Immunolocalization of electroneutral Na⁺-HCO₃^- cotransporters in human and rat salivary glands

Patterns of salivary HCO₃ secretion vary widely among species and among individual glands. In particular, virtually nothing is known about the molecular identity of the HCO₃^- transporters involved in human salivary secretion. We have therefore examined the distribution of several known members of the Na⁺-HCO₃^- cotransporter (NBC) family in the parotid and submandibular glands. By use of a combination of RT-PCR and immunoblotting analyses, the electroneutral cotransporters NBC3 and NBCn1 mRNA and protein expression were detected in both human and rat tissues. Immunohistochemistry demonstrated that NBC3 was present at the apical membranes of acinar and duct cells in both human and rat parotid and submandibular glands. NBCn1 was strongly expressed at the basolateral membrane of striated duct cells but not in the acinar cells in the human salivary glands, whereas little or no NBCn1 labeling was observed in the rat salivary glands. The presence of NBCn1 at the basolateral membrane of human striated duct cells suggests that it may contribute to ductal HCO₃^- secretion. In contrast, the expression of NBC3 at the apical membranes of acinar and duct cells in both human and rat salivary glands indicates a possible role of this isoform in HCO₃^- salvage under resting conditions.