Sinoatrial node myocytes (SAMs) act as cardiac pacemaker cells by firing spontaneous action potentials (APs) that initiate each heartbeat. The funny current (I(f)) 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 I(f) during the cardiac cycle in mouse SAMs. We found that I(f) 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, I(f) 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 I(f), consistent with a contribution of I(f) to the fight-or-flight increase in heart rate. These properties were confirmed by heterologously expressed HCN4 channels and by mathematical models of I(f). Modeling further suggested that the slow rates of activation and deactivation of the HCN4 isoform underlie the persistent activity of I(f) during the sinoatrial AP. These results establish a new conceptual framework for the role of I(f) 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.
