Kcnq Channels in Airway Smooth Muscle

Abstract

The role of KCNQ (Kv7) channels is well established in neurons, where they play dominant roles in control of resting membrane potentials and cell excitability. Recent studies have revealed expression of KCNQ channels in different types of smooth muscle. Here we focused on airway smooth muscle (ASM), whose membrane potential is primarily, determined by BK-type K+ channels. RT-PCR and immunostaining suggest that KCNQ 1, 4 and 5 are the predominant subtypes in rodent airway. To investigate the contribution of KCNQ channels in cholinergic-induced ASM contraction, we patch-clamped freshly isolated ASM cells and isolated KCNQ current as the non-inactivating component at the end of 2s depolarizations, with BK channels blocked with 1μM paxilline. The KCNQ current was enhanced by the KCNQ channel opener, flupirtine (10μM), and abolished by XE991 (10μM). Although XE991 depolarized the resting membrane potential, it did not affect that of ASM cells pre-treated with carbachol. Similarly, XE991 did not effect carbachol-evoked contractions, whereas flupirtine induced a significant relaxation. The flupirtine effect is likely via opposing voltage-dependent Ca2+ influx, since no effect of flupertine occurred in a low K+ (1mM) bathing solution. Pre-treatment with the muscarinic M3-receptor antagonist, fumarate (5nM), to prevent Gq/11-mediated PIP2 hydrolysis did not reveal an effect of XE991 on contractions, suggesting the mechanism of KCNQ inhibition in ASM is not due to PIP2 depletion. Interestingly, an XE991-dependent increase in contractility was revealed in mice with the β1 BK channel subunit knocked out, an effect enhanced by the M3 receptor antagonist. Thus, KCNQ channels may normally play a secondary role to BK channels in control of cholinergic evoked contractions in ASM; however, given that alteration in BK β1-subunit expression is known to occur in asthma, KCNQ channels in ASM cells may be potential targets for therapeutic intervention in respiratory disease.