Functional polarity of Na+/H+ and Cl-/HCO3- in a rat cholangiocyte cell line

Intrahepatic bile duct cells (cholangiocytes) play an important role in the secretion and alkalinization of bile. Both Na+/H+ exchange (NHE) and Cl-/H CO3- exchange (AE) contribute to these functions, but their functional dist ribution between the apical and basolateral membrane domains remains specul ative. We have addressed this issue in a normal rat cholangiocyte cell line (NRC-1), which maintains a polarized distribution of membrane markers. Gen e expression of AE and NHE isoforms was studied by RT-PCR. For functional s tudies, cells were placed in a chamber that allowed separate perfusion of t he apical and basolateral aspect of the epithelial sheet; intracellular pH (pH(i)) was measured by 2',7'-bis(2-carboxyethyl)-5(6)-carboxyfluorescein m icrofluorometry. In HCO3--CO(2)free medium and in the presence of apical am iloride, pH(i) recovery from an acid load was Na+ dependent and was inhibit ed by basolateral amiloride and by HOE-642 (10 mu M), consistent with basol ateral localization of the NHE1 isoform, which had clearly expressed mRNA. Apical Na+ readmission induced a slow pH(i) recovery that was inhibited by apical administration of 1 mM HOE-642 or amiloride. Among the apical NHE is oforms, NHE2 but not NHE3 gene expression was detected. The AE1 gene was no t expressed, but two different variants of AE2 mRNAs (AE2a and AE2b) were d etected; pH(i) experiments disclosed AE activities at both sides of the mem brane, but only apical AE was activated by cAMP. In conclusion, these studi es provide the first functional description of acid-base transporters in a polarized cholangiocyte cell line. NHE1, NHE2, AE2a, and AE2b isoforms are expressed and show different membrane polarity, functional properties, and sensitivity to inhibitors. These observations add a considerable level of c omplexity to current models of electrolyte transport in cholangiocytes.