Zhao H, Xu X, Diaz J, Muallem S
Department of Physiology, University of Texas Southwestern Medical Center, Dallas 75235, USA.
J Biol Chem. 1995 Aug 18;270(33):19599-605.
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.
通过研究下颌下唾液腺孤立小叶内导管和灌注主导管中[Na⁺]i、[K⁺]i和pHi的调节，对唾液腺导管中跨上皮离子转运的机制进行了表征和定位。开发了一种快速制备小叶内导管的新方法。pHi的测量揭示了小叶内导管细胞中存在Na⁺/H⁺和Cl⁻/HCO₃⁻交换活性。我们未能获得证据证明导管细胞腔膜中存在推测的耦联K⁺/H⁺交换活性。相反，发现了一种依赖细胞外K⁺的途径，该途径介导H⁺/HCO₃⁻和Na⁺的转运。同一腺体的腺泡细胞中不存在该途径，且在5mM细胞外K⁺中孵育的未受刺激的导管细胞中该途径具有活性。因此，用哇巴因抑制Na⁺泵会导致导管细胞而非腺泡细胞中快速且大量的Na⁺内流。利用可实验操作的主导管进行灌注实验和pHi测量，首次对唾液腺导管中的离子转运体进行了直接定位。导管的腔膜和基底外侧膜表达不同的Na⁺/N⁺和Cl⁻/HCO₃⁻交换体。两种膜中的Na⁺/H⁺交换活性相似，而腔膜中的Cl⁻/HCO₃⁻交换活性高于基底外侧膜。灌注的主导管还用于将新发现的依赖细胞外K⁺的H⁺/HCO₃⁻和Na⁺转运途径定位到腔膜，这表明该途径可能在唾液腺导管系统的Na⁺重吸收、K⁺和HCO₃⁻分泌中起重要作用。
