Chemical Properties
colourless to white crystalline powder with a slight
Uses
Acetyl-β-methylcholine (methacholine), an analog of acetylcholine, is use as a receptor agonist to study processes such as airway hyperresponsiveness associated with asthma.
Uses
antihypertensive, antianginal
Definition
ChEBI: Methacholine chloride is a quaternary ammonium salt. It contains a methacholine.
General Description
Methacholine chloride occurs as colorless or white crystalsor as a white crystalline powder. It is odorless or has aslight odor and is very deliquescent. It is freely soluble inwater, alcohol, or chloroform, and its aqueous solution isneutral to litmus and bitter. It is hydrolyzed rapidly in alkalinesolutions. Solutions are relatively stable to heat and willkeep for at least 2 or 3 weeks when refrigerated to delaygrowth of molds.
Biochem/physiol Actions
Metabolically stable analog of acetylcholine; muscarinic agonist. Its ability to constrict airway smooth muscle is used to assess bronchial reactivity in the laboratory and clinically.
Clinical Use
Methacholine, acetyl β-methylcholine, is marketed as the racemic mixture. It is a selective
muscarinic agonist with very little activity at nicotinic receptors. Although methacholine chloride is marketed as the racemic mixture, the
S-(+)-enantiomer is 240-fold more potent than the R-(–)-isomer at muscarinic receptors. In addition, AChE hydrolyzes S-(+)–methacholine at approximately 54% the rate of acetylcholine, whereas the R-(–)–enantiomer is a weak inhibitor. Methacholine chloride is used via inhalation for the diagnosis of asthma. The resulting bronchospasm may be relieved with bronchodilators. Methacholine chloride is available as a powder that is reconstituted for inhalation.
Purification Methods
It forms white hygroscopic needles from Et2O and is soluble in H2O, EtOH and CHCl3. It decomposes readily in alkaline solutions and slowly in H2O. It should be handled and stored in a dry atmosphere. The bromide is less hygroscopic, and the picrate has m 129.5-131o (from EtOH). [racemate: Annis & Ely Biochem J 53 34 1953, IR of iodide: Hansen Acta Chem Scand 13 155 1959, Beilstein 4 IV 1670.]
