Enhancement of Ca2+ Influx and Ciliary Beating by Membrane Hyperpolarization due to ATP-Sensitive K+ Channel Opening in Mouse Airway Epithelial Cells

Among the several types of cells composing the airway epithelium, the ciliary cells are responsible for one of the most important defense mechanisms of the airway epithelium: the transport of inhaled particles back up into the throat by coordinated ciliary movement. Changes in the cytoplasmic Ca2+ concentration ([Ca2+](i)) are the main driving force controlling the ciliary activity. In mouse ciliary cells, membrane hyperpolarization from -20 to -60 mV under whole-cell voltage-clamp induced a slow but significant [Ca2+](i) rise in a reversible manner. This rise was completely inhibited by the removal of Ca2+ from the extracellular solution. Application of diazoxide, an ATP-dependent K+ channel opener, dose-dependently induced a membrane hyperpolarization (EC50 = 2.3 mu M), which was prevented by the addition of 5 mu M glibenclamide. An inwardly rectifying current was elicited by the application of 10 mu M diazoxide and suppressed by subsequent addition of 5 mu M glibenclamide. Moreover, the application of 10 mu M diazoxide induced a significant [Ca2+](i) rise and facilitated ciliary movement. Multi-cell reverse-transcription polymerase chain reaction analyses and immunocytochemical staining suggested that the subunit combination of Kir6.2/SUR2B and possibly also Kir6.1/SUR2B is expressed in ciliary cells. The confocal Ca2+ imaging analyses suggested that the [Ca2+](i) rise induced by diazoxide occurred preferentially in the apical submembrane region. In conclusion, the application of a K-ATP channel opener to airway ciliary cells induces membrane hyperpolarization and thereby induces a [Ca2+](i) rise via the facilitation of Ca2+ influx through the non-voltage-dependent Ca2+ permeable channels. Therefore, a K-ATP opener may be beneficial in facilitating ciliary movement.