CALCIUM-STIMULATED CL- SECRETION IN CALU-3 HUMAN AIRWAY CELLS REQUIRES CFTR

Human airway serous cells secrete antibiotic-rich fluid, but, in cysti c fibrosis (CF), Cl--dependent fluid secretion is impaired by defects in CF transmembrane conductance regulator (CFTR) Cl- channels. Typical ly, CF disrupts adenosine 3',5'-cyclic monophosphate (cAMP)-mediated C l- 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 tha psigargin to elevate intracellular Ca2+ concentration ([Ca2+](i)) in C alu-3 cells, an airway cell line bearing some similarities to serous c ells. 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 stimu lated variable and smaller increases. Delta I-sc was decreased by baso lateral bumetanide, quinidine, barium, or diphenyl-amine-2-carboxylate (DPAC) but was unaffected by high apical concentrations of 4,4'-diiso thiocyanostilbene-2,2'-disulfonic acid (DIDS), 4,4'-dinitrostilbene-2, 2'-disulfonic acid, and calixarene. I-sc was measured after permeabili zing the basolateral membrane and establishing transmembrane ion gradi ents. 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 i n the apical membrane and display similar to 60% of maximum conductanc e at rest. Thus elevated [Ca2+](i) increases K+ conductance to force C l- through open CFTR channels. We hypothesize that loss of CFTR channe ls causes diminution of cholinergically mediated gland secretions in C F.