Heterogeneity within the ventricular wall. Electrophysiology and pharmacology of epicardial, endocardial, and M cells

Drug-induced long QT syndrome

The drug-induced long QT syndrome is a distinct clinical entity that has evolved from an electrophysiologic curiosity to a centerpiece in drug regulation and development. This evolution reflects an increasing recognition that a rare adverse drug effect can profoundly upset the balance between benefit and risk that goes into the prescription of a drug by an individual practitioner as well as the approval of a new drug entity by a regulatory agency. This review will outline how defining the central mechanism, block of the cardiac delayed-rectifier potassium current I(Kr), has contributed to defining risk in patients and in populations. Models for studying risk, and understanding the way in which clinical risk factors modulate cardiac repolarization at the molecular level are discussed. Finally, the role of genetic variants in modulating risk is described.

Interventricular heterogeneity in rat heart responses to hypoxia: the tuning of glucose metabolism, ion gradients, and function

The matching of energy supply and demand under hypoxic conditions is critical for sustaining myocardial function. Numerous reports indicate that basal energy requirements and ion handling may differ between the ventricles. We hypothesized that ventricular response to hypoxia shows interventricular differences caused by the heterogeneity in glucose metabolism and expression and activity of ion transporters. Thus we assessed glucose utilization rate, ATP, sodium and potassium concentrations, Na, K-ATPase activity, and tissue reduced:oxidized glutathione (GSH/GSSG) content in the right and left ventricles before and after the exposure of either the whole animals or isolated blood-perfused hearts to hypoxia. The hypoxia-induced boost in glucose utilization was more pronounced in the left ventricle compared with the right one. ATP levels in the right ventricle of hypoxic heart were lower than those in the left ventricle. Left ventricular sodium content was higher, and hydrolytic Na, K-ATPase activity was reduced compared with the right ventricle. Administration of the Na, K-ATPase blocker ouabain caused rapid increase in the right ventricular Na(+) and elimination of the interventricular Na(+) gradients. Exposure of the hearts to hypoxia made the interventricular heterogeneity in the Na(+) distribution even more pronounced. Furthermore, systemic hypoxia caused oxidative stress that was more pronounced in the right ventricle as revealed by GSH/GSSG ratios. On the basis of these findings, we suggest that the right ventricle is more prone to hypoxic damage, as it is less efficient in recruiting glucose as an alternative fuel and is particularly dependent on the efficient Na, K-ATPase function.

The effect of stimulation frequency on the transmural ventricular monophasic action potential in yellowfin tuna Thunnus albacares

Monophasic action potentials (MAPs) were recorded from the spongy and compact layers of the yellowfin tuna Thunnus albacares ventricle as stimulation frequency was increased. MAP duration decreased with increase in stimulation frequency in both the spongy and compact myocardial layers, but no significant difference in MAP duration was observed between the layers.

Early Repolarization Disease

Sudden cardiac death (SCD) is defined as an unexpected natural death from a cardiac cause within a short time period, generally less than or equal to 1 hour from the onset of symptoms, in a person without any prior condition that seems to result in instantaneous fatality. Although such a rapid death process is attributed to cardiac arrhythmia, arrhythmia often represents the final common event in a series of events precipitated by known (95%) or unknown (5%) cardiac disorder. Electrocardiographic early repolarization involving the inferolateral leads, which was labeled benign until recently, is the latest of the primary electrical cardiac diseases discovered to have significantly high prevalence in SCD cases. Careful evaluation of patients having early repolarization associated with unexplained syncope, family history of SCD, or idiopathic ventricular arrhythmias is recommended.

Transmural differences in respiratory capacity across the rat left ventricle in health, aging, and streptozotocin-induced diabetes mellitus: evidence that mitochondrial dysfunction begins in the subepicardium

In diabetic cardiomyopathy, ventricular dysfunction occurs in the absence of hypertension or atherosclerosis and is accompanied by altered myocardial substrate utilization and depressed mitochondrial respiration. It is not known if mitochondrial function differs across the left ventricular (LV) wall in diabetes. In the healthy heart, the inner subendocardial region demonstrates higher rates of blood flow, oxygen consumption, and ATP turnover compared with the outer subepicardial region, but published transmural respirometric measurements have not demonstrated differences. We aim to measure mitochondrial function in Wistar rat LV to determine the effects of age, streptozotocin-diabetes, and LV layer. High-resolution respirometry measured indexes of respiration in saponin-skinned fibers dissected from the LV subendocardium and subepicardium of 3-mo-old rats after 1 mo of streptozotocin-induced diabetes and 4-mo-old rats following 2 mo of diabetes. Heart rate and heartbeat duration were measured under isoflurane-anesthesia using a fetal-Doppler, and transmission electron microscopy was employed to observe ultrastructural differences. Heart rate decreased with age and diabetes, whereas heartbeat duration increased with diabetes. While there were no transmural respirational differences in young healthy rat hearts, both myocardial layers showed a respiratory depression with age (30-40%). In 1-mo diabetic rat hearts only subepicardial respiration was depressed, whereas after 2 mo diabetes, respiration in subendocardial and subepicardial layers was depressed and showed elevated leak (state 2) respiration. These data provide evidence that mitochondrial dysfunction is first detectable in the subepicardium of diabetic rat LV, whereas there are measureable changes in LV mitochondria after only 4 mo of aging.

Chaos in the genesis and maintenance of cardiac arrhythmias

Dynamical chaos, an irregular behavior of deterministic systems, has been widely shown in nature. It also has been demonstrated in cardiac myocytes in many studies, including rapid pacing-induced irregular beat-to-beat action potential alterations and slow pacing-induced irregular early afterdepolarizations, etc. Here we review the roles of chaos in the genesis of cardiac arrhythmias, the transition to ventricular fibrillation, and the spontaneous termination of fibrillation, based on evidence from computer simulation of mathematical models and experiments of animal models.

Electrophysiological differences between the epicardium and the endocardium of the left atrium

BACKGROUND

Electrophysiological properties of the atrial endocardium compared to epicardium are not well understood. The purpose of this study was to compare the electrophysiological properties and vulnerability to arrhythmia induction from these regions.

METHODS AND RESULTS

Transseptal endocardial and percutaneous epicardial mapping were performed in a porcine model (n = 7). Two opposing 4-mm electrophysiological catheters were positioned endocardially and epicardially. A circular mapping catheter (CMC) was positioned at the ostium of the common inferior pulmonary vein (CIPV) recording left atrial (LA)-PV potentials. Endocardial and epicardial effective refractory periods (ERPs) at two basic cycle lengths (CLs) of 600 and 400 ms were recorded from four anatomic locations (CIPV, LA appendage, right superior PV, and LA posterior wall). Atrial repetitive response (ARR) induction was also tested from endocardial and epicardial sites. Overall, 254 ERP measurements (mean 36.3 per animal) and 84 induction attempts (mean 12 per animal) were performed. The ERP was significantly shorter in the epicardium compared to the endocardium at basic CL of 400 ms (P = 0.006) but not at CL of 600 ms (P = 0.2). In addition, only the epicardium demonstrated ERP shortening when the CL of the basic drive was shortened (P = 0.03). ARR could be induced more often from the epicardium (P = 0.002) and fibrillatory activity with epicardial/endocardial dissociation was recorded (n = 3). Also, the earliest PV activation site on the CMC was noted to be different in 16.5% of cases during epicardial and endocardial pacing.

CONCLUSION

The electrophysiological characteristics of the atrial epicardium are different from the endocardium with a shorter ERP and more frequent ARR induction by programed stimulation.

Microvolt T-wave alternans during Holter monitoring in children and adolescents

BACKGROUND

Time-domain microvolt T-wave alternans (TWA) has been described as a noninvasive marker of sudden cardiac death in adults. The incidence of TWA in pediatric populations has not been defined well. The aim of the study was to determine peculiarities of TWA in children.

METHODS

We examined 68 healthy patients-newborns (20) and children in age group of 7-17 years (48)-and 85 pediatric patients: ventricular premature beats-65; dilated cardiomyopathy (DCMP)-2; long QT syndrome (LQTS)-10; Brugada syndrome (BrS)-5, catecholaminergic ventricular tachycardia (CVT)-3. All underwent Holter monitoring (HM) with definition of the peak value of TWA by modified moving average method.

RESULTS

In healthy newborns, TWA was 32 +/- 8 (12-55) microV (HR 123-156 bmp). In healthy children (7-17 years) it was 30 +/- 11 (10-l 55) microV, (HR 64-132 bmp) without any differences between boys and girls. In all group of patients, TWA were significantly higher (P < 0.05) than in healthy. Circadian peak of TWA was found (90%) in a second part of day and at sleep (8%). Among them 60% (LQTS, BrS, and DCPM) had TWA > 55 microV.

CONCLUSION

Time-domain TWA during HM in children was independent of age, gender, and heart rate. In 94% healthy children, values of TWA do not exceed 55 microV but 20-50% children with cardiac pathology had TWA more than 55 microV. Night circadian type of TWA in diseases with risk of life-threatening arrhythmias associated with TWA was more than 55 microV.

Effect of activation sequence on transmural patterns of repolarization and action potential duration in rabbit ventricular myocardium

Although transmural heterogeneity of action potential duration (APD) is established in single cells isolated from different tissue layers, the extent to which it produces transmural gradients of repolarization in electrotonically coupled ventricular myocardium remains controversial. The purpose of this study was to examine the relative contribution of intrinsic cellular gradients of APD and electrotonic influences to transmural repolarization in rabbit ventricular myocardium. Transmural optical mapping was performed in left ventricular wedge preparations from eight rabbits. Transmural patterns of activation, repolarization, and APD were recorded during endocardial and epicardial stimulation. Experimental results were compared with modeled data during variations in electrotonic coupling. A transmural gradient of APD was evident during endocardial stimulation, which reflected differences previously seen in isolated cells, with the longest APD at the endocardium and the shortest at the epicardium (endo: 165 ± 5 vs. epi: 147 ± 4 ms; P < 0.05). During epicardial stimulation, this gradient reversed (epi: 162 ± 4 vs. endo: 148 ± 6 ms; P < 0.05). In both activation sequences, transmural repolarization followed activation and APD shortened along the activation path such that significant transmural gradients of repolarization did not occur. This correlation between transmural activation time and APD was recapitulated in simulations and varied with changes in intercellular coupling, confirming that it is mediated by electrotonic current flow between cells. These data suggest that electrotonic influences are important in determining the transmural repolarization sequence in rabbit ventricular myocardium and that they are sufficient to overcome intrinsic differences in the electrophysiological properties of the cells across the ventricular wall.

Early repolarization syndrome – a new electrical disorder associated with sudden cardiac death –

Early repolarization (ER), consisting of a J-point elevation, notching or slurring of the terminal portion of the R wave (J wave), and tall/symmetric T wave, is a common finding on the 12-lead electrocardiogram. For decades, it has been considered as benign, barring sporadic case reports and basic electrophysiology research that suggested a critical role of the J wave in the pathogenesis of idiopathic ventricular fibrillation (VF). In 2007-2008, a high prevalence of ER in patients with idiopathic VF was reported and subsequent studies reinforced the results. This review summarizes the current state of knowledge concerning ER syndrome associated with sudden cardiac death. 

Complex structure of electrophysiological gradients emerging during long-duration ventricular fibrillation in the canine heart

Long-duration ventricular fibrillation (LDVF) in the globally ischemic heart is a common setting of cardiac arrest. Electrical heterogeneities during LDVF may affect outcomes of defibrillation and resuscitation. Previous studies in large mammalian hearts have investigated the role of Purkinje fibers and electrophysiological gradients between the endocardium (Endo) and epicardium (Epi). Much less is known about gradients between the right ventricle (RV) and left ventricle (LV) and within each chamber during LDVF. We studied the transmural distribution of the VF activation rate (VFR) in the RV and LV and at the junction of RV, LV, and septum (Sep) during LDVF using plunge needle electrodes in opened-chest dogs. We also used optical mapping to analyze the Epi distribution of VFR, action potential duration (APD), and diastolic interval (DI) during LDVF in the RV and LV of isolated hearts. Transmural VFR gradients developed in both the RV and LV, with a faster VFR in Endo. Concurrently, large VFR gradients developed in Epi, with the fastest VFR in the RV-Sep junction, intermediate in the RV, and slowest in the LV. Optical mapping revealed a progressively increasing VFR dispersion within both the LV and RV, with a mosaic presence of fully inexcitable areas after 4-8 min of LDVF. The transmural, interchamber, and intrachamber VFR heterogeneities were of similar magnitude. In both chambers, the inverse of VFR was highly correlated with DI, but not APD, at all time points of LDVF. We conclude that the complex VFR gradients during LDVF in the canine heart cannot be explained solely by the distribution of Purkinje fibers and are related to regional differences in the electrical depression secondary to LDVF.

Changes in the calcium current among different transmural regions contributes to action potential heterogeneity in rat heart

To clarify the transmural heterogeneity of action potential (AP) time course, we examined the regulation of L-type Ca(2+) current (I(Ca,L)) by voltage and Ca(2+)-dependent mechanisms. Currents were recorded using patch clamp of single rat subepicardial (EPI) and subendocardial (ENDO) of left ventricular, right ventricular (RV) and septal (SEP) cardiomyocytes. Voltage clamp commands were derived from ENDO and EPI APs or rectangular voltage pulses. During rectangular pulses, peak I(Ca,L) was significantly greater in EPI than in other cells. The inactivation of I(Ca,L) by Ca(2+)-dependent mechanisms (suppressed by ryanodine and BAPTA) was present in all cells but greater in extent in ENDO and SEP cells. Activation and inactivation curves for all regions show subtle differences that are Ca(2+) sensitive, with Ca(2+) inactivation shifting the activation variables negative by approximately 7 mV and inactivation variables positive by 2-7 mV (EPI being least, RV greatest). In AP-clamps, the peak I(Ca,L) was significantly smaller in ENDO than in EPI cells, while the integrated current was significantly larger in ENDO than in EPI cells. The results are discussed with regard to the interplay of AP time course and net Ca(2+) influx.

Docosahexaenoic acid has influence on action potentials and transient outward potassium currents of ventricular myocytes

Background

There are many reports about the anti-arrhythmic effects of ω-3 polyunsaturated fatty acids, however, the mechanisms are still not completely delineated. The purpose of this study was to investigate the characteristics of action potentials and transient outward potassium currents (I_to) of Sprague-Dawley rat ventricular myocytes and the effects of docosahexaenoic acid (DHA) on action potentials and I_to.

Methods

The calcium-tolerant rat ventricular myocytes were isolated by enzyme digestion. Action potentials and I_to of epicardial, mid-cardial and endocardial ventricular myocytes were recorded by whole-cell patch clamp technique.

Results

1. Action potential durations (APDs) were prolonged from epicardial to endocardial ventricular myocytes (P < 0.05). 2. I_to current densities were decreased from epicardial to endocardial ventricular myocytes, which were 59.50 ± 15.99 pA/pF, 29.15 ± 5.53 pA/pF, and 12.29 ± 3.62 pA/pF, respectively at +70 mV test potential (P < 0.05). 3. APDs were gradually prolonged with the increase of DHA concentrations from 1 μmol/L to 100 μmol/L, however, APDs changes were not significant as DHA concentrations were in the range of 0 μmol/L to 1 μmol/L. 4. I_to currents were gradually reduced with the increase of DHA concentrations from 1 μmol/L to 100 μmol/L, and its half-inhibited concentration was 5.3 μmol/L. The results showed that there were regional differences in the distribution of action potentials and I_to in rat epicardial, mid-cardial and endocardial ventricular myocytes. APDs were prolonged and I_to current densities were gradually reduced with the increase of DHA concentrations.

Conclusion

The anti-arrhythmia mechanisms of DHA are complex, however, the effects of DHA on action potentials and I_to may be one of the important causes.

Heterogeneous memory in restitution of action potential duration in pig ventricles

BACKGROUND

Restitution of action potential duration and memory importantly affect electrical stability in ventricles. Studies have reported heterogeneous restitution among different regions of the ventricles. However, existence of heterogeneity in memory is not as well investigated.

METHODS

Transmembrane potentials were recorded in endocardial and epicardial tissues from both ventricles of farm pigs. Pacing protocols with sinusoidally changing diastolic intervals were used to reveal hysteresis in restitution, from which quantitative measures of memory were calculated.

RESULTS

Larger measures of hysteresis were observed in the endocardium than the epicardium (P < .05): loop thickness (in milliseconds), 26.9 vs 16.2; overall tilt, 0.376 vs 0.249; and loop area (in square milliseconds), 7288 vs 4146. Except for overall tilt, no significant differences in these measures were observed between ventricles.

CONCLUSION

Heterogeneity in memory exists in pig ventricles. Because regions with the steepest restitution may also have the largest memory, our results suggest that heterogeneity in memory should also be factored in when predicting electrical stability.

J wave syndromes

The J wave, also referred to as an Osborn wave, is a deflection immediately following the QRS complex of the surface ECG. When partially buried in the R wave, the J wave appears as J-point elevation or ST-segment elevation. Several lines of evidence have suggested that arrhythmias associated with an early repolarization pattern in the inferior or mid to lateral precordial leads, Brugada syndrome, or arrhythmias associated with hypothermia and the acute phase of ST-segment elevation myocardial infarction are mechanistically linked to abnormalities in the manifestation of the transient outward current (I(to))-mediated J wave. Although Brugada syndrome and early repolarization syndrome differ with respect to the magnitude and lead location of abnormal J-wave manifestation, they can be considered to represent a continuous spectrum of phenotypic expression that we propose be termed J-wave syndromes. This review summarizes our current state of knowledge concerning J-wave syndromes, bridging basic and clinical aspects. We propose to divide early repolarization syndrome into three subtypes: type 1, which displays an early repolarization pattern predominantly in the lateral precordial leads, is prevalent among healthy male athletes and is rarely seen in ventricular fibrillation survivors; type 2, which displays an early repolarization pattern predominantly in the inferior or inferolateral leads, is associated with a higher level of risk; and type 3, which displays an early repolarization pattern globally in the inferior, lateral, and right precordial leads, is associated with the highest level of risk for development of malignant arrhythmias and is often associated with ventricular fibrillation storms.

Detection of U wave activity in healthy volunteers by high-resolution magnetocardiography

INTRODUCTION

The purpose of our study was to prove the existence of the U wave using magnetocardiograms (MCGs).

METHODS

The 31-channel MCGs of 25 healthy volunteers were recorded. The onset of the U wave was defined by newly developed spatial correlation analysis; and the end, by different approaches.

RESULTS

A U wave could be proved in all volunteers. In 10 volunteers (heart rate, 57 +/- 19 beats/min) in whom the U wave was found to be separated from the following P wave, the U wave's end could be determined as a threshold value (U wave duration, 310 +/- 24 milliseconds). In 15 volunteers (heart rate, 70 +/- 38 beats/min), the end of the U waves was concealed by a continuous transition of the U waves into the following P waves.

CONCLUSIONS

The U wave seems to be a regular phenomenon and has a distinct spatiotemporal assembly.

Repolarization of the cardiac action potential. Does an increase in repolarization capacity constitute a new anti-arrhythmic principle?

The cardiac action potential can be divided into five distinct phases designated phases 0-4. The exact shape of the action potential comes about primarily as an orchestrated function of ion channels. The present review will give an overview of ion channels involved in generating the cardiac action potential with special emphasis on potassium channels involved in phase 3 repolarization. In humans, these channels are primarily K(v)11.1 (hERG1), K(v)7.1 (KCNQ1) and K(ir)2.1 (KCNJ2) being the responsible alpha-subunits for conducting I(Kr), I(Ks) and I(K1). An account will be given about molecular components, biophysical properties, regulation, interaction with other proteins and involvement in diseases. Both loss and gain of function of these currents are associated with different arrhythmogenic diseases. The second part of this review will therefore elucidate arrhythmias and subsequently focus on newly developed chemical entities having the ability to increase the activity of I(Kr), I(Ks) and I(K1). An evaluation will be given addressing the possibility that this novel class of compounds have the ability to constitute a new anti-arrhythmic principle. Experimental evidence from in vitro, ex vivo and in vivo settings will be included. Furthermore, conceptual differences between the short QT syndrome and I(Kr) activation will be accounted for.

Transmural dispersion of repolarization in failing and nonfailing human ventricle

RATIONALE

Transmural dispersion of repolarization has been shown to play a role in the genesis of ventricular tachycardia and fibrillation in different animal models of heart failure (HF). Heterogeneous changes of repolarization within the midmyocardial population of ventricular cells have been considered an important contributor to the HF phenotype. However, there is limited electrophysiological data from the human heart.

OBJECTIVE

To study electrophysiological remodeling of transmural repolarization in the failing and nonfailing human hearts.

METHODS AND RESULTS

We optically mapped the action potential duration (APD) in the coronary-perfused scar-free posterior-lateral left ventricular free wall wedge preparations from failing (n=5) and nonfailing (n=5) human hearts. During slow pacing (S1S1=2000 ms), in the nonfailing hearts we observed significant transmural APD gradient: subepicardial, midmyocardial, and subendocardial APD80 were 383+/-21, 455+/-20, and 494+/-22 ms, respectively. In 60% of nonfailing hearts (3 of 5), we found midmyocardial islands of cells that presented a distinctly long APD (537+/-40 ms) and a steep local APD gradient (27+/-7 ms/mm) compared with the neighboring myocardium. HF resulted in prolongation of APD80: 477+/-22 ms, 495+/-29 ms, and 506+/-35 ms for the subepi-, mid-, and subendocardium, respectively, while reducing transmural APD80 difference from 111+/-13 to 29+/-6 ms (P<0.005) and presence of any prominent local APD gradient. In HF, immunostaining revealed a significant reduction of connexin43 expression on the subepicardium.

CONCLUSIONS

We present for the first time direct experimental evidence of a transmural APD gradient in the human heart. HF results in the heterogeneous prolongation of APD, which significantly reduces the transmural and local APD gradients.

Suppression of spirals and turbulence in inhomogeneous excitable media

Suppression of spiral and turbulence in inhomogeneous media due to local heterogeneity with higher excitability is investigated numerically. When the inhomogeneity is small, control tactics by boundary periodic forcing (BPF) is effective against the existing spiral and turbulence. When the inhomogeneity of excitability is large, a rotating electric field (REF) is utilized to "smooth" regional heterogeneity based on driven synchronization. Consequently, a control approach combining BPF with REF is proposed to suppress the spiral and turbulence. The underlying mechanism of successful suppression is discussed in terms of dispersion relation.

Drug-induced arrhythmias and sudden cardiac death: implications for the pharmaceutical industry

Following a series of high profile withdrawals from the market, the ability of medications to induce potentially fatal arrhythmias is a significant problem facing the pharmaceutical industry. Current preclinical cardiac safety assays are based on the assumption that blockade of a single repolarizing K(+) channel alone precipitates drug-induced arrhythmias, however, current findings point to a range of more complex arrhythmogenic mechanisms. This review begins by exploring clinical findings and potential mechanisms underlying drug-induced sudden cardiac death and then goes on to assess current and explore future strategies to detect cardiotoxicity at the preclinical stage.

Arrhythmia phenotype in mouse models of human long QT

Enhanced dispersion of repolarization (DR) was proposed as a unifying mechanism, central to arrhythmia genesis in the long QT (LQT) syndrome. In mammalian hearts, K(+) channels are heterogeneously expressed across the ventricles resulting in 'intrinsic' DR that may worsen in long QT. DR was shown to be central to the arrhythmia phenotype of transgenic mice with LQT caused by loss of function of the dominant mouse K(+) currents. Here, we investigated the arrhythmia phenotype of mice with targeted deletions of KCNE1 and KCNH2 genes which encode for minK/IsK and Merg1 (mouse homolog of human ERG) proteins resulting in loss of function of I(Ks) and I(Kr), respectively. Both currents are important human K(+) currents associated with LQT5 and LQT2. Loss of minK, a protein subunit that interacts with KvLQT1, results in a marked reduction of I(Ks) giving rise to the Jervell and Lange-Nielsen syndrome and the reduced KCNH2 gene reduces MERG and I(Kr). Hearts were perfused, stained with di-4-ANEPPS and optically mapped to compare action potential durations (APDs) and arrhythmia phenotype in homozygous minK (minK(-/-)) and heterozygous Merg1 (Merg(+/-)) deletions and littermate control mice. MinK(-/-) mice has similar APDs and no arrhythmias (n = 4). Merg(+/-) mice had prolonged APDs (from 20 +/- 6 to 32 +/- 9 ms at the base, p < 0.01; from 18 +/- 5 to 25 +/- 9 ms at the apex, p < 0.01; n = 8), longer refractory periods (RP) (36 +/- 14 to 63 +/- 27 at the base, p < 0.01 and 34 +/- 5 to 53 +/- 21 ms at the apex, p < 0.03; n = 8), higher DR 10.4 +/- 4.1 vs. 14 +/- 2.3 ms, p < 0.02) and similar conduction velocities (n = 8). Programmed stimulation exposed a higher propensity to VT in Merg(+/-) mice (60% vs. 10%). A comparison of mouse models of LQT based on K(+) channel mutations important to human and mouse repolarization emphasizes DR as a major determinant of arrhythmia vulnerability.

Tachycardia-induced early afterdepolarizations: insights into potential ionic mechanisms from computer simulations

Although early afterdepolarizations (EADs) are classically thought to occur at slow heart rates, mounting evidence suggests that EADs may also occur at rapid heart rates produced by tachyarrhythmias, due to Ca overload of the sarcoplasmic reticulum (SR) leading to spontaneous SR Ca release. We hypothesized that the mechanism of tachycardia-induced EADs depends on the spatial and temporal morphology of spontaneous SR Ca release, and tested this hypothesis in computer simulations using a ventricular action potential mathematical model. Using two previously suggested spontaneous release morphologies, we found two distinct tachycardia-induced EAD mechanisms: one mechanistically similar to bradycardia-induced EADs, the other to delayed afterdepolarizations (DADs).

Interaction between transcription factors Iroquois proteins 4 and 5 controls cardiac potassium channel Kv4.2 gene transcription

AIMS

The homeobox transcription factor, Iroquois protein 5 (Irx5), plays an essential role in the generation of region-selective expression of Kv4.2 gene across the left ventricular wall of rodent hearts. Here, we analyse molecular mechanisms underlying the Irx5-induced regulation of the rat Kv4.2 promoter.

METHODS AND RESULTS

The mRNA levels for Irx members in various heart regions were assessed by RT-PCR. A luciferase reporter gene with the rat Kv4.2 promoter was used to test the effects of Irx members on channel promoter activity. Irx3 and Irx5 mRNAs were differentially distributed across the left ventricular wall, whereas Irx4 message was equally abundant in various ventricular regions. Irx5, but not Irx3 or Irx4, increased Kv4.2 promoter activity in 10T1/2 fibroblasts, whereas the transcription factor decreased promoter activity in neonatal ventricular myocytes. These effects were mediated by the C-terminal portion of Irx5. Irx4 appeared to inhibit the Irx5-induced increase in channel promoter activity in 10T1/2 cells. The N-terminal region of Irx4 was necessary and sufficient for this inhibition. Furthermore, when endogenous Irx4 expression was suppressed with siRNA, Irx5 increased channel promoter activity in neonatal myocytes.

CONCLUSION

These results indicate that Irx5 possesses the ability to activate the Kv4.2 promoter. The abundant Irx4 expression throughout the rat ventricle may play a role in the inverse relationship between Irx5 and Kv4.2 levels across the left ventricular wall.

T(p-e)/QT ratio as an index of arrhythmogenesis

An increasing number of basic and clinical studies have suggested that the interval from the peak to the end of the electrocardiographic T wave (T(p-e)) may correspond to the transmural dispersion of repolarization and that amplification of the T(p-e) interval is associated with malignant ventricular arrhythmias. In this review, we outline the utility of the T(p-e) interval and the T(p-e)/QT ratio as an electrocardiographic index of arrhythmogenesis for both congenital and acquired ion channel disease leading to ventricular arrhythmias. In healthy individuals, the T(p-e)/QT ratio has a mean value of approximately 0.21 in the precordial leads and it remains relatively constant between the heart rates from 60 to 100 beats per minute. Interestingly, the T(p-e)/QT ratio is significantly greater in the patients at risk for arrhythmic event such as those with long QT syndrome, Brugada syndrome, short QT syndrome, and also in patients with organic heart disease such as acute myocardial infarction. Functional reentry is the underlying mechanism for arrhythmogenesis associated with an increased T(p-e)/QT ratio.

Chemical ablation of the Purkinje system causes early termination and activation rate slowing of long-duration ventricular fibrillation in dogs

Endocardial mapping has suggested that Purkinje fibers may play a role in the maintenance of long-duration ventricular fibrillation (LDVF). To determine the influence of Purkinje fibers on LDVF, we chemically ablated the Purkinje system with Lugol solution and recorded endocardial and transmural activation during LDVF. Dog hearts were isolated and perfused, and the ventricular endocardium was exposed and treated with Lugol solution (n = 6) or normal Tyrode solution as a control (n = 6). The left anterior papillary muscle endocardium was mapped with a 504-electrode (21 x 24) plaque with electrodes spaced 1 mm apart. Transmural activation was recorded with a six-electrode plunge needle on each side of the plaque. Ventricular fibrillation (VF) was induced, and perfusion was halted. LDVF spontaneously terminated sooner in Lugol-ablated hearts than in control hearts (4.9 +/- 1.5 vs. 9.2 +/- 3.2 min, P = 0.01). After termination of VF, both the control and Lugol hearts were typically excitable, but only short episodes of VF could be reinduced. Endocardial activation rates were similar during the first 2 min of LDVF for Lugol-ablated and control hearts but were significantly slower in Lugol hearts by 3 min. In control hearts, the endocardium activated more rapidly than the epicardium after 4 min of LDVF with wave fronts propagating most often from the endocardium to epicardium. No difference in transmural activation rate or wave front direction was observed in Lugol hearts. Ablation of the subendocardium hastens VF spontaneous termination and alters VF activation sequences, suggesting that Purkinje fibers are important in the maintenance of LDVF.

Subepicardial phase 0 block and discontinuous transmural conduction underlie right precordial ST-segment elevation by a SCN5A loss-of-function mutation

Two mechanisms are generally proposed to explain right precordial ST-segment elevation in Brugada syndrome: 1) right ventricular (RV) subepicardial action potential shortening and/or loss of dome causing transmural dispersion of repolarization; and 2) RV conduction delay. Here we report novel mechanistic insights into ST-segment elevation associated with a Na(+) current (I(Na)) loss-of-function mutation from studies in a Dutch kindred with the COOH-terminal SCN5A variant p.Phe2004Leu. The proband, a man, experienced syncope at age 22 yr and had coved-type ST-segment elevations in ECG leads V1 and V2 and negative T waves in V2. Peak and persistent mutant I(Na) were significantly decreased. I(Na) closed-state inactivation was increased, slow inactivation accelerated, and recovery from inactivation delayed. Computer-simulated I(Na)-dependent excitation was decremental from endo- to epicardium at cycle length 1,000 ms, not at cycle length 300 ms. Propagation was discontinuous across the midmyocardial to epicardial transition region, exhibiting a long local delay due to phase 0 block. Beyond this region, axial excitatory current was provided by phase 2 (dome) of the M-cell action potentials and depended on L-type Ca(2+) current ("phase 2 conduction"). These results explain right precordial ST-segment elevation on the basis of RV transmural gradients of membrane potentials during early repolarization caused by discontinuous conduction. The late slow-upstroke action potentials at the subepicardium produce T-wave inversion in the computed ECG waveform, in line with the clinical ECG.

The link between repolarisation alternans and ventricular arrhythmia: does the cellular phenomenon extend to the clinical problem?

T-wave alternans is considered a potentially useful clinical marker for the risk of ventricular arrhythmia in patients with heart disease. Cellular repolarisation alternans is thought to underlie T-wave alternans, and moreover, to cause re-entrant ventricular arrhythmia. This review examines the experimental and clinical evidence linking repolarisation alternans and T-wave alternans with the occurrence of ventricular arrhythmia. Repolarisation alternans, manifest as alternating changes in action potential duration, is observed in isolated ventricular cardiomyocytes and in multicellular preparations. Its underlying causes are discussed particularly with respect to the role of intracellular Ca(2+). The repolarisation alternans observed at the single cell level is compared to the alternating behaviour observed in isolated multicellular preparations including the perfused ventricular wedge and Langendorff perfused heart. The evidence concerning spatial differences in repolarisation alternans is considered, particularly the situation where adjacent regions of myocardium exhibit repolarisation alternans of different phases. This extreme behaviour, known as discordant alternans, is thought to produce marked gradients of repolarisation that can precipitate unidirectional block and re-entrant ventricular arrhythmias. Finally, the difficulties in extrapolating between experimental models of alternans and arrhythmias and the clinical manifestation are discussed. The areas where experimental evidence is weak are highlighted, and areas for future research are outlined.

Mouse models of human arrhythmia syndromes

Sudden cardiac death stemming from ventricular arrhythmogenesis is one of the major causes of mortality in the developed world. Congenital and acquired forms of long QT syndrome (LQTS) are in turn associated with life threatening arrhythmias. Over the past decade our understanding of arrhythmogenic mechanisms in the setting of these diseases has increased greatly due to the creation of a number of animal models. Of these, the genetically amenable mouse has proved to be a particularly powerful tool. This review summarizes the congenital and acquired LQTS and describes the various mouse models that have been created to further probe arrhythmogenic mechanisms.

Inhomogeneity in the response to mechanical stimulation: cardiac muscle function and gene expression

Mechanical stimulation has important consequences for myocardial function. However, this stimulation and the response to it, is not uniform. The right ventricle is thinner walled and operates at lower pressure than the left ventricle. Within the ventricles, differences in the orientation of myocardial fibres exist. These differences produce inhomogeneity in the stress and strain between and across the ventricles. Possibly as a result of these variations in mechanical stimulation, there are well characterised inhomogeneities in gene expression and protein function within the ventricular myocardium, for example in the transient outward K+ current and its associated Kv channels. Perhaps not surprisingly, it is becoming apparent that gradients of expression and function exist for proteins that are intimately involved in the response to mechanical stimulation in the heart, for example in the left ventricle of the rat there is a transmural gradient in mRNA and current density of the mechanosensitive two-pore domain K+ channel TREK-1 (ENDO>EPI). In healthy hearts it is assumed that these gradients are important for normal function and therefore that their disruption in diseased myocardium is involved in the dysfunction that occurs.

Ionic, molecular, and cellular bases of QT-interval prolongation and torsade de pointes

Torsade de pointes (TdP) is a life-threatening arrhythmia that develops as a consequence of a reduction in the repolarization reserve of cardiac cells leading to amplification of electrical heterogeneities in the ventricular myocardium as well as to the development of early after depolarization-induced triggered activity. Electrical heterogeneities within the ventricles are due to differences in the time course of repolarization of the three predominant cell types that make up the ventricular myocardium, giving rise to transmural voltage gradients and a dispersion of repolarization that contributes to the inscription of the electrocardiographic T wave. A number of non-antiarrhythmic drugs and antiarrhythmic agents with class III actions and/or the various mutations and cardiomyopathies associated with the long QT syndrome reduce net repolarizing current and amplify spatial dispersion of repolarization, thus creating the substrate for re-entry. This results in a prolongation of the QT interval, abnormal T waves, and development of TdP. Agents that prolong the QT interval but do not cause an increase in transmural dispersion of repolarization (TDR) do not induce TdP, suggesting that QT prolongation is not the sole or optimal determinant for arrhythmogenesis. This article reviews recent advances in our understanding of these mechanisms, particularly the role of TDR in the genesis of drug-induced TdP, and examines how these may guide us towards development of safer drugs.

Cardiac resynchronization therapy and its potential proarrhythmic effect

Cardiac resynchronization therapy (CRT) has become an established adjunctive treatment to optimal pharmacologic therapy in patients with advanced chronic heart failure (CHF), diminished left ventricular (LV) function and intraventricular conduction delay. Although CRT has been shown to improve ventricular hemodynamics, quality of life and exercise capacity, there is some evidence that it may rarely potentiate ventricular arrhythmias. As CRT is considered for an expanded population of CHF patients, and left-sided pacing is considered as an option for pacemaker-indicated patients (potentially without defibrillator backup), the effect of these pacing modalities on the incidence of ventricular tachyarrhythmia must be systematically studied and mechanistically understood. Strategies to prospectively predict the proarrhythmic potential of LV epicardial pacing need to be developed, and therapy accordingly individualized. This review attempts to summarize the current information on proarrhythmia in resynchronization therapy.

Impact of study design on proarrhythmia prediction in the SCREENIT rabbit isolated heart model

INTRODUCTION

Prediction of the propensity of a compound to induce Torsades de Pointes continues to be a formidable challenge to the pharmaceutical industry. Development of an in vitro model for assessment of proarrhythmic potential offers the advantage of higher throughput and reduced compound quantity requirements when compared to in vivo studies. A rabbit isolated heart model (SCREENIT) has been reported to identify compounds with proarrhythmic potential based on the observance of compound-induced triangulation and instability of the monophasic action potential (MAP), ectopic beats, and reverse-use dependence of prolongation of the MAP duration. Previous reports have indicated that this model qualitatively identifies proarrhythmic compounds and suggest the use of this model to assign safety margins for human clinical use. The intent of this series of studies was to evaluate the impact of study design on the proarrhythmic concentration predicted by this model.

METHODS

Nine compounds of varying proarrhythmic potential and a negative control were tested in a blinded fashion using a series of different experimental protocols: Compounds were tested at multiple concentration ranges and extended perfusion times were also evaluated.

RESULTS

In general when the dataset is viewed as a whole, the model did identify proarrhythmic compounds, however the concentration at which action potential prolongation, triangulation, instability, reverse-use dependence and ectopic beats occurred often varied based on the concentration range selected. Further analysis using extended compound perfusion times demonstrated that variability may be due in part to lack of adequate equilibration of compound with the cardiac tissue.

DISCUSSION

We report that the model correctly identified proarrhythmic agents in a qualitative manner, but that study design impacts the proarrhythmic concentration derived from the model.