INI-CET 2015 — 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]

Q: What is the drug of choice for pregnancy-induced hypertension?

Alpha-methyldopa. **Explanation:** **Alpha-methyldopa** is the traditional drug of choice for chronic hypertension in pregnancy. It is a centrally acting alpha-2 adrenergic agonist that reduces sympathetic outflow. Its preference in pregnancy stems from its long-standing safety profile and the absence of long-term adverse effects on fetal development or neonatal hemodynamics. **Analysis of Options:** * **Atenolol (Option A):** Beta-blockers, particularly Atenolol, are generally avoided in early pregnancy as they are associated with **fetal growth restriction (IUGR)** and placental complications. * **Nitroprusside (Option B):** It is contraindicated in pregnancy (except in extreme emergencies) due to the risk of **fetal cyanide poisoning**. * **Enalapril (Option C):** ACE inhibitors and ARBs are strictly **contraindicated** in pregnancy. They are teratogenic and can cause fetal renal dysgenesis, oligohydramnios, and skull hypoplasia. **High-Yield Clinical Pearls for NEET-PG:** * **First-line agents:** While Methyldopa is the classic answer, current guidelines (like ACOG) also recommend **Labetalol** (often preferred for its faster onset) and **Nifedipine** (long-acting) as first-line options. * **Acute Hypertensive Crisis in Pregnancy:** The drugs of choice are **IV Labetalol** or **IV Hydralazine**. * **Preeclampsia Prophylaxis:** Low-dose **Aspirin** (started before 16 weeks) is used in high-risk patients. * **Magnesium Sulfate ($MgSO_4$):** This is the drug of choice for preventing and treating seizures in **Eclampsia**, not for blood pressure control. [Practice more Pharmacology PYQs on OnCourse]

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Methacholine acts primarily at which receptor?

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What is the drug of choice for pregnancy-induced hypertension?