Block of I(Ks) does not induce early afterdepolarization activity but promotes beta-adrenergic agonist-induced delayed afterdepolarization activity

INTRODUCTION: An early afterdepolarization (EAD)-induced triggered beat is thought to precipitate torsade de pointes (TdP) in the long QT syndrome (LQTS). Previous studies demonstrated the development of EAD activity and dispersion of repolarization under LQT2 (reduced I(Kr)) and LQT3 (augmented late I(Na)), but not LQT1 (reduced I(Ks)), conditions. The present study examines these electrophysiologic characteristics during I(Ks) block. METHODS AND RESULTS: Canine epicardial (Epi), M, and endocardial (Endo) tissues and Purkinje fibers isolated from the canine left ventricle were studied using standard microelectrode recording techniques. The I(Ks) blocker chromanol 293B (293B, 30 microM), produced a homogeneous rate-independent prolongation of action potential duration (APD) in Epi, M, and Endo, but little to no APD prolongation in Purkinje. Chromanol 293B 1 to 30 microM failed to induce EADs or delayed afterdepolarizations (DADs) in any of the four tissue types. Isoproterenol (ISO, 0.1 to 1.0 microM) in the presence of 293B 30 microM significantly prolonged the APD of the M cell (basic cycle length > or = 1 sec), abbreviated that of Purkinje, and caused little change in that of Epi and Endo. The combination of 293B 30 microM and ISO 0.2 microM did not induce EADs in any of the four tissue types, but produced DAD activity in 4 of 8 Epi, 7 of 10 M cells, and 3 of 8 Endo. CONCLUSION: Our results indicate that I(Ks) block alone or in combination with beta-adrenergic stimulation does not induce EADs in any of the four canine ventricular tissue types, but that the combination of the two induces DADs as well as accentuated dispersion of repolarization.     

Pause induced early afterdepolarizations in the long QT syndrome: a simulation study.

OBJECTIVE: The long QT syndrome (LQTS) is characterized by prolonged repolarization and propensity to syncope and sudden death due to polymorphic ventricular tachycardias such as torsade de pointes (TdP). The exact mechanism of TdP is unclear, but pause-induced early afterdepolarizations (EADs) have been implicated in its initiation. In this study we investigate the mechanism of pause-induced EADs following pacing at clinically relevant rates and characterize the sensitivity of different cell types (epicardial, midmyocardial, and endocardial) to EAD development. METHODS: Simulations were conducted using the Luo-Rudy (LRd) model of the mamalian ventricular action potential (AP). Three cell types--epicardial, midmyocardial (M), and enocardial--are represented by altering the channel density of the slow delayed rectifier current, IKs. LQTS is modelled by enhanced late sodium current (LQT3), or reduced density of functional channels that conduct IKr (LQT2) and IKs (LQT1). The cell is paced 40 times at a constant Basic Cycle Length (BCL) of 500 ms. Following a 1500 ms pause, an additional single stimulus is applied. RESULTS: Our results demonstrate that pause-induced EADs develop preferentially in M cells under conditions of prolonged repolarization. These EADs develop at plateau potentials ('plateau EADs'). Mechanistic investigation shows that prolongation of the plateau phase of the post-pause AP due to a smaller delayed rectifier potassium current, IKs' and enhancement of the sodium-calcium exchange current, INaCa, allows for the reactivation of the L-type calcium current, ICa(L), which depolarizes the membrane to generate the EAD. CONCLUSIONS: APD is a very important determinant of arrhythmogenesis and its prolongation, either due to acquired or congenital LQTS, can result in the appearance of EADs. The formation of pause-induced EADs preferentially in M cells suggests a possible role for these cells in the generation of arrhythmias that are associated with abnormalities of repolarization (e.g., TdP). The ionic mechanism of pause-induced EADs involves reactivation of the L-type calcium current during the prolonged plateau of the post-pause AP.     

Prominent I(Ks) in epicardium and endocardium contributes to development of transmural dispersion of repolarization but protects against development of early afterdepolarizations

INTRODUCTION: Previous studies from our laboratory demonstrated (1) a much larger I(Ks) and (2) inability to induce early afterdepolarization (EAD) activity in epicardial and endocardial cells versus M cells. This study tests the hypothesis that these two characteristics are interrelated. METHODS AND RESULTS: Standard and floating microelectrode techniques were used to record transmembrane activity from the canine left ventricular epicardial, M, and endocardial regions in isolated tissue slices and arterially perfused wedge preparations. The I(Kr) blocker E-4031 (1 to 10 microM) caused prominent prolongation of action potential duration (APD) and induced EADs in tissues isolated from the M region, but not those from epicardium or endocardium, causing a large transmural dispersion of APD. In contrast, the I(Ks) blocker chromanol 293B (10 to 30 microM) produced moderate prolongation of APD without EADs in all three tissue types. The combination of E-4031 (1 microM) and chromanol 293B (30 microM) resulted in profound prolongation of APD and the development of EADs in all three tissue types. In the perfused wedge, neither E-4031 nor chromanol 293B alone could induce EADs. In combination, the two drugs caused significant prolongation of APD and EADs in all three transmural regions. CONCLUSION: Our results support the hypothesis that a prominent I(Ks) is responsible for the ability of epicardium and endocardium to resist some but not all of the arrhythmogenic effects of I(Kr) block. The data highlight the critical importance of I(Ks) in the canine heart and the significant role of electrotonic interactions in minimizing the development of an arrhythmogenic substrate when repolarization reserve is reduced.     

Differential effects of beta-adrenergic agonists and antagonists in LQT1, LQT2 and LQT3 models of the long QT syndrome

OBJECTIVES: To define the cellular mechanisms responsible for the development of life-threatening arrhythmias in response to sympathetic activity in the congenital and acquired long QT syndromes (LCQTS). METHODS: Transmembrane action potentials (AP) from epicardial (EPI), M and endocardial (ENDO) cells and a transmural electrocardiogram were simultaneously recorded from an arterially perfused wedge of canine left ventricle. We examined the effect of beta-adrenergic agonists and antagonists on action potential duration (APD90), transmural dispersion of repolarization (TDR) and the development of Torsade de Pointes (TdP) in models of LQT1, LQT2 and LQT3 forms of LQTS. RESULTS: I(Ks) block with chromanol 293B (LQT1) homogeneously prolonged APD90 of the three cell types without increasing TDR. Addition of isoproterenol prolonged QT and APD90 of M but abbreviated that of EPI and ENDO, causing a persistent increase in TDR; Torsade de Pointes developed or could be induced only in the presence of isoproterenol. I(Kr) block with d-sotalol (LQT2) and augmentation of late I(Na) with ATX-II (LQT3) prolonged APD90 of M more than EPI and ENDO, causing increases in QT and TDR. TdP developed in the absence of isoproterenol. In LQT2 isoproterenol initially prolonged, then abbreviated, the APD90 of M but always abbreviated EPI, thus transiently increasing TDR and the incidence of TdP. In LQT3, isoproterenol always abbreviated APD90 of the three cell types, causing a persistent decrease in TDR and suppression of TdP. The arrhythmogenic as well as protective actions of isoproterenol were reversed by propranolol. CONCLUSIONS: Our data suggest that beta-adrenergic stimulation induces TdP by increasing transmural dispersion of repolarization in LQT1 and LQT2 but suppresses TdP by decreasing dispersion in LQT3. The data indicate that beta-blockers are protective in LQT1 and LQT2 but may facilitate TdP in LQT3.     

Early afterdepolarizations induced by isoproterenol in patients with congenital long QT syndrome.

BACKGROUND. Several recent experimental and clinical studies have shown that early afterdepolarizations (EADs) are important in the genesis of QTU prolongation and ventricular tachyarrhythmias (VTs) in patients with long QT syndrome. On the other hand, sympathetic stimulation is well known to contribute to the genesis of QTU prolongation and VTs in patients with congenital long QT syndrome. The present study was performed to examine the influence of isoproterenol on the genesis of EADs and on the action potential durations and QTU intervals in patients with congenital long QT syndrome. METHODS AND RESULTS. We recorded monophasic action potentials (MAPs) with a contact electrode during right atrial pacing at a constant cycle length of 500 msec before and after continuous isoproterenol infusion (1 microgram/min). MAPs were obtained from the right and left ventricular endocardium in six patients with congenital long QT syndrome (LQT group, 18 recording sites) and in eight control patients (control group, 19 recording sites). Although no EADs were recorded from either group during the control state, MAP duration at 90% repolarization (MAPD90) was significantly longer in the LQT group (n = 18) than in the control group (n = 19) (275 +/- 36 versus 231 +/- 22 msec; p less than 0.0005). Isoproterenol induced EADs in four of the six LQT patients (five of 18 recording sites) but not in the eight control patients (zero of 19 recording sites). The appearance of EADs in the LQT group was associated with an increased amplitude of the late component of the TU complex, and the corrected QT (QTc) interval was prolonged by isoproterenol from 543 +/- 53 to 600 +/- 30 msec 1/2 (n = 6; p less than 0.05). Isoproterenol also prolonged the MAPD90 from 275 +/- 36 to 304 +/- 50 msec in the LQT group (n = 18; p less than 0.005), whereas it shortened the MAPD90 from 231 +/- 22 to 224 +/- 25 msec in the control group (n = 19; p less than 0.05). Moreover, isoproterenol increased the dispersion of MAPD90 (difference between the longest MAPD90 and the shortest MAPD90 in each patient) from 30 +/- 5 to 62 +/- 35 msec in the LQT group (n = 6; p = 0.08), whereas it did not change the dispersion of MAPD90 in the control group (n = 8; 25 +/- 14 versus 27 +/- 14 msec). CONCLUSIONS. These results suggest that patients with congenital long QT syndrome have primary repolarization abnormalities and that EADs induced by isoproterenol play an important role in the exaggeration of these repolarization abnormalities.     

Effect of the ATP-sensitive K⁺ channel opener nicorandil in a canine model of proarrhythmia

Increased action potential duration (APD) induces early afterdepolarization (EAD) in vitro and torsade de pointes in vivo, and ATP-sensitive K(+) channel openers decrease APD in cardiac tissue. We tested whether the ATP-sensitive K(+) channel opener nicorandil has antiarrhythmic effects on class III antiarrhythmic drug-induced ventricular arrhythmia. In 10 anesthetized dogs with chronic atrioventricular block, we recorded monophasic action potentials (MAPs) from the left and right ventricular (LV and RV) endocardium. The class III antiarrhythmic drug nifekalant (1 mg/kg, IV) was administered at 5 minute intervals (total doses; 2-6 mg/kg) until the appearance of EADs, premature ventricular contractions (PVCs), or polymorphic ventricular tachycardias (PVTs). Five minutes after the end of nifekalant administration, nicorandil (1.0 mg/kg) was administered over 5 minutes. Nifekalant decreased the ventricular escape rate from 75 ± 5 beats/minute to 45 ± 10 beats/minute and increased RV-MAP duration (MAPD) from 217 ± 32 msec to 308 ± 2 msec (P < 0.01) and LV-MAPD from 232 ± 32 msec to 353 ± 82 msec (P < 0.01). EADs were recorded in 9 dogs, frequent premature ventricular contractions (PVCs) developed in 10 dogs, incessant PVTs developed in 3 dogs, and monomorphic ventricular tachycardia developed in 3 dogs after nifekalant administration. Nicorandil decreased RV-MAPD to 267 ± 57 msec and LV-MAPD to 279 ± 44 msec. It suppressed EADs, decreased the incidence of PVCs, and abolished PVT. Nicorandil may be clinically useful for treatment of PVCs and PVTs accompanying acquired long QT syndrome.     

Cellular and ionic mechanism for drug-induced long QT syndrome and effectiveness of verapamil

OBJECTIVES: We examined the cellular and ionic mechanism for QT prolongation and subsequent Torsade de Pointes (TdP) and the effect of verapamil under conditions mimicking KCNQ1 (I(Ks) gene) defect linked to acquired long QT syndrome (LQTS). BACKGROUND: Agents with an I(Kr)-blocking effect often induce marked QT prolongation in patients with acquired LQTS. Previous reports demonstrated a relationship between subclinical mutations in cardiac K+ channel genes and a risk of drug-induced TdP. METHODS: Transmembrane action potentials from epicardial (EPI), midmyocardial (M), and endocardial (ENDO) cells were simultaneously recorded, together with a transmural electrocardiogram, at a basic cycle length of 2,000 ms in arterially perfused feline left ventricular preparations. RESULTS: The I(Kr) block (E-4031: 1 micromol/l) under control conditions (n = 5) prolonged the QT interval but neither increased transmural dispersion of repolarization (TDR) nor induced arrhythmias. However, the I(Kr) blocker under conditions with I(Ks) suppression by chromanol 293B 10 micromol/l mimicking the KCNQ1 defect (n = 10) preferentially prolonged action potential duration (APD) in EPI rather than M or ENDO, thereby dramatically increasing the QT interval and TDR. Spontaneous or epinephrine-induced early afterdepolarizations (EADs) were observed in EPI, and subsequent TdP occurred only under both I(Ks) and I(Kr) suppression. Verapamil (0.1 to 5.0 micromol/l) dose-dependently abbreviated APD in EPI more than in M and ENDO, thereby significantly decreasing the QT interval, TDR, and suppressing EADs and TdP. CONCLUSIONS: Subclinical I(Ks) dysfunction could be a risk of drug-induced TdP. Verapamil is effective in decreasing the QT interval and TDR and in suppressing EADs, thus preventing TdP in the model of acquired LQTS.     

L-type calcium current recovery versus ventricular repolarization: preserved membrane-stabilizing mechanism for different QT intervals across species

BACKGROUND: Long QT syndrome is associated with early after-depolarization (EAD) that may result in torsade de pointes (TdP). Interestingly, the corrected QT interval seems to be proportional to body mass across species under physiologic conditions. OBJECTIVE: The purpose of this study was to test whether recovery of L-type calcium current (I(Ca,L)), the primary charge carrier for EADs, from its inactivated state matches ventricular repolarization time and whether impairment of this relationship leads to development of EAD and TdP. METHODS: Transmembrane action potentials from the epicardium, endocardium, or subendocardium were recorded simultaneously with a transmural ECG in arterially perfused left ventricular wedges isolated from cow, dog, rabbit, and guinea pig hearts. I(Ca,L) recovery was examined using action potential stimulation in isolated left ventricular myocytes. RESULTS: The ventricular repolarization time (action potential duration at 90% repolarization [APD(90)]), ranging from 194.7 +/- 1.8 ms in guinea pig to 370.2 +/- 9.9 ms in cows, was linearly related to the thickness of the left ventricular wall among the species studied. The time constants (tau) of I(Ca,L) recovery were proportional to APD(90), making the ratios of tau to APD(90) fall into a relatively narrow range among these species despite markedly different ventricular repolarization time. Drugs with risk for TdP in humans were shown to impair this intrinsic balance by either prolongation of the repolarization time and/or acceleration of I(Ca,L) recovery, leading to the appearance of EADs capable of initiating TdP. CONCLUSION: An adequate balance between I(Ca,L) recovery and ventricular repolarization serves as a "physiologic stabilizer" of ventricular action potentials in repolarization phases.     

Transmural dispersion of repolarization as a key factor of arrhythmogenicity in a novel intact heart model of LQT3

BACKGROUND: Congenital and acquired long QT syndrome (LQTS) are caused by abnormalities of ionic currents underlying ventricular repolarization. For a better understanding of the mechanisms by which functional electrical instability at the level of the whole heart leads to torsade de pointes (TdP), a novel model of LQT3 was developed and the role of transmural dispersion of repolarization for the development of proarrhythmia was evaluated. METHODS AND RESULTS: In 11 Langendorff-perfused rabbit hearts, veratridine (0.1-0.5 microM), an inhibitor of sodium channel inactivation, led to a concentration-dependent increase in QT-interval and simultaneously recorded monophasic ventricular action potentials (MAPs) (p<0.05) and thereby mimicked LQT3. Veratridine reproducibly induced early afterdepolarizations (EADs) and TdP after lowering potassium concentration. In bradycardic (AV-blocked) hearts, the increase in MAP duration showed marked regional differences. It was significantly more pronounced on the left endocardium as compared to left or right epicardium. This resulted in a significant increase in dispersion of repolarization (24% at 0.1 microM, 92% at 0.25 microM, 208% at 0.5 microM; p<0.01). Left ventricular transmural dispersion of repolarization increased significantly more than interventricular dispersion (104 to 33 ms at 0.5 microM veratridine; p<0.05). CONCLUSION: By inhibition of sodium channel inactivation, veratridine mimics LQT3 in this intact heart model. In bradycardic, hypokalemic hearts, it reproducibly induced EADs and TdP in the setting of significantly increased left ventricular transmural dispersion of repolarization. Based on these experimental data, reduction of transmural dispersion of repolarization may be considered an important target for the prevention of TdP in patients with LQT3.     

Cellular basis for long QT,transmural dispersion of repolarization,and torsade de pointes in the long QT syndrome

Genetic studies have identified four forms of congenital long QT syndrome (LQTS) caused by mutations in ion channel genes located on chromosomes 3 (LQT3), 7 (LQT2), 11 (LQT1), and 21 (LQT5). Preliminary clinical studies have reported different phenotypic electrocardiographic patterns and different sensitivity to pacing or pharmacological therapy for each genotype. A transmural electrocardiogram and transmembrane action potentials from epicardial, M, and endocardial cells were simultaneously recorded from an arterially perfused wedge of canine left ventricle. Isoproterenol (100 nmol/L) in the presence of chromanol 293B (30 μmol/L), an I_Ks blocker (LQT1 model), produced a preferential prolongation of M-cell action potential duration (APD), resulting in an increase in transmural dispersion of repolarization (TDR) and a broad-based T wave, as commonly seen in LQT1 patients. D-Sotalol (100 μmol/L), an I_Kr blocker (LQT2 model), and ATX-II (20 nmol/L), an agent that augments late I_Na (LQT3 model), also produced a preferential prolongation of M-cell APD, an increase in TDR, and low-amplitude T wave with a bifurcated appearance (LQT2), and late-appearing T wave (LQT3), respectively. APD-, QT-, and TDR-rate relations were much steeper in the LQT3 model than in either the LQT1 or LQT2 model, whereas the rate relations in the LQT1 and LQT2 models were both steeper than those under control conditions. Spontaneous and programmed electrical stimulation-induced torsade de pointes (TdP) were observed in all 3 models. Propranolol (1 μmol/L), a beta blocker, completely prevented the effect of isoproterenol to persistently or transiently increase TDR and to induce TdP in the LQT1 and LQT2 models, but facilitated TdP in the LQT3 model. Mexiletine, a class IB Na⁺ channel blocker, dose-dependently (2–20 μmol/L) abbreviated the QT and APD more in the LQT3 model, but decreased TDR and suppressed TdP in the 3 models.     

Cellular consequences of HERG mutations in the long QT syndrome: precursors to sudden cardiac death

BACKGROUND: A variety of mutations in HERG, the major subunit of the rapidly activating component of the cardiac delayed rectifier I(Kr), have been found to underlie the congenital Long-QT syndrome, LQT2. LQT2 may give rise to severe arrhythmogenic phenotypes leading to sudden cardiac death. OBJECTIVE: We attempt to elucidate the mechanisms by which heterogeneous LQT2 genotypes can lead to prolongation of the action potential duration (APD) and consequently the QT interval on the ECG. METHODS: We develop Markovian models of wild-type (WT) and mutant I(Kr) channels and incorporate these models into a comprehensive model of the cardiac ventricular cell. RESULTS: Using this virtual transgenic cell model, we describe the effects of HERG mutations on the cardiac ventricular action potential (AP) and provide insight into the mechanism by which each defect results in a net loss of repolarizing current and prolongation of APD. CONCLUSIONS: This study demonstrates which mutations can prolong APD sufficiently to generate early afterdepolarizations (EADs), which may trigger life-threatening arrhythmias. The severity of the phenotype is shown to depend on the specific kinetic changes and how they affect I(Kr) during the time course of the action potential. Clarifying how defects in HERG can lead to impaired cellular electrophysiology can improve our understanding of the link between channel structure and cellular function.     

Sex modulates the arrhythmogenic substrate in prepubertal rabbit hearts with Long QT 2

Females have a greater susceptibility to Torsade de Pointes in congenital and drug-induced long QT syndrome (LQTS) that has been attributed to the modulation of ion channel expression by sex hormones. However, little is known regarding sex differences in pre-puberty, that is, before the surge of sexual hormones. In patients with congenital LQTS types 1 and 2, male children tend to have a greater occurrence of adverse events, especially in 10-15 year olds, than their female counterpart. To evaluate whether the rabbit model of drug-acquired LQTS exhibits similar age dependences, hearts of prepubertal rabbits were perfused, mapped optically to record action potentials (APs) and treated with an I(Kr) blocker, E4031 to elicit LQTS2. As expected, AP durations (APD) were significantly longer in female (n = 18) than male hearts (n = 10), at long cycle length. Surprisingly, E4031 (50-250 nM) induced a greater prolongation of APDs in male than in female hearts, and in both genders reversed the direction of repolarization (apex --> base to base --> apex), enhancing dispersions of repolarization. Furthermore, in male hearts, E4031 (0.5 microM) elicited early afterdepolarizations (EADs) that progressed to polymorphic ventricular tachycardia (PVT) (n = 7/10) and were interrupted by isoproterenol (40 nM) and prevented by propranolol (0.5-2.5 microM). In female hearts, E4031 (0.5 microM) produced marked prolongations of APDs yet few EADs with no progression to PVT (n = 16/18). Thus, sex differences are opposite in prepubertal versus adult rabbits with respect to E4031-induced APD prolongation, EADs and PVT, underscoring the fact that APD prolongation alone is insufficient to predict arrhythmia susceptibility.     

The ionic mechanisms of long QT interval in diabetic rabbits

Objective Abnormal QT prolongation associated with arrhythmias is considered the major cardiac electrical disorder and a significant predictor of mortality in diabetic patients. The precise ionic mechanisms for diabetic QT prolongation remained unclear. The present study was designed to analyze the changes of ventricular repolarization and the underlying ionic mechanisms in diabetic rabbit hearts. Methods Diabetes was induced by a single injection ofalloxan （145mg/kg, Lv. ）. After the development of diabetes （10 weeks）, ECG was measured. Whole-cell patch-clamp technique was applied to record the action potential duration （APD50, APD90）, slowly activating outward rectifying potassium current （IKs）, L-type calcium current （ICa-L） and inward rectifying potassium current （IK1）. Results The action potential duration （APD50 and APD90） of ventricular myocytes was obviously prolonged from 271.5＋32.3 ms and 347.8＋36.3 ms to 556.6~72.5 ms and 647.9~72.2 ms respectively （P〈 0.05）. Meanwhile the normalized peak current densities of IKs in ventricular myocytes investigated by whole-cell patch clamp was smaller in diabetic rabbits than that in control group at test potential of＋50mV （1.27~0.20 pA/pF vs 3.08~0.67 pA/pF, P〈0.05）. And the density of the ICa-L was increased apparently at the test potential of 10 mV （-2.67~0.41 pA/pF vs -5.404-1.08 pA/pF, P〈0.05）. Conclusion Ventricular repolarization was prolonged in diabetic rabbits, it may be partly due to the increased L-type calcium current and reduced slow delayed rectifier K＋ current （IKs） （J Geriatr Cardio12010; 7：25-29）.     

Effect of estrogen on ventricular repolarization in menopausal patients with syndrome X and effects of nicorandil.

Syndrome X may exhibit myocardial ischemia and is associated with estrogen deficiency. We sought to assess the possible role of estrogen in modulating the characteristics of ventricular repolarization by measurement of QT interval and QT dispersion in patients with syndrome X. We prospectively used 12-lead electrocardiograms and echocardiograms to study 52 consecutive menopausal patients with syndrome X (group subdivided into subgroup 1a, 32 patients who received nicorandil, an adenosine triphosphate-sensitive potassium ion channel opener; subgroup 1b, 20 patients without dosing nicorandil). For comparisons, a control group consisted of age-matched and echocardiographic left ventricular mass index-matched 20 healthy menopausal women. Baseline QT intervals and QT dispersion were similar between the 2 groups (subgroup 1a and controls). After administration of estrogen, there was significant prolongation of maximal QTc intervals and reduction in QT or QTc dispersion compared with baseline in patients with syndrome X. The changes returned to baseline after nicorandil administration. Control subjects had no changes with administration of estrogen. Thus, estrogen modulates characteristics of ventricular repolarization, which appears to be mediated by blocking adenosine triphosphate-sensitive potassium ion channel. The effects of estrogen on QT intervals may be different between menopausal women with or without syndrome X.     

LQT2模型室性心律失常电生理机制研究

为探讨心室肌跨壁复极离散度 (TDR)和心脏兴奋的恢复性质在长QT综合征 (LQTS)室性心律失常发生过程中的作用 ,应用冠状小动脉灌流的兔左室肌楔形组织块标本 ,分模型组和对照组 ,采用浮置玻璃微电极法同步记录心室肌内、外膜心肌细胞动作电位和跨壁心电图。模型组以 30 μmol/L的d sotalol台氏液灌流 ,制备LQT2模型。对照组以标准台氏液灌流。结果 :模型组和对照组比较TDR有显著性差异 (83.6± 14 .0msvs 4 8.6± 5 .3ms,P <0 .0 1,n =10 )。模型组内、外膜动作电位时程 (APD)恢复曲线最大斜率均大于 1,而对照组均小于 1,两组间APD恢复曲线最大斜率比较有显著性差异 (P <0 .0 1,n =2 0 )。模型组在S1S2 程序刺激下尖端扭转型室性心动过速的发生率为70 %。对照组无 1例发生室性心律失常。结论 :心脏兴奋的恢复性质和心室肌TDR均参与了LQT2室性心律失常的发生。     

Effects of Sodium Channel Block with Mexiletine to Reverse Action Potential Prolongation in In Vitro Models of the Long QT Syndrome

Sodium Channel Block in In Vitro Models of LQTS. Introduction: Recent clinical studies have reported a greater effectiveness of sodium channel block with mexiletine to abbreviate the QT interval in patients with the chromosome 3 variant (SCN5A, LQT3) of the long QT syndrome (LQTS) than those with the chromosome 7 form of the disease (HERG, LQT2), suggesting the possibility of gene-specific therapy for the two distinct forms of the congenital LQTS. Experimental studies using the arterially perfused left ventricular wedge preparation have confirmed these clinical observations on the QT interval but have gone on to further demonstrate a potent effect of mexiletine to reduce dispersion of repolarization and prevent torsades de pointes (TdP) in both LQT2 and LQT3 models. A differential action of sodium channel block on the three ventricular cell types is thought to mediate these actions of mexiletine. This study provides a test of this hypothesis by examining the effects of mexiletine in isolated canine ventricular epicardial, endocardial, and M region tissues under conditions that mimic the SCN5A and HERG gene defects. Methods and Results: We used standard microelectrode techniques to record transmembrane activity from endocardial, epicardial, mid-myocardial, and transmural strips isolated from the canine left ventricle, d-Sotalol, an I_kr blocker, was used to mimic the HERG defect (LQT2), and ATX-II, which increases late Na channel current, was used to mimic the SCN5A defect (LQT3). d-Sotalol (100 μM) preferentially prolonged the action potential of the mid-myocardial M cell (APD₉₀, increased from 340 ± 65 to 623 ± 203 msec) as did ATX-II (10 to 20 nM; APD₉₀, increased from 325 ± 51 to 580 ± 178 msec; basic cycle length = 2000 msec), thus causing a marked increase in transmural dispersion of repolarization (TDR). Mexiletine (2 to 20 μM) dose-dependently reversed the ATX-II-induced prolongation of APD₉₀, in all three cell types. Mexiletine also reversed the d-sotalol-induced prolongation of the M cell action potential duration (APD), but bad little effect on the action potential of epicardium and endocardium. Due to its preferential effect to abbreviate the action potential of M cells, mexiletine reduced the dispersion of repolarization in both models. Low concentrations of mexiletine (5 to 10 μM) totally suppressed early afterdepolarization (EAD) and KAD-induced triggered activity in both models. Conclusions: Our results indicate that the actions of mexiletine are both cell and model specific, but that sodium channel block with mexiletine is effective in reducing transmural differences in APD and in abolishing triggered activity induced by d-sotalol and ATX-II. The data suggest that mexiletine's actions to reduce TDR and prevent the induction of spontaneous and programmed stimulation-induced TdP in these models are due to a preferential effect of the drug to abbreviate the APD of the M cell and to suppress the development of EADs. The data provide further support for the hypothesis that block of the late sodium current may be of value in the treatment of LQT2 as well as LQT3 and perhaps other congenital and acquired (drug-induced) forms of LQTS.     

Triggers of ventricular tachyarrhythmias and therapeutic effects of nicorandil in canine models of LQT2 and LQT3 syndromes

OBJECTIVES: We sought to identify the triggers of ventricular tachyarrhythmia (VTA) in experimental models of long QT type 2 (LQT2) and long QT type 3 (LQT3) syndromes. BACKGROUND: Most adverse cardiac events occurring in the long QT type 1 syndrome are related to sympathetic nerve activity. In contrast, various factors may trigger VTA in patients with LQT2 and LQT3. METHODS: The mode of onset of VTA and therapeutic effects of the potassium-adenosine triphosphate channel opener nicorandil were compared in canine models of LQT2 and LQT3, using three induction protocols: 1) bradycardia produced by atrioventricular block (BRADY); 2) programmed ventricular stimulation; and 3) electrical stimulation of the left stellate ganglion (left stellate stimulation [LSS]). Transmural unipolar electrograms were recorded, and the activation-recovery interval (ARI) was measured. RESULTS: Ventricular tachyarrhythmias developed during BRADY in all six experiments in the LQT3 model, but in none of the six experiments in LQT2. Programmed ventricular stimulation induced VTA in two experiments of the LQT2 model, but in none of the LQT3 experiments. Stimulation of the left stellate ganglion induced VTA in three experiments in LQT2 and in two experiments in LQT3. Nicorandil caused greater shortening of ARI and greater attenuation of transmural ARI dispersion in the LQT2 model than in the LQT3 model. After treatment with nicorandil, a single VTA was induced in the LQT2 model by LSS, whereas in the LQT3 model, VTA remained inducible by BRADY in four experiments and LSS in one experiment. CONCLUSIONS: An abrupt increase in sympathetic activity appeared arrhythmogenic in both models. Nicorandil attenuated the heterogeneity of ventricular repolarization and suppressed the induction of VTA in the LQT2 model, but had a limited therapeutic effect in the LQT3 model.     

Comparative pharmacology of guinea pig cardiac myocyte and cloned hERG (I(Kr)) channel

INTRODUCTION: This study used whole-cell, patch clamp techniques on isolated guinea pig ventricular myocytes and HEK293 cells expressing cloned human ether-a-go-go-related gene (hERG) to examine the action of drugs causing QT interval prolongation and torsades de pointes (TdP) in man. Similarities and important differences in drug actions on cardiac myocytes and cloned hERG I(Kr) channels were established. Qualitative actions of the drugs on cardiac myocytes corresponded with results obtained from Purkinje fibers and measurement of QT interval prolongation in animal and human telemetry studies. METHODS AND RESULTS: Adult guinea pig ventricular myocytes were isolated by enzymatic digestion. Cells were continuously perfused with Tyrode's solution at 33-35 degrees C. Recordings were made using the whole-cell, patch clamp technique. Action potentials (APs) were elicited under current clamp. Voltage clamp was used to study the effect of drugs on I(Kr) (rapidly activating delayed rectifier potassium current), I(Na) (sodium current), and I(Ca) (L-type calcium current). Dofetilide increased the myocyte action potential duration (APD) in a concentration-dependent manner, with a pIC50 of 7.3. Dofetilide 1 microM elicited early afterdepolarizations (EADs) but had little affect on I(Ca) or I(Na). E-4031 increased APD in a concentration-dependent manner, with a pIC50 of 7.2. In contrast, 10 microM loratadine, desloratadine, and cetirizine had little effect on APD or I(Kr). Interestingly, cisapride displayed a biphasic effect on myocyte APD and inhibited I(Ca) at 1 microM. Even at this high concentration, cisapride did not elicit EADs. A number of AstraZeneca compounds were tested on cardiac myocytes, revealing a mixture of drug actions that were not observed in hERG currents in HEK293 cells. One compound, particularly AR-C0X, was a potent blocker of myocyte AP (pIC50 of 8.4). AR-C0X also elicited EADs in cardiac myocytes. The potencies of the same set of drugs on the cloned hERG channel also were assessed. The pIC50 values for dofetilide, E-4031, terfenadine, loratadine, desloratadine, and cetirizine were 6.8, 7.1, 7.3, 5.1, 5.2, and <4, respectively. Elevation of temperature from 22 to 35 degrees C significantly enhanced the current kinetics and amplitudes of hERG currents and resulted in approximately fivefold increase in E-4031 potency. CONCLUSION: Our study demonstrates the advantages of cardiac myocytes over heterologously expressed hERG channels in predicting QT interval prolongation and TdP in man. The potencies of some drugs in cardiac myocytes were similar to hERG, but only myocytes were able to detect important changes in APD characteristics and display EADs predictive of arrhythmia development. We observed similar qualitative drug profiles in cardiac myocytes, dog Purkinje fibers, and animal and human telemetry studies. Therefore, isolated native cardiac myocytes are a better predictor of drug-induced QT prolongation and TdP than heterologously expressed hERG channels. Isolated cardiac myocytes, when used with high-throughput patch clamp instruments, may have an important role in screening potential cardiotoxic compounds in the early phase of drug discovery. This would significantly reduce the attrition rate of drugs entering preclinical and/or clinical development. The current kinetics and amplitudes of the cloned hERG channel were profoundly affected by temperature, significantly altering the potency of one drug (E-4031). This finding cautions against routine drug testing at room temperature compared to physiologic temperature when using the cloned hERG channel.     

Transmural electrophysiological heterogeneities underlying arrhythmogenesis in heart failure

Although expression of numerous ion channels is altered in heart failure (HF), mechanisms by which dysfunction at the ionic and molecular levels lead to ventricular tachyarrhythmias in HF are unknown. Previously, we found that transmural heterogeneities of repolarization play a critical role in the genesis of polymorphic ventricular tachycardia (PVT) when QT interval was prolonged in LQT2. Because QT interval is also prolonged in HF, we hypothesized that transmural heterogeneities are a mechanism of PVT in HF. Optical action potentials were measured simultaneously from cells spanning the entire transmural wall of arterially perfused canine wedge preparations. Wedges were isolated from dogs without (control, n=5) and with HF (n=8) produced by rapid ventricular pacing. In HF, action potential duration (APD) prolongation was markedly heterogeneous across the transmural wall, and was characterized by disproportionate APD prolongation of midmyocardial (M) cells. APD prolongation of M cells accounted for QT-interval prolongation, and caused significant increases (P<0.01) in spatial gradients of repolarization across the ventricular wall from 4.3+/-2.1 (control) to 12.4+/-3.5 ms/mm (HF). Enhanced gradients were directly responsible for development of functional conduction block, leading to PVT in 63% of HF wedges but in no controls (P<0.03). Moreover, intramural decremental conduction and block of the premature impulse, preceded each episode of PVT, and always occurred at the border between M-cell and subepicardial zones, where repolarization gradients were highest. Selective prolongation of APD within M cells underlies several key features of the HF phenotype, including QT-interval prolongation, transmural heterogeneity of repolarization, and susceptibility to conduction block and reentrant PVT.     

Verapamil prevents torsade de pointes by reduction of transmural dispersion of repolarization and suppression of early afterdepolarizations in an intact heart model of LQT3

Background In long QT syndrome (LQTS), prolongation of the QT–interval is associated with sudden cardiac death resulting from potentially life–threatening polymorphic tachycardia of the torsade de pointes (TdP) type. Experimental as well as clinical reports support the hypothesis that calcium channel blockers such as verapamil may be an appropriate therapeutic approach in LQTS. We investigated the electrophysiologic mechanism by which verapamil suppresses TdP, in a recently developed intact heart model of LQT3.Methods and results In 8 Langendorff–perfused rabbit hearts, veratridine (0.1 µM), an inhibitor of sodium channel inactivation, led to a marked increase in QT–interval and simultaneously recorded monophasic ventricular action potentials (MAPs) (p < 0.05) thereby mimicking LQT3. In bradycardic (AV–blocked) hearts, simultaneous recording of up to eight epi– and endocardial MAPs demonstrated a significant increase in total dispersion of repolarization (56%, p < 0.05) and reverse frequency–dependence. After lowering potassium concentration, veratridine reproducibly led to early afterdepolarizations (EADs) and TdP in 6 of 8 (75%) hearts. Additional infusion of verapamil (0.75 µM) suppressed EADs and consecutively TdP in all hearts. Verapamil significantly shortened endocardial but not epicardial MAPs which resulted in significant reduction of ventricular transmural dispersion of repolarization.Conclusions Verapamil is highly effective in preventing TdP via shortening of endocardial MAPs, reduction of left ventricular transmural dispersion of repolarization and suppression of EADs in an intact heart model of LQT3. These data suggest a possible therapeutic role of verapamil in the treatment of LQT3 patients.