EXAMINATION OF RAT SALIVARY-GLANDS FOR THE PRESENCE OF THE AQUAPORIN CHIP

The aquaporin CHIP, AQP-CHIP, is a 28-kDa integral membrane protein th at functions as a water channel in kidney and red blood cells. We have examined rat salivary glands for the presence of this molecule. In No rthern blot analyses an AQP-CHIP cDNA hybrid ized to rat kidney mRNA a pproximately 20-fold more than to mRNA from the rat parotid gland. Wes tern 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 encodi ng AQP-CHIP was also isolated from an adult rat parotid gland cDNA lib rary. Immunolocalization studies using thin and ultrathin cryosections and light and electron microscopy, however, revealed that no glandula r 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 an d basolateral membranes. The present study demonstrates that AQP-CHIP is not involved directly in fluid secretion from salivary epithelial c ells. Further, this study provides strong evidence that AQP-CHIP is an endothelial cell water channel found within salivary gland tissue whi ch may contribute to wa ter permeability in the capillary beds.