The drug ivabradine lowers heart rate in stable angina and heart failure by selectively blocking which channel without affecting cardiac contractility or conduction?
- A L-type voltage-gated calcium channel in SA nodal cells
- B Slowly activating delayed rectifier potassium channel (IKs)
- C Transient outward potassium current (Ito) in ventricular myocytes
- D Funny current (If, HCN) channel in sinoatrial node pacemaker cells ✓
Explanation
Ivabradine specifically blocks HCN (hyperpolarization-activated cyclic nucleotide-gated) channels carrying the If ('funny') current in SA node pacemaker cells. This current flows inward during diastolic depolarisation and is essential for spontaneous pacemaker activity. Blocking If slows the rate of diastolic depolarisation and thus reduces heart rate (pure negative chronotropy) without affecting myocardial contractility, AV conduction, or blood pressure. L-type calcium channel blockers (verapamil, diltiazem) reduce both heart rate and contractility. Its use is indicated when heart rate remains >70 bpm despite optimised beta-blockade in HFrEF or stable angina.
Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.
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Written and medically reviewed by the StethoPrep medical team.