Munck L K
Department of Medical Physiology, Panum Institute, University of Copenhagen, Denmark.
Biochim Biophys Acta. 1995 Jul 17;1241(2):195-213. doi: 10.1016/0304-4157(95)00005-c.
The unidirectional influx of amino acids, D-glucose and ions across the brush-border membrane of the small intestine of different species has been measured in vitro with emphasis on characterization of topographic and species differences and on chloride dependence. The regional differences in transport along the small intestine are outlined and shown to be caused by variation in transport capacity, while the apparent affinity constants are unchanged. Rabbit small intestine is unique by exhibiting maximal rates of transport in the distal ileum and a very steep decline in the oral direction from where tissues are normally harvested for preparation of brush-border membrane vesicles. Transport in the guinea pig and rat is much more constant throughout the small intestine. Since the capacity of nutrient carriers is regulated by their substrates it is possible that bacterial breakdown of peptides and proteins in rabbit distal ileum increases the concentration of amino acids leading to an upregulation of the carriers. Chloride dependence is a characteristics of the carrier rather than the transported amino acid, and is used to improve the classification of amino acid carriers in rabbit small intestine. In this species the imino acid carrier, the beta-amino acid carrier, and the beta-alanine carrier, which should be renamed the B0,+ carrier, are chloride-dependent. The steady-state mucosal uptake of classical substrates for these carriers in biopsies from the human duodenum is also chloride-dependent. The carrier of beta-amino acids emerges as ubiquitous and chloride-dependent, and evidence of cotransport with both sodium and chloride is reviewed. A sodium:chloride:2-methyl-aminoisobutyric acid coupling stoichiometry of approx. 2:1:1 is suggested by ion activation studies. Direct measurements of coupled ion fluxes in rabbit distal ileum confirm that sodium, chloride and 2-methyl-aminoisobutyric acid are cotransported on the imino acid carrier with an identical influx stoichiometry. Control experiments and reference to the literature on the electrophysiology of the small intestine exclude alterations of the membrane potential as a feasible explanation of the chloride dependence. Thus, it is concluded that chloride is cotransported with both sodium and 2-methyl-aminoisobutyric acid across the brush-border membrane of rabbit distal ileum.
已在体外测量了不同物种小肠刷状缘膜上氨基酸、D-葡萄糖和离子的单向流入，重点是表征地形和物种差异以及对氯离子的依赖性。概述了沿小肠转运的区域差异，并表明其是由转运能力的变化引起的，而表观亲和常数不变。兔小肠的独特之处在于，其在回肠末端表现出最大转运速率，并且从通常用于制备刷状缘膜囊泡的组织采集部位向口腔方向转运速率急剧下降。豚鼠和大鼠小肠的转运在整个小肠中更为恒定。由于营养载体的能力受其底物调节，因此兔回肠末端肽和蛋白质的细菌分解可能会增加氨基酸浓度，从而导致载体上调。对氯离子的依赖性是载体的特征，而非所转运氨基酸的特征，可用于改进兔小肠中氨基酸载体的分类。在该物种中，亚氨基酸载体、β-氨基酸载体和β-丙氨酸载体（应重新命名为B0,+载体）对氯离子具有依赖性。在人十二指肠活检中，这些载体的经典底物的稳态黏膜摄取也依赖于氯离子。β-氨基酸载体是普遍存在且依赖于氯离子的，本文综述了其与钠和氯离子共转运的证据。离子激活研究表明，钠:氯:2-甲基氨基异丁酸的偶联化学计量比约为2:1:1。对兔回肠末端偶联离子通量的直接测量证实，钠、氯和2-甲基氨基异丁酸在亚氨基酸载体上以相同的流入化学计量比共转运。对照实验以及对小肠电生理学文献的参考排除了膜电位改变作为氯离子依赖性合理解释的可能性。因此，得出结论：氯离子与钠和2-甲基氨基异丁酸一起跨兔回肠末端的刷状缘膜共转运。
