Examination of rat salivary glands for the presence of the aquaporin CHIP

The aquaporin CHIP, AQP-CHIP, is a 28-kDa integral membrane protein that functions as a water channel in kidney and red blood cells. We have examined rat salivary glands for the presence of this molecule. In Northern blot analyses an AQP-CHIP cDNA hybridized to rat kidney mRNA approximately 20-fold more than to mRNA from the rat parotid gland. Western blot analyses, using antisera against human AQP-CHIP, showed that AQP-CHIP was present in crude membranes from rat salivary glands to a much lesser extent than in rat kidney membranes. A 2.8 kb cDNA encoding AQP-CHIP was also isolated from an adult rat parotid gland cDNA library. Immunolocalization studies using thin and ultrathin cryosections and light and electron microscopy, however, revealed that no glandular epithelium was labeled. In contrast, significant immunolabeling was present in microvascular endothelial cells. Non-fenestrated endothelia of capillaries and venules exhibited labeling on both their apical and basolateral membranes. The present study demonstrates that AQP-CHIP is not involved directly in fluid secretion from salivary epithelial cells. Further, this study provides strong evidence that AQP-CHIP is an endothelial cell water channel found within salivary gland tissue which may contribute to water permeability in the capillary beds.