|Baldwin, Ransom - Randy|
Submitted to: Journal of Nutrition
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/6/2009
Publication Date: 3/12/2009
Citation: El-Dadi, S.W., Baldwin, R.L., Mcleod, K.R., Sunny, N.E., Bequette, B.J. 2009. Glutamate is the major anaplerotic substrate in the tricarboxylic acid cycle of isolated rumen epithelial and duodenal mucosal cells from beef cattle. Journal of Nutrition. 139(5):869-875.
Interpretive Summary: The metabolism of nutrients by the epithelial lining of the digestive tract in ruminants plays an important role in the net efficiency of nutrient use by the whole animal. However, while changes in diet are known to change nutrient use efficiencies at the whole animal and gastrointestinal tissue levels, little is understood regarding the control of metabolic changes in these tissues. We have new technologies which allow for the monitoring of a multitude of metabolic outcomes for individual substrates, including glucose and amino acids. This technology was used in conjunction with isolated cell culture techniques to assess differences in nutrient use that occur when high efficiency (concentrate based) or low efficiency (forage based) diets are provided to growing bulls. Using this approach critical control points in metabolism can be identified and a molecular basis for the changes can begin to be identified. We evaluated glucose, glutamate, glutamine, leucine or valine use in a complex basal media. Glucose metabolism in rumen epithelial cells isolated from high concentrate fed bulls was greater than for bulls fed high forage diets. Although glutamate’s contribution to cell energy generation was significant, it was apparent that other substrates available in the media also contributed to the maintenance of cell energy metabolism. Diet composition alters glucose, glutamate and leucine catabolism by the isolated cells. The use of glucose for energy in the cell may play a role in providing precursor substrates to other important metabolic pathways. In ruminant digestive tract epithelia, catabolism of glucose, primarily to lactate, serves a role in preserving glucose carbon skeletons from further catabolism and thereby allows for re-synthesis of glucose by the liver. Understanding the complex control of the digestion and absorption process is necessary to better adapt dietary regimes for improved nutrient use efficiency in production rations and is fundamental to developing ideal phenotypes for future selection of animals with higher nutrient use efficiencies and intervention strategies.
Technical Abstract: This study aimed to determine the contribution of substrates to tricarboxylic acid (TCA) cycle fluxes in rumen epithelial (REC) and duodenal mucosal (DMC) cells isolated from bulls (n = 6) fed either a 75% forage (HF) or 75% concentrate (HC) diet. In separate incubations, [13C6]glucose, [13C5]glutamate, [13C5]glutamine, [13C6]leucine or [13C5]valine were added in increasing concentrations to basal media containing SCFA and a complete mixture of amino acids. Lactate, pyruvate and TCA cycle intermediates were analyzed by gas chromatography-mass spectrometry followed by 13C-mass isotopomer distribution analysis. Glucose metabolism accounted for 8 to 15% of lactate flux in REC from HF fed bulls compared with 21 to 30% in REC from HC and in DMC from bulls fed both diets (P < 0.05). For both cell types, an increasing proportion (3 to 63%) of a-ketoglutarate flux derived from glutamate, whereas glutamine contributed <3% (P < 0.05). Although leucine and valine were catabolized to their respective keto-acids, these were not further metabolized to TCA cycle intermediates. Glucose, glutamine, leucine and valine catabolism by ruminant GIT cells has been previously demonstrated, but in this study, their catabolism via the TCA cycle was limited. Further, although glutamate’s contribution to TCA cycle fluxes was significant, it was apparent that other substrates available in the media also contributed to the maintenance of TCA fluxes. Lastly, the results suggest that diet composition alters glucose, glutamate and leucine catabolism by the TCA cycle of REC and DMC.