It is highly active at all of the muscarinic receptors, but has little effect on the
nicotinic receptors. Methacholine has a charged
quaternary amine structure, rendering it insoluble in 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 relative resistance to
acetylcholinesterases.
The chemical structure of methacholine is identical to
acetylcholine but with a methyl group on the beta carbon (hence being called acetyl-β-methylcholine), this β-methyl group provides selectivity toward
muscarinic receptors as compared to
nicotinic receptors. The quaternary ammonium group is essential for activity. The ester makes it susceptible to the enzyme
acetylcholine esterase.[2][unreliable source?]
Contraindications
Use of methacholine is contraindicated in patients with recent heart attack or stroke, uncontrolled hypertension, known severe airway disease, or aortic aneurysm. It may be used with caution by nursing or pregnant mothers and patients taking certain medications for
myasthenia gravis.[3]
^Valentin Popa (2001). "ATS guidelines for methacholine and exercise challenge testing". American Journal of Respiratory and Critical Care Medicine. 163 (1): 292–293.
doi:
10.1164/ajrccm.163.1.16310b.
PMID11208661.
External links
"Methacholine". Drug Information Portal. U.S. National Library of Medicine.