Regional gradation of L-type calcium currents in the feline heart with a healed myocardial infarct

Mechanism of Ventricular Tachycardia Occurring in Chronic Myocardial Infarction Scar

Cardiac arrest is the leading cause of death in the more economically developed countries. Ventricular tachycardia associated with myocardial infarct is a prominent cause of cardiac arrest. Ventricular arrhythmias occur in 3 phases of infarction: during the ischemic event, during the healing phase, and after the scar matures. Mechanisms of arrhythmias in these phases are distinct. This review focuses on arrhythmia mechanisms for ventricular tachycardia in mature myocardial scar. Available data have shown that postinfarct ventricular tachycardia is a reentrant arrhythmia occurring in circuits found in the surviving myocardial strands that traverse the scar. Electrical conduction follows a zigzag course through that area. Conduction velocity is impaired by decreased gap junction density and impaired myocyte excitability. Enhanced sympathetic tone decreases action potential duration and increases sarcoplasmic reticular calcium leak and triggered activity. These elements of the ventricular tachycardia mechanism are found diffusely throughout scar. A distinct myocyte repolarization pattern is unique to the ventricular tachycardia circuit, setting up conditions for classical reentry. Our understanding of ventricular tachycardia mechanisms continues to evolve as new data become available. The ultimate use of this information would be the development of novel diagnostics and therapeutics to reliably identify at-risk patients and prevent their ventricular arrhythmias.

Ventricular Arrhythmias in Ischemic Cardiomyopathy-New Avenues for Mechanism-Guided Treatment

Ischemic heart disease is the most common cause of lethal ventricular arrhythmias and sudden cardiac death (SCD). In patients who are at high risk after myocardial infarction, implantable cardioverter defibrillators are the most effective treatment to reduce incidence of SCD and ablation therapy can be effective for ventricular arrhythmias with identifiable culprit lesions. Yet, these approaches are not always successful and come with a considerable cost, while pharmacological management is often poor and ineffective, and occasionally proarrhythmic. Advances in mechanistic insights of arrhythmias and technological innovation have led to improved interventional approaches that are being evaluated clinically, yet pharmacological advancement has remained behind. We review the mechanistic basis for current management and provide a perspective for gaining new insights that centre on the complex tissue architecture of the arrhythmogenic infarct and border zone with surviving cardiac myocytes as the source of triggers and central players in re-entry circuits. Identification of the arrhythmia critical sites and characterisation of the molecular signature unique to these sites can open avenues for targeted therapy and reduce off-target effects that have hampered systemic pharmacotherapy. Such advances are in line with precision medicine and a patient-tailored therapy.

Modeling the Electrophysiological Properties of the Infarct Border Zone

Ventricular arrhythmias (VA) in patients with myocardial infarction (MI) are thought to be associated with structural and electrophysiological remodeling within the infarct border zone (BZ). Personalized computational models have been used to investigate the potential role of the infarct BZ in arrhythmogenesis, which still remains incompletely understood. Most recent models have relied on experimental data to assign BZ properties. However, experimental measurements vary significantly resulting in different computational representations of this region. Here, we review experimental data available in the literature to determine the most prominent properties of the infarct BZ. Computational models are then used to investigate the effect of different representations of the BZ on activation and repolarization properties, which may be associated with VA. Experimental data obtained from several animal species and patients with infarct show that BZ properties vary significantly depending on disease's stage, with the early disease stage dominated by ionic remodeling and the chronic stage by structural remodeling. In addition, our simulations show that ionic remodeling in the BZ leads to large repolarization gradients in the vicinity of the scar, which may have a significant impact on arrhythmia simulations, while structural remodeling plays a secondary role. We conclude that it is imperative to faithfully represent the properties of regions of infarction within computational models specific to the disease stage under investigation in order to conduct in silico mechanistic investigations.

Exendin-4 inhibits structural remodeling and improves Ca2+ homeostasis in rats with heart failure via the GLP-1 receptor through the eNOS/cGMP/PKG pathway

The glucagon-like peptide-1 receptor (GLP-1R) agonist exendin-4 is a long-acting analog of GLP-1, which stimulates insulin secretion and is clinically used in the treatment of type 2 diabetes. Previous studies have demonstrated that GLP-1 agonists and analogs serve as cardioprotective factors in various conditions. Disturbances in calcium cycling are characteristic of heart failure (HF); therefore, the aim of this study was to investigate the effect of exendin-4 (a GLP-1 mimetic) on the regulation of calcium handling and to identify the underlying mechanisms in an HF rat model after myocardial infarction (MI). Rats underwent surgical ligation of the left anterior descending coronary artery or sham surgery prior to infusion with vehicle, exendin-4, or exendin-4 and exendin9-39 for 4 weeks. Exendin-4 treatment decreased MI size, suppressed chamber dilation, myocyte hypertrophy, and fibrosis and improved in vivo heart function in the rats subjected to MI. Exendin-4 resulted in an increase in circulating GLP-1 and GLP-1R in ventricular tissues. Additionally, exendin-4 activated the eNOS/cGMP/PKG signaling pathway and inhibited the Ca²⁺/calmodulin-dependent kinase II (CaMKII) pathways. Myocytes isolated from exendin-4-treated hearts displayed higher Ca²⁺ transients, higher sarcoplasmic reticulum Ca²⁺ content, and higher l-type Ca²⁺ current densities than MI hearts. Exendin-4 treatment restored the protein expression of sarcoplasmic reticulum Ca²⁺ uptake ATPase (SERCA2a), phosphorylated phospholamban (PLB) and Cav1.2 and decreased the levels of phosphorylated ryanodine receptor (RyR). Moreover, the favorable effects of exendin-4 were significantly inhibited by exendin9-39 (a GLP-1 receptor antagonist). Exendin-4 treatment of an HF rat model after MI inhibited cardiac and cardiomyocytes progressive remodeling. In addition, Ca²⁺ handling and its molecular modulation were also improved by exendin-4 treatment. The beneficial effects of exendin-4 on cardiac remodeling may be mediated through activation of the eNOS/cGMP/PKG pathway.

Current Perspectives of Electrical Remodeling and Its Therapeutic Implications

Electrical remodeling involves alterations in the electrophysiologic milieu of myocardium in various disease states, such as ventricular hypertrophy, heart failure, atrial tachyarrhythmias, myocardial ischemia, and infarction that are associated with cardiac arrhythmias. Although research in this area dates back to early part of the 19th century, we still lack the exact knowledge of ionic remodeling, the role of various genes and channel proteins, and their relevance for the newer antiarrhythmic therapies. Structural remodeling may also have an impact on the electrical remodeling process, although differences in both structural and electrical remodeling are associated with different disease states. Various electrophysiologic, cellular, and structural alterations, including anisotropic conduction, increased intracellular calcium levels, and gap junction remodeling predispose to increased dispersion of action potential duration and refractoriness. This constitutes a favorable substrate for early and late afterdepolarizations and reentrant arrhythmias. Studying the role of ionic remodeling in the initiation and propagation of cardiac arrhythmias has significant relevance for developing newer antiarrhythmic therapies, for identifying patients at risk of developing fatal arrhythmias, and for implementing effective preventive measures. Further research is required to understand the specific effects of individual ion channel remodeling, to understand the signal transduction mechanisms, and to address whether detrimental effects of electrical remodeling can be altered.

Ventricular tachycardia from the healed myocardial infarction scar: validation of an animal model and utility of gene therapy

Life-threatening ventricular arrhythmias generally occur in the setting of structural heart disease. Current clinical options for patients at risk for these rhythm disturbances are limited. We developed a porcine model of inducible ventricular tachycardia originating in the border region of a healed myocardial infarction scar. After validating the model, we assessed gene transfer techniques, focusing on local modification of border zone tissues. We found that gene transfer of the dominant negative KCNH2-G628S mutation to the anteroseptal infarct border caused localized prolongation of effective refractory period in the target region and eliminated all ventricular arrhythmia inducibility. In this work, we characterize the animal model and review the gene transfer results.

Regulation of the ankyrin-B-based targeting pathway following myocardial infarction

AIMS

Ion channel reorganization is a critical step in the pro-arrhythmogenic remodelling process that occurs in heart disease. Ankyrin-B (AnkB) is required for targeting and stabilizing ion channels, exchangers, and pumps. Despite a wealth of knowledge implicating the importance of AnkB in human cardiovascular physiology, nothing is known regarding the role of AnkB in common forms of acquired human disease.

METHODS AND RESULTS

We present the first report of AnkB regulation following myocardial infarction (MI). AnkB protein levels were reduced in the infarct border zone 5 days following coronary artery occlusion in the canine. We also observed a dramatic increase in AnkB mRNA levels 5 days post-occlusion. Surprisingly, the expression of the upstream AnkB cytoskeletal component beta2-spectrin was unchanged in post-infarct tissues. However, protein levels and/or membrane expression of downstream AnkB-associated ion channels and transporters Na+/K+ ATPase, Na+/Ca2+ exchanger, and IP3 receptor were altered 5 days post-occlusion. Interestingly, protein levels of the protein phosphatase 2A, an AnkB-associated signalling protein, were significantly affected 5 days post-occlusion. AnkB and PP2A protein levels recovered by 14 days post-occlusion, whereas Na+/K+ ATPase levels recovered by 2 months post-occlusion.

CONCLUSION

These findings reveal the first evidence of ankyrin remodelling following MI and suggest an unexpected divergence point for regulation between ankyrin and the underlying cytoskeletal network. These findings suggest a logical, but unexpected, molecular mechanism underlying ion channel and transporter remodelling following MI.

Arrhythmogenic Ion-Channel Remodeling in the Heart: Heart Failure, Myocardial Infarction, and Atrial Fibrillation

Rhythmic and effective cardiac contraction depends on appropriately timed generation and spread of cardiac electrical activity. The basic cellular unit of such activity is the action potential, which is shaped by specialized proteins (channels and transporters) that control the movement of ions across cardiac cell membranes in a highly regulated fashion. Cardiac disease modifies the operation of ion channels and transporters in a way that promotes the occurrence of cardiac rhythm disturbances, a process called “arrhythmogenic remodeling.” Arrhythmogenic remodeling involves alterations in ion channel and transporter expression, regulation and association with important protein partners, and has important pathophysiological implications that contribute in major ways to cardiac morbidity and mortality. We review the changes in ion channel and transporter properties associated with three important clinical and experimental paradigms: congestive heart failure, myocardial infarction, and atrial fibrillation. We pay particular attention to K+, Na+, and Ca2+channels; Ca2+transporters; connexins; and hyperpolarization-activated nonselective cation channels and discuss the mechanisms through which changes in ion handling processes lead to cardiac arrhythmias. We highlight areas of future investigation, as well as important opportunities for improved therapeutic approaches that are being opened by an improved understanding of the mechanisms of arrhythmogenic remodeling.

Remodeled cardiac calcium channels

Cardiac calcium channels play a pivotal role in the proper functioning of cardiac cells. In response to various pathologic stimuli, they become remodeled, changing how they function, as they adapt to their new environment. Specific features of remodeled channels depend upon the particular disease state. This review will summarize what is known about remodeled cardiac calcium channels in three disease states: hypertrophy, heart failure and atrial fibrillation. In addition, it will review the recent advances made in our understanding of the function of the various molecular building blocks that contribute to the proper functioning of the cardiac calcium channel.

Effect of input resistance voltage-dependency on DC estimate of membrane capacitance in cardiac myocytes

The measure of membrane capacitance (C(m)) in cardiac myocytes is of primary importance as an index of their size in physiological and pathological conditions, and for the understanding of their excitability. Although a plethora of very accurate methods has been developed to access C(m) value in single cells, cardiac electrophysiologists still use, in the majority of laboratories, classical direct current techniques as they have been established in the early days of cardiac cellular electrophysiology. These techniques are based on the assumption that cardiac membrane resistance (R(m)) is constant, or changes negligibly, in a narrow potential range around resting potential. Using patch-clamp whole-cell recordings, both in current-clamp and voltage-clamp conditions, and numerical simulations, we document here the voltage-dependency of R(m), up to -45% of its resting value for 10-mV hyperpolarization, in resting rat ventricular myocytes. We show how this dependency makes classical protocols to misestimate C(m) in a voltage-dependent manner (up to 20% errors), which can dramatically affect C(m)-based calculations on cell size and on intracellular ion dynamics. We develop a simple mechanistic model to fit experimental data and obtain voltage-independent estimates of C(m), and we show that accurate estimates can also be extrapolated from the classical approach.

Dynamic remodeling of K+ and Ca2+ currents in cells that survived in the epicardial border zone of canine healed infarcted heart

Action potentials (APs) of the epicardial border zone (EBZ) cells from the day 5 infarcted heart continue to be altered by day 14 postocclusion, namely, they shortened. However, by 2 mo, EBZ APs appear "normal," yet conduction of wave fronts remains abnormal. We hypothesize that the changes in transmembrane APs are due to a change in the distribution of ion channels in either density or function. Thus we focused on the changes in Ca2+ and K+ currents in cells isolated from the 14-day (IZ14d) and 2-mo (IZ2m) EBZ and compared them with those occurring in cells from the same hearts but remote (Rem) from the EBZ. Whole cell voltage-clamp techniques were used to measure and compare Ca2+ and K+ currents in cells from the different groups. Ca2+ current densities remain reduced in cells of the 14-day and 2-mo infarcted heart and the kinetic changes previously identified in the 5-day heart begin to, but do not recover to, cells from noninfarcted epicardium (NZ) values. Importantly, I(Ca,L) in both the EBZ and Rem regions still show a slowed recovery from inactivation. Furthermore, during the remodeling process, there is an increased expression of T-type Ca2+ currents, but only regionally, and only within a specific time window postmyocardial infarction (MI). Regional heterogeneity in beta-adrenergic responsiveness of I(Ca,L) exists between EBZ and remote cells of the 14-day hearts, but this regional heterogeneity is gone in the healed infarcted heart. In IZ14d, the transient outward K+ current (Ito) begins to reemerge and is accompanied by an upregulated tetraethylammonium-sensitive outward current. By 2-mo postocclusion, Ito and sustained outward K+ current have completed the reverse remodeling process. During the healing process post-MI, canine epicardial cells downregulate the fast Ito but compensate by upregulating a K+ current that in normal cells is minimally functional. For recovering I(Ca,L) of the 14-day and 2-mo EBZ cells, voltage-dependent processes appear to be reset, such that I(Ca,L) "window" current occurs at hyperpolarized potentials. Thus dynamic changes in both Ca2+ and K+ currents contribute to the altered AP observed in 14-day fibers and may account for return of APs of 2 mo EBZ fibers.

Altered excitation-contraction coupling in myocytes from remodeled myocardium after chronic myocardial infarction

Following myocardial infarction (MI), the left ventricle undergoes progressive dilatation and eccentric hypertrophy, i.e., remodeling, which is greater in the adjacent than the remote region. The cellular mechanisms underlying these regional differences were studied. One (n=5) and 8 weeks (n=8) after anteroapical MI in sheep, cardiac myocytes were isolated from the adjacent and remote regions. At 8 weeks after MI, myocyte function in the remote region was not different from values either in sham controls (n=3) or animals 1 week after MI. At 8 weeks after MI, myocyte contractile function (% contraction) was decreased, P<0.01, in the adjacent region (6.4+/-0.4%), as compared with the remote region (8.8+/-0.5%) and was associated with decreased amplitude of Ca(2+)transients (adjacent, 0.69+/-0.09 v remote, 1.08+/-0.20, P<0.05) and L-type Ca(2+)current density (adjacent, 3.6+/-0.2 v remote, 4.8+/-0.2 pA/pF, P<0.05). Relaxation was also impaired significantly in myocytes from the adjacent region, associated with decreased protein levels of SERCA2a. The myocytes were hypertrophied more in the adjacent region than the remote region. Furthermore, focal areas of central myofibrillar lysis and increased glycogen deposition were observed in the adjacent region. These results indicate that impaired excitation-contraction coupling underlies dysfunction of myocytes from the adjacent non-infarcted myocardium after chronic MI, even in the absence of heart failure. Hypertrophy is implicated as the mechanism, since these changes were noted at 8 weeks, but not at 1 week after MI.

Characteristics of I(K) and its response to quinidine in experimental healed myocardial infarction

INTRODUCTION

Mechanisms and drug treatment of serious ventricular arrhythmias in patients with healed myocardial infarction (HMI) are incompletely understood, in part because the electrophysiology and pharmacology of myocytes from noninfarcted regions of HMI hearts are not well characterized.

METHODS AND RESULTS

We studied the delayed rectifier potassium current (I(K)) and quinidine responsiveness of single left ventricular subendocardial myocytes isolated from the region remote to the border zone of healed infarct myocardium (4 to 6 mm from scar edge) in cat hearts 2 months after coronary artery occlusion. Subendocardial cells isolated from corresponding regions of normal cat hearts provided controls. I(K) activation and tail currents were recorded using whole cell, voltage clamp techniques. Membrane capacitance of cells remote to HMI (187 +/- 7 pF) was significantly greater than normal (155 +/- 6 pF; P < 0.001). Action potential durations (APDs) recorded from myocytes in remote regions were prolonged (APD90 = 247 +/- 10 msec) compared to normal (214 +/- 11 msec; P < 0.05). Quinidine (1 microM) significantly prolonged APD90 in normal cells but not in remote cells. Density of I(K) (tail current) was significantly decreased in remote cells (3.1 +/- 0.3 pA/pF) compared to normal (3.9 +/- 0.3 pA/pF; P < 0.05), and voltage-dependent activation of I(K) was shifted in the positive direction. Quinidine had significantly less incremental blocking effect on I(K) already blunted by regional hypertrophy compared to its effect on normal cells in remote cells. IC50 shifted to 0.95 microM in remote cells compared with 0.50 microM in normal cells.

CONCLUSION

Cells in noninfarct region remote from the scar are hypertrophied and display altered electrophysiology. Their reduced I(K) responsiveness to quinidine may explain, in part, failure of quinidine to prolong APD in such cells. Moreover, dispersion of repolarization may be decreased by the effect of quinidine on normal cells.

The search for novel antiarrhythmic strategies. Sicilian Gambit

The past fifty years of antiarrhythmic drug development have seen limited success in prolonging life and reducing morbidity. It is likely that arrhythmias are in most instances final common pathways through which changes in the cardiac substrate and in trigger mechanisms are expressed. We propose that the development and administration of therapies for the arrhythmias themselves, while offering a panacea for a disease entity that has evolved and is being overtly manifested, is also an admission of failure to identify and prevent evolution of the substrate and triggers such that arrhythmias can occur. We suggest that while strategies for treatment and prevention of recurrence of arrhythmias still warrant exploration, greater hope for the future lies in identifying means for earlier diagnosis of the arrhythmogenic substrate and triggers, and in developing therapies that are "upstream" to the arrhythmia and prevent their initial expression. Means to achieve this end are suggested, using specific arrhythmias as examples. Similarly, to increase the likelihood that clinical studies of new therapies can be successfully concluded and interpreted, we suggest new approaches to patient selection, risk stratification, trial endpoints, outcome events and trial methodologies.