Interleukin‐4 activates large‐conductance, calcium‐activated potassium (BKCa) channels in human airway smooth muscle cells

Contraction of airway smooth muscle cells contributes to airway obstruction in the inflammatory disease, asthma. Large-conductance, calcium-activated potassium (BKCa) channels are abundant in normal airway smooth muscle cells and modulate membrane potential, electromechanical coupling and contraction (Savaria et al. 1992; Snetkov et al. 1996; Wang et al. 1997; Semenov et al. 2006). Activation of BKCa channels allows an efflux of potassium ions from the cell which hyperpolarizes the plasma membrane and favours smooth muscle cell relaxation (Kotlikoff & Kamm, 1996). A fundamental property of BKCa channels is their high sensitivity to and rapid activation by increases in calcium. These channels are also regulated by protein phosphorylations, G–proteins and membrane voltage (Kume et al. 1989; Wang et al. 1997; Vergara et al. 1998). Owing to their importance in regulating contraction and relaxation, there is interest in understanding how inflammatory mediators regulate BKCa channel activity in airway smooth muscle cells (Savaria et al. 1992; Kotlikoff & Kamm, 1996). Interleukin (IL)-4 and IL-13 are inflammatory mediators that clearly have important roles in the pathogenesis of allergic inflammation (Grunig et al. 1998; Wills-Karp et al. 1998; Barnes, 2001). However, even though it is clear that IL-4 promotes the asthma phenotype, there is some evidence that IL-4 has unexpected inhibitory effects on calcium signalling in airway smooth muscle (Madison & Ethier, 2001). Specifically, studies from bovine trachealis cells have shown that IL-4, but not IL-13, inhibits the calcium signalling of contractile agonists by depleting sarcoplasmic reticulum (SR) calcium through ryanodine receptors over exposures as short as approximately 20 min (Madison & Ethier, 2001; Ethier et al. 2005; Ethier & Madison, 2006). Since consistent increases in cytosolic calcium were not detected by calcium imaging techniques, those findings suggested the possibility that IL-4 might be causing relatively small, difficult to detect and possibly localized, changes in subplasmalemmal calcium to regulate the function of plasma membrane ion channels. Since BKCa channels are plasma membrane ion channels that are highly sensitive to changes in subplasmalemmal calcium, we hypothesized that IL-4 might be activating BKCa channel activity in airway smooth muscle cells. Therefore, we tested that hypothesis by using on-cell patch clamping of BKCa channels in cultured human bronchial smooth muscle cells (HBSMC), a cell type previously shown to express abundant BKCa channels (Snetkov et al. 1996).