Cystic fibrosis transmembrane conductance regulator regulates luminal Cl-/HCO3- exchange in mouse submandibular and pancreatic ducts

We have demonstrated previously the regulation of Cl-/HCO3- exchange activi ty by the cystic fibrosis transmembrane conductance regulator (CFTR) in mod el systems of cells stably or transiently transfected with CFTR (Lee, M. G. , Wigley, W. C., Zeng, W., Noel, L. E., Marino, C, R,, Thomas, P. J., and M uallem, S, (1999) J. Biol. Chem. 274, 3414-3421), In the present work we ex amine the significance of this regulation in cells naturally expressing CFT R. These include the human colonic T84 cell line and the mouse submandibula r gland and pancreatic ducts, tissues that express high levels of CFTR in t he luminal membrane. As in heterologous expression systems, stimulation of T84 cells with forskolin increased the Cl-/HCO3- exchange activity independ ently of CFTR Cl- channel activity. Freshly isolated submandibular gland du cts from wild type mice showed variable Cl-/HCO3- exchange activity. Measur ement of [Cl-](i) revealed that this was largely the result of variable ste ady-state [Cl-](i). Membrane depolarization with 5 mM Ba2+ or 100 mM K+ inc reased and stabilized [Cl-](i). Under depolarized conditions wild type and Delta F/Delta F mice had comparable basal Cl-/HCO3- exchange activity. Nota bly, stimulation with forskolin increased Cl-/HCO3- exchange activity in su bmandibular gland ducts from wild type but not Delta F/Delta F mice. Microp erfusion of the main pancreatic duct showed Cl-/HCO3- exchange activity in both the basolateral and luminal membranes. Stimulation of ducts from wild type animals with forskolin had no effect on basolateral but markedly stimu lated luminal Cl-/HCO3- exchange activity. By contrast, forskolin had no ef fect on either basolateral or luminal Cl-/HCO3- exchange activity of ducts from Delta F/Delta F animals. We conclude that CFTR regulates luminal Cl-/H CO3- exchange activity in CFTR-expressing cells, and we discuss the possibl e physiological significance of these findings regarding cystic fibrosis.