Blocks If currents through HCN channels in sinoatrial node cells (IC50s = 1.83, 2.21, 1.9, and 1.88 µM, respectively for HCN1-4 in vitro); reduces heart rate from 600 to 200 beats per minute with an ED50 value of 1.8 mg/kg and induces increasing arrhythmia at concentrations >10 mg/kg when assessed through telemetric ECG recording in mice.