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  • Until now five muscarinic receptor M

    2024-04-02

    Until now, five muscarinic receptor (M1-5) genes have been cloned, but, M1-3 receptors are functionally recognized in the lungs. Smooth muscle contraction is mainly mediated by M3. M2 may also participate in the contraction via Gi protein which leads to a decrease in cAMP. On the other hand, M2 is located on postganglionic nerves and acts as feedback inhibitory receptors to inhibit ACh release from nerves. M2 dysfunction is thought to be involved in airway hyperresponsiveness in asthma. M1 is detected on airway parasympathetic ganglion cells and shown to mediate facilitatory neurotransmission and also identified in airway submucosal glands. Nicotinic receptors are also stimulated by ACh and multiple isotypes of nicotinic receptors including muscle and neuronal types have been identified. Nicotinic receptors are ligand-gated ion channels and are localized to parasympathetic ganglia, where they facilitate neurotransmission (Fig. 1). Nicotinic receptors are also present in various cells including airway epithelial cells, macrophages, lymphocytes and type 2 innate lymphoid cells, and the stimulation of nicotinic receptors mainly causes putative anti-inflammatory effects, but also causes mucus formation in airway epithelial cells and a proliferative effect on tumors. Currently available LAMAs show similar affinity to M1, M2, and M3. However, LAMAs show greater kinetical selectivity for M3 and M1 than M2 because they have a longer half-life of dissociation from M3 and M1 than that from M2. The bronchodilatory effect of LAMA is mainly attributed to M3 inhibition. M3 receptors are also involved in mucus secretion and, therefore, LAMAs are known to inhibit mucus secretion. One of the LAMAs, tiotropium or aclidinium, has been also demonstrated to attenuate cough. This antitussive effect of tiotropium is explained by the inhibition of transient receptor potential vanilloid 1 (TRPV1) -mediated depolarization of afferent sensory nerves through a mechanism unrelated to its anticholinergic activity.
    Non-neuronal ACh Recently, the existence of all components of the ACh synthesis system including high-affinity choline transporter 1 (CHT1), choline acetyltransferase (ChAT) and vesicular AG-126 transporter (VAChT), has been detected on various types of cells, such as epithelial cells, fibroblasts and inflammatory cells (Fig. 2A). Stimulation such as cigarette smoke, increases the levels of ChAT in airway epithelial cells. Muscarinic receptors are detected in epithelial cells and other structural cells including fibroblast and smooth muscle cells, and inflammatory cells. These lines of evidence suggest that non-neuronal cells also synthesize and release ACh, which may participate in various biological reactions in an autocrine or paracrine manner (Fig. 2B). A recent study using VAChT-deficient mice demonstrated that a decrease of VAChT causes airway inflammation, suggesting that endogenously released ACh might be necessary to maintain homeostasis in the lung. On the other hand, cigarette smoke has been shown to stimulate ACh synthesis in airway epithelial cells, which induces the release of inflammatory mediators from airway epithelial cells in an autocrine manner. In human fibroblasts from smokers and patients with COPD, the expression of ChAT has been also shown to be higher than that in healthy subjects. These results suggest that non-neuronal ACh could potentiate airway inflammation under an inflammatory condition and participate in the pathophysiology of airway diseases.
    Inflammatory effects of ACh on structural and inflammatory cells Recent cell culture and animal studies have demonstrated that ACh could potentiate airway inflammation and remodeling in airway diseases (Fig. 3).
    Effect of anti-muscarinic drugs on the airway inflammation in human Until now, there have been several studies that evaluated the effects of anti-muscarinic drugs on the airway and systemic inflammation in COPD patients. However, there is no evidence about the inhibitory effect of anti-muscarinic drugs on the inflammation in COPD patients. Although treatment with tiotropium in COPD patients decreased the annual rate of exacerbations, the amount of IL-6 and myeloperoxidase in sputum was not reduced, but the concentration of CXCL8 and MMP-9 was increased in the tiotropium treatment group. Tiotropium treatment also did not affect the serum level of IL-6 or C-reactive protein. These results might have been affected by the method of using sputum because tiotropium could decrease the amount of mucus secretion leading to a higher concentration of the mediators in sputum.