates of free amino acid absorption by the gastrointestinal tract and extraction by the liver were not different from those observed during the pre-feeding period or under frequent feeding conditions. Splanchnic amino acid metabolism in ruminants appears to be independent of feeding frequency and associated absorptive events (e.g. post-prandial supply of energy substrates). |
