Zeuthen Thomas
Institute of Medical Physiology, The Panum Institute, University of Copenhagen, Denmark.
Int Rev Cytol. 2002;215:285-317. doi: 10.1016/s0074-7696(02)15013-3.
The group of leaky epithelia, such as proximal tubule and small intestine, have several common properties in regard to salt and water transport. The fluid transport is isotonic, the transport rate increases in dilute solutions, and water can be transported uphill. Yet, it is difficult to find common features that could form the basis for a general transport model. The direction of transepithelial water transport does not correlate with the direction of the primary active Na+ transport, or with the ultrastucture as defined by the location of apical and basolateral membranes, of the junctional complex and the lateral intercellular spaces. The presence of specific water channels, aquaporins, increases the water permeability of the epithelial cell membranes, i.e., the kidney proximal tubule. Yet other leaky epithelia, for example, the retinal pigment epithelium, have no known aquaporins. There is, however, a general correlation between the direction of transepithelial transport and the direction of transport via cotransporters of the symport type. A simple epithelial model based on water permeabilities, a hyperosmolar compartment and restricted salt diffusion, is unable to explain epithelial transport phenomena, in particular the ability for uphill water transport. The inclusion of cotransporters as molecular water pumps in these models alleviates this problem.
渗漏上皮组织，如近端小管和小肠，在盐和水的转运方面具有几个共同特性。液体转运是等渗的，在稀溶液中转运速率增加，并且水可以逆浓度梯度转运。然而，很难找到能够构成通用转运模型基础的共同特征。跨上皮水转运的方向与原发性主动钠转运的方向不相关，也与由顶端和基底外侧膜的位置、连接复合体和细胞间外侧间隙所定义的超微结构不相关。特定水通道水孔蛋白的存在增加了上皮细胞膜的水通透性，即肾近端小管。然而，其他渗漏上皮组织，例如视网膜色素上皮，没有已知的水孔蛋白。然而，跨上皮转运的方向与通过同向转运体类型的共转运体的转运方向之间存在普遍相关性。基于水通透性、高渗隔室和受限盐扩散的简单上皮模型无法解释上皮转运现象，特别是逆浓度梯度水转运的能力。在这些模型中纳入共转运体作为分子水泵可缓解这一问题。
