Calcium-stimulated Cl- secretion in Calu-3 human airway cells requires CFTR

Human airway serous cells secrete antibiotic-rich fluid, but, in cystic fibrosis (CF), Cl--dependent fluid secretion is impaired by defects in CF transmembrane conductance regulator (CFTR) Cl- channels. Typically, CF disrupts adenosine 3',5'-cyclic monophosphate (cAMP)-mediated Cl- secretion but spares Ca2+-mediated secretion. However, in CF airway glands, Ca2+-mediated secretion is also greatly reduced. To determine the basis of Ca2+-mediated Cl- secretion in serous cells, we used thapsigargin to elevate intracellular Ca2+ concentration ([Ca2+](i)) in Calu-3 cells, an airway cell line bearing some similarities to serous cells. Cells were cultured using conventional and air interface methods. Short-circuit current (I-sc) and transepithelial conductance (G(te)) were measured in confluent cell layers. Thapsigargin stimulated large, sustained changes (Delta) in I-sc and G(te), whereas forskolin stimulated variable and smaller increases. Delta I-sc was decreased by basolateral bumetanide, quinidine, barium, or diphenyl-amine-2-carboxylate (DPAC) but was unaffected by high apical concentrations of 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS), 4,4'-dinitrostilbene-2,2'-disulfonic acid, and calixarene. I-sc was measured after permeabilizing the basolateral membrane and establishing transmembrane ion gradients. Unstimulated apical membranes displayed high Cl- conductance (G(Cl)) that was decreased by DPAC but not by DIDS. Apical G(Cl) could be increased by elevating intracellular cAMP concentration but not [Ca2+](i). We conclude that CFTR channels are the exclusive G(Cl) pathway in the apical membrane and display similar to 60% of maximum conductance at rest. Thus elevated [Ca2+](i) increases K+ conductance to force Cl- through open CFTR channels. We hypothesize that loss of CFTR channels causes diminution of cholinergically mediated gland secretions in CF.