Portal recovery of intraruminally infused short-chain fatty acids in sheep

Three rumen fistulated and catheterized sheep were used to study portal recovery of continuously intraruminally infused short-chain fatty acids (SCFA). Each sheep received six treatments: 0, 60, 120, 180, 240 and 300 mmol h(-1) SCFA infused for 8 h in a semi-fasted state. The infused SCFA had a molar proportion of 65:20:15 (acetate:propionate:butyrate). [1-C-13]Na-acetate was infused intravenously during the experiment for measurement of portal drained viscera (PDV) metabolism and irreversible loss rate (ILR) of acetate. Portal blood flow was measured by ultrasonic flowprobes. Portal blood, arterial blood and rumen liquid were sampled during the last 4 h of the experiments. A model of individual metabolite fluxes in the portal drained viscera is presented. The absorption rate of acetate, measured as portal net appearance plus PDV metabolism, and irreversible loss rate of acetate showed a linear response to infusion rate and accounted for 54 +/- 8% and 73 +/- 9% of infused acetate, respectively. The B-L flux of arterial acetate, i.e. the flux from the blood to the lumen of the gastrointestinal tract, was estimated to be 5 +/- 1% of infused acetate, whereas the metabolism of arterial acetate in the PDV tissues increased with increasing infusion rate only at the lower infusion rates. Portal net appearance of propionate and D-3-hydroxybutyrate showed a linear response to infusion rate and accounted for 62 +/- 7% and 21 +/- 3% of infused propionate and butyrate, respectively. The portal net appearance of butyrate showed a quadratic response to infusion rate of butyrate. The average recovery of butyrate reached 25 +/- 2% at the highest infusion level. The implications of the results in relation to feed evaluation are discussed.