In-vivo electrophysiological study in mice with chronic anterior myocardial infarction

Effect of trehalose on heart functions in rats model after myocardial infarction: assessment of novel intraventricular pressure and heart rate variability

Background

Myocardial infarctions remain a leading cause of global deaths. Developing novel drugs to target cardiac remodeling after myocardial injury is challenging. There is an increasing interest in exploring natural cardioprotective agents and non-invasive tools like intraventricular pressure gradients (IVPG) and heart rate variability (HRV) analysis in myocardial infarctions. Trehalose (TRE), a natural disaccharide, shows promise in treating atherosclerosis, myocardial infarction, and neurodegenerative disorders.

Objectives

The objective of this study was to investigate the effectiveness of TRE in improving cardiac functions measured by IVPG and HRV and reducing myocardial remodeling following myocardial infarction in rat model.

Methods

Rats were divided into three groups: sham, myocardial infarction (MI), and trehalose-treated MI (TRE) groups. The animals in the MI and TRE groups underwent permanent ligation of the left anterior descending artery. The TRE group received 2% trehalose in their drinking water for four weeks after the surgery. At the end of the experiment, heart function was assessed using conventional echocardiography, novel color M-mode echocardiography for IVPG evaluation, and HRV analysis. After euthanasia, gross image scoring, histopathology, immunohistochemistry, and quantitative real-time PCR were performed to evaluate inflammatory reactions, oxidative stress, and apoptosis.

Results

The MI group exhibited significantly lower values in multiple IVPG parameters. In contrast, TRE administration showed an ameliorative effect on IVPG changes, with results comparable to the sham group. Additionally, TRE improved HRV parameters, mitigated morphological changes induced by myocardial infarction, reduced histological alterations in wall mass, and suppressed inflammatory reactions within the infarcted heart tissues. Furthermore, TRE demonstrated antioxidant, anti-apoptotic and anti-fibrotic properties.

Conclusion

The investigation into the effect of trehalose on a myocardial infarction rat model has yielded promising outcomes, as evidenced by improvements observed through conventional echocardiography, histological analysis, and immunohistochemical analysis. While minor trends were noticed in IVPG and HRV measurements. However, our findings offer valuable insights and demonstrate a correlation between IVPG, HRV, and other traditional markers of echo assessment in the myocardial infarction vs. sham groups. This alignment suggests the potential of IVPG and HRV as additional indicators for future research in this field.

Cardiac ischemia and reperfusion in mice: a comprehensive hemodynamic, electrocardiographic and electrophysiological characterization

Malignant ventricular arrhythmias (VA) after acute myocardial infarction remain a major threat. Aim of this study was to characterize the electrophysiological and autonomic sequelae of cardiac ischemia and reperfusion (I/R) in mice during the first week post incident. Left ventricular function was serially assessed using transthoracic echocardiography. VA were quantified by telemetric electrocardiogram (ECG) recordings and electrophysiological studies on the 2nd and 7th day after I/R. Cardiac autonomic function was evaluated by heart rate variability (HRV) and heart rate turbulence (HRT). Infarct size was quantified by planimetric measures. I/R caused significant myocardial scarring and diminished left ventricular ejection fraction. The ECG intervals QRS, QT, QT_c, and JT_c were prolonged in I/R mice. Both spontaneous VA scored higher and the inducibility of VA was raised in I/R mice. An analysis of HRV and HRT indicated a relative reduction in parasympathetic activity and disturbed baroreflex sensitivity up to 7 days after I/R. In summary, during the first week after I/R, the murine heart reflects essential features of the human heart after myocardial infarction, including a greater vulnerability for VA and a decreased parasympathetic tone accompanied by decelerated depolarization and repolarization parameters.

A systematic review on reporting of refinement measures in mouse ECG telemetry implantation surgery

Telemetric monitoring is used in many scientific fields, such as cardiovascular research, neurology, endocrinology, as well as animal welfare research. Nowadays, implanted electrocardiogram (ECG) radiotelemetry units are the gold standard for monitoring ECG traces, heart rate and heart rate variability in freely moving mice. Telemetry technology can be a valuable tool when studies utilize it adequately, while prioritizing animal welfare. Recently, concerns have been raised in many research fields, including animal research, regarding the reproducibility of research findings, with insufficient reporting being one of the underlying causes.A systematic review was performed by making use of three literature databases, in order to include all publications until 31.12.2019, where the surgical placing of ECG recording telemetry devices in adult mice was involved. Data extracted from the publications included selected items recommended by the ARRIVE guidelines. We focused on aspects related to the refinement of the surgery and experimental conditions that aim to improve animal welfare. In general, the quality of reporting was low in the analyzed 234 publications. Based on our analyses, we assume there has been no improvement in this field's reporting quality since 2010 when the ARRIVE guidelines on reporting were introduced. Additionally, even though expert recommendations on telemetry surgery refinement have been available since many years now, no increase in uptake (or reporting) of these measures prior (e.g., acclimatization), during (e.g., asepsis) or after (e.g., social housing) the surgery could be observed.

Heart Rate Variability Reveals Altered Autonomic Regulation in Response to Myocardial Infarction in Experimental Animals

The analysis of beating rate provides information on the modulatory action of the autonomic nervous system on the heart, which mediates adjustments of cardiac function to meet hemodynamic requirements. In patients with myocardial infarction, alterations of heart rate variability (HRV) have been correlated to the occurrence of arrhythmic events and all-cause mortality. In the current study, we tested whether experimental rodent models of myocardial infarction recapitulate dynamics of heart rate variability observed in humans, and constitute valid platforms for understanding mechanisms linking autonomic function to the development and manifestation of cardiovascular conditions. For this purpose, HRV was evaluated in two engineered mouse lines using electrocardiograms collected in the conscious, restrained state, using a tunnel device. Measurements were obtained in naïve mice and animals at 3–∼28 days following myocardial infarction, induced by permanent coronary artery ligation. Two mouse lines with inbred and hybrid genetic background and, respectively, homozygous (Homo) and heterozygous (Het) for the MerCreMer transgene, were employed. In the naïve state, Het female and male mice presented prolonged RR interval duration (∼9%) and a ∼4-fold increased short- and long-term RR interval variability, with respect to sex-matched Homo mice. These differences were abrogated by pharmacological interventions inhibiting the sympathetic and parasympathetic axes. At 3–∼14 days after myocardial infarction, RR interval duration increased in Homo mice, but was not affected in Het animals. In contrast, Homo mice had minor modifications in HRV parameters, whereas substantial (> 50%) reduction of short- and long-term RR interval variation occurred in Het mice. Interestingly, ex vivo studies in isolated organs documented that intrinsic RR interval duration increased in infarcted vs. non-infarcted Homo and Het hearts, whereas RR interval variation was not affected. In conclusion, our study documents that, as observed in humans, myocardial infarction in rodents is associated with alterations in heart rhythm dynamics consistent with sympathoexcitation and parasympathetic withdrawal. Moreover, we report that mouse strain is an important variable when evaluating autonomic function via the analysis of HRV.

Nanoengineered Sprayable Therapy for Treating Myocardial Infarction

We report the development, as well as the in vitro and in vivo testing, of a sprayable nanotherapeutic that uses surface engineered custom-designed multiarmed peptide grafted nanogold for on-the-spot coating of an infarcted myocardial surface. When applied to mouse hearts, 1 week after infarction, the spray-on treatment resulted in an increase in cardiac function (2.4-fold), muscle contractility, and myocardial electrical conductivity. The applied nanogold remained at the treatment site 28 days postapplication with no off-target organ infiltration. Further, the infarct size in the mice that received treatment was found to be <10% of the total left ventricle area, while the number of blood vessels, prohealing macrophages, and cardiomyocytes increased to levels comparable to that of a healthy animal. Our cumulative data suggest that the therapeutic action of our spray-on nanotherapeutic is highly effective, and in practice, its application is simpler than other regenerative approaches for treating an infarcted heart.

Overexpression of protein phosphatase 2A in a murine model of chronic myocardial infarction leads to increased adverse remodeling but restores the regulation of β-catenin by glycogen synthase kinase 3β

BACKGROUND/OBJECTIVES

Increased activity of cardiac protein phosphatases is an important feature in human heart failure. Several different protein phosphatases (PP) are involved in the regulation of excitation-contraction-coupling of the myocardium. Protein phosphatase 2A (PP2A) is a serine/threonine phosphatase consisting of a dimeric core enzyme and tissue-specific subunits. In this study we used transgenic mice overexpressing PP2A to further investigate the role of PP2A in cardiac remodeling after myocardial infarction.

METHODS AND RESULTS

Adult male CD-1 mice overexpressing the catalytic subunit α of PP2A (αMHC-PP2A; TG) underwent chronic LAD-ligation or sham surgery, respectively; wildtype littermates (WT) were used as controls. Cardiac function was determined by echocardiography before and 28 days after LAD-ligation. 28 days after MI, the animals were sacrificed and cardiac remodeling was analyzed in histological sections and by Western blots. PP2A overexpression leads to dilated cardiomyopathy in mice, and increased cardiomyocyte hypertrophy and fibrosis of the remote myocardium can be seen after myocardial infarction. However, we found an improved survival of TG in the subacute phase after MI in comparison to WT. On the molecular level, TG shows reduced expression of SERCA and CaMKII alpha both under basal condition as well 28 days after MI. Additionally, the regulation of the Akt/GSK3β/β-catenin pathway is severely disturbed in TG at baseline where a significant activation of Akt is found that coincides with the typical phosphorylation of GSK3β. However, this does not lead to the accumulation of β-catenin - on the contrary: phosphorylation-induced degradation of β-catenin is significantly enhanced.

CONCLUSION

Transgenic overexpression of myocardial PP2A causes adverse remodeling which coincides with a disruption of the classical Akt/GSK3/β-catenin pathway under baseline conditions that is restored to normal values in chronic myocardial infarction. Even so overall survival of TG after myocardial infarction was not constrained and survival after day 2 post MI was improved.

Cardiac electrophysiology and the susceptibility to sustained ventricular tachycardia in intact, conscious mice

Cardiac electrophysiological dysfunction is a major cause of death in humans. Accordingly, electrophysiological testing is routinely performed in intact, conscious, humans to evaluate arrhythmias and disorders of cardiac conduction. However, to date, in vivo electrophysiological studies in mice are limited to anesthetized open-chest or closed-chest preparations. However, cardiac electrophysiology in anesthetized mice or mice with surgical trauma may not adequately represent what occurs in conscious mice. Accordingly, an intact, conscious murine model of cardiac electrophysiology has the potential to be of major importance for advancing the concepts and methods that drive cardiovascular therapies. Therefore, we describe, for the first time, the use of an intact, conscious, murine model of cardiac electrophysiology. The conscious mouse model permits measurements of atrioventricular interval, sinus cycle length, sinus node recovery time (SNRT), SNRT corrected for spontaneous sinus cycle, Wenckebach cycle length, the ventricular effective refractory period (VERP) and the electrical stimulation threshold to induce sustained ventricular tachyarrhythmias in an intact, complex model free of the confounding influences of anesthetics and surgical trauma. This is an important consideration because anesthesia and surgical trauma markedly reduced cardiac output and heart rate as well as altered cardiac electrophysiology parameters. Most importantly, anesthesia and surgical trauma significantly increased the VERP and virtually eliminated the ability to induce sustained ventricular tachyarrhythmias. Accordingly, the methodology allows for the accurate documentation of cardiac electrophysiology in complex, conscious mice and may be adopted for advancing the concepts and ideas that drive cardiovascular research.

Treatment with bone morphogenetic protein 2 limits infarct size after myocardial infarction in mice

Various strategies have been devised to reduce the clinical consequences of myocardial infarction, including acute medical care, revascularization, stem cell transplantations, and more recently, prevention of cardiomyocyte cell death. Activation of embryonic signaling pathways is a particularly interesting option to complement these strategies and to improve the functional performance and survival rate of cardiomyocytes. Here, we have concentrated on bone morphogenetic protein 2 (BMP-2), which induces ectopic formation of beating cardiomyocytes during development in the mesoderm and protects neonatal cardiomyocytes from ischemia-reperfusion injury. In a mouse model of acute myocardial infarction, an i.v. injection of BMP-2 reduced infarct size in mice when given after left anterior descending artery ligation. Mice treated with BMP-2 are characterized by a reduced rate of apoptotic cardiomyocytes both in the border zone of the infarcts and in the remote myocardium. In vitro, BMP-2 increases the frequency of spontaneously beating neonatal cardiomyocytes and the contractile performance under electrical pacing at 2 Hz, preserves cellular adenosine triphosphate stores, and decreases the rate of apoptosis despite the increased workload. In addition, BMP-2 specifically induced phosphorylation of Smad1/5/8 proteins and protected adult cardiomyocytes from long-lasting hypoxia-induced cellular damage and oxidative stress without activation of the cardiodepressant transforming growth factor-β pathway. Our data suggest that BMP-2 treatment may have considerable therapeutic potential in individuals with acute and chronic myocardial ischemia by improving the contractility of cardiomyocytes and preventing cardiomyocyte cell death.

Kir6.2 is not the mitochondrial KATP channel but is required for cardioprotection by ischemic preconditioning

ATP-sensitive K(+) (KATP) channels that contain K(+) inward rectifier subunits of the 6.2 isotype (Kir6.2) are important regulators of the cardiac response to ischemia-reperfusion (I/R) injury. Opening of these channels is implicated in the cardioprotective mechanism of ischemic preconditioning (IPC), but debate surrounds the contribution of surface KATP (sKATP) versus mitochondrial KATP (mKATP) channels. While responses to I/R injury and IPC have been examined in Kir6.2(-/-) mice before, breeding methods and other technical obstacles may have confounded interpretations. The aim of this study was to elucidate the role of Kir6.2 in cardioprotection and mKATP activity, using conventionally bred Kir6.2(-/-) mice with wild-type littermates as controls. We found that perfused hearts from Kir6.2(-/-) mice exhibited a normal baseline response to I/R injury, were not protected by IPC, and showed a blunted response to the IPC mimetic drug diazoxide. These data suggest that the loss of IPC in Kir6.2(-/-) hearts is not due to an underlying difference in I/R sensitivity. Furthermore, mKATP channel activity was identical in cardiac mitochondria isolated from wild-type versus Kir6.2(-/-) mice, suggesting no role for Kir6.2 in the mKATP. Collectively, these data indicate that Kir6.2 is required for the full response to IPC or diazoxide but is not involved in mKATP formation.

Cardiac electrophysiology in mice: a matter of size

Over the last decade, mouse models have become a popular instrument for studying cardiac arrhythmias. This review assesses in which respects a mouse heart is a miniature human heart, a suitable model for studying mechanisms of cardiac arrhythmias in humans and in which respects human and murine hearts differ. Section I considers the issue of scaling of mammalian cardiac (electro) physiology to body mass. Then, we summarize differences between mice and humans in cardiac activation (section II) and the currents underlying the action potential in the murine working myocardium (section III). Changes in cardiac electrophysiology in mouse models of heart disease are briefly outlined in section IV, while section V discusses technical considerations pertaining to recording cardiac electrical activity in mice. Finally, section VI offers general considerations on the influence of cardiac size on the mechanisms of tachy-arrhythmias.

Angiotensin II does not acutely regulate conduction velocity in rat atrial tissue

AIM

Atrial angiotensin II (Ang II) levels are increased in atrial fibrillation and are believed to be important in the pathogenesis of atrial arrhythmias. Ang II reduces intercellular coupling by inhibiting gap junctions (connexins) and may thereby increase the risk of reentry arrhythmia. The aim of the current study was to investigate the acute effect of Ang II on conduction velocity (CV) in atrial tissue from normal and chronically infarcted rats.

METHODS

Contractile force was measured and CV was determined from the conduction time between electrodes placed on the tissue preparation. Expression of AT1a and AT1b receptors was examined by real-time PCR.

RESULTS

Acute stimulation with Ang II did not affect CV in tissue from auricle or atrial free wall. A transient 6.5 ± 3.6% increase in resting tension was observed in atrial free wall preparations, indicating that receptors are present and functional in the free wall preparation. The difference between free wall and auricle was probably not caused by differences in receptor expression since equal amounts of AT1 mRNA were present. To test if myocardial infarction (MI) sensitizes the atrium to Ang II, free atrial wall from rats subjected to 4-5 weeks ventricular MI was examined. Although CV was significantly reduced by MI, no effect on CV of Ang II was seen.

CONCLUSION

Ang II does not acutely regulate CV in tissue preparations from the free wall of the left atria or the left auricle. Although ventricular MI reduces CV, this does not sensitize the atria to Ang II.

In vivo cardioprotection by S-nitroso-2-mercaptopropionyl glycine

The reversible S-nitrosation and inhibition of mitochondrial complex I is a potential mechanism of cardioprotection, recruited by ischemic preconditioning (IPC), S-nitrosothiols, and nitrite. Previously, to exploit this mechanism, the mitochondrial S-nitrosating agent S-nitroso-2-mercaptopropionyl glycine (SNO-MPG) was developed, and protected perfused hearts and isolated cardiomyocytes against ischemia-reperfusion (IR) injury. In the present study, the murine left anterior descending coronary artery (LAD) occlusion model of IR injury was employed, to determine the protective efficacy of SNO-MPG in vivo. Intraperitoneal administration of 1 mg/kg SNO-MPG, 30 min prior to occlusion, significantly reduced myocardial infarction and improved EKG parameters, following 30 min occlusion plus 2 or 24 h reperfusion. SNO-MPG protected to the same degree as IPC, and notably was also protective when administered at reperfusion. Cardioprotection was accompanied by increased mitochondrial protein S-nitrosothiol content, and inhibition of complex I, both of which were reversed after 2 h reperfusion. Finally, hearts from mice harboring a heterozygous mutation in the complex I NDUSF4 subunit were refractory to protection by either SNO-MPG or IPC, suggesting that a fully functional complex I, capable of reversible inhibition is critical for cardioprotection. Overall, these results are consistent with a role for mitochondrial S-nitrosation and complex I inhibition in the cardioprotective mechanism of IPC and SNO-MPG in vivo.

Ex vivo expanded hematopoietic progenitor cells improve cardiac function after myocardial infarction: role of beta-catenin transduction and cell dose

Cell-based therapy after myocardial infarction (MI) is a promising therapeutic option but the relevant cell subsets and dosage requirements are poorly defined. We hypothesized that cell therapy for myocardial infarction is improved by ex vivo expansion and high-dose transplantation of defined hematopoietic progenitor cells (HPCs). Since beta-catenin promotes self-renewal of stem cells we evaluated the therapeutic efficacy of beta-catenin-mediated ex vivo expansion of mouse HPCs in a mouse model of myocardial ischemia/reperfusion followed by intraarterial cell delivery. The impact of cell dose was determined by comparing a low-dose (LD, 5 x 10(5) cells) vs. a high-dose (HD, 1 x 10(7) cells) cell transplantation regimen of beta-catenin-HPCs. The impact of beta-catenin modification of HPCs was determined by comparing control-transduced HPCs (GFP-HPCs) vs. transgenic beta-catenin-HPCs. HD beta-catenin-HPCs significantly improved LV function and end-systolic and end-diastolic dimensions as compared to saline and LD beta-catenin-HPCs. Furthermore, while treatment with HD GFP-HPC resulted in a modest cardiac improvement the application of beta-catenin-HPCs was superior, resulting in a significant improvement in EF, FS and LVESD over saline and control GFP-HPC treatment. Although myocardial engraftment of HPCs was only transient, as determined by cell quantification after dye labeling, beta-catenin-HPC treatment significantly decreased infarct size, reduced cardiomyocyte apoptosis and increased capillary angiogenesis in vitro and in vivo. Ex vivo expanded HPCs improve cardiac function and remodeling post MI in a cell number- and beta-catenin-dependent manner.

Reduced delayed rectifier K+ current, altered electrophysiology, and increased ventricular vulnerability in MLP-deficient mice

BACKGROUND

Mice with a knockout (KO) of muscle LIM protein (MLP) exhibit many morphologic and clinical features of human cardiomyopathy. In humans, MLP-expression is downregulated both in ischemic and dilative cardiomyopathy. In this study, we investigated the effects of MLP on the electrophysiologic phenotype in vivo and on outward potassium currents.

METHODS AND RESULTS

MLP-deficient (MLPKO) and wild-type (MLPWT) mice were subjected to long-term electrocardiogram (ECG) recording and in vivo electrophysiologic study. The whole-cell, patch-clamp technique was applied to measure voltage dependent outward K+ currents in isolated cardiomyocytes. Long-term ECG revealed a significant prolongation of RR mean (108 +/- 9 versus 99 +/- 5 ms), P (16 +/- 3 versus 14 +/- 1 ms), QRS (17 +/- 3 versus 13 +/- 1 ms), QT (68 +/- 8 versus 46 +/- 7 ms), QTc (66 +/- 6 versus 46 +/- 7 ms), JT (51 +/- 7 versus 34 +/- 7 ms), and JTc (49 +/- 5 versus 33 +/- 7 ms) in MLPKO versus MLPWT mice (P < .05). During EP study, QT (80 +/- 8 versus 58 +/- 7 ms), QTc (61 +/- 6 versus 45 +/- 5 ms), JT (62 +/- 9 versus 43 +/- 6 ms), and JTc (47 +/- 5 versus 34 +/- 5 ms) were also significantly prolonged in MLPKO mice (P < .05). Nonsustained VT was inducible in 9/16 MLPKO versus 2/15 MLPWT mice (P < .05). Analysis of outward K+ currents in revealed a significantly reduced density of the slowly inactivating outward K+ current IK, slow in MLPKO mice (11 +/- 5 pA/pF versus 18 +/- 7 pA/pF; P < .05).

CONCLUSION

Mice with KO of MLP exhibit significant prolongation of atrial and ventricular conduction and an increased ventricular vulnerability. A reduction in repolarizing outward K+ currents may be responsible for these alterations.

Cellular Cardiomyoplasty: Improvement of Left Ventricular Function Correlates with the Release of Cardioactive Cytokines

A growing number of studies are reporting beneficial effects of the transplantation of alleged cardiac stem cells into diseased hearts after myocardial infarction. However, the mechanisms by which transplanted cells might help to promote repair of cardiac tissue are not understood and might involve processes different from the differentiation of transplanted cells into cardiomyocytes. We have compared the effects exerted by skeletal myoblasts (which are not able to form new cardiomyocytes) and ESC-derived cardiomyocytes after implantation into infarcted mouse hearts by echocardiographic follow-up and histological analysis and related these effects to the release of cardioactive cytokines. We found that both cell types led to a long-lasting improvement of left ventricle function and to an improvement of tissue architecture. Since no relevant amounts of myoblast-derived cells were present in infarcted hearts 28 days after transplantation, we investigated the release of cytokines from implanted cells both before and after transplantation into infarcted hearts. ESC-derived cardiomyocytes and myoblasts secreted substantial amounts of interleukin (IL)-1alpha, IL-6, tumor necrosis factor-beta, and oncostatin M, which strongly supported survival and protein synthesis of cultured cardiomyocytes. We postulate that the beneficial effects of the transplantation of myoblasts and cardiomyocytes on heart function and morphology only partially (if at all) depend on the integration of transplanted cells into the myocardium but do depend on the release of a complex blend of cardioactive cytokines.

Lack of JunD Promotes Pressure Overload–Induced Apoptosis, Hypertrophic Growth, and Angiogenesis in the Heart

Background— The Jun family of activator protein 1 (AP-1) transcription factors (c-Jun, JunB, and JunD) is involved in fundamental biological processes such as proliferation, apoptosis, tumor angiogenesis, and hypertrophy. The role of individual AP-1 transcription factors in the stressed heart is not clear. In the present study we analyzed the role of JunD in survival, hypertrophy, and angiogenesis in the pressure-overloaded mouse heart after thoracic aortic constriction.

Methods and Results— Mice lacking JunD (knockout [KO]) showed increased mortality and enhanced cardiomyocyte apoptosis and fibrosis associated with increased levels of hypoxia-induced factor-1α, vascular endothelial growth factor (VEGF), p53, and Bax protein and reduced levels of Bcl-2 protein after 7 days of severe pressure overload compared with wild-type (WT) siblings. Cardiomyocyte hypertrophy in surviving KO mice was enhanced compared with that in WT mice. Chronic moderate pressure overload for 12 weeks caused enhanced left ventricular hypertrophy in KO mice, and survival and interstitial fibrosis were comparable with WT mice. Cardiac function, 12 weeks after operation, was comparable among shams and pressure-overloaded mice of both genotypes. In addition, KO mice exposed to chronic pressure overload showed higher cardiac capillary density associated with increased protein levels of VEGF.

Conclusions— Thus, JunD limits cardiomyocyte hypertrophy and protects the pressure-overloaded heart from cardiac apoptosis. These beneficial effects of JunD, however, are associated with antiangiogenic properties.

Female mice lacking estrogen receptor beta display prolonged ventricular repolarization and reduced ventricular automaticity after myocardial infarction

BACKGROUND

Major gender-based differences in the incidence of ventricular tachyarrhythmia after myocardial infarction have been shown in humans. Although the underlying mechanisms are unclear, earlier studies suggest that estrogen receptor-mediated effects play a major role in this process.

METHODS AND RESULTS

We examined the effect of estrogen receptor alpha (ERalpha) and estrogen receptor beta (ERbeta) on the electrophysiological phenotype in female mice with and without chronic anterior myocardial infarction. There was no significant difference in overall mortality, infarct size, and parameters of left ventricular remodeling when we compared infarcted ERalpha-deficient and ERbeta-deficient mice with infarcted wild-type animals. In the 12-hour telemetric ECG recording 6 weeks after myocardial infarction, surface ECG parameters did not show significant differences in comparisons of ERalpha-deficient mice versus wild-type controls, infarcted versus noninfarcted ERalpha-deficient mice, and infarcted ERalpha-deficient versus infarcted wild-type mice. However, infarcted ERbeta-deficient versus noninfarcted ERbeta-deficient mice showed a significant prolongation of the QT (61+/-6 versus 48+/-8 ms; P<0.05) and QTc intervals (61+/-7 versus 51+/-9 ms; P<0.05) and the JT (42+/-6 versus 31+/-4 ms; P<0.05) and JTc intervals (42+/-7 versus 33+/-4 ms; P<0.05). Furthermore, infarcted ERbeta-deficient versus infarcted wild-type mice showed a significant prolongation of the QT (61+/-6 versus 53+/-8 ms; P<0.05) and QTc intervals (61+/-7 versus 53+/-7 ms; P<0.05) and the JT (42+/-6 versus 31+/-5 ms; P<0.05) and JTc intervals (42+/-7 versus 31+/-5 ms; P<0.05), accompanied by a significant decrease of ventricular premature beats (7+/-21/h versus 71+/-110/h; P<0.05). Finally, real-time polymerase chain reaction-based quantitative analysis of mRNA levels showed a significantly lower expression of Kv4.3 (coding for I(to)) in ERbeta-deficient mice (P<0.05).

CONCLUSIONS

Estrogen receptor beta deficiency results in prolonged ventricular repolarization and decreased ventricular automaticity in female mice with chronic myocardial infarction.

Regulation of Proangiogenic Factor CCN1 in Cardiac Muscle

BACKGROUND

CCN1, a potent proangiogenic factor, is induced in the vasculature by tissue injury, angiotensin II (Ang II), and growth factor stimulation. Because these conditions occur in myocardial ischemia and pressure overload, we investigated the regulation of CCN1 in cardiomyocytes in vitro and in the heart in vivo.

METHODS AND RESULTS

Ang II, signaling via the angiotensin type 1 (AT1) receptor, and alpha1-adrenergic stimulation with phenylephrine induced CCN1 expression in ventricular cardiomyocytes isolated from 1- to 3-day-old rats. Cell culture supernatant of Ang II-treated cardiomyocytes induced migration of smooth muscle cells, which was abolished by neutralizing antibody to CCN1. Ang II- and phenylephrine-mediated induction of CCN1 expression in cardiomyocytes was completely abolished by inhibition of MEK/extracellular signal-regulated kinases (ERK) or protein kinase C (PKC). Likewise, mechanical stretch induced CCN1 expression in cardiomyocytes, an effect that was prevented by AT1 receptor blockade or PKC inhibition. Similarly, pressure overload in vivo upregulated myocardial CCN1 expression levels via AT1 receptor- and PKC-dependent mechanisms. After myocardial infarction in mice, CCN1 expression was strongly induced in both ischemic and remote left ventricular myocardium. Marked CCN1 protein expression was noted in cardiomyocytes of patients with end-stage ischemic cardiomyopathy but was almost absent in nonfailing human myocardium.

CONCLUSIONS

Pressure overload, ischemia, and neurohormonal factors, such as Ang II or alpha1-adrenergic stimuli, induce myocardial expression of CCN1, a potent proangiogenic factor, supporting the notion that CCN1 may play an important role in the adaptation of the heart to cardiovascular stress.

Spontaneous and inducible ventricular arrhythmias after myocardial infarction in mice

INTRODUCTION

Remodeling of gap junctions has been implicated in development of ventricular arrhythmias following myocardial infarction (MI) but the specific contribution of reduced electrical coupling is not known. We addressed this question using hearts from mice heterozygous for a connexin43 null allele (Cx43(+/-)).

METHODS

To determine whether Cx43-deficient mice exhibit increased spontaneous ventricular arrhythmias in the setting of chronic ischemic heart disease, radiofrequency transmitters were implanted in wild-type and Cx43(+/-) mice 2 days or 9 weeks after left anterior descending coronary artery ligation or sham operations. ECGs were recorded from unanesthetized, unrestrained mice 1 and 10 weeks after MI. Isolated, perfused hearts excised 1 and 10 weeks after MI were subjected to programmed electrical stimulation to induce arrhythmias.

RESULTS AND CONCLUSIONS

Hearts with infarcts exhibited more spontaneous and inducible arrhythmias, but there was no significant difference between wild-type and Cx43-deficient mice. Fewer hearts exhibited spontaneous ventricular tachycardia (VT) in vivo than were inducible in vitro, suggesting that structural and functional substrates for inducible VT in isolated hearts may not be sufficient for initiation and maintenance of sustained VT in vivo. Previous studies have shown that Cx43-deficient mice exhibit more VT than wild-type mice during acute regional ischemia. Mice with MI exhibit increased arrhythmias. However, reduced coupling in Cx43-deficient mice does not significantly enhance spontaneous or inducible VT after MI.

Role of interleukin-6 for LV remodeling and survival after experimental myocardial infarction

Circulating levels of interleukin (IL)-6 are elevated after myocardial infarction (MI) and associated with increased morbidity and mortality. Its myocardial expression post-MI suggests a pathophysiological role in this condition. To explore the role of endogenous IL-6, we analyzed MI size, left ventricular (LV) remodeling, and mortality after permanent coronary ligation in IL-6 knockout mice (IL-6-/-) and wild-type controls (WT). Six weeks after MI, IL-6-/- and WT had similar mortality rates, MI sizes, LV remodeling, and LV dysfunction in vivo, determined by catheterization. Infarct size 24 h post-MI, shown by 2,3,5-triphenyltetrazolium chloride (TTC) staining, was similar at 24 h. Treatment with exogenous IL-6 did not alter MI size in WT. Infarction resulted in marked phosphorylation of STAT3, without differences between genotypes. Leukemia inhibitory factor (LIF) protein was increased 48 h post-MI in IL-6-/-, and angiotensin II and AT1 receptor (AT1R) protein were strongly increased in IL-6-/- baseline and post-MI, suggesting compensatory up-regulation. Lack of IL-6 does not affect long-term MI size or LV function, remodeling, and survival. In mice lacking IL-6, other members of the IL-6 family such as LIF and other factors signaling via JAK/STAT such as angiotensin may act in a compensatory manner to activate the JAK/STAT pathway, thereby maintaining STAT3 phosphorylation, which is crucial for the cellular effects of IL-6 cytokines.

Implications of ventricular arrhythmia vulnerability during murine electrophysiology studies

Programmed ventricular stimulation is being performed for the provocation of ventricular arrhythmias in genetically engineered mice. Despite the high level of interest in this area of translational research, little attention has been given to differentiating between selectivity and specificity of induced ventricular tachycardia (VT) in phenotypically normal mice. We aimed to assess factors that may enhance inducibility of VT in wild-type (WT) mice. In vivo intracardiac electrophysiological studies (EPS) were performed in 230 WT mice of 4 strains. An octapolar electrode catheter was inserted into a jugular vein and advanced to the right atrium and ventricle. Baseline ventricular conduction, refractoriness, and arrhythmia inducibility were assessed using programmed electrical stimulation (PES) and burst pacing. We found that nonsustained VT (> or =4 beats) was inducible in 68/230 (30%) mice. Duration of VT was 1.6 +/- 2.4 s, and the longest episode lasted 24 s. VT inducibility differed by strain and age. Ventricular effective refractory period (VERP) was shorter in mice with inducible VT (44 +/- 12 ms) compared with noninducible mice (61 +/- 16 ms, P < 0.001). VERP increased with age (P < 0.001), albeit with strain-related variability. We conclude that nonsustained VT in WT mice is reproducibly inducible and common. Genetic background variability may predispose certain strains to a higher incidence of arrhythmia induction. EPS methods impact prevalence and specificity of inducible VT. Increased VT inducibility was seen with shorter coupling intervals and application of tightly coupled extrastimuli techniques. These factors should be carefully considered when analyzing PES and burst pacing data in murine models to minimize false positives and optimize accuracy.

Age and anesthetic effects on murine electrocardiography

Murine models offer potential insights regarding human cardiac disease, but efficient and reliable methods for phenotype evaluation are necessary. We employed non-invasive electrocardiography (ECG) in mice, investigating statistical reliability of these parameters with respect to anesthetic and animal age. Mice (C57BL/6, 8 or 48 weeks) were anesthetized by ketamine/xylazine (K/X, 80/10 mg/kg ip) or by inhalation anesthetic (halothane, HAL; sevoflurane, SEV) and 6 lead ECGs were recorded. P wave duration and QT interval was significantly prolonged with K/X compared to HAL and SEV, indicating slowed atrial and ventricular conduction. P-R interval (atrio-ventricular conduction) was significantly increased in aged mice under all anesthetics. Heart rate was inversely correlated to QT interval and P wave duration. We also detected significant age effects with respect to optimal approaches for QT interval corrections. Power analysis showed 4-fold higher number of mice/group, were required for K/X, to achieve identical statistical sensitivity. These data demonstrate the importance of anesthetic selection for relevant and reliable ECG analysis in mice and illustrate the selective influences of anesthetics and age on cardiac conductance in this species.