The airway effects of released acetylcholine are mediated via activation of three distinct muscarinic receptor subtypes: M^ in parasympathetic ganglia, mucous glands and alveolar walls; autoinhibitory M2, in parasympathetic nerve terminals; and M3, in airway smooth muscle, mucus glands, and airway epithelium.
Although atropine and related compounds possess bronchodilator activity, their use is associated with the typical spectrum of anticholinergic side effects (see Chapter 13), and they are no longer used in the treatment of asthma. To improve the clinical utility of anti-cholinergics, quaternary amine derivatives of atropine were developed. By virtue of their positive charge, these drugs are absorbed poorly across mucosal surfaces and thus produce fewer side effects than atropine, especially when given by inhalation.
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If you suffer with asthma, you will no doubt be familiar with the uncomfortable sensations as your bronchial tubes begin to narrow and your muscles around them start to tighten. A sticky mucus known as phlegm begins to produce and increase within your bronchial tubes and you begin to wheeze, cough and struggle to breathe.