Net portal appearance of volatile fatty acids in sheep intraruminally infused with mixtures of acetate, propionate, isobutyrate, butyrate, and valerate

The net portal appearance of volatile fatty acids (VFA) was investigated in four ruminally fistulated and multicatheterized sheep. During the experime nts, the sheep were fed once every hour for 14 h and intraruminally infused with mixtures of VFA for the 12 h commencing 2 h after the initiation of t he hourly feeding protocol. Paired arterial and portal blood samples were o btained hourly during the last 6 h of the experiments. In the control treat ment (1), only water was infused intraruminally. In Treatments 2 through 4, the intraruminal infusion rates of propionate (40 mmol/h), isobutyrate (5 mmol/h), and valerate (5 mmol/ h) were unchanged. In Treatments 2, 3, and 4 , the acetate infusion rate was 100, 60, and 20 mmol/h, respectively, and t he butyrate infusion rate was 10, 30, and 50 mmol/h, respectively. Thus, th e infusion rate of VFA carbon was constant across Treatments 2 through 4. P ortal recovery estimated from the increased net portal appearance in Treatm ents 2 through 4 compared to the control treatment was 85% for propionate a nd 60% for isobutyrate, and these recoveries were unaffected by treatment. The portal recovery of butyrate increased (from 21 to 32%) with increasing infusion rate of butyrate and decreasing infusion rate of acetate, as did t he portal recovery of valerate (from 14 to 31%). The portal recovery of ace tate was 55%, when measured as net portal appearance. Thus, it seems that t he capacity for beta-oxidation in ruminal epithelium is limited, which woul d explain the increasing portal recovery of butyrate and valerate with incr easing infusion rate of butyrate, when infusion rate of VFA carbon is uncha nged.