Simulation and mechanistic investigation of the arrhythmogenic role of the late sodium current in human heart failure

Heart failure constitutes a major public health problem worldwide. The electrophysiological remodeling of failing hearts sets the stage for malignant arrhythmias, in which the role of the late Na⁺ current (I_NaL) is relevant and is currently under investigation. In this study we examined the role of I_NaL in the electrophysiological phenotype of ventricular myocytes, and its proarrhythmic effects in the failing heart. A model for cellular heart failure was proposed using a modified version of Grandi et al. model for human ventricular action potential that incorporates the formulation of I_NaL. A sensitivity analysis of the model was performed and simulations of the pathological electrical activity of the cell were conducted. The proposed model for the human I_NaL and the electrophysiological remodeling of myocytes from failing hearts accurately reproduce experimental observations. The sensitivity analysis of the modulation of electrophysiological parameters of myocytes from failing hearts due to ion channels remodeling, revealed a role for I_NaL in the prolongation of action potential duration (APD), triangulation of the shape of the AP, and changes in Ca²⁺ transient. A mechanistic investigation of intracellular Na⁺ accumulation and APD shortening with increasing frequency of stimulation of failing myocytes revealed a role for the Na⁺/K⁺ pump, the Na⁺/Ca²⁺ exchanger and I_NaL. The results of the simulations also showed that in failing myocytes, the enhancement of I_NaL increased the reverse rate-dependent APD prolongation and the probability of initiating early afterdepolarizations. The electrophysiological remodeling of failing hearts and especially the enhancement of the I_NaL prolong APD and alter Ca²⁺ transient facilitating the development of early afterdepolarizations. An enhanced I_NaL appears to be an important contributor to the electrophysiological phenotype and to the dysregulation of [Ca²⁺]_i homeostasis of failing myocytes.

Nanoscale organization of junctophilin-2 and ryanodine receptors within peripheral couplings of rat ventricular cardiomyocytes

The peripheral distributions of the cardiac ryanodine receptor (RyR) and a junctional protein, junctophilin-2 (JPH2), were examined using single fluorophore localization-based super-resolution microscopy in rat ventricular myocytes. JPH2 was strongly associated with RyR clusters. Estimates of the colocalizing fraction of JPH labeling with RyR was ~90% within 30 nm of RyR clusters. This is comparable to fractions estimated from confocal data (~87%). Similarly, most RyRs were associated with JPH2 labeling in super-resolution images (~81% within 30 nm of JPH2 clusters). The shape of associated RyR clusters and JPH2 clusters were very similar, but not identical, suggesting that JPH2 is dispersed throughout RyR clusters and that the packing of JPH2 into junctions and the assembly of RyR clusters are tightly linked.

Conditional ablation of Ezh2 in murine hearts reveals its essential roles in endocardial cushion formation, cardiomyocyte proliferation and survival

Ezh2 is a histone trimethyltransferase that silences genes mainly via catalyzing trimethylation of histone 3 lysine 27 (H3K27Me3). The role of Ezh2 as a regulator of gene silencing and cell proliferation in cancer development has been extensively investigated; however, its function in heart development during embryonic cardiogenesis has not been well studied. In the present study, we used a genetically modified mouse system in which Ezh2 was specifically ablated in the mouse heart. We identified a wide spectrum of cardiovascular malformations in the Ezh2 mutant mice, which collectively led to perinatal death. In the Ezh2 mutant heart, the endocardial cushions (ECs) were hypoplastic and the endothelial-to-mesenchymal transition (EMT) process was impaired. The hearts of Ezh2 mutant mice also exhibited decreased cardiomyocyte proliferation and increased apoptosis. We further identified that the Hey2 gene, which is important for cardiomyocyte proliferation and cardiac morphogenesis, is a downstream target of Ezh2. The regulation of Hey2 expression by Ezh2 may be independent of Notch signaling activity. Our work defines an indispensible role of the chromatin remodeling factor Ezh2 in normal cardiovascular development.

A localized meshless approach for modeling spatial-temporal calcium dynamics in ventricular myocytes

Spatial–temporal calcium dynamics due to calcium release, buffering and re-uptaking plays a central role in studying excitation–contraction (E–C) coupling in both normal and diseased cardiac myocytes. In this paper, we employ a meshless method, namely, the local radial basis function collocation method (LRBFCM), to model such calcium behaviors by solving a nonlinear system of reaction–diffusion partial differential equations. In particular, a simplified structural unit containing a single transverse tubule (T-tubule) and its surrounding half sarcomeres is investigated using the meshless method. Numerical results are compared with those generated by finite element methods, showing the capability and efficiency of the LRBFCM in modeling calcium dynamics in ventricular myocytes. The single T-tubule model is also extended to the whole-cell scale with T-tubules excluded to demonstrate the scalability of the proposed meshless method in handling very large domains. The experiments have shown that the LRBFCM is suitable to multiscale modeling of calcium dynamics in ventricular myocytes with high accuracy and efficiency.

Ghrelin stimulates angiogenesis via GHSR1a-dependent MEK/ERK and PI3K/Akt signal pathways in rat cardiac microvascular endothelial cells

Ghrelin, an endogenous ligand of the growth hormone secretagogue receptor (GHSR), is thought to exert a protective effect on the cardiovascular system, specifically by promoting vascular endothelial cell function such as cell proliferation, migration, survival and angiogenesis. However, the effect of ghrelin on angiogenesis and the corresponding mechanisms have not yet been extensively studied in cardiac microvascular endothelial cells (CMECs) isolated from left ventricular myocardium of adult Sprague-Dawley (SD) rats. In our study, we found that ghrelin and GHSR are constitutively expressed in CMECs. Ghrelin significantly increases CMECs proliferation, migration, and in vitro angiogenesis. The ghrelin-induced angiogenic process was accompanied by phosphorylation of ERK and Akt. MEK inhibitor PD98059 abolished ghrelin-induced phosphorylation of ERK, but had no effect on Akt phosphorylation. PI3K inhibitor LY294002 abolished ghrelin-induced phosphorylation of Akt, but had no effect on ERK phosphorylation. Ghrelin-induced angiogenesis was partially blocked by treatment with PD98059 or LY294002. In addition, this angiogenic effect was almost completely inhibited by PD98059+LY294002. Pretreatment with GHSR1a blocker [D-Lys3]-GHRP-6 abolished ghrelin-induced phosphorylation of ERK, Akt and in vitro angiogenesis. In conclusion, this is the first demonstration that ghrelin stimulates CMECs angiogenesis through GHSR1a-mediated MEK/ERK and PI3K/Akt signal pathways, indicating that two pathways are required for full angiogenic activity of ghrelin. This study suggests that ghrelin may play an important role in myocardial angiogenesis.

Quantitative comparison of myocardial fiber structure between mice, rabbit, and sheep using diffusion tensor cardiovascular magnetic resonance

BACKGROUND

Accurate interpretations of cardiac functions require precise structural models of the myocardium, but the latter is not available always and for all species. Although scaling or substitution of myocardial fiber information from alternate species has been used in cardiac functional modeling, the validity of such practice has not been tested.

METHODS

Fixed mouse (n = 10), rabbit (n = 6), and sheep (n = 5) hearts underwent diffusion tensor imaging (DTI). The myocardial structures in terms of the left ventricular fiber orientation helix angle index were quantitatively compared between the mouse rabbit and sheep hearts.

RESULTS

The results show that significant fiber structural differences exist between any two of the three species. Specifically, the subepicardial fiber orientation, and the transmural range and linearity of fiber helix angles are significantly different between the mouse and either rabbit or sheep. Additionally, a significant difference was found between the transmural helix angle range between the rabbit and sheep. Across different circumferential regions of the heart, the fiber orientation was not found to be significantly different.

CONCLUSIONS

The current study indicates that myocardial structural differences exist between different size hearts. An immediate implication of the present findings for myocardial structural or functional modeling studies is that caution must be exercised when extrapolating myocardial structures from one species to another.

Distinct roles of microRNA-1 and -499 in ventricular specification and functional maturation of human embryonic stem cell-derived cardiomyocytes

BACKGROUND

MicroRNAs (miRs) negatively regulate transcription and are important determinants of normal heart development and heart failure pathogenesis. Despite the significant knowledge gained in mouse studies, their functional roles in human (h) heart remain elusive.

METHODS AND RESULTS

We hypothesized that miRs that figure prominently in cardiac differentiation are differentially expressed in differentiating, developing, and terminally mature human cardiomyocytes (CMs). As a first step, we mapped the miR profiles of human (h) embryonic stem cells (ESCs), hESC-derived (hE), fetal (hF) and adult (hA) ventricular (V) CMs. 63 miRs were differentially expressed between hESCs and hE-VCMs. Of these, 29, including the miR-302 and -371/372/373 clusters, were associated with pluripotency and uniquely expressed in hESCs. Of the remaining miRs differentially expressed in hE-VCMs, 23 continued to express highly in hF- and hA-VCMs, with miR-1, -133, and -499 displaying the largest fold differences; others such as miR-let-7a, -let-7b, -26b, -125a and -143 were non-cardiac specific. Functionally, LV-miR-499 transduction of hESC-derived cardiovascular progenitors significantly increased the yield of hE-VCMs (to 72% from 48% of control; p<0.05) and contractile protein expression without affecting their electrophysiological properties (p>0.05). By contrast, LV-miR-1 transduction did not bias the yield (p>0.05) but decreased APD and hyperpolarized RMP/MDP in hE-VCMs due to increased I(to), I(Ks) and I(Kr), and decreased I(f) (p<0.05) as signs of functional maturation. Also, LV-miR-1 but not -499 augmented the immature Ca(2+) transient amplitude and kinetics. Molecular pathway analyses were performed for further insights.

CONCLUSION

We conclude that miR-1 and -499 play differential roles in cardiac differentiation of hESCs in a context-dependent fashion. While miR-499 promotes ventricular specification of hESCs, miR-1 serves to facilitate electrophysiological maturation.

Ca2+ dynamics in the mitochondria - state of the art

The importance of [Ca2+] in the mitochondrial matrix, [Ca2+]mito, had been proposed by early work of Carafoli and others [1 ], [2 ] and [3 ]. The key suggestion in the 1970s [4 ] was that regulatory [Ca2+]mito played a role in controlling the rate of activation of tricarboxylic acid cycle dehydrogenases, important in the regulation of ATP production by the electron transport chain (ETC) during oxidative phosphorylation. This view is now established [5 ] and [6 ] and the key questions currently debated are to what extent do the mitochondria acquire and release Ca2+, and what impact do mitochondria have on the dynamic Ca2+ signal in the cardiac ventricular myocyte [7 ]. Although investigations of Ca2+ dynamics in mitochondria have been problematic, disparate and inconclusive, they have also been both provocative and exciting. A recent special issue of this journal presented contrasting perspectives on the speed, extent and mechanisms of changes in [Ca2+]mito, and how these changes may influence cellular spatio-temporal [Ca2+]i dynamics [8 ]. An audio discussion is also available online [9 ]. The uncertain nature of the signaling pathways is noted in Table 1 (see below) which shows mitochondrial proteins and processes that are of current focus and which remain contentious. Each of the “items” listed is largely unsettled, or is a “work in progress”. There may be advocates for opposing positions noted or recent discoveries that must still be tested at multiple levels by diverse laboratories. Currently, the first item, the mitochondrial sodium/calcium exchanger (NCLX) [10 ], appears the most solid with respect to the molecular identification and physiological function, whereas, the recently described candidates of the mitochondrial Ca2+ uniporter (MCU) [11 ] and [12 ] still need to be verified and broadly examined by the scientific community.

[Effect of salvianolic acid B and tetrahydropalmatine on the L-type calcium channel of rat ventricular myocytes]

OBJECTIVE

To observe the effects of the separate and joint use of salvianolic acid B (SalB) and tetrahydropalmatine (THP) on the L-type calcium channel of rat ventricular myocytes.

METHODS

Single isolated ventricular myocytes of rats were obtained using acute enzymolysis separation. The current of the L-type calcium channel was recorded using whole-cell patch clamp technique. Changes of the current peak value of the calcium channel (the vertical distance between the peak value point after activation of the calcium electric current and the electric current track after complete inactivation) were observed before and after medication.

RESULTS

The inhibition rate of using SalB (at the dose of 1, 10, and 100 micromol/L) alone on the current peak value of the calcium channel was respectively (25.3 +/- 16.4)% (n=4), (44.6 +/- 24.0)% (n=6), and (86.0 +/- 20.4)% (n =4). That of using THP (at the dose of 10, 30, and 100 micromol/L) alone on the current peak value of the calcium channel was respectively (22.2 +/- 6.4)% (n=5), (27.4 +/- 1.6)% (n= 3), and (51.0 +/- 23.0)% (n=9). The inhibition potency of joint use of SalB (1 micromol/L) and THP (10 micromol/L) on the current peak value of the calcium channel was stronger than using SalB (1 micromol/L) alone or THP (10 micromol/L) alone, showing statistical difference ( P< 0.05). Atropine hydrochloric acid (14 mmol/L) could reverse the inhibition of THP on the L-type calcium channel, while strengthening the inhibition of SalB.

CONCLUSIONS

Both SalB and THP showed inhibition on the L-type calcium channel of rat ventricular myocytes. They could generate synergistic effects. Besides, their action mechanisms for regulating the L-type calcium channel were different.

Phospholamban binds with differential affinity to calcium pump conformers

To investigate the mechanism of regulation of sarco-endoplasmic reticulum Ca(2+)-ATPase (SERCA) by phospholamban (PLB), we expressed Cerulean-SERCA and yellow fluorescent protein (YFP)-PLB in adult rabbit ventricular myocytes using adenovirus vectors. SERCA and PLB were localized in the sarcoplasmic reticulum and were mobile over multiple sarcomeres on a timescale of tens of seconds. We also observed robust fluorescence resonance energy transfer (FRET) from Cerulean-SERCA to YFP-PLB. Electrical pacing of cardiac myocytes elicited cytoplasmic Ca(2+) elevations, but these increases in Ca(2+) produced only modest changes in SERCA-PLB FRET. The data suggest that the regulatory complex is not disrupted by elevations of cytosolic calcium during cardiac contraction (systole). This conclusion was also supported by parallel experiments in heterologous cells, which showed that FRET was reduced but not abolished by calcium. Thapsigargin also elicited a small decrease in PLB-SERCA binding affinity. We propose that PLB is not displaced from SERCA by high calcium during systole, and relief of functional inhibition does not require dissociation of the regulatory complex. The observed modest reduction in the affinity of the PLB-SERCA complex with Ca(2+) or thapsigargin suggests that the binding interface is altered by SERCA conformational changes. The results are consistent with multiple modes of PLB binding or alternative binding sites.

Comparison of the effects of a transient outward potassium channel activator on currents recorded from atrial and ventricular cardiomyocytes

INTRODUCTION

NS5806 activates the transient outward potassium current (I(to) ) in canine ventricular cells. We compared the effects of NS5806 on canine atrial versus ventricular tissues and myocytes.

METHODS AND RESULTS

NS5806 (10 μM) was evaluated in arterially perfused canine right atrial and right ventricular wedge preparations. In ventricular wedges NS5806 (10 μM) accentuated phase 1 in epicardium (Epi), with little effect in endocardium (Endo), resulting in augmented J-waves on the ECG. In contrast, application of NS5806 (10 μM) to atrial preparations had no effect on phase 1 repolarization but significantly decreased upstroke velocity (dV/dt) and depressed excitability, consistent with sodium channel block. Current and voltage-clamp recordings were made in the absence and presence of NS5806 in (10 μM) enzymatically dissociated atrial and ventricular myocytes. In ventricular myocytes, NS5806 increased I(to) magnitude by 80% and 16% in Epi and Endo, respectively (at +40 mV). In atrial myocytes, NS5806 increased peak I(to) by 25% and had no effect on the sustained current, I(Kur) . Under control conditions, I(Na) density in atrial myocytes was nearly double that in ventricular myocytes. NS5806 caused a shift in steady-state mid-inactivation (V(1/2)) from -73.9 ± 0.27 to -77.3 ± 0.21 mV in ventricular and from -82.6 ± 0.12 to -85.1 ± 0.11 mV in atrial cells, resulting in reduction of I(Na) in both cell types. Expression of mRNA encoding putative I(Na) and I(to) channel subunits was evaluated by qPCR.

CONCLUSION

NS5806 produces a prominent augmentation of I(to) with little effect on I(Na) in the ventricles, but a potent inhibition of I(Na) with little augmentation of I(to) in atria.

Heterogeneity in SDF-1 expression defines the vasculogenic potential of adult cardiac progenitor cells

Rationale

The adult myocardium has been reported to harbor several classes of multipotent progenitor cells (CPCs) with tri-lineage differentiation potential. It is not clear whether c-kit+CPCs represent a uniform precursor population or a more complex mixture of cell types.

Objective

To characterize and understand vasculogenic heterogeneity within c-kit+presumptive cardiac progenitor cell populations.

Methods and Results

c-kit+, sca-1+ CPCs obtained from adult mouse left ventricle expressed stem cell-associated genes, including Oct-4 and Myc, and were self-renewing, pluripotent and clonogenic. Detailed single cell clonal analysis of 17 clones revealed that most (14/17) exhibited trilineage differentiation potential. However, striking morphological differences were observed among clones that were heritable and stable in long-term culture. 3 major groups were identified: round (7/17), flat or spindle-shaped (5/17) and stellate (5/17). Stellate morphology was predictive of vasculogenic differentiation in Matrigel. Genome-wide expression studies and bioinformatic analysis revealed clonally stable, heritable differences in stromal cell-derived factor-1 (SDF-1) expression that correlated strongly with stellate morphology and vasculogenic capacity. Endogenous SDF-1 production contributed directly to vasculogenic differentiation: both shRNA-mediated knockdown of SDF-1 and AMD3100, an antagonist of the SDF-1 receptor CXC chemokine Receptor-4 (CXCR4), reduced tube-forming capacity, while exogenous SDF-1 induced tube formation by 2 non-vasculogenic clones. CPCs producing SDF-1 were able to vascularize Matrigel dermal implants in vivo, while CPCs with low SDF-1 production were not.

Conclusions

Clonogenic c-kit+, sca-1+ CPCs are heterogeneous in morphology, gene expression patterns and differentiation potential. Clone-specific levels of SDF-1 expression both predict and promote development of a vasculogenic phenotype via a previously unreported autocrine mechanism.

Nicotinamide-rich diet improves physical endurance by up-regulating SUR2A in the heart

SUR2A is an ATP-binding protein that serves as a regulatory subunit of cardioprotective ATP-sensitive K(+) (K(ATP) ) channels. Based on signalling pathway regulating SUR2A expression and SUR2A role in regulating numbers of fully assembled K(ATP) channels, we have suggested that nicotinamide-rich diet could improve physical endurance by stimulating SUR2A expression. We have found that mice on nicotinamide-rich diet significantly improved physical endurance, which was associated with significant increase in expression of SUR2A. Transgenic mice with solely overexpressed SUR2A on control diet had increased physical endurance in a similar manner as the wild-type mice on nicotinamide-rich diet. The experiments focused on action membrane potential and intracellular Ca(2+) concentration have demonstrated that increased SUR2A expression was associated with the activation of sarcolemmal K(ATP) channels and steady Ca(2+) levels in cardiomyocytes in response to β-adrenergic stimulation. In contrast, the same challenge in the wild-type was characterized by a lack of the channel activation and rise in intracellular Ca(2+) . Nicotinamide-rich diet was ineffective to increase physical endurance in mice lacking K(ATP) channels. This study has shown that nicotinamide-rich diet improves physical endurance by increasing expression of SUR2A and that this is a sole mechanism of the nicotinamide-rich diet effect. The obtained results suggest that oral nicotinamide is a regulator of SUR2A expression and has a potential as a drug that can improve physical endurance in conditions where this effect would be desirable.

Device for co-culture of sympathetic neurons and cardiomyocytes using microfabrication

Rat superior cervical ganglion (SCG) neurons and ventricular myocytes (VMs) were co-cultured separately in a minichamber placed on a microelectrode-array (MEA) substrate. The minichamber, fabricated photolithographically using polydimethylsiloxane (PDMS), had 2 compartments, 16 microcompartments and 8 microconduits. The SCG neurons were seeded into one of the compartments and all of the microcompartments using a glass pipette controlled by a micromanipulator and a microinjector. The VMs were seeded into the other compartment. Three days after seeding of the VMs, the SCG neurons were still confined to one compartment and all of the microcompartments, and the neurites of the SCG neurons had connected with the VMs via the microconduits. Constant-voltage stimulation, using a train of biphasic square pulses (1 ms at +1 V, followed by -1 ms at 1 V), was applied to the SCG neurons in the microcompartments using 16 electrodes. Evoked responses were observed in several electrodes while electrical stimulation was applied to the SCG neurons. Two-way analysis of variance (ANOVA) revealed that the frequency of the stimulation pulses had significant effects in increasing the beat rate of the VMs, and that the interaction between the frequency and the number of the pulses also had a significant effect on the ratio. No significant increases in the beat rate were observed when propranolol, a β-adrenergic receptor antagonist, was added to the culture medium. These results suggest that synaptic pathways were formed between the SCG neurons and the VMs, and that this co-culture device can be utilized for studies of network-level interactions between sympathetic neurons and cardiomyocytes.

Modulation of material properties of a decellularized myocardial matrix scaffold

Injectable materials offer the potential for minimally invasive therapy for myocardial infarction (MI), either as an acellular scaffold or as a cell delivery vehicle. A recently developed myocardial matrix hydrogel, derived from decellularized porcine ventricular tissue, has the potential to aid in cardiac repair following an MI. Herein, we set out to study the effects of cross-linking on the cardiac hydrogel stiffness, degradation properties, cellular migration, and catheter injectability in vitro. Cross-linking increased stiffness, while slowing degradation and cellular migration through the gels. Additionally, the cross-linked material was pushed through a clinically relevant catheter. These results demonstrate that the material properties of myocardial matrix can be tuned via cross-linking, while maintaining appropriate viscosity for catheter injectability.

Effects of rosiglitazone on the configuration of action potentials and ion currents in canine ventricular cells

BACKGROUND AND PURPOSE

In spite of its widespread clinical application, there is little information on the cellular cardiac effects of the antidiabetic drug rosiglitazone in larger experimental animals. In the present study therefore concentration-dependent effects of rosiglitazone on action potential morphology and the underlying ion currents were studied in dog hearts.

EXPERIMENTAL APPROACH

Standard microelectrode techniques, conventional whole cell patch clamp and action potential voltage clamp techniques were applied in enzymatically dispersed ventricular cells from dog hearts.

KEY RESULTS

At concentrations ≥10 µM rosiglitazone decreased the amplitude of phase-1 repolarization, reduced the maximum velocity of depolarization and caused depression of the plateau potential. These effects developed rapidly and were readily reversible upon washout. Rosiglitazone suppressed several transmembrane ion currents, concentration-dependently, under conventional voltage clamp conditions and altered their kinetic properties. The EC(50) value for this inhibition was 25.2 ± 2.7 µM for the transient outward K(+) current (I(to)), 72.3 ± 9.3 µM for the rapid delayed rectifier K(+) current (I(Kr)) and 82.5 ± 9.4 µM for the L-type Ca(2+) current (I(Ca) ) with Hill coefficients close to unity. The inward rectifier K(+) current (I(K1)) was not affected by rosiglitazone up to concentrations of 100 µM. Suppression of I(to), I(Kr), and I(Ca) was confirmed also under action potential voltage clamp conditions.

CONCLUSIONS AND IMPLICATIONS

Alterations in the densities and kinetic properties of ion currents may carry serious pro-arrhythmic risk in case of overdose with rosiglitazone, especially in patients having multiple cardiovascular risk factors, like elderly diabetic patients.

PITX2 insufficiency leads to atrial electrical and structural remodeling linked to arrhythmogenesis

BACKGROUND

Pitx2 is a homeobox transcription factor that plays a pivotal role in early left/right determination during embryonic development. Pitx2 loss-of-function mouse mutants display early embryonic lethality with severe cardiac malformations, demonstrating the importance of Pitx2 during cardiogenesis. Recently, independent genome-wide association studies have provided new evidence for a putative role of PITX2 in the adult heart. These studies have independently reported several risk variants close to the PITX2 locus on chromosome 4q25 that are strongly associated with atrial fibrillation in humans.

METHODS AND RESULTS

We show for the first time that PITX2C expression is significantly decreased in human patients with sustained atrial fibrillation, thus providing a molecular link between PITX2 loss of function and atrial fibrillation. In addition, morphological, molecular, and electrophysiological characterization of chamber-specific Pitx2 conditional mouse mutants reveals that atrial but not ventricular chamber-specific deletion of Pitx2 results in differences in the action potential amplitude and resting membrane potential in the adult heart as well as ECG characteristics of atrioventricular block. Lack of Pitx2 in atrial myocardium impairs sodium channel and potassium channel expression, mediated in part by miRNA misexpression.

CONCLUSIONS

This study thus identifies Pitx2 as an upstream transcriptional regulator of atrial electric function, the insufficiency of which results in cellular and molecular changes leading to atrial electric and structural remodeling linked to arrhythmogenesis.

Simulation of the undiseased human cardiac ventricular action potential: model formulation and experimental validation

Cellular electrophysiology experiments, important for understanding cardiac arrhythmia mechanisms, are usually performed with channels expressed in non myocytes, or with non-human myocytes. Differences between cell types and species affect results. Thus, an accurate model for the undiseased human ventricular action potential (AP) which reproduces a broad range of physiological behaviors is needed. Such a model requires extensive experimental data, but essential elements have been unavailable. Here, we develop a human ventricular AP model using new undiseased human ventricular data: Ca(2+) versus voltage dependent inactivation of L-type Ca(2+) current (I(CaL)); kinetics for the transient outward, rapid delayed rectifier (I(Kr)), Na(+)/Ca(2+) exchange (I(NaCa)), and inward rectifier currents; AP recordings at all physiological cycle lengths; and rate dependence and restitution of AP duration (APD) with and without a variety of specific channel blockers. Simulated APs reproduced the experimental AP morphology, APD rate dependence, and restitution. Using undiseased human mRNA and protein data, models for different transmural cell types were developed. Experiments for rate dependence of Ca(2+) (including peak and decay) and intracellular sodium ([Na(+)](i)) in undiseased human myocytes were quantitatively reproduced by the model. Early afterdepolarizations were induced by I(Kr) block during slow pacing, and AP and Ca(2+) alternans appeared at rates >200 bpm, as observed in the nonfailing human ventricle. Ca(2+)/calmodulin-dependent protein kinase II (CaMK) modulated rate dependence of Ca(2+) cycling. I(NaCa) linked Ca(2+) alternation to AP alternans. CaMK suppression or SERCA upregulation eliminated alternans. Steady state APD rate dependence was caused primarily by changes in [Na(+)](i), via its modulation of the electrogenic Na(+)/K(+) ATPase current. At fast pacing rates, late Na(+) current and I(CaL) were also contributors. APD shortening during restitution was primarily dependent on reduced late Na(+) and I(CaL) currents due to inactivation at short diastolic intervals, with additional contribution from elevated I(Kr) due to incomplete deactivation.

[Effects of docosahexaenoic acid on sodium channel current and transient outward potassium channel current in rat ventricular myocytes]

OBJECTIVE

To investigate the effects of docosahexaenoic acid (DHA) on sodium channel current (I(Na)) and transient outward potassium channel current (I(to)) in rat ventricular myocytes and to evaluate potential anti-arrhythmic mechanisms of DHA.

METHODS

I(Na) and I(to) of individual ventricular myocytes were recorded by patch-clamp technique in whole-cell configuration at room temperature. Effects of DHA at various concentrations (0, 20, 40, 60, 80, 100 and 120 micromol/L) on I(Na) and I(to) were observed.

RESULTS

(1) I(Na) was blocked in a concentration-dependent manner by DHA, stably inactivated curves were shifted to the left, and recover time from inactivation was prolonged while stably activated curves were not affected by DHA. At -30 mV, I(Na) was blocked to (1.51 ± 1.32)%, (21.13 ± 4.62)%, (51.61 ± 5.73)%, (67.62 ± 6.52)%, (73.49 ± 7.59)% and (79.95 ± 7.62)% in the presence of above DHA concentrations (all P < 0.05, n = 20), and half-effect concentration (EC(50)) of DHA on I(Na) was (47.91 ± 1.57)micromol/L. (2) I(to) were also blocked in a concentration-dependent manner by DHA, stably inactivated curves were shifted to the left, and recover time from inactivation was prolonged with increasing concentrations of DHA, and stably activated curves were not affected by DHA. At +70 mV, I(to) was blocked to (2.61 ± 0.26)%, (21.79 ± 4.85)%, (63.11 ± 6.57)%, (75.52 ± 7.26)%, (81.82 ± 7.63)% and (84.33 ± 8.25)%, respectively, in the presence of above DHA concentrations (all P < 0.05, n = 20), and the EC(50) of DHA on I(to) was (49.11 ± 2.68)micromol/L.

CONCLUSION

The blocking effects of DHA on APD and I(to) may serve as one of the anti-arrhythmia mechanisms of DHA.

The involvement of N-G,N-G-dimethyarginine dimethylhydrolase 1 in the proliferative effect of Astragali radix on cardiac cells

AIM OF THE STUDY

Astragali radix (AR) is a widely used traditional medicine in oriental countries for treating various diseases including cardiovascular disease (CVD). We investigated the effects of AR extracts on rat cardiomyocytes and H9C2 cardiac cells as well as identified many target genes that mediate the effect of AR.

MATERIALS AND METHODS

The effect of AR extracts on cell proliferation was assessed and cDNA microarray technique was used to analyse the differential gene expressions upon AR treatment in cardiac cells. One of the selected target genes was over-expressed to elucidate its role in cell proliferation.

RESULTS

AR was shown to promote the proliferation of neonatal rat cardiomyocytes and H9C2 cells. Results of cDNA microarray hybridization showed that N-G,N-G-dimethylarginine dimethylaminohydrolase 1 (DDAH1) gene was up-regulated in AR-treated H9C2 cells and the results were further confirmed by reverse transcription polymerase chain reaction. Over-expression of DDAH1 gene in H9C2 cells significantly enhances the cell proliferation. Moreover, a drastic drop of DDAH1 expression in rat ventricular myocardium was observed from day 3 to day 5 after birth, which is the critical transition of cardiomyocytes from hyperplastic to hypertrophic growth.

CONCLUSIONS

AR promotes cardiac cell proliferation and up-regulates the DDAH1, an enzyme that metabolized the endogenous nitric oxide (NO) synthase inhibitor. The effect of AR on the metabolism of NO deserves future investigation.

Effect of polydatin on action potential in ventricular papillary muscle of rat and the underlying ionic mechanism

It is proved that polydatin has cardioprotection against ischemia-induced arrhythmia, but the electrophysiological mechanism is not clear. The aim of the present study was to investigate the effect of polydatin on action potential (AP) in ventricular papillary muscle and the underlying ionic mechanism in rat using intracellular recording and whole-cell patch clamp techniques. The results showed: (1) In normal papillary muscles, polydatin (50 and 100 µmol/L) shortened duration of 50% repolarization (APD(50)) and duration of 90% repolarization (APD(90)) in a concentration-dependent manner (P<0.01). But polydatin had no effects on resting potential (RP), overshoot (OS), amplitude of action potential (APA) and maximal rate of depolarization in phase 0 (V(max)) in normal papillary muscles (P>0.05). (2) In partially depolarized papillary muscles, polydatin (50 µmol/L) not only shortened APD(50) and APD(90) (P<0.05), but also decreased OS, APA and V(max) (P<0.05). (3) After pretreatment with glibenclamide (10 µmol/L), an ATP-sensitive K(+) channel blocker, the electrophysiological effect of polydatin (50 µmol/L) was partially inhibited. (4) Pretreatment with N(G)-nitro-L-arginine methyl ester (L-NAME, 1 mmol/L), a nitric oxide (NO) synthase inhibitor, failed to abolish the effect of polydatin (50 µmol/L) on AP. (5) Polydatin (25, 50, 75 and 100 µmol/L) decreased L-type Ca(2+) current in ventricular myocytes in a concentration-dependent manner (P<0.05). (6) Polydatin (50 µmol/L) increased ATP-sensitive K(+) current in ventricular myocytes (P<0.05). The results suggest that polydatin can shorten the repolarization of AP in normal papillary muscle and inhibit AP in partially depolarized papillary muscle, which might be related to the blocking of L-type Ca(2+) channel and the opening of ATP-sensitive K(+) channel.

The aqueous extract, not organic extracts, of Terminalia arjuna bark exerts cardiotonic effect on adult ventricular myocytes

The bark of Terminalia arjuna (TA) has been used for centuries in ayurvedic medicine as cardiotonics for treatment of cardiac disorders. It became recently available as over-the-counter supplements marketed for maintaining a healthy heart. However, the cellular mechanism of its cardiotonic effect remains undefined. The present study was designed to investigate the physicochemical property and inotropic effect of the aqueous extract of TA bark (TA(AqE)) on adult rat ventricular myocytes in comparison with extracts prepared sequentially with organic solvents (organic extracts). The kinetics of myocyte contraction and caffeine-induced contraction were analyzed to assess the effect of TA(AqE) on sarcoplasmic reticular (SR) function. The inotropic effect of TA(AqE) was also compared with that of known cardiotonics, isoproterenol (ISO) and ouabain (Ouab). We found that TA(AqE) decoctions exerted positive inotropy, accelerated myocyte relaxation and increased caffeine-induced contraction concentration-dependently. In contrast, TA organic extracts caused interruption of excitability and arrhythmias without consistent inotropic action. In conclusion, TA(AqE)-induced cardiotonic action via enhancing SR function, a unique action minimizing the occurrence of arrhythmias, makes TA(AqE) a promising and relatively safe cardiotonic beneficial to the healthy heart and the treatment for chronic heart disease. The cardiotonic effect of TA(AqE) is consistent with the therapeutic property of TA bark used in ayurvedic medicine. The method of administration and/or selective omission of hydrophobic components from bark powder could be crucial to the efficacy and safety of TA bark in cardiac therapy and uses as over-the-counter supplements.

Effects of β-adrenoceptor stimulation on delayed rectifier K(+) currents in canine ventricular cardiomyocytes

BACKGROUND AND PURPOSE

While the slow delayed rectifier K(+) current (I(Ks)) is known to be enhanced by the stimulation of β-adrenoceptors in several mammalian species, phosphorylation-dependent regulation of the rapid delayed rectifier K(+) current (I(Kr)) is controversial.

EXPERIMENTAL APPROACH

In the present study, therefore, the effect of isoprenaline (ISO), activators and inhibitors of the protein kinase A (PKA) pathway on I(Kr) and I(Ks) was studied in canine ventricular myocytes using the whole cell patch clamp technique.

KEY RESULTS

I (Kr) was significantly increased (by 30-50%) following superfusion with ISO, forskolin or intracellular application of PKA activator cAMP analogues (cAMP, 8-Br-cAMP, 6-Bnz-cAMP). Inhibition of PKA by Rp-8-Br-cAMP had no effect on baseline I(Kr). The stimulating effect of ISO on I(Kr) was completely inhibited by selective β₁-adrenoceptor antagonists (metoprolol and CGP-20712A), by the PKA inhibitor Rp-8-Br-cAMP and by the PKA activator cAMP analogues, but not by the EPAC activator 8-pCPT-2'-O-Me-cAMP. In comparison, I(Ks) was increased threefold by the activation of PKA (by ISO or 8-Br-cAMP), and strongly reduced by the PKA inhibitor Rp-8-Br-cAMP. The ISO-induced enhancement of I(Ks) was decreased by Rp-8-Br-cAMP and completely inhibited by 8-Br-cAMP.

CONCLUSIONS AND IMPLICATIONS

The results indicate that the stimulation of β₁-adrenoceptors increases I(Kr), similar to I(Ks), via the activation of PKA in canine ventricular cells.

Stochastic simulation of cardiac ventricular myocyte calcium dynamics and waves

A three dimensional model of calcium dynamics in the rat ventricular myocyte was developed to study the mechanism of calcium homeostasis and pathological calcium dynamics during calcium overload. The model contains 20,000 calcium release units (CRUs) each containing 49 ryanodine receptors. The model simulates calcium sparks with a realistic spontaneous calcium spark rate. It suggests that in addition to the calcium spark-based leak, there is an invisible calcium leak caused by the stochastic opening of a small number of ryanodine receptors in each CRU without triggering a calcium spark. The model also explores the mechanism of calcium wave propagation between release sites under the conditions of calcium overload.

[Structural reaction of cardiomyocytes in response to loading on voluntary running wheel]

The aims of this study were to characterize the peculiarities of structural reorganization of right and left ventricular myocardium in response of physical load. Male laboratory white rats (2 month old) were choused in free spinning running wheel (n=5) for 1 month and 2 month (n=5), non-running animals (n=5) serve as control group. Blood acid-base balance and gases were carried out as tissue hypoxia control by Astrup micromethod. Heart weight and ventricular index were measured after rats killing by overdose of intraperitoneally injected pentobarbitone sodium. Cardiomyocytes morphological response were studied by histological and electron microscopical data, also by Selye staining results. Collecting data indicates that heart weight, cardiac muscle fibre cross-sectional area and ventricular index are significantly greater after 1 month physical load modeled by voluntary wheel running in this group. Acid-base balance parameters suggest the low tissues hypoxia and alveolar hypoventilation level that provides a physiologically suitable model to cardiac muscle hypertrophy in rats. In opposite, 2 month duration load acts on the right ventricular cardiomyocytes, including fuchsinophilic dystrophia and homogenization of myofibrils that we can refereed as "physical load induced cardiomyopathy" of right ventricle, so we conclude that this is not physiologically suitable exercise response.

Effect of ethanol on action potential and ionic membrane currents in rat ventricular myocytes

AIM

Even though alcohol intoxication is often linked to arrhythmias, data describing ethanol effect on cardiac ionic channels are rare. In addition, ethanol is used as a solvent of hydrophobic compounds in experimental studies. We investigated changes of the action potential (AP) configuration and main ionic membrane currents in rat cardiomyocytes under 20-1500 m(M) ethanol.

METHODS

  Experiments were performed on enzymatically isolated rat right ventricular myocytes using the whole cell patch-clamp technique at room temperature.

RESULTS

  Ethanol reversibly decelerated the upstroke velocity and decreased AP amplitude and duration at 0.2 and 3 Hz. The fast sodium current I(Na) , l-type calcium current I(Ca) and transient outward potassium current I(to) were reversibly inhibited in a concentration-dependent manner (50% inhibition at 446 ± 12, 553 ± 49 and 1954 ± 234 m(M), respectively, with corresponding Hill coefficients 3.1 ± 0.3, 1.1 ± 0.2 and 0.9 ± 0.1). Suppression of I(Na) and I(Ca) magnitude was slightly voltage dependent. The effect on I(Ca) and I(to) was manifested mainly as an acceleration of their apparent inactivations with a decreased slow and fast time constant respectively. As a consequence of marked differences in n(H) , sensitivity of the currents to ethanol at 10% inhibition decreases in the following order: I(Ca) (75 mm, 3.5‰), I(to) (170 m(M), 7.8‰) and I(Na) (220 m(M), 10.1‰).

CONCLUSION

  Our results suggest a slight inhibition of all the currents at ethanol concentrations relevant to deep alcohol intoxication. The concentration dependence measured over a wide range may serve as a guideline when using ethanol as a solvent.

Electroporation-induced inward current in voltage-clamped guinea pig ventricular myocytes

Electroporation induced by high-strength electrical fields has long been used to investigate membrane properties and facilitate transmembrane delivery of molecules and genes for research and clinical purposes. In the heart, electric field-induced passage of ions through electropores is a factor in defibrillation and postshock dysfunction. Voltage-clamp pulses can also induce electroporation, as exemplified by findings in earlier studies on rabbit ventricular myocytes: Long hyperpolarizations to ≤-110 mV induced influx of marker ethidium and irregular inward currents that were as large with external NMDG(+) as Na(+). In the present study, guinea pig ventricular myocytes were bathed with NMDG(+), Na(+) or NMDG(+) + La(3+) solution (36°C) and treated with five channel blockers. Hyperpolarization of myocytes in NMDG(+) solution elicited an irregular inward current (I (ep)) that reversed at -21.5 ± 1.5 mV. In myocytes hyperpolarized with 200-ms steps every 30 s, I (ep) occurred in "episodes" that lasted for one to four steps. Boltzmann fits to data on the incidence of I (ep) per experiment indicate 50% incidence at -129.7 ± 1.4 mV (Na(+)) and -146.3 ± 1.6 mV (NMDG(+)) (slopes ≈-7.5 mV). I (ep) amplitude increased with negative voltage and was larger with Na(+) than NMDG(+) (e.g., -2.83 ± 0.34 vs. -1.40 ± 0.22 nA at -190 mV). La(3+) (0.2 mM) shortened episodes, shifted 50% incidence by -35 mV and decreased amplitude, suggesting that it inhibits opening/promotes closing of electropores. We compare our findings with earlier ones, especially in regard to electropore selectivity. In the Appendix, relative permeabilities and modified excluded-area theory are used to derive estimates of electropore diameters consistent with reversal potential -21.5 mV.

Angiotensin II induces afterdepolarizations via reactive oxygen species and calmodulin kinase II signaling

Renin-angiotensin system inhibitors significantly reduce the incidence of arrhythmias. However, the underlying mechanism(s) is not well understood. We aim to test the hypothesis that angiotensin II (Ang II) induces early afterdepolarizations (EADs) and triggered activities (TAs) via the nicotinamide adenine dinucleotide phosphate (NADPH) oxidase-ROS-calmodulin kinase II (CaMKII) pathway. ROS production was analyzed in the isolated rabbit myocytes loaded with ROS dye. Ang II (1-2 μM) increased ROS fluorescence in myocytes, which was abolished by Ang II type 1 receptor blocker losartan, NADPH oxidase inhibitor apocynin, and antioxidant MnTMPyP, respectively. Action potentials were recorded using the perforated patch-clamp technique. EADs emerged in 27 out of 41 (66%) cells at 15.8 ± 1.6 min after Ang II (1-2 μM) perfusion. Ang II-induced EADs were eliminated by losartan, apocynin, or trolox. The CaMKII inhibitor KN-93 (n=6) and inhibitory peptide (AIP) (n=4) also suppressed Ang II-induced EADs, whereas the inactive analogue KN-92 did not. Nifedipine, a blocker of L-type Ca current (I(Ca)(2+)(,L)), or ranolazine, an inhibitor of late Na current (I(Na)(+)), abolished Ang II-induced EADs. The effects of Ang II on major membrane currents were evaluated using voltage clamp. While Ang II at same concentrations had no significant effect on total outward K(+) current, it enhanced I(Ca.L) and late I(Na), which were attenuated by losartan, apocynin, trolox, or KN-93. We conclude that Ang II induces EADs via intracellular ROS production through NADPH oxidase, activation of CaMKII, and enhancement of I(Ca,L) and late I(Na). These results provide evidence supporting a link between renin-angiotensin system and cardiac arrhythmias.

Characterization of a novel MK3 splice variant from murine ventricular myocardium

p38 MAP kinase (MAPK) isoforms alpha, beta, and gamma, are expressed in the heart. p38alpha appears pro-apoptotic whereas p38beta is pro-hypertrophic. The mechanisms mediating these divergent effects are unknown; hence elucidating the downstream signaling of p38 should further our understanding. Downstream effectors include MAPK-activated protein kinase (MK)-3, which is expressed in many tissues including skeletal muscles and heart. We cloned full-length MK3 (MK3.1, 384 aa) and a novel splice variant (MK3.2, 266 aa) from murine heart. For MK3.2, skipping of exons 8 and 9 resulted in a frame-shift in translation of the first 85 base pairs of exon 10 followed by an in-frame stop codon. Of 3 putative phosphorylation sites for p38 MAPK, only Thr-203 remained functional in MK3.2. In addition, MK3.2 lacked nuclear localization and export signals. Quantitative real-time PCR confirmed the presence of these mRNA species in heart and skeletal muscle; however, the relative abundance of MK3.2 differed. Furthermore, whereas total MK3 mRNA was increased, the relative abundance of MK3.2 mRNA decreased in MK2(-/-) mice. Immunoblotting revealed 2 bands of MK3 immunoreactivity in ventricular lysates. Ectopically expressed MK3.1 localized to the nucleus whereas MK3.2 was distributed throughout the cell; however, whereas MK3.1 translocated to the cytoplasm in response to osmotic stress, MK3.2 was degraded. The p38alpha/beta inhibitor SB203580 prevented the degradation of MK3.2. Furthermore, replacing Thr-203 with alanine prevented the loss of MK3.2 following osmotic stress, as did pretreatment with the proteosome inhibitor MG132. In vitro, GST-MK3.1 was strongly phosphorylated by p38alpha and p38beta, but a poor substrate for p38delta and p38gamma. GST-MK3.2 was poorly phosphorylated by p38alpha and p38beta and not phosphorylated by p38delta and p38gamma. Hence, differential regulation of MKs may, in part, explain diverse downstream effects mediated by p38 signaling.

Effects of GH secretagogues on contractility and Ca2+ homeostasis of isolated adult rat ventricular myocytes

Ghrelin and its synthetic analogue hexarelin are specific ligands of GH secretagogue receptor (GHS-R) and induce a variety of cardiovascular protective and cardiac positive inotropic effects. The signaling system underlying immediate effects of both GHSs in cardiomyocytes remains undefined. In the present study, we investigated the immediate effects of GHSs on isolated ventricular myocyte shortening, intracellular Ca(2+) ([Ca(2+)](i)) transients, and the L-type Ca(2+) current (I(Ca,L)). Putative intracellular signalling cascades were studied with specific receptor and signalling blockers. In fresh isolated adult Wistar rat ventricular myocytes, GHSs produced a positive inotropic effect in a concentration-dependent manner and increased the amplitude of [Ca(2+)](i) transients and the I(Ca,L). The positive inotropic response was abolished by the GHS-R1a antagonist [D-Lys(3)]-GH-releasing peptide-6 (10 microm). GHS-induced increase in the I(Ca,L) was abolished by [D-Lys(3)]-GH-releasing peptide-6 and protein kinase C inhibitor, chelerythrine chloride (5 microm), but not by protein kinase A inhibitor, KT 5720 (10 microm). We conclude that hexarelin and ghrelin increase the I(Ca,L), through GHS-R1a receptor and protein kinase C signalling cascade, which contribute to its direct positive inotropic effect on cardiomyocytes.

Computational modelling of the initiation and development of spontaneous intracellular Ca2+ waves in ventricular myocytes

Intracellular Ca(2+) dynamics provides excitation-contraction coupling in cardiac myocytes. Under pathological conditions, spontaneous Ca(2+) release events can lead to intracellular Ca(2+) travelling waves, which can break, giving transitory or persistent intracellular re-entrant Ca(2+) scroll waves. Intracellular Ca(2+) waves can trigger cellular delayed after-depolarizations of membrane potential, which if they occur in a cluster of a few hundred neighbouring myocytes may lead to cardiac arrhythmia. Quantitative prediction of the initiation and propagation of intracellular Ca(2+) waves requires the dynamics of Ca(2+)-induced Ca(2+) release, and the intracellular spatial distribution of Ca(2+) release units (CRUs). The spatial distribution of ryanodine receptor clusters within a few sarcomeres was reconstructed directly from confocal imaging measurements. It was then embedded into a three-dimensional ventricular cell model, with a resting membrane potential and simple stochastic Ca(2+)-induced Ca(2+) release dynamics. Isotropic global Ca(2+) wave propagation can be produced within the anisotropic intracellular architecture, by isotropic local Ca(2+) diffusion, and the branching Z-disc structure providing inter Z-disc pathways for Ca(2+) propagation. The branching Z-disc provides a broader spatial distribution of ryanodine receptor clusters across Z-discs, which reduces the likelihood of wave initiation by spontaneous Ca(2+) releases. Intracellular Ca(2+) dynamics during catecholaminergic polymorphic ventricular tachycardia (CPVT) was simulated phenomenologically by increasing the Ca(2+) sensitivity factor of the CRU, which results in an increased rate of Ca(2+) release events. Flecainide has been shown to prevent arrhythmias in a murine model of CPVT and in patients. The modelled actions of flecainide on the time course of Ca(2+) release events prevented the initiation of Ca(2+) waves.

The collagen receptor DDR2 is expressed during early cardiac development

Discoidin Domain Receptor 2 (DDR2) is a receptor tyrosine kinase which has been shown to regulate cell migration upon binding its ligand, collagen. Expression studies determined that DDR2 mRNA and protein are present in the atrioventricular canal during epithelial-mesenchymal transformation (EMT) and the receptor is expressed in both activated endothelial and migrating mesenchymal cells in vivo.

Structure-function relationship of king cobra cathelicidin

King cobra cathelicidin (OH-CATH) is composed of 34 amino acid residues having strong antibacterial and very weak hemolytic activities as reported by us recently. OH-CATH can be served as a valuable template to develop novel therapeutic drugs. In this study, OH-CATH and six of its analogs were synthesized to explore their structure-function relationships based on their bactericidal and hemolytic activities. Experimental results of OH-CATH(3-34) and OH-CATH(5-34) indicated that the N-terminal 4 amino acid residues of OH-CATH played an important role on its hemolytic activity but had weak effects on its bactericidal activity. Among OH-CATH and its analogs, OH-CATH(5-34) had the lowest hemolytic activity while maintained strong antimicrobial activity. To evaluate its potential usage, the biological activities of OH-CATH(5-34) were compared with those of pexiganan. The bactericidal activity of OH-CATH(5-34) against 5 different species (11 laboratory strains) was 2-4 times stronger than that of pexiganan (4-16 microg/ml vs 8-32 microg/ml). Hemolytic activity of OH-CATH(5-34) against human erythrocytes was 0.69% while that of pexiganan was 16.5% at the dosage of 200 microg/ml. OH-CATH(5-34) showed very weak cytotoxic activities against primary rabbit ventricular endothelial cells and four human cancer cell lines whereas pexiganan showed strong cytotoxic activity against these five cell lines (IC(50)=20-90 microg/ml). The intravenous LD(50) value of OH-CATH(5-34) on mice was 7-fold higher than that of pexiganan (175 mg/kg vs 25mg/kg). Taken together, our results suggested that OH-CATH(5-34) should be considered as an excellent candidate for developing therapeutic drugs.

Effects of ropinirole on action potential characteristics and the underlying ion currents in canine ventricular myocytes

In spite of its widespread clinical application, there is little information on the cellular cardiac effects of the dopamine receptor agonist ropinirole. In the present study, therefore, the concentration-dependent effects of ropinirole on action potential morphology and the underlying ion currents were studied in enzymatically dispersed canine ventricular cardiomyocytes using standard microelectrode, conventional whole-cell patch clamp, and action potential voltage clamp techniques. At concentrations > or = 1 microM, ropinirole increased action potential duration (APD(90)) and suppressed the rapid delayed rectifier K(+) current (I (Kr)) with an IC(50) value of 2.7 +/- 0.25 microM and Hill coefficient of 0.92 +/- 0.09. The block increased with increasing depolarizations to more positive voltages, but paradoxically, the activation of I (Kr) was accelerated by 3 muM ropinirole (time constant decreased from 34 +/- 4 to 14 +/- 1 ms). No significant changes in the fast and slow deactivation time constants were observed with ropinirole. At higher concentrations, ropinirole decreased the amplitude of early repolarization (at concentrations > or = 10 microM), reduced the maximum rate of depolarization and caused depression of the plateau (at concentrations > or = 30 microM), and shortened APD measured at 50% repolarization (at 300 microM) indicating a concentration-dependent inhibition of I (to), I (Na), and I (Ca). Suppression of I (Kr), I (to), and I (Ca) has been confirmed under conventional patch clamp and action potential voltage clamp conditions. I (Ks) and I (K1) were not influenced significantly by ropinirole at concentrations less than 300 microM. All these effects of ropinirole were fully reversible upon washout. The results indicate that ropinirole treatment may carry proarrhythmic risk for patients with inherited or acquired long QT syndrome due to inhibition of I (Kr)-especially in cases of accidental overdose or intoxication.

Remodelling of Ca2+ handling organelles in adult rat ventricular myocytes during long term culture

It is well known that for cardiomyocytes, isolation and culturing induce largely unknown remodelling processes. We analysed changes in the structure of cell compartments with optical techniques such as confocal microscopy and fluorescence redistribution after photobleaching employing adenoviral-mediated transduction of targeted fluorescent proteins and small molecule dyes. We identified characteristic remodelling processes: the T-tubular membrane system was gradually lost by a process referred to as "sequential pinching off", in an outward direction. Mitochondria fell in one of three classes, very small (0.9 microm length), medium long (1.8 microm) or extended shape (3.6 microm) organelles. Over the culturing time mitochondria gradually fused. Bleaching of individual mitochondria revealed association between apparently separated mitochondria by "tunnelling" via sub-resolution organelle-tubes. This tunnelling process was increasing over the culturing time. A gradual loss of the cross-striation arrangement in the endoplasmic/sarcoplasmic reticulum was visualised. Analysis of large populations of Ca(2+) sparks by video-rate confocal 2D-scanning revealed significant albeit small changes of these elementary SR-Ca(2+) release events in adult cardiomyocytes that could be related to changes in SR-Ca(2+) content rather than resting Ca(2+) concentration. In conclusion, primary isolated cardiomyocytes from adult hearts undergo a well-defined, but reproducible subcellular remodelling during optimised long term culture.

Cell swelling impairs dye coupling in adult rat ventricular myocytes. Cell volume as a regulator of cell communication

The influence of cell swelling on cell communication was investigated in cardiomyocytes isolated from the ventricle of adult rats. Measurements of dye coupling were performed in cell pairs using intracellular dialysis of Lucifer Yellow CH. The pipette was attached to one cell of the pair and after a gig ohm seal was achieved, the membrane was ruptured by a brief suction allowing the dye to diffuse from the pipette into the cell. Fluorescence of the dye in the injected as well as in non-dialyzed cell of the pair was continuously monitored. The results indicate that in cell pairs exposed to hypotonic solution the cell volume was increased by about 60% within 35 min and the dye coupling was significantly reduced by cell swelling. Calculation of gap junction permeability (P(j)) assuming an the intracellular volume accessible to intracellular diffusion of the dye as 12% of total cell volume, showed an average P(j) value of 0.16 ± 0.04 × 10(-4) cm/s (n = 35) in the control and 0.89 ± 1.1 × 10(-5) cm (n = 40) for cells exposed to hypotonic solution (P < 0.05). Similar results were found assuming intracellular volumes accessible to the dye of 20 and 30% of total cell volume, respectively. Cell swelling did not change the rate of intracellular diffusion of the dye. The results which indicate that cell volume is an important regulator of gap junction permeability, have important implications to myocardial ischemia and heart failure as well as to heart pharmacology because changes in cell volume caused by drugs and transmitters can impair cell communication with consequent generation of slow conduction and cardiac arrhythmias.

Hypothermia improves ventricular myocyte contractility under conditions of normal perfusion and after an interval of ischemia

AIM

Recent investigations have reported improved myocardial function during hypothermia following resuscitation from cardiac arrest. The effects of hypothermia on myocyte contractility were investigated under conditions of normal perfusion and after a 10min interval of ischemia.

METHODS

Ventricular myocytes were obtained from 10 male Sprague-Dawley rats weighing 400+/-50g. The myocytes were randomized to be perfused at: 37 degrees C, 34 degrees C, 32 degrees C, or 30 degrees C. A subsequent set of myocytes was subjected to 10min of ischemia at 37 degrees C, prior to being randomized to reperfusion at: 37 degrees C, 34 degrees C, 32 degrees C or 30 degrees C. Myocyte contractility was expressed as length-shortening percentage. Intracellular Ca(2+) transients were assessed in a separate group of myocytes preloaded with Fura-2/AM. Sensitivity to Ca(2+) was tested by increasing perfusate Ca(2+) content, i.e. 0.5mM, 1mM and 2mM.

RESULTS

During normal perfusion and following reperfusion after 10min of ischemia, myocyte contractility increased at 34 degrees C compared to 37 degrees C (P<0.01). When the perfusion temperature was decreased to 32 degrees C and 30 degrees C, contractility further increased (P<0.001). Intracellular Ca(2+) transients were greater during perfusion at 34 degrees C compared to those at 37 degrees C (P<0.001) and further increased at 30 degrees C (P<0.001). Increases in extracellular Ca(2+) concentration from 0.5mM to 2mM resulted in greater myocyte contractility during perfusion at 30 degrees C compared to that observed at 37 degrees C (P<0.001). Effects of hypothermia on intracellular Ca(2+) transients and sensitivity to Ca(2+) persisted after ischemia.

CONCLUSIONS

Hypothermia improved myocyte contractility, intracellular Ca(2+) transients and sensitivity to Ca(2+) under conditions of normal perfusion and following reperfusion after 10min of ischemia.

Phosphodiesterase 8A (PDE8A) regulates excitation-contraction coupling in ventricular myocytes

In ventricular myocytes, activation of protein kinase A (PKA) by 3'-5' cyclic adenosine monophosphate (cAMP) increases the force of contraction by increasing L-type Ca(2+) channel currents (I(Ca)) and sarcoplasmic reticulum (SR) Ca(2+) release during excitation-contraction coupling. Cyclic-nucleotide phosphodiesterases (PDEs) comprise a large family of enzymes whose role in the cell is to regulate the spatial and temporal profile of cAMP signals by controlling the degradation of this second messenger. At present, however, the molecular identity and functional roles of the PDEs expressed in ventricular myocytes are incompletely understood. Here, we tested the hypothesis that PDE8A plays a critical role in the modulation of at least one compartment of cAMP and hence PKA activity during beta-adrenergic receptor (betaAR) activation in ventricular myocytes. Consistent with this hypothesis, we found that PDE8A transcript and protein are expressed in ventricular myocytes. Our data indicate that evoked [Ca(2+)](i) transients and I(Ca) increased to a much larger extent in PDE8A null (PDE8A(-/-)) than in wild-type (WT) myocytes during beta-adrenergic signaling activation. In addition, Ca(2+) spark activity was higher in PDE8A(-/-) than in WT myocytes. Our data indicate that PDE8A is a novel cardiac PDE that controls one or more pools of cAMP implicated in regulation of Ca(2+) movement through cardiomyocyte.

Three-dimensional culture alters primary cardiac cell phenotype

The directed formation of complex three-dimensional (3D) tissue architecture is a fundamental goal in tissue engineering and regenerative medicine. The growth of cells in 3D structures is expected to influence cellular phenotype and function, especially relative cell distribution, expression profiles, and responsiveness to exogenous signals; however, relatively few studies have been carried out to examine the effects of 3D reaggregation on cells from critical target organs, like the heart. Accordingly, we cultured primary cardiac ventricular cells in a 3D model system using a serum-free medium to test the hypothesis that expression profiles, multicellular organizational pathways, tissue maturation markers, and responsiveness to hormone stimulation were significantly altered in stable cell populations grown in 3D versus 2D culture. We found that distinct multi-cellular structures formed in 3D in conjunction with changes in mRNA expression profile, up-regulation of endothelial cell migratory pathways, decreases in the expression of fetal genes (Nppa and Ankrd1), and increased sensitivity to tri-iodothyronine stimulation when compared to parallel 2D cultures comprising the same cell populations. These results indicate that the culture of primary cardiac cells in 3D aggregates leads to physiologically relevant alterations in component cell phenotype consistent with cardiac ventricular tissue formation and maturation.

Phenomenological modeling of cell-to-cell and beat-to-beat variability in isolated Guinea Pig ventricular myocytes

Experimental action potential (AP) recordings in isolated ventricular myoctes display significant temporal beat-to-beat variability in morphology and duration. Furthermore, significant cell-to-cell differences in AP also exist even for isolated cells originating from the same region of the same heart. However, current mathematical models of ventricular AP fail to replicate the temporal and cell-to-cell variability in AP observed experimentally. In this study, we propose a novel mathematical framework for the development of phenomenological AP models capable of capturing cell-to-cell and temporal variabilty in cardiac APs. A novel stochastic phenomenological model of the AP is developed, based on the deterministic Bueno-Orovio/Fentonmodel. Experimental recordings of AP are fit to the model to produce AP models of individual cells from the apex and the base of the guinea-pig ventricles. Our results show that the phenomenological model is able to capture the considerable differences in AP recorded from isolated cells originating from the location. We demonstrate the closeness of fit to the available experimental data which may be achieved using a phenomenological model, and also demonstrate the ability of the stochastic form of the model to capture the observed beat-to-beat variablity in action potential duration.

In vivo human 3D cardiac fibre architecture: reconstruction using curvilinear interpolation of diffusion tensor images

In vivo imaging of the cardiac 3D fibre architecture is still a challenge, but it would have many clinical applications, for instance to better understand pathologies and to follow up remodelling after therapy. Recently, cardiac MRI enabled the acquisition of Diffusion Tensor images (DTI) of 2D slices. We propose a method for the complete 3D reconstruction of cardiac fibre architecture in the left ventricular myocardium from sparse in vivo DTI slices. This is achieved in two steps. First we map non-linearly the left ventricular geometry to a truncated ellipsoid. Second, we express coordinates and tensor components in Prolate Spheroidal System, where an anisotropic Gaussian kernel regression interpolation is performed. The framework is initially applied to a statistical cardiac DTI atlas in order to estimate the optimal anisotropic bandwidths. Then, it is applied to in vivo beating heart DTI data sparsely acquired on a healthy subject. Resulting in vivo tensor field shows good correlation with literature, especially the elevation (helix) angle transmural variation. To our knowledge, this is the first reconstruction of in vivo human 3D cardiac fibre structure. Such approach opens up possibilities in terms of analysis of the fibre architecture in patients.

[Development and structure of intraventricular formations of the human heart during the intrauterine development]

The aim of this study was to establish the origins of development and to examine the structure of the intraventricular formations of the human heart during the intrauterine development. In the study of the serial sections of 57 human embryos of 6-70 mm parietococcygeal length, no subdivision of ventricular myocardium on external, middle and inner layers was detected. This means that not only the carneal trabecules but also the papillary muscles are formed by the bundles of cardiomyocytes originating from the whole thickness of the myocardium. The multilevel pattern of trabecular network of cardiac ventricles at the early stages of embryogenesis is described. It is suggested that the orientation of the carneal trabecules within the trabecular network is associated with the structure of the cardiac tube, which gives rise to the heart during development.

miR133a regulates cardiomyocyte hypertrophy in diabetes

BACKGROUND

Diabetic cardiomyopathy, characterized by cardiac hypertrophy and contractile dysfunction, eventually leads to heart failure. We have previously shown that alterations of a number of key molecules are involved in producing cardiomyocyte hypertrophy in diabetes. The aim of the present study was to determine whether microRNAs (miRNA) play a role in mediating altered gene expression and structural/functional deficits in the heart in diabetes.

METHODS

STZ-induced diabetic mice were haemodynamically investigated after 2 months of diabetes to establish the development of cardiomyopathy. The tissues were then examined for gene expression and microRNA analysis. We further investigated neonatal rat cardiomyocytes to identify the mechanisms of glucose-induced hypertrophy and the potential role of miR133a.

RESULTS

Diabetic mice showed myocardial contractile dysfunction and augmented mRNA expression of atrial and brain natriuretic peptides (ANP, BNP), MEF2A and MEF2C, SGK1 and IGF1R compared to age- and sex-matched controls. Cardiac tissues from these mice showed alteration of multiple miRNAs by array analysis including miR133a, which was confirmed by RT-PCR. In vitro exposure of cardiomyocytes to high levels of glucose produced hypertrophic changes and reduced expression of miRNA133a. Finally, transfection of miR133a mimics prevented altered gene expression and hypertrophic changes.

CONCLUSION

Data from these studies demonstrate a novel glucose-induced mechanism regulating gene expression and cardiomyocyte hypertrophy in diabetes which is mediated through miR133a.

The development of a new computational model for the electromechanics of the human ventricular myocyte

In this work we present a new electromechanical cardiac myocyte model tailored to reproduce the electrical and force generating activities of human ventricular myocytes. The model was created by coupling two existing models: the ten Tusscher electrophysiology model and the Rice myofilament mechanics model. The parameters of the new model were adjusted in order to replicate the available experimental data for human myocytes. The main challenges in this work were the strong feedbacks between the models, the high non-linearity of the models and mainly the lack of human data to make the adjustments.

4-Hydroxytamoxifen inhibits K(+) currents in mouse ventricular myocytes

Tamoxifen is a widely used chemotherapeutic agent, which has been associated with prolongation of the QT interval. Other studies have reported that acute exposure to tamoxifen can reduce cardiac K(+) currents. However, in vivo tamoxifen is largely metabolized and most of its activity is attributable to its major metabolite, 4-hydroxytamoxifen (4OH-tamoxifen). Accordingly, in the present study, we performed voltage-clamp experiments to directly investigate the effects of 4OH-tamoxifen on the repolarizing K(+) currents in adult mouse ventricular myocytes in order to determine whether the effects of tamoxifen on repolarization could be ascribed to 4OH-tamoxifen. K(+) currents were recorded before and after acute exposure to 4OH-tamoxifen (0.5, 1 and 10microM). 4OH-tamoxifen reduced the density of the Ca(2+)-independent transient outward (I(to)), the ultrarapid delayed rectifier (I(Kur)) and the inward rectifier (I(K1)) K(+) currents (by up to 43%, 41% and 26%, respectively) but had no significant effect on the steady-state outward K(+) current (I(ss)). Voltage dependence of steady-state inactivation and reactivation time of I(to) and I(Kur) were not affected by 4OH-tamoxifen. Experiments using the pure estrogen receptor antagonist, ICI 182,780 and the inhibitor of gene transcription, actinomycin D, were undertaken to assess the involvement of estrogen receptor. Administered alone these compounds did not affect the density of K(+) currents. Moreover, pretreatment of the cells with ICI 182,780 or actinomycin D did not prevent the inhibitory response to 4OH-tamoxifen. Overall, 4OH-tamoxifen reduced K(+) currents in mouse ventricle and this effect is unrelated to gene transcription and does not involve interaction of the drug with estrogen receptor.

Caveolae act as membrane reserves which limit mechanosensitive I(Cl,swell) channel activation during swelling in the rat ventricular myocyte

Background

Many ion channels are preferentially located in caveolae where compartmentalisation/scaffolding with signal transduction components regulates their activity. Channels that are mechanosensitive may be additionally dependent on caveolar control of the mechanical state of the membrane. Here we test which mechanism underlies caveolar-regulation of the mechanosensitive I_Cl,swell channel in the adult cardiac myocyte.

Methodology/Principal Findings

Rat ventricular myocytes were exposed to solution of 0.02 tonicity (T; until lysis), 0.64T for 10–15 min (swelling), and/or methyl-β-cyclodextrin (MBCD; to disrupt caveolae). MBCD and 0.64T swelling reduced the number of caveolae visualised by electron microscopy by 75 and 50% respectively. MBCD stimulated translocation of caveolin 3 from caveolae-enriched buoyant membrane fractions, but both caveolin 1 and 3 remained in buoyant fractions after swelling. I_Cl,swell inhibition in control cells decreased time to half-maximal volume (t_0.5,vol; 0.64T), consistent with a role for I_Cl,swell in volume regulation. MBCD-treated cells showed reduced time to lysis (0.02T) and t_0.5,vol (0.64T) compared with controls. The negative inotropic response to swelling (an index of I_Cl,swell activation) was enhanced by MBCD.

Conclusions/Significance

These data show that disrupting caveolae removes essential membrane reserves, which speeds swelling in hyposmotic conditions, and thereby promotes activation of I_Cl,swell. They illustrate a general principle whereby caveolae as a membrane reserve limit increases in membrane tension during stretch/swelling thereby restricting mechanosensitive channel activation.

Electrophysiological development of transplanted embryonic stem cell-derived cardiomyocytes in the hearts of syngeneic mice

BACKGROUND AND AIMS

We undertook this study in order to investigate the electrophysiological properties of grafted mouse embryonic stem cell (ES)-derived cardiomyocytes in mouse hearts.

METHODS

We generated transgenic D3 ES cells that carry the alpha-myosin heavy-chain promoter-driven enhanced green fluorescent protein (EGFP) gene. ES-derived cardiomyocytes (10 days) were purified by fluorescent-activated cell sorting and then transplanted into the left ventricle of syngeneic mice. Finally, hearts were removed and the EGFP+ cardiomyocytes were dissociated from the host heart for patch clamp study.

RESULTS

Morphological studies showed that EGFP+ cardiomyocytes were round and small before transplantation. Majority of cells were larger and longer with clear cross striations at the fourth week. Colocalization of EGFP and 4',6-diamidino-2-phenylindole-labeled nuclei of transplanted cells with cardiomyocyte markers for cardiac troponin T, as detected by immunofluorescent microscopy, indicated the survival of grafted cells. The patch clamp study revealed that the ES-derived cardiomyocytes possessed pacemaker-like action potential (AP) before transplantation. Four weeks after transplantation, grafted cells retained the characteristic of intermediate embryonic ventricular-like AP distinct from triangular AP of adult mouse ventricular myocytes, along with the loss of cellular excitability and downregulation of pacemaker current, suggesting that these grafted cells were not as mature as native ventricular cells.

CONCLUSIONS

Transplanted ES-derived cardiomyocytes display accelerated differentiation and loss of automaticity, indicating that the long-term effectiveness of ES cell-based biological pacemakers can also be problematic.

Origin of cardiac fibroblasts and the role of periostin

Cardiac fibroblasts are the most populous nonmyocyte cell type within the mature heart and are required for extracellular matrix synthesis and deposition, generation of the cardiac skeleton, and to electrically insulate the atria from the ventricles. Significantly, cardiac fibroblasts have also been shown to play an important role in cardiomyocyte growth and expansion of the ventricular chambers during heart development. Although there are currently no cardiac fibroblast-restricted molecular markers, it is generally envisaged that the majority of the cardiac fibroblasts are derived from the proepicardium via epithelial-to-mesenchymal transformation. However, still relatively little is known about when and where the cardiac fibroblasts cells are generated, the lineage of each cell, and how cardiac fibroblasts move to reside in their final position throughout all four cardiac chambers. In this review, we summarize the present understanding regarding the function of Periostin, a useful marker of the noncardiomyocyte lineages, and its role during cardiac morphogenesis. Characterization of the cardiac fibroblast lineage and identification of the signals that maintain, expand and regulate their differentiation will be required to improve our understanding of cardiac function in both normal and pathophysiological states.

[Eukaryotic expression vector pcDNA3-HERG transfection inhibits angiotensin II induced neonatal rabbit ventricular myocyte hypertrophy in vitro]

OBJECTIVE

To explore the effects of eukaryotic expression vector pcDNA3-HERG transfection on angiotensin II (Ang II) induced myocyte hypertrophy in cultured neonatal rabbit ventricular myocytes.

METHODS

Neonatal rabbit ventricular myocytes and eukaryotic expression vector pcDNA3-HERG transfected ventricular myocytes were cultured in Dulbecco's-modified Eagle medium (DMEM), containing 1% fetal bovine serum (FBS) for 6 h, then stimulated with Ang II (10(-7) mol/L) for 48 h. Control ventricular myocytes were cultured in Dulbecco's-modified Eagle medium (DMEM), containing 1% fetal bovine serum (FBS) for 54 h. At 6 and 54 h, myocyte hypertrophic parameters including myocyte volume, total protein content and membrane capacitance, action potential duration (APD) and Calcineurin (CaN) activity were measured.

RESULTS

Compared to control myocytes, APD at 90% repolarization (APD(90)) was prolonged by 19.8% (P < 0.01), without signs of myocyte hypertrophy at 6 h post Ang II stimulation, APD(90) was prolonged by 22.1% (P < 0.01), myocyte volume, total protein content and membrane capacitance and CaN activity were significantly increased by 40.4%, 40.4%, 38.2% and 114.7% respectively (all P < 0.01) at 48 h after Ang II stimulation. HERG gene transfection upregulated I(HERG) tail current (3.6-fold higher than I(Kr)-rapidly activating delayed rectifier potassium current, P < 0.01). HERG gene transfection also accelerated and repolarization and a shortened APD(90) and inhibited myocyte hypertrophy and CaN activation induced by Ang II.

CONCLUSIONS

Ang II induced prolongation of APD(90) is directly associated with myocyte hypertrophy by increasing the Ca(2+) influx and resulting in the increment of intracellular Ca(2+) and activation of CaN reaction pathway.