In the sinoatrial node, the rapid upstroke (phase 0) of the action potential is primarily mediated by the influx of calcium ions through L-type channels, unlike in ventricular myocytes. Which pharmacological agent most specifically targets this phase in the SA node to reduce heart rate?
- A Verapamil ✓
- B Lidocaine
- C Tetrodotoxin
- D Amiodarone
Correct answer: A. Verapamil
Explanation
SA node phase 0 depends on L-type calcium channels, not fast sodium channels. Verapamil, a non-dihydropyridine calcium channel blocker, slows SA node firing by blocking these channels. Tetrodotoxin and lidocaine block fast sodium channels, which are absent in SA node cells. Amiodarone has multiple effects but is not the most specific for this phase.
Reference: Katzung Basic and Clinical Pharmacology, 15th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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