Identification and localization of aquaporin water channels in human salivary glands

Aquaporin (AQP) water channels are expressed in a variety of fluid-transpor ting epithelia and are likely to play a significant role in salivary secret ion. Our aim was to identify and localize the aquaporins expressed in human salivary glands. Total RNA was extracted from human parotid, submandibular , sublingual, and labial glands and from human brain. Expression of aquapor in mRNA was assessed by RT-PCR using specific primers for human AQP1, AQP3, AQP4, and AQP5. All four aquaporins were detected by RT-PCR in all of the glands, and the sequences were confirmed after further amplification with n ested primers. Cleaned PCR products were then used as P-32-labeled cDNA pro bes in a semiquantitative Northern blot analysis using glyceraldehyde-3-pho sphate dehydrogenase as reference. Only AQP1, AQP3, and AQP5 mRNAs were pre sent at significant levels. AQP localization was determined by immunohistoc hemistry on paraffin sections using affinity-purified primary antibodies an d peroxidase-linked secondary antibodies. Each salivary gland type showed a broadly similar staining pattern: AQP1 was localized to the capillary endo thelium and myoepithelial cells; AQP3 was present in the basolateral membra nes of both mucous and serous acinar cells; AQP4 was not detected; and AQP5 was expressed in the luminal and canalicular membranes of both types of ac inar cell. We conclude that AQP3 and AQP5 together may provide a pathway fo r transcellular osmotic water flow in the formation of the primary saliva.