Methacholine

Methacholine

Methacholine chemical structure

Methacholine is a synthetic choline ester that acts as a non-selective muscarinic receptor agonist in the parasympathetic nervous system. It is primarily used in the diagnosis of asthma through a test known as the methacholine challenge test.

Pharmacology

Methacholine is a quaternary ammonium compound, which means it carries a permanent positive charge. This characteristic limits its ability to cross the blood-brain barrier, thus reducing central nervous system effects. Methacholine mimics the action of the neurotransmitter acetylcholine by binding to muscarinic receptors, which are a subtype of acetylcholine receptors.

Mechanism of Action

Methacholine acts by stimulating muscarinic receptors, leading to the activation of the parasympathetic nervous system. This results in bronchoconstriction, increased bronchial secretions, and other effects typical of parasympathetic stimulation. The bronchoconstriction effect is utilized in the methacholine challenge test to assess airway hyperreactivity.

Clinical Use

Methacholine Challenge Test

The methacholine challenge test is a diagnostic tool used to evaluate airway hyperreactivity, which is a hallmark of asthma. During the test, the patient inhales increasing concentrations of methacholine aerosol. The degree of bronchoconstriction is measured using spirometry, specifically looking at the forced expiratory volume in one second (FEV1). A significant decrease in FEV1 indicates a positive test, suggesting the presence of asthma.

Side Effects

The administration of methacholine can lead to side effects due to its action on muscarinic receptors. Common side effects include:

• Bronchoconstriction
• Cough
• Wheezing
• Chest tightness
• Headache
• Dizziness

These effects are generally mild and resolve after the test is completed. However, in rare cases, severe bronchoconstriction can occur, necessitating the availability of bronchodilators and medical supervision during the test.

Contraindications

Methacholine should not be used in individuals with:

• Severe asthma
• Recent myocardial infarction
• Uncontrolled hypertension
• Known hypersensitivity to methacholine or other choline esters

Related Pages

• Asthma
• Bronchoconstriction
• Muscarinic receptor
• Parasympathetic nervous system