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

Abstract

Significance The funny current (If) is critical for spontaneous activity in cardiac pacemaker cells; however, its precise role remains enigmatic because it activates mostly outside the physiological voltage range and its kinetics are slow relative to the cardiac cycle. If is typically considered as an inward current; however, we show that If is persistently active in pacemaker cells. Once opened, the small fraction of ion channels that conduct If do not reclose. Consequently, If flows both inward and outward and, paradoxically, conducts a large fraction of the net charge movement. These results establish a new conceptual framework for pacemaking in which voltage-dependent gating of If is minimal and If contributes to spontaneous pacemaker activity by providing driving force in both directions. 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.