The Effect of Oxytocin on Subcellular Localization of Aquaporin-2 in Rat Kidney: an Ultrastructural Immunocytochemical Study

Kim, Joon Yong; Kim, Wan Young; Han, Ki Hwan; Kim, Jin; Kim, Gheun Ho; Han, Jin Suk

Abstract

BACKGROUND
The aquaporin-2 (AQP2) water channel is mainly located in the apical plasma membrane of epithelial cells in the connecting tubule and collecting ducts, but there has been some evidence of a moderate amount of basolateral localization of AQP2 in these nephron segments. Previous in vitro microperfusion studies showed that oxytocin has an antidiuretic action most likely mediated by the vasopressin V2 receptor (V2R) in rat inner medullary collecting duct. METHODS: By using ultrastructural preembedding immunocytochemistry with 1 nm immunogold in male Sprague-Dawley rat kidneys, we investigated the acute (60 min) effect of oxytocin (10 U) on the subcellular localization of AQP2 and tested whether the effect of oxytocin is prevented by a V2R antagonist, OPC-31260 (OPC). RESULTS: In control rat kidneys, AQP2 was mainly expressed in the apical plasma membrane and subapical vesicles in the connecting tubule (CNT) cells, principal cells of cortical (CCD), outer medullary collecting duct (OMCD) and initial part of inner medullary collecting duct (IMCD), and IMCD cells of terminal part of IMCD. Basolateral AQP2 labeling was observed in the CNT cells and IMCD cells. In contrast, there was little basolateral AQP2 labeling in the CCD and OMCD principal cells. Oxytocin treatment induced apical immunolabeling of AQP2 and caused an increase of AQP2 immunolabeling in the basal part including basolateral plasma membrane in the CNT cells, principal cells of CCD, OMCD and initial part of IMCD, and IMCD cells of terminal part of IMCD. Pretreatment of rats with a V2R antagonist OPC before oxytocin treatment caused translocation of AQP2 from the apical plasma membrane to the subapical vesicles. However, AQP2 labeling of basolateral plasma membrane was unchanged or slightly increased. CONCLUSION: Oxytocin induces an increase of AQP2 expression not only in the apical plasma membrane but also in the basolateral plasma membrane. Pretreatment with a V2R antagonist blocked redistribution of apical AQP2 immunolabeling, but did not cause retrieval of AQP2 from the basolateral plasma membrane. These results suggest that apical and basal targeting of AQP2 are regulated by different mechanisms.