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

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Abstract
Crossbred wether lambs were used to assess the effect of altered forage:concentrate 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 treatments consisting of either 75% forage or 75% concentrate fed once daily at either .099 or .181 Mcal ME ·(kg BW.75)−1 · d−1 for 52 d. After a 52-d feeding period, isolated rumen epithelial cells (IREC) were incubated in the presence of an oxidizable substrate with a single 14C label (acetate, propionate, butyrate, glucose, glutamate, and glutamine) at concentrations ranging from .1 to 50 mM, and substrate oxidation to 14CO2 or metabolism to µ-hydroxybutyrate (µ-HBA), acetoacetate, pyruvate, and lactate was determined. For all substrates, oxidation to CO2 was concentration-dependent and saturable within the physiological range. Differences in substrate oxidation to CO2 by IREC at specific substrate concentrations did not affect Vmax (maximal rate of substrate oxidation, nmol oxidized to CO2 · 1 × 106 cells−1 · 90 min−1) and Kox (concentration of substrate at which half Vmax oxidation rate is achieved, mmoles/L) estimates for the dietary treatments. Production of µ-HBA from butyrate by IREC from the lambs fed 75% forage was not affected by ME intake; however, production was elevated by high ME intake of the 75% concentrate diet (diet × intake interaction; P < .02). 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. Lactate and pyruvate production from glucose, glutamate, and propionate were generally unaffected by dietary treatment; however, rate of glutamine metabolism to lactate and pyruvate by IREC was increased with increased ME intake. The observed changes in metabolite production rates across groups did not affect the predicted Vmax and Kox parameter estimates. The estimated Kox values 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 Kox 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:concentrate ratio of the diet.

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