Permeabilization via the P2X7 purinoreceptor reveals the presence of a Ca2+-activated Cl- conductance in the apical membrane of murine tracheal epithelial cells

Calcium-activated Cl- secretion is an important modulator of regulated ion transport in murine airway epithelium and is mediated by an unidentified Ca2+-stimulated Cl- channel. We have transfected immortalized murine tracheal epithelial cells with the cDNA encoding the permeabilizing P2X(7) purinoreceptor (P2X(7)-R) to selectively permeabilize the basolateral membrane and thereby isolate the apical membrane Ca2+-activated Cl- current. In P2X(7)-R-permeabilized cells, we have demonstrated that UTP stimulates a Cl- current across the apical membrane of CF and normal murine tracheal epithelial cells. The magnitude of the UTP-stimulated current was significantly greater in CF than in normal cells. Ion substitution studies demonstrated that the current exhibited a permselectivity sequence of Cl- > I- > Br- > gluconate(-). We have also determined a rank order of potency for putative Cl- channel blockers: niflumic acid greater than or equal to 5-nitro-2-(3-phenylpropylamino)benzoic acid > 4,4'-diisothiocyanostilbene-2,2'-disulfonate > glybenclamide much greater than diphenlyamine-2-carboxylate, tamoxifen, and p-tetra-sulfonato-tetra-methoxy-calix[4]arene. Complete characterization of this current and the corresponding single channel properties could lead to the development of a new therapy to correct the defective airway surface liquid in cystic fibrosis patients.