Department of Physiology, University of Arizona Health Sciences Center, Tucson, Arizona, USA.
Compr Physiol. 2012 Jul;2(3):2063-86. doi: 10.1002/cphy.c110019.
The thin limbs of the loop of Henle, which comprise the intermediate segment, connect the proximal tubule to the distal tubule and lie entirely within the renal medulla. The descending thin limb consists of at least two or three morphologically and functionally distinct subsegments and participates in transepithelial transport of NaCl, urea, and water. Only one functionally distinct segment is recognized for the ascending thin limb, which carries out transepithelial transport of NaCl and urea in the reabsorptive and/or secretory directions. Membrane transporters involved with passive transcellular Cl, urea, and water fluxes have been characterized for thin limbs; however, these pathways do not account for all transepithelial fluid and solute fluxes that have been measured in vivo. The paracellular pathway has been proposed to play an important role in transepithelial Na and urea fluxes in defined thin-limb subsegments. As the transport pathways become clearer, the overall function of the thin limbs is becoming better understood. Primary and secondary signaling pathways and protein-protein interactions are increasingly recognized as important modulators of thin-limb cell function and cell metabolism. These functions must be investigated under diverse extracellular conditions, particularly for those cells of the deep inner medulla that function in an environment of wide variation in hyperosmolality. Transgenic mouse models of several key water and solute transport proteins have provided significant insights into thin-limb function. An understanding of the overall architecture of the medulla, including juxtapositions of thin limbs with collecting ducts, thick ascending limbs, and vasa recta, is essential for understanding the role of the kidney in maintaining Na and water homeostasis, and for understanding the urine concentrating mechanism.
细的亨勒袢升支（loop of Henle）的分支，构成了中间段，连接近端小管和远端小管，完全位于肾髓质内。降支细段至少有两到三个形态和功能不同的亚段，参与 NaCl、尿素和水的跨上皮转运。升支细段只有一个功能不同的段，进行 NaCl 和尿素的跨上皮转运，无论是在吸收方向还是分泌方向。已经鉴定出参与 Cl、尿素和水被动跨细胞转运的膜转运蛋白，但是这些途径并不能解释所有在体内测量到的跨上皮液和溶质通量。已经提出细胞旁途径在特定的细段亚段的跨上皮 Na 和尿素通量中发挥重要作用。随着转运途径变得更加清晰，细段的整体功能也越来越被理解。初级和次级信号通路和蛋白-蛋白相互作用被越来越多地认为是调节细段细胞功能和细胞代谢的重要因素。这些功能必须在不同的细胞外条件下进行研究，特别是对于那些在高渗环境中功能广泛变化的深层内髓质细胞。几种关键的水和溶质转运蛋白的转基因小鼠模型为细段功能提供了重要的见解。了解髓质的整体结构，包括细段与集合管、升支粗段和直小血管的毗邻关系，对于理解肾脏在维持 Na 和水稳态中的作用以及理解尿液浓缩机制至关重要。
