Verbavatz J M
Service de Biologie Cellulaire, Centre d'Etudes de Saclay, Gif-sur-Yvette.
C R Seances Soc Biol Fil. 1995;189(2):191-7.
The high water permeability of some specialized cells is accounted for by the presence of specific water channels in their plasma membranes. These channels, that play an important role in water homeostasis are now known as aquaporins. Mammalian kidney, in particular contains several forms of aquaporins. There are constitutive water channels (aquaporin 1), which are involved in the bulk of water reabsorption in the kidney. Aquaporin 1 is also found in red blood cells, some endothelial cells and several other tissues. Other water channels (aquaporin 2) are involved in the fine tuning of the final osmolarity of urine in mammalian kidney or amphibian urinary bladder. The latter are regulated by a membrane shuttle mechanism of vesicles called aggrephores, that contain the water channels. This mechanism is under hormonal control. Upon stimulation, aggrephores rapidly fuse with the apical plasma membrane of target epithelial cells, which increases their water permeability. After hormone withdrawal, aggrephores are internalized by endocytosis. Finally some kidney water channels (aquaporin 3) are localized in the basolateral plasma membrane of the same cells, where they contribute to the high transepithelial water permeability. Five classes of aquaporins are now known--they all belong to the same family of transmembrane proteins. Unlike previous members of the family, aquaporin 5, that was cloned most recently, is involved in the water secretion that occurs in lacrimal and salivary glands.
某些特化细胞的高水渗透性是由其质膜中存在特定的水通道所致。这些在水稳态中起重要作用的通道现在被称为水通道蛋白。特别是哺乳动物的肾脏含有几种形式的水通道蛋白。有组成型水通道（水通道蛋白1），它参与肾脏中的大部分水重吸收。水通道蛋白1也存在于红细胞、一些内皮细胞和其他几种组织中。其他水通道（水通道蛋白2）参与哺乳动物肾脏或两栖动物膀胱中尿液最终渗透压的微调。后者受称为聚集小泡的囊泡的膜穿梭机制调节，聚集小泡含有水通道。这种机制受激素控制。受到刺激时，聚集小泡迅速与靶上皮细胞的顶端质膜融合，从而增加其水渗透性。激素撤除后，聚集小泡通过内吞作用被内化。最后，一些肾脏水通道（水通道蛋白3）定位于同一细胞的基底外侧质膜，它们在其中有助于高跨上皮水渗透性。现在已知五类水通道蛋白——它们都属于同一跨膜蛋白家族。与该家族先前的成员不同，最近克隆的水通道蛋白5参与泪腺和唾液腺中的水分泌。
