Expression of aquaporins in the renal connecting tubule

The renal connecting tubule (CNT) is a distinct segment that occurs between the distal convoluted tubule (DCT) and the cortical collecting duct. On th e basis of its characterization in rabbit it is widely believed that connec ting tubule cells have a low permeability to water and do not respond to va sopressin. Here we utilize segment-specific markers and specific aquaporin antibodies to characterize expression of water channels in CNT of the rat b y immunocytochemistry. Colocalization of aquaporin 2 (AQP2), AQP3, and AQP4 with Na+,Ca2+ exchanger (NCX), a transporter characteristic of the connect ing tubule, gave heterogeneous labeling. There was aquaporin labeling in ma ny but not all regions labeled by NCX. Colocalization of AQP2 with AQP3 and with AQP4 showed that AQP3 and AQP4 labeling were always accompanied by AQ P2. Immunogold labeling and electron microscopy showed that NCX-labeled cel ls with AQP2 labeling had the morphology of CNT cells, whereas NCX-labeled cells without AQP2 labeling were DCT cells. The latter regions were identif ied as the late region of the DCT known as DCT2. Additionally, regions of C NT lacking AQP2 labeling could be identified in Brattleboro rats not treate d with vasopressin but not in such animals chronically treated with deamino -Cys(1),D-Arg(8)-vasopressin (dDAVP). Quantitative analysis of labeling was consistent with expression of AQP2 over a longer region of CNT after dDAVP exposure.