The human bronchial cell line16HBE14o-was used as a model of airway epithelial cells to study the Ca2+-dependent Cl- secretion and the identity of KCa channels involved in the generation of a favorable driving force for Cl- exit. After ionomycin application, a calcium-activated short-circuit current (7 SC) developed, presenting a transient peak followed by a plateau phase. Both phases were inhibited to different degrees by NFA, glybenclamide and NPPB but DIDS was only effective on the peak phase. 86Rb effluxes through both apical and basolateral membranes were stimulated by calcium, blocked by charybdotoxin, clotrimazole and TPA. 1-EBIO, a SK-channel opener, stimulated 86Rb effluxes. Block of basolateral K Ca channels resulted in Tsc inhibition but, while reduced, Tsc was still observed if mucosal Cl- was lowered. Among SK family members, only SK4 and SK1 mRNAs were detected by RT-PCR. KCNQ1 mRNAs were also identified, but involvement of KcAMP channels in Cl - secretion was unlikely, since cAMP application had no effect on 86Rb effluxes. Moreover, chromanol 293B or clofilium, specific inhibitors of KCNQ1 channels, had no effect on cAMP-dependent Isc. In conclusion, two distinct components of Cl- secretion were identified by a pharmacological approach after a Cai2+ rise. KCa channels presenting the pharmacology of SK4 channels are present on both apical and basolateral membranes, but it is the basolateral SK4-like channels that play a major role in calcium-dependent chloride secretion in 16HBE14o- cells.