Some observations on metabolite production and utilizationin vivoby the gut and liver of adult dairy cows

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Abstract
SUMMARY: Metabolite production rates were determined in the portal vein and hepatic veins of catheterized adult dairy cows maintained under normal conditions of husbandry. The production rates were calculated from the metabolite concentrations in arterial, portal and hepatic-venous blood, and from the rates of blood flow in the portal vein and the hepatic veins. In general, qualitatively similar metabolite production was observed in three non-lactating and two lactating cows, but production rates tended to be higher in the lactating cows due to higher blood flow rates.About half the lactate utilized by the liver was absorbed from the gut, while the other half was derived from endogenous sources. Lactate absorbed from the gut was quantitatively of less significance than propionate as a substrate for hepatic metabolism. Even in cows that were well fed, there was a net production of ketone bodies from the liver that was almost as great as the net production from the gut. However, while the liver produced D-3-hydroxybutyrate it took up acetoacetate. In the lactating cows, acetate, propionate and butyrate were absorbed from the gut in the proportions of 9·5:2·5:1, respectively. About 90% and 80% of the absorbed propionate and butyrate, respectively, were taken up by the liver.On the routine hay/concentrate diet there was little net uptake or output of glucose by the gut. However, there was consistent production of glucose by the liver, amounting to a maximum, in this study, of about 11 mol/24 h, assuming constant glucose production throughout the 24 h day. The rate of carbon dioxide appearance in the portal vein was about twice that of oxygen uptake by the gut. The liver used oxygen, and produced carbon dioxide, at nearly equal rates. The uptake of glucogenic substrates by the liver accounted adequately for the observed rates of hepatic gluconeogenesis.In one of the lactating cows it was observed that there was a net production of acetate by the liver that amounted to half the net absorption of acetate from the gut (ethyl alcohol was not the precursor of this acetate) and that (glutamine + asparagine) and serine had the highest rates of appearance in the portal vein and the highest rates of uptake by the liver.