Kikeri D, Sun A, Zeidel M L, Hebert S C
Department of Medicine Brigham and Women's Hospital, Boston, Massachusetts.
Nature. 1989 Jun 8;339(6224):478-80. doi: 10.1038/339478a0.
Classically, there is a direct correlation between the lipophilic nature of a molecule and its rate of permeation across a biological membrane, so cell membranes should be more permeable to small, neutral molecules than they are to charged molecular species of similar size. Consequently, the distribution of NH+4 in biological systems is generally believed to be due to the rapid diffusion and equilibration of lipophilic NH3 across cell membranes and the accumulation of NH+4 to be governed by pH differences between compartments. Here we report that renal tubule cells from the medullary thick ascending limb of Henle have an apical membrane which is not only virtually impermeable to NH3, but is also highly permeable to NH+4. These remarkable properties have been incorporated into a model which explains how this renal epithelium can mediate vectorial movement of NH+4 between compartments of equal pH.
传统上，分子的亲脂性与其跨生物膜的渗透速率之间存在直接关联，因此细胞膜对小的中性分子的通透性应高于对类似大小的带电分子的通透性。因此，生物系统中NH₄⁺的分布通常被认为是由于亲脂性NH₃跨细胞膜的快速扩散和平衡，而NH₄⁺的积累则受不同隔室间pH差异的控制。在此我们报告，来自髓袢升支粗段的肾小管细胞具有顶端膜，该膜不仅实际上对NH₃不可渗透，而且对NH₄⁺具有高度通透性。这些显著特性已被纳入一个模型，该模型解释了这种肾上皮如何在pH相同的隔室之间介导NH₄⁺的定向移动。
