Metabolism of propionate in the tissues of the sheep gut

1. The extent of propionate metabolism during absorption from the gut and the amounts of L-lactate formed and glucose utilized by the portal-drained viscera were determined in conscious sheep from measurements of portal venous blood flow and portal venous and aortic metabolite concentrations. The sheep were fasted overnight and given primed continuous intraruminal infusions of volatile fatty acids (VFA) at two rates, supplying propionate at 40.0 and 79.9 mmol/h. Measurements were made during the 5th and 6th hours of the infusion, when rumen liquor VFA concentrations were constant. 2. The rate of L-lactate formation by the portal-drained viscera was not affected by the VFA infusions and accounted for approximately 15% of the probably total lactate entry rate. 3. Considerable amounts of glucose were taken up by the portal-drained viscera, amounting to approximately 35% of the probable glucose entry rate. If this glucose was metabolized through the glycolytic pathway, this would at all times have accounted for the amounts of L-lactate formed. 4. Portal venous blood flow was positively correlated with VFA infusion rates and with the net amount of propionate appearing in the portal blood. 5. It is concluded that although propionate may be metabolized by the rumen epithelium, the unique pathway of L-lactate formation from propionate is of limited quantitative significance to the animal, although it may be of importance to the rumen epithelium itself.