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BACKGROUND: Bradycardic agents are of interest for the treatment of ischemic heart disease and heart failure, as heart rate is an important determinant of myocardial oxygen consumption. OBJECTIVES: The purpose of this study was to investigate the propensity of hydroxychloroquine (HCQ) to cause bradycardia. METHODS: We assessed the effects of HCQ on (1) cardiac beating rate in vitro (mice); (2) the "funny" current (If) in isolated guinea pig sinoatrial node (SAN) myocytes (1, 3, 10 µM); (3) heart rate and blood pressure in vivo by acute bolus injection (rat, dose range 1-30 mg/kg), (4) blood pressure and ventricular function during feeding (mouse, 100 mg/kg/d for 2 wk, tail cuff plethysmography, anesthetized echocardiography). RESULTS: In mouse atria, spontaneous beating rate was significantly (P < .05) reduced (by 9% ± 3% and 15% ± 2% at 3 and 10 µM HCQ, n = 7). In guinea pig isolated SAN cells, HCQ conferred a significant reduction in spontaneous action potential firing rate (17% ± 6%, 1 μM dose) and a dose-dependent reduction in If (13% ± 3% at 1 µM; 19% ± 2% at 3 µM). Effects were also observed on L-type calcium ion current (ICaL) (12% ± 4% reduction) and rapid delayed rectifier potassium current (IKr) (35% ± 4%) at 3 µM. Intravenous HCQ decreased heart rate in anesthetized rats (14.3% ± 1.1% at 15mg/kg; n = 6) without significantly reducing mean arterial blood pressure. In vivo feeding studies in mice showed no significant change in systolic blood pressure nor left ventricular function. CONCLUSIONS: We have shown that HCQ acts as a bradycardic agent in SAN cells, in atrial preparations, and in vivo. HCQ slows the rate of spontaneous action potential firing in the SAN through multichannel inhibition, including that of If.

Original publication




Journal article


Heart Rhythm

Publication Date





2186 - 2194


Arrhythmia, Electrophysiology, Funny current, Heart failure, Heart rate, Hydroxychloroquine, I(f), Ion channels, Pacemaker, Action Potentials, Animals, Bradycardia, Disease Models, Animal, Electrophysiological Phenomena, Enzyme Inhibitors, Guinea Pigs, Heart Atria, Heart Failure, Heart Rate, Hydroxychloroquine, Male, Mice, Patch-Clamp Techniques, Rats, Rats, Sprague-Dawley, Sinoatrial Node