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A series of recent studies have demonstrated that expression of aquaporin-2 (AQP2), the vasopressin-regulated water channel of the kidney collecting duct, is greatly reduced in acquired forms of nephrogenic diabetes insipidus (NDI). In some forms of NDI, there is also impaired delivery of these channels to the apical plasma membrane, where they permit water reabsorption from the urine. The combination of these factors is likely to underlie the urinary concentrating defect that defines these conditions. Direct infusion of vasopressin causes an increase in AQP2 expression, probably via a rise in cytosolic adenosine 3:5-cyclic phosphate, which also acts as the second messenger, triggering the delivery of AQP2 to the plasma membrane. However, it is clear from the studies described that there are also vasopressin-independent pathways that regulate the expression of AQP2, some of which appear to reflect intranephric changes, whereas others involve systemic signals. These studies also show that recovery of AQP2 expression, even after correction of the underlying condition, can be slow, consistent with the clinical observation that recovery of urinary-concentrating ability often takes weeks or months. An understanding of the cellular signals and mechanisms responsible for the decrease in AQP2 expression may make it possible to develop treatments for this common clinical problem. |
