Expression of the Na⁺-HCO₃^- cotransporter and its role in pH_i regulation in guinea pig salivary glands

Patterns of salivary HCO₃^- secretion vary and depend on species and gland types. However, the identities of the transporters involved in HCO₃^- transport and the underlying mechanism of intracellular pH (pH_i) regulation in salivary glands still remain unclear. In this study, we examined the expression of the Na⁺- HCO₃^- cotransporter (NBC) and its role in pH_i regulation in guinea pig salivary glands, which can serve as an experimental model to study HCO₃^- transport in human salivary glands. RT-PCR, immunohistochemistry, and pH_i measurements from BCECF-AM-loaded cells were performed. The amiloride-sensitive Na ⁺/H⁺ exchanger (NHE) played a putative role in pH _i regulation in salivary acinar cells and also appeared to be involved in regulation in salivary ducts. In addition to NHE, NBC also played a role in pH_i regulation in both acini and ducts. In the parotid gland, NBC1 was functionally expressed in the basolateral membrane (BLM) of acinar cells and the luminal membrane (LM) of ducts. In the submandibular gland, NBC1 was expressed only in the BLM of ducts. NBC1 expressed in these two types of salivary glands takes up HCO₃^- and is involved in pH _i regulation. Although NBC3 immunoreactivity was also detected in submandibular gland acinar cells and in the ducts of both glands, it is unlikely that NBC3 plays any role in pH_i regulation. We conclude that NBC1 is functionally expressed and plays a role in pH_i regulation in guinea pig salivary glands but that its localization and role are different depending on the type of salivary glands.