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Distribution of aquaporin water channels AQP1 and AQP5 in the ductal system of the human pancreas

¹ Molecular Oral Biology Research Group, Department of Oral Biology, Semmelweis University and Hungarian Academy of Sciences, Budapest, Hungary
² Institute of Experimental Medicine, Hungarian Academy of Sciences, Budapest, Hungary
³ The Water and Salt Research Centre, University of Aarhus, Denmark
⁴ Department of Physiology, Dongguk University School of Medicine, Korea
⁵ School of Biological Sciences, University of Manchester, UK

Correspondence to:
Correspondence to:
Dr S Nielsen, The Water and Salt Research Center, Building 233, University of Aarhus, DK-8000 Aarhus, Denmark;
sn{at}ana.au.dk

ABSTRACT
Background: The exocrine pancreas secretes large volumes of isotonic fluid, most of which originates from the ductal system. The role of aquaporin (AQP) water channels in this process is unknown.

Methods: Expression and localisation of known AQP isoforms was examined in normal human pancreas, pancreatic adenocarcinoma, and pancreatic cell lines of ductal origin (Capan-1, Capan-2, and HPAF) using reverse transcriptase-polymerase chain reaction and immunohistochemistry.

Results: Messenger RNAs for AQP1, -3, -4, -5, and -8 were detected in normal pancreas and in pancreatic adenocarcinoma. The cell lines expressed AQP3, -4, and -5 but lacked AQP1 and AQP8. Immunohistochemistry of normal pancreas revealed that AQP1 is strongly expressed in centroacinar cells and in both the apical and basolateral domains of intercalated and intralobular duct epithelia. AQP1 expression declined with distance along the small interlobular ducts and was not detectable in larger interlobular ducts. AQP3 and AQP4 were not detectable by immunohistochemistry. AQP5 was observed at the apical membrane of intercalated duct cells and also in duct associated mucoid glands. AQP8 was confined to the apical pole of acinar cells. Both AQP1 and AQP5 were colocalised with cystic fibrosis transmembrane conductance regulator (CFTR) at the apical membrane of intercalated duct cells.

Conclusions: AQP1 and AQP5 are strongly expressed in the intercalated ducts of the human pancreas. Their distribution correlates closely with that of CFTR, a marker of ductal electrolyte secretion. This suggests that fluid secretion is concentrated in the terminal branches of the ductal tree and that both AQP1 and AQP5 may play a significant role.

Keywords: aquaporin water channels; pancreatic duct; fluid secretion; cystic fibrosis transmembrane conductance regulator

Abbreviations: AQP, aquaporin; CCK, cholecystokinin; CFTR, cystic fibrosis transmembrane conductance regulator; PBS, phosphate buffered saline; RT-PCR, reverse transcriptase-polymerase chain reaction

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