Fordtran J S, Rector F C, Carter N W
J Clin Invest. 1968 Apr;47(4):884-900. doi: 10.1172/JCI105781.
The present studies were designed to characterize sodium transport in the jejunum and ileum of humans with respect to the effects of water flow, sodium concentration, addition of glucose and galactose, and variations in aniomic composition of luminal fluid. In the ileum, sodium absorption occurred against very steep electrochemical gradients (110 mEq/liter, 5-15 mv), was unaffected by the rate or direction of water flow, and was not stimulated by addition of glucose, galactose, or bicarbonate. These findings led to the conclusion that there is an efficiently active sodium transport across a membrane that is relatively impermeable to sodium. In contrast, jejunal sodium (chloride) absorption can take place against only the modest concentration gradient of 13 mEq/liter, was dramatically influenced by water movement, and was stimulated by addition of glucose, galactose, and bicarbonate. The stimulatory effect of glucose and galactose was evident even when net water movement was inhibited to zero by mannitol. These observations led to the conclusion that a small fraction of jejunal sodium absorption was mediated by active transport coupled either to active absorption of bicarbonate or active secretion of hydrogen ions. The major part of sodium absorption, i.e. sodium chloride absorption, appeared to be mediated by a process of bulk flow of solution along osmotic pressure gradients. The stimulatory effect of glucose and galactose, even at zero water flow, was explained by a model in which the active transport of monosaccharide generates a local osmotic force for the absorption of solution (NaCl and water) from the jejunal lumen, which, in the presence of mannitol, is counterbalanced by a reverse flow of pure solvent (H(2)O) through a parallel set of channels which are impermeable to sodium. Support for the model was obtained by the demonstration that glucose and bicarbonate stimulated the absorption of the nonactively transported solute urea even when net water flow was maintained at zero by addition of mannitol to luminal contents.
本研究旨在探讨水流、钠浓度、葡萄糖和半乳糖的添加以及肠腔液离子组成变化对人体空肠和回肠钠转运的影响。在回肠中，钠的吸收是逆着非常陡峭的电化学梯度（110 mEq/升，5 - 15 mV）进行的，不受水流速率或方向的影响，也不受葡萄糖、半乳糖或碳酸氢盐添加的刺激。这些发现得出结论，存在一种高效的跨膜主动钠转运，该膜对钠相对不通透。相比之下，空肠钠（氯）的吸收仅能逆着13 mEq/升的适度浓度梯度进行，受水的移动显著影响，并受葡萄糖、半乳糖和碳酸氢盐添加的刺激。即使通过甘露醇将净水流抑制至零，葡萄糖和半乳糖的刺激作用仍然明显。这些观察结果得出结论，空肠钠吸收的一小部分是由主动转运介导的，该主动转运与碳酸氢盐的主动吸收或氢离子的主动分泌相关联。钠吸收的主要部分，即氯化钠吸收，似乎是由溶液沿渗透压梯度的大量流动过程介导的。葡萄糖和半乳糖即使在零水流时的刺激作用，可通过一个模型来解释，即单糖的主动转运产生局部渗透压，促进空肠腔内溶液（NaCl和水）的吸收，在存在甘露醇的情况下，纯溶剂（H₂O）通过一组对钠不通透的平行通道的反向流动与之平衡。通过证明即使在向肠腔内容物中添加甘露醇使净水流保持为零的情况下，葡萄糖和碳酸氢盐仍能刺激非主动转运溶质尿素的吸收，从而获得了对该模型的支持。
