Expression of the aquaporin 8 water channel in a rat salivary epithelial cell.

Aquaporins are a family of water channels considered to play an important role in fluid transport across plasma membranes. Among the reported isoforms, relatively little is known about the functional role of aquaporin 8 (AQP8), and there are no cell lines known to express the AQP8 protein. We report here that the rat submandibular epithelial cell line, SMIE, expresses AQP8. Using RT-PCR, the presence of mRNA for AQP8 was demonstrated in these cells. Confocal immunofluorescence experiments revealed that the AQP8 protein is primarily present in the apical membranes of SMIE cells. When grown as a polarized monolayer on collagen coated polycarbonate filters, and exposed on their apical surface to different hyperosmotic (440, 540, or 640 mOsm) solutions, net fluid movement across SMIE cells was 8-25-fold that seen under isosmotic conditions. Similarly, when grown on coverslips and then exposed to a hypertonic solution, SMIE cells shrunk as a function of time. Together, these results suggest that SMIE cells endogenously express functional AQP8 water channels.