BK channel beta1 subunits regulate airway contraction secondary to M2 muscarinic acetylcholine receptor mediated depolarization.

摘要

The large conductance calcium- and voltage-activated potassium channel (BK channel) and its smooth muscle-specific beta1 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 beta1 subunit enhances cholinergic-evoked trachea contractions. Here, we demonstrate that the enhanced contraction of the BK beta1 knockout can be ascribed to a defect in BK channel opposition of M2 receptor-mediated contractions. Indeed, the enhanced contraction of beta1 knockout is eliminated by specific M2 receptor antagonism. The role of BK beta1 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 beta1 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 beta1 knockout or by paxilline block of BK channels. Normalization of beta1 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/beta1 channels is to oppose cholinergic M2 receptor-mediated depolarization and activation of calcium channels by restricting excitation-contraction coupling to more negative voltage ranges.