Characteristics of Ivabradine-Sensitive Currents in Mouse Sinoatrial Node Myocytes

Abstract

Cardiac pacemaking is driven by spontaneous action potentials (APs) in sinoatrial node myocytes (SAMs). The funny current (If) is thought play a role in the generation of pacemaker activity in SAMs, but the degree to which it contributes is incompletely understood. In this study, we used AP clamp experiments and the If blocker, ivabradine, to investigate the role of If in pacemaking. Spontaneous APs were recorded from acutely-isolated SAMs from mice, and ∼10 s of the AP recording was incorporated into a voltage clamp command. This AP clamp protocol elicited both inward and outward ivabradine-sensitive currents in SAMs. If was defined as the ivabradine-sensitive current remaining in the presence of a pharmacological cocktail of sodium, calcium and potassium channel blockers. Measurable inward ivabradine-sensitive current was evident during diastole in 57% of all cells and was generally larger in cells with the most negative diastolic voltages. Although we found that 30 μM ivabradine also blocked both L- and T-type calcium currents in SAMs, the majority of the ivabradine-sensitive inward current in the AP clamp experiments appeared to be If because control experiments showed that the drug cocktail blocked ∼85% of the calcium currents. Ivabradine also blocked a substantial outward current during the AP upstroke and repolarization phases. This outward ivabradine-sensitive current likely includes a contribution of If, which results from its relatively slow deactivation. However some of the outward ivabradine-sensitive current may also reflect off-target block of potassium currents by ivabradine. We are currently assaying the contribution of potassium currents to the outward ivabradine-sensitive current. Our results suggest that If is active not only during diastole, but also throughout the AP upstroke and repolarization phases of the mouse SAM AP.