Temporal effects of ruminal infusion of propionic acid on hepatic metabolism in cows in the postpartum period

摘要

A faster rate of infusion of propionic acid into therumen of cows in the postpartum period increased mealsize compared with a slower rate of infusion in a previousexperiment. Because propionate is anaplerotic andstimulates oxidation of acetyl coenzyme A (CoA) in theliver, and hepatic oxidation has been linked to satiety,this result was opposite to our expected response. Wethen hypothesized that the faster rate of infusion mighthave saturated the pathway for propionate metabolismin hepatocytes resulting in lower first-pass extraction bythe liver. Because we were measuring feeding behavior,we could not sample blood and liver tissue over time inthat experiment. Therefore, to determine the temporaleffects of propionic acid (PA) infusion on hepatic metabolismand plasma metabolites over the time courseof a meal, we infused 1.25 mol of PA (2.5 L of 0.5M PA)over 5 min (FST) or 15 min (SLW) into the rumen. Weevaluated response to PA infusions both before feeding,when ruminal PA production by rumen microbes islower and hepatic acetyl CoA concentration is greater,and 4 h after feeding, when PA production is greaterand hepatic acetyl CoA concentration is lower. Bloodand liver samples were collected before, and after 5,15, and 30 min of infusion. Contrary to our hypothesis,the rate of PA infusion into the rumen did not affectplasma propionate concentration, indicating the FSTeffects on feeding behavior were not because of a limitationon propionate uptake by the liver. However, FSTincreased plasma glucose and insulin concentrationsfaster than SLW, resulting in a reduction in plasmanonesterified fatty acid concentration during the timeframe of meals. Decreased plasma nonesterified fattyacid concentration during infusion likely decreased thesupply of acetyl CoA for oxidation in the liver. TheFST treatment also increased fumarate concentrationat 5 min after the initiation of infusion but did notaffect oxaloacetate concentration compared with SLW,consistent with a limitation to propionate metabolismat that reaction. A metabolic bottleneck at the malatedehydrogenase reaction for FST compared with SLWwould further contribute to a reduction in hepaticoxidation within the time frame of a meal, allowinggreater meal size, consistent with the hepatic oxidationtheory and our previous results.