Analysis of the expression of aquaporin-1 and aquaporin-9 in pig liver tissue: comparison with rat liver tissue.

Aquaporins (AQPs) are cellular proteins involved with the movement of water across cell membranes and are fundamentally important to the fluid transport in the bile ducts and ductules of the liver. An immunohistochemical analysis of AQP-1 and AQP-9 was undertaken to describe their expression in fetal and adult pig liver, while immunoreagents specific to some other AQPs were screened for their efficacy on pig liver tissues. Anti-AQP-1 antibody reacted with the bile duct of the portal space and the bile ductules at the periphery of the liver lobules. Histological identification of bile ductules was confirmed by positive reactivity with anti-cytokeratin-7 and antilaminin immunostaining. Anti-AQP-1 signals were also pronounced in the endothelium of the portal space blood vessels and peripheral distributing venules. Antibody to AQP-9 reacted strongly with small ductules peripheral to the liver lobules, but only weakly with the bile ducts of the portal space. Anti-AQP-adipose antibody bound to the smooth muscle cells of the arteries in the portal space and sporadically with certain binucleated cells in the liver lobule. Antibodies to AQP-3, AQP-4, AQP-7, and AQP-8 were nonreactive with any of the tissues of the adult pig liver. For comparative purposes, immunohistochemical analysis of rat liver tissue was done with the anti-AQP-1 and AQP-9 antibodies. Anti-AQP-1 reacted weakly with the rat liver's bile ducts, but robustly with the endothelium of the liver's veins and arteries. It also reacted strongly with the central vein of the rat liver lobules, and, because the staining was continuous with hepatic sinusoids, it appeared that the reactivity was specific to the endothelial cells. Anti-AQP-9 antibodies reacted with rat hepatocytes and was not associated with the canaliculi, as judged by concurrent phalloidin staining of actin. The results indicate that specific AQPs are expressed in the tissues of the pig liver and that AQP-9 expression is distinct from its expression in the rat liver.