Epac1 Mediates Cardiac Repolarization And Arrhythmogenesis During Chronic Heart Failure

BackgroundChronic heart failure（CHF）has been a global health problem,which is the end stage of multiple heart diseases such as hypertention,coronary heart disease,valvulopathy,and cardiomyopathy.CHF patients are susceptible to ventricular tachycardia and ventricular fibrillation,leading to sudden cardiac death（SCD）.The detailed mechanism of ventricular arrhythmia（VA）in CHF remains unclear.One of the characteristics in the cardiomyocytes of CHF is slow repolarization,which results in longer action potential duration（APD）and corrected QT interval（QTc）.Rapid delayed-rectifier potassium current(I_Kr)is an important component of AP,participating in phase 3 repolarization.β-adrenergic receptor（β-AR）is a typical G protein-coupled receptor,which could regulate I_Kr currents.Kaele et al demonstrated thatβ1-AR regulated I_Kr via cAMP/PKA pathway.In CHF,β-AR suppressed I_Krr and contributed to longer AP.Cyclic adenosine 3’,5’-monophosphate（cAMP）is a widely accepted second messenger in cells.Exchange proteins directly activated by cAMP（Epacs）are effectors of cAMP,acting as guanine nucleotide exchange factors for small G proteins.Epacs family is consisted of Epac1 and Epac2,and the former one is expressed with high levels of expression in the heart.Epac proteins function with a PKA-independent manner,taking part in regulation of vascular tone,vascular smooth muscle cells proliferation and migration,vascular endothelial barrier function,vascular inflammation,excitation–contraction coupling,pathological cardiac remodeling and cardiac ischemia.Epac proteins are also involved in arrhythmogenesis.Epacs could alter slow delayed rectifier potassium current(I_Ks),transient receptor potential canonical 3 and 4 channels（TRPC3,4）and induce calcium leakd in sarcoplasmic reticulum.Based on current studies,we want to ask if Epac proteins are essential for I_Kr?Is Epac regulation of I_Kr indispensable in ventricular arrhythmia in CHF?If so,which isoform is more important?ObjectiveWe aimed to demonstrate the role of Epac1 in ventricular arrhythmia in chronic heart failure.The study will provide theoretical basis and experimental evidence for Epac1 as a new target for prevention and treatment of ventricular arrhythmias and sudden cardiac death in chronic heart failure.MethodsThe chronic heart failure model of guinea pigs was established by transverse aortic constriction（TAC）.The left ventricular diameter and left ventricular ejection fraction（LVEF）were examined by echocardiography;the heart rate,QT interval and corrected QT interval（QTc）were measured by electrocardiogram.AP and I_Krr currents were recorded in primary cardiomyocytes of adult guinea pigs using patch clamp technique.Effective refractoriness period（ERP）was tested in Lagendorff perfused hearts by programmed electrical stimulation（PES）.The mRNA and protein levels of Epacs were detected in left ventricle tissues by real-time quantative PCR and western blot.Osmotic minipumps filled with 8-pCPT were implanted in animals to perform in vivo chronic Epac treatment.Echocardiography,electrocardiogram,whole cell patch clamp and PES were performed to evaluate the effects of chronic Epac stimulation.Selective Epac1 inhibitor CE3F4,Epac2 inhibitor ESI 05 and nonselective Epac inhibitor ESI 09 were applied in Epac stimulated cardiomyocytes for I_Kr currents.Effective Epac inhibitor was used in chronic heart failure myocytes and I_Kr currents were collected by whole cell patch clamp technique.Results1.The chronic heart failure model of guinea pigs was successfully established by TAC;2.APD was longer and I_Kr current was smaller in chronic heart failure myocytes;3.ERP was longer in chronic heart failure;4.Epac1 was upregulated in TAC-induced heart failure;5.The QTc duration was longer after chronic Epac stimulation;6.APD was longer after chronic Epac stimulation,and I_Kr current was depressed which was restored by Epac1 inhibitor CE3F4 and Epac1/2 inhibitor ESI 09 but not Epac2 inhibitor ESI 05;7.I_Kr current is not changed in acute Epac activation;8.Hearts were more susceptible to malignant arrhythmia after chronic 8-pCPT perfusion;9.CE3F4 rescued the I_Krr drop in chronic heart failure.ConclusionsEpac mediates ventricular repolarization directly in guinea pigs,of which Epac1plays the leading role.Epac1 inhibition rescues the abnormity of repolarization in chronic heart failure.