Long-term regulation of aquaporins in the kidney

The discovery of the aquaporin family of water channels has greatly improve d our understanding of how water crosses epithelial cells, particularly in the kidney. The study of the mechanisms involved in the regulation of colle cting duct water permeability, in particular, has advanced very rapidly sin ce the identification and characterization of aquaporin-2 (AQP2) in 1993. O ne of the more surprising findings has been the dramatic long-term changes that are seen in the abundance of this protein, as well as the recognition that these changes represent a way of modulating the acute antidiuretic eff ects of vasopressin. Furthermore, such changes seem to be of etiological an d pathological significance in a number of clinical disorders of water bala nce. This review focuses on the various conditions in which AQP2 expression is altered (either increased or decreased) and on what this can tell us ab out the signals and mechanisms controlling these changes Ultimately, this m ay be of great value in the clinical management of water balance disorders. Evidence is also now beginning to emerge that there are similar changes in the expression of other renal aquaporins, which had previously been though t to provide an essentially constitutive water permeability pathway, sugges ting that they too should be considered as regulatory factors in the contro l of body water balance.