Metacholine

Metacholine (also spelt methacholine) is a synthetic choline ester that acts as a non-selective muscarinic receptor agonist in the parasympathetic nervous system. It is highly active at all of the muscarinic receptors, but has little effect on the nicotinic receptors. Metacholine has a charged quaternary amine structure, rendering it insoluble to lipid cell membranes. Clinically, this means that it will not cross the blood-brain barrier and has poor absorption from the gastrointestinal tract. It is broken down at a relatively slow rate within the body, due to its resistance to acetylcholinesterases.

The primary clinical use of methacholine is to diagnose bronchial hyperreactivity, which occurs in asthma. This is accomplished through the metacholine challenge test. Other therapeutic uses are limited by its adverse cardiovascular effects, such as bradycardia and hypotension, which arise from its function as a cholinomimetic.

Use of metacholine, as well as all other muscarinic receptor agonists, is contraindicated in patients with coronary insufficiency, gastroduodenal ulcers, and incontinence. The parasympathomimetic action of this drug will exacerbate the symptoms of these disorders.

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