DISTRIBUTION OF AQUAPORIN-4 WATER CHANNEL EXPRESSION WITHIN RAT-KIDNEY

The aquaporins are a family of transmembrane proteins that function as molecular water channels. Recently, a mercurial-insensitive water cha nnel [MIWC or aquaporin-4 (AQP4)] has been cloned, and its mRNA was fo und to be expressed strongly in kidney inner medulla and several nonre nal tissues. We prepared affinity-purified polyclonal antipeptide anti bodies to AQP4 to define the regional distribution and cellular locati on of this water channel within the kidney. Immunoblotting of membrane fractions from different regions of the kidney revealed strongest exp ression in the base of the renal inner medulla, with detectable levels also in the inner medullary tip, but little or no expression in the o uter medulla or cortex. Immunocytochemistry (light microscopy) reveale d renal AQP4 labeling exclusively in the collecting duct principal cel ls, chiefly in the proximal two-thirds of the inner medullary collecti ng duct (IMCD). Little or no expression was seen in the outer medullar y and cortical collecting ducts. Immunoelectron microscopy demonstrate d AQP4 labeling of the basolateral membrane of IMCD cells, with relati vely little labeling of intracellular vesicles. Differential centrifug ation of inner medullary homogenates also revealed a lack of distribut ion to the vesicle-enriched fraction, which contains the vasopressin-r egulated water channel, aquaporin-2. In contrast to aquaporin-2 and aq uaporin-3, water restriction of rats did not increase the level of AQP 4 expression. These results suggest a possible role for AQP4 in the ba solateral exit of water from the IMCD.