Evaluation On Antiarrhythmic Effect Of HERG Channel Opener In Arterially Perfused Rabbit Left Ventricular Wedge Preparation

Arrhythmia is the leading cause of mortality in cardiovascular diseases,especially drug-induced arrhythmia has been greatly concerned by thepharmaceutical companies. It has been found that a number of drugs such asantineoplastic, psychotroptic drug, antibiotics, antihistaminics andcardiovascular drugs could cause the delay in ventricular repolarization andresult in the prolongation of the QT interval on electrocardiogram （ECG）,which is known as acquired long QT syndrome（LQTS）with high risk ofarrhythmias. Thus, drug-induced arrhythmia has emerged as one of the mostsignificant concerns in drug safety and a main obstacle to new drugdevelopment.In recent years, studies have shown that the main cause of drug-inducedQT-interval prolongation is that these drugs block the rapidly activatingdelayed rectifier potassium channel, conducting the current Ikr, encoded byhuman ether-a-go-go related gene （hERG）, which plays an important role incardiac repolarization. The drug block of the Ikrchannel reducesthe repolarization current and delays the cardiac repolarization. Theprolongation of cardiac repolarization favors the genesis of earlyafter-depolarization（EAD）and triggered activity. It is often associated with anincrease in transmural dispersion of repolarization （TDR）, which can greatlyincrease the risk of ventricular tachycardia, especially torsade de pointes（TdP）.In view of drug-induced QT-interval prolongation, and in order to reducethe risk of drug discovery and development, manypharmaceutical companies pay more attention to screen for hERG channelblocker with diverse chemical structure from different therapeutic areas.During this process, some compounds were found to activate the hERG channel. Studies have shown that hERG channel openers increase thehERG current and thus shorten the action potential duration （APD） viadifferent mechanisms and may have value in the treatment of cardiacarrhythmias associated with LQTS. ICA-105574（ICA）, the hERG channelopener we used this study, enhances hERG current via the mechanism thatprevent or limit the inactivation gating process. The previous studies of hERGchannel opener indicated that the hERG channel opener maybe have thepotential to modulate the drug-induced arrhythmia.The aim of our study was to design the effect on APD,TDR and ECG, aswell as the effect on the drug-induced arrhythmiaswhich was characterized byEAD and TdP. The results will provide evidence to the potential use of hERGchannel opener.Part1Electrophysiological effect of ICA on normal rabbit myocardialtissueObjective: To observe the effect of ICA on APD,TDR in arteriallyperfused rabbit ventricular wedge preparation.Methods: Female New Zealand White rabbits around3⁴months ofage, weighing2.0².5kg, anticoagulated with heparin（600⁸00IU/kg）,and10¹5minutes later anesthetized by intravenous administration ofpentobarbital sodium （30³5mg/kg）. The chest was opened via a leftthoracotomy, and then the heart was excised and placed in a cardioplegicsolution consisting of cold （4°C） cardioplegic solution. The left circumflexbranch of the coronary artery was then cannulated by a self made cannula andperfused with the cardioplegic solution. Once adequate perfusion wasestablished, the cannula was tied with silk sutures, and then quickly cutoff the non-perfused parts of the myocardium, make a5¹0mm wide,3³.5mm thick,10²0mm long, ventricle muscle wedge preparation. Then,the preparation was placed in a small tissue bath that containing normalTyrode’s solution. Using the arterially perfused rabbit left ventricular wedgepreparation to record the action potential of endocardium and transmuralelectrocardiogram （ECG） simultaneously. The action potential at90% repolarization (APD₉₀),Tp-e（the time between T peak and T end,representative of TDR）were measured before and10²0minutes after thepresence of ICA at different concentration1,5,10μmol/L. And the changes ofAPD₉₀, Tp-e before and after the presence of ICA（10μmol/L）at thestimulatory frequencies of0.2Hz,1Hz and2Hz were measured.Results:（1）The APD₉₀in epicardial and endocardial myocardium wererespectively shorted by34.6%and45.5%（p<0.05） in the presence of ICA（10μmol/L）, suggesting that the effect was more potent that in endocardialmyocardium. Therefore, we recorded the endocardial APD₉₀and ECGsimultaneously at the following studies.（2）The APD₉₀at the concentration of1μmol/L,5μmol/L,10μmol/Lwere respectively shorted by10.4%,21.8%,40.8%under the stimulatoryfrequencies of0.5Hz.（3）The APD₉₀at the stimulatory frequencies of0.2,1.0,2.0Hz wererespectively shorted by34.2%，18.4%,17.7%in the presence of ICA（10μmol/L）, and the Tp-e have no significant change.（4）ICA（10μmol/L）induced EAD at the stimulatory frequencies of2.0Hz, and the incidence was4/6.Conclusion: ICA shortened the APD₉₀in a concentration-dependentmanner, but it had no significant effect on TDR. ICA induced EAD at thestimulatory frequencies of2.0Hz, and it may have the arrhythmogenicpotentials.Part2Effect of ICA-105574on moxifloxacin and dofetilide induced EAD,TdPObjective: To estimate the effect of ICA on moxifloxacin and dofetilideinduced EAD.Methods: The methods were the same as part1. The action potentials inendocardium and transmural electrocardiogram were recorded simultaneouslybefore and after the presence of Moxifloxacin（100μmol/L） and Dofetilide（100nmol/L）, and then addition of ICA（10μmol/L）. The change of APD₉₀and Tp-e/QT ratio（predictor of TdP） were measured before and after thepresence of ICA（10μmol/L）, and whether EAD could be induced ormodulated. The stimulatory frequencies was2.0Hz in this part.Results:（1） Moxifloxacin（100μmol/L） produced a prolongation in both APD₉₀（from299±13ms to434±22ms, p<0.05） and Tp-e interval（from75±5ms to199±8ms, p<0.01）. The EAD incidence was3/6. Tp-e/QT ratio was increasedby68.7%. After the addition of ICA（10μmol/L）, both APD₉₀（266±12ms,p>0.05compared with control） and Tp-e interval （90±4ms,p>0.05comparedwith control） were shortened to the normal level. The Tp-e/QT ratio had nosignificant change. There were no EAD induced.（2） Dofetilide（100nmol/L） prolonged both APD₉₀（from270±8ms to386±3ms, p<0.05） and Tp-e interval（from105±10ms to302±11ms, p<0.01）,the EAD incidence was4/6. Tp-e/QT ratio was increased by86%. After theaddition of ICA（10μmol/L）, both of APD₉₀（283±3ms, p>0.05comparedwith control） and Tp-e interval （98±6ms,p>0.05compared with control） wereshortened to the normal level. The Tp-e/QT ratio have no significant changecompared with control. There were no EAD observed after the perfusion withICA（10μmol/L）.Conclusion: ICA can antagonized the EAD induced by moxifloxacin anddofetilide. However, at the high concentration,ICA（10μmol/L） inducedEAD, suggesting its arrhythmogenic potentials.