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Methacholine chloride is the β-methyl homolog of acetylcholine and differs from the latter primarily in its greater duration and selectivity of action. Bronchial smooth muscle contains significant parasympathetic (cholinergic) innervation.
Bronchoconstriction occurs when the vagus nerve is stimulated and acetylcholine is released from the nerve endings. Muscle constriction is essentially confined to the local site of release because acetylcholine is rapidly inactivated by acetylcholinesterase.
Compared with acetylcholine, methacholine chloride is more slowly hydrolyzed by acetylcholinesterase and is almost totally resistant to inactivation by nonspecific cholinesterase or pseudocholinesterase. When a sodium chloride solution containing methacholine chloride is inhaled, subjects with asthma are markedly more sensitive to methacholine-induced bronchoconstriction than are healthy subjects. This difference in response is the pharmacologic basis for the Provocholine® (methacholine chloride powder for inhalation) inhalation diagnostic challenge. However, it should be recognized that methacholine challenge may occasionally be positive after influenza, upper respiratory infections or immunizations, in very young or very old patients, or in patients with chronic lung disease (cystic fibrosis, sarcoidosis, tuberculosis, chronic obstructive pulmonary disease). The challenge may also be positive in patients with allergic rhinitis without asthma, in smokers, in patients after exposure to air pollutants, or in patients who have had or wil in the future develop asthma.
There are no metabolic and pharmacokinetic data available on methacholine chloride.
Last reviewed on RxList: 9/25/2008
This monograph has been modified to include the generic and brand name in many instances.
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