Bidirectional flow of the funny current (If) during the pacemaking cycle in murine sinoatrial node myocytes

Colin H. Peters, Pin W. Liu, Stefano Morotti, Stephanie C. Gantz, Eleonora Grandi, Bruce P. Bean, Catherine Proenza

Research output: Contribution to journalArticlepeer-review


Sinoatrial node myocytes (SAMs) act as cardiac pacemaker cells by firing spontaneous action potentials (APs) that initiate each heartbeat. The funny current (If) is critical for the generation of these spontaneous APs; however, its precise role during the pacemaking cycle remains unresolved. Here, we used the AP-clamp technique to quantify If during the cardiac cycle in mouse SAMs. We found that If is persistently active throughout the sinoatrial AP, with surprisingly little voltage-dependent gating. As a consequence, it carries both inward and outward current around its reversal potential of −30 mV. Despite operating at only 2 to 5% of its maximal conductance, If carries a substantial fraction of both depolarizing and repolarizing net charge movement during the firing cycle. We also show that β-adrenergic receptor stimulation increases the percentage of net depolarizing charge moved by If, consistent with a contribution of If to the fight-or-flight increase in heart rate. These properties were confirmed by heterologously expressed HCN4 channels and by mathematical models of If. Modeling further suggested that the slow rates of activation and deactivation of the HCN4 isoform underlie the persistent activity of If during the sinoatrial AP. These results establish a new conceptual framework for the role of If in pacemaking, in which it operates at a very small fraction of maximal activation but nevertheless drives membrane potential oscillations in SAMs by providing substantial driving force in both inward and outward directions.

Original languageEnglish (US)
Article numberPNAS 2021 Vol. 118 No. 28 e2104668118
JournalProceedings of the National Academy of Sciences of the United States of America
Issue number28
StatePublished - Jul 13 2021


  • AP clamp
  • Cardiac pacemaking
  • Funny current
  • HCN channel
  • Sinoatrial node

ASJC Scopus subject areas

  • General


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