The funny current (If) in sinoatrial node pacemaker cells is carried primarily by which ion channel, and what is its key gating characteristic?

• A T-type calcium channel; activated during early phase 4 depolarization
• B Kir3.1/Kir3.4 channel; activated by muscarinic M2 receptor stimulation
• C TREK-1 channel; activated by membrane stretch and arachidonic acid
• D HCN channel; activated by hyperpolarization and blocked by ivabradine ✓

Explanation

The If (funny current) flows through HCN (hyperpolarization-activated cyclic nucleotide-gated) channels. These channels are paradoxically activated by hyperpolarization below approximately -50 mV and carry a mixed Na+/K+ inward current that depolarises the pacemaker cell toward threshold. Ivabradine selectively blocks HCN channels from inside the cell, slowing the heart rate without affecting contractility. T-type Ca²⁺ channels contribute to late phase 4 but are not the If channel; Kir3 mediates IKACh (vagal slowing), distinct from If.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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