| Anion selectivity of the cystic fibrosis conductance transmembrane conductance regulator ( CFTR ) and other channels and parallel pathways expressed endogenously in apical membranes of polarized Calu 3 epithelial monolayers was studied under control conditions and during cAMP stimulation . ^^^ Thus , CFTR appears to be the major apical anion conductance pathway in Calu 3 cells , and its conduction properties are independent of the expression system . ^^^ CFTR in Calu 3 cells also conducts physiologically relevant anions , but not ATP , PO 4 or SO 4 . ^^^ |