Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 1, 1999
Publication Date: N/A
Interpretive Summary: The rumen epithelium is the tissue which lines the largest of the four fore stomachs in the ruminant animal (cows and sheep). Rumen epithelial cells are important because they are responsible for the absorption of nutrients from the rumen into the blood. The rumen cells use the absorbed nutrients for their own energy needs. When ruminants are fed diets that are mostly forage, as opposed to mostly grain, the rumen epithelium uses more energy and when the amount of energy they are fed is increased they also use more energy. We tested to see if the increase in energy use by the rumen cells was due to changes in the metabolism of specific nutrients. Rumen cells were placed into media that contained different amounts of volatile fatty acids (nutrients that are produced in the rumen), glucose, and amino acids (which are present in the blood). We found that the metabolism of these substrates did not change when the lambs were fed different diets or different amounts of energy. We also showed that the cells can use different amounts of substrates depending on the amount in the media. We found that the rumen epithelium should use acetate, propionate and butyrate because the amount of substrate in the rumen is very high but, the amount of glucose, glutamine and glutamate uses should be much lower because they can not use them when only small amounts are present, as they are in the blood. These numbers are important for the development of future experiments because they will determine the appropriate amount of each nutrient to use to ask other questions. Also, because the amounts of metabolism did not change when the diet was changed we think that the increase in energy use is due to growth and not to specific changes in cell metabolism.
Technical Abstract: Twenty-eight lambs were used to assess the effect of altered forage-to- concentrate ratio and metabolizable energy (ME) intake on the metabolism of substrates by rumen epithelial cells (REC). Lambs were assigned to treatments consisting of diets containing either 75% orchard grass (OGD) or 75% concentrate (CD) fed at either .099 (LE) or .181 (HE) Mcal MEù(kg BW.75)-1ùd-1 for 52 d. Lambs were slaughtered and REC were isolated (IREC) and incubated with radiolabeled substrates at concentrations ranging from .1 to 50 mmol/L and oxidation to carbon dioxide, or metabolism to beta- hydroxybutyrate (BHBA), acetoacetate (AcAc), pyruvate and lactate was determined. Differences in substrate oxidation to CO2 by IREC at specific substrate concentrations did not affect Vmax (maximal rate of substrate oxidation, nmoles oxidized to CO2 ù1X106 cells-1ù90 min-1) and Kox (concentration of substrate at which half Vmax oxidation rate is achieved, mmoles/L) estimates. Butyrate oxidation to BHBA by IREC from the OGD lambs was unaffected by ME intake, however production was elevated by HE intake of CD (Diet Intake interaction; P < .02) while oxidation to AcAc by IREC from HE lambs were greater (P =.001) than LE. The changes in metabolite production rates did not affect predicted Vmax and Kox estimates The Kox values corroborate the contention that VFA are the primary oxidizable fuels used by REC while showing glucose, glutamate and glutamine should not be oxidized extensively in vivo due to the high Kox relative to substrate concentrations in vivo. In conclusion, capacity of IREC to oxidize substrates was largely unaffected by ME intake or dietary forage-to-concentrate ratio of the diet.