1303 Braz J Med Biol Res 32(10) 1999 Aquaporins in human colon Brazilian Journal of Medical and Biological Research (1999) 32: 1303-1313 ISSN 0100-879X Functional characterization and localization of AQP3 in the human colon Laboratorio de Fisiopatogenia, Departamento de Fisiología, Facultad de Medicina, Universidad Nacional de Buenos Aires, Buenos Aires, Argentina C. Silberstein, A. Kierbel, G. Amodeo, E. Zotta, F. Bigi, D. Berkowski and C. Ibarra Abstract Water channels or aquaporins (AQPs) have been identified in a large variety of tissues. Nevertheless, their role in the human gastrointesti- nal tract, where their action is essential for the reabsorption and secretion of water and electrolytes, is still unclear. The purpose of the present study was to investigate the structure and function of water channels expressed in the human colon. A cDNA fragment of about 420 bp with a 98% identity to human AQP3 was amplified from human stomach, small intestine and colon by reverse transcription polymerase chain reaction (RT-PCR) and a transcript of 2.2 kb was expressed more abundantly in colon than in jejunum, ileum and stomach as indicated by Northern blots. Expression of mRNA from the colon of adults and children but not from other gastrointestinal regions in Xenopus oocytes enhanced the osmotic water permeability, and the urea and glycerol transport in a manner sensitive to an antisense AQP3 oligonucleotide, indicating the presence of functional AQP3. Immunocytochemistry and immunofluorescence studies in human colon revealed that the AQP3 protein is restricted to the villus epithelial cells. The immunostaining within these cells was more intense in the apical than in the basolateral membranes. The presence of AQP3 in villus epithelial cells suggests that AQP3 is implicated in water absorption across human colonic surface cells. Correspondence C. Ibarra Departamento de Fisiología Facultad de Medicina, UBA Paraguay 2155, 7º piso 1121 Buenos Aires Argentina Fax: + 54-1-963-6287 E-mail: ibarra@fmed.uba.ar Part of these data were presented at the International Symposium of the Molecular Physiology of Water Transport, Paris, France, April 27-30, 1997. Research supported by the Universidad de Buenos Aires (TM23) and the Consejo Nacional de Investigaciones Científicas y Técnicas (PIP No. 867/98). Received February 12, 1999 Accepted July 26, 1999 Key words Absorptive epithelium Human colon Water channels Aquaporins AQ P3 Introduction Approximately 9 liters per day of water are involved in the digestion process and due to the great absorptive capacity of the intes- tine, only 0.2 liters are excreted with the stool (1). The opposite can also occur, i.e., the small and large intestine can secrete large amounts of water in the presence of toxins or endogenous secretagogues (2,3). It is gener- ally accepted that these important fluid trans- fers are associated with salt reabsorption or secretion. Nevertheless, it is still a subject of controversy whether the absorptive processes are located in the surface (and villus) cells, if the secretory processes are located in the crypt cells (4,5) and if the transport-associ- ated water movements are transcellular or paracellular (6,7). The recent characterization and cloning of aquaporins (AQPs) have introduced a new approach to this problem. These structures