Immunolocalization of the water channel, aquaporin-5 (AQP5), in the rat digestive system.

Aquaporin-5 (AQP5), an isoform of membrane water channel aquaporins, is expressed in the salivary and lacrimal glands. We surveyed the expression and immunohistochemical localization of AQP5 in the rat digestive system. RT-PCR analysis revealed that AQP5 is expressed in the submandibular gland, tongue, gastric corpus, pyloric region, duodenum, and liver. Immunofluorescence microscopy using AQP5-specific antibodies showed that AQP5 protein is present in the minor salivary glands of the tongue, the pyloric glands, and duodenal glands. To distinguish apical and basolateral domains of the plasma membrane of epithelial cells, double-immunofluorescence staining for AQP5 and tight junction protein occludin was performed. In the minor salivary gland, AQP5 was present in both the serous and mixed secretory end portions. AQP5 was found in the apical membrane of the secretory cells including intercellular secretory canaliculi demarcated with occludin. At higher magnifications, omega-shaped indentations of AQP5 labeling were seen along the apical membrane, suggesting a dynamic process for the apical membrane in exocytosis. Only weak labeling for AQP5 was detected in the basolateral domain. In the stomach, AQP5 was detected in the apical membrane of the pyloric gland secretory cells. In the duodenum, AQP5 was restricted to duodenal glands, where it was localized to the apical membrane. AQP5 was not detected in the intestinal glands or cells in the villi. These observations show that AQP5 is localized mainly in the apical membrane, including intercellular secretory canaliculi of secretory cells in the minor salivary glands, pyloric glands, and duodenal glands. AQP5 appears to play an important role in water transfer in these glands.