Simulation of action potentials from metabolically impaired cardiac myocytes. Role of ATP-sensitive K+ current

In silico study of the mechanisms of hypoxia and contractile dysfunction during ischemia and reperfusion of hiPSC cardiomyocytes

Interconnected mechanisms of ischemia and reperfusion (IR) has increased the interest in IR in vitro experiments using human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs). We developed a whole-cell computational model of hiPSC-CMs including the electromechanics, a metabolite-sensitive sarcoplasmic reticulum Ca2+-ATPase (SERCA) and an oxygen dynamics formulation to investigate IR mechanisms. Moreover, we simulated the effect and action mechanism of levosimendan, which recently showed promising anti-arrhythmic effects in hiPSC-CMs in hypoxia. The model was validated using hiPSC-CM and in vitro animal data. The role of SERCA in causing relaxation dysfunction in IR was anticipated to be comparable to its function in sepsis-induced heart failure. Drug simulations showed that levosimendan counteracts the relaxation dysfunction by utilizing a particular Ca2+-sensitizing mechanism involving Ca2+-bound troponin C and Ca2+ flux to the myofilament, rather than inhibiting SERCA phosphorylation. The model demonstrates extensive characterization and promise for drug development, making it suitable for evaluating IR therapy strategies based on the changing levels of cardiac metabolites, oxygen and molecular pathways.

Myocardial ischemia simulation based on a multi-scale heart electrophysiology model

BACKGROUND

Myocardial ischemia, caused by insufficient myocardial blood supply, is a leading cause of human death worldwide. Therefore, it is crucial to prioritize the prevention and treatment of this condition. Mathematical modeling is a powerful technique for studying heart diseases.

OBJECTIVE

The aim of this study was to discuss the quantitative relationship between extracellular potassium concentration and the degree of myocardial ischemia directly related to it.

METHODS

A human cardiac electrophysiological multiscale model was developed to calculate action potentials of all cells simultaneously, enhancing efficiency over traditional reaction-diffusion models.

RESULTS

Contrary to the commonly held view that myocardial ischemia is caused by an increase in extracellular potassium concentration, our simulation results indicate that level 1 ischemia is associated with a decrease in extracellular potassium concentration.

CONCLUSION

This unusual finding provides a new perspective on the mechanisms underlying myocardial ischemia and has the potential to lead to the development of new diagnostic and treatment strategies.

The mechanisms of potassium loss in acute myocardial ischemia: New insights from computational simulations

Acute myocardial ischemia induces hyperkalemia (accumulation of extracellular potassium), a major perpetrator of lethal reentrant ventricular arrhythmias. Despite considerable experimental efforts to explain this pathology in the last decades, the intimate mechanisms behind hyperkalemia remain partially unknown. In order to investigate these mechanisms, we developed a novel computational model of acute myocardial ischemia which couples a) an electrophysiologically detailed human cardiomyocyte model that incorporates modifications to account for ischemia-induced changes in transmembrane currents, with b) a model of cardiac tissue and extracellular K ⁺ transport. The resulting model is able to reproduce and explain the triphasic time course of extracellular K ⁺ concentration within the ischemic zone, with values of [ K + ] o close to 14 mmol/L in the central ischemic zone after 30 min. In addition, the formation of a [ K + ] o border zone of approximately 1.2 cm 15 min after the onset of ischemia is predicted by the model. Our results indicate that the primary rising phase of [ K + ] o is mainly due to the imbalance between K ⁺ efflux, that increases slightly, and K ⁺ influx, that follows a reduction of the NaK pump activity by more than 50%. The onset of the plateau phase is caused by the appearance of electrical alternans (a novel mechanism identified by the model), which cause an abrupt reduction in the K ⁺ efflux. After the plateau, the secondary rising phase of [ K + ] o is caused by a subsequent imbalance between the K ⁺ influx, which continues to decrease slowly, and the K ⁺ efflux, which remains almost constant. Further, the study shows that the modulation of these mechanisms by the electrotonic coupling is the main responsible for the formation of the ischemic border zone in tissue, with K ⁺ transport playing only a minor role. Finally, the results of the model indicate that the injury current established between the healthy and the altered tissue is not sufficient to depolarize non-ischemic cells within the healthy tissue.

Analysis of vulnerability to reentry in acute myocardial ischemia using a realistic human heart model

Electrophysiological alterations of the myocardium caused by acute ischemia constitute a pro-arrhythmic substrate for the generation of potentially lethal arrhythmias. Experimental evidence has shown that the main components of acute ischemia that induce these electrophysiological alterations are hyperkalemia, hypoxia (or anoxia in complete artery occlusion), and acidosis. However, the influence of each ischemic component on the likelihood of reentry is not completely established. Moreover, the role of the His-Purkinje system (HPS) in the initiation and maintenance of arrhythmias is not completely understood. In the present work, we investigate how the three components of ischemia affect the vulnerable window (VW) for reentry using computational simulations. In addition, we analyze the role of the HPS on arrhythmogenesis. A 3D biventricular/torso human model that includes a realistic geometry of the central and border ischemic zones with one of the most electrophysiologically detailed model of ischemia to date, as well as a realistic cardiac conduction system, were used to assess the VW for reentry. Four scenarios of ischemic severity corresponding to different minutes after coronary artery occlusion were simulated. Our results suggest that ischemic severity plays an important role in the generation of reentries. Indeed, this is the first 3D simulation study to show that ventricular arrhythmias could be generated under moderate ischemic conditions, but not in mild and severe ischemia. Moreover, our results show that anoxia is the ischemic component with the most significant effect on the width of the VW. Thus, a change in the level of anoxia from moderate to severe leads to a greater increment in the VW (40 ms), in comparison with the increment of 20 ms and 35 ms produced by the individual change in the level of hyperkalemia and acidosis, respectively. Finally, the HPS was a necessary element for the generation of approximately 17% of reentries obtained. The retrograde conduction from the myocardium to HPS in the ischemic region, conduction blocks in discrete sections of the HPS, and the degree of ischemia affecting Purkinje cells, are suggested as mechanisms that favor the generation of ventricular arrhythmias.

Investigating two kinds of cellular alternans and corresponding TWA induced by impaired calcium cycling in myocardial ischemia

BACKGROUND

The utility of T wave alternans (TWA) in identifying arrhythmia risk has been demonstrated. During myocardial ischemia (MI), TWA could be induced by cellular alternans. However, the relationship between cellular alternans patterns and TWA patterns in MI has not been investigated thoroughly.

METHODS

We set MI conditions to simulate alternans. Either prolonging Ca²⁺ release or increasing spark-induced sparks (secondary sparks) can give rise to different patterns of APD alternans and TWA. In addition, different ischemic zones and reduced conduction velocity are also considered in one dimensional simulation.

RESULTS

Delay of Ca²⁺ release can produce discordant Ca²⁺-driven alternans in single cell simulation. Increasing secondary sparks leads to concordant alternans. Correspondingly, morphology and magnitude of TWA vary in two different cellular alternans. Epi ischemia results in alternans concentrating in the first half of T wave. Endo and transmural ischemia lead to fluctuations in the second half of T wave. In addition, slowing conduction velocity has no effect on TWA magnitude.

CONCLUSION

Specific ionic channel dysfunction and ischemic zones affect TWA patterns.

Cardiac transmembrane ion channels and action potentials: cellular physiology and arrhythmogenic behavior

Cardiac arrhythmias are among the leading causes of mortality. They often arise from alterations in the electrophysiological properties of cardiac cells and their underlying ionic mechanisms. It is therefore critical to further unravel the pathophysiology of the ionic basis of human cardiac electrophysiology in health and disease. In the first part of this review, current knowledge on the differences in ion channel expression and properties of the ionic processes that determine the morphology and properties of cardiac action potentials and calcium dynamics from cardiomyocytes in different regions of the heart are described. Then the cellular mechanisms promoting arrhythmias in congenital or acquired conditions of ion channel function (electrical remodeling) are discussed. The focus is on human-relevant findings obtained with clinical, experimental, and computational studies, given that interspecies differences make the extrapolation from animal experiments to human clinical settings difficult. Deepening the understanding of the diverse pathophysiology of human cellular electrophysiology will help in developing novel and effective antiarrhythmic strategies for specific subpopulations and disease conditions.

Elucidating the relationship between arrhythmia and ischemic heterogeneity: an in silico study

Dialysis causes blood flow defects in the heart that may augment electrophysiological heterogeneity in the form of increased number of ischemic zones in the human left ventricle. We computationally tested whether a larger number of ischemic zones aggravate arrhythmia using a 2D electrophysiological model of the human ventricle.A human ventricle cardiomyocyte model capable of simulating ischemic action potentials was adapted in this study. The cell model was incorporated into a spatial 2D model consisting of known number of ischemic zones. Inter-cellular gap junction coupling within ischemic zones was reduced to simulate slow conduction. Arrhythmia severity was assessed by inducing a re-entry, and quantifying the ensuing breakup and tissue pacing rates.Ischemia elevated the isolated cardiomyocyte's resting potential and reduced its action potential duration. In the absence of ischemic zones, the propensity in the 2D model to induce multiple re-entrant waves was low. The inclusion of ischemic zones provided the substrate for initiation of re-entrant waves leading to fibrillation. Dominant frequency, which measured the highest rate of pacing in the tissue, increased drastically with the inclusion of multiple ischemic zones. Re-entrant wave tip maximum numbers increased from 1 tip (no ischemic zone) to 34 tips when a large number (20) of ischemic zones were included. Computational limiting factors of our platform were identified using software profiling.Clinical significance. Dialysis may promote deleterious arrhythmias by increasing tissue level action potential dispersion.

Pro-arrhythmic Effects of Hydrogen Sulfide in Healthy and Ischemic Cardiac Tissues: Insight From a Simulation Study

Hydrogen sulfide (H₂S), an ambient air pollutant, has been reported to increase cardiac events in patients with cardiovascular diseases, but the underlying mechanisms remain not elucidated. This study investigated the pro-arrhythmic effects of H₂S in healthy and ischemic conditions. Experimental data of H₂S effects on ionic channels (including the L-type Ca²⁺ channel and ATP-sensitive K⁺ channel) were incorporated into a virtual heart model to evaluate their integral action on cardiac arrhythmogenesis. It was shown that H₂S depressed cellular excitability, abbreviated action potential duration, and augmented tissue’s transmural dispersion of repolarization, resulting in an increase in tissue susceptibility to initiation and maintenance of reentry. The observed effects of H₂S on cardiac excitation are more remarkable in the ischemic condition than in the healthy condition. This study provides mechanistic insights into the pro-arrhythmic effects of air pollution (H₂S), especially in the case with extant ischemic conditions.

Trace Element Determination and Cardioprotection of Terminalia pallida Fruit Ethanolic Extract in Isoproterenol Induced Myocardial Infarcted Rats by ICP-MS

The trace elements and minerals in Terminalia pallida fruit ethanolic extract (TpFE) were determined by the instrument inductively coupled plasma-mass spectrometry (ICP-MS), and the cardioprotection of TpFE against isoproterenol (ISO)-administered rats was studied. Rats were pretreated with TpFE (100, 300, and 500 mg/kg bw) for 30 days, with concurrent administration of ISO (85 mg/kg bw) for two consecutive days. The levels of trace elements and minerals in TpFE were below the permitted limits of World Health Organization standards. ISO administration significantly increased the heart weight and cardiac marker enzymes in serum, xanthine oxidase, sodium, and calcium in the heart, whereas significantly decreased body weight, reduced glutathione, glutathione-S-transferase, superoxide dismutase, and potassium in the heart. Oral pretreatment of TpFE significantly prevented the ISO-induced alterations. This is the first report that revealed the determination of trace elements and mineral nutrients of TpFE by ICP-MS which plays a principal role in the herbal drug discovery for the treatment of cardiovascular diseases.

Mechanism underlying impaired cardiac pacemaking rhythm during ischemia: A simulation study

Ischemia in the heart impairs function of the cardiac pacemaker, the sinoatrial node (SAN). However, the ionic mechanisms underlying the ischemia-induced dysfunction of the SAN remain elusive. In order to investigate the ionic mechanisms by which ischemia causes SAN dysfunction, action potential models of rabbit SAN and atrial cells were modified to incorporate extant experimental data of ischemia-induced changes to membrane ion channels and intracellular ion homeostasis. The cell models were incorporated into an anatomically detailed 2D model of the intact SAN-atrium. Using the multi-scale models, the functional impact of ischemia-induced electrical alterations on cardiac pacemaking action potentials (APs) and their conduction was investigated. The effects of vagal tone activity on the regulation of cardiac pacemaker activity in control and ischemic conditions were also investigated. The simulation results showed that at the cellular level ischemia slowed the SAN pacemaking rate, which was mainly attributable to the altered Na+-Ca2+ exchange current and the ATP-sensitive potassium current. In the 2D SAN-atrium tissue model, ischemia slowed down both the pacemaking rate and the conduction velocity of APs into the surrounding atrial tissue. Simulated vagal nerve activity, including the actions of acetylcholine in the model, amplified the effects of ischemia, leading to possible SAN arrest and/or conduction exit block, which are major features of the sick sinus syndrome. In conclusion, this study provides novel insights into understanding the mechanisms by which ischemia alters SAN function, identifying specific conductances as contributors to bradycardia and conduction block.

T wave axis deviation and QRS-T angle - Controversial indicators of incident coronary heart events

Abnormal orientation of the T-wave axis and increased angle between the QRS complex (depolarization) and the T-wave (repolarization) have long been assumed to provide a global measure of repolarization abnormality, and have been used to assess ventricular repolarization. The ability of the T wave axis deviation and the QRS-T angle to predict incident coronary heart events was examined in several studies. However, conflicting results have led to significant controversy in the literature concerning their purported ability. Potential explanations involve true variation between study populations, non-standardized cut-off values, different baseline cardiovascular risk levels or different patterns of confounding by other concomitant cardiovascular risk factors. In the present article we will attempt to briefly present the rationale and pathophysiology behind these indices, summarize existing knowledge regarding their prognostic significance and their correlation with established cardiovascular disease risk factors. Further prospective studies are necessary to confirm or refute whether T-wave axis deviation, QRS-T angle and ventricular gradient may in the future serve as indicators of incident coronary heart events and mortality, both in populations with higher prevalence of subclinical advanced atherosclerotic heart disease and in apparently healthy subjects.

Electrophysiological properties of computational human ventricular cell action potential models under acute ischemic conditions

Acute myocardial ischemia is one of the main causes of sudden cardiac death. The mechanisms have been investigated primarily in experimental and computational studies using different animal species, but human studies remain scarce. In this study, we assess the ability of four human ventricular action potential models (ten Tusscher and Panfilov, 2006; Grandi et al., 2010; Carro et al., 2011; O'Hara et al., 2011) to simulate key electrophysiological consequences of acute myocardial ischemia in single cell and tissue simulations. We specifically focus on evaluating the effect of extracellular potassium concentration and activation of the ATP-sensitive inward-rectifying potassium current on action potential duration, post-repolarization refractoriness, and conduction velocity, as the most critical factors in determining reentry vulnerability during ischemia. Our results show that the Grandi and O'Hara models required modifications to reproduce expected ischemic changes, specifically modifying the intracellular potassium concentration in the Grandi model and the sodium current in the O'Hara model. With these modifications, the four human ventricular cell AP models analyzed in this study reproduce the electrophysiological alterations in repolarization, refractoriness, and conduction velocity caused by acute myocardial ischemia. However, quantitative differences are observed between the models and overall, the ten Tusscher and modified O'Hara models show closest agreement to experimental data.

Depth Attenuation Degree Based Visualization for Cardiac Ischemic Electrophysiological Feature Exploration

Although heart researches and acquirement of clinical and experimental data are progressively open to public use, cardiac biophysical functions are still not well understood. Due to the complex and fine structures of the heart, cardiac electrophysiological features of interest may be occluded when there is a necessity to demonstrate cardiac electrophysiological behaviors. To investigate cardiac abnormal electrophysiological features under the pathological condition, in this paper, we implement a human cardiac ischemic model and acquire the electrophysiological data of excitation propagation. A visualization framework is then proposed which integrates a novel depth weighted optic attenuation model into the pathological electrophysiological model. The hidden feature of interest in pathological tissue can be revealed from sophisticated overlapping biophysical information. Experiment results verify the effectiveness of the proposed method for intuitively exploring and inspecting cardiac electrophysiological activities, which is fundamental in analyzing and explaining biophysical mechanisms of cardiac functions for doctors and medical staff.

Rabbit-specific computational modelling of ventricular cell electrophysiology: Using populations of models to explore variability in the response to ischemia

Computational modelling, combined with experimental investigations, is a powerful method for investigating complex cardiac electrophysiological behaviour. The use of rabbit-specific models, due to the similarities of cardiac electrophysiology in this species with human, is especially prevalent. In this paper, we first briefly review rabbit-specific computational modelling of ventricular cell electrophysiology, multi-cellular simulations including cellular heterogeneity, and acute ischemia. This mini-review is followed by an original computational investigation of variability in the electrophysiological response of two experimentally-calibrated populations of rabbit-specific ventricular myocyte action potential models to acute ischemia. We performed a systematic exploration of the response of the model populations to varying degrees of ischemia and individual ischemic parameters, to investigate their individual and combined effects on action potential duration and refractoriness. This revealed complex interactions between model population variability and ischemic factors, which combined to enhance variability during ischemia. This represents an important step towards an improved understanding of the role that physiological variability may play in electrophysiological alterations during acute ischemia.

Activation of β1-adrenoceptor triggers oxidative stress mediated myocardial membrane destabilization in isoproterenol induced myocardial infarcted rats: 7-hydroxycoumarin and its counter action

Activation of β1-adrenoceptor stimulates myocardial membrane destabilization in isoproterenol induced rats. Male albino Wistar rats were pre and co-treated with 7-hydroxycoumarin (16mg/kg body weight) daily for 8 days. Myocardial infarction was induced into rats by the subcutaneous administration of isoproterenol (100mg/kg body weight) at an interval of 24h daily for a period of two days (7th and 8th day). The levels/activities of serum cardiac troponin-T, lactate dehydrogenase and the concentrations of heart lipid peroxidation products were significantly increased and the antioxidant status was significantly decreased in isoproterenol induced rats. Furthermore, the activity of sodium/potassium-dependent adenosine triphosphatase was significantly decreased and the activities of calcium and magnesium-dependent adenosine triphosphatases were significantly increased in the heart of isoproterenol induced myocardial infarcted rats. Isoproterenol induced rats also revealed increased concentrations of sodium and calcium and decreased concentrations of potassium in the heart. 7-hydroxycoumarin pre- and co-treatment showed considerable impact on all biochemical parameters assessed. Also, 7-HC greatly reduced the infarct size of the myocardium. The in vitro study confirmed its potent free radical scavenging activity. Thus, the present study revealed that 7-HC attenuates myocardial membrane destabilization by reinstating the activities/levels of adenosine triphosphatases and minerals in isoproterenol induced rats by inhibiting oxidative stress. These effects are attributed to the membrane stabilizing and free radical scavenging properties of 7-hydroxycoumarin.

EFFECTS OF ACUTE GLOBAL ISCHEMIA ON RE-ENTRANT ARRHYTHMOGENESIS: A SIMULATION STUDY

Sudden cardiac death is mainly caused by arrhythmogenesis. For a functional abnormal heart, such as an ischemic heart, the probability of arrhythmia occurring is greatly increased. During myocardial ischemia, re-entry is prone to degenerate into ventricular fibrillation (VF). Therefore it has important meaning to investigate the intricate mechanisms underlying VF under an ischemic condition in order to better facilitate therapeutic interventions. In this paper, to analyze the functional influence of acute global ischemia on cardiac electrical activity and subsequently on re-entrant arrhythmogenesis, we take into account three main pathophysiological consequences of ischemia: hyperkalaemia, acidosis, and anoxia, and develop a 3D human ventricular ischemic model that combines a detailed biophysical description of the excitation kinetics of human ventricular cells with an integrated geometry of human ventricular tissue which incorporates fiber direction anisotropy and the stimulation activation sequence. The results show that under acute global ischemia, the tissue excitability and the slope of ventricular cellular action potential duration restitution (APDR) are greatly decreased. As a result, the complexity of VF activation patterns is reduced. For the three components of ischemia, hyperkalaemia is the dominant contributor to the stability of re-entry under acute global ischemia. Increasing [K+]o acts to prolong the cell refractory period, reduce the tissue excitability and slow the conduction velocity. Our results also show that VF can be eliminated by decreasing cellular excitability, primarily by elevating the concentration value of extracellular K+.

Three-dimensional cardiac computational modelling: methods, features and applications

The combination of computational models and biophysical simulations can help to interpret an array of experimental data and contribute to the understanding, diagnosis and treatment of complex diseases such as cardiac arrhythmias. For this reason, three-dimensional (3D) cardiac computational modelling is currently a rising field of research. The advance of medical imaging technology over the last decades has allowed the evolution from generic to patient-specific 3D cardiac models that faithfully represent the anatomy and different cardiac features of a given alive subject. Here we analyse sixty representative 3D cardiac computational models developed and published during the last fifty years, describing their information sources, features, development methods and online availability. This paper also reviews the necessary components to build a 3D computational model of the heart aimed at biophysical simulation, paying especial attention to cardiac electrophysiology (EP), and the existing approaches to incorporate those components. We assess the challenges associated to the different steps of the building process, from the processing of raw clinical or biological data to the final application, including image segmentation, inclusion of substructures and meshing among others. We briefly outline the personalisation approaches that are currently available in 3D cardiac computational modelling. Finally, we present examples of several specific applications, mainly related to cardiac EP simulation and model-based image analysis, showing the potential usefulness of 3D cardiac computational modelling into clinical environments as a tool to aid in the prevention, diagnosis and treatment of cardiac diseases.

Catecholamine toxicity triggers myocardial membrane destabilization in rats: thymol and its counter action

The present manuscript deals with the protective action of thymol against isoproterenol induced cardiotoxicity by reinstating ATPases and minerals as evidenced by decreased myocardial infarct size.

Multiscale computational analysis of the bioelectric consequences of myocardial ischaemia and infarction

Ischaemic heart disease is considered as the single most frequent cause of death, provoking more than 7 000 000 deaths every year worldwide. A high percentage of patients experience sudden cardiac death, caused in most cases by tachyarrhythmic mechanisms associated to myocardial ischaemia and infarction. These diseases are difficult to study using solely experimental means due to their complex dynamics and unstable nature. In the past decades, integrative computational simulation techniques have become a powerful tool to complement experimental and clinical research when trying to elucidate the intimate mechanisms of ischaemic electrophysiological processes and to aid the clinician in the improvement and optimization of therapeutic procedures. The purpose of this paper is to briefly review some of the multiscale computational models of myocardial ischaemia and infarction developed in the past 20 years, ranging from the cellular level to whole-heart simulations.

In silico ischaemia-induced reentry at the Purkinje-ventricle interface

AIMS

This computational modelling work illustrates the influence of hyperkalaemia and electrical uncoupling induced by defined ischaemia on action potential (AP) propagation and the incidence of reentry at the Purkinje-ventricle interface in mammalian hearts.

METHODS AND RESULTS

Unidimensional and bidimensional models of the Purkinje-ventricle subsystem, including ischaemic conditions (defined as phase 1B) in the ventricle and an ischaemic border zone, were developed by altering several important electrophysiological parameters of the Luo-Rudy AP model of the ventricular myocyte. Purkinje electrical activity was modelled using the equations of DiFrancesco and Noble. Our study suggests that an extracellular potassium concentration [K(+)]o >14 mM and a slight decrease in intercellular coupling induced by ischaemia in ventricle can cause conduction block from Purkinje to ventricle. Under these conditions, propagation from ventricle to Purkinje is possible. Thus, unidirectional block (UDB) and reentry can result. When conditions of UDB are met, retrograde propagation with a long delay (320 ms) may re-excite Purkinje cells, and give rise to a reentrant pathway. This induced reentry may be the origin of arrhythmias observed in phase 1B ischaemia.

CONCLUSION

In a defined setting of ischaemia (phase 1B), a small amount of uncoupling between ventricular cells, as well as between Purkinje and ventricular tissue, may induce UDBs and reentry. Hyperkalaemia is also confirmed to be an important factor in the genesis of reentrant rhythms, since it regulates the range of coupling in which UDBs may be induced.

Transmural IK(ATP) heterogeneity as a determinant of activation rate gradient during early ventricular fibrillation: mechanistic insights from rabbit ventricular models

BACKGROUND

Activation rate (AR) gradients develop during ventricular fibrillation (VF), with the highest AR on the surface near Purkinje system (PS) terminals (endocardium in humans and rabbits and epicardium in pigs). The application of glibenclamide to block adenosine triphosphate (ATP)-sensitive potassium current (IK(ATP)) before VF induction eliminates transmural AR gradients and prevents the induction of sustained arrhythmia. It remains unclear whether the PS, which is resistant to ischemia, is also a factor in AR heterogeneity.

OBJECTIVE

To dissect IK(ATP) and PS contributions to AR gradients during VF by using detailed computer simulations.

METHODS

We constructed rabbit ventricular models with either subendocardial or subepicardial PS terminals. Physiologically relevant IK(ATP) gradients were implemented, and early VF was induced and observed.

RESULTS

Prominent AR gradients were observed only in models with large IK(ATP) gradients. The critical underlying factor of AR gradient maintenance was refractoriness in low-IK(ATP) regions, which blocked the propagation of action potentials from high-IK(ATP) regions. The PS played no role in transmural AR gradient maintenance, but did cause local spatial heterogeneity of AR on the surface adjacent to terminals. Simulated glibenclamide application during VF led to spontaneous arrhythmia termination within a few seconds in most cases, which builds on previous experimental findings of anti-VF properties of glibenclamide pretreatment.

CONCLUSION

Differential IK(ATP) across the ventricular wall is an important factor underlying AR gradients during VF; thus, higher epicardial AR in pigs is most likely due to an abundance of epicardial IK(ATP). For terminating early VF, our results suggest that IK(ATP) modulation is a stronger target than Purkinje ablation.

Pro- and antiarrhythmic effects of ATP-sensitive potassium current activation on reentry during early afterdepolarization-mediated arrhythmias

BACKGROUND

Under conditions promoting early afterdepolarizations (EADs), ventricular tissue can become bi-excitable, that is, capable of wave propagation mediated by either the Na current (INa) or the L-type calcium current (ICa,L), raising the possibility that ICa,L-mediated reentry may contribute to polymorphic ventricular tachycardia (PVT) and torsades de pointes. ATP-sensitive K current (IKATP) activation suppresses EADs, but the effects on ICa,L-mediated reentry are unknown.

OBJECTIVE

To investigate the effects of IKATP activation on ICa,L-mediated reentry.

METHODS

We performed optical voltage mapping in cultured neonatal rat ventricular myocyte monolayers exposed to BayK8644 and isoproterenol. The effects of pharmacologically activating IKATP with pinacidil were analyzed.

RESULTS

In 13 monolayers with anatomic ICa,L-mediated reentry around a central obstacle, pinacidil (50 μM) converted ICa,L-mediated reentry to INa-mediated reentry. In 33 monolayers with functional ICa,L-mediated reentry (spiral waves), pinacidil terminated reentry in 17, converted reentry into more complex INa-mediated reentry resembling fibrillation in 12, and had no effect in 4. In simulated 2-dimensional bi-excitable tissue in which ICa,L- and INa-mediated wave fronts coexisted, slow IKATP activation (over minutes) reliably terminated rotors but rapid IKATP activation (over seconds) often converted ICa,L-mediated reentry to INa-mediated reentry resembling fibrillation.

CONCLUSIONS

IKATP activation can have proarrhythmic effects on EAD-mediated arrhythmias if ICa,L-mediated reentry is present.

Alterations in ventricular K(ATP) channel properties during aging

Coronary heart disease remains the principle cause of mortality in the United States. During aging, the efficiency of the cardiovascular system is decreased and the aged heart is less tolerant to ischemic injury. ATP-sensitive K(+) (K(ATP)) channels protect the myocardium against ischemic damage. We investigated how aging affects cardiac K(ATP) channels in the Fischer 344 rat model. Expression of K(ATP) channel subunit mRNA and protein levels was unchanged in hearts from 26-month-old vs. 4-month-old rats. Interestingly, the mRNA expression of several other ion channels (> 80) was also largely unchanged, suggesting that posttranscriptional regulatory mechanisms occur during aging. The whole-cell K(ATP) channel current density was strongly diminished in ventricular myocytes from aged male rat hearts (also observed in aged C57BL/6 mouse myocytes). Experiments with isolated patches (inside-out configuration) demonstrated that the K(ATP) channel unitary conductance was unchanged, but that the inhibitory effect of cytosolic ATP on channel activity was enhanced in the aged heart. The mean patch current was diminished, consistent with the whole-cell data. We incorporated these findings into an empirical model of the K(ATP) channel and numerically simulated the effects of decreased cytosolic ATP levels on the human action potential. This analysis predicts lesser activation of K(ATP) channels by metabolic impairment in the aged heart and a diminished action potential shortening. This study provides insights into the changes in K(ATP) channels during aging and suggests that the protective role of these channels during ischemia is significantly compromised in the aged individual.

Regulation of cardiac ATP-sensitive potassium channel surface expression by calcium/calmodulin-dependent protein kinase II

Cardiac ATP-sensitive potassium (K(ATP)) channels are key sensors and effectors of the metabolic status of cardiomyocytes. Alteration in their expression impacts their effectiveness in maintaining cellular energy homeostasis and resistance to injury. We sought to determine how activation of calcium/calmodulin-dependent protein kinase II (CaMKII), a central regulator of calcium signaling, translates into reduced membrane expression and current capacity of cardiac K(ATP) channels. We used real-time monitoring of K(ATP) channel current density, immunohistochemistry, and biotinylation studies in isolated hearts and cardiomyocytes from wild-type and transgenic mice as well as HEK cells expressing wild-type and mutant K(ATP) channel subunits to track the dynamics of K(ATP) channel surface expression. Results showed that activation of CaMKII triggered dynamin-dependent internalization of K(ATP) channels. This process required phosphorylation of threonine at 180 and 224 and an intact (330)YSKF(333) endocytosis motif of the K(ATP) channel Kir6.2 pore-forming subunit. A molecular model of the μ2 subunit of the endocytosis adaptor protein, AP2, complexed with Kir6.2 predicted that μ2 docks by interaction with (330)YSKF(333) and Thr-180 on one and Thr-224 on the adjacent Kir6.2 subunit. Phosphorylation of Thr-180 and Thr-224 would favor interactions with the corresponding arginine- and lysine-rich loops on μ2. We concluded that calcium-dependent activation of CaMKII results in phosphorylation of Kir6.2, which promotes endocytosis of cardiac K(ATP) channel subunits. This mechanism couples the surface expression of cardiac K(ATP) channels with calcium signaling and reveals new targets to improve cardiac energy efficiency and stress resistance.

The ischemic heart: what causes ectopic beating?

The mechanisms by which spontaneous electrical activity originates in the ischemic heart and leads to arrhythmia remain unknown, however mechanical stretch of the diseased region has been hypothesized to play a role. The goal of this study is to investigate the conditions that favor the initiation of stretch-induced premature beats in the ischemic heart. We employ a mathematical model of the ischemic cell subjected to stretch. The study found that upon stretch, spontaneous beats occur in the ischemic cell, which are due to the stretch-induced re-activation of the L-type calcium current.

Computational approaches to understand cardiac electrophysiology and arrhythmias

Cardiac rhythms arise from electrical activity generated by precisely timed opening and closing of ion channels in individual cardiac myocytes. These impulses spread throughout the cardiac muscle to manifest as electrical waves in the whole heart. Regularity of electrical waves is critically important since they signal the heart muscle to contract, driving the primary function of the heart to act as a pump and deliver blood to the brain and vital organs. When electrical activity goes awry during a cardiac arrhythmia, the pump does not function, the brain does not receive oxygenated blood, and death ensues. For more than 50 years, mathematically based models of cardiac electrical activity have been used to improve understanding of basic mechanisms of normal and abnormal cardiac electrical function. Computer-based modeling approaches to understand cardiac activity are uniquely helpful because they allow for distillation of complex emergent behaviors into the key contributing components underlying them. Here we review the latest advances and novel concepts in the field as they relate to understanding the complex interplay between electrical, mechanical, structural, and genetic mechanisms during arrhythmia development at the level of ion channels, cells, and tissues. We also discuss the latest computational approaches to guiding arrhythmia therapy.

Toward an integrative computational model of the Guinea pig cardiac myocyte

The local control theory of excitation-contraction (EC) coupling asserts that regulation of calcium (Ca²⁺) release occurs at the nanodomain level, where openings of single L-type Ca²⁺ channels (LCCs) trigger openings of small clusters of ryanodine receptors (RyRs) co-localized within the dyad. A consequence of local control is that the whole-cell Ca²⁺ transient is a smooth continuous function of influx of Ca²⁺ through LCCs. While this so-called graded release property has been known for some time, its functional importance to the integrated behavior of the cardiac ventricular myocyte has not been fully appreciated. We previously formulated a biophysically based model, in which LCCs and RyRs interact via a coarse-grained representation of the dyadic space. The model captures key features of local control using a low-dimensional system of ordinary differential equations. Voltage-dependent gain and graded Ca²⁺ release are emergent properties of this model by virtue of the fact that model formulation is closely based on the sub-cellular basis of local control. In this current work, we have incorporated this graded release model into a prior model of guinea pig ventricular myocyte electrophysiology, metabolism, and isometric force production. The resulting integrative model predicts the experimentally observed causal relationship between action potential (AP) shape and timing of Ca²⁺ and force transients, a relationship that is not explained by models lacking the graded release property. Model results suggest that even relatively subtle changes in AP morphology that may result, for example, from remodeling of membrane transporter expression in disease or spatial variation in cell properties, may have major impact on the temporal waveform of Ca²⁺ transients, thus influencing tissue level electromechanical function.

Influence of ischemic core muscle fibers on surface depolarization potentials in superfused cardiac tissue preparations: a simulation study

Thin-walled cardiac tissue samples superfused with oxygenated solutions are widely used in experimental studies. However, due to decreased oxygen supply and insufficient wash out of waste products in the inner layers of such preparations, electrophysiological functions could be compromised. Although the cascade of events triggered by cutting off perfusion is well known, it remains unclear as to which degree electrophysiological function in viable surface layers is affected by pathological processes occurring in adjacent tissue. Using a 3D numerical bidomain model, we aim to quantify the impact of superfusion-induced heterogeneities occurring in the depth of the tissue on impulse propagation in superficial layers. Simulations demonstrated that both the pattern of activation as well as the distribution of extracellular potentials close to the surface remain essentially unchanged. This was true also for the electrophysiological properties of cells in the surface layer, where most relevant depolarization parameters varied by less than 5.5 %. The main observed effect on the surface was related to action potential duration that shortened noticeably by 53 % as hypoxia deteriorated. Despite the known limitations of such experimental methods, we conclude that superfusion is adequate for studying impulse propagation and depolarization whereas repolarization studies should consider the influence of pathological processes taking place at the core of tissue sample.

Measuring and evaluating the role of ATP-sensitive K+ channels in cardiac muscle

Since ion channels move electrical charge during their activity, they have traditionally been studied using electrophysiological approaches. This was sometimes combined with mathematical models, for example with the description of the ionic mechanisms underlying the initiation and propagation of action potentials in the squid giant axon by Hodgkin and Huxley. The methods for studying ion channels also have strong roots in protein chemistry (limited proteolysis, the use of antibodies, etc.). The advent of the molecular cloning and the identification of genes coding for specific ion channel subunits in the late 1980s introduced a multitude of new techniques with which to study ion channels and the field has been rapidly expanding ever since (e.g. antibody development against specific peptide sequences, mutagenesis, the use of gene targeting in animal models, determination of their protein structures) and new methods are still in development. This review focuses on techniques commonly employed to examine ion channel function in an electrophysiological laboratory. The focus is on the K(ATP) channel, but many of the techniques described are also used to study other ion channels.

COMPUTER MODELS OF DEPOLARIZATION ALTERATIONS INDUCED BY MYOCARDIAL ISCHEMIA: THE EFFECT OF SUPERIMPOSED ISCHEMIC INHOMOGENEITIES ON PROPAGATION IN SPACE AND TIME-FREQUENCY DOMAINS

A two-dimensional modified Luo-Rudy model was created to represent a 40 mm by 40 mm slab of myocardial tissue. An inhomogeneity was introduced to simulate acute myocardial ischemia, with components of hyperkalemia, acidosis and anoxia. Simulations were carried out for various degrees of ischemia, to study both the interaction of the propagation front with the inhomogeneity, and the reconstructed signals. The simulations utilized a modified LR model, with a realistic anisotropy of myocardial tissue. Each cluster (.4 mm ×.4 mm) was given bulk electric properties, Rx and Ry (25Ω and 250Ω, respectively). The slab was stimulated and the 2D depolarization pattern was computed by numerical integration. To study ischemia, a circular inhomogeneity with concentric regions (ro=12.8 mm{border zone, BZ} , ri=11.2 mm{extreme zone, EZ} ) regions was introduced in the model. From the 2D simulations and the regional action potentials (AP), unipolar and bipolar lead potentials were reconstructed. Time-frequency decomposition was performed on the lead signals by wavelet analysis. Isochrone and (dV/dt) max maps were obtained to study depolarization. Our results indicate that spatial inhomogeneities yield dramatic spatial dispersion of the wavefront and are the origin of mid-frequency intra-QRS components in cardiac signals. Severe APD shortening and spatial distortion of the isochrone and upstroke maps are also observed.

Reduction in number of sarcolemmal KATP channels slows cardiac action potential duration shortening under hypoxia

The cardiovascular system operates under demands ranging from conditions of rest to extreme stress. One mechanism of cardiac stress tolerance is action potential duration shortening driven by ATP-sensitive potassium (K(ATP)) channels. K(ATP) channel expression has a significant physiologic impact on action potential duration shortening and myocardial energy consumption in response to physiologic heart rate acceleration. However, the effect of reduced channel expression on action potential duration shortening in response to severe metabolic stress is yet to be established. Here, transgenic mice with myocardium-specific expression of a dominant negative K(ATP) channel subunit were compared with littermate controls. Evaluation of K(ATP) channel whole cell current and channel number/patch was assessed by patch clamp in isolated ventricular cardiomyocytes. Monophasic action potentials were monitored in retrogradely perfused, isolated hearts during the transition to hypoxic perfusate. An 80-85% reduction in cardiac K(ATP) channel current density results in a similar magnitude, but significantly slower rate, of shortening of the ventricular action potential duration in response to severe hypoxia, despite no significant difference in coronary flow. Therefore, the number of functional cardiac sarcolemmal K(ATP) channels is a critical determinant of the rate of adaptation of myocardial membrane excitability, with implications for optimization of cardiac energy consumption and consequent cardioprotection under conditions of severe metabolic stress.

Unique properties of the ATP-sensitive K⁺ channel in the mouse ventricular cardiac conduction system

Background- The specialized cardiac conduction system (CCS) expresses a unique complement of ion channels that confer a specific electrophysiological profile. ATP-sensitive potassium (K(ATP)) channels in these myocytes have not been systemically investigated. Methods and Results- We recorded K(ATP) channels in isolated CCS myocytes using Cntn2-EGFP reporter mice. The CCS K(ATP) channels were less sensitive to inhibitory cytosolic ATP compared with ventricular channels and more strongly activated by MgADP. They also had a smaller slope conductance. The 2 types of channels had similar intraburst open and closed times, but the CCS K(ATP) channel had a prolonged interburst closed time. CCS K(ATP) channels were strongly activated by diazoxide and less by levcromakalim, whereas the ventricular K(ATP) channel had a reverse pharmacological profile. CCS myocytes express elevated levels of Kir6.1 but reduced Kir6.2 and SUR2A mRNA compared with ventricular myocytes (SUR1 expression was negligible). SUR2B mRNA expression was higher in CCS myocytes relative to SUR2A. Canine Purkinje fibers expressed higher levels of Kir6.1 and SUR2B protein relative to the ventricle. Numeric simulation predicts a high sensitivity of the Purkinje action potential to changes in ATP:ADP ratio. Cardiac conduction time was prolonged by low-flow ischemia in isolated, perfused mouse hearts, which was prevented by glibenclamide. Conclusions- These data imply a differential electrophysiological response (and possible contribution to arrhythmias) of the ventricular CCS to K(ATP) channel opening during periods of ischemia.

Protective effect of Lagenaria siceraria (Mol) against membrane-bound enzyme alterations in isoproterenol-induced cardiac damage in rats

This study was aimed at evaluating the preventive role of the ethanolic extract of Lagenaria siceraria (Mol) fruit on membrane-bound enzymes, such as sodium potassium-dependent adenosine triphosphatase (Na(+)/K(+) ATPase), calcium-dependent adenosine triphosphatase (Ca(2+) ATPase) and magnesium-dependent adenosine triphosphatase (Mg(2+) ATPase) on isoproterenol (ISO)-induced myocardial infarction (MI) in rats. Male albino Wistar rats were pretreated with the ethanolic extract of L. siceraria (Mol) fruit (125, 250 and 500 mg kg(-1) body weight) for a period of 30 days. After the treatment period, ISO (85mg kg(-1) body weight) was subcutaneously injected into rats at 24-h intervals for 2 days. ISO-induced rats showed a significant (p < 0.05) decrease in the activity of Na(+)/K(+) ATPase and an increase in the activities of Ca(2+) and Mg(2+) ATPases in the heart tissues. Pre-treatment with the ethanolic extract of L. siceraria (Mol) fruit for a period of 30 days exhibited a significant (p < 0.05) effect in ISO-induced rats. Thus, our study shows that the ethanolic extract of L. siceraria (Mol) fruit has membrane-stabilising role in ISO-induced MI in rats.

Experiment-model interaction for analysis of epicardial activation during human ventricular fibrillation with global myocardial ischaemia

We describe a combined experiment-modelling framework to investigate the effects of ischaemia on the organisation of ventricular fibrillation in the human heart. In a series of experimental studies epicardial activity was recorded from 10 patients undergoing routine cardiac surgery. Ventricular fibrillation was induced by burst pacing, and recording continued during 2.5 min of global cardiac ischaemia followed by 30 s of coronary reflow. Modelling used a 2D description of human ventricular tissue. Global cardiac ischaemia was simulated by (i) decreased intracellular ATP concentration and subsequent activation of an ATP sensitive K⁺ current, (ii) elevated extracellular K⁺ concentration, and (iii) acidosis resulting in reduced magnitude of the L-type Ca²⁺ current I(Ca,L). Simulated ischaemia acted to shorten action potential duration, reduce conduction velocity, increase effective refractory period, and flatten restitution. In the model, these effects resulted in slower re-entrant activity that was qualitatively consistent with our observations in the human heart. However, the flattening of restitution also resulted in the collapse of many re-entrant waves to several stable re-entrant waves, which was different to the overall trend we observed in the experimental data. These findings highlight a potential role for other factors, such as structural or functional heterogeneity in sustaining wavebreak during human ventricular fibrillation with global myocardial ischaemia.

Exercise-induced expression of cardiac ATP-sensitive potassium channels promotes action potential shortening and energy conservation

Physical activity is one of the most important determinants of cardiac function. The ability of the heart to increase delivery of oxygen and metabolic fuels relies on an array of adaptive responses necessary to match bodily demand while avoiding exhaustion of cardiac resources. The ATP-sensitive potassium (K(ATP)) channel has the unique ability to adjust cardiac membrane excitability in accordance with ATP and ADP levels, and up-regulation of its expression that occurs in response to exercise could represent a critical element of this adaption. However, the mechanism by which K(ATP) channel expression changes result in a beneficial effect on cardiac excitability and function remains to be established. Here, we demonstrate that an exercise-induced rise in K(ATP) channel expression enhanced the rate and magnitude of action potential shortening in response to heart rate acceleration. This adaptation in membrane excitability promoted significant reduction in cardiac energy consumption under escalating workloads. Genetic disruption of normal K(ATP) channel pore function abolished the exercise-related changes in action potential duration adjustment and caused increased cardiac energy consumption. Thus, an expression-driven enhancement in the K(ATP) channel-dependent membrane response to alterations in cardiac workload represents a previously unrecognized mechanism for adaptation to physical activity and a potential target for cardioprotection.

Automaticity in acute ischemia: bifurcation analysis of a human ventricular model

Acute ischemia (restriction in blood supply to part of the heart as a result of myocardial infarction) induces major changes in the electrophysiological properties of the ventricular tissue. Extracellular potassium concentration ([K(o)(+)]) increases in the ischemic zone, leading to an elevation of the resting membrane potential that creates an "injury current" (I(S)) between the infarcted and the healthy zone. In addition, the lack of oxygen impairs the metabolic activity of the myocytes and decreases ATP production, thereby affecting ATP-sensitive potassium channels (I(Katp)). Frequent complications of myocardial infarction are tachycardia, fibrillation, and sudden cardiac death, but the mechanisms underlying their initiation are still debated. One hypothesis is that these arrhythmias may be triggered by abnormal automaticity. We investigated the effect of ischemia on myocyte automaticity by performing a comprehensive bifurcation analysis (fixed points, cycles, and their stability) of a human ventricular myocyte model [K. H. W. J. ten Tusscher and A. V. Panfilov, Am. J. Physiol. Heart Circ. Physiol. 291, H1088 (2006)] as a function of three ischemia-relevant parameters [K(o)(+)], I(S), and I(Katp). In this single-cell model, we found that automatic activity was possible only in the presence of an injury current. Changes in [K(o)(+)] and I(Katp) significantly altered the bifurcation structure of I(S), including the occurrence of early-after depolarization. The results provide a sound basis for studying higher-dimensional tissue structures representing an ischemic heart.

Protective effect of fish oil on changes in the activities of membrane-bound ATPases and mineral status in experimentally induced myocardial infarction in Wistar rats

The present study evaluated the protective effect of fish oil in isoproterenol-induced myocardial infarction in rats. The results of the present study indicate that the IPH administration decreases the activities of membrane-bound ATPases compared to control animals. Fish oil pretreatment brought about significant increase in the activity of these membrane-bound ATPases in IPH (isoproterenol hydrochloride)-treated animals. Significant increase in serum potassium level with concomitant decrease in the values of sodium, magnesium, and calcium were observed in IPH-treated rats compared to control rats, fish oil pretreatment reversed these changes to near normal. Significant elevation of sodium and calcium levels with concomitant decrease in the levels of potassium and magnesium were observed in the myocardial tissue of IPH-administered rats compared to control rats, fish oil pretreatment followed by IPH administration brought these levels to near normal. The levels of lipid peroxidation (LPO) in both serum and tissue were increased in IPH-treated rats compared with control rats, whereas pretreatment with fish oil in IPH-treated rats maintained near-normal LPO levels. The results of the present study reveals that the pretreatment of fish maintains the activities of membrane-bound ATPases and the mineral levels at near normal by the inhibition of lipid peroxidation.

Exploring the role of pH in modulating the effects of lidocaine in virtual ischemic tissue

Lidocaine is a class I antiarrhytmic drug that blocks Na(+) channels and exists in both neutral and charged forms at a physiological pH. In this work, a mathematical model of pH and the frequency-modulated effects of lidocaine has been developed and incorporated into the Luo-Rudy model of the ventricular action potential. We studied the effects of lidocaine on Na(+) current, maximum upstroke velocity, and conduction velocity and demonstrated that a decrease of these parameters was dependent on pH, frequency, and concentration. We also tested the action of lidocaine under pathological conditions. Specifically, we investigated its effects on conduction block under acute regional ischemia. Our results in one-dimensional fiber simulations showed a reduction of the window of block in the presence of lidocaine, thereby highlighting the role of reduced conduction velocity and safe conduction. This reduction may be related to the antifibrillatory effects of the drug by hampering wavefront fragmentation. In bidimensional acute ischemic tissue, lidocaine increased the vulnerable window for reentry and exerted proarrhythmic effects. In conclusion, the present simulation study used a newly formulated model of lidocaine, which considers pH and frequency modulation, and revealed the mechanisms by which lidocaine facilitates the onset of reentries. The results of this study also help to increase our understanding of the potential antifibrillatory effects of the drug.

(—)-Epigallocatechin gallate (EGCG) prevents isoprenaline-induced cardiac toxicity by stabilizing cardiac marker enzymes and membrane-bound ATPases

Intake of tea flavonoids has been reported to reduce the incidence of cardiovascular disease. The present study was undertaken to investigate the preventive effect of (—)epigallocatechin gallate (EGCG) on heart weight, cardiac marker enzymes, membrane-bound ATPases and electrolytes in isoprenaline (ISO)-induced myocardial infarcted (MI) Wistar rats. Rats subcutaneously administered ISO 100 mgkg−1 at intervals of 24 h for 2 days resulted in significant increases in heart weight and the activities of cardiac marker enzymes such as creatine kinase, creatine kinase-MB, lactate dehydrogenase (LDH), aspartate transaminase and alanine transaminase in serum, and significant decreases in the activities of these enzymes in the myocardium. ISO injection also increased levels of LDH isoenzymes (LDH 1 and LDH 2). The activity of Na+/K+ ATPase was decreased significantly and the activities of Ca2+ and Mg2+ ATPases were increased significantly in ISO-induced MI rats. Furthermore, the levels of potassium were lowered and the levels of sodium and calcium were increased in ISO-induced MI rats. Prior treatment with EGCG (10, 20 and 30 mgkg−1) daily for a period of 21 days reduced the effects of ISO on heart weight, activities of cardiac marker enzymes and membrane bound-ATPases and levels of LDH 1 and LDH 2 and electrolytes. Thus, EGCG exhibits beneficial effects on these enzymes and electrolytes. The observed effects may be due to the antioxidant and membrane-stabilizing effects of EGCG in ISO-induced MI rats.

Modeling cardiac action potential shortening driven by oxidative stress-induced mitochondrial oscillations in guinea pig cardiomyocytes

Ischemia-induced shortening of the cardiac action potential and its heterogeneous recovery upon reperfusion are thought to set the stage for reentrant arrhythmias and sudden cardiac death. We have recently reported that the collapse of mitochondrial membrane potential (DeltaPsi(m)) through a mechanism triggered by reactive oxygen species (ROS), coupled to the opening of sarcolemmal ATP-sensitive potassium (K(ATP)) channels, contributes to electrical dysfunction during ischemia-reperfusion. Here we present a computational model of excitation-contraction coupling linked to mitochondrial bioenergetics that incorporates mitochondrial ROS-induced ROS release with coupling between the mitochondrial energy state and electrical excitability mediated by the sarcolemmal K(ATP) current (I(K,ATP)). Whole-cell model simulations demonstrate that increasing the fraction of oxygen diverted from the respiratory chain to ROS production triggers limit-cycle oscillations of DeltaPsi(m), redox potential, and mitochondrial respiration through the activation of a ROS-sensitive inner membrane anion channel. The periods of transient mitochondrial uncoupling decrease the cytosolic ATP/ADP ratio and activate I(K,ATP), consequently shortening the cellular action potential duration and ultimately suppressing electrical excitability. The model simulates emergent behavior observed in cardiomyocytes subjected to metabolic stress and provides a new tool for examining how alterations in mitochondrial oxidative phosphorylation will impact the electrophysiological, contractile, and Ca(2+) handling properties of the cardiac cell. Moreover, the model is an important step toward building multiscale models that will permit investigation of the role of spatiotemporal heterogeneity of mitochondrial metabolism in the mechanisms of arrhythmogenesis and contractile dysfunction in cardiac muscle.

The relative role of refractoriness and source-sink relationship in reentry generation during simulated acute ischemia

During acute myocardial ischemia, reentrant episodes may lead to ventricular fibrillation (VF), giving rise to potentially mortal arrhythmias. VF has been traditionally related to dispersion of refractoriness and more recently to the source-sink relationship. Our goal is to theoretically investigate the relative role of dispersion of refractoriness and source-sink mismatch in vulnerability to reentry in the specific situation of regional myocardial acute ischemia. The electrical activity of a regionally ischemic tissue was simulated using a modified version of the Luo-Rudy dynamic model. Ischemic conditions were varied to simulate the time-course of acute ischemia. Our results showed that dispersion of refractoriness increased with the severity of ischemia. However, no correlation between dispersion of refractoriness and the width of the vulnerable window was found. Additionally, in approximately 50% of the reentries, unidirectional block (UDB) took place in cells completely recovered from refractoriness. We examined patterns of activation after premature stimulation and they were intimately related to the source-sink relationship, quantified by the safety factor (SF). Moreover, the isoline where the SF dropped below unity matched the area where propagation failed. It was concluded that the mismatch of the source-sink relationship, rather than solely refractoriness, was the ultimate cause of the UDB leading to reentry. The SF represents a very powerful tool to study the mechanisms responsible for reentry.

Glycolytic oscillations in isolated rabbit ventricular myocytes

Previous studies have shown that glycolysis can oscillate periodically, driven by feedback loops in regulation of key glycolytic enzymes by free ADP and other metabolites. Here we show both theoretically and experimentally in cardiac myocytes that when the capacity of oxidative phosphorylation and the creatine kinase system to buffer the cellular ATP/ADP ratio is suppressed, glycolysis can cause large scale periodic oscillations in cellular ATP levels (0.02-0.067 Hz), monitored from glibenclamide-sensitive changes in action potential duration or intracellular free Mg2+. Action potential duration oscillations originate primarily from glycolysis, since they 1) occur in the presence of cyanide or rotenone, 2) are suppressed by iodoacetate, 3) are accompanied by at most very small mitochondrial membrane potential oscillations, and 4) exhibit an anti-phase relationship to NADH fluorescence. By uncoupling energy supply-demand balance, glycolytic oscillations may promote injury and electrophysiological heterogeneity during acute metabolic stresses, such as acute myocardial ischemia in which both oxidative phosphorylation and creatine kinase activity are inhibited.

Ameliorative prospective of alpha-mangostin, a xanthone derivative from Garcinia mangostana against beta-adrenergic cathecolamine-induced myocardial toxicity and anomalous cardiac TNF-alpha and COX-2 expressions in rats

Altered membrane integrity and inflammation play a key role in cardiovascular damage. We investigated the salubrious effect of exogenously administered alpha-mangostin against beta-adrenergic cathecolamine-induced cardiovascular toxicity with special reference to membrane ATPases, lysosomal hydrolases and inflammatory mediators TNF-alpha and Cyclooxygenase-2 (COX-2) expressions in albino rats. Induction of rats with isoproterenol (150mg/kg body wt, i.p.) for 2 days resulted in a significant increase in the activities of serum and cardiac lysosomal hydrolases (beta-d-glucuronidase, beta-d-galactosidase, beta-d-N-acetylglucosaminidase, acid phosphatase and cathepsin-D). A significant increase in cardiac levels of sodium, calcium with a decrease in the level of potassium paralleled by abnormal activities of membrane-bound phosphatases (Na(+)-K(+) ATPase, Ca(2+) ATPase and Mg(2+) ATPase) were observed in the heart of ISO-administered rats. Cardiac TNF-alpha and COX-2 expressions were assessed by Western blotting. Cardiac TNF-alpha and COX-2 expressions were significantly elevated in ISO-intoxicated rats. Pre-co-treatment with alpha-mangostin (200mg/kg body wt.) orally for 8 days significantly attenuated these abnormalities and restored the levels to near normalcy when compared to ISO intoxicated group of rats. In conclusion, alpha-mangostin preserves the myocardial membrane integrity and extenuates anomalous TNF-alpha and COX-2 expressions by mitigating ISO-induced oxidative stress and cellular damage effectively. Restoration of cellular normalcy accredits the cytoprotective role of alpha-mangostin.

Mechanistic investigation into the arrhythmogenic role of transmural heterogeneities in regional ischaemia phase 1A

AIMS

Studies of arrhythmogenesis during ischemia have focused primarily on reentrant mechanisms manifested on the epicardial surface. The goal of this study was to use a physiologically-accurate model of acute regional ischemia phase 1A to determine the contribution of ischaemia-induced transmural electrophysiological heterogeneities to arrhythmogenesis following left anterior descending artery occlusion.

METHODS AND RESULTS

A slice through a geometrical model of the rabbit ventricles was extracted and a model of regional ischaemia developed. The model included a central ischaemic zone incorporating transmural gradients of I(K(ATP)) activation and [K+]o, surrounded by ischaemic border zones (BZs), with the degree of ischaemic effects varied to represent progression of ischaemia 2-10 min post-occlusion. Premature stimulation was applied over a range of coupling intervals to induce re-entry. The presence of ischaemic BZs and a transmural gradient in I(K(ATP)) activation provided the substrate for re-entrant arrhythmias. Increased dispersion of refractoriness and conduction velocity in the BZs with time post-occlusion led to a progressive increase in arrhythmogenesis. In the absence of a transmural gradient of I(K(ATP)) activation, re-entry was rarely sustained.

CONCLUSION

Knowledge of the mechanism by which specific electrophysiological heterogeneities underlie arrhythmogenesis during acute ischaemia could be useful in developing preventative treatments for patients at risk of coronary vascular disease.

A grid computing-based approach for the acceleration of simulations in cardiology

This paper combines high-performance computing and grid computing technologies to accelerate multiple executions of a biomedical application that simulates the action potential propagation on cardiac tissues. First, a parallelization strategy was employed to accelerate the execution of simulations on a cluster of personal computers (PCs). Then, grid computing was employed to concurrently perform the multiple simulations that compose the cardiac case studies on the resources of a grid deployment, by means of a service-oriented approach. This way, biomedical experts are provided with a gateway to easily access a grid infrastructure for the execution of these research studies. Emphasis is stressed on the methodology employed. In order to assess the benefits of the grid, a cardiac case study, which analyzes the effects of premature stimulation on reentry generation during myocardial ischemia, has been carried out. The collaborative usage of a distributed computing infrastructure has reduced the time required for the execution of cardiac case studies, which allows, for example, to take more accurate decisions when evaluating the effects of new antiarrhythmic drugs on the electrical activity of the heart.

The balance between inactivation and activation of the Na+-K+ pump underlies the triphasic accumulation of extracellular K+ during myocardial ischemia

Ischemia-induced hyperkalemia (accumulation of extracellular K(+)) predisposes the heart to the development of lethal reentrant ventricular arrhythmias. This phenomenon exhibits a triphasic time course and is thought to be mediated by a combination of three mechanisms: 1) increased cellular K(+) efflux, 2) decreased cellular K(+) influx, and 3) shrinkage of the extracellular space. These ischemia-induced electrophysiological changes are driven by an impaired cellular metabolism. However, the relative contributions of these mechanisms, as well as the origin of the triphasic profile, have proven to be difficult to determine experimentally. In this study, the changes in metabolite concentrations that arise during 15 min of zero-flow global ischemia were incorporated into a dynamic model of cellular electrophysiology, which was extended to include a metabolically sensitive description of the Na(+)-K(+) pump and ATP-sensitive K(+) channel, in addition to cell volume regulation. The coupling of altered K(+) fluxes and cell volume regulation enables an integrative simulation of ischemic hyperkalemia. These simulations were able to quantitatively reproduce experimental measurements of the accumulation of extracellular K(+) during 15 min of simulated ischemia, both with respect to the degree of K(+) loss as well as the triphasic time course. Analysis of the model indicates that the inhibition of the Na(+)-K(+) pump is the dominant factor underlying this hyperkalemic behavior, accounting for approximately 85% of the observed extracellular K(+) accumulation. It was found that the balance between activation and inhibition of the Na(+)-K(+) pump, affected by the changing metabolite and ion concentrations (in particular, [ADP]), give rise to the triphasic profile associated with ischemic hyperkalemia.

Modelling the cardiac transverse-axial tubular system

The transverse-axial tubular system (TATS) of cardiac ventricular myocytes is a complex network of tubules that arises as invaginations of the surface membrane; it appears to form a specialised region of cell membrane that is particularly important for excitation-contraction coupling. However, much remains unknown about the structure and role of the TATS. In this brief review we use experimental data and computer modelling to address the following key questions: (i) What fraction of the cell membrane is within the TATS? (ii) Is the composition of the TATS membrane the same as the surface membrane? (iii) How good is electrical coupling between the surface and TATS membranes? (iv) What fraction of each current is within the TATS? (v) How important is the complex structure of the TATS network? (vi) What is the effect of current inhomogeneity on lumenal ion concentrations? (vii) Does the TATS contribute to the functional changes observed in heart failure? Although there are many areas in which experimental evidence is lacking, computer models provide a method to assess and predict the possible function of the TATS; such models suggest that although the surface and TATS membranes are electrically well coupled, concentration of ion flux pathways within the TATS, coupled to restricted diffusion, may result in the ionic composition in the TATS lumen being different from that in the bulk extracellular space, and varying with activity and in pathological conditions.

Vulnerability to reentry in a regionally ischemic tissue: a simulation study

Sudden cardiac death is mainly provoked by arrhythmogenic processes. During myocardial ischemia many malignant arrhythmias, such as reentry, take place and can degenerate into ventricular fibrillation. It is thus of great interest to unravel the intricate mechanisms underlying the initiation and maintenance of a reentry. In this computational study, we analyze the probability of reentry during different stages of the acute phase of ischemia. We also aimed at the understanding of the role of its main components: hypoxia, hyperkalemia, and acidosis analyzing the intricate ionic mechanisms responsible for reentry generation. We simulated the electrical activity of a ventricular tissue affected by regional ischemia based on a modified version of the Luo-Rudy model (LRd00). The ischemic conditions were varied to simulate different stages of this pathology. After premature stimulation, we evaluated the vulnerability to reentry. We obtained an unimodal behavior for the vulnerable window as ischemia progressed, peaking at the eighth minute after the onset of ischemia where the vulnerable window yielded 58 ms. Under more severe conditions the vulnerable window decreased and became zero for minute 8.75. The present work provides insight into the mechanisms of reentry generation during ischemia, highlighting the role of acidosis and hypoxia when hyperkalemia is present.