Functional coupling of apical Cl-/HCO3- exchange with CFTR in stimulated HCO3- secretion by guinea pig interlobular pancreatic duct
A. K Stewart,
S. L Alper
Pancreatic ductal epithelium produces a HCO3–-rich fluid. HCO3– transport across ductal apical membranes has been proposed to be mediated by both SLC26-mediated Cl–/HCO3– exchange and CFTR-mediated HCO3– conductance, with proportional contributions determined in part by axial changes in gene expression and luminal anion composition. In this study we investigated the characteristics of apical Cl–/HCO3– exchange and its functional interaction with Cftr activity in isolated interlobular ducts of guinea pig pancreas. BCECF-loaded epithelial cells of luminally microperfused ducts were alkalinized by acetate prepulse or by luminal Cl– removal in the presence of HCO3–-CO2. Intracellular pH recovery upon luminal Cl– restoration (nominal Cl–/HCO3– exchange) in cAMP-stimulated ducts was largely inhibited by luminal dihydro-DIDS (H2DIDS), accelerated by luminal CFTR inhibitor inh-172 (CFTRinh-172), and was insensitive to elevated bath K+ concentration. Luminal introduction of CFTRinh-172 into sealed duct lumens containing BCECF-dextran in HCO3–-free, Cl–-rich solution enhanced cAMP-stimulated HCO3– secretion, as calculated from changes in luminal pH and volume. Luminal Cl– removal produced, after a transient small depolarization, sustained cell hyperpolarization of ~15 mV consistent with electrogenic Cl–/HCO3– exchange. The hyperpolarization was inhibited by H2DIDS and potentiated by CFTRinh-172. Interlobular ducts expressed mRNAs encoding CFTR, Slc26a6, and Slc26a3, as detected by RT-PCR. Thus Cl–-dependent apical HCO3– secretion in pancreatic duct is mediated predominantly by an Slc26a6-like Cl–/HCO3– exchanger and is accelerated by inhibition of CFTR. This study demonstrates functional coupling between Cftr and Slc26a6-like Cl–/HCO3– exchange activity in apical membrane of guinea pig pancreatic interlobular duct.