Parasympathetic nerve activation of M3 and M2 muscarinic acetylcholine receptors initiates and modulates calcium release from the sarcoplasmic reticulum to control airway smooth muscle contraction. Here we investigate M2 acetylcholine receptors that also contribute to contraction through depolarization and recruitment of voltage-dependent calcium channels (VDCCs). We find that the calcium- and voltage-activated potassium channel (BK channel) and its β1 accessory subunit are important proteins that oppose M2-mediated contraction of airway smooth muscle. BK channels contribute to a negative baseline membrane voltage from which M2-mediated depolarization only weakly activates VDCCs. The role of BK β1 to oppose M2 signalling is evidenced by a greater than fourfold increase in the contribution of L-type VDCCs to contraction that otherwise does not occur with M2 receptor antagonist or with β1 containing BK channels. These findings provide a better understanding of how cholinergic second messenger signalling impinges on voltage-dependent mechanisms and excitation-contraction coupling of smooth muscle. Abstract The large conductance calcium- and voltage-activated potassium channel (BK channel) and its smooth muscle-specific β1 subunit regulate excitation-contraction coupling in many types of smooth muscle cells. However, the relative contribution of BK channels to control of M2- or M3-muscarinic acetylcholine receptor mediated airway smooth muscle contraction is poorly understood. Previously, we showed that knockout of the BK channel β1 subunit enhances cholinergic-evoked trachea contractions. Here, we demonstrate that the enhanced contraction of the BK β1 knockout can be ascribed to a defect in BK channel opposition of M2 receptor-mediated contractions. Indeed, the enhanced contraction of β1 knockout is eliminated by specific M2 receptor antagonism. The role of BK β1 to oppose M2 signalling is evidenced by a greater than fourfold increase in the contribution of L-type voltage-dependent calcium channels to contraction that otherwise does not occur with M2 antagonist or with β1 containing BK channels. The mechanism through which BK channels oppose M2-mediated recruitment of calcium channels is through a negative shift in resting voltage that offsets, rather than directly opposes, M2-mediated depolarization. The negative shift in resting voltage is reduced to similar extents by BK β1 knockout or by paxilline block of BK channels. Normalization of β1 knockout baseline voltage with low external potassium eliminated the enhanced M2-receptor mediated contraction. In summary, these findings indicate that an important function of BK/β1 channels is to oppose cholinergic M2 receptor-mediated depolarization and activation of calcium channels by restricting excitation-contraction coupling to more negative voltage ranges.
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