pH-dependent inhibition of K₂P3.1 prolongs atrial refractoriness in whole hearts

In isolated human atrial cardiomyocytes, inhibition of K2P3.1 K(+) channels results in action potential (action potential duration (APD)) prolongation. It has therefore been postulated that K2P3.1 (KCNK3), together with K2P9.1 (KCNK9), could represent novel drug targets for the treatment of atrial fibrillation (AF). However, it is unknown whether these findings in isolated cells translate to the whole heart. The purposes of this study were to investigate the expression levels of KCNK3 and KCNK9 in human hearts and two relevant rodent models and determine the antiarrhythmic potential of K2P3.1 inhibition in isolated whole-heart preparations. By quantitative PCR, we found that KCNK3 is predominantly expressed in human atria whereas KCNK9 was not detectable in heart human tissue. No differences were found between patients in AF or sinus rhythm. The expression in guinea pig heart resembled humans whereas rats displayed a more uniform expression of KCNK3 between atria and ventricle. In voltage-clamp experiments, ML365 and A293 were found to be potent and selective inhibitors of K2P3.1, but at pH 7.4, they failed to prolong atrial APD and refractory period (effective refractory period (ERP)) in isolated perfused rat and guinea pig hearts. At pH 7.8, which augments K2P3.1 currents, pharmacological channel inhibition produced a significant prolongation of atrial ERP (11.6 %, p = 0.004) without prolonging ventricular APD but did not display a significant antiarrhythmic effect in our guinea pig AF model (3/8 hearts converted on A293 vs 0/7 hearts in time-matched controls). These results suggest that when K2P3.1 current is augmented, K2P3.1 inhibition leads to atrial-specific prolongation of ERP; however, this ERP prolongation did not translate into significant antiarrhythmic effects in our AF model.

Whole-Heart Construct Cultivation Under 3D Mechanical Stimulation of the Left Ventricle

Today the concept of Whole-Heart Tissue Engineering represents one of the most promising approaches to the challenge of synthesizing functional myocardial tissue. At the current state of scientific and technological knowledge it is a principal task to transfer findings of several existing and widely investigated models to the process of whole-organ tissue engineering. Hereby, we present the first bioreactor system that allows the integrated 3D biomechanical stimulation of a whole-heart construct while allowing for simultaneous controlled perfusion of the coronary system.

CD14 as a Mediator of the Mineralocorticoid Receptor-Dependent Anti-apolipoprotein A-1 IgG Chronotropic Effect on Cardiomyocytes

In vitro and animal studies point to autoantibodies against apolipoprotein A-1 (anti-apoA-1 IgG) as possible mediators of cardiovascular (CV) disease involving several mechanisms such as basal heart rate interference mediated by a mineralocorticoid receptor-dependent L-type calcium channel activation, and a direct pro-inflammatory effect through the engagement of the toll-like receptor (TLR) 2/CD14 complex. Nevertheless, the possible implication of these receptors in the pro-arrhythmogenic effect of anti-apoA-1 antibodies remains elusive. We aimed at determining whether CD14 and TLRs could mediate the anti-apoA-1 IgG chronotropic response in neonatal rat ventricular cardiomyocytes (NRVC). Blocking CD14 suppressed anti-apoA-1 IgG binding to NRVC and the related positive chronotropic response. Anti-apoA-1 IgG alone induced the formation of a TLR2/TLR4/CD14 complex, followed by the phosphorylation of Src, whereas aldosterone alone promoted the phosphorylation of Akt by phosphatidylinositol 3-kinase (PI3K), without affecting the chronotropic response. In the presence of both aldosterone and anti-apoA-1 IgG, the localization of TLR2/TLR4/CD14 was increased in membrane lipid rafts, followed by PI3K and Src activation, leading to an L-type calcium channel-dependent positive chronotropic response. Pharmacological inhibition of the Src pathway led to the decrease of L-type calcium channel activity and abrogated the NRVC chronotropic response. Activation of CD14 seems to be a key regulator of the mineralocorticoid receptor-dependent anti-apoA-1 IgG positive chronotropic effect on NRVCs, involving relocation of the CD14/TLR2/TLR4 complex into lipid rafts followed by PI3K and Src-dependent L-type calcium channel activation.

Differential Expression Levels of Integrin α6 Enable the Selective Identification and Isolation of Atrial and Ventricular Cardiomyocytes

Rationale

Central questions such as cardiomyocyte subtype emergence during cardiogenesis or the availability of cardiomyocyte subtypes for cell replacement therapy require selective identification and purification of atrial and ventricular cardiomyocytes. However, current methodologies do not allow for a transgene-free selective isolation of atrial or ventricular cardiomyocytes due to the lack of subtype specific cell surface markers.

Methods and Results

In order to develop cell surface marker-based isolation procedures for cardiomyocyte subtypes, we performed an antibody-based screening on embryonic mouse hearts. Our data indicate that atrial and ventricular cardiomyocytes are characterized by differential expression of integrin α6 (ITGA6) throughout development and in the adult heart. We discovered that the expression level of this surface marker correlates with the intracellular subtype-specific expression of MLC-2a and MLC-2v on the single cell level and thereby enables the discrimination of cardiomyocyte subtypes by flow cytometry. Based on the differential expression of ITGA6 in atria and ventricles during cardiogenesis, we developed purification protocols for atrial and ventricular cardiomyocytes from mouse hearts. Atrial and ventricular identities of sorted cells were confirmed by expression profiling and patch clamp analysis.

Conclusion

Here, we introduce a non-genetic, antibody-based approach to specifically isolate highly pure and viable atrial and ventricular cardiomyocytes from mouse hearts of various developmental stages. This will facilitate in-depth characterization of the individual cellular subsets and support translational research applications.

Alcohol and Apoptosis: Friends or Foes?

Alcohol abuse causes 79,000 deaths stemming from severe organ damage in the United States every year. Clinical manifestations of long-term alcohol abuse on the cardiac muscle include defective contractility with the development of dilated cardiomyopathy and low-output heart failure; which has poor prognosis with less than 25% survival for more than three years. In contrast, low alcohol consumption has been associated with reduced risk of cardiovascular disease, however the mechanism of this phenomenon remains elusive. The aim of this study was to determine the significance of apoptosis as a mediating factor in cardiac function following chronic high alcohol versus low alcohol exposure. Adult rats were provided 5 mM (low alcohol), 100 mM (high alcohol) or pair-fed non-alcohol controls for 4–5 months. The hearts were dissected, sectioned and stained with cresyl violet or immunohistochemically for caspase-3, a putative marker for apoptosis. Cardiomyocytes were isolated to determine the effects of alcohol exposure on cell contraction and relaxation. High alcohol animals displayed a marked thinning of the left ventricular wall combined with elevated caspase-3 activity and decreased contractility. In contrast, low alcohol was associated with increased contractility and decreased apoptosis suggesting an overall protective mechanism induced by low levels of alcohol exposure.

The role of nitric oxide in regulation of leukocyte migration into the heart tissue in vitro

Characteristics of leukocyte migration have been studied during the incubation of the right and left ventricles of rat heart explants in autologous blood plasma. Within the first 60 min, the leukocyte amount in the medium increases. Moreover, it is associated with cell release from the heart tissue. During further incubation, the cell release decreases; after 3 h of incubation, the cells begin to migrate back into the heart tissue. However, neutrophil migration does not change. Sodium nitrite, being a donor of nitric oxide significantly, reduces the leukocyte migration from the heart explants into the incubation medium, especially from left ventricle explants.

Turbulent states and their transitions in mathematical models for ventricular tissue: the effects of random interstitial fibroblasts

We study the dynamical behaviors of two types of spiral- and scroll-wave turbulence states, respectively, in two-dimensional (2D) and three-dimensional (3D) mathematical models, of human, ventricular, myocyte cells that are attached to randomly distributed interstitial fibroblasts; these turbulence states are promoted by (a) the steep slope of the action-potential-duration-restitution (APDR) plot or (b) early afterdepolarizations (EADs). Our single-cell study shows that (1) the myocyte-fibroblast (MF) coupling G_{j} and (2) the number N_{f} of fibroblasts in an MF unit lower the steepness of the APDR slope and eliminate the EAD behaviors of myocytes; we explore the pacing dependence of such EAD suppression. In our 2D simulations, we observe that a spiral-turbulence (ST) state evolves into a state with a single, rotating spiral (RS) if either (a) G_{j} is large or (b) the maximum possible number of fibroblasts per myocyte N_{f}^{max} is large. We also observe that the minimum value of G_{j}, for the transition from the ST to the RS state, decreases as N_{f}^{max} increases. We find that, for the steep-APDR-induced ST state, once the MF coupling suppresses ST, the rotation period of a spiral in the RS state increases as (1) G_{j} increases, with fixed N_{f}^{max}, and (2) N_{f}^{max} increases, with fixed G_{j}. We obtain the boundary between ST and RS stability regions in the N_{f}^{max}-G_{j} plane. In particular, for low values of N_{f}^{max}, the value of G_{j}, at the ST-RS boundary, depends on the realization of the randomly distributed fibroblasts; this dependence decreases as N_{f}^{max} increases. Our 3D studies show a similar transition from scroll-wave turbulence to a single, rotating, scroll-wave state because of the MF coupling. We examine the experimental implications of our study and propose that the suppression (a) of the steep slope of the APDR or (b) EADs can eliminate spiral- and scroll-wave turbulence in heterogeneous cardiac tissue, which has randomly distributed fibroblasts.

Cardioprotective HIF-1α-frataxin signaling against ischemia-reperfusion injury

Previous studies have demonstrated the protective signaling of hypoxia-inducible factor (HIF)-1 α against ischemia-reperfusion (I/R) injury in the heart. In the present study, we provide further evidence for a cardioprotective mechanism by HIF-1α against I/R injury exerted via the mitochondrial protein frataxin, which regulates mitochondrial Fe-S cluster formation. Disruption of frataxin has been found to induce mitochondrial iron overload and subsequent ROS production. We observed that frataxin expression was elevated in mice hearts subjected to I/R injury, and this response was blunted in cardiomyocyte-specific HIF-1α knockout (KO) mice. Furthermore, these HIF-1α KO mice sustained extensive cardiac damage from I/R injury compared with control mice. Similarly, reduction of HIF-1α by RNA inhibition resulted in an attenuation of frataxin expression in response to hypoxia in H9C2 cardiomyocytes. Therefore, we postulated that HIF-1α transcriptionally regulates frataxin expression in response to hypoxia and offers a cardioprotective mechanism against ischemic injury. Our promoter activity and chromatin immunoprecipitation assays confirmed the presence of a functional hypoxia response element in the frataxin promoter. Our data also suggest that increased frataxin mitigated mitochondrial iron overload and subsequent ROS production, thus preserving mitochondrial membrane integrity and viability of cardiomyocytes. We postulate that frataxin may exert its beneficial effects by acting as an iron storage protein under hypoxia and subsequently facilitates the maintenance of mitochondrial membrane potential and promotes cell survival. The findings from our study revealed that HIF-1α-frataxin signaling promotes a protective mechanism against hypoxic/ischemic stress.

Implementation of Contraction to Electrophysiological Ventricular Myocyte Models, and Their Quantitative Characterization via Post-Extrasystolic Potentiation

Heart failure (HF) affects over 5 million Americans and is characterized by impairment of cellular cardiac contractile function resulting in reduced ejection fraction in patients. Electrical stimulation such as cardiac resynchronization therapy (CRT) and cardiac contractility modulation (CCM) have shown some success in treating patients with HF. Computer simulations have the potential to help improve such therapy (e.g. suggest optimal lead placement) as well as provide insight into the underlying mechanisms which could be beneficial. However, these myocyte models require a quantitatively accurate excitation-contraction coupling such that the electrical and contraction predictions are correct. While currently there are close to a hundred models describing the detailed electrophysiology of cardiac cells, the majority of cell models do not include the equations to reproduce contractile force or they have been added ad hoc. Here we present a systematic methodology to couple first generation contraction models into electrophysiological models via intracellular calcium and then compare the resulting model predictions to experimental data. This is done by using a post-extrasystolic pacing protocol, which captures essential dynamics of contractile forces. We found that modeling the dynamic intracellular calcium buffers is necessary in order to reproduce the experimental data. Furthermore, we demonstrate that in models the mechanism of the post-extrasystolic potentiation is highly dependent on the calcium released from the Sarcoplasmic Reticulum. Overall this study provides new insights into both specific and general determinants of cellular contractile force and provides a framework for incorporating contraction into electrophysiological models, both of which will be necessary to develop reliable simulations to optimize electrical therapies for HF.

Finite-Element Extrapolation of Myocardial Structure Alterations Across the Cardiac Cycle in Rats

Myocardial microstructures are responsible for key aspects of cardiac mechanical function. Natural myocardial deformation across the cardiac cycle induces measurable structural alteration, which varies across disease states. Diffusion tensor magnetic resonance imaging (DT-MRI) has become the tool of choice for myocardial structural analysis. Yet, obtaining the comprehensive structural information of the whole organ, in 3D and time, for subject-specific examination is fundamentally limited by scan time. Therefore, subject-specific finite-element (FE) analysis of a group of rat hearts was implemented for extrapolating a set of initial DT-MRI to the rest of the cardiac cycle. The effect of material symmetry (isotropy, transverse isotropy, and orthotropy), structural input, and warping approach was observed by comparing simulated predictions against in vivo MRI displacement measurements and DT-MRI of an isolated heart preparation at relaxed, inflated, and contracture states. Overall, the results indicate that, while ventricular volume and circumferential strain are largely independent of the simulation strategy, structural alteration predictions are generally improved with the sophistication of the material model, which also enhances torsion and radial strain predictions. Moreover, whereas subject-specific transversely isotropic models produced the most accurate descriptions of fiber structural alterations, the orthotropic models best captured changes in sheet structure. These findings underscore the need for subject-specific input data, including structure, to extrapolate DT-MRI measurements across the cardiac cycle.

[Simultaneous separation of primary cardiomyocytes and cardiac fibroblasts from neonatal rats with density gradient centrifugation]

To improve a fast and high-quality isolation method for culturing the primary cardiomyocyte and fibroblast in vitro, the neonatal Wistar rats were decapitated accordingly and left ventricles were isolated under the sterile condition. The ventricles were chopped and digested in the enzyme solution containing 0.5 mg/mL type II collagenase. During this process, the digesting time, frequency and stirring speed, centrifuging frequency and speed were strictly controlled. The cardiomyocytes were separated from the cardiac fibroblast by using the Percoll density gradient centrifugation. The cell viability was tested by staining with 0.2% trypan blue. The purity of cardiomyocytes and fibroblasts were determined by immunoflourescent staining with anti-cTnI, anti-Vimentin and anti-α-SMA antibodies. The results indicated that with this protocol, the viability and purity of cardiomyocytes were 92% and 95%. The automobile pulse of the adhered cardiomyocyte was visible. For fibroblasts, the cell viability and purity were 96% and 94%. Our results demonstrate that this advanced isolation method is reproducible, and can simultaneously produce high-quality primary cardiomyocytes and fibroblasts for the future study.

Calcium response of KCl-excited populations of ventricular myocytes from the European sea bass (Dicentrarchus labrax): a promising approach to integrate cell-to-cell heterogeneity in studying the cellular basis of fish cardiac performance

Climate change challenges the capacity of fishes to thrive in their habitat. However, through phenotypic diversity, they demonstrate remarkable resilience to deteriorating conditions. In fish populations, inter-individual variation in a number of fitness-determining physiological traits, including cardiac performance, is classically observed. Information about the cellular bases of inter-individual variability in cardiac performance is scarce including the possible contribution of excitation-contraction (EC) coupling. This study aimed at providing insight into EC coupling-related Ca(2+) response and thermal plasticity in the European sea bass (Dicentrarchus labrax). A cell population approach was used to lay the methodological basis for identifying the cellular determinants of cardiac performance. Fish were acclimated at 12 and 22 °C and changes in intracellular calcium concentration ([Ca(2+)]i) following KCl stimulation were measured using Fura-2, at 12 or 22 °C-test. The increase in [Ca(2+)]i resulted primarily from extracellular Ca(2+) entry but sarcoplasmic reticulum stores were also shown to be involved. As previously reported in sea bass, a modest effect of adrenaline was observed. Moreover, although the response appeared relatively insensitive to an acute temperature change, a difference in Ca(2+) response was observed between 12- and 22 °C-acclimated fish. In particular, a greater increase in [Ca(2+)]i at a high level of adrenaline was observed in 22 °C-acclimated fish that may be related to an improved efficiency of adrenaline under these conditions. In conclusion, this method allows a rapid screening of cellular characteristics. It represents a promising tool to identify the cellular determinants of inter-individual variability in fishes' capacity for environmental adaptation.

Isolation and study of cardiac nuclei from canine myocardium and adult ventricular myocytes

The nuclear envelope encloses the genome as well as the molecular machinery responsible for both the replication and transcription of DNA as well as the maturation of nascent RNA. Recent studies ascribe a growing number of functions to the nuclear membrane, in addition to sequestering the DNA, through receptors and their effectors, ion channels, as well as ion pumps and transporters located within the nuclear membrane itself. Despite the obvious structural and functional importance of the nucleus, certain aspects remain poorly understood due to the challenges associated with its accessibility in vivo, as well as isolating nuclei intact and with sufficient purity from cardiac cells to permit studies in vitro. Here, we present a detailed protocol for isolation of intact nuclei from both myocardial tissue and freshly isolated adult ventricular cardiomyocytes. These methods are based on partial permeabilization of plasma membrane with digitonin and cell disruption, followed by differential and discontinuous sucrose density centrifugation. These preparations provide for rapid separation of nonnuclear membranes and cytosol from nuclei.

Quantitative Decomposition of Dynamics of Mathematical Cell Models: Method and Application to Ventricular Myocyte Models

Mathematical cell models are effective tools to understand cellular physiological functions precisely. For detailed analysis of model dynamics in order to investigate how much each component affects cellular behaviour, mathematical approaches are essential. This article presents a numerical analysis technique, which is applicable to any complicated cell model formulated as a system of ordinary differential equations, to quantitatively evaluate contributions of respective model components to the model dynamics in the intact situation. The present technique employs a novel mathematical index for decomposed dynamics with respect to each differential variable, along with a concept named instantaneous equilibrium point, which represents the trend of a model variable at some instant. This article also illustrates applications of the method to comprehensive myocardial cell models for analysing insights into the mechanisms of action potential generation and calcium transient. The analysis results exhibit quantitative contributions of individual channel gating mechanisms and ion exchanger activities to membrane repolarization and of calcium fluxes and buffers to raising and descending of the cytosolic calcium level. These analyses quantitatively explicate principle of the model, which leads to a better understanding of cellular dynamics.

Effect of acute and prolonged alcohol administration on Mg(2+) homeostasis in cardiac cells

Alcoholic cardiomyopathy represents a major clinical complication in chronic alcoholics. Previous studies from our laboratory indicate that acute and chronic exposure of liver cells to ethanol results in a major loss of cellular Mg(2+) as a result of alcohol oxidation. We investigated whether exposure to ethanol induces a similar Mg(2+) loss in cardiac cells. The results indicate that chronic exposure to a 6% ethanol-containing diet depleted cardiac myocytes of >25% of their cellular Mg(2+) content. Acute ethanol exposure, instead, induced a time- and dose-dependent manner of Mg(2+) extrusion from perfused hearts and collagenase-dispersed cardiac ventricular myocytes. Pretreatment with chlormethiazole prevented ethanol-induced Mg(2+) loss to a large extent, suggesting a role of ethanol oxidation via cyP4502E1 in the process. Magnesium extrusion across the sarcolemma occurred via the amiloride-inhibited Na(+)/Mg(2+) exchanger. Taken together, our data indicate that Mg(2+) extrusion also occurs in cardiac cells exposed to ethanol as a result of alcohol metabolism by cyP4502E1. The extrusion, which is mediated by the Na(+)/Mg(2+) exchanger, only occurs at doses of ethanol ≥0.1%, and depends on ethanol-induced decline in cellular ATP. The significance of Mg(2+) extrusion for the onset of alcoholic cardiomyopathy remains to be elucidated.

Stimulus-dependent regulation of nuclear Ca2+ signaling in cardiomyocytes: a role of neuronal calcium sensor-1

In cardiomyocytes, intracellular calcium (Ca²⁺) transients are elicited by electrical and receptor stimulations, leading to muscle contraction and gene expression, respectively. Although such elevations of Ca²⁺levels ([Ca²⁺]) also occur in the nucleus, the precise mechanism of nuclear [Ca²⁺] regulation during different kinds of stimuli, and its relationship with cytoplasmic [Ca²⁺] regulation are not fully understood. To address these issues, we used a new region-specific fluorescent protein-based Ca²⁺ indicator, GECO, together with the conventional probe Fluo-4 AM. We confirmed that nuclear Ca²⁺ transients were elicited by both electrical and receptor stimulations in neonatal mouse ventricular myocytes. Kinetic analysis revealed that electrical stimulation-elicited nuclear Ca²⁺ transients are slower than cytoplasmic Ca²⁺ transients, and chelating cytoplasmic Ca²⁺ abolished nuclear Ca²⁺ transients, suggesting that nuclear Ca²⁺ are mainly derived from the cytoplasm during electrical stimulation. On the other hand, receptor stimulation such as with insulin-like growth factor-1 (IGF-1) preferentially increased nuclear [Ca²⁺] compared to cytoplasmic [Ca²⁺]. Experiments using inhibitors revealed that electrical and receptor stimulation-elicited Ca²⁺ transients were mainly mediated by ryanodine receptors and inositol 1,4,5-trisphosphate receptors (IP3Rs), respectively, suggesting different mechanisms for the two signals. Furthermore, IGF-1-elicited nuclear Ca²⁺ transient amplitude was significantly lower in myocytes lacking neuronal Ca²⁺ sensor-1 (NCS-1), a Ca²⁺ binding protein implicated in IP₃R-mediated pathway in the heart. Moreover, IGF-1 strengthened the interaction between NCS-1 and IP₃R. These results suggest a novel mechanism for receptor stimulation-induced nuclear [Ca²⁺] regulation mediated by IP3R and NCS-1 that may further fine-tune cardiac Ca²⁺ signal regulation.

Resistance training regulates cardiac function through modulation of miRNA-214

Aims: To determine the effects of resistance training (RT) on the expression of microRNA (miRNA)-214 and its target in sarcoplasmic reticulum Ca²⁺-ATPase (SERCA2a), and on the morphological and mechanical properties of isolated left ventricular myocytes. Main methods: Male Wistar rats were divided into two groups (n = 7/group): Control (CO) or trained (TR). The exercise-training protocol consisted of: 4 × 12 bouts, 5×/week during 8 weeks, with 80% of one repetition maximum. Key findings: RT increased the left ventricular myocyte width by 15% and volume by 12%, compared with control animals (p < 0.05). The time to half relaxation and time to peak were 8.4% and 4.4% lower, respectively, in cells from TR group as compared to CO group (p < 0.05). RT decreased miRNA-214 level by 18.5% while its target SERCA2a expression were 18.5% higher (p < 0.05). Significance: Our findings showed that RT increases single left ventricular myocyte dimensions and also leads to faster cell contraction and relaxation. These mechanical adaptations may be related to the augmented expression of SERCA2a which, in turn, may be associated with the epigenetic modification of decreased miRNA-214 expression.

Numerical evaluation of myofiber orientation and transmural contractile strength on left ventricular function

The left ventricle (LV) of the heart is composed of a complex organization of cardiac muscle fibers, which contract to generate force and pump blood into the body. It has been shown that both the orientation and contractile strength of these myofibers vary across the ventricular wall. The hypothesis of the current study is that the transmural distributions of myofiber orientation and contractile strength interdependently impact LV pump function. In order to quantify these interactions a finite element (FE) model of the LV was generated, which incorporated transmural variations. The influences of myofiber orientation and contractile strength on the Starling relationship and the end-systolic (ES) apex twist of the LV were assessed. The results suggest that reductions in contractile strength within a specific transmural layer amplified the effects of altered myofiber orientation in the same layer, causing greater changes in stroke volume (SV). Furthermore, when the epicardial myofibers contracted the strongest, the twist of the LV apex was greatest, regardless of myofiber orientation. These results demonstrate the important role of transmural distribution of myocardial contractile strength and its interplay with myofiber orientation. The coupling between these two physiologic parameters could play a critical role in the progression of heart failure.

Changes in intracellular calcium concentration influence beat-to-beat variability of action potential duration in canine ventricular myocytes

The aim of the present work was to study the influence of changes in intracellular calcium concentration ([Ca(2+)]i) on beat-to-beat variability (short term variability, SV) of action potential duration (APD) in isolated canine ventricular cardiomyocytes. Series of action potentials were recorded from enzymatically isolated canine ventricular cells using conventional microelectrode technique. Drug effects on SV were evaluated as relative SV changes determined by plotting the drug-induced changes in SV against corresponding changes in APD and comparing these data to the exponential SV-APD function obtained with inward and outward current injections. Exposure of myocytes to the Ca(2+) chelator BAPTA-AM (5 μM) decreased, while Ca(2+) ionophore A23187 (1 μM) increased the magnitude of relative SV. Both effects were primarily due to the concomitant changes in APD. Relative SV was reduced by BAPTA-AM under various experimental conditions including pretreatment with veratridine, BAY K8644, dofetilide or E-4031. Contribution of transient changes of [Ca(2+)]i due to Ca(2+) released from the sarcoplasmic reticulum (SR) was studied using 10 μM ryanodine and 1 μM cyclopiazonic acid: relative SV was reduced by both agents. Inhibition of the Na(+)-Ca(2+) exchanger by 1 μM SEA0400 increased relative SV. It is concluded that elevation of [Ca(2+)]i increases relative SV significantly. More importantly, Ca(2+) released from the SR is an important component of this effect.

[STEM CELLS PLAY NO CONSIDERABLE ROLE IN CARDIOMYOCYTE REPOPULATION OF ADULT HUMAN HEART]

There are two viewpoints concerning cardiac regeneration. One assumes that the myocardium of an adult human heart has a weak regenerative capacity. According to another, myocardium can renew at a high rate due to the presence of resident stem cells. This study was aimed to test the role of stem cells in myocardium repopulation in adult humans of different age by examining the distribution of cardiomyocytes as to their size and ploidy. Cytofluorimetry and interferometry were used to determine the dry weight, volume and ploidy of myocytes isolated from the left ventricle of the normal heart of 12 men aged 20-30 years (n = 7) and 40-50 years (n = 5). Dry weight of cardiomyocytes made up 6906 ± 182 pg (10(-12) g) aged 20-30 years and 9126 ± 263 pg in men aged 40-50 years. There were no cells with an intermediate volume between amplifying and mature myocytes. The number of candiomyocytes in the left ventricle made up (3.18 ± 0.05) x 10(9) cells in the age group 20-30 years and (2.06 ± 0.6) x 10(9) cells in the age group 40-50 years. Most of the myocyte population was represented by mononucleate cells with tetraploid nuclei (41.3%). Proportion of myocytes of different ploidy classes did not change in the interval from 20 to 50 years. Our results strongly suggest that stem cells of the heart are not involved in the regeneration of human myocardium during aging. The function of the aging heart is mostly compensated by the hypertrophy of the remaining myocytes.

Engineering 3D bio-artificial heart muscle: the acellular ventricular extracellular matrix model

Current therapies in left ventricular systolic dysfunction and end-stage heart failure include mechanical assist devices or transplant. The development of a tissue-engineered integrative platform would present a therapeutic option that overcomes the limitations associated with current treatment modalities. This study provides a foundation for the fabrication and preliminary viability of the acellular ventricular extracellular matrix (AVEM) model. Acellular ventricular extracellular matrix was fabricated by culturing 4 million rat neonatal cardiac cells around an excised acellular ventricular segment. Acellular ventricular extracellular matrix generated a maximum spontaneous contractile force of 388.3 μN and demonstrated a Frank-Starling relationship at varying pretensions. Histologic assessment displayed cell cohesion and adhesion within the AVEM as a result of passive cell seeding.

Assessing cardiomyocyte proliferative capacity in the newt heart and primary culture

Unlike humans, adult newts possess extraordinary abilities to functionally regenerate lost and injured organs, including cardiac muscle. The most remarkable feature of mature newt cardiomyocytes is their ability to reenter the cell cycle, undergo cell division, and serve as a reservoir for progenitor cells. There are, however, a number of unsolved questions concerning the cellular and molecular mechanisms that underlie this plasticity; for example, we still lack a deeper understanding of the cell-inherent properties of newt cardiomyocytes and to what degree they differ from their mammalian counterparts. Along with considerable morphological changes at the wound site, a striking feature shared by different regenerating tissues in the newt is an extensive and dynamic remodeling of the extracellular environment. The dynamic signaling between cardiomyocytes and extracellular environment is of eminent importance in the control of the differentiated state of the cell, but the molecular details remain elusive. In this chapter, we describe methods to assess cardiomyocyte proliferation in vivo and enrich primary cardiomyocytes from newt hearts to study their behavior, taking extracellular matrix components into consideration.

[Acquirement and evaluation of murine ventricular extracellular matrix]

Cardiac extracellular matrix (ECM), generated from the process of decellularization, has been widely considered as an ideal source of biological scaffolds. However, current ECM preparations are generally difficult to be applied to generate cardiac tissue. Our research was aimed to improve decellularization protocols to prepare cardiac ECM slices. Adult murine ventricular tissues were embedded in low melting agarose and cut into 300 μm slices, and then were divided randomly into three groups: normal cardiac tissue, SDS treated group (0.1% SDS) and SDS+Triton X-100 treated group (0.1% SDS+0.5% Triton X-100). Total RNA content and protein content quantification, HE staining and immunostaining were used to evaluate the removal of cell components and preservation of vital ECM components. Furthermore, murine embryonic stem cell-derived cardiomyocytes (mES-CMs) and mouse embryonic fibroblasts (MEFs) were co-cultured with ECM slices to evaluate biocompatibility. The relative residual RNA and protein contents of ECM slices significantly decreased after decellularization. HE staining showed that SDS+Triton X-100 treatment better destroyed cellular structure and removed nuclei of ECM slices, compared with SDS treatment. Immunostaining showed that collagen IV and laminin were better preserved and presented better similarity to original cardiac tissue in ECM slices acquired by SDS+Triton X-100 treatment. However, collagen IV and laminin were significantly decreased and arranged disorderly in SDS treated group. We observed effective survival (≥ 12 days) of MEFs and mES-CMs on ECM slices acquired by SDS+Triton X-100 treatment, and signs of integration, whereas those signs were not found in SDS treated group. We concluded that, compared with traditional SDS method, new combined protocol (SDS+Triton X-100) generated ECM slices with better component and structural preservation, as well as better biocompatibility.

The role of dyadic organization in regulation of sarcoplasmic reticulum Ca(2+) handling during rest in rabbit ventricular myocytes

The dyadic organization of ventricular myocytes ensures synchronized activation of sarcoplasmic reticulum (SR) Ca(2+) release during systole. However, it remains obscure how the dyadic organization affects SR Ca(2+) handling during diastole. By measuring intraluminal SR Ca(2+) ([Ca(2+)]SR) decline during rest in rabbit ventricular myocytes, we found that ∼76% of leaked SR Ca(2+) is extruded from the cytosol and only ∼24% is pumped back into the SR. Thus, the majority of Ca(2+) that leaks from the SR is removed from the cytosol before it can be sequestered back into the SR by the SR Ca(2+)-ATPase (SERCA). Detubulation decreased [Ca(2+)]SR decline during rest, thus making the leaked SR Ca(2+) more accessible for SERCA. These results suggest that Ca(2+) extrusion systems are localized in T-tubules. Inhibition of Na(+)-Ca(2+) exchanger (NCX) slowed [Ca(2+)]SR decline during rest by threefold, however did not prevent it. Depolarization of mitochondrial membrane potential during NCX inhibition completely prevented the rest-dependent [Ca(2+)]SR decline. Despite a significant SR Ca(2+) leak, Ca(2+) sparks were very rare events in control conditions. NCX inhibition or detubulation increased Ca(2+) spark activity independent of SR Ca(2+) load. Overall, these results indicate that during rest NCX effectively competes with SERCA for cytosolic Ca(2+) that leaks from the SR. This can be explained if the majority of SR Ca(2+) leak occurs through ryanodine receptors in the junctional SR that are located closely to NCX in the dyadic cleft. Such control of the dyadic [Ca(2+)] by NCX play a critical role in suppressing Ca(2+) sparks during rest.

Vangl2-regulated polarisation of second heart field-derived cells is required for outflow tract lengthening during cardiac development

Planar cell polarity (PCP) is the mechanism by which cells orient themselves in the plane of an epithelium or during directed cell migration, and is regulated by a highly conserved signalling pathway. Mutations in the PCP gene Vangl2, as well as in other key components of the pathway, cause a spectrum of cardiac outflow tract defects. However, it is unclear why cells within the mesodermal heart tissue require PCP signalling. Using a new conditionally floxed allele we show that Vangl2 is required solely within the second heart field (SHF) to direct normal outflow tract lengthening, a process that is required for septation and normal alignment of the aorta and pulmonary trunk with the ventricular chambers. Analysis of a range of markers of polarised epithelial tissues showed that in the normal heart, undifferentiated SHF cells move from the dorsal pericardial wall into the distal outflow tract where they acquire an epithelial phenotype, before moving proximally where they differentiate into cardiomyocytes. Thus there is a transition zone in the distal outflow tract where SHF cells become more polarised, turn off progenitor markers and start to differentiate to cardiomyocytes. Membrane-bound Vangl2 marks the proximal extent of this transition zone and in the absence of Vangl2, the SHF-derived cells are abnormally polarised and disorganised. The consequent thickening, rather than lengthening, of the outflow wall leads to a shortened outflow tract. Premature down regulation of the SHF-progenitor marker Isl1 in the mutants, and accompanied premature differentiation to cardiomyocytes, suggests that the organisation of the cells within the transition zone is important for maintaining the undifferentiated phenotype. Thus, Vangl2-regulated polarisation and subsequent acquisition of an epithelial phenotype is essential to lengthen the tubular outflow vessel, a process that is essential for on-going cardiac morphogenesis.

Congenital heart defects are common, affecting almost 1% of all live births. Many of these affect the outflow region, where the aorta and pulmonary trunk connect with the main ventricular chambers. Congenital heart defects arise from disruption of normal developmental processes and can be modelled in mice. Thus, studying normal development, together with mouse mutants that develop heart malformations, should shed light on why these common anomalies arise. We have studied cardiac development in a mouse mutant for the Vangl2 gene, a key component of the planar cell polarity (PCP) pathway. This pathway controls the orientations of cells in epithelia and during directional cell migration. Here, we show that PCP signalling is required by cells derived from the second heart field, which forms the outflow tract walls. We show that in the absence of Vangl2, the cells within the distal outflow tract walls are non-polarised and disorganised. As a consequence the outflow tract is shortened and does not align properly with the ventricles. Thus, we show why disruption of a key PCP gene leads to outflow tract malformations. This is important for understanding heart development, but also more generally for understanding how PCP signalling regulates growth of tubular structures.

Salvia miltiorrhiza (Danshen) inhibits L-type calcium current and attenuates calcium transient and contractility in rat ventricular myocytes

ETHNOPHARMACOLOGICAL RELEVANCE

Salvia miltiorrhiza (SM, Danshen), a traditional Chinese herbal drug, has been widely used for hundreds of years to treat coronary artery disease.

MATERIALS AND METHODS

We studied the effects of SM on the L-type Ca(2+) current (ICa-L) with whole-cell patch-clamp technique in rat ventricular myocytes, and its influence on Ca(2+) transient and contractility using video-based edge detection and dual excitation fluorescence photomultiplier systems as well.

RESULTS

Exposure to SM solution caused a concentration- and voltage-dependent blockade of ICa-L, and the dose of SM solution (10g/l) decreased the maximal inhibitory effect of 35.2±1.2%. However, SM solution did not significantly change the current-voltage relationship or reversal potential of ICa-L, nor did it altered the activation and inactivation gating properties of cardiac Ca(2+) channels. Meanwhile, SM decreased the amplitude of myocyte shortening and the peak value of Ca(2+) transient with a significant decrease in the time to 90% of the baseline (Tr), but the time to 10% of the peak (Tp) was not dramatically prolonged.

CONCLUSIONS

The results indicated that SM significantly inhibited L-type Ca(2+) channels, decreased [Ca(2+)]i and contractility in adult rat cardiac myocytes. These findings may be relevant to the cardioprotective efficacy of SM.

Irx4 identifies a chamber-specific cell population that contributes to ventricular myocardium development

BACKGROUND

The ventricular myocardium is the most prominent layer of the heart, and the most important for mediating cardiac physiology. Although the ventricular myocardium is critical for heart function, the cellular hierarchy responsible for ventricle-specific myocardium development remains unresolved.

RESULTS

To determine the pattern and time course of ventricular myocardium development, we investigated IRX4 protein expression, which has not been previously reported. We identified IRX4+ cells in the cardiac crescent, and these cells were positive for markers of the first or second heart fields. From the onset of chamber formation, IRX4+ cells were restricted to the ventricular myocardium. This expression pattern persisted into adulthood. Of interest, we observed that IRX4 exhibits developmentally regulated dynamic intracellular localization. Throughout prenatal cardiogenesis, and up to postnatal day 4, IRX4 was detected in the cytoplasm of ventricular myocytes. However, between postnatal days 5–6, IRX4 translocated to the nucleus of ventricular myocytes.

CONCLUSIONS

Given the ventricle-specific expression of Irx4 in later stages of heart development, we hypothesize that IRX4+ cells in the cardiac crescent represent the earliest cell population in the cellular hierarchy underlying ventricular myocardium development.

Role of inositol-1,4,5-trisphosphate receptor in the regulation of calcium transients in neonatal rat ventricular myocytes

This study determined the regulatory effect of inositol 1,4,5-trisphosphate receptors (IP3Rs) on the basal Ca(2+) transients in cardiomyocytes. In cultured neonatal rat ventricular myocytes (NRVMs) at different densities, we used confocal microscopy to assess the effect of IP3Rs on the endogenous spontaneous Ca(2+) oscillations through specific activation of IP3Rs with myo-IP3 hexakis (butyryloxymethyl) ester (IP3BM), a membrane permeable IP3, and interference of IP3R expression with shRNA. We found that NRVMs at the monolayer state displayed coordinated Ca(2+) transients with less rate, shorter duration, and higher amplitude compared to single NRVMs. In addition, monolayer NRVMs exhibited 4 or 10 times more increased Ca(2+) transients in response to phenylephrine, an α-adrenergic receptor agonist, or IP3BM than single NRVMs did, while the transient pattern remained unaltered, suggesting that the sensitivity of intracellular Ca(2+) response to IP3R activation is different between single and monolayer NRVMs. However, interference of IP3R expression with shRNA reduced the frequency and amplitude of the spontaneous Ca(2+) fluctuates similarly in both densities of NRVMs, resembling the effects of ryanodine receptor inhibition by ryanodine or tetracaine. Our findings suggest that IP3Rs are involved, in part, in the regulation of native Ca(2+) transients, in profiles of their initiation and Ca(2+) release extent, in developing cardiomyocytes. In addition, caution should be paid in evaluating the behavior of Ca(2+) signaling in primary cultured cardiomyocytes at different densities.

An efficient finite element approach for modeling fibrotic clefts in the heart

Advanced medical imaging technologies provide a wealth of information on cardiac anatomy and structure at a paracellular resolution, allowing to identify microstructural discontinuities which disrupt the intracellular matrix. Current state-of-the-art computer models built upon such datasets account for increasingly finer anatomical details, however, structural discontinuities at the paracellular level are typically discarded in the model generation process, owing to the significant costs which incur when using high resolutions for explicit representation. In this study, a novel discontinuous finite element (dFE) approach for discretizing the bidomain equations is presented, which accounts for fine-scale structures in a computer model without the need to increase spatial resolution. In the dFE method, this is achieved by imposing infinitely thin lines of electrical insulation along edges of finite elements which approximate the geometry of discontinuities in the intracellular matrix. Simulation results demonstrate that the dFE approach accounts for effects induced by microscopic size scale discontinuities, such as the formation of microscopic virtual electrodes, with vast computational savings as compared to high resolution continuous finite element models. Moreover, the method can be implemented in any standard continuous finite element code with minor effort.

Dual effect of ethanol on inward rectifier potassium current IK1 in rat ventricular myocytes

Alcohol consumption may result in electrocardiographic changes and arrhythmias. Important role of modifications of the inward rectifier potassium current I(K1) in arrhythmogenesis is well established. Considering lack of relevant data, we aimed at studying the effect of 0.2-200 mM ethanol on I(K1) in enzymatically isolated rat right ventricular myocytes using the whole cell patch-clamp technique at 23±1°C. Ethanol reversibly affected I(K1) in a dual way. At a very low concentration of 0.8 mM (≈~0.004%), ethanol significantly decreased IK1 by 6.9±2.7%. However, at concentrations of ethanol ≥20 mM (≈0.09%), I(K1) was conversely significantly increased (by 16.6±4.0% at 20 mM and 24.5±2.4% at 80 mM). The steady-state I(K1) increase was regularly preceded by its transient decrease at the beginning of ethanol application. Under 2 and 8 mM ethanol, I(K1) was decreased at the steady-state in some cells but increased in others. Both effects were voltage-independent. In agreement with the observed effects of ethanol on I(K1), a transient action potential (AP) prolongation followed by its final shortening were observed after the application of ethanol in a low concentration of 8 mM (≈0.04%). Under the effect of 0.8 mM ethanol, only AP prolongation was apparent which agreed well with the above described I(K1) decrease. Other AP characteristics remained unaltered in both concentrations. These observations corresponded with the results of mathematical simulations in a model of the rat ventricular myocyte. To summarize, changes of the cardiac I(K1) under ethanol at concentrations relevant to the current alcohol consumption were first demonstrated in ventricular myocytes in this study. The observed dual ethanol effect suggests at least two underlying mechanisms that remain to be clarified. The ethanol-induced I(K1) changes might contribute to the reported alterations of cardiac electrophysiology related to alcohol consumption.

Frataxin deficiency in neonatal rat ventricular myocytes targets mitochondria and lipid metabolism

Friedreich ataxia (FRDA) is a hereditary disease caused by deficient frataxin expression. This mitochondrial protein has been related to iron homeostasis, energy metabolism, and oxidative stress. Patients with FRDA experience neurologic alterations and cardiomyopathy, which is the leading cause of death. The specific effects of frataxin depletion on cardiomyocytes are poorly understood because no appropriate cardiac cellular model is available to researchers. To address this research need, we present a model based on primary cultures of neonatal rat ventricular myocytes (NRVMs) and short-hairpin RNA interference. Using this approach, frataxin was reduced down to 5 to 30% of control protein levels after 7 days of transduction. At this stage the activity and amount of the iron-sulfur protein aconitase, in vitro activities of several OXPHOS components, levels of iron-regulated mRNAs, and the ATP/ADP ratio were comparable to controls. However, NRVMs exhibited markers of oxidative stress and a disorganized mitochondrial network with enlarged mitochondria. Lipids, the main energy source of heart cells, also underwent a clear metabolic change, indicated by the increased presence of lipid droplets and induction of medium-chain acyl-CoA dehydrogenase. These results indicate that mitochondria and lipid metabolism are primary targets of frataxin deficiency in NRVMs. Therefore, they contribute to the understanding of cardiac-specific mechanisms occurring in FRDA and give clues for the design of cardiac-specific treatment strategies for FRDA.

SKF-96365 strongly inhibits voltage-gated sodium current in rat ventricular myocytes

SKF-96365 (1-(beta-[3-(4-methoxy-phenyl) propoxy]-4-methoxyphenethyl)-1H-imidazole hydrochloride) is a general TRPC channel antagonist commonly used to characterize the potential functions of TRPC channels in cardiovascular system. Recent reports showed that SKF-96365 induced a reduction in cardiac conduction. The present study investigates whether the reduced cardiac conduction caused by SKF-96365 is related to the blockade of voltage-gated sodium current (I Na) in rat ventricular myocytes using the whole-cell patch voltage-clamp technique. It was found that SKF-96365 inhibited I Na in rat ventricular myocytes in a concentration-dependent manner. The compound (1 μM) negatively shifted the potential of I Na availability by 9.5 mV, increased the closed-state inactivation of I Na, and slowed the recovery of I Na from inactivation. The inhibition of cardiac I Na by SKF-96365 was use-dependent and frequency-dependent, and the IC₅₀ was decreased from 1.36 μM at 0.5 Hz to 1.03, 0.81, 0.61, 0.56 μM at 1, 2, 5, 10 Hz, respectively. However, the selective TRPC3 antagonist Pyr3 decreased cardiac I Na by 8.5% at 10 μM with a weak use and frequency dependence. These results demonstrate that the TRPC channel antagonist SKF-96365 strongly blocks cardiac I Na in use-dependent and frequency-dependent manners. Caution should be taken for interpreting the alteration of cardiac electrical activity when SKF-96365 is used in native cells as a TRPC antagonist.

Reducing endoglin activity limits calcineurin and TRPC-6 expression and improves survival in a mouse model of right ventricular pressure overload

BACKGROUND

Right ventricular (RV) failure is a major cause of mortality worldwide and is often a consequence of RV pressure overload (RVPO). Endoglin is a coreceptor for the profibrogenic cytokine, transforming growth factor beta 1 (TGF-β1). TGF-β1 signaling by the canonical transient receptor protein channel 6 (TRPC-6) was recently reported to stimulate calcineurin-mediated myofibroblast transformation, a critical component of cardiac fibrosis. We hypothesized that reduced activity of the TGF-β1 coreceptor, endoglin, limits RV calcineurin expression and improves survival in RVPO.

METHODS AND RESULTS

We first demonstrate that endoglin is required for TGF-β1-mediated calcineurin/TRPC-6 expression and up-regulation of alpha-smooth muscle antigen (α-SMA), a marker of myofibroblast transformation, in human RV fibroblasts. Using endoglin haploinsufficient mice (Eng(+/-)) we show that reduced endoglin activity preserves RV function, limits RV fibrosis, and attenuates activation of the calcineurin/TRPC-6/α-SMA pathway in a model of angio-obliterative pulmonary hypertension. Next, using Eng(+/-) mice or a neutralizing antibody (Ab) against endoglin (N-Eng) in wild-type mice, we show that reduced endoglin activity improves survival and attenuates RV fibrosis in models of RVPO induced by pulmonary artery constriction. To explore the utility of targeting endoglin, we observed a reversal of RV fibrosis and calcineurin levels in wild-type mice treated with a N-Eng Ab, compared to an immunoglobulin G control.

CONCLUSION

These data establish endoglin as a regulator of TGF-β1 signaling by calcineurin and TRPC-6 in the RV and identify it as a potential therapeutic target to limit RV fibrosis and improve survival in RVPO, a common cause of death in cardiac and pulmonary disease.

Elevated numbers of CD163+ macrophages in hearts of simian immunodeficiency virus-infected monkeys correlate with cardiac pathology and fibrosis

The role of macrophage activation, traffic, and accumulation on cardiac pathology was examined in 23 animals. Seventeen animals were simian immunodeficiency virus (SIV) infected, 12 were CD8 lymphocyte depleted, and the remaining six were uninfected controls (two CD8 lymphocyte depleted, four nondepleted). None of the uninfected controls had cardiac pathology. One of five (20%) SIV-infected, non-CD8 lymphocyte-depleted animals had minor cardiac pathology with increased numbers of macrophages in ventricular tissue compared to controls. Seven of the 12 (58%) SIV-infected, CD8 lymphocyte-depleted animals had cardiac pathology in ventricular tissues, including macrophage infiltration and myocardial degeneration. The extent of fibrosis (measured as the percentage of collagen per tissue area) was increased 41% in SIV-infected, CD8 lymphocyte-depleted animals with cardiac pathology compared to animals without pathological abnormalities. The number of CD163+ macrophages increased significantly in SIV-infected, CD8 lymphocyte-depleted animals with cardiac pathology compared to ones without pathology (1.66-fold) and controls (5.42-fold). The percent of collagen (percentage of collagen per total tissue area) positively correlated with macrophage numbers in ventricular tissue in SIV-infected animals. There was an increase of BrdU+ monocytes in the heart during late SIV infection, regardless of pathology. These data implicate monocyte/macrophage activation and accumulation in the development of cardiac pathology with SIV infection.

Apamin does not inhibit human cardiac Na+ current, L-type Ca2+ current or other major K+ currents

Background

Apamin is commonly used as a small-conductance Ca²⁺-activated K⁺ (SK) current inhibitor. However, the specificity of apamin in cardiac tissues remains unclear.

Objective

To test the hypothesis that apamin does not inhibit any major cardiac ion currents.

Methods

We studied human embryonic kidney (HEK) 293 cells that expressed human voltage-gated Na⁺, K⁺ and Ca²⁺ currents and isolated rabbit ventricular myocytes. Whole-cell patch clamp techniques were used to determine ionic current densities before and after apamin administration.

Results

Ca²⁺ currents (CACNA1c+CACNB2b) were not affected by apamin (500 nM) (data are presented as median [25^th percentile;75^th percentile] (from –16 [–20;–10] to –17 [–19;–13] pA/pF, P = NS), but were reduced by nifedipine to –1.6 [–3.2;–1.3] pA/pF (p = 0.008). Na⁺ currents (SCN5A) were not affected by apamin (from –261 [–282;–145] to –268 [–379;–132] pA/pF, P = NS), but were reduced by flecainide to –57 [–70;–47] pA/pF (p = 0.018). None of the major K⁺ currents (I_Ks, I_Kr, I_K1 and I_to) were inhibited by 500 nM of apamin (KCNQ1+KCNE1, from 28 [20]; [37] to 23 [18]; [32] pA/pF; KCNH2+KCNE2, from 28 [24]; [30] to 27 [24]; [29] pA/pF; KCNJ2, from –46 [–48;–40] to –46 [–51;–35] pA/pF; KCND3, from 608 [505;748] to 606 [454;684]). Apamin did not inhibit the I_Na or I_CaL in isolated rabbit ventricular myocytes (I_Na, from –67 [–75;–59] to –68 [–71;–59] pA/pF; I_CaL, from –16 [–17;–14] to –14 [–15;–13] pA/pF, P = NS for both).

Conclusions

Apamin does not inhibit human cardiac Na⁺ currents, L-type Ca²⁺ currents or other major K⁺ currents. These findings indicate that apamin is a specific SK current inhibitor in hearts as well as in other organs.

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.

Cell-based delivery of dATP via gap junctions enhances cardiac contractility

The transplantation of human pluripotent stem cell-derived cardiomyocytes (hPSC-CMs) is a promising strategy to treat myocardial infarction and reverse heart failure, but to date the contractile benefit in most studies remains modest. We have previously shown that the nucleotide 2-deoxyadenosine triphosphate (dATP) can substitute for ATP as the energy substrate for cardiac myosin, and increasing cellular dATP content by globally overexpressing ribonucleotide reductase (R1R2) can dramatically enhance cardiac contractility. Because dATP is a small molecule, we hypothesized that it would diffuse readily between cells via gap junctions and enhance the contractility of neighboring coupled wild type cells. To test this hypothesis, we performed studies with the goals of (1) validating gap junction-mediated dATP transfer in vitro and (2) investigating the use of R1R2-overexpressing hPSC-CMs in vivo as a novel strategy to increase cardiac function. We first performed intracellular dye transfer studies using dATP conjugated to fluorescein and demonstrated rapid gap junction-mediated transfer between cardiomyocytes. We then cocultured wild type cardiomyocytes with either cardiomyocytes or fibroblasts overexpressing R1R2 and saw more than a twofold increase in the extent and rate of contraction of wild type cardiomyocytes. Finally, we transplanted hPSC-CMs overexpressing R1R2 into healthy uninjured rat hearts and noted an increase in fractional shortening from 41±4% to 53±5% just five days after cell transplantation. These findings demonstrate that dATP is an inotropic factor that spreads between cells via gap junctions. Our data suggest that transplantation of dATP-producing hPSC-CMs could significantly increase the effectiveness of cardiac cell therapy.

Effects of glucocorticoid exposure on growth and structural maturation of the heart of the preterm piglet

Inadequate maintenance of systemic blood flow in neonates following preterm birth is associated with increased morbidity and mortality, and may be due in part to structural immaturity of the myocardium. Maternal glucocorticoid administration is associated with improved cardiovascular function, and possibly promotes structural maturation of the myocardium. This study assessed the structural maturity of the myocardium in male and female preterm and term piglets, and preterm piglets exposed to a regimen of maternal glucocorticoids as used clinically. In preterm, term and glucocorticoid exposed preterm piglets cardiomyocyte maturity was examined by measuring the proportion of binucleated myocytes and the volumes of single living ventricular cardiomyocytes with fluorescence microscopy. Ventricular apoptosis and proliferation were measured by immunohistochemistry. Preterm piglet hearts had fewer binucleated myocytes, smaller myocytes, and more proliferative and fewer apoptotic nuclei than term hearts. Maternal glucocorticoid treatment resulted in increased binucleation with no increase in myocyte volume, and levels of proliferation and apoptosis that were more similar to the term heart. Atrial weights were increased and in female piglets there was an increase in the ratio of left to right ventricular weight. The observed changes in atrial mass and myocyte structural maturation correlated with changes in cardiac function of isolated hearts of littermates. In conclusion, the association between increased myocardial maturation following glucocorticoid exposure, improved cardiac function in littermates, and clinical improvement in human neonatal cardiac function exposed to antenatal glucocorticoids, suggests that glucocorticoid exposure contributes to improved cardiovascular function in preterm infants by promoting myocardial structural maturity.

Comprehensive analyses of ventricular myocyte models identify targets exhibiting favorable rate dependence

Reverse rate dependence is a problematic property of antiarrhythmic drugs that prolong the cardiac action potential (AP). The prolongation caused by reverse rate dependent agents is greater at slow heart rates, resulting in both reduced arrhythmia suppression at fast rates and increased arrhythmia risk at slow rates. The opposite property, forward rate dependence, would theoretically overcome these parallel problems, yet forward rate dependent (FRD) antiarrhythmics remain elusive. Moreover, there is evidence that reverse rate dependence is an intrinsic property of perturbations to the AP. We have addressed the possibility of forward rate dependence by performing a comprehensive analysis of 13 ventricular myocyte models. By simulating populations of myocytes with varying properties and analyzing population results statistically, we simultaneously predicted the rate-dependent effects of changes in multiple model parameters. An average of 40 parameters were tested in each model, and effects on AP duration were assessed at slow (0.2 Hz) and fast (2 Hz) rates. The analysis identified a variety of FRD ionic current perturbations and generated specific predictions regarding their mechanisms. For instance, an increase in L-type calcium current is FRD when this is accompanied by indirect, rate-dependent changes in slow delayed rectifier potassium current. A comparison of predictions across models identified inward rectifier potassium current and the sodium-potassium pump as the two targets most likely to produce FRD AP prolongation. Finally, a statistical analysis of results from the 13 models demonstrated that models displaying minimal rate-dependent changes in AP shape have little capacity for FRD perturbations, whereas models with large shape changes have considerable FRD potential. This can explain differences between species and between ventricular cell types. Overall, this study provides new insights, both specific and general, into the determinants of AP duration rate dependence, and illustrates a strategy for the design of potentially beneficial antiarrhythmic drugs.

Several drugs intended to treat cardiac arrhythmias have failed because of unfavorable rate-dependent properties. That is, the drugs fail to alter electrical activity at fast heart rates, where this would be beneficial, but they do affect electrical activity at slow rates, where this is unwanted. In targeted studies, several agents have been shown to exhibit these unfavorable properties, suggesting that these rate-dependent responses may be intrinsic to ventricular muscle. To determine whether drugs with desirable rate-dependent properties could be rationally designed, we performed comprehensive and systematic analyses of several heart cell models. These analyses calculated the rate-dependent properties of changes in any model parameter, thereby generating simultaneously a large number of model predictions. The analyses showed that targets with favorable rate-dependent properties could indeed be identified, and further simulations uncovered the mechanisms underlying these behaviors. Moreover, a quantitative comparison of results obtained in different models provided new insight in why a given drug applied to different species, or to different tissue types, might produce different rate-dependent behaviors. Overall this study shows how a comprehensive and systematic approach to heart cell models can both identify novel targets and produce more general insight into rate-dependent alterations to cardiac electrical activity.

[Antiarrhythmic effect of TJ0711]

To study the antiarrhythmic effect of the newly developed alpha/beta-blocker TJ0711, a variety of animal models of arrhythmia were induced by CaCl2, ouabain and ischemia/reperfusion. Glass microelectrode technique was used to observe action potentials of right ventricular papillary muscle of guinea pig. The onset time of arrhythmia induced by CaCl2 was significantly prolonged by TJ0711 at 0.75, 1.5 and 3 mg x kg(-1) doses. TJ0711 (1.5 and 3 mg x kg(-1)) can significantly shorten the ventricular tachycardia (VT) and ventricular fibrillation (VF) duration, the incidence of VF and mortality were significantly reduced. On ischemia-reperfusion-induced arrhythmic model, TJ0711 (0.25, 0.5, 1 and 2 mg x kg(-1)) can significantly reduce the ventricular premature contraction (PVC), VT, VF incidence, mortality, arrhythmia score with a dose-dependent manner. At the same time, rats serum lactate dehydrogenase (LDH) and creatine kinase (CK) activities decreased significantly by TJ0711 (1 and 2 mg x kg(-1)). Ouabain could cause arrhythmia in guinea pigs, when TJ0711 (0.375, 0.75, 1.5 and 3 mg x kg(-1)) was given, the doses of ouabain inducing a variety of arrhythmia PVC, VT, VF, cardiac arrest (CA) were significantly increased with a dose-dependent manner. In the TJ0711 0.1-30 micromol x L(-1) concentration range, guinea pig right ventricular papillary muscle action potential RP (rest potential), APA (action potential amplitude) and V(max) (maximum velocity of depolarization) were not significantly affected. APD20, APD50 and APD90 had a shortening trend but no statistical difference with the increase of TJ0711 concentration. TJ0711 has antiarrhythmic effect on the sympathetic nerve excitement and myocardial cell high calcium animal arrhythmia model. Myocardial action potential zero phase conduction velocity and resting membrane potential were not inhibited by TJ0711. APD20, APD50 and APD90 were shortened by TJ0711 at high concentration. Its antiarrhythmic action mechanism may be besides the action of blocking beta1 receptor, may also have a strong selective blocking action on alpha1 receptor and reducing intracellular calcium concentration.

Ionic mechanism underlying distinctive excitability in atrium and ventricle of the heart

Cellular excitability is an important physiological factor in maintaining normal cardiac activity. The present study was designed to investigate the ionic mechanism underlying different excitability in atrial and ventricular myocytes of guinea pig heart using a whole-cell patch configuration. We found that excitability is lower in ventricular myocytes than that in atrial myocytes. Although the density of voltage-gated fast Na(+) current (INa) was lower in ventricular myocytes, it would not correlate to the lower excitability since its availability was greater than that in atrial myocytes around threshold potential. Classical inward rectifier K(+) current (IK1) was greater in ventricular myocytes than that in atrial myocytes, which might contribute in part to the lower excitability. In addition, the transient outward K(+) current with inward rectification (Itoir) elicited by depolarization was greater in ventricular myocytes than that in atrial myocytes and might contribute to the lower excitability. In ventricular myocytes, Ba(2+) at 5 µmol/L significantly inhibited Itoir, enhanced excitability, and shifted the threshold potential of INa activation to more negative, and the effect was independent of affecting INa. Our results demonstrate the novel information that in addition to classical IK1, Itoir plays a major role in determining the distinctive excitability in guinea pig atrial and ventricular myocytes and maintaining cardiac excitability. More effort is required to investigate whether increase of Itoir would be protective via reducing excitability.

Cycle length restitution in sinoatrial node cells: a theory for understanding spontaneous action potential dynamics

Normal heart rhythm (sinus rhythm) is governed by the sinoatrial node, a specialized and highly heterogeneous collection of spontaneously active myocytes in the right atrium. Sinoatrial node dysfunction, characterized by slow and/or asynchronous pacemaker activity and even failure, is associated with cardiovascular disease (e.g. heart failure, atrial fibrillation). While tremendous progress has been made in understanding the molecular and ionic basis of automaticity in sinoatrial node cells, the dynamics governing sinoatrial nodel cell synchrony and overall pacemaker function remain unclear. Here, a well-validated computational model of the mouse sinoatrial node cell is used to test the hypothesis that sinoatrial node cell dynamics reflect an inherent restitution property (cycle length restitution) that may give rise to a wide range of behavior from regular periodicity to highly complex, irregular activation. Computer simulations are performed to determine the cycle length restitution curve in the computational model using a newly defined voltage pulse protocol. The ability of the restitution curve to predict sinoatrial node cell dynamics (e.g., the emergence of irregular spontaneous activity) and susceptibility to termination is evaluated. Finally, ionic and tissue level factors (e.g. ion channel conductances, ion concentrations, cell-to-cell coupling) that influence restitution and sinoatrial node cell dynamics are explored. Together, these findings suggest that cycle length restitution may be a useful tool for analyzing cell dynamics and dysfunction in the sinoatrial node.

Crebanine inhibits voltage-dependent Na+ current in guinea-pig ventricular myocytes

AIM

To study the effects of crebanine on voltage-gated Na(+) channels in cardiac tissues.

METHODS

Single ventricular myocytes were enzymatically dissociated from adult guinea-pig heart. Voltage-dependent Na(+) current was recorded using the whole cell voltage-clamp technique.

RESULTS

Crebanine reversibly inhibited Na(+) current with an IC50 value of 0.283 mmol·L(-1) (95% confidence range: 0.248-0.318 mmol·L(-1)). Crebanine at 0.262 mmol·L(-1) caused a negative shift (about 12 mV) in the voltage-dependence of steady-state inactivation of Na(+) current, and retarded its recovery from inactivation, but did not affect its activation curve.

CONCLUSION

In addition to blocking other voltage-gated ion channels, crebanine blocked Na(+) channels in guinea-pig ventricular myocytes. Crebanine acted as an inactivation stabilizer of Na(+) channels in cardiac tissues.

Egr-1 upregulates Siva-1 expression and induces cardiac fibroblast apoptosis

The early growth response transcription factor Egr-1 controls cell specific responses to proliferation, differentiation and apoptosis. Expression of Egr-1 and downstream transcription is closely controlled and cell specific upregulation induced by processes such as hypoxia and ischemia has been previously linked to multiple aspects of cardiovascular injury. In this study, we showed constitutive expression of Egr-1 in cultured human ventricular cardiac fibroblasts, used adenoviral mediated gene transfer to study the effects of continuous Egr-1 overexpression and studied downstream transcription by Western blotting, immunohistochemistry and siRNA transfection. Apoptosis was assessed by fluorescence microscopy and flow cytometry in the presence of caspase inhibitors. Overexpression of Egr-1 directly induced apoptosis associated with caspase activation in human cardiac fibroblast cultures in vitro assessed by fluorescence microscopy and flow cytometry. Apoptotic induction was associated with a caspase activation associated loss of mitochondrial membrane potential and transient downstream transcriptional up-regulation of the pro-apoptotic gene product Siva-1. Suppression of Siva-1 induction by siRNA partially reversed Egr-1 mediated loss of cell viability. These findings suggest a previously unknown role for Egr-1 and transcriptional regulation of Siva-1 in the control of cardiac accessory cell death.

Stimulation of ICa by basal PKA activity is facilitated by caveolin-3 in cardiac ventricular myocytes

L-type Ca channels (LTCC), which play a key role in cardiac excitation–contraction coupling, are located predominantly at the transverse (t-) tubules in ventricular myocytes. Caveolae and the protein caveolin-3 (Cav-3) are also present at the t-tubules and have been implicated in localizing a number of signaling molecules, including protein kinase A (PKA) and β₂-adrenoceptors. The present study investigated whether disruption of Cav-3 binding to its endogenous binding partners influenced LTCC activity. Ventricular myocytes were isolated from male Wistar rats and LTCC current (I_Ca) recorded using the whole-cell patch-clamp technique. Incubation of myocytes with a membrane-permeable peptide representing the scaffolding domain of Cav-3 (C3SD) reduced basal I_Ca amplitude in intact, but not detubulated, myocytes, and attenuated the stimulatory effects of the β₂-adrenergic agonist zinterol on I_Ca. The PKA inhibitor H-89 also reduced basal I_Ca; however, the inhibitory effects of C3SD and H-89 on basal I_Ca amplitude were not summative. Under control conditions, myocytes stained with antibody against phosphorylated LTCC (pLTCC) displayed a striated pattern, presumably reflecting localization at the t-tubules. Both C3SD and H-89 reduced pLTCC staining at the z-lines but did not affect staining of total LTCC or Cav-3. These data are consistent with the idea that the effects of C3SD and H-89 share a common pathway, which involves PKA and is maximally inhibited by H-89, and suggest that Cav-3 plays an important role in mediating stimulation of I_Ca at the t-tubules via PKA-induced phosphorylation under basal conditions, and in response to β₂-adrenoceptor stimulation.

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Basal L type calcium current was reduced by interfering with caveolin-3 binding.

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L type calcium current is tonically regulated by PKA phosphorylation.

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Interfering with caveolin-3 binding reduced beta2 adrenergic stimulation of I_Ca.

A quantitative comparison of the behavior of human ventricular cardiac electrophysiology models in tissue

Numerical integration of mathematical models of heart cell electrophysiology provides an important computational tool for studying cardiac arrhythmias, but the abundance of available models complicates selecting an appropriate model. We study the behavior of two recently published models of human ventricular action potentials, the Grandi-Pasqualini-Bers (GPB) and the O'Hara-Virág-Varró-Rudy (OVVR) models, and compare the results with four previously published models and with available experimental and clinical data. We find the shapes and durations of action potentials and calcium transients differ between the GPB and OVVR models, as do the magnitudes and rate-dependent properties of transmembrane currents and the calcium transient. Differences also occur in the steady-state and S1–S2 action potential duration and conduction velocity restitution curves, including a maximum conduction velocity for the OVVR model roughly half that of the GPB model and well below clinical values. Between single cells and tissue, both models exhibit differences in properties, including maximum upstroke velocity, action potential amplitude, and minimum diastolic interval. Compared to experimental data, action potential durations for the GPB and OVVR models agree fairly well (although OVVR epicardial action potentials are shorter), but maximum slopes of steady-state restitution curves are smaller. Although studies show alternans in normal hearts, it occurs only in the OVVR model, and only for a narrow range of cycle lengths. We find initiated spiral waves do not progress to sustained breakup for either model. The dominant spiral wave period of the GPB model falls within clinically relevant values for ventricular tachycardia (VT), but for the OVVR model, the dominant period is longer than periods associated with VT. Our results should facilitate choosing a model to match properties of interest in human cardiac tissue and to replicate arrhythmia behavior more closely. Furthermore, by indicating areas where existing models disagree, our findings suggest avenues for further experimental work.

Effects of endogenous cannabinoid anandamide on excitation-contraction coupling in rat ventricular myocytes

A role for anandamide (N-arachidonoyl ethanolamide; AEA), a major endocannabinoid, in the cardiovascular system in various pathological conditions has been reported in earlier reports. In the present study, the effects of AEA on contractility, Ca2+ signaling, and action potential (AP) characteristics were investigated in rat ventricular myocytes. Video edge detection was used to measure myocyte shortening. Intracellular Ca2+ was measured in cells loaded with the fluorescent indicator fura-2 AM. AEA (1 μM) caused a significant decrease in the amplitudes of electrically evoked myocyte shortening and Ca2+ transients. However, the amplitudes of caffeine-evoked Ca2+ transients and the rate of recovery of electrically evoked Ca2+ transients following caffeine application were not altered. Biochemical studies in sarcoplasmic reticulum (SR) vesicles from rat ventricles indicated that AEA affected Ca2+ -uptake and Ca2+ -ATPase activity in a biphasic manner. [3H]-ryanodine binding and passive Ca2+ release from SR vesicles were not altered by 10 μM AEA. Whole-cell patch-clamp technique was employed to investigate the effect of AEA on the characteristics of APs. AEA (1 μM) significantly decreased the duration of AP. The effect of AEA on myocyte shortening and AP characteristics was not altered in the presence of pertussis toxin (PTX, 2 μg/ml for 4 h), AM251 and SR141716 (cannabinoid type 1 receptor antagonists; 0.3 μM) or AM630 and SR 144528 (cannabinoid type 2 receptor antagonists; 0.3 μM). The results suggest that AEA depresses ventricular myocyte contractility by decreasing the action potential duration (APD) in a manner independent of CB1 and CB2 receptors.

Dynamic clamp as a tool to study the functional effects of individual membrane currents

Today, the patch-clamp technique is the main technique in electrophysiology to record action potentials or membrane current from isolated cells, using a patch pipette to gain electrical access to the cell. The common recording modes of the patch-clamp technique are current clamp and voltage clamp. In the current clamp mode, the current injected through the patch pipette is under control while the free-running membrane potential of the cell is recorded. Current clamp allows for measurements of action potentials that may either occur spontaneously or in response to an injected stimulus current. In voltage clamp mode, the membrane potential is held at a set level through a feedback circuit, which allows for the recording of the net membrane current at a given membrane potential.A less common configuration of the patch-clamp technique is the dynamic clamp. In this configuration, a specific non-predetermined membrane current can be added to or removed from the cell while it is in free-running current clamp mode. This current may be computed in real time, based on the recorded action potential of the cell, and injected into the cell. Instead of being computed, this current may also be recorded from a heterologous expression system such as a HEK-293 cell that is voltage-clamped by the free-running action potential of the cell ("dynamic action potential clamp"). Thus, one may directly test the effects of an additional or mutated membrane current, a synaptic current or a gap junctional current on the action potential of a patch-clamped cell. In the present chapter, we describe the dynamic clamp on the basis of its application in cardiac cellular electrophysiology.

Current recordings at the single channel level in adult mammalian isolated cardiomyocytes

This chapter describes appropriate methods to investigate mammalian cardiac channels properties at the single channel level. Cell isolation is performed from new born or adult heart by enzymatic digestion on minced tissue or using the Langendorff apparatus. Isolation proceeding is suitable for rabbit, rat, and mouse hearts. In addition, isolation of human atrial cardiomyocytes is described. Such freshly isolated cells or cells maintained in primary culture are suitable for patch-clamp studies. Here we describe the single channel variants of the patch-clamp technique (cell-attached, inside-out, outside-out) used to investigate channel properties. Proceedings for the evaluation of biophysical properties such as conductance, ionic selectivity, regulations by extracellular and intracellular mechanisms are described. To illustrate the study, we provide an example by the characterization of a calcium-activated non-selective cation channel (TRPM4).