Evidence for location of the CFTR in human placental apical membrane vesicles

Ion transport (36Cl uptake) and immunochemical studies were undertaken to detect the cystic fibrosis transmembrane conductance regulator (CFTR) in apical membrane vesicles prepared from human placenta. 36Cl uptake into membrane vesicles was studied in the absence and presence of inwardly directed potassium gradients and valinomycin (Ko = Ki and Ko > Ki, where Ko is potassium concentration outside and Ki is potassium concentration inside the vesicles). The sensitivities of 36Cl uptake to the inhibitors 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS), bumetanide, and diphenylamine-2-carboxylate were investigated. Each compound significantly inhibited uptake under both sets of conditions. Additional inhibition of 36Cl uptake was found when the compounds were added together, indicating that they were acting at least partly on different components of the 36Cl uptake. The DIDS- and bumetanide-insensitive component of transport was more selective for Cl than I. These findings suggested that this component may, at least in part, represent Cl transport via CFTR. Addition of adenosine 5'-O-(3-thiotriphosphate) (0.8 mM) led to a decrease in total 36Cl uptake but masked in the overall decrease was an increase in the DIDS- and bumetanide-insensitive component of 36Cl uptake. Western blot analysis of the apical membrane proteins with an antibody specific for a region of human CFTR detected a protein band of approximately 190 kDa. These ion transport and immunochemical studies provide evidence that CFTR is located in human placental apical membrane vesicles.