Influence of Vitamin B-12 Status on Hepatic Propionic Acid Uptake in Sheep

Abstract

Pregnant ewes were fed a depletion diet low in cobalt (0.06 ppm) for 3½ months: Chronic catheters were implanted 8 weeks postpartum and 7 experiments were performed on these nonlactating vitamin B-12-depleted sheep (de-B12: 340 ± 30 ng vitamin B-12 per gram wet liver) prior to repletion by intramuscular injection of hydroxocobalamin. Six experiments were then repeated after vitamin B-12 repletion (re-B12: 2220 ± 50 ng vitamin B-12 per gram wet liver). The hepatic extraction ratios (HER) in continuously fed sheep were 0.81 and 0.77 for de-B12 and re-B12 corresponding to net hepatic uptakes of 460 ± 50 and 440 ± 40 µmol propionate per minute, respectively. Continuous infusion of unlabeled propionate into a mesenteric vein at 1 mmol/minute reduced the HER, yet this depression was greatest for re-B12 (0.74 vs. 0.63 for de-B12 and re-B12, respectively). Net hepatic uptake of propionate was increased (1145 ± 100 vs. 985 ± 95 µmol/minute, respectively), although vitamin B-12 status was without effect. It is concluded that the ability of liver to extract propionate is not affected at vitamin B-12 concentrations greater than 250 ng/g wet liver. However, when propionate entry rate was enhanced by intramesenteric infusion, the livers of de-B12 sheep had a greater capacity to remove propionate suggesting that alternate routes of metabolism may occur.