Role of sodium ion in transport of folic acid in the small intestine.
Role of sodium ion in transport of folic acid in the small intestine.
复制标题
钠离子在小肠转运叶酸中的作用。
DOI：
10.1152/ajpgi.1986.251.2.g218
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发表时间：
1986
期刊：
影响因子：
--
通讯作者：
Rosenberg,IH
中科院分区：
文献类型：
--
作者：
Zimmerman,J;Selhub,J;Rosenberg,IH
用“内流”小室和分离的刷状缘膜囊泡两种技术分析了钠对叶酸跨肠腔膜转运的影响。在无Na+的介质中预孵育组织对叶酸内流的影响并不一致，前提是测试液中存在Na+。用胆碱+取代试验溶液中的Na+，显著减少了叶酸的内流。然而，当在无Na+溶液中平衡的肠片暴露于以Na+、Li+、K+、Rb+、Cs+、Tris+或Gu+为主要阳离子的试验液中时，叶酸内流并未显著减少。浓度依赖性研究表明，Rb+取代Na+不影响叶酸饱和转运机制。相反，非饱和叶酸摄取的减少解释了在Rb+存在时观察到的轻微的内流减少。刷状缘膜囊泡的实验表明，在外加Na+的情况下，甲氨蝶呤的摄取显著高于在K+的情况下的摄取，但在K+加呋喃霉素的情况下与甲氨蝶呤的摄取没有差别。这些数据表明，叶酸运输的可饱和成分不依赖于Na+，叶酸通过管腔膜的非饱和运输部分是通过一种传导途径发生的，该途径涉及一种带负电荷的叶酸和一种阳离子，其膜通透性影响叶酸的运输速率。Na+在体内这一过程中的重要性来自于这样一个事实，即Na+是小肠吸收部位可获得的最有价值的阳离子。
The effect of sodium on folate transport across the intestinal luminal membrane was analyzed using two techniques: the "influx" chamber and isolated brush-border membrane vesicles. Preincubation of tissue in Na+-free medium did not have a consistent effect on folic acid influx provided that Na+ was present in the test solution. Replacement of Na+ in the test solution by choline+ resulted in a significant reduction of folic acid influx. However, when intestinal sheets that had been equilibrated in Na+-free solution were exposed to test solution containing either Na+, Li+, K+, Rb+, Cs+, Tris+, or guanidinium+ as main cations, folic acid influx was not significantly decreased. Concentration-dependence studies showed that replacement of Na+ by Rb+ did not affect the saturable mechanism of folate transport. Rather, a decrease in nonsaturable folic acid uptake accounted for the slightly reduced influx observed in the presence of Rb+. Experiments with brush-border membrane vesicles revealed that methotrexate uptake was significantly higher in the presence of external Na+ than in the presence of K+, but was not different from uptake in the presence of K+ plus valinomycin. These data suggest that the saturable component of folate transport is not Na+ dependent, and nonsaturable transport of folic acid across the luminal membrane occurs in part through a conductive pathway that involves a negatively charged species of folate and a cation whose membrane permeability affects the rate of folate transport. The importance of Na+ in this process in vivo derives from the fact that Na+ is the most permeant cation available at the absorptive site in the small intestine.