Shachar-Hill Bruria, Hill Adrian E
Physiological Laboratory, Cambridge University, United Kingdom.
Int Rev Cytol. 2002;215:319-50. doi: 10.1016/s0074-7696(02)15014-5.
The evidence that a major fraction of water crosses the paracellular route during isotonic fluid transfer is reviewed together with a description of the theory and experimental results derived from extracellular probe studies. Four transporting epithelia which have been studied using the method are gallbladder, intestine, Malpighian tubule, and salivary gland. It is concluded that paracellular probe flows are not due to simple convection generated by osmotic flow through the junctions but are generated by active fluid transport within the junction: a mechano-osmotic process. The geometry of the pathway involved would indicate that some salt accompanies the paracellular fluid, representing a hypo-osmotic flow. Transport of salt by the cell route, which may be accompanied by some water, represents a hypertonic flow. The problem then becomes one of balancing the two to produce an isotonic transportate. We suggest, using recent data from knockout mice, that some aquaporins are functioning in different epithelial tissues as osmo-comparators within a feedback loop that regulates the paracellular fluid flow rate. This results in an overall quasi-isotonic transport by the epithelium. The model is applied to forward-facing systems such as proximal tubule and backward-facing systems such as exocrine glands.
本文回顾了等渗液体转移过程中大部分水通过细胞旁途径的证据，并描述了细胞外探针研究得出的理论和实验结果。使用该方法研究过的四种转运上皮组织分别是胆囊、肠道、马氏管和唾液腺。得出的结论是，细胞旁探针流动并非由于通过连接点的渗透流产生的简单对流，而是由连接点内的主动液体运输产生的：一种机械渗透过程。所涉及途径的几何结构表明，一些盐伴随着细胞旁液体，代表低渗流。通过细胞途径运输的盐可能伴随着一些水，代表高渗流。那么问题就变成了平衡两者以产生等渗转运物。我们利用基因敲除小鼠的最新数据表明，一些水通道蛋白在不同的上皮组织中作为反馈回路中的渗透压比较器发挥作用，该反馈回路调节细胞旁液体流速。这导致上皮组织进行整体准等渗运输。该模型适用于正向系统，如近端小管，以及反向系统，如外分泌腺。
