Electrical remodeling in cardiac hypertrophy

Engineered small extracellular vesicle-mediated NOX4 siRNA delivery for targeted therapy of cardiac hypertrophy

Small-interfering RNA (siRNA) therapy is considered a powerful therapeutic strategy for treating cardiac hypertrophy, an important risk factor for subsequent cardiac morbidity and mortality. However, the lack of safe and efficient in vivo delivery of siRNAs is a major challenge for broadening its clinical applications. Small extracellular vesicles (sEVs) are a promising delivery system for siRNAs but have limited cell/tissue-specific targeting ability. In this study, a new generation of heart-targeting sEVs (CEVs) has been developed by conjugating cardiac-targeting peptide (CTP) to human peripheral blood-derived sEVs (PB-EVs), using a simple, rapid and scalable method based on bio-orthogonal copper-free click chemistry. The experimental results show that CEVs have typical sEVs properties and excellent heart-targeting ability. Furthermore, to treat cardiac hypertrophy, CEVs are loaded with NADPH Oxidase 4 (NOX4) siRNA (siNOX4). Consequently, CEVs@siNOX4 treatment enhances the in vitro anti-hypertrophic effects by CEVs with siRNA protection and heart-targeting ability. In addition, the intravenous injection of CEVs@siNOX4 into angiotensin II (Ang II)-treated mice significantly improves cardiac function and reduces fibrosis and cardiomyocyte cross-sectional area, with limited side effects. In conclusion, the utilization of CEVs represents an efficient strategy for heart-targeted delivery of therapeutic siRNAs and holds great promise for the treatment of cardiac hypertrophy.

Prognostic implications of structural heart disease and premature ventricular contractions in recovery of exercise

Premature ventricular contractions (PVCs) during recovery of exercise stress testing are associated with increased cardiovascular mortality, but the cause remains unknown. We aimed to evaluate the association of PVCs during recovery with echocardiographic abnormalities, and their combined prognostic performance. Echocardiographic abnormalities [reduced left ventricular (LV) ejection fraction, valvular heart disease, LV dilatation, LV hypertrophy, or increased filling pressures] and PVCs during recovery were identified among patients having undergone both echocardiography and exercise stress test. Among included patients (n = 3106, age 59 ± 16 years, 55% males), PVCs during recovery were found in 1327 (43%) patients, among which the prevalence of echocardiographic abnormalities was increased (58% vs. 43%, p < 0.001). Overall, PVCs during recovery were associated with increased cardiovascular mortality (219 total events, 7.9 [5.4–11.1] years follow-up; adjusted hazard ratio (HR [95% confidence interval]) 1.6 [1.2–2.1], p < 0.001). When analyzed in combination with either presence or absence of echocardiographic abnormalities, PVCs during recovery were associated with increased risk when such were present (HR 3.3 [1.9–5.5], p < 0.001) but not when absent (HR 1.5 [0.8–2.8], p = 0.22), in reference to those with neither. Our findings provide mechanistic insights to the increased CV risk reported in patients with PVCs during recovery.

ECG in left ventricular hypertrophy: A change in paradigm from assessing left ventricular mass to its electrophysiological properties

Left ventricular hypertrophy (LVH) detected electrocardiographically is documented as an independent cardiovascular risk factor. However, the reasoning for using electrocardiography (ECG) for LVH detection is frequently referring to its low cost and availability, which should compensate for the main problem of the ECG criteria for LVH detection (ECG-LVH) - the high number of ECG false negative results and the resulting low sensitivity. This opinion paper is focused on the scientific evidence for advocating the usefulness of ECG in LVH assessment. The classical paradigm assumes that the increased left ventricular mass generates a stronger electrical field that has to be reflected in the increased QRS amplitude. However, the solid angle theorem postulates that the recorded ECG voltage depends not only on the extent of the activation front that is increased in LVH, but also on the electrical characteristics of myocardium. There is an accumulated evidence from animal and clinical studies documenting significant alterations of structural and functional properties of hypertrophied myocardium, both of cardiomyocytes as well as of interstitium. These alterations are associated with significant changes of active and passive electrical properties of myocardium modifying the resultant QRS amplitudes. The new paradigm should consider the altered electrical properties of hypertrophied myocardium in interpreting the whole spectrum of QRS patterns seen in LVH patients: the increased QRS voltage, the QRS voltage within normal limits, occurrence of left axis deviation and left bundle branch block. Thus further research is necessary for utilizing the unique diagnostic information provided by ECG: to link the agreements as well as the disagreements between ECG and imaging methods findings to pathophysiological processes and patho-anatomical backgrounds, to the risk assessment and the clinical status of patients with LVH.

Effects of Trandolapril on Structural, Contractile and Electrophysiological Remodeling in Experimental Volume Overload Heart Failure

Chronic volume overload induces multiple cardiac remodeling processes that finally result in eccentric cardiac hypertrophy and heart failure. We have hypothesized that chronic angiotensin-converting enzyme (ACE) inhibition by trandolapril might affect various remodeling processes differentially, thus allowing their dissociation. Cardiac remodeling due to chronic volume overload and the effects of trandolapril were investigated in rats with an aortocaval fistula (ACF rats). The aortocaval shunt was created using a needle technique and progression of cardiac remodeling to heart failure was followed for 24 weeks. In ACF rats, pronounced eccentric cardiac hypertrophy and contractile and proarrhythmic electrical remodeling were associated with increased mortality. Trandolapril substantially reduced the electrical proarrhythmic remodeling and mortality, whereas the effect on cardiac hypertrophy was less pronounced and significant eccentric hypertrophy was preserved. Effective suppression of electrical proarrhythmic remodeling and mortality but not hypertrophy indicates that the beneficial therapeutic effects of ACE inhibitor trandolapril in volume overload heart failure might be dissociated from pure antihypertrophic effects.

Scaffolds and Extracellular Vesicles as a Promising Approach for Cardiac Regeneration after Myocardial Infarction

Clinical studies have demonstrated the regenerative potential of stem cells for cardiac repair over the past decades, but their widespread use is limited by the poor tissue integration and survival obtained. Natural or synthetic hydrogels or microcarriers, used as cell carriers, contribute to resolving, in part, the problems encountered by providing mechanical support for the cells allowing cell retention, survival and tissue integration. Moreover, hydrogels alone also possess mechanical protective properties for the ischemic heart. The combined effect of growth factors with cells and an appropriate scaffold allow a therapeutic effect on myocardial repair. Despite this, the effects obtained with cell therapy remain limited and seem to be equivalent to the effects obtained with extracellular vesicles, key actors in intercellular communication. Extracellular vesicles have cardioprotective effects which, when combined proangiogenic properties with antiapoptotic and anti-inflammatory actions, make it possible to act on all the damages caused by ischemia. The evolution of biomaterial engineering allows us to envisage their association with new major players in cardiac therapy, extracellular vesicles, in order to limit undesirable effects and to envisage a transfer to the clinic. This new therapeutic approach could be associated with the release of growth factors to potentialized the beneficial effect obtained.

Missing Link between Molecular Aspects of Ventricular Arrhythmias and QRS Complex Morphology in Left Ventricular Hypertrophy

The aim of this opinion paper is to point out the knowledge gap between evidence on the molecular level and clinical diagnostic possibilities in left ventricular hypertrophy (LVH) regarding the prediction of ventricular arrhythmias and monitoring the effect of therapy. LVH is defined as an increase in left ventricular size and is associated with increased occurrence of ventricular arrhythmia. Hypertrophic rebuilding of myocardium comprises interrelated processes on molecular, subcellular, cellular, tissue, and organ levels affecting electrogenesis, creating a substrate for triggering and maintaining arrhythmias. The knowledge of these processes serves as a basis for developing targeted therapy to prevent and treat arrhythmias. In the clinical practice, the method for recording electrical phenomena of the heart is electrocardiography. The recognized clinical electrocardiogram (ECG) predictors of ventricular arrhythmias are related to alterations in electrical impulse propagation, such as QRS complex duration, QT interval, early repolarization, late potentials, and fragmented QRS, and they are not specific for LVH. However, the simulation studies have shown that the QRS complex patterns documented in patients with LVH are also conditioned remarkably by the alterations in impulse propagation. These QRS complex patterns in LVH could be potentially recognized for predicting ventricular arrhythmia and for monitoring the effect of therapy.

Pathophysiology of Calcium Mediated Ventricular Arrhythmias and Novel Therapeutic Options with Focus on Gene Therapy

Cardiac arrhythmias constitute a major health problem with a huge impact on mortality rates and health care costs. Despite ongoing research efforts, the understanding of the molecular mechanisms and processes responsible for arrhythmogenesis remains incomplete. Given the crucial role of Ca²⁺-handling in action potential generation and cardiac contraction, Ca²⁺ channels and Ca²⁺ handling proteins represent promising targets for suppression of ventricular arrhythmias. Accordingly, we report the different roles of Ca²⁺-handling in the development of congenital as well as acquired ventricular arrhythmia syndromes. We highlight the therapeutic potential of gene therapy as a novel and innovative approach for future arrhythmia therapy. Furthermore, we discuss various promising cellular and mitochondrial targets for therapeutic gene transfer currently under investigation.

Understanding Key Mechanisms of Exercise-Induced Cardiac Protection to Mitigate Disease: Current Knowledge and Emerging Concepts

The benefits of exercise on the heart are well recognized, and clinical studies have demonstrated that exercise is an intervention that can improve cardiac function in heart failure patients. This has led to significant research into understanding the key mechanisms responsible for exercise-induced cardiac protection. Here, we summarize molecular mechanisms that regulate exercise-induced cardiac myocyte growth and proliferation. We discuss in detail the effects of exercise on other cardiac cells, organelles, and systems that have received less or little attention and require further investigation. This includes cardiac excitation and contraction, mitochondrial adaptations, cellular stress responses to promote survival (heat shock response, ubiquitin-proteasome system, autophagy-lysosomal system, endoplasmic reticulum unfolded protein response, DNA damage response), extracellular matrix, inflammatory response, and organ-to-organ crosstalk. We summarize therapeutic strategies targeting known regulators of exercise-induced protection and the challenges translating findings from bench to bedside. We conclude that technological advancements that allow for in-depth profiling of the genome, transcriptome, proteome and metabolome, combined with animal and human studies, provide new opportunities for comprehensively defining the signaling and regulatory aspects of cell/organelle functions that underpin the protective properties of exercise. This is likely to lead to the identification of novel biomarkers and therapeutic targets for heart disease.

Peroxisome proliferator-activated receptor-γ coactivator 1 α1 induces a cardiac excitation-contraction coupling phenotype without metabolic remodelling

KEY POINTS

Transcriptional co-activator PGC-1α1 has been shown to regulate energy metabolism and to mediate metabolic adaptations in pathological and physiological cardiac hypertrophy but other functional implications of PGC-1α1 expression are not known. Transgenic PGC-1α1 overexpression within the physiological range in mouse heart induces purposive changes in contractile properties, electrophysiology and calcium signalling but does not induce substantial metabolic remodelling. The phenotype of the PGC-1α1 transgenic mouse heart recapitulates most of the functional modifications usually associated with the exercise-induced heart phenotype, but does not protect the heart against load-induced pathological hypertrophy. Transcriptional effects of PGC-1α1 show clear dose-dependence with diverse changes in genes in circadian clock, heat shock, excitability, calcium signalling and contraction pathways at low overexpression levels, while metabolic genes are recruited at much higher PGC-1α1 expression levels. These results imply that the physiological role of PGC-1α1 is to promote a beneficial excitation-contraction coupling phenotype in the heart.

ABSTRACT

The transcriptional coactivator PGC-1α1 has been identified as a central factor mediating metabolic adaptations of the heart. However, to what extent physiological changes in PGC-1α1 expression levels actually contribute to the functional adaptation of the heart is still mostly unresolved. The aim of this study was to characterize the transcriptional and functional effects of physiologically relevant, moderate PGC-1α1 expression in the heart. In vivo and ex vivo physiological analysis shows that expression of PGC-1α1 within a physiological range in mouse heart does not induce the expected metabolic alterations, but instead induces a unique excitation-contraction (EC) coupling phenotype recapitulating features typically seen in physiological hypertrophy. Transcriptional screening of PGC-1α1 overexpressing mouse heart and myocyte cultures with higher, acute adenovirus-induced PGC-1α1 expression, highlights PGC-1α1 as a transcriptional coactivator with a number of binding partners in various pathways (such as heat shock factors and the circadian clock) through which it acts as a pleiotropic transcriptional regulator in the heart, to both augment and repress the expression of its target genes in a dose-dependent fashion. At low levels of overexpression PGC-1α1 elicits a diverse transcriptional response altering the expression state of circadian clock, heat shock, excitability, calcium signalling and contraction pathways, while metabolic targets of PGC-1α1 are recruited at higher PGC-1α1 expression levels. Together these findings demonstrate that PGC-1α1 elicits a dual effect on cardiac transcription and phenotype. Further, our results imply that the physiological role of PGC-1α1 is to promote a beneficial EC coupling phenotype in the heart.

Vascular Endothelial Growth Factor-B Induces a Distinct Electrophysiological Phenotype in Mouse Heart

Vascular endothelial growth factor B (VEGF-B) is a potent mediator of vascular, metabolic, growth, and stress responses in the heart, but the effects on cardiac muscle and cardiomyocyte function are not known. The purpose of this study was to assess the effects of VEGF-B on the energy metabolism, contractile, and electrophysiological properties of mouse cardiac muscle and cardiac muscle cells. In vivo and ex vivo analysis of cardiac-specific VEGF-B TG mice indicated that the contractile function of the TG hearts was normal. Neither the oxidative metabolism of isolated TG cardiomyocytes nor their energy substrate preference showed any difference to WT cardiomyocytes. Similarly, myocyte Ca²⁺ signaling showed only minor changes compared to WT myocytes. However, VEGF-B overexpression induced a distinct electrophysiological phenotype characterized by ECG changes such as an increase in QRSp time and decreases in S and R amplitudes. At the level of isolated TG cardiomyocytes, these changes were accompanied with decreased action potential upstroke velocity and increased duration (APD_60–70). These changes were partly caused by downregulation of sodium current (I_Na) due to reduced expression of Na_v1.5. Furthermore, TG myocytes had alterations in voltage-gated K⁺ currents, namely decreased density of transient outward current (I_to) and total K⁺ current (I_peak). At the level of transcription, these were accompanied by downregulation of K_v channel-interacting protein 2 (Kcnip2), a known modulatory subunit for K_v4.2/3 channel. Cardiac VEGF-B overexpression induces a distinct electrophysiological phenotype including remodeling of cardiomyocyte ion currents, which in turn induce changes in action potential waveform and ECG.

Protection of cardiac cell-to-cell coupling attenuate myocardial remodeling and proarrhythmia induced by hypertension

Gap junction connexin channels are important determinants of myocardial conduction and synchronization that is crucial for coordinated heart function. One of the main risk factors for cardiovascular events that results in heart attack, congestive heart failure, stroke as well as sudden arrhythmic death is hypertension. Mislocalization and/or dysfunction of specific connexin-43 channels due to hypertension-induced myocardial remodeling have been implicated in the occurrence of life-threatening arrhythmias and heart failure in both, humans as well as experimental animals. Recent studies suggest that down-regulation of myocardial connexin-43, its abnormal distribution and/or phosphorylation might be implicated in this process. On the other hand, treatment of hypertensive animals with cardioprotective drugs (e.g. statins) or supplementation with non-pharmacological compounds, such as melatonin, omega-3 fatty acids and red palm oil protects from lethal arrhythmias. The antiarrhythmic effects are attributed to the attenuation of myocardial connexin-43 abnormalities associated with preservation of myocardial architecture and improvement of cardiac conduction. Findings uncover novel mechanisms of cardioprotective (antihypertensive and antiarrhythmic) effects of compounds that are used in clinical settings. Well-designed trials are needed to explore the antiarrhythmic potential of these compounds in patients suffering from hypertension.

Limits of ventricular function: from athlete's heart to a failing heart

The interest in the study of ventricular function has grown considerably in the last decades. In this review, we analyse the extreme values of ventricular function as obtained with Doppler echocardiography. We mainly focus on the parameters that have been used throughout the history of Doppler echocardiography to assess left ventricular (LV) systolic and diastolic function. The 'athlete's heart' would be the highest expression of ventricular function whereas its lowest expression is represented by the failing heart, independently from the original aetiology leading to this condition. There are, however, morphological similarities (dilation and hypertrophy) between the athlete's and the failing heart, which emerge as physiological and pathophysiological adaptations, respectively. The introduction of new assessment techniques, specifically speckle tracking, may provide new insight into the properties that determine ventricular filling, specifically left ventricular twisting. The concept of ventricular function must be always considered, although it may not be always possible to distinguish the normal heart of sedentary individuals from that of highly trained hearts based solely on echocardiographic or basic studies.

Myocardial remodeling and bioelectric changes in tachycardia-induced heart failure in dogs

In this study, electrical and structural remodeling of ventricles was examined in tachycardia-induced heart failure (HF). We studied two groups of weight-matched adult male mongrel dogs: a sham-operated control group (n=5) and a pacing group (n=5) that underwent ventricular pacing at 230 bpm for 3 weeks. Clinical symptoms of congestive HF were observed in both groups. Their hemodynamic parameters were determined and the severity of the HF was evaluated by M-mode echocardiography. Changes in heart morphology were observed by scanning electron and light microscopy. Ventricular action potential duration (APD), as well as the 50 and 90% APD were measured in both groups. All dogs exhibited clinical symptoms of congestive HF after rapid right ventricular pacing for 3 weeks. These data indicate that rapid, right ventricular pacing produces a useful experimental model of low-output HF in dogs, characterized by biventricular pump dysfunction, biventricular cardiac dilation, and non-ischemic impairment of left ventricular contractility. Electrical and structural myocardial remodeling play an essential role in congestive HF progression, and should thus be prevented.

Left ventricular hypertrophy: The relationship between the electrocardiogram and cardiovascular magnetic resonance imaging

Conventional assessment of left ventricular hypertrophy (LVH) using the electrocardiogram (ECG), for example, by the Sokolow-Lyon, Romhilt-Estes or Cornell criteria, have relied on assessing changes in the amplitude and/or duration of the QRS complex of the ECG to quantify LV mass. ECG measures of LV mass have typically been validated by imaging with echocardiography or cardiovascular magnetic resonance imaging (CMR). However, LVH can be the result of diverse etiologies, and LVH is also characterized by pathological changes in myocardial tissue characteristics on the genetic, molecular, cellular, and tissue level beyond a pure increase in the number of otherwise normal cardiomyocytes. For example, slowed conduction velocity through the myocardium, which can be due to diffuse myocardial fibrosis, has been shown to be an important determinant of conventional ECG LVH criteria regardless of LV mass. Myocardial tissue characterization by CMR has emerged to not only quantify LV mass, but also detect and quantify the extent and severity of focal or diffuse myocardial fibrosis, edema, inflammation, myocarditis, fatty replacement, myocardial disarray, and myocardial deposition of amyloid proteins (amyloidosis), glycolipids (Fabry disease), or iron (siderosis). This can be undertaken using CMR techniques including late gadolinium enhancement (LGE), T1 mapping, T2 mapping, T2* mapping, extracellular volume fraction (ECV) mapping, fat/water-weighted imaging, and diffusion tensor CMR. This review presents an overview of current and emerging concepts regarding the diagnostic possibilities of both ECG and CMR for LVH in an attempt to narrow gaps in our knowledge regarding the ECG diagnosis of LVH.

Dietary omega-3 fatty acids attenuate myocardial arrhythmogenic factors and propensity of the heart to lethal arrhythmias in a rodent model of human essential hypertension

OBJECTIVE

Hypertension-induced myocardial remodeling is known to be associated with increased risk for malignant arrhythmias and alterations in electrical coupling protein, connexin-43 (Cx43), may be involved. We investigated whether omega-3 fatty acids intake affects abnormalities of Cx43 as well as protein kinase C (PKC) signaling and myosin heavy chain (MyHC) profile at the early and late stage of hypertension in the context of the heart's susceptibility to ventricular fibrillation and ability to restore sinus rhythm.

METHODS

Untreated young and old male spontaneously hypertensive rats (SHRs) and age-matched normotensive rats were compared with animals supplemented by omega-3 (eicosapentaneoic acid + docosahexaneoic acid, 200 mg/kg body weight/day) for 2 months. Left ventricular tissues were taken for examination of subcellular integrity of gap junctions, Cx43 mRNA and protein expression, PKCε and PKCδ as well as MyHC determination. Electrically inducible ventricular fibrillation and sinus rhythm restoration (SRR) were examined on Langedorff-perfused heart preparation.

RESULTS

Omega-3 intake significantly reduced cardiovascular risk factors, suppressed inducible ventricular fibrillation, and facilitated SRR in hypertensive rats. Supplementation attenuated lateralization and internalization of Cx43, suppressed elevated Cx43 mRNA, enhanced total Cx43 protein expression and/or expression of its functional phosphorylated forms as well as the expression of cardioprotective PKC-ε and suppressed pro-apoptotic PKC-δ isoform. Moreover, the omega-3 diet normalized MyHC profiles in SHR at early stage of disease and old nonhypertensive rats, but failed to do so in old SHR at late stage of disease.

CONCLUSION

Findings suggest that amelioration of myocardial Cx43-related abnormalities, positive modulation of PKC pathways, and normalization of MyHC can significantly contribute to the antiarrhythmic effects of omega-3 in rat model mimicking human essential hypertension. Our results support the prophylactic use of omega-3 to minimize cardiovascular risk and sudden arrhythmic death.

Gene therapies for arrhythmias in heart failure

In this article, we review recent advances in our understanding of arrhythmia mechanisms in the failing heart. We focus on changes in repolarization, conduction, and intracellular calcium cycling because of their importance to the vast majority of clinical arrhythmias in heart failure. We highlight recent efforts to combat arrhythmias using gene-based approaches that target ion channel, gap junction, and calcium cycling proteins. We further discuss the advantages and limitations associated with individual approaches.

Pathological ventricular remodeling: mechanisms: part 1 of 2

Despite declines in heart failure morbidity and mortality with current therapies, rehospitalization rates remain distressingly high, substantially affecting individuals, society, and the economy. As a result, the need for new therapeutic advances and novel medical devices is urgent. Disease-related left ventricular remodeling is a complex process involving cardiac myocyte growth and death, vascular rarefaction, fibrosis, inflammation, and electrophysiological remodeling. Because these events are highly interrelated, targeting a single molecule or process may not be sufficient. Here, we review molecular and cellular mechanisms governing pathological ventricular remodeling.

Mitochondrial function in cardiac hypertrophy

Cardiac hypertrophic program is a chronic, complex process, and occurs in response to long-term increases of hemodynamic load related to a variety of pathophysiological conditions. Mitochondria, known as "the cellular power plants", occupy about one-third of cardiomyocyte volume and supply roughly 90% of the adenosine triphosphate (ATP). Impairment of energy metabolism has been regarded as one of the main pathogenesis of cardiac hypertrophy. Thus, we summarize here the molecular events of mitochondrial adaptations, including the mitochondrial genesis, ATP generation, ROS signaling and Ca(2+) homeostasis in cardiac hypertrophy, expecting that this effort will shed new light on understanding the maladaptive cardiac remodeling.

Hypertrophic reprogramming of the left ventricle: translation to the ECG

Hypertrophic growth of the heart occurs in many clinical scenarios, and it confers substantially increased risk of untoward sequelae. Among them, transition to ventricular dilation, wall thinning, contractile dysfunction, and a clinical syndrome of heart failure are paramount. Left ventricular hypertrophy (LVH) is typically diagnosed by either electrocardiography or echocardiography. However, these two means of assessing hypertrophic transformation of the left ventricle can sometimes disagree. At one level, this may not be surprising as the two methodologies are based on entirely divergent signals: electrical potential between two places on the surface of the skin and ultrasound energy reflected from the ventricle itself. Echocardiography is an effective means of assessing ventricular mass, which is a cardinal feature of LVH. Importantly, however, LVH is characterized by a wide range of remodeling events beyond simple increases in muscle mass. Electrocardiographic changes in LVH are reflective of the electrophysiological aspects of hypertrophic transformation. Here, I present an overview of the complex biology of left ventricular hypertrophy with an eye toward enhancing our understanding of its ECG manifestations.

N-acetylcysteine prevents electrical remodeling and attenuates cellular hypertrophy in epicardial myocytes of rats with ascending aortic stenosis

Pressure overload is associated with cardiac hypertrophy and electrical remodeling. Here, we investigate the effects of the antioxidant N-acetylcysteine (NAC) on the cellular cardiac electrophysiology of female Sprague-Dawley rats with ascending aortic stenosis (AS). Rats were treated with NAC (1 g/kg body weight) or control solution 1 week before the intervention and in the week following AS or sham operation. Seven days after the operation, blood pressure and left ventricular pressure were measured before the heart was excised. Single cells were isolated from epicardial and endocardial layers of the left ventricular free wall and investigated using the whole-cell patch-clamp technique. Systolic blood pressure and left ventricular peak pressure were not significantly altered in the NAC group. NAC reduced the increase (p < 0.001) in the relative left ventricular weight (p < 0.05) as well as the increase (p < 0.001) in cell capacitance in epicardial (p < 0.05), but not in endocardial myocytes of AS animals. The L-type Ca(2+) current (I (CaL)) was significantly increased by AS in epicardial (+19 % at 0 mV, p < 0.01) but not in endocardial myocytes. NAC completely prevented this increase in I (CaL) (p < 0.01). The current density of the transient outward K(+) current (I (to)) was not affected by AS or NAC. Action potential duration to 90 % repolarization was significantly prolonged in epicardial (p < 0.01) as well as in endocardial (p < 0.001) cells of AS animals. NAC prevented the AP prolongation in epicardial myocytes only (p < 0.05). We conclude that reducing oxidative stress in pressure overload can prevent electrical remodeling and ameliorate hypertrophy in epicardial but not in endocardial myocytes.

Mechanisms of ventricular arrhythmias: a dynamical systems-based perspective

Defining the cellular electrophysiological mechanisms for ventricular tachyarrhythmias is difficult, given the wide array of potential mechanisms, ranging from abnormal automaticity to various types of reentry and kk activity. The degree of difficulty is increased further by the fact that any particular mechanism may be influenced by the evolving ionic and anatomic environments associated with many forms of heart disease. Consequently, static measures of a single electrophysiological characteristic are unlikely to be useful in establishing mechanisms. Rather, the dynamics of the electrophysiological triggers and substrates that predispose to arrhythmia development need to be considered. Moreover, the dynamics need to be considered in the context of a system, one that displays certain predictable behaviors, but also one that may contain seemingly stochastic elements. It also is essential to recognize that even the predictable behaviors of this complex nonlinear system are subject to small changes in the state of the system at any given time. Here we briefly review some of the short-, medium-, and long-term alterations of the electrophysiological substrate that accompany myocardial disease and their potential impact on the initiation and maintenance of ventricular arrhythmias. We also provide examples of cases in which small changes in the electrophysiological substrate can result in rather large differences in arrhythmia outcome. These results suggest that an interrogation of cardiac electrical dynamics is required to provide a meaningful assessment of the immediate risk for arrhythmia development and for evaluating the effects of putative antiarrhythmic interventions.

Role of cardiac stem cells in cardiac pathophysiology: a paradigm shift in human myocardial biology

For nearly a century, the human heart has been viewed as a terminally differentiated postmitotic organ in which the number of cardiomyocytes is established at birth, and these cells persist throughout the lifespan of the organ and organism. However, the discovery that cardiac stem cells live in the heart and differentiate into the various cardiac cell lineages has changed profoundly our understanding of myocardial biology. Cardiac stem cells regulate myocyte turnover and condition myocardial recovery after injury. This novel information imposes a reconsideration of the mechanisms involved in myocardial aging and the progression of cardiac hypertrophy to heart failure. Similarly, the processes implicated in the adaptation of the infarcted heart have to be dissected in terms of the critical role that cardiac stem cells and myocyte regeneration play in the restoration of myocardial mass and ventricular function. Several categories of cardiac progenitors have been described but, thus far, the c-kit-positive cell is the only class of resident cells with the biological and functional properties of tissue specific adult stem cells.

Four genetic loci influencing electrocardiographic indices of left ventricular hypertrophy

BACKGROUND

Presence of left ventricular hypertrophy on an ECG (ECG-LVH) is widely assessed clinically and provides prognostic information in some settings. There is evidence for significant heritability of ECG-LVH. We conducted a large-scale gene-centric association analysis of 4 commonly measured indices of ECG-LVH.

METHODS AND RESULTS

We calculated the Sokolow-Lyon index, Cornell product, 12-lead QRS voltage sum, and 12-lead QRS voltage product in 10 256 individuals from 3 population-based cohorts and typed their DNA using a customized gene array (the Illumina HumanCVD BeadChip 50K array), containing 49 094 genetic variants in ≈2100 genes of cardiovascular relevance. We followed-up promising associations in 11 777 additional individuals. We identified and replicated 4 loci associated with ECG-LVH indices: 3p22.2 (SCN5A, rs6797133, P=1.22 × 10(-7)) with Cornell product and 12q13.3 (PTGES3, rs2290893, P=3.74 × 10(-8)), 15q25.2 (NMB, rs2292462, P=3.23 × 10(-9)), and 15q26.3 (IGF1R, rs4966014, P=1.26 × 10(-7)) with the 12-lead QRS voltage sum. The odds ratio of being in the top decile for the 12-lead QRS voltage sum for those carrying 6 trait-raising alleles at the 12q13.3, 15q25.2, and 15q26.3 loci versus those carrying 0 to 1 alleles was 1.60 (95% CI: 1.20 to 2.29). Lead single-nucleotide polymorphisms at the 12q13.3 and 15q25.2 loci showed significant expression quantitative trait loci effects in monocytes.

CONCLUSIONS

These findings provide novel insights into the genetic determination of ECG-LVH. The findings could help to improve our understanding of the mechanisms determining this prognostically important trait.

Diacylglycerol kinase ζ inhibits ventricular tachyarrhythmias in a mouse model of heart failure

BACKGROUND

Diacylglycerol kinase ζ (DGKζ) inhibited atrial tachyarrhythmias in a mouse model of heart failure (HF) in our study. However, whether DGKζ prevents the HF-induced ventricular tachyarrhythmia (VT) is unknown.

METHODS AND RESULTS

Effects of DGKζ on VT using transgenic mice with transient cardiac expression of activated G protein α(q) (Gα(q)-TG; model of HF) were elucidated and double transgenic mice with cardiac-specific overexpression of both DGKζ and the activated Gα(q) (Gα(q)/DGKζ-TG) were used. Premature ventricular contraction (PVC) and/or VT were frequently observed in Gα(q)-TG mice but not in Gα(q)/DGKζ-TG and wild-type (WT) mice (P<0.01). Protein expressions of canonical transient receptor potential (TRPC) channels 3 and 6 increased in Gα(q)-TG hearts compared with WT and Gα(q)/DGKζ-TG hearts. SK&F96365, a TRPC channel blocker, decreased the number of PVC and prevented VT in anesthetized Gα(q)-TG mice (P<0.05). 1-oleoyl-2-acyl-sn-glycerol (OAG), a diacylglycerol analogue, increased the number of PVC in isolated Gα(q)-TG hearts compared with WT hearts and induced VT in Gα(q)-TG hearts (P<0.01). SK&F96365 decreased the number of PVC and prevented VT in isolated Gα(q)-TG hearts (P<0.01) even in the presence of OAG. Early afterdepolarization (EAD)-induced triggered activity was frequently observed in single Gα(q)-TG ventricular myocytes. Moreover, SK&F96365 prevented the EAD.

CONCLUSIONS

These results demonstrated that DGKζ inhibited VT in a mouse model of HF and suggest that TRPC channels participate in VT induction in failing hearts.

Cardiac electrical remodeling in health and disease

Electrical remodeling of the heart takes place in response to both functional (altered electrical activation) and structural (including heart failure and myocardial infarction) stressors. These electrophysiological changes produce a substrate that is prone to malignant ventricular arrhythmias. Understanding the cellular and molecular mechanisms of electrical remodeling is important in elucidating potential therapeutic targets designed to alter maladaptive electrical remodeling. For example, altered patterns of electrical activation lead primarily to electrical remodeling, without significant structural remodeling. By contrast, secondary remodeling arises in response to a structural insult. In this article we review cardiac electrical remodeling (predominantly in the ventricle) with an emphasis on the mechanisms causing these adaptations. These mechanisms suggest novel therapeutic targets for the management or prevention of the most devastating manifestation of heart disease, sudden cardiac death (SCD).

Deciphering Arrhythmia Mechanisms - Tools of the Trade

Pathophysiological remodeling of cardiac function occurs at multiple levels, spanning the spectrum from molecular and sub-cellular changes to those occurring at the organ-system levels. Of key importance to arrhythmias are changes in electrophysiological and calcium handling properties at the tissue level. In this review, we discuss how high-resolution optical action potential and calcium transient imaging has advanced our understanding of basic arrhythmia mechanisms associated with multiple cardiovascular disorders, including the long QT syndrome, heart failure, and ischemia-reperfusion injury. We focus on the role of repolarization gradients (section 1) and calcium mediated triggers (section 2) in the initiation and maintenance of complex arrhythmias in these settings.

Hypertrophic phenotype in cardiac cell assemblies solely by structural cues and ensuing self-organization

In vitro models of cardiac hypertrophy focus exclusively on applying "external" dynamic signals (electrical, mechanical, and chemical) to achieve a hypertrophic state. In contrast, here we set out to demonstrate the role of "self-organized" cellular architecture and activity in reprogramming cardiac cell/tissue function toward a hypertrophic phenotype. We report that in neonatal rat cardiomyocyte culture, subtle out-of-plane microtopographic cues alter cell attachment, increase biomechanical stresses, and induce not only structural remodeling, but also yield essential molecular and electrophysiological signatures of hypertrophy. Increased cell size and cell binucleation, molecular up-regulation of released atrial natriuretic peptide, altered expression of classic hypertrophy markers, ion channel remodeling, and corresponding changes in electrophysiological function indicate a state of hypertrophy on par with other in vitro and in vivo models. Clinically used antihypertrophic pharmacological treatments partially reversed hypertrophic behavior in this in vitro model. Partial least-squares regression analysis, combining gene expression and functional data, yielded clear separation of phenotypes (control: cells grown on flat surfaces; hypertrophic: cells grown on quasi-3-dimensional surfaces and treated). In summary, structural surface features can guide cardiac cell attachment, and the subsequent syncytial behavior can facilitate trophic signals, unexpectedly on par with externally applied mechanical, electrical, and chemical stimulation.

Human umbilical cord blood derived stem cells repair doxorubicin-induced pathological cardiac hypertrophy in mice

In the present study, we investigated the cardiomyogenic potential of human umbilical cord blood (hUCB)-derived stem cells and whether stem cell treatment repairs the pathological hypertrophy induced by doxorubicin (DOX) in cultured neonatal rat cardiomyocytes (NRCM) and in mouse hearts. hUCB, which were labeled with cell tracker dye, were co-cultured with isolated NRCM in vitro. After 48h of incubation, the red stained hUCB cells (30%) contracted rhythmically and synchronously (physical examination). These differentiated hUCB also expressed cardiac specific alpha-actinin and showed diffused expression of connexin 43 and N-cadherin, thereby suggesting a tight electrical coupling among hUCB cells and myocytes. When co-cultured, hUCB also reversed the pathological effects induced by DOX in NRCM and in mice as seen by RT-PCR, immunoblot analysis and immunocytochemistry. hUCB migrated and integrated into the hearts of mice that were treated with DOX after intravenous injection and reversed the expression of pathological hypertrophic markers induced by DOX in mice. Further, we observed a shift from pathological hypertrophy towards physiological hypertrophy by hUCB in DOX-challenged mice. hUCB treatment in mice decreased DOX-induced increase of heart weight to body mass ratio and fibrosis. Taken together, these findings suggest the potential therapeutic use of hUCB in reversing heart failure conditions.

Hyperpolarization-activated cation channels: from genes to function

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels comprise a small subfamily of proteins within the superfamily of pore-loop cation channels. In mammals, the HCN channel family comprises four members (HCN1-4) that are expressed in heart and nervous system. The current produced by HCN channels has been known as I(h) (or I(f) or I(q)). I(h) has also been designated as pacemaker current, because it plays a key role in controlling rhythmic activity of cardiac pacemaker cells and spontaneously firing neurons. Extensive studies over the last decade have provided convincing evidence that I(h) is also involved in a number of basic physiological processes that are not directly associated with rhythmicity. Examples for these non-pacemaking functions of I(h) are the determination of the resting membrane potential, dendritic integration, synaptic transmission, and learning. In this review we summarize recent insights into the structure, function, and cellular regulation of HCN channels. We also discuss in detail the different aspects of HCN channel physiology in the heart and nervous system. To this end, evidence on the role of individual HCN channel types arising from the analysis of HCN knockout mouse models is discussed. Finally, we provide an overview of the impact of HCN channels on the pathogenesis of several diseases and discuss recent attempts to establish HCN channels as drug targets.

Role of TGF-beta on cardiac structural and electrical remodeling

The type β transforming growth factors (TGF-βs) are involved in a number of human diseases, including heart failure and myocardial arrhythmias. In fact, during the last 20 years numerous studies have demonstrated that TGF-β affects the architecture of the heart under both normal and pathological conditions. Moreover, TGF-β signaling is currently under investigation, with the aim of discovering potential therapeutic roles in human disease. In contrast, only few studies have investigated whether TGF-β affects electrophysiological properties of the heart. This fact is surprising since electrical remodeling represents an important substrate for cardiac disease. This review discusses the potential role of TGF-β on cardiac excitation-contraction (EC) coupling, action potentials, and ion channels. We also discuss the effects of TGF-β on cardiac development and disease from structural and electrophysiological points of view.

Impact of dihydrotestosterone on L-type calcium channels in human ventricular cardiomyocytes

OBJECTIVES

Reports of testosterone effects on cardiovascular morbidity remain contradictory. Besides modulating cardiovascular risk factors recent evidence indicates direct actions of testosterone on cardiac tissue. However, the impact on human cardiac L-type calcium channels that play a central role in electro-mechanical coupling is unknown.

METHODS AND RESULTS

Human ventricular myocytes were isolated from patients undergoing heart transplantation. Patch-clamp experiments in whole-cell configuration were performed to evaluate the effect of dihydrotestosterone on cardiac L-type calcium current I(Ca,L). Treatment of cultured cardiomyocytes with dihydrotestosterone 100 nmol/L for 24-30 h increased the whole-cell I(Ca,L) current density from 2.32 +/- 0.17 pA/pF (n = 11) to 3.21 +/- 0.17 pA/pF (n = 14) at +10 mV (p = 0.01) without shifting the current-voltage relation. This effect was associated with a 1.35-fold higher expression of the pore-forming Ca(V)1.2 (alpha1c) subunit of L-type calcium channels in dihydrotestosterone-treated myocytes compared with controls (p = 0.03).

CONCLUSIONS

Dihydrotestosterone treatment increased L-type calcium current density by the upregulation of Ca(V)1.2 in human ventricular myocytes. These data provide a possible explanation for dihydrotestosterone effects on the cardiovascular system in androgenic steroid abuse.

Arrhythmia mechanisms in the failing heart

BACKGROUND

Heart failure (HF) claims over 200,000 lives annually in the United States alone. Approximately 50% of these deaths are sudden and unexpected, and presumably the consequence of lethal ventricular tachyarrhythmias. Electrical remodeling that occurs at the cellular and tissue network levels predisposes patients with HF to malignant arrhythmias. Our limited understanding of fundamental arrhythmia mechanisms has hampered the development of effective treatment strategies for these patients.

METHODS AND CONCLUSIONS

In this review, we outline recent advances in our understanding of arrhythmia mechanisms in the failing heart, highlighting various aspects of remodeling of ion channels, calcium handling proteins, and gap junction-related molecules. As will be discussed, these changes promote the prolongation of the action potential, the enhancement of spatio-temporal gradients of repolarization, the formation of calcium-mediated triggers and conduction abnormalities, all of which combine to form an electrophysiological substrate that is ripe for the genesis of lethal arrhythmias and sudden cardiac death.

Ca2+-Operated Transcriptional Networks: Molecular Mechanisms and In Vivo Models

Calcium is the most universal signal used by living organisms to convey information to many different cellular processes. In this review we present well-known and recently identified proteins that sense and decode the calcium signal and are key elements in the nucleus to regulate the activity of various transcriptional networks. When possible, the review also presents in vivo models in which the genes encoding these calcium sensors-transducers have been modified, to emphasize the critical role of these Ca2+-operated mechanisms in many physiological functions.

Dietary n-3 Polyunsaturated Fatty Acids and Direct Renin Inhibition Improve Electrical Remodeling in a Model of High Human Renin Hypertension

We compared the effect n-3 polyunsaturated fatty acids (PUFAs) with direct renin inhibition on electrophysiological remodeling in angiotensin II–induced cardiac injury. We treated double-transgenic rats expressing the human renin and angiotensinogen genes (dTGRs) from week 4 to 7 with n-3 PUFA ethyl-esters (Omacor; 25-g/kg diet) or a direct renin inhibitor (aliskiren; 3 mg/kg per day). Sprague-Dawley rats were controls. We performed electrocardiographic, magnetocardiographic, and programmed electrical stimulation. Dietary n-3 PUFAs increased the cardiac content of eicosapentaenoic and docosahexaenoic acid. At week 7, mortality in dTGRs was 31%, whereas none of the n-3 PUFA- or aliskiren-treated dTGRs died. Systolic blood pressure was modestly reduced in n-3 PUFA-treated (180±3 mm Hg) compared with dTGRs (208±5 mm Hg). Aliskiren-treated dTGRs and Sprague-Dawley rats were normotensive (110±3 and 119±6 mm Hg, respectively). Both n-3 PUFA–treated and untreated dTGRs showed cardiac hypertrophy and increased atrial natriuretic peptide levels. Prolonged QRS and QT c intervals and increased T-wave dispersion in dTGRs were reduced by n-3 PUFAs or aliskiren. Both treatments reduced arrhythmia induction from 75% in dTGRs to 17% versus 0% in Sprague-Dawley rats. Macrophage infiltration and fibrosis were reduced by n-3 PUFAs and aliskiren. Connexin 43, a mediator of intermyocyte conduction, was redistributed to the lateral cell membranes in dTGRs. n-3 PUFAs and aliskiren restored normal localization to the intercalated disks. Thus, n-3 PUFAs and aliskiren improved electrical remodeling, arrhythmia induction, and connexin 43 expression, despite a 70-mm Hg difference in blood pressure and the development of cardiac hypertrophy.

Structural and electrical remodeling as therapeutic targets in heart failure

Heart failure is a progressive clinical syndrome that is characterized by remodeling of the myocardium in response to various stress signals. The past several years has seen remarkable progress in unraveling the molecular and cellular mechanisms of structural and electrical remodeling in HF. Improved understanding of the molecular mechanism of myocardial remodeling has resulted in improved HF therapies and revealed potentially novel therapeutic targets. This review discusses the mechanisms of myocardial remodeling in HF and their clinical manifestations. Current and investigational HF therapies targeting these mechanisms also will be discussed.

The role of cardiac tissue alignment in modulating electrical function

INTRODUCTION

Most cardiac arrhythmias are associated with pathology-triggered ion channel remodeling. However, multicellular effects, for example, exaggerated anisotropy and altered cell-to-cell coupling, can also indirectly affect action potential morphology and electrical stability via changed electrotonus. These changes are particularly relevant in structural heart disease, including hypertrophy and infarction. Recent computational studies showed that electrotonus factors into stability by altering dynamic properties (restitution). We experimentally address the question of how cell alignment and connectivity alter tissue function and whether these effects depend on the direction of wave propagation.

METHODS AND RESULTS

We show that cardiac cell arrangement can alter electrical stability in an in vitro cardiac tissue model by mechanisms both dependent and independent of the direction of wave propagation, and local structural remodeling can be felt beyond a space constant. Notably, restitution of action potential duration (APD) and conduction velocity was significantly steepened in the direction of cell alignment. Furthermore, prolongation of APD and calcium transient duration was found in highly anisotropic cell networks, both for longitudinal and transverse propagation. This is in contrast to expected correlation between wave propagation direction and APD based on electrotonic effects only, but is consistent with our findings of increased cell size and secretion of atrial natriuretic factor, a hypertrophy marker, in the aligned structures.

CONCLUSION

Our results show that anisotropic structure is a potent modulator of electrical stability via electrotonus and molecular signaling. Tissue alignment must be taken into account in experimental and computational models of arrhythmia generation and in designing effective treatment therapies.

Electrical and structural remodeling in left ventricular hypertrophy-a substrate for a decrease in QRS voltage?

Electrical remodeling in advanced stages of cardiovascular diseases creates a substrate for triggering and maintenance of arrhythmias. The electrical remodeling is a continuous process initiated already in the early stages of cardiological pathology. The aim of this opinion article was to discuss the changes in electrical properties of myocardium in left ventricular hypertrophy (LVH), with special focus on its early stage, as well as their possible reflection in the QRS amplitude of the electrocardiogram. It critically appraises the classical hypothesis related to the QRS voltage changes in LVH. The hypothesis of the relative voltage deficit is discussed in the context of supporting evidence from clinical studies, animal experiments, and simulation studies. The underlying determinants of electrical impulse propagation which may explain discrepancies between "normal" ECG findings and increased left ventricular size/mass in LVH are reviewed.

Remodeling of Outward K+Currents in Pressure-Overload Heart Failure

OBJECTIVES

Outward K+ currents are critical determinants of action potential repolarization and the site of action of a number of electrophysiologically active drugs. Further, expression and processing of the channels underlying these currents is altered in heart disease. Here, we investigated the native transmural gradient of outward K+ currents in murine left ventricle (LV) and delineated disease-related remodeling of these currents in heart failure (HF).

METHODS

Pressure-overload heart failure was induced in mice by thoracic aortic constriction. Outward K+ currents were recorded using the whole-cell patch clamp technique in acutely dissociated ventricular myocytes.

RESULTS

Unambiguous gradients of outward K+ current density and Kv4.2 protein abundance were observed across the wall of the LV, with significantly larger current density and protein levels in subepicardial (SEP) myocytes, compared with subendocardial (SEN) myocytes. Voltage dependences of current activation and inactivation were similar in SEP and SEN myocytes. In failing LV, however, outward K+ current density was significantly decreased in SEP but not in SEN cells leading to elimination of the native transmural gradient. In failing LV, the voltage dependences of K+ current activation and inactivation were not altered. However, current inactivation (decay) was significantly accelerated and recovery from inactivation was significantly slowed. Consistent with this, Western blot analysis revealed a decrease in KChIP2 protein abundance in failing LV.

CONCLUSIONS

This is the first report of HF-related remodeling of outward K+ currents in murine LV. Similar to humans, disease-related remodeling occurs differentially across the murine ventricular wall, leading to loss of the native gradient of repolarization. Together with slowed recovery from inactivation, these alterations likely promote abnormal impulse conduction, a major proarrhythmic mechanism.

Crosstalk between L-type calcium channels and ZnT-1, a new player in rate-dependent cardiac electrical remodeling

Crosstalk between two membrane transport systems is an established mechanism underlying regulation. In this study, we investigated the interaction between ZnT-1, a putative plasma membrane zinc transporter, and L-type voltage-dependent calcium channels (LTCC). In the atrium of the myocardium decreased activity of the LTCC is a dominant feature of patients with atrial fibrillation. The trigger for this inhibition has been attributed to the rapid firing rates and consequent calcium overload in the atrial cardiomyocytes. However, the underlying mechanism of LTCC inhibition is still to be elucidated. Here, we showed that the expression of ZnT-1 inhibits the activity of L-type channels during electrical remodeling induced by rapid pacing. (i) Direct manipulations of ZnT-1 expression in cultured cardiomyocytes either by ZnT-1 overexpression or by ZnT-1 silencing with siRNA, decreased or enhanced, respectively, the barium influx through the LTCC. (ii) Co-expression of ZnT-1 with LTCC in Xenopus oocytes decreased whole cell barium current through LTCC. (iii) Rapid pacing of cultured cardiomyocytes (4 h, 100 ms cycle) increased ZnT-1 protein expression and inhibited the voltage-dependent divalent cation influx through the LTCC. Moreover, silencing ZnT-1 with siRNA prevented the rapid pacing induced inhibition of the LTCC (iv) Atrial pacing of anesthetized adult rats (4 h, 50 ms cycle) led to a significant increase in atrial ZnT-1 protein expression in parallel with the typical decrease of the refractory period in the atria. Taken together, these findings demonstrate that crosstalk between ZnT-1 and the L-type calcium channels may underlie atrial response to rapid pacing, suggesting that ZnT-1 is a significant participant in rate-dependent cardiac electrical remodeling.

Angiotensin II-induced sudden arrhythmic death and electrical remodeling

Rats harboring the human renin and angiotensinogen genes (dTGR) feature angiotensin (ANG) II/hypertension-induced cardiac damage and die suddenly between wk 7 and 8. We observed by electrocardiogram (ECG) telemetry that ventricular tachycardia (VT) is a common terminal event in these animals. Our aim was to investigate electrical remodeling. We used ECG telemetry, noninvasive cardiac magnetic field mapping (CMFM) at wk 5 and 7, and performed in vivo programmed electrical stimulation at wk 7. We also investigated whether or not losartan (Los; 30 mg x kg(-1) x day(-1)) would prevent electrical remodeling. Cardiac hypertrophy and systolic blood pressure progressively increased in dTGR compared with Sprague-Dawley (SD) controls. Already by wk 5, untreated dTGR showed increased perivascular and interstitial fibrosis, connective tissue growth factor expression, and monocyte infiltration compared with SD rats, differences that progressed through time. Left-ventricular mRNA expression of potassium channel subunit Kv4.3 and gap-junction protein connexin 43 were significantly reduced in dTGR compared with Los-treated dTGR and SD. CMFM showed that depolarization and repolarization were prolonged and inhomogeneous. Los ameliorated all disturbances. VT could be induced in 88% of dTGR but only in 33% of Los-treated dTGR and could not be induced in SD. Untreated dTGR show electrical remodeling and probably die from VT. Los treatment reduces myocardial remodeling and predisposition to arrhythmias. ANG II target organ damage induces VT.

Remodeling of early-phase repolarization: a mechanism of abnormal impulse conduction in heart failure

BACKGROUND

The early phase of action potential (AP) repolarization is critical to impulse conduction in the heart because it provides current for charging electrically coupled cells. In the present study we tested the impact of heart failure-associated electrical remodeling on AP propagation.

METHODS AND RESULTS

Subepicardial, midmyocardial, and subendocardial myocytes were enzymatically dissociated from control and pressure-overload failing left ventricle (LV), and APs were recorded. A unique coupling-clamp technique was used to electrically couple 2 isolated myocytes with a controlled value of coupling conductance (Gc). In sham-operated mice, AP duration manifested a clear transmural gradient, with faster repolarization in subepicardial myocytes than in subendocardial myocytes. AP propagation from subendocardial to subepicardial myocytes required less Gc compared with conduction in the opposite direction. In failing heart, AP morphology was dramatically altered, with a significantly elevated plateau potential and prolonged AP duration. Spatially nonuniform alteration of AP duration in failing heart blunted the transmural gradient of repolarization. Furthermore, increased pacing rate prolonged AP duration exclusively in myocytes from failing heart, and the critical conductance required for successful AP propagation decreased significantly at high frequencies. Finally, in failing heart, asymmetry of transmural electrical propagation was abolished.

CONCLUSIONS

In failing heart, preferential conduction from subendocardial to subepicardial myocytes is lost, and failing myocytes manifest facilitated AP propagation at fast rates. Together, these electrical remodeling responses may promote conduction of premature impulses and heighten the risk of malignant arrhythmia, a prominent feature of heart failure.

Ion channels as novel therapeutic targets in heart failure

Electrophysiological remodeling in heart failure (HF) is characterized by major changes in ion channel function and expression that alter the electrical phenotype and predispose to the development of lethal ventricular tachyarrhythmias. In this article, we provide a review of our current understanding of HF-induced ion channel dysfunction by highlighting changes in potassium and sodium currents, pumps, and exchangers as well as calcium handling proteins. We further relate these changes in ion channel function to abnormalities in impulse generation, conduction, and repolarization with a view towards identifying potentially novel targets for anti-arrhythmic therapy for this public health epidemic.

Hypertrophy of the heart: a new therapeutic target?

Recent studies call into question the necessity of hypertrophic growth of the heart as a "compensatory" response to hemodynamic stress. These findings, coupled with recent progress in dissecting the molecular bases of hypertrophy, raise the prospect of suppressing hypertrophy without provoking circulatory insufficiency. In this article, we focus on signaling pathways that hold promise as potential targets for therapeutic intervention. We also summarize observations from animal models and clinical trials that suggest benefit from an antihypertrophic strategy.