EXPRESSION OF CFTR AND A CAMP-STIMULATED CHLORIDE SECRETORY CURRENT IN CULTURED HUMAN FETAL ALVEOLAR EPITHELIAL-CELLS

During development the fetal lung secretes fluid that is osmotically linked to chloride (Cl-) transport. One possible pathway for Cl- secretion across the fetal pulmonary epithelium is through the cystic fibrosis transmembrane conductance regulator (CFTR). CFTR is expressed in epithelia and functions as a Cl- channel regulated by cyclic adenosine monophosphate (cAMP)-dependent protein kinase and intracellular ATP. Previous studies have shown that CFTR mRNA is expressed throughout the human fetal pulmonary epithelium and CFTR protein can be immunoprecipitated from human fetal lung homogenates. In cultured fetal lung tissue explants, CFTR mRNA was localized to alveolar epithelial cells. To test the hypothesis that fetal alveolar epithelial cells express functional CFTR, we immunolocalized CFTR in human fetal lung and looked for evidence of Cl- secretion in cultured alveolar epithelial cell monolayers. Monoclonal anti-CFTR antibodies localized CFTR in cultured lung explants to the epithelial cells, predominantly at the apical surface. Bioelectric properties of cultured monolayers of midgestation fetal alveolar epithelial cells were measured in modified Ussing chambers. In unstimulated monolayers, transepithelial electrical potential difference (Psi(t))= -1.1 +/- 0.1 mV, transepithelial resistance (R(t)) = 768 +/- 58 Omega.cm(2), and short-circuit current (I-sc) = 1.9 +/- 0.2 mu A/cm(2) (mean +/- SE, n = 17). Addition of amiloride to the apical surface significantly decreased basal I-sc. Apical diphenylamine-2-carboxylate (DPC), a Cl- channel inhibitor, caused no significant change in basal I-sc. In the presence of apical amiloride, isoproterenol significantly increased I-sc a response that was inhibited by apical DPC and submucosal bumetanide. The cAMP agonists forskolin and IBMX also stimulated I-sc. These results suggest that human fetal alveolar epithelial cells express CFTR, which mediates cAMP-stimulated transepithelial Cl- secretion.