HCO₃^- Salvage Mechanisms in the Submandibular Gland Acinar and Duct Cells

In the present work, we characterized H⁺ and HCO ₃^- transport mechanisms in the submandibular salivary gland (SMG) ducts of wild type, NHE2-/-, NHE3-/-, and NHE2-/-;NHE3-/- double knock-out mice. The bulk of recovery from an acid load across the luminal membrane (LM) of the duct was mediated by a Na⁺-dependent HOE and ethyl-isopropyl-amiloride (EIPA)-inhibitable and 4,4′ -diisothiocyanostilbene-2,2′-disulfonic acid (DIDS)-insensitive mechanism. HCO₃^- increased the rate of luminal Na ⁺-dependent pH_i recovery but did not change inhibition by HOE and EIPA or the insensitivity to DIDS. Despite expression of NHE2 and NHE3 in the LM of the duct, the same activity was observed in ducts from wild type and all mutant mice. Measurements of Na⁺-dependent OH^- and/or HCO₃^- cotransport (NBC) activities in SMG acinar and duct cells showed separate DIDS-sensitive/EIPA-insensitive and DIDS-insensitive/ EIPA-sensitive NBC activities in both cell types. Functional and immunocytochemical localization of these activities in the perfused duct indicated that pNBC1 probably mediates the DIDS-sensitive/EIPA-insensitive transport in the basolateral membrane, and splice variants of NBC3 probably mediate the DIDS-insensitive/ EIPA-sensitive NBC activity in the LM of duct and acinar cells. Notably, the acinar cell NBC3 variants transported HCO ₃^- but not OH^-. By contrast, duct cell NBC3 transported both OH^- and HCO₃^-. Accordingly, reverse transcription-polymerase chain reaction analysis revealed that both cell types expressed mRNA for pNBC1. However, the acini expressed mRNA for the NBC3 splice variants NBCn1C and NBCn1D, whereas the ducts expressed mRNA for NCBn1B. Based on these findings we propose that the luminal NBCs in the HCO ₃^- secreting SMG acinar and duct cells function as HCO₃^- salvage mechanisms at the resting state. These studies emphasize the complexity but also begin to clarify the mechanism of HCO₃^- homeostasis in secretory epithelia.