Cooper Gordon J, Zhou Yuehan, Bouyer Patrice, Grichtchenko Irina I, Boron Walter F
Department of Cellular and Molecular Physiology, Yale University School of Medicine, New Haven, CT 06520-8026, USA.
J Physiol. 2002 Jul 1;542(Pt 1):17-29. doi: 10.1113/jphysiol.2002.023218.
For almost a century it was generally assumed that the lipid phases of all biological membranes are freely permeable to gases. However, recent observations challenge this dogma. The apical membranes of epithelial cells exposed to hostile environments, such as gastric glands, have no demonstrable permeability to the gases CO2 and NH3. Additionally, the water channel protein aquaporin 1 (AQP1), expressed at high levels in erythrocytes, can increase membrane CO2 permeability when expressed in Xenopus oocytes. Similarly, nodulin-26, which is closely related to AQP1, can act as a conduit for NH3. A key question is whether aquaporins, which are abundant in virtually every tissue that transports O2 and CO2 at high levels, ever play a physiologically significant role in the transport of small volatile molecules. Preliminary data are consistent with the hypothesis that AQP1 enhances the reabsorption of HCO3- by the renal proximal tubule by increasing the CO2 permeability of the apical membrane. Other preliminary data on Xenopus oocytes heterologously expressing the electrogenic Na+-HCO3- cotransporter (NBC), AQP1 and carbonic anhydrases are consistent with the hypothesis that the macroscopic cotransport of Na+ plus two HCO3- occurs as NBC transports Na+ plus CO3(2-) and AQP1 transports CO2 and H2O. Although data - obtained on AQP1 reconstituted into liposomes or on materials from AQP1 knockout mice - appear inconsistent with the model that AQP1 mediates substantial CO2 transport in certain preparations, the existence of unstirred layers or perfusion-limited conditions may have masked the contribution of AQP1 to CO2 permeability.
近一个世纪以来，人们普遍认为所有生物膜的脂质相都可让气体自由透过。然而，最近的观察结果对这一教条提出了挑战。暴露于恶劣环境（如胃腺）的上皮细胞的顶端膜对二氧化碳和氨气没有明显的通透性。此外，在红细胞中高水平表达的水通道蛋白水通道蛋白1（AQP1），当在非洲爪蟾卵母细胞中表达时，可增加膜对二氧化碳的通透性。同样，与AQP1密切相关的结节蛋白26可作为氨气的通道。一个关键问题是，在几乎每个高水平运输氧气和二氧化碳的组织中都大量存在的水通道蛋白，是否在小挥发性分子的运输中发挥生理上重要的作用。初步数据与以下假设一致：AQP1通过增加顶端膜对二氧化碳的通透性来增强肾近端小管对碳酸氢根的重吸收。关于非洲爪蟾卵母细胞异源表达电中性钠-碳酸氢根共转运体（NBC）、AQP1和碳酸酐酶的其他初步数据与以下假设一致：钠加两个碳酸氢根的宏观共转运发生在NBC转运钠加碳酸根以及AQP1转运二氧化碳和水时。尽管关于重组到脂质体中的AQP1或来自AQP1基因敲除小鼠的材料所获得的数据似乎与AQP1在某些制剂中介导大量二氧化碳运输的模型不一致，但未搅动层的存在或灌注受限条件可能掩盖了AQP1对二氧化碳通透性的贡献。
