Immunolocalization of anion exchanger AE2 and Na+-HCO3- cotransporter in rat parotid and submandibular glands

Salivary glands secrete K+ and HCO3- and reabsorb Na+ and Cl-, but the iden tity of transporters involved in HCO3- transport remains unclear. We invest igated localization of Cl-/HCO3- exchanger isoform AE2 and of Na+-HC3-, cot ransporter (NBC) in rat parotid gland (PAR) and submandibular gland (SMG) b y immunoblot and immunocytochemical techniques. Immunoblotting of PAR and S MG plasma membranes with specific antibodies against mouse kidney AE2 and r at kidney NBC revealed protein bands at similar to 160 and 180 kDa for AE2 and similar to 130 kDa far NBC, as expected for the AE2 full-length protein and consistent with the apparent molecular mass of NBC in several tissues other than kidney. Immunostaining of fixed PAR and SMG tissue sections reve aled specific basolateral staining of PAR acinar cells for AE2 and NBC, but in SMG acinar cells only basolateral AE2 labeling was observed. No AE2 exp ression was detected in any ducts. Striated, intralobular, and main duct ce lls of both glands showed NBC expression predominantly at basolateral membr anes, with some cells being apically stained. In SMG duct cells, NBC staini ng exhibited a gradient of distribution from basolateral localization in mo re proximal parts of the ductal tree to apical localization toward distal p arts of the ductal tree. Both immunoblotting signals and immunostaining wer e abolished in preabsorption experiments with the respective antigens. Thus the mechanisms of fluid and anion secretion in salivary acinar cells may b e different between PAR and SMG, and, because NBC was detected in acinar an d duct cells, it may play a more important role in transport of HCO3- by ra t salivary duct cells than previously believed.