Nicotinic antagonists

Nicotinic Antagonists - The Two Blockades

• Two major sites of action: Autonomic ganglia & Neuromuscular Junction (NMJ).
• Ganglionic Blockers: (e.g., Hexamethonium, Mecamylamine) Block autonomic ganglia, causing broad, complex effects. Rarely used clinically due to non-specificity.
• Neuromuscular Blockers (NMBs): Target nicotinic receptors at the NMJ for skeletal muscle paralysis, primarily in anesthesia.

• Non-Depolarizing (Competitive) Blockade:
  • Compete with ACh for NMJ receptors.
  • Reversal: ↑ ACh levels with cholinesterase inhibitors (e.g., Neostigmine).
• Depolarizing Blockade:
  • Succinylcholine: Binds to and activates the nicotinic receptor, causing persistent depolarization.
  • Phase I: Initial fasciculations, then flaccid paralysis. Potentiated by cholinesterase inhibitors.
  • Phase II: Receptor desensitization. Resembles non-depolarizing block; may be reversed by cholinesterase inhibitors.

⭐ Succinylcholine can cause life-threatening hyperkalemia in patients with burns, crush injuries, or extensive denervation. Avoid use in these settings.

Non-Depolarizing Blockers - Competitive Crew

• Mechanism: Competitive antagonists at the neuromuscular junction (NMJ) nicotinic receptors ($N_M$). They prevent acetylcholine (ACh) from binding, leading to flaccid paralysis.
• Drugs: "-curonium" or "-curium" endings. (e.g., Pancuronium, Rocuronium, Vecuronium, Atracurium, Cisatracurium).
• Clinical Use: Adjuvant in anesthesia for skeletal muscle relaxation.
• Reversal: Overcome blockade by increasing ACh. Use acetylcholinesterase inhibitors (e.g., Neostigmine, Edrophonium). Always co-administer with an antimuscarinic (e.g., Atropine, Glycopyrrolate) to prevent systemic muscarinic effects (bradycardia, salivation).
• Key Side Effects:
  • Atracurium/Tubocurarine: Can trigger histamine release → hypotension, flushing, bronchoconstriction.
  • Pancuronium: Moderate muscarinic receptor blockade → tachycardia.

⭐ Atracurium and its isomer Cisatracurium are inactivated spontaneously in plasma (Hofmann elimination), making them ideal for patients with hepatic or renal failure.

Depolarizing Blockers - Succinylcholine's Twist

• Mechanism: Binds to postsynaptic nicotinic acetylcholine receptors (nAChR) at the neuromuscular junction (NMJ), mimicking acetylcholine (ACh).
• Structure: Essentially two ACh molecules linked end-to-end.
• Metabolism: Rapidly hydrolyzed by plasma butyrylcholinesterase (pseudocholinesterase), not acetylcholinesterase (AChE).

Two-Phase Blockade

Clinical Pearls

• Use: Rapid-sequence intubation due to fast onset (~1 min) and short duration (~10 min).
• Adverse Effects:
  • ⚠️ Hyperkalemia: Risk in patients with burns, crush injuries, or neuromuscular disease due to upregulation of nAChRs.
  • Malignant Hyperthermia: Especially with halogenated anesthetics. Treat with dantrolene.
  • Increased intraocular and intragastric pressure.

⭐ Exam Favorite: Succinylcholine can cause life-threatening hyperkalemia in patients with extensive tissue damage (burns, trauma) or denervation (Guillain-Barré, ALS), leading to cardiac arrest. Avoid in these settings.

High‑Yield Points - ⚡ Biggest Takeaways

• Ganglionic blockers (e.g., hexamethonium) act on both autonomic ganglia; effects depend on the dominant tone of the target organ.
• Neuromuscular blockers are either depolarizing (succinylcholine) or non-depolarizing (e.g., rocuronium, vecuronium).
• Succinylcholine causes initial muscle fasciculations (Phase I block), followed by flaccid paralysis (Phase II block).
• Non-depolarizing agents are competitive antagonists at the NMJ; their blockade is reversed by AChE inhibitors.
• Beware of malignant hyperthermia with succinylcholine and histamine release with atracurium.