Atrial fibrillation (AF) is frequently associated with β-adrenergic stimulation, especially in patients with structural heart diseases. The objective of this study was to determine the synergism of late sodium current (late I Na ) and Ca 2+ /calmodulin-dependent protein kinase (CaMKII)-mediated arrhythmogenic activities in β-adrenergic overactivation-associated AF. Monophasic action potential, conduction properties, protein phosphorylation, ion currents and cellular trigger activities were measured from rabbit-isolated hearts, atrial tissue and atrial myocytes, respectively. Isoproterenol (ISO, 1–15 nM) increased atrial conduction inhomogeneity index, phospho-Na v 1.5 and phospho-CaMKII protein levels and late I Na by 108%, 65%, 135% and 87%, respectively, and induced triggered activities and episodes of AF in all hearts studied ( p < 0.05). Sea anemone toxin II (ATX-II, 2 nM) was insufficient to induce any atrial arrhythmias, whereas the propensities of AF were greater in hearts treated with a combination of ATX-II and ISO. Ranolazine, eleclazine and KN-93 abolished ISO-induced AF, attenuated the phosphorylation of Na v 1.5 and CaMKII, and reversed the increase of late I Na ( p < 0.05) in a synergistic mode. Overall, late I Na in association with the activation of CaMKII potentiates β-adrenergic stimulation-induced AF and the inhibition of both late I Na and CaMKII exerted synergistic anti-arrhythmic effects to suppress atrial arrhythmic activities associated with catecholaminergic activation. This article is part of the theme issue ‘The heartbeat: its molecular basis and physiological mechanisms’.

Introduction: Idarucizumab is a humanized monoclonal antibody fragment that specifically binds to dabigatran with high affinity and reverses its anticoagulant effect. This study investigated the pharmacokinetics (PK) and pharmacodynamics (PD) of idarucizumab in healthy Chinese subjects at steady state of dabigatran and explored the effect of idarucizumab on PK and PD of dabigatran. Methods: Twelve subjects received dabigatran etexilate treatment alone (220 mg twice daily, b.i.d., oral). After a washout period, the 12 subjects again received dabigatran etexilate (220 mg b.i.d., oral) and idarucizumab (2.5 ? 2.5 g, intravenous) 2 h after the last administration of dabigatran etexilate. Results: The geometric mean (gMean) values of area under the plasma concentration-time curve (AUC 0-? ) and maximum concentration (C max ) were 44,200 nmol h/L and 30,900 nmol/ L, respectively. An amount of 35.3 lmol of idarucizumab, corresponding to 33.8% of the total dose, was excreted by urine over 72 h. The area under the effect (AUEC above,2-12 ) in the presence and absence of idarucizumab was close to zero for all coagulation parameters, diluted thrombin time (dTT), ecarin clotting time (ECT), activated partial thromboplastin time (aPTT), and thrombin time (TT), which indicated the reversal of dabigatran anticoagulation by idarucizumab. There were no serious adverse events reported in this study. No subject tested positive for anti-idarucizumab antibodies. Conclusion: Idarucizumab was well tolerated and no subject tested positive for anti-idarucizumab antibodies in this study. PK and PD of idarucizumab in healthy Chinese subjects at a steady state of dabigatran were comparable with those in Japanese and Caucasian subjects. Clinical registration: ClinicalTrials.gov Identifier No. NCT03086356.Zining Wang and Xia Zhao contributed equally to this work.