Immunolocalization of anion exchanger AE2 and Na⁺-HCO^-₃ cotransporter in rat parotid and submandibular glands

Salivary glands secrete K⁺ and HCO^-₃ and reabsorb Na⁺ and Cl^-, but the identity of transporters involved in HCO^-₃ transport remains unclear. We investigated localization of Cl^-/HCO^-₃ exchanger isoform AE2 and of Na⁺-HCO^-₃ cotransporter (NBC) in rat parotid gland (PAR) and submandibular gland (SMG) by immunoblot and immunocytochemical techniques. Immunoblotting of PAR and SMG plasma membranes with specific antibodies against mouse kidney AE2 and rat kidney NBC revealed protein bands at ~160 and 180 kDa for AE2 and ~130 kDa for 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 revealed specific basolateral staining of PAR acinar cells for AE2 and NBC, but in SMG acinar cells only basolateral AE2 labeling was observed. No AE2 expression was detected in any ducts. Striated, intralobular, and main duct cells of both glands showed NBC expression predominantly at basolateral membranes, with some cells being apically stained. In SMG duct cells, NBC staining exhibited a gradient of distribution from basolateral localization in more proximal parts of the ductal tree to apical localization toward distal parts of the ductal tree. Both immunoblotting signals and immunostaining were abolished in preabsorption experiments with the respective antigens. Thus the mechanisms of fluid and anion secretion in salivary acinar cells may be different between PAR and SMG, and, because NBC was detected in acinar and duct cells, it may play a more important role in transport of HCO^-₃ by rat salivary duct cells than previously believed.