Involvement of Allosteric Effect and KCa Channels in Crosstalk between β₂-Adrenergic and Muscarinic M₂ Receptors in Airway Smooth Muscle.

Hiroaki Kume,Yuji Tohda,Yuji Higashimoto,Yukihiro Noda,Takaaki Isoya,Osamu Nishiyama

doi:10.3390/ijms19071999

Hiroaki Kume, Yuji Tohda + Show 4 more

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https://doi.org/10.3390/ijms19071999

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Abstract

To advance the development of bronchodilators for asthma and chronic obstructive pulmonary disease (COPD), this study was designed to investigate the mechanism of functional antagonism between β2-adrenergic and muscarinic M2 receptors, focusing on allosteric effects and G proteins/ion channels coupling. Muscarinic receptor antagonists (tiotropium, glycopyrronium, atropine) synergistically enhanced the relaxant effects of β2-adrenergic receptor agonists (procaterol, salbutamol, formoterol) in guinea pig trachealis. This crosstalk was inhibited by iberitoxin, a large-conductance Ca2+-activated K+ (KCa) channel inhibitor, whereas it was increased by verapamil, a L-type voltage-dependent Ca2+ (VDC) channel inhibitor; additionally, it was enhanced after tissues were incubated with pertussis or cholera toxin. This synergism converges in the G proteins (Gi, Gs)/KCa channel/VDC channel linkages. Muscarinic receptor antagonists competitively suppressed, whereas, β2-adrenergic receptor agonists noncompetitively suppressed muscarinic contraction. In concentration-inhibition curves for β2-adrenergic receptor agonists with muscarinic receptor antagonists, EC50 was markedly decreased, and maximal inhibition was markedly increased. Hence, muscarinic receptor antagonists do not bind to allosteric sites on muscarinic receptors. β2-Adrenergic receptor agonists bind to allosteric sites on these receptors; their intrinsic efficacy is attenuated by allosteric modulation (partial agonism). Muscarinic receptor antagonists enhance affinity and efficacy of β2-adrenergic action via allosteric sites in β2-adrenergic receptors (synergism). In conclusion, KCa channels and allosterism may be novel targets of bronchodilator therapy for diseases such as asthma and COPD.

Highlights

Since the functional antagonism between β2-adrenergic and muscarinic receptors regulates airway smooth muscle tone, investigation of this antagonism may play a key role for progressing bronchodilator therapy for asthma and chronic obstructive pulmonary disease (COPD) [1,2,3,4]
When procaterol (10 nM) was applied to the tissues pre-contracted by MCh (10 μM) in the presence of tiotropium (1 nM), the inhibitory effects of the combination of procaterol and tiotropium were markedly enhanced (Figure 1A), and values of percent inhibition were increased to 80.8 ± 9.0% [95% confidence interval (CI): 73.27–88.33] (n = 8, Figure 1B)
This study used physiological methods to demonstrate that G proteins/KCa channel/voltage-dependent Ca2+ (VDC) channel linkages are an essential process in the synergistic effects induced by crosstalk between β2-adrenergic and muscarinic M2 receptors

Summary

Introduction

Since the functional antagonism between β2-adrenergic and muscarinic receptors regulates airway smooth muscle tone, investigation of this antagonism may play a key role for progressing bronchodilator therapy for asthma and chronic obstructive pulmonary disease (COPD) [1,2,3,4]. The combination of β2-adrenergic receptor agonists with muscarinic receptor antagonists markedly enhances relaxant effects on muscarinic contraction [4,11,21]. This effect is due to crosstalk between β2-adrenergic and muscarinic receptors (G protein-coupled receptors: GPCRs), and is involved in Ca2+ signaling mediated by KCa channels [4,11] and protein kinase C (PKC) [22,23,24]. There is a pharmacological rationale for combining β2-adrenergic receptor agonists and muscarinic receptor antagonists as a bronchodilator therapy for asthma and COPD [4,11,21,24,26]. Clinical trials have indicated that this combination therapy is beneficial to these diseases [27,28,29]

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