INI-CET 2014 — Pharmacology

Q: Methacholine acts primarily at which receptor?

M3. ***M3*** - **Methacholine** is a non-selective muscarinic agonist that acts at all muscarinic receptors, but its **primary clinically relevant effects**, particularly in the airways (e.g., bronchoconstriction in asthma challenge tests), are mediated through **M3 receptors**. - **M3 receptors** are Gq-coupled and lead to **smooth muscle contraction**, glandular secretion, and vasodilation via nitric oxide release. *M1* - **M1 receptors** are primarily found in the central nervous system and autonomic ganglia, playing a role in **neuronal excitation** and **autonomic transmission**. - While methacholine can activate M1 receptors, this is not its predominant site of action for clinical uses. *M2* - **M2 receptors** are found in the heart and are Gi-coupled, leading to a **decrease in heart rate** and contractility. - While methacholine acts on M2 receptors, its most notable effects (like bronchoconstriction) are not primarily mediated through M2. *M4* - **M4 receptors** are primarily located in the central nervous system, where they modulate neurotransmitter release and contribute to **motor control** and cognition. - Methacholine has some affinity for M4, but it is not the primary receptor responsible for its clinically relevant effects. [Practice more Pharmacology PYQs on OnCourse]

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Methacholine acts primarily at which receptor?

Q2Easy

Which of the following conditions is NOT treated with a cholinomimetic agent?

INI-CET 2014 - Pharmacology INI-CET Practice Questions and MCQs

Question 1: Methacholine acts primarily at which receptor?

A. M1
B. M2
C. M3 (Correct Answer)
D. M4

Explanation: ***M3*** - **Methacholine** is a non-selective muscarinic agonist that acts at all muscarinic receptors, but its **primary clinically relevant effects**, particularly in the airways (e.g., bronchoconstriction in asthma challenge tests), are mediated through **M3 receptors**. - **M3 receptors** are Gq-coupled and lead to **smooth muscle contraction**, glandular secretion, and vasodilation via nitric oxide release. *M1* - **M1 receptors** are primarily found in the central nervous system and autonomic ganglia, playing a role in **neuronal excitation** and **autonomic transmission**. - While methacholine can activate M1 receptors, this is not its predominant site of action for clinical uses. *M2* - **M2 receptors** are found in the heart and are Gi-coupled, leading to a **decrease in heart rate** and contractility. - While methacholine acts on M2 receptors, its most notable effects (like bronchoconstriction) are not primarily mediated through M2. *M4* - **M4 receptors** are primarily located in the central nervous system, where they modulate neurotransmitter release and contribute to **motor control** and cognition. - Methacholine has some affinity for M4, but it is not the primary receptor responsible for its clinically relevant effects.

Question 2: Which of the following conditions is NOT treated with a cholinomimetic agent?

A. Bradycardia (Correct Answer)
B. Glaucoma
C. Myasthenia gravis
D. Post-surgical atony or ileus

Explanation: **Explanation:** Cholinomimetic agents (cholinergic agonists) mimic the action of acetylcholine (ACh) by stimulating muscarinic and nicotinic receptors. To answer this question, one must understand the physiological effects of Parasympathetic Nervous System (PNS) activation. **1. Why Bradycardia is the Correct Answer:** Stimulation of **M2 receptors** in the heart (specifically at the SA node) leads to a decrease in heart rate (negative chronotropy). Therefore, cholinomimetics would **worsen** bradycardia. In clinical practice, **anticholinergics** (like Atropine) are used to treat bradycardia, not cholinomimetics. **2. Why the other options are incorrect (Uses of Cholinomimetics):** * **Glaucoma:** Direct agonists like **Pilocarpine** cause miosis and contraction of the ciliary muscle, which opens the trabecular meshwork and facilitates the drainage of aqueous humor, reducing intraocular pressure. * **Myasthenia Gravis:** This condition involves autoantibodies against NMJ nicotinic receptors. Indirect cholinomimetics (AChE inhibitors) like **Pyridostigmine** or **Neostigmine** increase the concentration of ACh at the synaptic cleft, improving muscle strength. * **Post-surgical Atony/Ileus:** Drugs like **Bethanechol** (a muscarinic agonist) stimulate **M3 receptors** in the GI tract and bladder, promoting peristalsis and bladder emptying in non-obstructive conditions. **High-Yield NEET-PG Pearls:** * **Bethanechol:** "B" for Bowel and Bladder (used for postoperative ileus/urinary retention). * **Pilocarpine:** Drug of choice for acute angle-closure glaucoma. * **Edrophonium:** A short-acting AChE inhibitor used in the **Tensilon Test** for diagnosing Myasthenia Gravis (though largely replaced by antibody testing). * **Contraindications:** Cholinomimetics should be avoided in patients with **Asthma** (causes bronchoconstriction) and **Peptic Ulcer Disease** (increases gastric acid secretion).