Aquaporins in rat pancreatic interlobular ducts

The aquaporin (AQP) family of water channels is distributed ubiquitously in many epithelia and plays a fundamental role in transmembrane water transport. The aim of this study is to identify the water transport pathway in pancreatic duct cells where most of the HCO₃^--rich fluid originates. Using digital videomicroscopy, we measured the osmotic water permeability (P_f) of pancreatic duct epithelium by exposing isolated rat interlobular ducts to the hypotonic solution (145 mosM). To identify mRNA and protein of AQPs expressed in duct cells, we conducted RT-PCR analysis and immunohistochemistry of the isolated duct and pancreas. The calculated P_f (160-230 μm/s) of the isolated ducts was significantly reduced to 16-35 μm/s by 80-90% with either basolateral or luminal applications of HgCl₂. Fluid secretion evoked by secretin was almost completely abolished by a basolateral or luminal application of HgCl₂. A large amount of AQP1 and small amounts of AQP5 transcripts were detected in the isolated duct cells by RT-PCR. AQP1, but not AQP5, immunoreactivity was present in both luminal and basolateral membranes of the interlobular duct cells. Mercury-sensitive water channels are present in both luminal and basolateral membranes of rat pancreatic ducts. AQP1 of the known AQPs appears to be the main water pathway in interlobular ducts.