Immunolocalization of aquaporin-8 in rat digestive organs and testis

Expression of aquaporin-8 mRNA has previously been shown in hepatocytes, pa ncreatic acinar cells, colon epithelial cells and seminiferous tubules of t he testis in the rat by in situ hybridization technique. However, immunoloc alization of this water channel has not yet been demonstrated. In the prese nt study, the localization of immunoreactive aquaporin-8 and expression of the mRNA were examined in rat organs (cerebrum, cerebellum, eve, salivary g land, heart, lung, liver, pancreas, esophagus, stomach. jejunum, ileum, col on, testis, ovary, kidney, spleen and lymphnode) by immunohistochemistry us ing an antibody against aquaporin-8 and ribonuclease protection assay. Aqua porin-8 was distinctly immunolocalized on the apical membranes of pancreati c acinar cells and mucosal epithelium of the colon and jejunum, In the live r, the bile canalicular membrane of hepatocytes was immunostained. In the t estis, immunoreactive aquaporin-8 was demonstrated on the luminal side of t he seminiferous tubules, At high magnification, the peroxidase reaction pro ducts appeared on the ramified cytoplasmic membrane of Sertoli cells surrou nding the residual bodies or spermatogenic cells. Specificity of the antibo dy was verified by Western blot analysis showing a minor similar to 28 kDa band (deduced deglycosylated form of aquaporin-8) and a major similar to 30 kDa band (glycosylated form) in these organs. The intensity of aquaporin-8 immunoreactivity was approximately comparable to that of aquaporin-8 mRNA expression in the liver, pancreas, colon, jejunum and testis, The aquaporin -8 mRNA expression in the hepatocytes was presumed to be closely associated with the structure of bile canaliculi since the message was detected in he patocytes immediately after isolation from the liver but not in cells follo wing cultivation for three days. The localization of immunoreactive aquaporin-8 indicated functions for this water channel in the secretion of bile and pancreatic juice, and the secre tion or absorption of water in the colon and jejunum, and the maturation or liberation of spermatogenic cells in the testis.