A large conductance, Ca²⁺-activated K⁺ channel in a human lung epithelial cell line (A549)

A large conductance, Ca²⁺-activated K⁺ channel in a human lung epithelial cell line (A549) was identified using the single channel patch clamp technique. Channel conductance was 242 ± 33 pS (n = 67) in symmetrical KCl (140 mM). The channel was activated by membrane depolarization and increased cytosolic Ca²⁺. High selectivity was observed for K⁺ over Rb⁺(0.49)> Cs⁺(0.14)> Na⁺(0.09). Open probability was significantly decreased by Ba²⁺ (5 mM) and quinidine (5 mM) to either surface, but TEA (5 mM) was only effective when added to the external surface. All effects were reversible. Increasing cytosolic Ca²⁺ concentration from 10^-7 to 10^-6 M caused an increase in open probability from near zero to fully activated. ATP decreased open probability at ~2 mM, but the effect was variable. The channel was almost always observed together with a smaller conductance channel, although they could both be seen individually. We conclude that A549 cells contain large conductance Ca²⁺-activated K⁺ channels which could explain a major fraction of the Kt conductance in human alveolar epithelial membranes.