Calu-3 human airway epithelial cells express intermediate conductance Ca²⁺-activated potassium channels that are important in regulating HCO₃^– and Cl^– secretion (Devor et al. 1999). Here we characterise recombinant human IK (hIK) channels, and compare these properties with those of an endogenous Ca²⁺-activated K⁺ current in a second human airway epithelial cell line, NCI-H292.

Electrophysiological recordings were made from a Chinese hamster ovary (CHO) hIK stable cell line and from NCI-H292 cells using conventional whole-cell patch-clamp techniques under symmetrical K⁺ conditions (144 mM K⁺). Internal (pipette) free Ca²⁺ was buffered at 1 µM unless otherwise stated. Currents were recorded using voltage ramps (1 s, -100 to +70 mV). Data are presented as means ± S.E.M. or geomean [95 % CI], n = 3-7. A large Ca²⁺-activated inwardly rectifying K⁺ current was observed in both CHOhIK (695 pA pF^-1 at -100 mV) and H292 (350 pA pF^-1 at -100 mV) cells which in each case reversed close to the calculated E_K (0 mV). Charybdotoxin (ChTx) and clotrimazole reversibly inhibited the hIK current with IC₅₀ values of 21 nM [12-37] and 24 nM [15-40], respectively. The H292 current was markedly reduced (98 ± 1 %) by 100 nM ChTx. The ChTx-insensitive current reversed at -11 ± 2 mV, suggesting the presence of a small Cl^– conductance. The selective BK blocker iberiotoxin (100 nM) reduced the CHOhIK and H292 currents by less than 10 %. At 100 nM [Ca²⁺]_i, 1-ethyl-2-benzimidazolinone (EBIO) and riluzole, known K⁺ channel activators, evoked concentration-dependent increases in currents with CHOhIK EC₅₀ values of 136 µM [80-231] and 21 µM [11-42] and H292 EC₅₀ values of 187 µM [147-239] and 36 µM [23-55], respectively. The potency of EBIO was dependent on [Ca²⁺]_i – EC₅₀ values in CHOhIK were 343 µM [186-632] and 12 µM [8-18] at 30 and 300 nM [Ca²⁺]_i, respectively. Using a specific hIK antibody and Western blotting, a band of the expected size was detected in cell extracts from the CHOhIK and H292 cells, but not from untransfected CHO cells. RT-PCR (Taqman) confirmed expression of hIK mRNA in a range of surface rich and secretory human tissues including lung and colon and the salivary glands.

We conclude that IK channels are abundantly expressed in the bronchial epithelial cell line NCI-H292 and that these channels are pharmacologically similar to recombinant hIK channels expressed in CHO cells. NCI-H292 cells may be a useful model system for exploring the role of IK channels in secretion and electrolyte transport.

Devor, D.C., Singh, A.K., Lambert, L.C., DeLuca, A., Frizzell, R.A. & Bridges, R.J. (1999). J. Gen. Physiol. 113, 743-760.