Aquaporin-2, a regulated water channel, is expressed in apical membranes of rat distal colon epithelium

Aquaporin-2 (AQP-2) is the vasopressin-regulated water channel expressed in the apical membrane of principal cells in the collecting duct and is invol ved in the urinary concentrating mechanism. In the rat distal colon, vasopr essin stimulates water absorption through an unknown mechanism. With the hy pothesis that AQP-2 could contribute to this vasopressin effect, we studied its presence in rat colonic epithelium. We used RT-PCR, in situ hybridizat ion, immunoblotting, and immunocytochemistry to probe for AQP-2 expression. An AQP-2 amplicon was obtained through RT-PCR of colon epithelium RNA, and in situ hybridization revealed AQP-2 mRNA in colonic crypts and, to a less er extent, in surface absorptive epithelial cells. AQP-2 protein was locali zed to the apical membrane of surface absorptive epithelial cells, where it colocalized with H+-K+-ATPase but not with Na+-K+-ATPase. AQP-2 was absent from the small intestine, stomach, and liver. Water deprivation increased the hybridization signal and the protein level (assessed by Western blot an alysis) for AQP-2 in distal colon. This was accompanied by increased p-chlo romercuriphenylsulfonic acid-sensitive water absorption. These results indi cate that AQP-2 is present in the rat distal colon, where it might be invol ved in a water-sparing mechanism. In addition, these results support the id ea that AQP-2, and probably other aquaporins, are involved in water absorpt ion in the colon.