Baldwin R L, McLeod K R
USDA, ARS, Livestock and Poultry Sciences Institute, Nutrient Conservation and Metabolism Laboratory, Beltsville, MD 20705, USA.
J Anim Sci. 2000 Mar;78(3):771-83. doi: 10.2527/2000.783771x.
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 x(kg BW(.75))(-1) x 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 beta-hydroxybutyrate (beta-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 x 1 x 10(6) cells(-1) x 90 min(-1)) and K(ox) (concentration of substrate at which half Vmax oxidation rate is achieved, mmoles/L) estimates for the dietary treatments. Production of beta-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 x 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 K(ox) parameter estimates. The estimated K(ox) 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 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:concentrate ratio of the diet.
使用杂种去势公羔羊，通过分离细胞系统评估改变的饲草与精料比例以及代谢能摄入量对瘤胃上皮底物代谢的影响。将羔羊（n = 28；体重20.1±3千克）随机分配到析因设计的日粮处理组，日粮处理包括每日一次饲喂75%饲草或75%精料，代谢能水平分别为0.099或0.181兆卡ME×(千克体重(0.75))(-1)×天(-1)，持续52天。经过52天的饲喂期后，将分离的瘤胃上皮细胞（IREC）在含有单一14C标记的可氧化底物（乙酸盐、丙酸盐、丁酸盐、葡萄糖、谷氨酸盐和谷氨酰胺）存在的情况下进行孵育，底物浓度范围为0.1至50毫摩尔，测定底物氧化为14CO2或代谢为β-羟基丁酸（β-HBA）、乙酰乙酸、丙酮酸和乳酸的情况。对于所有底物，氧化为CO2在生理范围内呈浓度依赖性且具有饱和性。在特定底物浓度下，IREC对底物氧化为CO2的差异并不影响日粮处理组Vmax（底物氧化最大速率，氧化为CO2的纳摩尔数×1×10(6)个细胞(-1)×90分钟(-1)）和K(ox)（达到最大氧化速率一半时的底物浓度，毫摩尔/升）的估计值。采食75%饲草的羔羊的IREC由丁酸盐产生β-HBA不受代谢能摄入量的影响；然而，采食75%精料日粮且代谢能摄入量高时，β-HBA的产生量增加（日粮×摄入量交互作用；P < 0.02）。采食高代谢能水平的羔羊的IREC由丁酸盐产生乙酰乙酸的量比采食低代谢能水平的羔羊的IREC产生的量更大（P = 0.001）。日粮处理一般不影响IREC由葡萄糖、谷氨酸盐和丙酸盐产生乳酸和丙酮酸；然而，随着代谢能摄入量增加，IREC将谷氨酰胺代谢为乳酸和丙酮酸的速率增加。各处理组间观察到的代谢产物产生速率变化并不影响预测的Vmax和K(ox)参数估计值。估计的K(ox)值证实挥发性脂肪酸是瘤胃上皮细胞使用的主要可氧化燃料，同时说明由于相对于体内底物浓度K(ox)较高，预计葡萄糖、谷氨酸盐和谷氨酰胺等其他底物在体内不会被大量氧化。总之，分离的瘤胃上皮细胞氧化底物的能力在很大程度上不受代谢能摄入量或日粮中饲草与精料比例的影响。
