Volatile fatty acid uptake and propionate metabolism in ruminant hepatocytes

Previous reports have demonstrated that butyrate inhibits metabolism of propionate by liver cells isolated from sheep and goats. Our objectives were to examine some possible mechanisms for this inhibition and to test for this inhibition in the bovine animal. Incorporation of label from 2.5 mM [2-(14)C]propionate into glucose (nmol propionate/mg cell DM/h) in the presence of 0, 1.25, and 2.5 mM butyrate was 107, 66, and 62 by goat hepatocytes and 79, 25, and 29 by calf hepatocytes; therefore, butyrate inhibited propionate metabolism at least as effectively in calves as in goats. In goat hepatocytes 1.25 mM butyrate reduced 1.25 mM propionate uptake to 46% of control, and 1.25 mM [2-(14)C] propionate incorporation into glucose to 44% of control. Propionate had no effect on butyrate uptake. Isovalerate and valerate tended to be cleared from the media to a greater extent than butyrate but had no effect on propionate uptake. Therefore, inhibition of propionate conversion to glucose by butyrate is specific and is not due to a general competition among VFA for metabolism. Butyrate inhibits hepatic propionate utilization generally, not specifically propionate conversion to glucose. Butyrate also inhibited propionate utilization by goat liver homogenates, indicating that butyrate inhibits propionate metabolism at a step subsequent to propionate transport across the hepatocyte plasma membrane.