Danfaer A, Tetens V, Agergaard N
Department of Animal Physiology and Biochemistry, Foulum Research Centre, Tjele, Denmark.
Comp Biochem Physiol B Biochem Mol Biol. 1995 Jun;111(2):201-10. doi: 10.1016/0305-0491(94)00242-m.
A short review describes quantitative aspects of glucose metabolism in lactating ruminants, including the contribution of different substrates to glucose synthesis as affected by nutritional and hormonal regulation. Gluconeogenesis from propionate (the major gluconeogenic substrate in the fed state) is mainly regulated by feed intake and increased propionate availability and is less affected by insulin and glucagon. Quantitative estimates of amino acid contribution to glucose synthesis are highly variable (from 2 to 40% of glucose flux), but no conclusive data exist by which this large variation can be explained. An experimental model is described by which nutrient exchanges across the liver as well as the contribution of different substrates to glucose synthesis can be quantitated. The novel approach is continuous infusion of differently labelled (3H and 14C) gluconeogenic substrates into multi-catheterized goats. Results are reported from a preliminary experiment using this model. A goat in mid-lactation was fed hay ad libitum, and the experimental treatments were continuous infusion into a mesenteric vein of either sodium propionate or a complete mixture of amino acids. Measurements were nutrient exchanges across portal-drained viscera and liver, whole body glucose flux rate, and hepatic gluconeogenesis from propionate, lactate and glycerol. Glucose synthesis from amino acids was estimated by difference. Net portal appearance and hepatic uptake of propionate and of amino acids were increased by propionate and amino acid infusions, respectively. Glucose flux rate and hepatic glucose output were not affected by treatments. With propionate infusion, the proportions of liver glucose derived from propionate and amino acids were 62 and 19%, respectively. With amino acid infusion, the corresponding figures were 24 and 36%. Glucagon concentration in portal blood plasma was 2.7 times higher on the amino acid than on the propionate treatment. It is concluded that the hepatic metabolism of propionate and glucogenic amino acids into glucose synthesis is regulated to a great extent by the availability of these substrates, and that glucagon may be involved in this regulation. It is further concluded that the described experimental approach is suitable for investigations of nutrient absorption and hepatic gluconeogenesis.
一篇简短综述描述了泌乳反刍动物葡萄糖代谢的定量方面，包括不同底物对葡萄糖合成的贡献，这些贡献受营养和激素调节的影响。丙酸（采食状态下主要的糖异生底物）的糖异生主要受采食量和丙酸可利用性增加的调节，受胰岛素和胰高血糖素的影响较小。氨基酸对葡萄糖合成贡献的定量估计差异很大（占葡萄糖通量的2%至40%），但尚无确凿数据能解释这种巨大差异。本文描述了一种实验模型，通过该模型可以对肝脏的营养物质交换以及不同底物对葡萄糖合成的贡献进行定量。这种新方法是将不同标记（3H和14C）的糖异生底物持续输注到多导管山羊体内。报告了使用该模型的初步实验结果。给一只处于泌乳中期的山羊随意投喂干草，实验处理是将丙酸钠或完整氨基酸混合物持续输注到肠系膜静脉中。测量指标包括门静脉引流内脏和肝脏的营养物质交换、全身葡萄糖通量率以及丙酸、乳酸和甘油的肝脏糖异生。通过差值估计氨基酸合成葡萄糖的量。丙酸盐和氨基酸输注分别增加了门静脉中丙酸盐和氨基酸的净出现量以及肝脏摄取量。葡萄糖通量率和肝脏葡萄糖输出不受处理影响。输注丙酸盐时，肝脏葡萄糖中来自丙酸盐和氨基酸的比例分别为62%和19%。输注氨基酸时，相应数字分别为24%和36%。门静脉血浆中胰高血糖素浓度在氨基酸处理组比丙酸盐处理组高2.7倍。结论是，丙酸和生糖氨基酸向葡萄糖合成的肝脏代谢在很大程度上受这些底物可利用性的调节，并且胰高血糖素可能参与了这种调节。进一步得出结论，所描述的实验方法适用于营养物质吸收和肝脏糖异生的研究。
