Localization of muscarinic receptor subtype mRNAs in human lung

Cholinergic sensing of allergen exposure by airway epithelium promotes type 2 immunity in the lungs

BACKGROUND

Nonneuronal cells, including epithelial cells, can produce acetylcholine (ACh). Muscarinic ACh receptor antagonists are used clinically to treat asthma and other medical conditions; however, knowledge regarding the roles of ACh in type 2 immunity is limited.

OBJECTIVE

Our aim was to investigate the roles of epithelial ACh in allergic immune responses.

METHODS

Human bronchial epithelial (HBE) cells were cultured with allergen extracts, and their ACh production and IL-33 secretion were studied in vitro. To investigate immune responses in vivo, naive BALB/c mice were treated intranasally with different muscarinic ACh receptor antagonists and then exposed intranasally to allergens.

RESULTS

At steady state, HBE cells expressed cellular components necessary for ACh production, including choline acetyltransferase and organic cation transporters. Exposure to allergens caused HBE cells to rapidly release ACh into the extracellular medium. Pharmacologic or small-interfering RNA-based blocking of ACh production or autocrine action through the M3 muscarinic ACh receptors in HBE cells suppressed allergen-induced ATP release, calcium mobilization, and extracellular secretion of IL-33. When naive mice were exposed to allergens, ACh was quickly released into the airway lumen. A series of clinical M3 muscarinic ACh receptor antagonists inhibited allergen-induced IL-33 secretion and innate type 2 immune response in the mouse airways. In a preclinical murine model of asthma, an ACh receptor antagonist suppressed allergen-induced airway inflammation and airway hyperreactivity.

CONCLUSIONS

ACh is released quickly by airway epithelial cells on allergen exposure, and it plays an important role in type 2 immunity. The epithelial ACh system can be considered a therapeutic target in allergic airway diseases.

Effects of long-acting muscarinic antagonists on promoting ciliary function in airway epithelium

Background

Mucociliary clearance (MCC) is an essential defense mechanism in airway epithelia for removing pathogens from the respiratory tract. Impaired ciliary functions and MCC have been demonstrated in asthma and chronic obstructive pulmonary disease (COPD). Long-acting muscarinic antagonists (LAMAs) are a major class of inhaled bronchodilators, which are used for treating asthma and COPD; however, the effects of LAMAs on ciliary function remain unclear. This study aimed to identify the effects of LAMAs on airway ciliary functions.

Methods

Wild-type BALB/c mice were treated with daily intranasal administrations of glycopyrronium for 7 days, and tracheal samples were collected. Cilia-driven flow and ciliary activity, including ciliary beat frequency (CBF), ciliary beating amplitude, effective stroke velocity, recovery stroke velocity and the ratio of effective stroke velocity to recovery stroke velocity, were analyzed by imaging techniques. Using in vitro murine models, tracheal tissues were transiently cultured in media with/without LAMAs, glycopyrronium or tiotropium, for 60 min. Cilia-driven flow and ciliary activity were then analyzed. Well-differentiated normal human bronchial epithelial (NHBE) cells were treated with glycopyrronium, tiotropium, or vehicle for 60 min, and CBF was evaluated. Several mechanistic analyses were performed.

Results

Intranasal glycopyrronium administration for 7 days significantly increased cilia-driven flow and ciliary activity in murine airway epithelium. In the murine tracheal organ culture models, treatment with glycopyrronium or tiotropium for 60 min significantly increased cilia-driven flow and ciliary activity in airway epithelium. Further, we confirmed that 60-min treatment with glycopyrronium or tiotropium directly increased CBF in well-differentiated NHBE cells. In the mechanistic analyses, neither treatment with glycopyrronium nor tiotropium affected intracellular calcium ion concentrations in well-differentiated NHBE cells. Glycopyrronium did not increase protein kinase A activity in well-differentiated NHBE cells. Moreover, glycopyrronium had no effect on extracellular adenosine triphosphate concentration.

Conclusions

LAMAs exert a direct effect on airway epithelium to enhance ciliary function, which may improve impaired MCC in asthma and COPD. Further investigations are warranted to elucidate the underlying mechanisms of the effects of LAMAs on the promotion of airway ciliary function.

Neuroimmune Pathophysiology in Asthma

Asthma is a chronic inflammation of lower airway disease, characterized by bronchial hyperresponsiveness. Type I hypersensitivity underlies all atopic diseases including allergic asthma. However, the role of neurotransmitters (NT) and neuropeptides (NP) in this disease has been less explored in comparison with inflammatory mechanisms. Indeed, the airway epithelium contains pulmonary neuroendocrine cells filled with neurotransmitters (serotonin and GABA) and neuropeptides (substance P[SP], neurokinin A [NKA], vasoactive intestinal peptide [VIP], Calcitonin-gene related peptide [CGRP], and orphanins-[N/OFQ]), which are released after allergen exposure. Likewise, the autonomic airway fibers produce acetylcholine (ACh) and the neuropeptide Y(NPY). These NT/NP differ in their effects; SP, NKA, and serotonin exert pro-inflammatory effects, whereas VIP, N/OFQ, and GABA show anti-inflammatory activity. However, CGPR and ACh have dual effects. For example, the ACh-M3 axis induces goblet cell metaplasia, extracellular matrix deposition, and bronchoconstriction; the CGRP-RAMP1 axis enhances Th2 and Th9 responses; and the SP-NK1R axis promotes the synthesis of chemokines in eosinophils, mast cells, and neutrophils. In contrast, the ACh-α7nAChR axis in ILC2 diminishes the synthesis of TNF-α, IL-1, and IL-6, attenuating lung inflammation whereas, VIP-VPAC1, N/OFQ-NOP axes cause bronchodilation and anti-inflammatory effects. Some NT/NP as 5-HT and NKA could be used as biomarkers to monitor asthma patients. In fact, the asthma treatment based on inhaled corticosteroids and anticholinergics blocks M3 and TRPV1 receptors. Moreover, the administration of experimental agents such as NK1R/NK2R antagonists and exogenous VIP decrease inflammatory mediators, suggesting that regulating the effects of NT/NP represents a potential novel approach for the treatment of asthma.

G Protein-Coupled Receptors in Asthma Therapy: Pharmacology and Drug Action

Asthma is a heterogeneous inflammatory disease of the airways that is associated with airway hyperresponsiveness and airflow limitation. Although asthma was once simply categorized as atopic or nonatopic, emerging analyses over the last few decades have revealed a variety of asthma endotypes that are attributed to numerous pathophysiological mechanisms. The classification of asthma by endotype is primarily routed in different profiles of airway inflammation that contribute to bronchoconstriction. Many asthma therapeutics target G protein-coupled receptors (GPCRs), which either enhance bronchodilation or prevent bronchoconstriction. Short-acting and long-acting β 2-agonists are widely used bronchodilators that signal through the activation of the β 2-adrenergic receptor. Short-acting and long-acting antagonists of muscarinic acetylcholine receptors are used to reduce bronchoconstriction by blocking the action of acetylcholine. Leukotriene antagonists that block the signaling of cysteinyl leukotriene receptor 1 are used as an add-on therapy to reduce bronchoconstriction and inflammation induced by cysteinyl leukotrienes. A number of GPCR-targeting asthma drug candidates are also in different stages of development. Among them, antagonists of prostaglandin D2 receptor 2 have advanced into phase III clinical trials. Others, including antagonists of the adenosine A2B receptor and the histamine H4 receptor, are in early stages of clinical investigation. In the past decade, significant research advancements in pharmacology, cell biology, structural biology, and molecular physiology have greatly deepened our understanding of the therapeutic roles of GPCRs in asthma and drug action on these GPCRs. This review summarizes our current understanding of GPCR signaling and pharmacology in the context of asthma treatment. SIGNIFICANCE STATEMENT: Although current treatment methods for asthma are effective for a majority of asthma patients, there are still a large number of patients with poorly controlled asthma who may experience asthma exacerbations. This review summarizes current asthma treatment methods and our understanding of signaling and pharmacology of G protein-coupled receptors (GPCRs) in asthma therapy, and discusses controversies regarding the use of GPCR drugs and new opportunities in developing GPCR-targeting therapeutics for the treatment of asthma.

The muscarinic antagonist gallamine induces proliferation of airway smooth muscle cells regardless of the cell phenotype

BACKGROUND

Muscarinic receptor antagonists are a usual treatment for chronic airway diseases, with increased bronchoconstriction, like asthma and chronic obstructive pulmonary disease. These diseases are usually accompanied by airway remodeling, involving airway smooth muscle cell (ASMC) proliferation. The purpose of this study was to examine the effect of the muscarinic receptor modulator gallamine on rabbit tracheal ASMC proliferation.

METHODS

ASMCs were incubated with gallamine (1 nM-10 mM), atropine (1 fM-10 mM), and/or acetylcholine (1 nM-1 mM), in the presence or absence of FBS (1% or 10%). Cell proliferation was estimated by incorporation of radioactive thymidine, the Cell Titer AQueous One Solution method and cell number counting after Trypan blue exclusion. The mechanisms mediating cell proliferation were studied using the PI3K and MAPK inhibitors LY294002 (20 μM) and PD98059 (100 μM), respectively. Cell phenotype was studied by indirect immunofluorescence for α-actin, Myosin Heavy Chain and desmin.

RESULTS

ASMC incubation with the muscarinic receptor allosteric modulator gallamine or the muscarinic receptor antagonist atropine increased methyl-[³H]thymidine incorporation and cell number in a dose-dependent manner. ASMC proliferation was mediated via PI3K and MAPK activation and was transient. Gallamine antagonized the mitogenic effect of 1% FBS. Furthermore, gallamine had a similar effect on contractile ASMCs, without synergizing with or affecting acetylcholine induced proliferation, or altering the percentage of ASMCs expressing contractile phenotype marker proteins.

CONCLUSIONS

Gallamine, in the absence of any agonist, has a transient mitogenic effect on ASMCs, regardless of the cell phenotype, mediated by the PI3K and the MAPK signaling pathways.

Cigarette smoke and non-neuronal cholinergic system in the airway epithelium of COPD patients

Acetylcholine (ACh), synthesized by Choline Acetyl-Transferase (ChAT), exerts its physiological effects via mAChRM3 in epithelial cells. We hypothesized that cigarette smoke affects ChAT, ACh, and mAChRM3 expression in the airways from COPD patients promoting airway disease. ChAT, ACh, and mAChRM3 were assessed: "ex vivo" in the epithelium from central and distal airways of COPD patients, Healthy Smoker (S) and Healthy Subjects (C), and "in vitro" in bronchial epithelial cells stimulated with cigarette smoke extract (CSE). In central airways, mAChRM3, ChAT, and ACh immunoreactivity was significantly higher in the epithelium from S and COPD than in C subjects. mAChRM3, ChAT, and ACh score of immunoreactivity was high in the metaplastia area of COPD patients. mAChRM3/ChAT and ACh/ChAT co-localization of immunoreactivity was observed in the bronchial epithelium from COPD. In vitro, CSE stimulation significantly increased mAChRM3, ChAT, and ACh expression and mAChRM3/ChAT and ACh/ChAT co-localization in 16HBE and NHBE, and increased 16HBE proliferation. Cigarette smoke modifies the levels of mAChMR3, ChAT expression, and ACh production in bronchial epithelial cells from COPD patients. Non-neuronal components of cholinergic system may have a role in the mechanism of bronchial epithelial cell proliferation, promoting alteration of normal tissue, and of related pulmonary functions.

Interactions between glycopyrronium and indacaterol on cholinergic neurotransmission and contractile response in bovine trachealis

BACKGROUND

Muscarinic-receptor antagonists and β-adrenoceptor agonists are used, alone or in combination, as first-line treatment for chronic obstructive pulmonary disease. Both drugs decrease airway smooth muscle tone by post-junctional mechanisms but they may have opposing effects on pre-junctional acetylcholine (ACh)-release.

METHODS

We studied the effects of the muscarinic-receptor antagonist glycopyrronium (GLY), the β-adrenoceptor agonist indacaterol (IND) and their combination on electrically-induced ACh-release and contractile response in isolated bovine trachealis. Data were analyzed by paired t-test and analysis of variance for repeated or independent measures with Newmann-Keuls post-hoc test when appropriate.

RESULTS

GLY 10-8 M decreased contractile response by 19 ± 6% (p = 0.010) without altering ACh-release. GLY 10-7 M and 10-6 M almost abolished contractile responses even if the ACh-release was increased by 27 ± 19% (p < 0.001) and 20 ± 8% (p = 0.004), respectively. IND 10-7 M had no significant effects on contractile response and ACh-release, whereas IND 10-6 M reduced contractile response by 24 ± 12% (p = 0.002) without altering ACh-release. IND 10-5 M decreased contractile response by 51 ± 17% (p < 0.001) and ACh-release by 22 ± 11% (p = 0.004). Co-incubation with GLY 10-8 M and IND 10-7 M did not alter ACh-release but inhibited contractile response by 41 ± 8% (p < 0.001). The latter effect was greater than with GLY 10-8 M, or IND 10-7 M, or IND 10-6 M given separately (p < 0.001 for all). The increment of ACh-release caused by GLY was attenuated by IND 10-5 M, though this did not affect contractile response.

CONCLUSIONS

At equimolar concentration, GLY alone attenuates airway smooth muscle contraction more than IND, despite an increased ACh-release. Combination of GLY with IND at submaximal concentrations has more than additive effect suggesting a synergistic post-junctional effect. Adding GLY to IND provides a greater inhibitory effect on airway smooth muscle contraction than increasing IND concentration.

Inhibition of airway surface fluid absorption by cholinergic stimulation

In upper airways airway surface liquid (ASL) depth and clearance rates are both increased by fluid secretion. Secretion is opposed by fluid absorption, mainly via the epithelial sodium channel, ENaC. In static systems, increased fluid depth activates ENaC and decreased depth inhibits it, suggesting that secretion indirectly activates ENaC to reduce ASL depth. We propose an alternate mechanism in which cholinergic input, which causes copious airway gland secretion, also inhibits ENaC-mediated absorption. The conjoint action accelerates clearance, and the increased transport of mucus out of the airways restores ASL depth while cleansing the airways. We were intrigued by early reports of cholinergic inhibition of absorption by airways in some species. To reinvestigate this phenomenon, we studied inward short-circuit currents (Isc) in tracheal mucosa from human, sheep, pig, ferret, and rabbit and in two types of cultured cells. Basal Isc was inhibited 20–70% by the ENaC inhibitor, benzamil. Long-lasting inhibition of ENaC-dependent Isc was also produced by basolateral carbachol in all preparations except rabbit and the H441 cell line. Atropine inhibition produced a slow recovery or prevented inhibition if added before carbachol. The mechanism for inhibition was not determined and is most likely multi-factorial. However, its physiological significance is expected to be increased mucus clearance rates in cholinergically stimulated airways.

Airway and lung remodelling in chronic pulmonary obstructive disease: a role for muscarinic receptor antagonists?

Lung tissue remodelling in chronic inflammatory lung diseases has long been regarded as a follow-up event to inflammation. Recent studies have indicated that, although airway and lung tissue remodelling is often independent of inflammation, it precedes or causes inflammation. None of the available therapies has a significant effect on airway and lung tissue remodelling in asthma, bronchiectasis, fibrosis and chronic obstructive pulmonary disease (COPD). The goal of stopping or reversing lung tissue remodelling is difficult, as the term summarizes the net effect of independent events, including (1) cell proliferation, (2) cell volume increase, (3) cell migration, (4) modified deposition and metabolism of specific extracellular matrix components, and (5) local action of infiltrated inflammatory cells. The extracellular matrix of the lung has a very high turnover, and thus small changes may accumulate to significant structural pathologies, which seem to be irreversible. The most important question is 'why are pathological changes of the lung structure irreversible and resistant to drugs?' Many drugs have the potential to reduce remodelling mechanisms in vitro but fail in clinical trials. New evidence suggests that muscarinic receptor inhibitors have the potential to improve lung function through modifying tissue remodelling. However, the role of muscarinic receptors in lung remodelling, especially their supportive role for other remodelling driving factors, needs to be further investigated. The focus of this review is the role of muscarinic receptors in lung tissue remodelling as it has been reported in the human lung.

Pulmonary effects of intravenous atropine induce ventilation perfusion mismatch

Atropine is used for a number of medical conditions, predominantly for its cardiovascular effects. Cholinergic nerves that innervate pulmonary smooth muscle, glands, and vasculature may be affected by anticholinergic medications. We hypothesized that atropine causes alterations in pulmonary gas exchange. We conducted a prospective interventional study with detailed physiologic recordings in anesthetized, spontaneously breathing rats (n = 8). Animals breathing a normoxic gas mixture titrated to a partial arterial pressure of oxygen of 110-120 were exposed to an escalating dose of intravenous atropine (0.001, 0.01, 0.1, 5.0, and 20.0 mg/kg body mass). Arterial blood gas measurements were recorded every 2 min (×5) at baseline, and following each of the 5 doses of atropine. In addition, the animals regional pulmonary blood flow was measured using neutron-activated microspheres. Oxygenation decreased immediately following intravenous administration of atropine, despite a small increase in the volume of inspired air with no change in respiratory rate. Arterial blood gas analysis showed an increase in pulmonary dysfunction, characterized by a widening of the alveolar-arteriole gradient (p < 0.003 all groups except for the lowest dose of atropine). The microsphere data demonstrates an abrupt and marked heterogeneity of pulmonary blood flow following atropine treatment. In conclusion, atropine was found to decrease pulmonary gas exchange in a dose-dependent fashion in this rat model.

CHRM2 but not CHRM1 or CHRM3 polymorphisms are associated with asthma susceptibility in Mexican patients

Asthma is a complex disease for which genetic predisposition has been widely documented. Considerable evidence supports the hypothesis that polymorphisms in the muscarinic-cholinergic (CHRM) genes could be involved in asthma pathogenesis, bronchial hyperresponsiveness, and mucus secretion. To determine whether single nucleotide polymorphisms (SNPs) or haplotypes in CHRM1, CHRM2, or CHRM3 are associated with asthma in Mexican pediatric population. We performed a case-control study including 398 pediatric cases with asthma and 450 healthy controls. We analyzed 19 SNPs distributed among these three genes. Two of the seven SNPs located in CHRM2, the 3' untranslated region rs8191992 and rs6962027, differed significantly in allele frequencies between patients with asthma and healthy controls [odds ratio (OR) 1.42, 95 % confidence interval (95 % CI) 1.14-1.77, P = 0.001, and OR 1.50, 95 % CI 1.21-1.87, P = 0.0002, respectively]. Statistical significance remained after multiple comparison corrections (P = 0.003 and P = 0.005, respectively). The haplotypes AA and TT, containing both major and minor alleles from rs8191992 and rs6962027, also differed between cases and controls. The haplotype AA occurred at a lower frequency in cases (OR 0.67, 95 % CI 0.53-0.85, P = 0.001) whereas the haplotype TT was overrepresented in cases compared to controls (28 vs 21 %, respectively; OR 1.46, 95 % CI 1.15-1.85, P = 0.002). No association was observed between CHRM1 or CHRM3 SNPs or haplotypes and asthma. CHRM2 polymorphisms are implicated in the genetic etiology of asthma.

Muscarinic receptors and their antagonists in COPD: anti-inflammatory and antiremodeling effects

Muscarinic receptors are expressed by most cell types and mediate cellular signaling of their natural ligand acetylcholine. Thereby, they control numerous central and peripheral physiological organ responses to neuronal activity. In the human lung, muscarinic receptors are predominantly expressed by smooth muscle cells, epithelial cells, and fibroblasts. Antimuscarinic agents are used for the treatment of chronic obstructive pulmonary disease and to a lesser extent for asthma. They are primarily used as bronchodilators, but it is now accepted that they are also associated with anti-inflammatory, antiproliferative, and antiremodeling effects. Remodeling of the small airways is a major pathology in COPD and impairs lung function through changes of the extracellular matrix. Glycosaminoglycans, particularly hyaluronic acid, and matrix metalloproteases are among extracellular matrix molecules that have been associated with tissue inflammation and remodeling in lung diseases, including chronic obstructive pulmonary disease and asthma. Since muscarinic receptors have been shown to influence the homeostasis of glycosaminoglycans and matrix metalloproteases, these molecules may be proved valuable endpoint targets in clinical studies for the pharmacological exploitation of the anti-inflammatory and antiremodeling effects of muscarinic inhibitors in the treatment of chronic obstructive pulmonary disease and asthma.

Regional quantification of muscarinic acetylcholine receptors and β-adrenoceptors in human airways

BACKGROUND AND PURPOSE Muscarinic acetylcholine receptors (mAChRs) and β-adrenoceptors in the airways and lungs are clinically important in chronic obstructive pulmonary disease (COPD) and asthma. However, the quantitative and qualitative estimation of these receptors by radioligand binding approaches in human airways has not yet been reported because of tissue limitations. EXPERIMENTAL APPROACH The regional distribution and relative proportion of mAChR and β-adrenoceptor subtypes were evaluated in human bronchus and lung parenchyma by a tissue segment binding method with [(3)H]-N-methylscopolamine ([(3)H]-NMS) for mAChRs and [(3)H]-CGP-12,177 for β-adrenoceptors. Functional responses to carbachol and isoprenaline were also analysed in the bronchus. KEY RESULTS The M(3) subtype predominantly occurred in the bronchus, but the density decreased from the segmental to subsegmental bronchus, and was absent in lung parenchyma. On the other hand, the M(1) subtype occurred in the lung only, and the M(2) subtype was distributed ubiquitously in the bronchus and lungs. β(2)-adrenoceptors were increased along the airways, and their densities in the subsegmental bronchus and lung parenchyma were approximately twofold higher than those of mAChRs in the same region. β(1)-adrenoceptors were also detected in lung parenchyma but not in the bronchus. The muscarinic contractions and adrenoceptor relaxations in both bronchial regions were mediated through M(3)-mAChRs and β(2)-adrenoceptors, respectively. CONCLUSIONS AND IMPLICATIONS From the present radioligand binding approach with intact tissue segments, we constructed a distribution map of mAChRs and β-adrenoceptors in human bronchus and lung parenchyma for the first time, providing important evidence for future pharmacotherapy and new drug development for respiratory disorders.

Luminal cholinergic signalling in airway lining fluid: a novel mechanism for activating chloride secretion via Ca²⁺-dependent Cl⁻ and K⁺ channels

BACKGROUND AND PURPOSE

Recent studies detected the expression of proteins involved in cholinergic metabolism in airway epithelial cells, although the function of this non-neuronal cholinergic system is not known in detail. Thus, this study focused on the effect of luminal ACh as a regulator of transepithelial ion transport in epithelial cells.

EXPERIMENTAL APPROACH

RT-PCR experiments were performed using mouse tracheal epithelial cells for ChAT and organic cation transporter (OCT) transcripts. Components of tracheal airway lining fluid were analysed with desorption electrospray ionization (DESI) MS. Effects of nicotine on mouse tracheal epithelial ion transport were examined with Ussing-chamber experiments.

KEY RESULTS

Transcripts encoding ChAT and OCT1-3 were detected in mouse tracheal epithelial cells. The DESI experiments identified ACh in the airway lining fluid. Luminal ACh induced an immediate, dose-dependent increase in the transepithelial ion current (EC₅₀: 23.3 µM), characterized by a transient peak and sustained plateau current. This response was not affected by the Na⁺-channel inhibitor amiloride. The Cl⁻-channel inhibitor niflumic acid or the K⁺-channel blocker Ba²⁺ attenuated the ACh effect. The calcium ionophore A23187 mimicked the ACh effect. Luminal nicotine or muscarine increased the ion current. Experiments with receptor gene-deficient animals revealed the participation of muscarinic receptor subtypes M₁ and M₃.

CONCLUSIONS AND IMPLICATIONS

The presence of luminal ACh and activation of transepithelial ion currents by luminal ACh receptors identifies a novel non-neuronal cholinergic pathway in the airway lining fluid. This pathway could represent a novel drug target in the airways.

Evidence for functional atypical nicotinic receptors that activate K+-dependent Cl- secretion in mouse tracheal epithelium

The present study focused on the influence of nicotinic acetylcholine receptors (nAChR) on ion transport processes in mouse tracheal epithelium. RT-PCR experiments revealed expression of the α3, α4, α5, α7, α9, α10, β2, and β4 nAChR subunits in mouse tracheal epithelium. In Ussing chamber recordings of mouse tracheae, apically applied nicotine (100 μM) induced a dose-dependent increase of the transepithelial short-circuit current (EC(50): 14.6 μM). The nicotine-induced effect (I(NIC)) was attenuated by mecamylamine (25 μM, apical) and methyllycaconitine (1 μM, apical). The nAChR agonist 1.1-dimethyl-4-phenylpiperatinium iodide (DMPP) (100 μM) revealed apical and basolateral location of the receptors. I(NIC) was not affected by the sodium channel inhibitor amiloride (10 μM, apical) or the cystic fibrosis transmembrane conductance regulator inhibitor CFTR(inh)-172 (20 μM, apical) but was reduced by the chloride channel inhibitor 5-nitro-2-(3-phenylpropylamino)benzoic acid (100 μM, apical), the Na(+)/K(+)/2Cl(-) cotransporter inhibitor bumetanide (200 μM, basolateral), the potassium channel inhibitor Ba(2+) (5 mM, basolateral), and 4.4'-diisothiocyanatostilbene-2.2'-disulfonate (100 μM, apical), indicating a contribution of Ca(2+)-activated chloride channels and potassium channels. Removal of extracellular Na(+) (apical) or Ca(2+) (apical) did not influence I(NIC) but reduced the DMPP effect. Experiments with the Ca(2+)-ionophore A23187, a mix of 3-isobutyl-1-methylxanthine and forskolin, or the inositol-1,4,5-triphospate (IP(3)) receptor inhibitor 2-aminoethyl-diphenyl-borinate (75 μM, apical) decreased I(NIC), indicating a nicotine-mediated increase of intracellular Ca(2+) and cAMP levels involving the IP(3) signaling pathway. These findings indicate the activity of Ca(2+)-permeable nAChRs and alternative metabotropic pathways by nAChR activation that mediate Cl(-) and K(+) transport in tracheal epithelium.

Muscarinic receptor agonists and antagonists: effects on cancer

Many epithelial and endothelial cells express a cholinergic autocrine loop in which acetylcholine acts as a growth factor to stimulate cell growth. Cancers derived from these tissues similarly express a cholinergic autocrine loop and ACh secreted by the cancer or neighboring cells interacts with M3 muscarinic receptors expressed on the cancer cells to stimulate tumor growth. Primary proliferative pathways involve MAPK and Akt activation. The ability of muscarinic agonists to stimulate, and M3 antagonists to inhibit tumor growth has clearly been demonstrated for lung and colon cancer. The ability of muscarinic agonists to stimulate growth has been shown for melanoma, pancreatic, breast, ovarian, prostate and brain cancers, suggesting that M3 antagonists will also inhibit growth of these tumors as well. As yet no clinical trials have proven the efficacy of M3 antagonists as cancer therapeutics, though the widespread clinical use and low toxicity of M3 antagonists support the potential role of these drugs as adjuvants to current cancer therapies.

Muscarinic receptor antagonists: effects on pulmonary function

In healthy lungs, muscarinic receptors control smooth muscle tone, mucus secretion, vasodilation, and inflammation. In chronic obstructive pulmonary disease (COPD) and asthma, cholinergic mechanisms contribute to increased bronchoconstriction and mucus secretion that limit airflow. This chapter reviews neuronal and nonneuronal sources of acetylcholine in the lung and the expression and role of M₁, M₂, and M₃ muscarinic receptor subtypes in lung physiology. It also discusses the evidence for and against the role of parasympathetic nerves in asthma, and the current use and therapeutic potential of muscarinic receptor antagonists in COPD and asthma.

Non-neuronal regulation and repertoire of cholinergic receptors in organs

Many studies on the cholinergic pathway have indicated that cholinergic receptors, which are widely expressed in various cells, play an important role in all body organs. In this review, we present the concept that cholinergic responses are regulated through a neuronal or non-neuronal mechanism. The neuronal mechanism is a system in which acetylcholine binds to cholinergic receptors on target cells through the nerves. In the non-neuronal mechanism, acetylcholine, produced by neighboring cells in an autocrine/paracrine manner, binds to cholinergic receptors on target cells. Both mechanisms subsequently lead to physiological and pathophysiological responses. We also investigated the subunits/subtypes of cholinergic receptors on target cells, physiological and pathophysiological responses of the organs via cholinergic receptors, and extracellular factors that alter the subtypes/subunits of cholinergic receptors. Collectively, this concept will elucidate how cholinergic responses occur and will help us conduct further experiments to develop new therapeutic agents.

Tiotropium bromide exerts anti-inflammatory activity in a cigarette smoke mouse model of COPD

Tiotropium bromide is a long acting muscarinic antagonist (LAMA), marketed under the brand name Spiriva, for the treatment of chronic obstructive pulmonary disease (COPD). Besides its proven direct bronchodilatory activity, recent clinical studies demonstrated that tiotropium is able to reduce the exacerbation rate and impact the clinical course of COPD. One significant pathological feature believed to be causative for the progressive nature of COPD is chronic pulmonary inflammation. The aim of the present study was to investigate the anti-inflammatory activity of tiotropium on cigarette smoke-induced pulmonary inflammation in mice. C57Bl/6 mice were exposed to cigarette smoke (CS) for four days with increasing exposure time for up to 6h per day to elicit pulmonary inflammation and mediator release. One hour before smoke exposure, animals were treated with tiotropium by inhalation (0.01-0.3mg/mL) for 5 min; 18h after the last CS exposure a bronchoalveolar lavage was performed. Tiotropium concentration-dependently inhibited pulmonary neutrophilic inflammation with an IC(50) of 0.058 mg/mL and a maximum inhibition of 60% at 0.3mg/mL. Furthermore, the CS-induced pulmonary release of leukotriene B(4), interleukin-6, keratinocyte-derived chemokine, monocyte chemotactic protein-1, macrophage inflammatory protein-1 alpha and -2, and tumor necrosis factor alpha was dose-dependently reduced. The bronchodilatory activity of tiotropium against acetycholine-induced bronchoconstriction was found to be in the same dose range as the anti-inflammatory activity with an IC(50) of 0.045 mg/mL and a maximum bronchodilation of 90% at 0.3mg/mL. Our data suggest that the beneficial effects of tiotropium on the course of COPD shown in patients may be associated with an anti-inflammatory activity.

Muscarinic receptor subtypes in cilia-driven transport and airway epithelial development

Ciliary beating of airway epithelial cells drives the removal of mucus and particles from the airways. Mucociliary transport and possibly airway epithelial development are governed by muscarinic acetylcholine receptors but the precise roles of the subtypes involved are unknown. This issue was addressed by determining cilia-driven particle transport, ciliary beat frequency, and the composition and ultrastructural morphology of the tracheal epithelium in M1-M5 muscarinic receptor gene-deficient mice. Knockout of M3 muscarinic receptors prevented an increase in particle transport speed and ciliary beat frequency in response to muscarine. Furthermore, the ATP response after application of muscarine was blunted. Pretreatment with atropine before application of muscarine restored the response to ATP. Additional knockout of the M2 receptor in these mice partially restored the muscarine effect, most likely through the M1 receptor, and normalised the ATP response. M1, M4 and M5 receptor-deficient mice exhibited normal responses to muscarine. None of the investigated mutant mouse strains had any impairment of epithelial cellular structure or composition. In conclusion, M3 receptors stimulate whereas M2 receptors inhibit cilia-driven particle transport. The M1 receptor increases cilia-driven particle transport if the M3 and M2 receptors are missing. None of the receptors is necessary for epithelial development.

The effect of asthma therapeutics on signalling and transcriptional regulation of airway smooth muscle function

SCOPE OF THE REVIEW

Our knowledge of the multifunctional nature of airway smooth muscle (ASM) has expanded rapidly in the last decade, but the underlying molecular mechanisms and how current therapies for obstructive airway diseases, such as asthma and chronic obstructive pulmonary disease (COPD), affect these are still being elucidated. Our current knowledge has built on the pharmacology of human ASM contraction and relaxation established prior to that and which is reviewed in detail elsewhere in this issue. The advent of methods to isolate and culture ASM cells, especially human ASM cells, has made it possible to study how they may contribute to airway remodelling through their synthetic, proliferative, and migratory capacities. Now the underlying molecular mechanisms of ASM growth factor secretion, extracellular matrix (ECM) production, proliferation and migration, as well as contraction and relaxation, are being determined. A complex network of signalling pathways leading to gene transcription in ASM cells permits this functional plasticity in healthy and diseased airways. This review is an overview of the effects of current therapies, and some of those in development, on key signalling pathways and transcription factors involved in these ASM functions.

Isolated porcine bronchi provide a reliable model for development of bronchodilator anti-muscarinic agents for human use

BACKGROUND AND PURPOSE

In human airways, muscarinic acetylcholine receptors (mAChRs) exert a predominant role in the control of airways resistance and anti-muscarinic agents are currently included in the pharmacological treatment of chronic obstructive pulmonary disease (COPD). However, the development of more effective mAChR antagonists is hampered by considerable species variability in the ultrastrucural and functional control of airway smooth muscle, making extrapolation of any particular animal model questionable. This study was designed to characterize the mAChRs in a bronchial preparation from pigs, animals considered to provide close models of human biology.

EXPERIMENTAL APPROACH

Smooth muscle bronchial strips were examined by electron microscopy in order to compare their neuromuscular structure with that of human bronchi and used to study the affinity of a series of selective mAChR antagonists, estimated as pKis in competition binding assays with NMS and pA2, by Schild analysis, in contractile experiments.

KEY RESULTS

Pharmacodynamic binding parameters and affinity profiles of a series of antagonists were consistent with the presence of a majority of M2 mAChRs along with a minor population of M3 mAChRs. Functionally, the highly significant correlation between postjunctional pA2 affinities and corresponding affinity constants at human recombinant M1-M5 subtypes indicated that smooth muscle contraction in porcine bronchi, as in human bronchi, was dependent on the M3 subtype.

CONCLUSION AND IMPLICATIONS

Based on the characterization of mAChRs, isolated porcine bronchi provide an additional experimental model for development of mAChR antagonists for the treatment of human airway dysfunctions.

Bronchodilatory effects of S-isopetasin, an antimuscarinic sesquiterpene of Petasites formosanus, on obstructive airway hyperresponsiveness

In the presence of neostigmine (0.1 microM), S-isopetasin competitively antagonized cumulative acetylcholine-induced contractions in guinea pig trachealis, because the slope [1.18+/-0.15 (n=6)] of Schild's plot did not significantly differ from unity. The pA2 value of S-isopetasin was calculated to be 4.62+/-0.05 (n=18). The receptor binding assay for muscarinic receptors of cultured human tracheal smooth muscle cells (HTSMCs) was performed using [3H]-N-methylscopolamine ([3H]-NMS). Saturation binding assays were carried out with [3H]-NMS in the presence (non-specific binding) and absence (total binding) of atropine (1 microM). Analysis of the Scatchard plot (y=0.247-1.306x, r2=0.95) revealed that the muscarinic receptor binding sites in cultured HTSMCs constituted a single population (n(H)=1.00). The equilibrium dissociation constant (Kd) and the maximal receptor density (B(max)) for [3H]-NMS binding were 766 pM and 0.189 pmol/mg of protein, respectively. The -logIC50 values of S-isopetasin, methoctramine, and 1,1-Dimethyl-4-diphenylacetoxypiperidinium iodide (4-DAMP) for displacing 0.4 nM [3H]-NMS-specific binding were 5.05, 6.25, and 8.56, respectively, which suggests that [3H]-NMS binding is predominantly on muscarinic M3 receptors of cultured HTSMCs. The inhibitory effects of S-isopetasin on enhanced pause (P(enh)) value were similar to that of ipratropium bromide, a reference drug. The duration of action of S-isopetasin (20 microM), also similar to that of ipratropium bromide (20 microM), was 3 h. In contrast to ipratropium bromide, which non-selectively acts on muscarinic receptors, S-isopetasin preferentially acts on muscarinic M3 receptors. In conclusion, S-isopetasin may be beneficial as a bronchodilator in the treatment of chronic obstructive pulmonary disease and asthma exacerbations.

p42/p44 MAP kinase activation is localized to caveolae-free membrane domains in airway smooth muscle

Caveolae are abundant plasma membrane invaginations in airway smooth muscle that may function as preorganized signalosomes by sequestering and regulating proteins that control cell proliferation, including receptor tyrosine kinases (RTKs) and their signaling effectors. We previously demonstrated, however, that p42/p44 MAP kinase, a critical effector for cell proliferation, does not colocalize with RTKs in caveolae of quiescent airway myocytes. Therefore, we investigated the subcellular sites of growth factor-induced MAP kinase activation. In quiescent myocytes, though epidermal growth factor receptor (EGFR) was almost exclusively found in caveolae, p42/p44 MAP kinase, Grb2, and Raf-1 were absent from these membrane domains. EGF induced concomitant phosphorylation of caveolin-1 and p42/p44 MAP kinase; however, EGF did not promote the localization of p42/p44 MAP kinase, Grb2, or Raf-1 to caveolae. Interestingly, stimulation of muscarinic M(2) and M(3) receptors that were enriched in caveolae-deficient membranes also induced p42/p44 MAP kinase phosphorylation, but this occurred in the absence of caveolin-1 phosphorylation. This suggests that the localization of receptors to caveolae and interaction with caveolin-1 is not directly required for p42/p44 MAP kinase phosphorylation. Furthermore, we found that EGF exposure induced rapid translocation of EGFR from caveolae to caveolae-free membranes. EGFR trafficking coincided temporally with EGFR and p42/p44 MAP kinase phosphorylation. Collectively, this indicates that although caveolae sequester some receptors associated with p42/p44 MAP kinase activation, the site of its activation is associated with caveolae-free membrane domains. This reveals that directed trafficking of plasma membrane EGFR is an essential element of signal transduction leading to p42/p44 MAP kinase activation.

Changes in muscarinic cholinergic receptor expression in human peripheral blood lymphocytes in allergic rhinitis patients

BACKGROUND

Parasympathetic nerves provide the dominant autonomic innervation of the upper and lower airways. They release acetylcholine that, activating post-junctional muscarinic receptors, causes bronchoconstriction, mucous secretion and vasodilation. Dysfunction of the upper and lower airways frequently coexist, and they appear to share key elements of pathogenesis.

OBJECTIVE

The present study has assessed the expression and pattern of cholinergic muscarinic receptor subtypes in peripheral blood lymphocytes harvested from allergic rhinitis patients with different degree of bronchial hyperresponsiveness detected by methacholine challenge test.

METHODS

Radioligand binding assay for determining the density of muscarinic cholinergic receptor subtypes; immunoblot analysis for assessing the characteristic of muscarinic cholinergic receptor subtype protein and immunocytochemical techniques for investigating the cellular localization of receptors.

RESULTS

An increased expression of M2 and M5 receptor proteins was observed in peripheral blood lymphocytes of allergic rhinitis patients in comparison with healthy control individuals. M3 receptor subtype decreased in allergic rhinitis patients with normal or mild responses to methacholine. A trend versus a return to normal value was found in moderate and severe responders. No changes of the M4 receptor subtype were found.

CONCLUSIONS AND CLINICAL IMPLICATIONS

Increase in M2 receptor expression correlated with disease severity and bronchial hyperreactivity. Changes in muscarinic cholinergic receptor expression in allergic rhinitis underline a role of cholinergic system of immune cells in allergic airway disease.

CAPSULE SUMMARY

Studies addressed to rhinitis and asthma have identified many similarities. Our results indicate that changes in peripheral blood lymphocyte muscarinic receptor expression may reflect the cholinergic involvement into allergic airway diseases.

Segment-dependent expression of muscarinic acetylcholine receptors and G-protein coupling in the equine respiratory tract

Muscarinic receptors are considered to be of comparable clinical importance in chronic obstructive pulmonary disease (COPD) in equines and in humans. At present, data are scarce on the expression and distribution of probable subtypes of these receptors and their signalling pathways in airway segments, including lung parenchyma and bronchial and tracheal epithelium with the underlying smooth muscle in horses. Specific [N-methyl-3H]scopolamine chloride ([3H]NMS) binding to all three tissues was saturable and of high affinity, with KD values ranging between 1.6+/-0.7 and 1.9+/-0.3 nmol/L. [3H]NMS binding identified a higher density of total muscarinic receptors (fmol/mg protein) in the trachea (720+/-59 nmol/L) than in bronchi (438+/-48 nmol/L) or lung (22 +/- 3 nmol/L). Competitive binding studies using [3H]NMS and the unlabelled subtype-selective antagonists pirenzepine and telenzepine (M1), methoctramine and himbacine (M2), 4-diphenylacetoxy-N-methylpiperidine (4-DAMP) (M3), tropicamide (M4) and mamba toxin (MT-3) (M4) indicated the presence of at least three muscarinic receptor subtypes in peripheral lung tissue (50:40:24-28%: M2>M3>M1), whereas in bronchus and trachea M2 subtypes (87-90%) predominated over M3 (14-22%), and M1 subtypes were lacking. No differences were found between tissues in high-affinity binding sites for carbachol in the absence (31-36%) or presence of guanosine 5'-triphosphate (GTP) (approximately 100%). Western blotting for G-protein alpha-subunits showed a much more robust expression of G(alphai1/2) in the trachea (with highest receptor density) than in the lung or bronchi, whereas G(alphas)-protein was dominantly expressed in bronchus. Concomitantly, carbachol inhibited isoproterenol- and GTP-stimulated adenylyl cyclase activity with increasing muscarinic receptor expression (trachea > bronchi > lung). We conclude that the expression and signalling pathways of muscarinic receptors in the equine respiratory tract are segment-dependent. These receptors might contribute to the pathogenesis of COPD in the horse and could provide potential drug targets for the therapeutic use of anticholinergics in this species.

Muscarinic receptor subtypes in neuronal and non-neuronal cholinergic function

1 Muscarinic M1-M5 receptors mediate the metabotropic actions of acetylcholine in the nervous system. A growing body of data indicate they also mediate autocrine functions of the molecule. The availability of novel and selective muscarinic agonists and antagonists, as well as in vivo gene disruption techniques, has clarified the roles of muscarinic receptors in mediating both functions of acetylcholine. 2 Selective M1 agonists or mixed M1 agonists/M2 antagonists may provide an approach to the treatment of cognitive disorders, while M3 antagonism, or mixed M2/M3 antagonists, are approved for the treatment of contractility disorders including overactive bladder and chronic obstructive pulmonary disease. Preclinical data suggest that selective agonism of the M4 receptor will provide novel anti-nociceptive agents, while therapeutics-based upon agonism or antagonism of the muscarinic M5 receptor have yet to be reported. 3 The autocrine functions of muscarinic receptors broadly fall into two areas - control of cell growth or proliferation and mediation of the release of chemical mediators from epithelial cells, ultimately causing muscle relaxation. The former particularly are involved in embryological development, oncogenesis, keratinocyte function and immune responsiveness. The latter regulate contractility of smooth muscle in the vasculature, airways and urinary bladder. 4 Most attention has focused on muscarinic M1 or M3 receptors which mediate lymphocyte immunoresponsiveness, cell migration and release of smooth muscle relaxant factors. Muscarinic M4 receptors are implicated in the regulation of keratinocyte adhesion and M2 receptors in stem cell proliferation and development. Little data are available concerning the M5 receptor, partly due to the difficulties in defining the subtype pharmacologically. 5 The autocrine functions of acetylcholine, like those in the nervous system, involve activation of several muscarinic receptor subtypes. Consequently, the role of these subtypes in autocrine, as well neuronal cholinergic systems, significantly expands their importance in physiology and pathophysiology.

Variable therapeutic response in asthma: a genetic perspective

Asthma is a complex clinical syndrome with multiple genetic and environmental factors contributing to its phenotypic expression. This etiological heterogeneity adds to the complexity when addressing variation in the response to antiasthma treatment. There is regular progress in the field of asthma pharmacogenetics in determining the efficacy and potential for adverse effects of the asthma medication from a patient's genetic background. This reveals that a clinically relevant variability in response to the asthma medications may be due to genetic determinants, and refers to the polymorphisms in the genes encoding either the drug targets, or the molecular component of the downstream signal transduction pathways responsible for drug actions. The major classes of asthma therapy, β-agonists, leukotriene antagonists and inhaled corticosteroids, demonstrate wide interindividual variability. The statistical issues, such as population stratification, sample size and statistical power, are crucial factors for the identification of significant biological marker(s) for patient's response. The aim of this review is to discuss the scientific rationale and outline the genetic impact on the variability in response to different asthma medications. In conclusion, despite of new developments and recent studies in asthma pharmacogenetics, significant gaps in knowledge still remain, and several replicate studies are needed in different populations to derive firm conclusions that may help to bring pharmacotyping into clinical practice.

Expression and muscarinic receptor coupling of Lyn kinase in cultured human airway smooth muscle cells

Src family tyrosine kinases are signaling intermediates in a diverse array of cellular events including cell differentiation, motility, proliferation, and survival. In nonairway smooth muscle cells, muscarinic receptors directly interact with Src family tyrosine kinases. As little is known about the expression and signaling of these Src family tyrosine kinases in human airway smooth muscle cells, we determined the expression of Src family members and characterized the muscarinic receptor-mediated activation of Lyn kinase in these cells. RT-PCR revealed mRNA transcripts for FYN, c-SRC, YES, FRK, and LYN. Fyn, c-Src, Yes, and Lyn were identified in cultured airway smooth muscle cells by immunoblot analysis. In both nontransformed human cultured airway smooth muscle cells and cells transduced with wild-type human Lyn kinase, carbachol increased Lyn kinase activity. Pertussis toxin pretreatment failed to block carbachol activation of Lyn kinase but did attenuate the carbachol-induced increase in ERK/MAPK phosphorylation. Moreover, carbachol inhibited adenylyl cyclase but failed to increase total inositol phosphate synthesis in these cells. The present study shows that Lyn kinase is expressed in human cultured airway smooth muscle cells at both the mRNA and protein levels and that carbachol, an M2 muscarinic receptor agonist in these cells, activates Lyn kinase by a pertussis toxin-insensitive signaling pathway.

Acetylcholine: a novel regulator of airway smooth muscle remodelling?

Increased airway smooth muscle mass is a pathological feature that asthma and chronic obstructive pulmonary disease (COPD) have in common. This increase has gained renewed interest in view of recent developments showing that airway smooth muscle, instead of solely being a contractile partner, is capable of interacting dynamically with its environment, especially under inflammatory conditions. Airway smooth muscle cells are able to proliferate, to migrate, and to secrete chemokines, cytokines, extracellular matrix proteins and growth factors, and most importantly, to adapt to these functions by changing its phenotype from contractile to proliferative/synthetic. Conversely, switching to a (hyper)contractile phenotype may also occur. A vast number of inflammatory stimuli regulate these functions and exert their effects via excitatory G(q) or G(i)-coupled receptors. Since acetylcholine activates muscarinic M(2) and M(3) receptors in the airway smooth muscle cell membrane, which are coupled to G(i) and G(q) proteins, respectively, and since acetylcholine release may be enhanced in airway inflammation, a pathophysiological role of acetylcholine related to the above processes and exceeding contraction could be envisaged. In this review, evidence in favour of this hypothesis, based on recent data that show a role for muscarinic receptors in modulating airway smooth muscle proliferation, contractility and contractile protein expression is discussed. Based on these findings, we postulate that endogenous acetylcholine contributes to airway remodeling in asthma and COPD.

The role of anticholinergics in chronic obstructive pulmonary disease

Anticholinergics are the bronchodilators of choice in the management of chronic obstructive pulmonary disease (COPD). They work by blocking muscarinic receptors in airway smooth muscle. Cholinergic tone appears to be the only reversible component of COPD. With the discovery of different muscarinic receptor subtypes, the development of more selective anticholinergics is possible. A major advance in this therapeutic area has been the discovery of tiotropium bromide, which has kinetic selectivity for M3 receptors as well as a duration of action of >24 hours. Once-daily administration of tiotropium is well tolerated and has shown significant advantages over ipratropium bromide, given 4 times daily, in the control of COPD.

The airway cholinergic system: physiology and pharmacology

The present review summarizes the current knowledge of the cholinergic systems in the airways with special emphasis on the role of acetylcholine both as neurotransmitter in ganglia and postganglionic parasympathetic nerves and as non-neuronal paracrine mediator. The different cholinoceptors, various nicotinic and muscarinic receptors, as well as their signalling mechanisms are presented. The complex ganglionic and prejunctional mechanisms controlling the release of acetylcholine are explained, and it is discussed whether changes in transmitter release could be involved in airway dysfunctions. The effects of acetylcholine on different target cells, smooth muscles, nerves, surface epithelial and secretory cells as well as mast cells are described in detail, including the receptor subtypes involved in signal transmission.

Role of muscarinic receptor subtypes in the constriction of peripheral airways: studies on receptor-deficient mice

In the airways, increases in cholinergic nerve activity and cholinergic hypersensitivity are associated with chronic obstructive pulmonary disease and asthma. However, the contribution of individual muscarinic acetylcholine receptor subtypes to the constriction of smaller intrapulmonary airways that are primarily responsible for airway resistance has not been analyzed. To address this issue, we used videomicroscopy and digital imaging of precision-cut lung slices derived from wild-type mice and mice deficient in either the M1 (mAChR1-/- mice), M2 (mAChR2-/- mice), or M3 receptor subtype (mAChR3-/- mice) or lacking both the M2 and M3 receptor subtypes (mAChR2/3-/- double-knockout mice). In peripheral airways from wild-type mice (mAChR+/+ mice), muscarine induced a triphasic concentration-dependent response, characterized by an initial constriction, a transient relaxation, and a sustained constriction. The bronchoconstriction was diminished by up to 60% in mAChR3-/- lungs and was completely abolished in mAChR2/3-/- lungs. The sustained bronchoconstriction was reduced in mAChR2-/- bronchi, and, interestingly, the transient relaxation was absent; the bronchoconstriction in response to 10-8 M muscarine was increased by 158% in mAChR1-/- mice. Quantitative reverse transcriptase-polymerase chain reaction analysis revealed that the disruption of specific mAChR genes had no significant effect on the expression levels of the remaining mAChR subtypes. These results demonstrate that cholinergic constriction of murine peripheral airways is mediated by the concerted action of the M2 and M3 receptor subtypes and suggest the existence of pulmonary M1 receptor activation, which counteracts cholinergic bronchoconstriction. Given the important role of muscarinic cholinergic mechanisms in pulmonary disease, these findings should be of considerable therapeutic relevance.

G protein-coupled receptor kinase 5 regulates airway responses induced by muscarinic receptor activation

G protein-coupled receptors (GPCRs) transduce extracellular signals into intracellular events. The waning responsiveness of GPCRs in the face of persistent agonist stimulation, or desensitization, is a necessary event that ensures physiological homeostasis. GPCR kinases (GRKs) are important regulators of GPCR desensitization. GRK5, one member of the GRK family, desensitizes central M(2) muscarinic receptors in mice. We questioned whether GRK5 might also be an important regulator of peripheral muscarinic receptor responsiveness in the cardiopulmonary system. Specifically, we wanted to determine the role of GRK5 in regulating muscarinic receptor-mediated control of airway smooth muscle tone or regulation of cholinergic-induced bradycardia. Tracheal pressure, heart rate, and tracheal smooth muscle tension were measured in mice having a targeted deletion of the GRK5 gene (GRK5(-/-)) and littermate wild-type (WT) control mice. Both in vivo and in vitro results showed that the airway contractile response to a muscarinic receptor agonist was not different between GRK5(-/-) and WT mice. However, the relaxation component of bilateral vagal stimulation and the airway smooth muscle relaxation resulting from beta(2)-adrenergic receptor activation were diminished in GRK5(-/-) mice. These data suggest that M(2) muscarinic receptor-mediated opposition of airway smooth muscle relaxation is regulated by GRK5 and is, therefore, excessive in GRK5(-/-) mice. In addition, this study shows that GRK5 regulates pulmonary responses in a tissue- and receptor-specific manner but does not regulate peripheral cardiac muscarinic receptors. GRK5 regulation of airway responses may have implications in obstructive airway diseases such as asthma or chronic obstructive pulmonary disease.

Muscarinic acetylcholine receptors and airway diseases

Parasympathetic nerves provide the dominant autonomic innervation of the airways. Release of acetylcholine from parasympathetic nerves activates postjunctional muscarinic receptors present on airway smooth muscle, submucosal glands, and blood vessels to cause bronchoconstriction, mucus secretion, and vasodilatation, respectively. Acetylcholine also feeds back onto prejunctional muscarinic receptors to enhance or inhibit further acetylcholine release. In asthma and chronic obstructive pulmonary disease, bronchoconstriction and mucus secretion is increased and the airways are hyperresponsive to contractile agents. These changes are due to increased parasympathetic nerve activity. The number and function of postjunctional muscarinic receptors in the airways are unchanged in animal models of asthma. Rather, it is the supply of acetylcholine to the postjunctional cells (smooth muscle and submucosal gland) that is increased. The increase in acetylcholine release occurs because prejunctional, inhibitory M(2) muscarinic receptors on the parasympathetic nerves are dysfunctional. M(2) muscarinic receptor dysfunction and subsequent airway hyperreactivity have been demonstrated to occur in animals in response to a variety of triggers, including antigen challenge, virus infection, ozone exposure, and vitamin A deficiency. In humans, there is evidence that loss of M(2) muscarinic receptor function is related to asthma. The mechanisms by which neuronal M(2) muscarinic receptor function is lost and its relevance to human airway disease are discussed in this review.

Pharmacological approaches to discovery and development of new mucolytic agents

Airway mucus is the secretory product of the mucous cells; it is a variable mixture of water, mucous glycoproteins, low molecular weight ions, proteins, and lipids, whose physical properties are important for airway defense. The factors that contribute to the physical properties of mucus are complex, and there are a number of pharmacological strategies that can potentially serve to improve the clearability of airway mucus. Novel mucoactive approaches include strategies for mucoregulation--decreasing the abnormal volume of mucus secretion--and medications designed to improve the cough clearability of airway secretions. In vitro results suggest potential benefits from the additive effects of selected combinations of mucoactive medications. Further studies are required to confirm these findings, to perform direct assessments of mucus clearability, and to extend the observations to patients with various types of pulmonary diseases where mucoactive treatments are required.

Muscarinic M2 receptors in acetylcholine-isoproterenol functional antagonism in human isolated bronchus

The muscarinic functional antagonism of isoproterenol relaxation and the contribution of muscarinic M2 receptors were examined in human isolated bronchus. In intact tissues, acetylcholine (ACh) precontraction decreased isoproterenol potency and maximal relaxation (-log EC50 shift = -1.49 +/- 0.16 and E(max) inhibition for 100 microM ACh = 30%) more than the same levels of histamine contraction. The M2 receptor-selective antagonist methoctramine (1 microM) reduced this antagonism in ACh- but not histamine-contracted tissues. Similar results were obtained for forskolin-induced relaxation. After selective inactivation of M3 receptors with 4-diphenylacetoxy-N-(2-chloroethyl)piperadine hydrochloric acid (30 nM), demonstrated by abolition of contractile and inositol phosphate responses to ACh, muscarinic recontractile responses were obtained in U-46619-precontracted tissues fully relaxed with isoproterenol. Methoctramine antagonized recontraction, with pK(B) (6.9) higher than in intact tissues (5.4), suggesting participation of M2 receptors. In M3-inactivated tissues, methoctramine augmented the isoproterenol relaxant potency in U-46619-contracted bronchus and reversed the ACh-induced inhibition of isoproterenol cAMP accumulation. These results indicate that M2 receptors cause indirect contraction of human bronchus by reversing sympathetically mediated relaxation and contribute to cholinergic functional antagonism.

Effects of albuterol enantiomers on ciliary beat frequency in ovine tracheal epithelial cells

beta(2)-Adrenergic agonists stimulate ciliary beat frequency (CBF), an integral part of mucociliary clearance. To evaluate the differential effects of albuterol enantiomers and their racemic mixture on ciliary function, CBF and intracellular calcium were measured at room temperature from single ovine airway epithelial cells with use of digital videomicroscopy. Baseline CBF was 7.2 +/- 0.2 (SE) Hz (n = 80 measurements). R-albuterol (10 microM to 1 mM) stimulated CBF in a dose-dependent manner to maximally 24.4 +/- 5.4% above baseline. Racemic albuterol stimulated CBF to maximally 12.8 +/- 3.6% above baseline, a significantly lower increase compared with R-albuterol alone, despite identical R-enantiomer amounts in both groups. Simultaneous recordings of intracellular calcium concentration and CBF from single cells indicated that the CBF increase in response to R-albuterol was mediated through beta-receptors and stimulation of protein kinase A, in a calcium-dependent and -independent fashion. S-albuterol had a negligible effect on CBF and did not change intracellular calcium. Together, these results suggest that R-albuterol is more efficacious than racemic albuterol in stimulating CBF. Thus S-albuterol may interfere with the ability of R-albuterol to increase CBF.

The Non-neuronal cholinergic system: an emerging drug target in the airways

The non-neuronal cholinergic system is widely expressed in human airways. Choline acetyltransferase (ChAT) and/or acetylcholine are demonstrated in more or less all epithelial surface cells (goblet cells, ciliated cells, basal cells), submucosal glands and airway smooth muscle fibres. Acetylcholine is also demonstrated in the effector cells of the immune system (lymphocytes, macrophages, mast cells). Epithelial, endothelial and immune cells express nicotinic and muscarinic receptors. Thus the cytomolecule acetylcholine can contribute to the regulation of basic cell functions via auto-/paracrine mechanisms (proliferation, differentiation, ciliary activity, secretion of water, ions and mucus, organization of the cytoskeleton, cell-cell contact). Acetylcholine also modulates immune functions (release of cytokines; proliferation, activation and inhibition of immune cells). Preliminary experimental evidence suggests that mucosal inflammation may be associated with raised acetylcholine levels, impairing cell and organ homeostasis. It should be considered that anti-muscarinic drugs which are applied for the treatment of chronic airway diseases antagonize the effect of both neuronal and non-neuronal acetylcholine. Non-neuronal acetylcholine, however, is still active, possibly directly within the cell cytosol and also via nicotinic receptors localized on various non-neuronal cells. It is an essential task to clarify the pathophysiological role of the non-neuronal cholinergic system in more detail to develop new drugs which can target the synthesis, release, inactivation and cellular activity of non-neuronal acetylcholine.

Effects of inhalation exposures to an M1-receptor agonist on ventilation in rhesus monkeys

Information was needed on effects of possible occupational inhalation exposure to an M1-receptor agonist (xanomeline) such as might occur during the manufacturing process. Both acute and repeated inhalation exposures to xanomeline were carried out in six male rhesus monkeys using a head-dome exposure system. Exposure concentrations ranged from 0.3 to 10 mg/m3. The exposure durations were up to 2 weeks. Decreases in tidal volume and increases in respiratory frequency were both time and concentration related during acute exposures. These effects were blocked with atropine pre-treatment. Correlation with pulmonary resistance measurements in two monkeys suggested that these were bronchoconstrictive changes that increased with severity with time at a given concentration and with concentration when measured after a constant exposure time. The dose-response was relatively steep with 10 mg/m3 becoming intolerable to the monkeys after approximately 15 minutes, but no measurable effects were observed at 0.3 mg/m3 after up to 4 hours of exposure. To investigate the effects of repeated exposures, monkeys were exposed for 4 hr/day, 5 days/wk for 2 weeks to 0.0 (air only), 0.3, and 1.2 mg xanomeline/m3 of air. When compared to the air-only exposure, 0.3 mg/m3 caused no significant changes in tidal volume. In contrast, 1.2 mg/m3 caused a rapid and significant decrease in tidal volume that was sustained throughout the 4-hr exposure. A slower rise in breathing frequency also occurred. Repeated exposures did not alter the effects seen after a single exposure. It is concluded that xanomeline, a M1-receptor agonist, can acutely alter normal ventilation in non-human primates at airborne concentrations > or = 0.6 mg/m3 and should be carefully controlled in a manufacturing environment. The no-observed-effect concentration was 0.3 mg/m3.

Muscarinic cholinoceptor subtypes mediating tracheal smooth muscle contraction and inositol phosphate generation in guinea pig and rat

The effects of the muscarinic cholinoceptor antagonists atropine (non-selective), pirenzepine (M1-selective), methoctramine (M2-selective) and 4-diphenylacetoxy-N-methylpiperidine methiodide (4-DAMP; M3-selective) were examined on the responsiveness of guinea pig and rat tracheal tissue to acetylcholine and carbachol. Results indicate that smooth muscle contraction in isolated tracheal tissue from both species was mediated primarily by muscarinic M3 cholinoceptors. The effects of atropine, pirenzepine and 4-DAMP were similar against the contractile actions of acetylcholine and carbachol in both species and in epithelium-intact and epithelium-denuded tissue. In contrast, differences in the effects of methoctramine in antagonising contractile responses to acetylcholine and carbachol were observed between the two species and following epithelium removal in the guinea pig. Thus, whilst this study has found that tracheal smooth muscle contraction in the guinea pig and rat is mediated primarily by muscarinic M3 cholinoceptors, anomalies in the functional inositol phosphate generation results obtained with the muscarinic cholinoceptor antagonists highlight species differences in the actions of acetylcholine and carbachol in eliciting smooth muscle contraction suggesting the possible existence of functional non-M3 muscarinic cholinoceptors.

Pharmacological characterization of the muscarinic receptor antagonist, glycopyrrolate, in human and guinea-pig airways

1. In this study we have evaluated the pharmacological profile of the muscarinic antagonist glycopyrrolate in guinea-pig and human airways in comparison with the commonly used antagonist ipratropium bromide. 2. Glycopyrrolate and ipratropium bromide inhibited EFS-induced contraction of guinea-pig trachea and human airways in a concentration-dependent manner. Glycopyrrolate was more potent than ipratropium bromide. 3. The onset of action (time to attainment of 50% of maximum response) of glycopyrrolate was similar to that obtained with ipratropium bromide in both preparations. In guinea-pig trachea, the offset of action (time taken for response to return to 50% recovery after wash out of the test antagonist) for glycopyrrolate (t1/2 [offset]=26.4+/-0.5 min) was less than that obtained with ipratropium bromide (81.2+/-3.7 min). In human airways, however, the duration of action of glycopyrrolate (t1/2 [offset]>96 min) was significantly more prolonged compared to ipratropium bromide (t1/2 [offset]= 59.2+/-17.8 min). 4. In competition studies, glycopyrrolate and ipratropium bromide bind human peripheral lung and human airway smooth muscle (HASM) muscarinic receptors with affinities in the nanomolar range (K1 values 0.5-3.6 nM). Similar to ipratropium bromide, glycopyrrolate showed no selectivity in its binding to the M1-M3 receptors. Kinetics studies, however, showed that glycopyrrolate dissociates slowly from HASM muscarinic receptors (60% protection against [3H]-NMS binding at 30 nM) compared to ipratropium bromide. 5. These results suggest that glycopyrrolate bind human and guinea-pig airway muscarinic receptors with high affinity. Furthermore, we suggest that the slow dissociation profile of glycopyrrolate might be the underlying mechanism by which this drug accomplishes its long duration of action.

Tiotropium bromide (Ba 679 BR), a novel long-acting muscarinic antagonist for the treatment of obstructive airways disease

Tiotropium bromide (Ba 679 BR) is a novel potent and long-lasting muscarinic antagonist that has been developed for the treatment of chronic obstructive airways disease (COPD). Binding studies with [3H]tiotropium bromide in human lung have confirmed that this is a potent muscarinic antagonist with equal affinity for M1-, M2- and M3-receptors and is approximately 10-fold more potent than ipratropium bromide. Tiotropium bromide dissociates very slowly from lung muscarinic receptors compared with ipratropium bromide. In vitro tiotropium bromide has a potent inhibitory effect against cholinergic nerve-induced contraction of guinea-pig and human airways, that has a slower onset than atropine or ipratropium bromide. After washout, however, tiotropium bromide dissociates extremely slowly compared with the dissociation of atropine and ipratropium bromide. Measurement of acetylcholine (ACh) release from guinea-pig trachea shows that tiotropium bromide, ipratropium bromide and atropine all increase ACh release on neural stimulation and that this effect is washed out equally quickly for the three antagonists. This confirms binding studies to transfected human muscarinic receptors which suggested that tiotropium bromide dissociates slowly from M3-receptors (on airway smooth muscle) but rapidly from M2 autoreceptors (on cholinergic nerve terminals). Clinical studies with inhaled tiotropium bromide confirm that it is a potent and long-lasting bronchodilator in COPD and asthma. Furthermore, it protects against cholinergic bronchoconstriction for > 24 h. This suggests that tiotropium bromide will be a useful bronchodilator, particularly in patients with COPD, and may be suitable for daily dosing. The selectivity for M3- over M2-receptors may also confer a clinical advantage.

Pharmacology of airway smooth muscle

Airway smooth muscle contributes to changes in airway caliber not only through the variations in its tone but also through its contribution to thickness of the airway wall. Until recently, most attention was paid to the agents that altered airway smooth muscle tone, their receptors, the signal transduction pathways they activated, and the mechanisms of contraction and relaxation themselves. Lately, the regulation of smooth muscle proliferation has received increasing attention, and, most recently, the possible role of smooth muscle as a source of inflammatory mediators has been recognized. Airway smooth muscle cells are now seen as playing an important interactive role with inflammatory and structural cells in the response to injury and repair of the airways.

Muscarinic receptor binding sites of the M4 subtype in porcine lung parenchyma

Muscarinic acetylcholine receptors regulate distal airway resistance and secretion. The subtype expressed in the lung in different species remains uncertain. It has recently become possible to identify the M4 subtype by careful comparison of antagonist affinities. We characterized the binding of [3H]quinuclidinyl benzilate ([3H]QNB) to muscarinic receptors in cell membranes from lung parenchyma of 2-8 week old pigs in comparison to cloned human M3 and M4 receptors expressed in COS cells, to M2 in rat atria and to M4 in bovine adrenal medulla. In porcine lung, [3H]QNB bound with high affinity (Kd = 95 +/- 9 pM) to a single homogeneous population of muscarinic receptor sites (Bmax = 340 +/- 10 fmol/mg protein). Competition studies showed that the affinity (expressed as pKi) of 3 selective blockers was in close agreement between pig lung and cloned human m4 (r = 0.996). A series of 10 blockers showed affinities closely matching reported values for M4 receptors of the adrenal medulla (r = 0.965). Conversely, affinity values in porcine lung differed significantly (P < 0.05, t-test) from those determined in parallel with either human cloned M3 or with rat atria expressing the M2 subtype. We conclude that pig lung muscarinic receptor binding sites most closely resemble the M4 subtype, in contrast to the M3 subtype typical of large airways in this species.