Immunolocalization of aquaporin homologs in mouse lacrimal glands.

It was shown recently that the aquaporin family of water channels exists in lacrimal gland cells. To determine the localization of aquaporin homologs and the pathophysiological modification of aquaporins by pilocarpine, a muscarinic agonist, we performed immunohistochemistry and immunoblotting in mouse lacrimal glands. By immunohistochemistry, aquaporin-4 (AQP4) and aquaporin-5 (AQP5) were found to be the lacrimal glands. AQP5 immunolabeling was detected in the apical membranes of the acinus and duct cells, while AQP4 was expressed in the basolateral membranes only. The tear secretion of mice systemically treated with pilocarpine was significantly (2.5-fold) higher than that of the saline-treated controls. The antibody to the AQP5 carboxy terminus showed high immunoreactivity on the apical membrane in the pilocarpine-treated lacrimal glands but not on that of saline-treated controls. However, the antibody to the extracellular domain of AQP5 showed similar immunolabeling in both groups of animals. In contrast, the immunoreactivity of AQP4 was not affected by pilocarpine stimulation. As shown by western blot analysis, the expression level of AQP5 on the apical membrane in the pilocarpine-stimulated lacrimal glands was not significantly different compared with the saline-treated controls. We conclude that AQP4 and AQP5 water channels are expressed on mouse lacrimal gland cells, with greater expression of AQP4 on the basolateral membrane and of AQP5 on the apical membrane. Furthermore, the AQP5 carboxy terminus region may undergo pathophysiological modification when tear secretion is increased by pilocarpine stimulation.