Expression and immunolocalization of aquaporin water channels in rat exocrine pancreas

Both the acinar and ductal cells of the pancreas secrete a near-isotonic fl uid and may thus be sites of aquaporin (AQP) water channel expression. Nort hern blot analysis of mRNA from whole rat pancreas revealed high levels of AQP1 and AQP8 expression, whereas lower levels of AQP4 and AQP5 expression were just detectable by RT-PCR Southern blot analysis. Immunohistochemistry showed that AQP1 is localized in the microvasculature, whereas AQP8 is con fined to the apical pole of the acinar cells. No labeling of acinar, ductal , or vascular tissue was detected with antibodies to AQP2-7. With immunoele ctron microscopy, AQP8 labeling was observed not only at the apical membran e of the acinar cells but also among small intracellular vesicles in the su bapical cytoplasm, suggesting that there may be regulated trafficking of AQ P8 to the apical plasma membrane. To evaluate the contribution of AQPs to t he membrane water permeability, video microscopy was used to measure the sw elling of acinar cells in response to hypotonic stress. Osmotic water perme ability was reduced by 90% following exposure to Hg2+. Since AQP8 is confin ed to the apical membrane, the marked effect of Hg2+ suggests that other wa ter channels may be expressed in the basolateral membrane.