BK Channels Regulate Contraction Secondary to M2 Muscarinic Acetylcholine Receptor Mediated Depolarization

Abstract

Airway smooth muscles (ASM) contraction is primarily mediated by M2 and M3 muscarinic acetylcholine receptors. We and others have shown that one of the functions of large conductance Ca2+- and voltage-activated (BK) potassium channel and the accessory β1 subunit is to moderate ASM excitation-contraction coupling by opposing cholinergic evoked depolarization and reducing activation of L-type voltage-dependent calcium channels. Here, we have employed airway contraction studies, and a mouse knockout of the BK channel β1 subunit to investigate fuctional consequences specifically downstream of M2 signaling. Our results indicate that effects of BK channels are downstream of M2 signaling since M2 receptor antagonist (100 nM AF-DX 116) eliminates the enhanced contraction of β1 KO trachea. We found that the mechanism of M2 antagonist evoked relaxation of β1 KO is a reduction of excitation-contraction coupling since pretreatment with L-type Ca2+ channels blocker Nifedipine or with low extracellular potassium solution that hyperpolarizes membrane potential occludes effect of M2 antagonist. M2 mediated membrane depolarization and the role of BK/β1 in controlling ASM membrane potentials were directly confirmed using sharp electrode recording of membrane voltage. In summary, these results indicate that the important role of BK/β1 during M2 activation is to oppose membrane depolarization and thereby attenuating Ca2+-influx through voltage dependent Ca2+-channels. Blocking M2 signaling largely made BK channels superfluous to control of ASM contraction.