Effects of diet forage : concentrate ratio and metabolizable energy intakeon isolated rumen epithelial cell metabolism in vitro

Crossbred wether lambs were used to assess the effect of altered forage:con centrate ratio and metabolizable energy intake on metabolism of substrates by ruminal epithelium using an isolated cell system. Lambs (n = 28; 20.1 +/ - 3 kg BW) were assigned randomly to a factorial arrangement of dietary tre atments consisting of either 75% forage or 75% concentrate fed once daily a t either .099 or .181 Meal ME (kg BW.75)(-1.)d(-1) for 52 d. After a 52-d f eeding period, isolated rumen epithelial cells (IREC) were incubated in the presence of an oxidizable substrate with a single C-14 label (acetate, pro pionate, butyrate, glucose, glutamate, and glutamine) at concentrations ran ging from .1 to 50 mM, and substrate oxidation to (CO2)-C-14 Or metabolism to beta-hydroxybutyrate (beta-HBA), acetoacetate, pyruvate, and lactate was determined. For all substrates, oxidation to CO2 was concentration-depende nt and saturable within the physiological range. Differences in substrate o xidation to CO2 by IREC at specific substrate concentrations did not affect V-max (maximal rate of substrate oxidation, nmol oxidized to CO2.1 x 10(6) cells(-1).90 min(-1)) and K-ox (concentration of substrate at which half V -max oxidation rate is achieved, mmoles/L) estimates for the dietary treatm ents. Production of beta-HBA from butyrate by IREC from the lambs fed 75% f orage was not affected by ME intake; however, production was elevated by hi gh ME intake of the 75% concentrate diet (diet x intake interaction; P < .0 2). Acetoacetate production from butyrate by IREC from lambs fed at high ME intake was greater (P = .001) than from those fed at low ME intake. Lactat e and pyruvate production from glucose, glutamate, and propionate were gene rally unaffected by dietary treatment; however, rate of glutamine metabolis m to lactate and pyruvate by IREC was increased with increased ME intake. T he observed changes in metabolite production rates across groups did not af fect the predicted V-max and K-ox parameter estimates. The estimated K-ox v alues corroborate that VFA are the primary oxidizable fuels used by ruminal epithelial cells while illustrating that other substrates such as glucose, glutamate, and glutamine would not be expected to be oxidized extensively in vivo due to the high K-ox relative to substrate concentrations in vivo. In conclusion, the capacity of isolated ruminal epithelial cells to oxidize substrates was largely unaffected by ME intake or dietary forage:concentra te ratio of the diet.