Human Ventricular Cells Research Articles

A Human Ventricular Myocyte Model with a Refined Representation of Excitation-Contraction Coupling

Cardiac Ca2+-induced Ca2+ release (CICR) occurs by a regenerative activation of ryanodine receptors (RyRs) within each Ca2+-releasing unit, triggered by the activation of L-type Ca2+ channels (LCCs). CICR is then terminated, most probably by depletion of Ca2+ in the junctional sarcoplasmic reticulum (SR). Hinch et al. previously developed a tightly coupled LCC-RyR mathematical model, known as the Hinch model, that enables simulations to deal with a variety of functional states of whole-cell populations of a Ca2+-releasing unit using a personal computer. In this study, we developed a membrane excitation-contraction model of the human ventricular myocyte, which we call the human ventricular cell (HuVEC) model. This model is a hybrid of the most recent HuVEC models and the Hinch model. We modified the Hinch model to reproduce the regenerative activation and termination of CICR. In particular, we removed the inactivated RyR state and separated the single step of RyR activation by LCCs into triggering and regenerative steps. More importantly, we included the experimental measurement of a transient rise in Ca2+ concentrations ([Ca2+], 10–15 μM) during CICR in the vicinity of Ca2+-releasing sites, and thereby calculated the effects of the local Ca2+ gradient on CICR as well as membrane excitation. This HuVEC model successfully reconstructed both membrane excitation and key properties of CICR. The time course of CICR evoked by an action potential was accounted for by autonomous changes in an instantaneous equilibrium open probability of couplons. This autonomous time course was driven by a core feedback loop including the pivotal local [Ca2+], influenced by a time-dependent decay in the SR Ca2+ content during CICR.

Nuclear Membranes ETB Receptors Mediate ET-1-induced Increase of Nuclear Calcium in Human Left Ventricular Endocardial Endothelial Cells.

In fetal human left ventricular endocardial endothelial cells (EECLs), both plasma membrane (PM) ET(A)R and ET(B)R were reported to mediate ET-1-induced increase of intracellular calcium [Ca](i); however, this effect was mediated by ET(A)R in right EECs (EECRs). In this study, we verified whether, as for the PM, nuclear membranes (NMs) ET-1 receptors activation in EECLs and EECRs induce an increase of nuclear calcium ([Ca](n)) and if this effect is mediated through the same receptor type as in PM. Using a plasmalemma-perforated technique and 3D confocal microscopy, our results showed that, as in PM intact cells, superfusion of nuclei of both cell types with cytosolic ET-1 induced a concentration-dependent sustained increase of [Ca](n). In EECRs, the ET(A)R antagonist prevented the effect of ET-1 on [Ca](n) without affecting EECLs. However, in both cell types, the effect of cytosolic ET-1 on [Ca](n) was prevented by the ETBR antagonist. In conclusion, both NMs' ET(A)R and ET(B)R mediated the effect of cytosolic ET-1 on [Ca](n) in EECRs. In contrast, only NMs' ET(B)R activation mediated the effect of cytosolic ET-1 in EECLs. Hence, the type of NMs' receptors mediating the effect of ET-1 on [Ca](n) are different from those of PM mediating the increase in [Ca](i).

Epigenetic Therapy for the Treatment of Hypertension-Induced Cardiac Hypertrophy and Fibrosis.

The development of heart failure is associated with changes in the size, shape, and structure of the heart that has a negative impact on cardiac function. These pathological changes involve excessive extracellular matrix deposition within the myocardial interstitium and myocyte hypertrophy. Alterations in fibroblast phenotype and myocyte activity are associated with reprogramming of gene transcriptional profiles that likely requires epigenetic alterations in chromatin structure. The aim of our work was to investigate the potential of a currently licensed anticancer epigenetic modifier as a treatment option for cardiac diseases associated with hypertension-induced cardiac hypertrophy and fibrosis. The effects of DNA methylation inhibition with 5-azacytidine (5-aza) were examined in a human primary fibroblast cell line and in a spontaneously hypertensive rat (SHR) model. The results from this work allude to novel in vivo antifibrotic and antihypertrophic actions of 5-aza. Administration of the DNA methylation inhibitor significantly improved several echocardiographic parameters associated with hypertrophy and diastolic dysfunction. Myocardial collagen levels and myocyte size were reduced in 5-aza-treated SHRs. These findings are supported by beneficial in vitro effects in cardiac fibroblasts. Collagen I, collagen III, and α-smooth muscle actin were reduced in a human ventricular cardiac fibroblast cell line treated with 5-aza. These findings suggest a role for epigenetic modifications in contributing to the profibrotic and hypertrophic changes evident during disease progression. Therapeutic intervention with 5-aza demonstrated favorable effects highlighting the potential use of this epigenetic modifier as a treatment option for cardiac pathologies associated with hypertrophy and fibrosis.

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.

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+.

Adipogenesis and epicardial adipose tissue: A novel fate of the epicardium induced by mesenchymal transformation and PPARγ activation

The hearts of many mammalian species are surrounded by an extensive layer of fat called epicardial adipose tissue (EAT). The lineage origins and determinative mechanisms of EAT development are unclear, in part because mice and other experimentally tractable model organisms are thought to not have this tissue. In this study, we show that mouse hearts have EAT, localized to a specific region in the atrial-ventricular groove. Lineage analysis indicates that this adipose tissue originates from the epicardium, a multipotent epithelium that until now is only established to normally generate cardiac fibroblasts and coronary smooth muscle cells. We show that adoption of the adipocyte fate in vivo requires activation of the peroxisome proliferator activated receptor gamma (PPARγ) pathway, and that this fate can be ectopically induced in mouse ventricular epicardium, either in embryonic or adult stages, by expression and activation of PPARγ at times of epicardium-mesenchymal transformation. Human embryonic ventricular epicardial cells natively express PPARγ, which explains the abundant presence of fat seen in human hearts at birth and throughout life.

A Novel Classification Method with Superior Prediction of Drug Arrhythmia Risk

Many drugs, both cardiac and non-cardiac, cause fatal arrhythmias. Current assays to evaluate arrhythmia risk based on block of the delayed rectifier K+ current (hERG) and QT interval prolongation are limited by poor specificity. We hypothesize that physiological, dynamical simulations of drug effects on heart cells will yield better algorithms to discriminate between arrhythmogenic and safe drugs. To test this, we model drug action in cardiomyocyte models based on drug interaction with 3 cardiac ion channels: hERG, the L-type Ca2+ channel, and the fast Na+ channel. For 67 distinct drugs, we measure 15 cellular physiological metrics under multiple pacing conditions and in the 3 ventricular cell types, each in 3 human ventricular cell models. Additionally, we evaluate the influences of gender and inter-individual variability on risk by conducting simulations in both female and male models, and in a simulated population of individuals, respectively. Principal component analysis of these drug effect metrics reveals distinct behaviors of arrhythmogenic drugs. This segregation of drugs in principal component space facilitates their classification into arrhythmogenic and safe categories. Our classification by this method is more sensitive and specific than the hERG block assay or, to our knowledge, any other published predictors, with an area under the receiver-operator characteristic of 0.9630. Furthermore, because it is based on cellular physiological metrics, our algorithm can predict the arrhythmogenicity of drugs with a range of cardiac molecular targets, beyond those of the drugs used to train the algorithm. In conclusion, we have developed a classification algorithm using dynamical heart cell models, suitable for cardiac safety assessments early in drug development, that outperforms current methods in the discrimination of arrhythmogenic and safe drugs.

Abstract 19273: Fibroblast-Derived TSP1, via CD47, Upregulates Connective Tissue Growth Factor to Promote Left Ventricular Fibrosis and Heart Failure

Background: An estimated 5.5 million individuals in the United States have left ventricular heart failure (LV HF). Therapeutics, while relieving symptoms and extending life in some cases, cannot resolve this process. LV HF is characterized by excessive accumulation of the extracellular matrix. The causes of this process remain incompletely understood. Our team has recently described a widely expressed cell surface receptor CD47 that is activated by its high affinity ligand, thrombospondin-1 (TSP1) to promote LV HF. However, the role of TSP1-CD47 signaling in promoting fibrosis, in general, and in the setting of LV HF, in specific, is unknown. Methods: Wild type mice expressing CD47 and mutated mice lacking CD47 (CD47 null) were subjected to transverse aortic constriction (TAC) for 4 weeks followed by open chest pressure-volume loop analysis of cardiac hemodynamics. Signal transduction was confirmed in isolated left ventricles (LV) and normal human ventricular fibroblast cell culture systems. Results: In biopsy samples from failing human hearts and in pre-clinical model of TAC-driven LV HF, TSP1-CD47 signaling was altered. Post-TAC wild type (CD47+/+) mice developed cardiac fibrosis, associated cardiac stiffness and HF. In contrast, CD47 null mice subjected to TAC showed enhanced cardiac function and decreased fibrosis concordant with suppression of TSP1. In cardiac resident fibroblasts, but not myocytes, hypoxia rapidly induced TSP1 protein, while treating with 7N3, a TSP1-based peptide mimetic that selectively binds CD47, increased connective tissue growth factor (CTGF) protein. Further, treating cardiac fibroblast with 7N3 increased fibroblast collagen production. Finally, CD47 blocking antibody mitigated established TAC-driven LV fibrosis and HF. Conclusion: These data identify a proximate role for activated cardiac fibroblast CD47 in promoting LV fibrosis associated with HF and suggest possible therapeutic opportunity.

EAD and DAD mechanisms analyzed by developing a new human ventricular cell model

It has long been suggested that the Ca2+-mechanisms are largely involved in generating the early afterdepolarization (EAD) as well as the delayed afterdepolarization (DAD). This view was examined in a quantitative manner by applying the lead potential analysis to a new human ventricular cell model. In this ventricular cell model, the tight coupled LCC-RyR model (CaRU) based on local control theory (Hinch et al. 2004) and ion channel models mostly based on human electrophysiological data were included to reproduce realistic Ca2+ dynamics as well as the membrane excitation. Simultaneously, the Ca2+ accumulation near the Ca2+ releasing site was incorporated as observed in real cardiac myocytes. The maximum rate of ventricular repolarization (−1.02 mV/ms) is due to IK1 (−0.55 mV/ms) and the rest is provided nearly equally by INCX (−0.20 mV/ms), INaL (−0.16 mV/ms) and INaT (−0.13 mV/ms). These INaL and INaT components are due to closure of the voltage gate, which remains partially open during the plateau potential. DADs could be evoked by applying high-frequency stimulations supplemented by a partial Na+/K+ pump inhibition, or by a microinjection of Ca2+. EADs was evoked by retarding the inactivation of INaL. The lead potential (VL) analysis revealed that IK1 and IKr played the primary role to reverse the AP repolarization to depolarizing limb of EAD. ICaL and INCX amplified EAD, while the remaining currents partially antagonized dVL/dt. The maximum rate of rise of EAD was attributable to the rapid activation of both ICaL (45.5%) and INCX (54.5%).

Electrical wave propagation in an anisotropic model of the left ventricle based on analytical description of cardiac architecture.

We develop a numerical approach based on our recent analytical model of fiber structure in the left ventricle of the human heart. A special curvilinear coordinate system is proposed to analytically include realistic ventricular shape and myofiber directions. With this anatomical model, electrophysiological simulations can be performed on a rectangular coordinate grid. We apply our method to study the effect of fiber rotation and electrical anisotropy of cardiac tissue (i.e., the ratio of the conductivity coefficients along and across the myocardial fibers) on wave propagation using the ten Tusscher–Panfilov (2006) ionic model for human ventricular cells. We show that fiber rotation increases the speed of cardiac activation and attenuates the effects of anisotropy. Our results show that the fiber rotation in the heart is an important factor underlying cardiac excitation. We also study scroll wave dynamics in our model and show the drift of a scroll wave filament whose velocity depends non-monotonically on the fiber rotation angle; the period of scroll wave rotation decreases with an increase of the fiber rotation angle; an increase in anisotropy may cause the breakup of a scroll wave, similar to the mother rotor mechanism of ventricular fibrillation.

Role of O‐GlcNAcylation in regulating mitophagy in cardiomyocytes (693.14)

Acute increases in the post‐translational protein modification O‐linked‐N‐acetylglucosamine (O‐GlcNAc) have been shown to protect cardiomyocytes against oxidative stress and this has been associated with O‐GlcNAcylation of mitochondrial proteins. Mitophagy, a type of cargo‐mediated autophagy, protects against oxidative stress by removing damaged mitochondria. Therefore the goal of this study was to determine whether O‐GlcNAcylation plays a role in regulating mitophagy in AC16 immortalized human ventricular cardiomyocyte cells. Cells were treated with 2,3‐dimethoxy‐1,4‐napthoquinone (DMNQ, 20 µM), a redox cycling agent which increases mitochondrial reactive oxygen species (ROS) and FCCP (0.5 µg/mL), a mitochondrial uncoupler commonly used to induce mitophagy. Following 3 hour treatment with DMNQ or FCCP, O‐GlcNAc levels were decreased and this was associated with increased levels of total PINK1 and increased mitochondrial Parkin. Increasing basal O‐GlcNAc levels for 2 hours with thiamet‐G (3 µM) a highly selective inhibitor for O‐GlcNAcase, attenuated the loss of O‐GlcNAc induced by both DMNQ and FCCP treatment and appeared to increase basal PINK1 levels. These results support the concept that protein O‐GlcNAcylation plays a role in regulating mitophagy and that this could contribute to the protection against oxidative stress. Future studies will include quantifying the effects of DMNQ and thiamet‐G on mitochondrial function using XF24 Extracellular Flux Analyzer as well as determining whether PINK1 or Parkin are targets for O‐GlcNAcylation.Grant Funding Source: Supported by NIH grants HL101192 and HL110366

Dynamic reorganization of the AC16 cardiomyocyte transcriptome in response to TNFα signaling revealed by integrated genomic analyses.

BackgroundDefining cell type-specific transcriptomes in mammals can be challenging, especially for unannotated regions of the genome. We have developed an analytical pipeline called groHMM for annotating primary transcripts using global nuclear run-on sequencing (GRO-seq) data. Herein, we use this pipeline to characterize the transcriptome of an immortalized adult human ventricular cardiomyocyte cell line (AC16) in response to signaling by tumor necrosis factor alpha (TNFα), which is controlled in part by NF-κB, a key transcriptional regulator of inflammation. A unique aspect of this work is the use of the RNA polymerase II (Pol II) inhibitor α-amanitin, which we used to define a set of RNA polymerase I and III (Pol I and Pol III) transcripts.ResultsUsing groHMM, we identified ~30,000 coding and non-coding transcribed regions in AC16 cells, which includes a set of unique Pol I and Pol III primary transcripts. Many of these transcripts have not been annotated previously, including enhancer RNAs originating from NF-κB binding sites. In addition, we observed that AC16 cells rapidly and dynamically reorganize their transcriptomes in response to TNFα stimulation in an NF-κB-dependent manner, switching from a basal state to a proinflammatory state affecting a spectrum of cardiac-associated protein-coding and non-coding genes. Moreover, we observed distinct Pol II dynamics for up- and downregulated genes, with a rapid release of Pol II into productive elongation for TNFα-stimulated genes. As expected, the TNFα-induced changes in the AC16 transcriptome resulted in corresponding changes in cognate mRNA and protein levels in a similar manner, but with delayed kinetics.ConclusionsOur studies illustrate how computational genomics can be used to characterize the signal-regulated transcriptome in biologically relevant cell types, providing new information about how the human genome is organized, transcribed and regulated. In addition, they show how α-amanitin can be used to reveal the Pol I and Pol III transcriptome. Furthermore, they shed new light on the regulation of the cardiomyocyte transcriptome in response to a proinflammatory signal and help to clarify the link between inflammation and cardiomyocyte function at the transcriptional level.

Computational Investigation into Female Susceptibility to Arousal Induced Arrhythmias in Long-Qt Syndrome Type 2

Auditory stimuli are a recognized trigger of arousal-induced torsade de pointes (TdP) in patients with Long QT Syndrome Type 2 (LQT2). There is also clear evidence that the effect of auditory stimulation to modulate ventricular repolarization is more pronounced in women than men, and that female sex is an independent risk factor for long-QT syndrome and increased propensity for torsade de pointes arrhythmias. Indeed, females with LQT2 experience a greater than nine-fold increase in the risk for arousal-triggered cardiac events compared with men in the same age group after the onset of adolescence. In order to reveal the interactions and contributions of rapid bursts of sympathetic activity, sex steroid hormones and differences in ion channels expression in male and female hearts, we have developed a model of the female cardiac cell that includes sex stereoid hormones and experimentally observed reduction in expression of Connexin 43 (Cx43) and repolarization currents IK1, IKr, IKs, and Ito compared to males. These changes were incorporated into the O'Hara-Rudy human ventricular cell model. In order to account for sympathetically mediated arousal, we also incorporated subcellular signaling pathways linked to beta-adrenergic receptors. Finally, Long-QT 2 mutations were incorporated into the model. The model simulations predict that the high estrogen levels and low progesterone levels that occur during the follicular phase of the menstrual cycle in females and the interplay between larger net inward currents set the stage for action potential prolongation that in combination with sudden bursts of beta-adrenergic signaling leads to a profoundly increased likelihood of arrhythmia triggers in females compared to males in singe cells and 1-D tissue. The model also predicts a high degree of protection by testosterone in males against arrhythmia triggering.

The variant hERG/R148W associated with LQTS is a mutation that reduces current density on co-expression with the WT

BackgroundA variant of the ether-à-go-go related channel (hERG), p.Arg148Trp (R148W) was found at heterozygous state in two infants who died from sudden infant death syndrome (SIDS), one with documented prolonged QTc and Torsade de Pointes (TdP), and in an adult woman with QTc >500ms, atrioventricular block and TdP. This variant was previously reported in cases of severe ventricular arrhythmia but very rarely in control subjects. Its classification as mutation or polymorphism awaited electrophysiological characterization. MethodsThe properties of this N-terminal, proximal domain, hERG variant were explored in Xenopus oocytes injected with the same amount of RNA encoding for either hERG/WT or hERG/R148W or their equimolar mixture. The human ventricular cell (TNNP) model was used to test the effects of changes in hERG current. ResultsR148W alone produced a current similar to the WT (369±76nA (mean±SEM), n=13 versus 342±55nA in WT, n=13), while the co-expression of 1/2 WT+1/2 R148W lowered the current by 29% versus WT (243±35nA, n=13, p<0.05). The voltage dependencies of steady-state activation and inactivation were not changed in the variant alone or in co-expression with the WT. The time constants of fast recovery from inactivation and of fast and slow deactivation analyzed between −120 and +20mV were not changed. The voltage-dependent distribution of the current amplitudes among fast-, slow- and non-deactivating fractions was unaltered. A 6.6% increase in APD90 from 323.5ms to 345ms was observed using the human cardiac ventricular myocyte model. ConclusionsSuch a decrease in hERG current as evidenced here when co-expressing the hERG/R148W variant with the WT may have predisposed to the observed long QT syndrome and associated TdP. Therefore, the heterozygous carriers of hERG/R148W may be at risk of cardiac sudden death.

Computational Optogenetics: Empirically-Derived Voltage- and Light-Sensitive Channelrhodopsin-2 Model

Channelrhodospin-2 (ChR2), a light-sensitive ion channel, and its variants have emerged as new excitatory optogenetic tools not only in neuroscience, but also in other areas, including cardiac electrophysiology. An accurate quantitative model of ChR2 is necessary for in silico prediction of the response to optical stimulation in realistic tissue/organ settings. Such a model can guide the rational design of new ion channel functionality tailored to different cell types/tissues. Focusing on one of the most widely used ChR2 mutants (H134R) with enhanced current, we collected a comprehensive experimental data set of the response of this ion channel to different irradiances and voltages, and used these data to develop a model of ChR2 with empirically-derived voltage- and irradiance- dependence, where parameters were fine-tuned via simulated annealing optimization. This ChR2 model offers: 1) accurate inward rectification in the current-voltage response across irradiances; 2) empirically-derived voltage- and light-dependent kinetics (activation, deactivation and recovery from inactivation); and 3) accurate amplitude and morphology of the response across voltage and irradiance settings. Temperature-scaling factors (Q10) were derived and model kinetics was adjusted to physiological temperatures. Using optical action potential clamp, we experimentally validated model-predicted ChR2 behavior in guinea pig ventricular myocytes. The model was then incorporated in a variety of cardiac myocytes, including human ventricular, atrial and Purkinje cell models. We demonstrate the ability of ChR2 to trigger action potentials in human cardiomyocytes at relatively low light levels, as well as the differential response of these cells to light, with the Purkinje cells being most easily excitable and ventricular cells requiring the highest irradiance at all pulse durations. This new experimentally-validated ChR2 model will facilitate virtual experimentation in neural and cardiac optogenetics at the cell and organ level and provide guidance for the development of in vivo tools.

Mitotic spindle orientation predicts outer radial glial cell generation in human neocortex

The human neocortex is increased in size and complexity as compared to most other species. Neocortical expansion has recently been attributed to protracted neurogenesis by outer radial glial (oRG) cells in the outer subventricular zone (oSVZ), a region present in humans but not in rodents. The mechanisms of human oRG cell generation are unknown, but are proposed to involve division of ventricular radial glial (vRG) cells; neural stem cells present in all developing mammals. Here we show that human vRG cells produce oRG cells and seed formation of the oSVZ via horizontal divisions, which occur more frequently in humans than in rodents. We further find that oRG cell mitotic behavior is cell intrinsic, and that the basal fiber, inherited by oRG cells after vRG division, determines cleavage angle. Our results suggest that altered regulation of mitotic spindle orientation increased oRG cell numbers, and ultimately neuronal numbers, during human brain evolution.

Antenatal architecture and activity of the human heart

We construct the components for a family of computational models of the electrophysiology of the human foetal heart from 60 days gestational age (DGA) to full term. This requires both cell excitation models that reconstruct the myocyte action potentials, and datasets of cardiac geometry and architecture. Fast low-angle shot and diffusion tensor magnetic resonance imaging (DT-MRI) of foetal hearts provides cardiac geometry with voxel resolution of approximately 100 µm. DT-MRI measures the relative diffusion of protons and provides a measure of the average intravoxel myocyte orientation, and the orientation of any higher order orthotropic organization of the tissue. Such orthotropic organization in the adult mammalian heart has been identified with myocardial sheets and cleavage planes between them. During gestation, the architecture of the human ventricular wall changes from being irregular and isotropic at 100 DGA to an anisotropic and orthotropic architecture by 140 DGA, when it has the smooth, approximately 120° transmural change in myocyte orientation that is characteristic of the adult mammalian ventricle. The DT obtained from DT-MRI provides the conductivity tensor that determines the spread of potential within computational models of cardiac tissue electrophysiology. The foetal electrocardiogram (fECG) can be recorded from approximately 60 DGA, and RR, PR and QT intervals between the P, R, Q and T waves of the fECG can be extracted by averaging from approximately 90 DGA. The RR intervals provide a measure of the pacemaker rate, the QT intervals an index of ventricular action potential duration, and its rate-dependence, and so these intervals constrain and inform models of cell electrophysiology. The parameters of models of adult human sinostrial node and ventricular cells that are based on adult cell electrophysiology and tissue molecular mapping have been modified to construct preliminary models of foetal cell electrophysiology, which reproduce these intervals from fECG recordings. The PR and QR intervals provide an index of conduction times, and hence propagation velocities (approx. 1-10 cm s(-1), increasing during gestation) and so inform models of tissue electrophysiology. Although the developing foetal heart is small and the cells are weakly coupled, it can support potentially lethal re-entrant arrhythmia.

Upregulated autophagy protects cardiomyocytes from oxidative stress-induced toxicity

Autophagy is a cellular self-digestion process that mediates protein quality control and serves to protect against neurodegenerative disorders, infections, inflammatory diseases and cancer. Current evidence suggests that autophagy can selectively remove damaged organelles such as the mitochondria. Mitochondria-induced oxidative stress has been shown to play a major role in a wide range of pathologies in several organs, including the heart. Few studies have investigated whether enhanced autophagy can offer protection against mitochondrially-generated oxidative stress. We induced mitochondrial stress in cardiomyocytes using antimycin A (AMA), which increased mitochondrial superoxide generation, decreased mitochondrial membrane potential and depressed cellular respiration. In addition, AMA augmented nuclear DNA oxidation and cell death in cardiomyocytes. Interestingly, although oxidative stress has been proposed to induce autophagy, treatment with AMA did not result in stimulation of autophagy or mitophagy in cardiomyocytes. Our results showed that the MTOR inhibitor rapamycin induced autophagy, promoted mitochondrial clearance and protected cardiomyocytes from the cytotoxic effects of AMA, as assessed by apoptotic marker activation and viability assays in both mouse atrial HL-1 cardiomyocytes and human ventricular AC16 cells. Importantly, rapamycin improved mitochondrial function, as determined by cellular respiration, mitochondrial membrane potential and morphology analysis. Furthermore, autophagy induction by rapamycin suppressed the accumulation of ubiquitinylated proteins induced by AMA. Inhibition of rapamycin-induced autophagy by pharmacological or genetic interventions attenuated the cytoprotective effects of rapamycin against AMA. We propose that rapamycin offers cytoprotection against oxidative stress by a combined approach of removing dysfunctional mitochondria as well as by degrading damaged, ubiquitinated proteins. We conclude that autophagy induction by rapamycin could be utilized as a potential therapeutic strategy against oxidative stress-mediated damage in cardiomyocytes.

Sudden arrhythmic death: from basic science to clinical practice

EDITORIAL article Front. Physiol., 25 November 2013Sec. Cardiac Electrophysiology Volume 4 - 2013 | https://doi.org/10.3389/fphys.2013.00339

Determination of Adequate Defibrillation of Ventricular Tachycardia Using Computer Simulation of Electroporation Aftershock Effects on Human Cardiomyocytes

Electric defibrillation is a life-saving therapy for clinically dangerous ventricular arrhythmias. Unfortunately, direct-current (DC) shock treatment often induces or worsens other tachyarrhythmias. The adverse effects of electric shock delivered to human ventricular cardiomyocytes are not fully understood. This study thus uses computer simulation to investigate the pathogenesis of the electrophoresis in epicardial and endocardial human tissue. The O'Hara-Rudy dynamic human ventricular cell model incorporated with Ohuchi's mathematical DC shock model is used. The effect of electroporation is described as a transmembrane pore, mimicking the reversible breakdown of the cell membrane. The aftershock effects on epiardial and endocardial ventricular cardiomyocytes are evaluated. The effects of delivering shock from the endocardium and epicardium using a multicellular one-dimensional strand model are also investigated. Ventricular tachycardia can be terminated by a low-strength shock to endocardial cardiomyocytes. However, an excessively strong shock to epicardial cardiomyocytes induces early afterdepolarization and is arrhythmogenic. The aftershock effect of the electroporation is serious for epicardial ventricular myocytes. The optimization of defibrillation energy delivery to the maximum membrane potential and the pathogenesis of the aftershock effect during ventricular tachycardia are also investigated. The aftershock effect of electroporation is more serious in epicardial cells than it is in endocardial cells. It is suggested that the DC shock be delivered to endocardium cells before the maximum membrane potential of the epicardial cell in a multicellular one-dimensional strand model is reached, which is before the electrocardiogram R wave spike.