Arrhythmogenic remodelling of activation and repolarization in the failing human heart

Renal denervation reverses ventricular structural and functional remodeling in failing rabbit hearts

Renal denervation (RDN) suppresses the activity of the renin-angiotensin-aldosterone system and inflammatory cytokines, leading to the prevention of cardiac remodeling. Limited studies have reported the effects of renal denervation on ventricular electrophysiology. We aimed to use optical mapping to evaluate the effect of RDN on ventricular structural and electrical remodeling in a tachycardia-induced cardiomyopathy rabbit model. Eighteen rabbits were randomized into 4 groups: sham control group (n = 5), renal denervation group receiving RDN (n = 5), heart failure group receiving rapid ventricular pacing for 1 month (n = 4), and RDN-heart failure group (n = 4). Rabbit hearts were harvested for optical mapping. Different cycle lengths were paced (400, 300, 250, 200, and 150 ms), and the results were analyzed. In optical mapping, the heart failure group had a significantly slower epicardial ventricular conduction velocity than the other three groups. The RDN-heart failure, sham control, and RDN groups had similar velocities. We then analyzed the 80% action potential duration at different pacing cycle lengths, which showed a shorter action potential duration as cycle length decreased (P for trend < 0.01), which was consistent across all groups. The heart failure group had a significantly longer action potential duration than the sham control and RDN groups. Action potential duration was shorter in the RDN-heart failure group than the heart failure group (P < 0.05). Reduction of conduction velocity and prolongation of action potential duration are significant hallmarks of heart failure, and RDN reverses these remodeling processes.

Apitherapy combination improvement of blood pressure, cardiovascular protection, and antioxidant and anti-inflammatory responses in dexamethasone model hypertensive rats

Hypertension-induced ventricular and vascular remodeling causes myocardial infarction, heart failure, and sudden death. Most available pharmaceutical products used to treat hypertension lead to adverse effects on human health. Limited data is available on apitherapy (bee products) combinations for treatment of hypertension. This study aims to evaluate the antihypertensive effects of combinations of natural apitherapy compounds used in the medical sector to treat a variety of diseases. Rats were assigned into six groups consisting of one control group and five hypertensive groups where hypertension (blood pressure > 140/90) was induced with dexamethasone. One of these groups was used as a hypertension model, while the remaining four hypertensive groups were treated with a propolis, royal jelly, and bee venom combination (PRV) at daily oral doses of 0.5, 1.0, and 2.0 mg/kg, and with losartan 10 mg/kg. The PRV combination at all doses decreased arterial blood pressure below the suboptimal value (p < 0.001), and PRV combination treatment improved dexamethasone-induced-ECG changes. The same treatment decreased angiotensin-II, endothelin-1, and tumor growth factor β serum levels in hypertensive rats. Additionally, PRV combination improved histopathological structure, and decreased serum levels of NF-kB and oxidative stress biomarkers. We concluded that PRV combination therapy may be used as a potential treatment for a variety of cardiovascular diseases.

Therapeutic Effects of Apamin as a Bee Venom Component for Non-Neoplastic Disease

Bee venom is a natural toxin produced by honeybees and plays an important role in defending bee colonies. Bee venom has several kinds of peptides, including melittin, apamin, adolapamine, and mast cell degranulation peptides. Apamin accounts for about 2%-3% dry weight of bee venom and is a peptide neurotoxin that contains 18 amino acid residues that are tightly crosslinked by two disulfide bonds. It is well known for its pharmacological functions, which irreversibly block Ca2+-activated K+ (SK) channels. Apamin regulates gene expression in various signal transduction pathways involved in cell development. The aim of this study was to review the current understanding of apamin in the treatment of apoptosis, fibrosis, and central nervous system diseases, which are the pathological processes of various diseases. Apamin's potential therapeutic and pharmacological applications are also discussed.

Role of Apamin-Sensitive Calcium-Activated Small-Conductance Potassium Currents on the Mechanisms of Ventricular Fibrillation in Pacing-Induced Failing Rabbit Hearts

BACKGROUND

Ventricular fibrillation (VF) during heart failure is characterized by stable reentrant spiral waves (rotors). Apamin-sensitive small-conductance calcium-activated potassium currents (I_KAS) are heterogeneously upregulated in failing hearts. We hypothesized that I_KAS influences the location and stability of rotors during VF.

METHODS AND RESULTS

Optical mapping was performed on 9 rabbit hearts with pacing-induced heart failure. The epicardial right ventricular and left ventricular surfaces were simultaneously mapped in a Langendorff preparation. At baseline and after apamin (100 nmol/L) infusion, the action potential duration (APD₈₀) was determined, and VF was induced. Areas with a >50% increase in the maximum action potential duration (ΔAPD) after apamin infusion were considered to have a high I_KAS distribution. At baseline, the distribution density of phase singularities during VF in high I_KAS distribution areas was higher than in other areas (0.0035±0.0011 versus 0.0014±0.0010 phase singularities/pixel; P=0.004). In addition, high dominant frequencies also colocalized to high I_KAS distribution areas (26.0 versus 17.9 Hz; P=0.003). These correlations were eliminated during VF after apamin infusion, as the number of phase singularities (17.2 versus 11.0; P=0.009) and dominant frequencies (22.1 versus 16.2 Hz; P=0.022) were all significantly decreased. In addition, reentrant spiral waves became unstable after apamin infusion, and the duration of VF decreased.

CONCLUSIONS

The I_KAS current influences the mechanism of VF in failing hearts as phase singularities, high dominant frequencies, and reentrant spiral waves all correlated to areas of high I_KAS. Apamin eliminated this relationship and reduced VF vulnerability.

Electrophysiology of Heart Failure Using a Rabbit Model: From the Failing Myocyte to Ventricular Fibrillation

Heart failure is a leading cause of death, yet its underlying electrophysiological (EP) mechanisms are not well understood. In this study, we use a multiscale approach to analyze a model of heart failure and connect its results to features of the electrocardiogram (ECG). The heart failure model is derived by modifying a previously validated electrophysiology model for a healthy rabbit heart. Specifically, in accordance with the heart failure literature, we modified the cell EP by changing both membrane currents and calcium handling. At the tissue level, we modeled the increased gap junction lateralization and lower conduction velocity due to downregulation of Connexin 43. At the biventricular level, we reduced the apex-to-base and transmural gradients of action potential duration (APD). The failing cell model was first validated by reproducing the longer action potential, slower and lower calcium transient, and earlier alternans characteristic of heart failure EP. Subsequently, we compared the electrical wave propagation in one dimensional cables of healthy and failing cells. The validated cell model was then used to simulate the EP of heart failure in an anatomically accurate biventricular rabbit model. As pacing cycle length decreases, both the normal and failing heart develop T-wave alternans, but only the failing heart shows QRS alternans (although moderate) at rapid pacing. Moreover, T-wave alternans is significantly more pronounced in the failing heart. At rapid pacing, APD maps show areas of conduction block in the failing heart. Finally, accelerated pacing initiated wave reentry and breakup in the failing heart. Further, the onset of VF was not observed with an upregulation of SERCA, a potential drug therapy, using the same protocol. The changes introduced at the cell and tissue level have increased the failing heart’s susceptibility to dynamic instabilities and arrhythmias under rapid pacing. However, the observed increase in arrhythmogenic potential is not due to a steepening of the restitution curve (not present in our model), but rather to a novel blocking mechanism.

Ventricular fibrillation (VF) is one of the leading causes of sudden death. During VF, the electrical wave of activation in the heart breaks up chaotically. Consequently, the heart is unable to contract synchronously and pump blood to the rest of the body. In our work we formulate and validate a model of heart failure (HF) that allows us to evaluate the arrhythmogenic potential of individual and combined electrophysiological changes. In diagnostic cardiology, the electrocardiogram (ECG) is one of the most commonly used tools for detecting abnormalities in the heart electrophysiology. One of our goals is to use our numerical model to link changes at the cellular and tissue level in a failing heart to a numerically computed ECG. This allows us to characterize the precursor to and the risk of VF. In order to understand the mechanisms underlying VF in HF, we design a test that simulates a HF patient performing physical exercise. We show that under fast heart rates with changes in pacing, HF patients are more prone to VF due to a new conduction blocking mechanism. In the long term, our mathematical model is suitable for investigating the effect of drug therapies in HF.

Kv4.3-Encoded Fast Transient Outward Current Is Presented in Kv4.2 Knockout Mouse Cardiomyocytes

Gradients of the fast transient outward K⁺ current (I_to,f) contribute to heterogeneity of ventricular repolarization in a number of species. Cardiac I_to,f levels and gradients change notably with heart disease. Human cardiac I_to,f appears to be encoded by the Kv4.3 pore-forming α-subunit plus the auxiliary KChIP2 β-subunit while mouse cardiac I_to,f requires Kv4.2 and Kv4.3 α-subunits plus KChIP2. Regional differences in cardiac I_to,f are associated with expression differences in Kv4.2 and KChIP2. Although I_to,f was reported to be absent in mouse ventricular cardiomyocytes lacking the Kv4.2 gene (Kv4.2-/-) when short depolarizing voltage pulses were used to activate voltage-gated K⁺ currents, in the present study, we showed that the use of long depolarization steps revealed a heteropodatoxin-sensitive I_to,f (at ~40% of the wild-type levels). Immunohistological studies further demonstrated membrane expression of Kv4.3 in Kv4.2-/- cardiomyocytes. Transmural I_to,f gradients across the left ventricular wall were reduced by ~3.5-fold in Kv4.2-/- heart, compared to wild-type. The I_to,f gradient in Kv4.2-/- hearts was associated with gradients in KChIP2 mRNA expression while in wild-type there was also a gradient in Kv4.2 expression. In conclusion, we found that Kv4.3-based I_to,f exists in the absence of Kv4.2, although with a reduced transmural gradient. Kv4.2-/- mice may be a useful animal model for studying Kv4.3-based I_to,f as observed in humans.

Ion Channels in the Heart

Optimal cardiac function depends on proper timing of excitation and contraction in various regions of the heart, as well as on appropriate heart rate. This is accomplished via specialized electrical properties of various components of the system, including the sinoatrial node, atria, atrioventricular node, His-Purkinje system, and ventricles. Here we review the major regionally determined electrical properties of these cardiac regions and present the available data regarding the molecular and ionic bases of regional cardiac function and dysfunction. Understanding these differences is of fundamental importance for the investigation of arrhythmia mechanisms and pharmacotherapy.

Human cardiac systems electrophysiology and arrhythmogenesis: iteration of experiment and computation

Human cardiac electrophysiology (EP) is a unique system for computational modelling at multiple scales. Due to the complexity of the cardiac excitation sequence, coordinated activity must occur from the single channel to the entire myocardial syncytium. Thus, sophisticated computational algorithms have been developed to investigate cardiac EP at the level of ion channels, cardiomyocytes, multicellular tissues, and the whole heart. Although understanding of each functional level will ultimately be important to thoroughly understand mechanisms of physiology and disease, cardiac arrhythmias are expressly the product of cardiac tissue-containing enough cardiomyocytes to sustain a reentrant loop of activation. In addition, several properties of cardiac cellular EP, that are critical for arrhythmogenesis, are significantly altered by cell-to-cell coupling. However, relevant human cardiac EP data, upon which to develop or validate models at all scales, has been lacking. Thus, over several years, we have developed a paradigm for multiscale human heart physiology investigation and have recovered and studied over 300 human hearts. We have generated a rich experimental dataset, from which we better understand mechanisms of arrhythmia in human and can improve models of human cardiac EP. In addition, in collaboration with computational physiologists, we are developing a database for the deposition of human heart experimental data, including thorough experimental documentation. We anticipate that accessibility to this human heart dataset will further human EP computational investigations, as well as encourage greater data transparency within the field of cardiac EP.

Representing variability and transmural differences in a model of human heart failure

During heart failure (HF) at the cellular level, the electrophysiological properties of single myocytes get remodeled, which can trigger the occurrence of ventricular arrhythmias that could be manifested in many forms such as early afterdepolarizations (EADs) and alternans (ALTs). In this paper, based on experimentally observed human HF data, specific ionic and exchanger current strengths are modified from a recently developed human ventricular cell model: the O'Hara-Virág-Varró-Rudy (OVVR) model. A new transmural HF-OVVR model is developed that incorporates HF changes and variability of the observed remodeling. This new heterogeneous HF-OVVR model is able to replicate many of the failing action potential (AP) properties and the dynamics of both [Ca(2+)]i and [Na(+)]i in accordance with experimental data. Moreover, it is able to generate EADs for different cell types and exhibits ALTs at modest pacing rate for transmural cell types. We have assessed the HF-OVVR model through the examination of the AP duration and the major ionic currents' rate dependence in single myocytes. The evaluation of the model comes from utilizing the steady-state (S-S) and S1-S2 restitution curves and from probing the accommodation of the HF-OVVR model to an abrupt change in cycle length. In addition, we have investigated the effect of chosen currents on the AP properties, such as blocking the slow sodium current to shorten the AP duration and suppress the EADs, and have found good agreement with experimental observations. This study should help elucidate arrhythmogenic mechanisms at the cellular level and predict unseen properties under HF conditions. In addition, this AP cell model might be useful for modeling and simulating HF at the tissue and organ levels.

The effects of remodeling with heart failure on mode of initiation of ventricular fibrillation and its spatiotemporal organization

PURPOSE

The effect of the heart failure substrate on the initiation of ventricular fibrillation (VF) and its resulting mechanism is not known. The objective of this study was to determine the effects of substrate on VF initiation and its spatiotemporal organization in the heart failure model.

METHODS

Optical action potentials were recorded from LV wedge preparations either from structurally normal hearts (control, n = 11) or from congestive heart failure (CHF; n = 7), at the epicardial surface, endocardial surface which included a papillary muscle, and a transmural cross section. Action potential duration (APD(80)) was determined, and VF was initiated. A fast Fourier transform was calculated, and the dominant frequency (DF) was determined.

RESULTS

The CHF group showed increased VF vulnerability (69 vs 26 %, p < 0.03), and every mapped surface showed an APD(80) gradient which included islands of higher APDs on the transmural surface (M cells) which was not observed in controls. VF in the CHF group was characterized by stable, discrete, high-DF areas that correlated to either foci or spiral waves located on the transmural surface at the site of the papillary muscle. Overall, the top 10 % of DFs correlated to an APD of 101 ms while the bottom 10 % of DFs correlated to an APD of 126 ms (p < 0.01).

CONCLUSIONS

In the CHF model, APD gradients correlated with an increased vulnerability to VF, and the highest stable DFs were located on the transmural surface which was not seen in controls. This indicates that the CHF substrate creates unique APD and DF characteristics.

Electrophysiological and structural remodeling in heart failure modulate arrhythmogenesis. 1D simulation study

Background

Heart failure is a final common pathway or descriptor for various cardiac pathologies. It is associated with sudden cardiac death, which is frequently caused by ventricular arrhythmias. Electrophysiological remodeling, intercellular uncoupling, fibrosis and autonomic imbalance have been identified as major arrhythmogenic factors in heart failure etiology and progression.

Objective

In this study we investigate in silico the role of electrophysiological and structural heart failure remodeling on the modulation of key elements of the arrhythmogenic substrate, i.e., electrophysiological gradients and abnormal impulse propagation.

Methods

Two different mathematical models of the human ventricular action potential were used to formulate models of the failing ventricular myocyte. This provided the basis for simulations of the electrical activity within a transmural ventricular strand. Our main goal was to elucidate the roles of electrophysiological and structural remodeling in setting the stage for malignant life-threatening arrhythmias.

Results

Simulation results illustrate how the presence of M cells and heterogeneous electrophysiological remodeling in the human failing ventricle modulate the dispersion of action potential duration and repolarization time. Specifically, selective heterogeneous remodeling of expression levels for the Na⁺/Ca²⁺ exchanger and SERCA pump decrease these heterogeneities. In contrast, fibroblast proliferation and cellular uncoupling both strongly increase repolarization heterogeneities. Conduction velocity and the safety factor for conduction are also reduced by the progressive structural remodeling during heart failure.

Conclusion

An extensive literature now establishes that in human ventricle, as heart failure progresses, gradients for repolarization are changed significantly by protein specific electrophysiological remodeling (either homogeneous or heterogeneous). Our simulations illustrate and provide new insights into this. Furthermore, enhanced fibrosis in failing hearts, as well as reduced intercellular coupling, combine to increase electrophysiological gradients and reduce electrical propagation. In combination these changes set the stage for arrhythmias.

Computational model of erratic arrhythmias in a cardiac cell network: the role of gap junctions

Cardiac morbidity and mortality increases with the population age. To investigate the underlying pathological mechanisms, and suggest new ways to reduce clinical risks, computational approaches complementing experimental and clinical investigations are becoming more and more important. Here we explore the possible processes leading to the occasional onset and termination of the (usually) non-fatal arrhythmias widely observed in the heart. Using a computational model of a two-dimensional network of cardiac cells, we tested the hypothesis that an ischemia alters the properties of the gap junctions inside the ischemic area. In particular, in agreement with experimental findings, we assumed that an ischemic episode can alter the gap junctions of the affected cells by reducing their average conductance. We extended these changes to include random fluctuations with time, and modifications in the gap junction rectifying conductive properties of cells along the edges of the ischemic area. The results demonstrate how these alterations can qualitatively give an account of all the main types of non-fatal arrhythmia observed experimentally, and suggest how premature beats can be eliminated in three different ways: a) with a relatively small surgical procedure, b) with a pharmacological reduction of the rectifying conductive properties of the gap-junctions, and c) by pharmacologically decreasing the gap junction conductance. In conclusion, our model strongly supports the hypothesis that non-fatal arrhythmias can develop from post-ischemic alteration of the electrical connectivity in a relatively small area of the cardiac cell network, and suggests experimentally testable predictions on their possible treatments.

Overexpression of Sema3a in myocardial infarction border zone decreases vulnerability of ventricular tachycardia post-myocardial infarction in rats

The expression of the chemorepellent Sema3a is inversely related to sympathetic innervation. We investigated whether overexpression of Sema3a in the myocardial infarction (MI) border zone could attenuate sympathetic hyper-innervation and decrease the vulnerability to malignant ventricular tachyarrhythmia (VT) in rats. Survived MI rats were randomized to phosphate buffered saline (PBS, n = 12); mock lentivirus (MLV, n = 13) and lentivirus-mediated overexpression of Sema3a (SLV, n = 13) groups. Sham-operated rats served as control group (CON, n = 20). Cardiac function and electrophysiological study (PES) were performed at 1 week later. Blood and tissue samples were collected for histological analysis, epinephrine (EPI), growth-associated factor 43 (GAP43) and tyrosine hydroxylase (TH) measurements. QTc intervals were significantly shorter in SLV group than in PBS and MLV groups (168.6 ± 7.8 vs. 178.1 ± 9.5 and 180.9 ± 8.2 ms, all P < 0.01). Inducibility of VT by PES was significantly lower in the SLV group [30.8% (4/13)] than in PBS [66.7% (8/12)] and MLV [61.5% (8/13)] groups (P < 0.05). mRNA and protein expressions of Sema3a were significantly higher and the protein expression of GAP43 and TH was significantly lower at 7 days after transduction in SLV group compared with PBS, MLV and CON groups. Myocardial EPI in the border zone was also significantly lower in SLV group than in PBS and MLV group (8.73 ± 1.30 vs. 11.94 ± 1.71 and 12.24 ± 1.54 μg/g protein, P < 0.001). Overexpression of Sema3a in MI border zone could reduce the inducibility of ventricular arrhythmias by reducing sympathetic hyper-reinnervation after infarction.