Na+, K+, and H+/HCO3- transport in submandibular salivary ducts. Membrane localization of transporters.

The mechanisms mediating transepithelial ion transport in salivary ducts were characterized and localized by studying the regulation of [Na+]i, [K+]i, and pHi in isolated intralobular ducts and perfused main ducts of the submandibular salivary gland. A new procedure was developed for the rapid preparation of intralobular ducts. Measurements of pHi revealed the presence of Na+/H+ and Cl-/HCO3- exchange activities in intralobular duct cells. We could not obtain evidence for a coupled K+/H+ exchange activity which was postulated to exist in the luminal membrane of duct cells. Rather, a Kout+-dependent pathway which mediates the transport of H+/HCO3- and Na+ was found. This pathway was absent from acinar cells of the same gland and was active in unstimulated duct cells incubated in 5mM Kout+. Accordingly, inhibition of the Na+ pump with ouabain resulted in rapid and large Na+ influx in duct but not acinar cells. Perfusion experiments with the experimentally accessible main duct and measurements of pHi were used to provide the first direct localization of ion transporters in salivary ducts. The luminal and basolateral membranes of the duct express separate Na+/N+ and Cl-/HCO3- exchangers. Na+/H+ exchange activity in both membranes was similar, whereas the luminal Cl-/HCO3- exchange activity was higher than that in the basolateral membrane. The perfused main dust was also used to localize the newly discovered Kout(+)-dependent H+/HCO3- and Na+ transport pathway to the luminal membrane, which suggests that this pathway may play an important role in Na+ reabsorption of K+ and HCO3- secretion by the salivary ductal system.