J. Terris et al., DISTRIBUTION OF AQUAPORIN-4 WATER CHANNEL EXPRESSION WITHIN RAT-KIDNEY, American journal of physiology. Renal, fluid and electrolyte physiology, 38(6), 1995, pp. 775-785
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.