Methacholine is a:(AIIMS May 2015, May 2014)
**Core Concept**
Methacholine is a synthetic cholinergic agonist that mimics the action of acetylcholine at muscarinic receptors, primarily M2 receptors in the heart. This leads to a decrease in heart rate and contractility.
**Why the Correct Answer is Right**
Methacholine induces bronchoconstriction by stimulating M3 muscarinic receptors in airway smooth muscle cells, causing an increase in intracellular calcium and subsequent muscle contraction. This results in a decrease in lung function and airway resistance. The mechanism of action is similar to that of acetylcholine, which is the neurotransmitter responsible for parasympathetic stimulation of the heart and airways.
**Why Each Wrong Option is Incorrect**
**Option A:** This is an incorrect statement regarding methacholine's primary mechanism of action. While it does have some effect on the heart, its primary use is as a bronchoconstrictor.
**Option B:** This is incorrect as methacholine does not primarily act as a beta-blocker. Its primary action is through muscarinic receptors.
**Option C:** This is incorrect as methacholine is not a direct-acting vasodilator. Its primary action is through muscarinic receptors in the airways.
**Clinical Pearl / High-Yield Fact**
Methacholine is used in the diagnosis of bronchial hyperresponsiveness, particularly in patients with asthma or chronic obstructive pulmonary disease (COPD). A positive response to methacholine challenge is indicative of airway hyperresponsiveness.
**Correct Answer: D. Methacholine is a bronchoconstrictor.**