Transient outward potassium current and Ca2+ homeostasis in the heart: beyond the action potential

Dengue virus infection induces inflammation and oxidative stress on the heart

OBJECTIVE

Dengue fever is one of the most important arboviral diseases in the world, and its severe forms are characterised by a broad spectrum of systemic and cardiovascular hallmarks. However, much remains to be elucidated regarding the pathogenesis triggered by Dengue virus (DENV) in the heart. Herein, we evaluated the cardiac outcomes unleashed by DENV infection and the possible mechanisms associated with these effects.

METHODS

A model of an adapted DENV-3 strain was used to infect male BALB/c mice to assess haemodynamic measurements and the functional, electrophysiological, inflammatory and oxidative parameters in the heart.

RESULTS

DENV-3 infection resulted in increased systemic inflammation and vascular permeability with consequent reduction of systolic blood pressure and increase in heart rate. These changes were accompanied by a decrease in the cardiac output and stroke volume, with a reduction trend in the left ventricular end-systolic and end-diastolic diameters and volumes. Also, there was a reduction trend in the calcium current density in the ventricular cardiomyocytes of DENV-3 infected mice. Indeed, DENV-3 infection led to leucocyte infiltration and production of inflammatory mediators in the heart, causing pericarditis and myocarditis. Moreover, increased reactive oxygen species generation and lipoperoxidation were also verified in the cardiac tissue of DENV-3 infected mice.

CONCLUSIONS

DENV-3 infection induced a marked cardiac dysfunction, which may be associated with inflammation, oxidative stress and electrophysiological changes in the heart. These findings provide new cardiac insights into the mechanisms involved in the pathogenesis triggered by DENV, contributing to the research of new therapeutic targets for clinical practice.

In-silico study of age-related ionic remodeling in human ventricular cardiomyocytes

Ageing is one of the dominant risk factors for cardiovascular diseases. A large number of experimental data is collected on the cellular remodeling in the ageing myocardium from mammals, but very little is known about the human cardiomyocytes. We used a combined electro-mechanical model of human ventricular cardiomyocytes and a population of models approach to investigate the variability in the response of cardiomyocytes to age-related changes in model parameters of the ionic currents.

To generate a control model population, we varied 9 ionic parameters and excluded model samples with biomarkers of cellular action potential (AP) and Ca2+ transient (CT) falling outside the physiological ranges. Using the control population of models, we evaluated the response to age-related reduction in the K+ transient outward current, SERCA pump, and an increase in the Na+Ca2+ exchange current and L-type Ca2+ current. Then, we randomly generated 60 age-related sets of the 4 parameters and applied each set to every model in the control population. We showed an increase in the frequency of repolarization anomalies (RA) and critical AP prolongation in the ageing model populations suggesting arrhythmogenic effects of the ionic remodeling. The population based approach allowed us to assess the pro-arrhythmic contribution of the ionic parameters in ageing cardiomyocytes.

Kv4.3 expression reverses ICa remodeling in ventricular myocytes of heart failure

Background

Ca²⁺/calmodulin-dependent protein kinase II (CaMKII)-dependent L-type calcium channel (LTCC) current (I_Ca) remodeling is an important contributor to the disruption of calcium homeostasis in heart failure (HF). We have reported that Kv4.3 proteins play an important role in delicate regulation of the membrane-associated CaMKII activity in ventricular myocytes. Here, we investigated the effect of in vivo Kv4.3 expression on I_Ca in HF left ventricular (LV) myocytes.

Results

Kv4.3 expression reduced overall CaMKII autophosphorylation with much greater reduction in the membrane compartmentalized CaMKII activity. I_Ca density in subepicardial (SEP) and subendocardial (SEN) myocytes was proportionately reduced, without changing the transmural gradient. While the time course of I_Ca decay was hastened, the voltage-dependence of I_Ca activation and inactivation, however, remained unchanged. I_Ca recovery from inactivation was significantly accelerated. In line with the partial inhibition of CaMKII, the frequency-dependent Ca²⁺-induced I_Ca facilitation was recovered in the HF myocytes transfected with Kv4.3.

Materials and Methods

Pressure-overload HF was induced by thoracic aortic banding. Kv4.3 expression was achieved by Ad-Kv4.3 injection in the LV myocardium. I_Ca was recorded in dissociated SEP and SEN myocytes using whole-cell patch clamp method.

Conclusions

Kv4.3 expression in HF ventricle can effectively reverse I_Ca remodeling via inhibition of the membrane-associated CaMKII, pointing to Kv4.3 restoration as a potential therapeutic approach for the disordered calcium regulation in HF.

Phase-contrast magnet resonance imaging reveals regional, transmural, and base-to-apex dispersion of mechanical dysfunction in patients with long QT syndrome

BACKGROUND

Regional dispersion of prolonged repolarization is a hallmark of long QT syndrome (LQTS). We have also revealed regional heterogeneities in mechanical dysfunction in transgenic rabbit models of LQTS.

OBJECTIVE

In this clinical pilot study, we investigated whether patients with LQTS exhibit dispersion of mechanical/diastolic dysfunction.

METHODS

Nine pediatric patients with genotyped LQTS (12.2 ± 3.3 years) and 9 age- and sex-matched healthy controls (10.6 ± 1.5 years) were subjected to phase-contrast magnetic resonance imaging to analyze radial (Vr) and longitudinal (Vz) myocardial velocities during systole and diastole in the left ventricle (LV) base, mid, and apex. Twelve-lead electrocardiograms were recorded to assess the heart rate-corrected QT (QTc) interval.

RESULTS

The QTc interval was longer in patients with LQTS than in controls (469.1 ± 39.4 ms vs 417.8 ± 24.4 ms; P < .01). Patients with LQTS demonstrated prolonged radial and longitudinal time-to-diastolic peak velocities (TTP), a marker for prolonged contraction duration, in the LV base, mid, and apex. The longer QTc interval positively correlated with longer time-to-diastolic peak velocities (correlation coefficient 0.63; P < .01). Peak diastolic velocities were reduced in LQTS in the LV mid and apex, indicating impaired diastolic relaxation. In patients with LQTS, regional (TTPmax-min) and transmural (TTPVz-Vr) dispersion of contraction duration was increased in the LV apex (TTPVz_max-min: 38.9 ± 25.5 ms vs 20.2 ± 14.7 ms; P = .07; TTPVz-Vr: -21.7 ± 14.5 ms vs -8.7 ± 11.3 ms; P < .05). The base-to-apex longitudinal relaxation sequence was reversed in patients with LQTS compared with controls (TTPVz_base-apex: 14.4 ± 14.9 ms vs -10.1 ± 12.7 ms; P < .01).

CONCLUSION

Patients with LQTS exhibit diastolic dysfunction with reduced diastolic velocities and prolonged contraction duration. Mechanical dispersion is increased in LQTS with an increased regional and transmural dispersion of contraction duration and altered apicobasal longitudinal relaxation sequence. LQTS is an electromechanical disorder, and phase-contrast magnetic resonance imaging Heterogeneity in mechanical dysfunction enables a detailed assessment of mechanical consequences of LQTS.

Changes in the action potential and transient outward potassium current in cardiomyocytes during acute cardiac rejection in rats

BACKGROUND

Acute cardiac rejection contributes to the changes in the electrophysiological properties of grafted hearts. However, the electrophysiological changes of cardiomyocytes during acute cardiac rejection are still unknown. An understanding of the electrophysiological mechanisms of cardiomyocytes could improve the diagnosis and treatment of acute cardiac rejection. So it is important to characterize the changes in the action potential (AP) and the transient outward potassium current (I_to ) in cardiomyocytes during acute cardiac rejection.

METHODS

Heterotopic heart transplantation was performed in allogeneic [Brown Norway (BN)-to-Lewis] and isogeneic (BN-to-BN) rats. Twenty models were established in each group. Ten recipients were sacrificed at the 2nd day and the other ten recipients were sacrificed at the 4^th day after the operation in each group. Histopathological examinations of the grafted hearts were performed in half of the recipients in each group randomly. The other half of the grafted hearts were excised rapidly and enzymatically dissociated to obtain single cardiomyocytes. The AP and I_to current were recorded using the whole cell patch-clamp technique.

RESULTS

Forty grafted hearts were successfully harvested and used in experiments. Histologic examination showed mild rejection at the 2^nd day and moderate rejection at the 4^th day in the allogeneic group after cardiac transplantation, while no evidence of histologic lesions of rejection were observed in the isogeneic group. Compared with the isogeneic group, the action potential duration (APD) of cardiomyocytes in the allogeneic group was significantly prolonged (APD₉₀ was 49.28±5.621 mV in the isogeneic group and 88.08±6.445 mV in the allogeneic group at the 2^nd day, P=0.0016; APD₉₀ was 59.34±5.183 mV in the isogeneic group and 104.0±9.523 mV in the allogeneic group at the 4^th day, P=0.0064). The current density of I_to was significantly decreased at the 4^th day after cardiac transplantation.

CONCLUSIONS

The APD of cardiomyocytes was significantly prolonged during acute cardiac rejection in rats, which might be partly attributed to decreased current densities of I_to .

Transgenic rabbit models to investigate the cardiac ion channel disease long QT syndrome

Long QT syndrome (LQTS) is a rare inherited channelopathy caused mainly by different mutations in genes encoding for cardiac K(+) or Na(+) channels, but can also be caused by commonly used ion-channel-blocking and QT-prolonging drugs, thus affecting a much larger population. To develop novel diagnostic and therapeutic strategies to improve the clinical management of these patients, a thorough understanding of the pathophysiological mechanisms of arrhythmogenesis and potential pharmacological targets is needed. Drug-induced and genetic animal models of various species have been generated and have been instrumental for identifying pro-arrhythmic triggers and important characteristics of the arrhythmogenic substrate in LQTS. However, due to species differences in features of cardiac electrical function, these different models do not entirely recapitulate all aspects of the human disease. In this review, we summarize advantages and shortcomings of different drug-induced and genetically mediated LQTS animal models - focusing on mouse and rabbit models since these represent the most commonly used small animal models for LQTS that can be subjected to genetic manipulation. In particular, we highlight the different aspects of arrhythmogenic mechanisms, pro-arrhythmic triggering factors, anti-arrhythmic agents, and electro-mechanical dysfunction investigated in transgenic LQTS rabbit models and their translational application for the clinical management of LQTS patients in detail. Transgenic LQTS rabbits have been instrumental to increase our understanding of the role of spatial and temporal dispersion of repolarization to provide an arrhythmogenic substrate, genotype-differences in the mechanisms for early afterdepolarization formation and arrhythmia maintenance, mechanisms of hormonal modification of arrhythmogenesis and regional heterogeneities in electro-mechanical dysfunction in LQTS.

Electro-mechanical dysfunction in long QT syndrome: Role for arrhythmogenic risk prediction and modulation by sex and sex hormones

Long QT syndrome (LQTS) is a congenital arrhythmogenic channelopathy characterized by impaired cardiac repolarization. Increasing evidence supports the notion that LQTS is not purely an "electrical" disease but rather an "electro-mechanical" disease with regionally heterogeneously impaired electrical and mechanical cardiac function. In the first part, this article reviews current knowledge on electro-mechanical (dys)function in LQTS, clinical consequences of the observed electro-mechanical dysfunction, and potential underlying mechanisms. Since several novel imaging techniques - Strain Echocardiography (SE) and Magnetic Resonance Tissue Phase Mapping (TPM) - are applied in clinical and experimental settings to assess the (regional) mechanical function, advantages of these non-invasive techniques and their feasibility in the clinical routine are particularly highlighted. The second part provides novel insights into sex differences and sex hormone effects on electro-mechanical cardiac function in a transgenic LQT2 rabbit model. Here we demonstrate that female LQT2 rabbits exhibit a prolonged time to diastolic peak - as marker for contraction duration and early relaxation - compared to males. Chronic estradiol-treatment enhances these differences in time to diastolic peak even more and additionally increases the risk for ventricular arrhythmia. Importantly, time to diastolic peak is particularly prolonged in rabbits exhibiting ventricular arrhythmia - regardless of hormone treatment - contrasting with a lack of differences in QT duration between symptomatic and asymptomatic LQT2 rabbits. This indicates the potential added value of the assessment of mechanical dysfunction in future risk stratification of LQTS patients.

Ion Channels in the Heart

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

Prolonged leptin treatment increases transient outward K⁺ current via upregulation of Kv4.2 and Kv4.3 channel subunits in adult rat ventricular myocytes

Circulating leptin levels are elevated in obesity and hyperleptinaemia has been postulated to be an independent risk factor for the development of cardiovascular diseases. Although many studies have been published on the mechanisms involved in the effects of leptin on cardiac function and pathological remodeling, scarce information is currently available analyzing the influence of prolonged leptin treatment on ionic cardiac channels remodeling in adult ventricular myocytes. Enzymatically isolated adult rat ventricular myocytes were treated with leptin or vehicle for 48h. Real-Time RT-PCR were used to analyze mRNA expression of Kir2.1, Cav1.2, Cav 3.1, Kv4.2 and Kv4.3 α-subunits and KChIP2 auxiliary subunit. The fast transient outward potassium channels (Itof) α-subunits Kv4.2, Kv4.3 and KChIP2 were analyzed by Western-blot. The fast transient outward potassium current and the action potentials were recorded in isolated myocytes by the whole-cell patch-clamp technique. Leptin treatment induced an up-regulation of Kv4.2, Kv4.3 and KChIP2 subunits mRNA expression. However, transcriptional levels of Kir2.1, Cav1.2, or Cav3.1 α-subunit channels were unmodified by leptin. Protein expression levels of Kv4.2, Kv4.3 and KChIP2 subunits were also increased by leptin. The electrophysiological study showed that leptin increases the fast transient outward potassium current amplitudes and densities shortening action potential duration. In addition, leptin activated Akt signaling in cardiomyocytes and this mechanism was involved in the effect of leptin on Itof channels. In conclusión, leptin increases both the expression and function of Itof channels in adult ventricular myocytes and this mechanism involves Akt signaling. Altogether these data suggest that leptin could exert beneficial or detrimental effects depending on the initial ventricular myocyte repolarizing reserve.

Spatial correlation of action potential duration and diastolic dysfunction in transgenic and drug-induced LQT2 rabbits

BACKGROUND

Enhanced dispersion of action potential duration (APD) is a major contributor to long QT syndrome (LQTS)-related arrhythmias.

OBJECTIVE

To investigate spatial correlations of regional heterogeneities in cardiac repolarization and mechanical function in LQTS.

METHODS

Female transgenic LQTS type 2 (LQT2; n = 11) and wild-type littermate control (LMC) rabbits (n = 9 without E4031 and n = 10 with E4031) were subjected to phase contrast magnetic resonance imaging to assess regional myocardial velocities. In the same rabbits' hearts, monophasic APDs were assessed in corresponding segments.

RESULTS

In LQT2 and E4031-treated rabbits, APD was longer in all left ventricular segments (P < .01) and APD dispersion was greater than that in LMC rabbits (P < .01). In diastole, peak radial velocities (Vr) were reduced in LQT2 and E4031-treated compared to LMC rabbits in LV base and mid (LQT2: -3.36 ± 0.4 cm/s, P < .01; E4031-treated: -3.24 ± 0.6 cm/s, P < .0001; LMC: -4.42 ± 0.5 cm/s), indicating an impaired diastolic function. Regionally heterogeneous diastolic Vr correlated with APD (LQT2: correlation coefficient [CC] 0.38, P = .01; E4031-treated: CC 0.42, P < .05). Time-to-diastolic peak Vr were prolonged in LQT2 rabbits (LQT2: 196.8 ± 2.9 ms, P < .001; E4031-treated: 199.5 ± 2.2 ms, P < .0001, LMC 183.1 ± 1.5), indicating a prolonged contraction duration. Moreover, in transgenic LQT2 rabbits, diastolic time-to-diastolic peak Vr correlated with APD (CC 0.47, P = .001). In systole, peak Vr were reduced in LQT2 and E4031-treated rabbits (P < .01) but longitudinal velocities or ejection fraction did not differ. Finally, random forest machine learning algorithms enabled a differentiation between LQT2, E4031-treated, and LMC rabbits solely based on "mechanical" magnetic resonance imaging data.

CONCLUSIONS

The prolongation of APD led to impaired diastolic and systolic function in transgenic and drug-induced LQT2 rabbits. APD correlated with regional diastolic dysfunction, indicating that LQTS is not purely an electrical but an electromechanical disorder.

Brugada-like syndrome in infancy presenting with rapid ventricular tachycardia and intraventricular conduction delay

BACKGROUND

Brugada syndrome is a potentially serious channelopathy that usually presents in adulthood and has only rarely been described in infancy. In the absence of metabolic or structural cardiac disease, rapid ventricular tachycardia (>200 bpm) and primary cardiac conduction disease are uncommon in infancy. We hypothesized that infants having rapid ventricular tachycardia and conduction abnormalities and not having structural or metabolic pathogeneses were likely to have mutations in depolarizing current channels.

METHODS AND RESULTS

A retrospective review of all clinical materials from a single institution over a 9-year period from all infants <2 years old and having a discharge diagnosis of ventricular tachycardia or ventricular fibrillation was performed. Among 32 infants fulfilling inclusion criteria, 12 had a structurally normal heart, and 9 of them had either prolonged QRS duration or Brugada pattern while in sinus rhythm. Of those 5 infants not having a definitive pathogenesis, electrophysiological testing had been performed in 4, and genetic testing had been performed in all 5 of those infants. During electrophysiological testing, a prolonged HV interval was present in 2 of 4, inducible ventricular tachycardia was present in 1 of 4, and a type 1 Brugada pattern was induced by intravenous procainamide in 3 of 4. Genetic testing revealed disease-causing mutations in depolarizing sodium (SCN5A) or calcium (CaCNB2b) channels in all 5 infants.

CONCLUSIONS

Infants having rapid ventricular tachycardia and conduction abnormalities in the absence of structural or metabolic abnormalities are likely to have disease-causing mutations in cardiac depolarizing channels.

Central role of PKCα in isoenzyme-selective regulation of cardiac transient outward current Ito and Kv4.3 channels

The transient outward current I(to) is an important determinant of the early repolarization phase. I(to) and its molecular basis Kv4.3 are regulated by adrenergic pathways including protein kinase C. However, the exact regulatory mechanisms have not been analyzed yet. We here analyzed isoenzyme specific regulation of Kv4.3 and I(to) by PKC. Kv4.3 channels were expressed in Xenopus oocytes and currents were measured with double electrode voltage clamp technique. Patch clamp experiments were performed in isolated rat cardiomyocytes. Unspecific PKC stimulation with PMA resulted in a reduction of Kv4.3 current. Similar effects could be observed after activation of conventional PKC isoforms by TMX. Both effects were reversible by pharmacological inhibition of the conventional PKC isoenzymes (Gö6976). In contrast, activation of the novel PKC isoforms (ingenol) did not significantly affect Kv4.3 current. Whereas TMX-induced PKC activation was not attenuated inhibition of PKCβ, inhibition of PKCα with HBDDE prevented inhibitory effects of both PMA and TMX. Accordingly, stimulatory effects of PMA and TMX could be mimicked by the α-isoenzyme selective PKC activator iripallidal. Further evidence for the central role of PKCα was provided with the use of siRNAs. We found that PKCα siRNA but not PKCβ siRNA abolished the TMX induced effect. In isolated rat cardiomyocytes, PMA dependent I(to) reduction could be completely abolished by pharmacologic inhibition of PKCα. In summary we show that PKCα plays a central role in protein kinase C dependent regulation of Kv4.3 current and native I(to). These results add to the current understanding of isoenzyme selective ion channel regulation by protein kinases.

Modulation of sarcoplasmic reticulum Ca(2+) cycling in systolic and diastolic heart failure associated with aging

Hypertension, atherosclerosis, and resultant chronic heart failure (HF) reach epidemic proportions among older persons, and the clinical manifestations and the prognoses of these worsen with increasing age. Thus, age per se is the major risk factor for cardiovascular disease. Changes in cardiac cell phenotype that occur with normal aging, as well as in HF associated with aging, include deficits in ss-adrenergic receptor (ss-AR) signaling, increased generation of reactive oxygen species (ROS), and altered excitation-contraction (EC) coupling that involves prolongation of the action potential (AP), intracellular Ca(2+) (Ca(i)(2+)) transient and contraction, and blunted force- and relaxation-frequency responses. Evidence suggests that altered sarcoplasmic reticulum (SR) Ca(2+) uptake, storage, and release play central role in these changes, which also involve sarcolemmal L-type Ca(2+) channel (LCC), Na(+)-Ca(2+) exchanger (NCX), and K(+) channels. We review the age-associated changes in the expression and function of Ca(2+) transporting proteins, and functional consequences of these changes at the cardiac myocyte and organ levels. We also review sexual dimorphism and self-renewal of the heart in the context of cardiac aging and HF.

Heart rate reduction with ivabradine prevents the global phenotype of left ventricular remodeling

The aim of this study was to investigate the effect of chronic heart rate (HR) reduction with the hyperpolarization-activated current inhibitor ivabradine on the global phenotype of left ventricular (LV) remodeling in a ligated rat model. Seven days after coronary artery ligation, Wistar rats received ivabradine (10 mg·kg−1·day−1 administered in drinking water) [myocardial infarction + ivabradine (MI+IVA), n = 22] or vehicle only (drinking water) (MI, n = 20) for 90 days. A sham group ( n = 20) was included for model validation. MI+IVA rats had 12% lower HR ( P < 0.01), improved LV volumes, 15% higher LV ejection fraction (LVEF, P < 0.01) than MI rats, and 33% reductions in both plasma atrial natriuretic peptide (ANP, P = 0.052) and cardiac hydroxyproline. Using patch-clamp, action potential duration was reduced and transient outward current density increased ( P < 0.05). Cardiac energy metabolism was also improved (+33% creatine phosphate, P < 0.001; +15% ATP; and +9% energy charge, P < 0.05). Significant correlations were found between HR and parameters of cardiac metabolism, ANP, and LVEF (all P < 0.05). The HR-reducing properties of ivabradine prevent changes in the global phenotype of LV remodeling in the rat, optimize energy consumption, and avoid electrophysiological and structural remodeling.

Control of Ca2+ release by action potential configuration in normal and failing murine cardiomyocytes

Cardiomyocytes from failing hearts exhibit spatially nonuniform or dyssynchronous sarcoplasmic reticulum (SR) Ca(2+) release. We investigated the contribution of action potential (AP) prolongation in mice with congestive heart failure (CHF) after myocardial infarction. AP recordings from CHF and control myocytes were included in a computational model of the dyad, which predicted more dyssynchronous ryanodine receptor opening during stimulation with the CHF AP. This prediction was confirmed in cardiomyocyte experiments, when cells were alternately stimulated by control and CHF AP voltage-clamp waveforms. However, when a train of like APs was used as the voltage stimulus, the control and CHF AP produced a similar Ca(2+) release pattern. In this steady-state condition, greater integrated Ca(2+) entry during the CHF AP lead to increased SR Ca(2+) content. A resulting increase in ryanodine receptor sensitivity synchronized SR Ca(2+) release in the mathematical model, thus offsetting the desynchronizing effects of reduced driving force for Ca(2+) entry. A modest nondyssynchronous prolongation of Ca(2+) release was nevertheless observed during the steady-state CHF AP, which contributed to increased time-to-peak measurements for Ca(2+) transients in failing cells. Thus, dyssynchronous Ca(2+) release in failing mouse myocytes does not result from electrical remodeling, but rather other alterations such as T-tubule reorganization.

Dynamic Kv4.3-CaMKII unit in heart: an intrinsic negative regulator for CaMKII activation

AIMS

Reduction of transient outward current (I(to)) and excessive activation of Ca(2+)/Calmodulin-dependent kinase II (CaMKII) are general features of ventricular myocytes in heart failure. We hypothesize that alterations of I(to) directly regulate CaMKII activation in cardiomyocytes.

METHODS AND RESULTS

A dynamic coupling of I(to) channel subunit Kv4.3 and inactive CaMKII was discovered in cardiomyocytes with the membrane predominant distribution by co-immunoprecipitation and fluorescence resonance energy transfer techniques. CaMKII dissociation from Kv4.3-CaMKII units caused a significant increase in CaMKII autophosphorylation and L-type calcium current (I(Ca)) facilitation. I(Ca) facilitation was blunted by the compartmental Ca²(+) chelator BAPTA but unaffected by bulk Ca²(+) chelator EGTA, implicating membrane-localized CaMKII. Kv4.3 overexpression reduced basal CaMKII autophosphorylation in myocytes and eliminated Ca²(+)-induced CaMKII activation. Kv4.3 blocks CaMKII activation by binding to the calmodulin binding sites, whereas Kv4.3 uncoupling releases these sites and leads to a substantial CaMKII activation.

CONCLUSION

Our results uncovered an important mechanism that regulates CaMKII activation in the heart and implicate I(to) channel alteration in pathological CaMKII activation.

Growth hormone secretagogues reduce transient outward K+ current via phospholipase C/protein kinase C signaling pathway in rat ventricular myocytes

Endogenous ghrelin and its synthetic counterpart hexarelin are peptide GH secretagogues (GHS) that exert a positive ionotropic effect in the cardiovascular system. The mechanism by which GHS modulate cardiac electrophysiology properties to alter myocyte contraction is poorly understood. In the present study, we examined whether GHS regulates the transient outward potassium current (I(to)) as well as the putative intracellular signaling cascade responsible for such regulation. GHS and experimental agents were applied locally onto freshly isolated adult Sprague-Dawley rat ventricular myocytes and action potential morphology and I(to) was recorded using nystatin-perforated whole-cell patch-clamp recording technique. Under current clamp, ghrelin and hexarelin (10 nm) significantly prolonged action potential duration. Under voltage clamp, hexarelin and ghrelin inhibited I(to) in a concentration-dependent manner. This inhibition was abolished in the presence of the GHS receptor (GHS-R) antagonist [D-Lys(3)]GH-releasing peptide-6 (10 microm) and GHS-R1a-specific antagonist BIM28163 (1 microm). GHS-induced I(to) inhibition was totally reversed by the phospholipase C inhibitor U73122 (5 microm) and protein kinase C inhibitors GO6983 (1 microm) and calphostin C (0.1 microm) but not by the cAMP antagonist Rp-cAMP (100 microm) or the PKA inhibitor H89 (1 microm). We conclude that hexarelin and ghrelin activate phospholipase C and protein kinase C signaling cascade through the stimulation of the GHS-R, resulting in a decrease in the I(to) current and subsequent prolongation of action potential duration.

Report and recommendations of the workshop of the European Centre for the Validation of Alternative Methods for Drug-Induced Cardiotoxicity

Cardiotoxicity is among the leading reasons for drug attrition and is therefore a core subject in non-clinical and clinical safety testing of new drugs. European Centre for the Validation of Alternative Methods held in March 2008 a workshop on "Alternative Methods for Drug-Induced Cardiotoxicity" in order to promote acceptance of alternative methods reducing, refining or replacing the use of laboratory animals in this field. This review reports the outcome of the workshop. The participants identified the major clinical manifestations, which are sensitive to conventional drugs, to be arrhythmias, contractility toxicity, ischaemia toxicity, secondary cardiotoxicity and valve toxicity. They gave an overview of the current use of alternative tests in cardiac safety assessments. Moreover, they elaborated on new cardiotoxicological endpoints for which alternative tests can have an impact and provided recommendations on how to cover them.