Hemicholinium-3 (HC-3) blocks cholinergic transmission. What is its exact site and mechanism of action?
- A Blocks vesicular acetylcholine transporter (VAChT), preventing ACh storage
- B Inhibits choline acetyltransferase, preventing ACh synthesis
- C Blocks the high-affinity choline uptake transporter at the presynaptic terminal ✓
- D Prevents Ca2+-dependent exocytosis of ACh vesicles
Correct answer: C. Blocks the high-affinity choline uptake transporter at the presynaptic terminal
Explanation
Hemicholinium-3 blocks the high-affinity sodium-dependent choline uptake transporter (CHT1) at the presynaptic cholinergic terminal, thereby depleting choline availability for ACh synthesis. Vesamicol blocks VAChT (option A); botulinum toxin prevents vesicle fusion/exocytosis (option D); choline acetyltransferase is not a clinically relevant drug target.
Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.
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