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 (Δ) in I sc and G te, whereas forskolin stimulated variable and smaller increases. Δ I sc was decreased by basolateral bumetanide, quinidine, barium, or diphenylamine-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 ∼60% of maximum conductance at rest. Thus elevated [Ca2+]iincreases 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.