Electrophysiological consequences of acute regional ischemia/reperfusion in neonatal rat ventricular myocyte monolayers

Bupleurum exerts antiarrhythmic effects by inhibiting L-type calcium channels in mouse ventricular myocytes

BACKGROUND

Bupleurum (Bup), is a traditional effective medicine to treat colds and fevers in clinics. Multiple studies have demonstrated that Bup exhibites various biological activities, including cardioprotective effects, anti-inflammatory, anticancer, antipyretic, antimicrobial, and antiviral effects, etc. Currently, the effects of Bup on cardiac electrophysiology have not been reported yet.

METHODS

Electrocardiogram recordings were used to investigate the effects of Bup on aconitine-induced arrhythmias. Patch-clamp techniques were used to explore the effects of Bup on APs and ion currents.

RESULTS

Bup reduced the incidence of ventricular fibrillation (VF) and delayed the onset time of ventricular tachycardia (VT) in mice. Additionally, Bup (40 mg/mL) suppressed DADs induced by high-Ca2+ and shortened action potential duration at 50 % completion of repolarization (APD50) and action potential duration at 90 % completion of repolarization (APD90) to 60.89 % ± 8.40 % and 68.94 % ± 3.24 % of the control, respectively. Moreover, Bup inhibited L-type calcium currents (ICa.L) in a dose-dependent manner, with an IC50 value of 25.36 mg/mL. Furthermore, Bup affected the gated kinetics of L-type calcium channels by slowing down steady-state activation, accelerating the steady-state inactivation, and delaying the inactivation-recovery process. However, Bup had no effects on the Transient sodium current (INa.T), ATX II-increased late sodium current (INa.L), transient outward current (Ito), delayed rectifier potassium current (IK), or inward rectifier potassium current (IK1).

CONCLUSION

Bup is an antiarrhythmic agent that may exert its antiarrhythmic effects by inhibiting L-type calcium channels.

Bridging the Translational Gap in Heart Failure Research: Using Human iPSC-derived Cardiomyocytes to Accelerate Therapeutic Insights

Heart failure (HF) remains a leading cause of death worldwide, with increasing prevalence and burden. Despite extensive research, a cure for HF remains elusive. Traditionally, the study of HF's pathogenesis and therapies has relied heavily on animal experimentation. However, these models have limitations in recapitulating the full spectrum of human HF, resulting in challenges for clinical translation. To address this translational gap, research employing human cells, especially cardiomyocytes derived from human-induced pluripotent stem cells (hiPSC-CMs), offers a promising solution. These cells facilitate the study of human genetic and molecular mechanisms driving cardiomyocyte dysfunction and pave the way for research tailored to individual patients. Further, engineered heart tissues combine hiPSC-CMs, other cell types, and scaffold-based approaches to improve cardiomyocyte maturation. Their tridimensional architecture, complemented with mechanical, chemical, and electrical cues, offers a more physiologically relevant environment. This review explores the advantages and limitations of conventional and innovative methods used to study HF pathogenesis, with a primary focus on ischemic HF due to its relative ease of modeling and clinical relevance. We emphasize the importance of a collaborative approach that integrates insights obtained in animal and hiPSC-CMs-based models, along with rigorous clinical research, to dissect the mechanistic underpinnings of human HF. Such an approach could improve our understanding of this disease and lead to more effective treatments.

Mitochondrial membrane potential instability on reperfusion after ischemia does not depend on mitochondrial Ca2+ uptake

Physiologic Ca2+ entry via the Mitochondrial Calcium Uniporter (MCU) participates in energetic adaption to workload but may also contribute to cell death during ischemia/reperfusion (I/R) injury. The MCU has been identified as the primary mode of Ca2+ import into mitochondria. Several groups have tested the hypothesis that Ca2+ import via MCU is detrimental during I/R injury using genetically-engineered mouse models, yet the results from these studies are inconclusive. Furthermore, mitochondria exhibit unstable or oscillatory membrane potentials (ΔΨm) when subjected to stress, such as during I/R, but it is unclear if the primary trigger is an excess influx of mitochondrial Ca2+ (mCa2+), reactive oxygen species (ROS) accumulation, or other factors. Here, we critically examine whether MCU-mediated mitochondrial Ca2+ uptake during I/R is involved in ΔΨm instability, or sustained mitochondrial depolarization, during reperfusion by acutely knocking out MCU in neonatal mouse ventricular myocyte (NMVM) monolayers subjected to simulated I/R. Unexpectedly, we find that MCU knockout does not significantly alter mCa2+ import during I/R, nor does it affect ΔΨm recovery during reperfusion. In contrast, blocking the mitochondrial sodium-calcium exchanger (mNCE) suppressed the mCa2+ increase during Ischemia but did not affect ΔΨm recovery or the frequency of ΔΨm oscillations during reperfusion, indicating that mitochondrial ΔΨm instability on reperfusion is not triggered by mCa2+. Interestingly, inhibition of mitochondrial electron transport or supplementation with antioxidants stabilized I/R-induced ΔΨm oscillations. The findings are consistent with mCa2+ overload being mediated by reverse-mode mNCE activity and supporting ROS-induced ROS release as the primary trigger of ΔΨm instability during reperfusion injury.

LRP6-mediated phosphorylation of connexin43 in myocardial infarction

Ventricular tachycardia (VT) and ventricular fibrillation are most causes of early death in patients with acute myocardial infarction (AMI). Conditional cardiac-specific low-density lipoprotein receptor-related protein 6 (LRP6)-knockout mice with connexin 43 (Cx43) reduction triggered the lethal ventricular arrhythmias. Thus, it is necessary for exploring whether LRP6 and its upstream genes circRNA1615 mediate the phosphorylation of Cx43 in VT of AMI. Here, we showed that circRNA1615 regulated the expression of LRP6 mRNA through sponge adsorption of miR-152-3p. Importantly, LRP6 interference fragments aggravated hypoxia injury of Cx43, while overexpression of LRP6 improved the phosphorylation of Cx43. Subsequently, interference with G-protein alpha subunit (Gα_s) downstream of LRP6 further inhibited the phosphorylation of Cx43, along with increasing VT. Our results demonstrated that LRP6 upstream genes circRNA1615 controlled the damage effect and VT in AMI, and LRP6 mediated the phosphorylation of Cx43 via Gα_s which played a role in VT of AMI.

Oxidative stress in the mitochondrial matrix underlies ischemia/reperfusion-induced mitochondrial instability

Ischemia and reperfusion affect multiple elements of cardiomyocyte electrophysiology, especially within the mitochondria. We previously showed that in cardiac monolayers, upon reperfusion after coverslip-induced ischemia, mitochondrial inner membrane potential (ΔΨ) unstably oscillates between polarized and depolarized states, and ΔΨ instability corresponds with arrhythmias. Here, through confocal microscopy of compartment-specific molecular probes, we investigate the mechanisms underlying the postischemic ΔΨ oscillations, focusing on the role of Ca²⁺ and oxidative stress. During reperfusion, transient ΔΨ depolarizations occurred concurrently with periods of increased mitochondrial oxidative stress (5.07 ± 1.71 oscillations/15 min, N = 100). Supplementing the antioxidant system with GSH monoethyl ester suppressed ΔΨ oscillations (1.84 ± 1.07 oscillations/15 min, N = 119, t test p = 0.027) with 37% of mitochondrial clusters showing no ΔΨ oscillations (versus 4% in control, odds ratio = 14.08, Fisher's exact test p < 0.001). We found that limiting the production of reactive oxygen species using cyanide inhibited postischemic ΔΨ oscillations (N = 15, t test p < 10^-5). Furthermore, ΔΨ oscillations were not associated with any discernable pattern in cell-wide oxidative stress or with the changes in cytosolic or mitochondrial Ca²⁺. Sustained ΔΨ depolarization followed cytosolic and mitochondrial Ca²⁺ increase and was associated with increased cell-wide oxidative stress. Collectively, these findings suggest that transient bouts of increased mitochondrial oxidative stress underlie postischemic ΔΨ oscillations, regardless of Ca²⁺ dynamics.

Serine-threonine protein phosphatase regulation of Cx43 dephosphorylation in arrhythmogenic disorders

Regulation of cell-to-cell communication in the heart by the gap junction protein Connexin43 (Cx43) involves modulation of Cx43 phosphorylation state by protein kinases, and dephosphorylation by protein phosphatases. Dephosphorylation of Cx43 has been associated with impaired intercellular coupling and enhanced arrhythmogenesis in various pathologic states. While there has been extensive study of the protein kinases acting on Cx43, there has been limited studies of the protein phosphatases that may underlie Cx43 dephosphorylation. The focus of this review is to introduce serine-threonine protein phosphatase regulation of Cx43 phosphorylation state and cell-to-cell communication, and its impact on arrhythmogenesis in the setting of chronic heart failure and myocardial ischemia, as well as on atrial fibrillation. We also discuss the therapeutic potential of modulating protein phosphatases to treat arrhythmias in these clinical settings.

Effects of post-treatment electroacupuncture on ventricular monophasic action potential and cardiac function in a rat model of ischemia/reperfusion injury

BACKGROUND

To determine the effects of post-treatment electroacupuncture (EA) on the electrophysiological properties of ventricular muscle in rats with ischemia/reperfusion (IR) injury.

METHODS

Male Sprague-Dawley (SD) rats were randomly assigned into sham-operated (SH), IR and IR + EA groups (n = 8 each). The IR model was generated by ligation of the left anterior descending (LAD) coronary artery for 30 min. After establishing the IR model, EA was administered at PC6 for 30 min while opening the coronary artery and allowing reperfusion for 30 min. Heart rate (HR), mean arterial pressure and monophasic action potential (MAP) of cardiac muscle in the outer membrane of the antetheca of the left ventricle before coronary artery ligation (T₀), after coronary artery ligation for 30 min (T₁) and after reperfusion for 30 min (T₂) were recorded. At the same time, ventricular electrophysiological parameters including ventricular effective refractory period (ERP), conduction velocity (CV) and ventricular fibrillation threshold (VFT) were measured. Then, the cardiac function and the levels of creatine kinase-muscle/brain (CK-MB) and cardiac troponin I (cTnI) were monitored. Based on these data, monophasic action potential amplitude (MAPA), the maximum depolarization velocity (V_max) and the MAP durations at 50% and 90% repolarization (MAPD₅₀ and MAPD₉₀) were calculated to determine the incidence of arrhythmia during reperfusion.

RESULTS

Compared with the SH group, the IR group showed an obviously decreased HR as well as reduced mean arterial pressure, V_max, CV, ERP and MAPA. All indices of cardiac function except left ventricular end-diastolic pressure (LVEDP) decreased (i.e. ventricular systolic pressure (LVSP), left ventricular ejection fraction (LVEF), fractional shortening (FS) and rate of the ventricular pressure rise/drop (±dp/dt)). Furthermore, the MAPD₅₀ and MAPD₉₀ were prolonged, and the levels of CK-MB and cTnI increased (p < 0.05). In comparison to the IR group, HR and the mean arterial pressure were increased. All indices of cardiac function except LVEDP increased (LVSP, LVEF, FS and ±dp/dt). V_max, CV, ERP and MAPA were also increased in the IR + EA group. However, MAPD₅₀ and MAPD₉₀ were distinctly shortened, and the levels of CK-MB and cTnI decreased (p < 0.05). There were no statistically significant differences in VFT between the three groups (p > 0.05).

CONCLUSION

EA post-treatment can relieve prolongation of repolarization and slowed depolarization of ventricular muscle during IR, thus decreasing the rate of incidence of reperfusion arrhythmia.

A Multi-Scale Computational Model for the Rat Ventricle: Construction, Parallelization, and Applications

BACKGROUND

Cardiovascular diseases are the top killer of human beings. The ventricular arrhythmia, as a type of malignant cardiac arrhythmias, typically leads to death if not treated within minutes. The multi-scale virtual heart provides an idealized tool for exploring the underlying mechanisms, by means of incorporating abundant experimental data at the level of ion channels and analyzing the subsequent pathological changes at organ levels. However, there are few studies on building a virtual heart model for rats-a species most widely used in experiments.

OBJECTIVE

To build a multi-scale computational model for rats, with detailed methodology for the model construction, computational optimization, and its applications.

METHODS

First, approaches for building multi-scale models ranging from cellular to 3-D organ levels are introduced, with detailed descriptions of handling the ventricular myocardium heterogeneity, geometry processing, and boundary conditions, etc. Next, for dealing with the expensive computational costs of 3-D models, optimization approaches including an optimized representation and a GPU-based parallelization method are introduced. Finally, methods for reproducing of some key phenomenon (e.g., electrocardiograph, spiral/scroll waves) are demonstrated.

RESULTS

Three types of heterogeneity, including the transmural heterogeneity, the interventricular heterogeneity, and the base-apex heterogeneity are incorporated into the model. The normal and reentrant excitation waves, as well as the corresponding pseudo-ECGs are reproduced by the constructed ventricle model. In addition, the temporal and spatial vulnerability to reentry arrhythmias are quantified based on the evaluation experiments of vulnerable window and the critical length.

CONCLUSIONS

The constructed multi-scale rat ventricle model is able to reproduce both the physiological and the pathological phenomenon in different scales. Evaluation experiments suggest that the apex is the most susceptible area to arrhythmias. The model can be a promising tool for the investigation of arrhythmogenesis and the screening of anti-arrhythmic drugs.

Gene and Cell Therapy for Cardiac Arrhythmias

PURPOSE

In the last decade, interest in gene therapy as a therapeutic technology has increased, largely driven by an exciting yet modest number of successful applications for monogenic diseases. Setbacks in the use of gene therapy for cardiac disease have motivated efforts to develop vectors with enhanced tropism for the heart and more efficient delivery methods. Although monogenic diseases are the logical target, cardiac arrhythmias represent a group of conditions amenable to gene therapy because of focal targets (biological pacemakers, nodal conduction, or stem cell-related arrhythmias) or bystander effects on cells not directly transduced because of electrical coupling.

METHODS

This review provides a contemporary narrative of the field of gene therapy for experimental cardiac arrhythmias, including those associated with stem cell transplant. Recent articles published in the English language and available through the PubMed database and other prominent literature are discussed.

FINDINGS

The promise of gene therapy has been realized for a handful of monogenic diseases and is actively being pursued for cardiac applications in preclinical models. With improved vectors, it is likely that cardiac disease will also benefit from this technology. Cardiac arrhythmias, whether inherited or acquired, are a group of conditions with a potentially lower threshold for phenotypic correction and as such hold unique potential as targets for cardiac gene therapy.

IMPLICATIONS

There has been a proliferation of research on the potential of gene therapy for cardiac arrhythmias. This body of investigation forms a strong basis on which further developments, particularly with viral vectors, are likely to help this technology progress along its translational trajectory.

Robust Cardiac Regeneration: Fulfilling the Promise of Cardiac Cell Therapy

PURPOSE

We review the history of cardiac cell therapy, highlighting lessons learned from initial adult stem cell (ASC) clinical trials. We present pluripotent stem cell-derived cardiomyocytes (PSC-CMs) as a leading candidate for robust regeneration of infarcted myocardium but identify several issues that must be addressed before successful clinical translation.

METHODS

We conducted an unstructured literature review of PubMed-listed articles, selecting the most comprehensive and relevant research articles, review articles, clinical trials, and basic or translation articles in the field of cardiac cell therapy. Articles were identified using the search terms adult stem cells, pluripotent stem cells, cardiac stem cell, and cardiac regeneration or from references of relevant articles, Articles were prioritized and selected based on their impact, originality, or potential clinical applicability.

FINDINGS

Since its inception, the ASC therapy field has been troubled by conflicting preclinical data, academic controversies, and inconsistent trial designs. These issues have damaged perceptions of cardiac cell therapy among investors, the academic community, health care professionals, and, importantly, patients. In hindsight, the key issue underpinning these problems was the inability of these cell types to differentiate directly into genuine cardiomyocytes, rendering them unable to replace damaged myocardium. Despite this, beneficial effects through indirect paracrine or immunomodulatory effects remain possible and continue to be investigated. However, in preclinical models, PSC-CMs have robustly remuscularized infarcted myocardium with functional, force-generating cardiomyocytes. Hence, PSC-CMs have now emerged as a leading candidate for cardiac regeneration, and unpublished reports of first-in-human delivery of these cells have recently surfaced. However, the cardiac cell therapy field's history should serve as a cautionary tale, and we identify several translational hurdles that still remain. Preclinical solutions to issues such as arrhythmogenicity, immunogenicity, and poor engraftment rates are needed, and next-generation clinical trials must draw on robust knowledge of mechanistic principles of the therapy.

IMPLICATIONS

The clinical transplantation of functional stem cell-derived heart tissue with seamless integration into native myocardium is a lofty goal. However, considerable advances have been made during the past 2 decades. Currently, PSC-CMs appear to be the best prospect to reach this goal, but several hurdles remain. The history of adult stem cell trials has taught us that shortcuts cannot be taken without dire consequences, and it is essential that progress not be hurried and that a worldwide, cross-disciplinary approach be used to ensure safe and effective clinical translation.

Antiarrhythmic effect of crotonoside by regulating sodium and calcium channels in rabbit ventricular myocytes

AIMS

Detect the antiarrhythmic effect of crotonoside (Cro).

MAIN METHODS

We used whole-cell patch-clamp techniques to detect the effects of Cro on action potentials (APs) and transmembrane ion currents in isolated rabbit left ventricular myocytes. We also verified the effect of Cro on ventricular arrhythmias caused by aconitine in vivo.

KEY FINDINGS

Cro reduced the maximum depolarization velocity (V_max) of APs and shortened the action potential duration (APD) in a concentration-dependent manner, but it had no significant effect on the resting membrane potential (RMP) or action potential amplitude (APA). It also inhibited the peak sodium current (I_Na) and L-type calcium current (I_CaL) in a concentration-dependent manner with half-maximal inhibitory concentrations (IC₅₀) of 192 μmol/L and 159 μmol/L, respectively. However, Cro had no significant effects on the inward rectifier potassium current (I_K1) or rapidly activating delayed rectifier potassium current (I_Kr). Sea anemone toxin II (ATX II) increased the late sodium current (I_NaL), but Cro abolished this effect. Moreover, Cro significantly abolished ATX II-induced early afterdepolarizations (EADs) and high extracellular Ca²⁺ concentration (3.6 mmol/L)-induced delayed afterdepolarizations (DADs). We also verified that Cro effectively delayed the onset time and reduced the incidence of ventricular arrhythmias caused by aconitine in vivo.

SIGNIFICANCE

These results revealed that Cro effectively inhibits I_Na, I_NaL, and I_CaL in ventricular myocytes. Cro has antiarrhythmic potential and thus deserves further study.

Arrhythmogenic risks of stem cell replacement therapy for cardiovascular diseases

Ischemic heart disease and congestive heart failure are major contributors to high morbidity and mortality. Approximately 1.5 million cases of myocardial infarction occur annually in the United States; the yearly incidence rate is approximately 600 cases per 100,000 people. Although significant progress to improve the survival rate has been made by medications and implantable medical devices, damaged cardiomyocytes are unable to be recovered by current treatment strategies. After almost two decades of research, stem cell therapy has become a very promising approach to generate new cardiomyocytes and enhance the function of the heart. Along with clinical trials with stem cells conducted in cardiac regeneration, concerns regarding safety and potential risks have emerged. One of the contentious issues is the electrical dysfunctions of cardiomyocytes and cardiac arrhythmia after stem cell therapy. In this review, we focus on the cell sources currently used for stem cell therapy and discuss related arrhythmogenic risk.

Pro-arrhythmic Effects of Hydrogen Sulfide in Healthy and Ischemic Cardiac Tissues: Insight From a Simulation Study

Hydrogen sulfide (H₂S), an ambient air pollutant, has been reported to increase cardiac events in patients with cardiovascular diseases, but the underlying mechanisms remain not elucidated. This study investigated the pro-arrhythmic effects of H₂S in healthy and ischemic conditions. Experimental data of H₂S effects on ionic channels (including the L-type Ca²⁺ channel and ATP-sensitive K⁺ channel) were incorporated into a virtual heart model to evaluate their integral action on cardiac arrhythmogenesis. It was shown that H₂S depressed cellular excitability, abbreviated action potential duration, and augmented tissue’s transmural dispersion of repolarization, resulting in an increase in tissue susceptibility to initiation and maintenance of reentry. The observed effects of H₂S on cardiac excitation are more remarkable in the ischemic condition than in the healthy condition. This study provides mechanistic insights into the pro-arrhythmic effects of air pollution (H₂S), especially in the case with extant ischemic conditions.

Sevoflurane Alleviates Reperfusion Arrhythmia by Ameliorating TDR and MAPD90 in Isolated Rat Hearts after Ischemia-Reperfusion

Objective

To investigate the effect of sevoflurane on the monophasic action potentials (MAPs) in isolated rat hearts after ischemia-reperfusion.

Methods

Twenty-four healthy SD male rats, weighing 280–320 g, were randomly divided into three groups after successful preparation of a Langendorff isolated heart perfusion model with a stabilization period perfusion of 15 min with Krebs–Henseleit (K–H) fluid (n = 8): the control group (group A, continuously perfused with K–H fluid for 105 min), the ischemia-reperfusion group (group B, continuously perfused with K–H fluid for 15 min, and then exposed to 60 min of global ischemia induced by Thomas solution followed by 30 min of reperfusion), and the sevoflurane group (group C, K–H fluid contained 1.0 MAC sevoflurane, and other procedures were same as in group B). Heart rate (HR) and MAPs including time course (MAPD₅₀ or MAPD₉₀) of the epicardium (Epi) and endocardium (Endo) were recorded at the time of balance perfusion for 15 min (T₀), continuous perfusion for 15 min (T₁), reperfusion for 15 min/continuous perfusion for 105 min (T₂), and reperfusion for 30 min/continuous perfusion for 120 min (T₃), and the transmural dispersion of repolarization (TDR) was calculated. The incidence of arrhythmia, time for restoration of spontaneous heart beat, and duration of arrhythmia were recorded during the period of reperfusion.

Results

HR in group B and group C was lower at T₂ and T₃ than that in group A, while that in group B was significantly lower than that in group A at T₂ and T₃, and HR in group C was higher than that in group B at T₂ and T₃ (P < 0.05). There was no difference of TDR in each group at T₀ and T₁ (P > 0.05), while TDR in group B was increased at T₂ and T₃ compared with that in group C and group A (P < 0.05). TDR in group C was decreased compared with that in group B at T₂ and T₃ (P < 0.05), while there was no such difference between group C and group A (P > 0.05). The time for restoration of spontaneous heart beat and duration of arrhythmia in group C were shorter than those in group B (P < 0.05), while cardiac arrhythmia scores in group B were higher than those in group C (P < 0.05). There was no difference of MAPD₅₀ in each group (P > 0.05). The MAPD₉₀ in group B was much longer than that in other groups at T₂ and T₃ (P < 0.05), while there was no such difference between group C and group A (P > 0.05). The prolonged MAPD₉₀ at T₂ and T₃ in group B strikingly differed from that at T₀ and T₁ (P < 0.05). Nevertheless, there was no such difference in other groups at different time points (P > 0.05).

Conclusion

Sevoflurane alleviates reperfusion arrhythmia induced by myocardial ischemia-reperfusion through the shortening of MPAD₉₀ in isolated rat hearts.

Restricted exchange microenvironments for cell culture

Metabolite diffusion in tissues produces gradients and heterogeneous microenvironments that are not captured in standard 2D cell culture models. Here we describe restricted exchange environment chambers (REECs) in which diffusive gradients are formed and manipulated on length scales approximating those found in vivo. In REECs, cells are grown in 2D in an asymmetric chamber (<50 μL) formed between a coverglass and a glass bottom cell culture dish separated by a thin (~100 μm) gasket. Diffusive metabolite exchange between the chamber and bulk media occurs through one or more openings micromachined into the coverglass. Cell-generated concentration gradients form radially in REECs with a single round opening (~200 μm diameter). At steady state only cells within several hundred micrometers of the opening experience metabolite concentrations that permit survival which is analogous to diffusive exchange near a capillary in tissue. The chamber dimensions, the openings' shape, size, and number, and the cellular density and metabolic activity define the gradient structure. For example, two parallel slots above confluent cells produce the 1D equivalent of a spheroid. Using REECs, we found that fibroblasts align along the axis of diffusion while MDCK cells do not. MDCK cells do, however, exhibit significant morphological variations along the diffusive gradient.

Icariin, a Novel Blocker of Sodium and Calcium Channels, Eliminates Early and Delayed Afterdepolarizations, As Well As Triggered Activity, in Rabbit Cardiomyocytes

Icariin, a flavonoid monomer from Herba Epimedii, has confirmed pharmacological and biological effects. However, its effects on arrhythmias and cardiac electrophysiology remain unclear. Here we investigate the effects of icariin on ion currents and action potentials (APs) in the rabbit myocardium. Furthermore, the effects of icariin on aconitine-induced arrhythmias were assessed in whole rabbits. Ion currents and APs were recorded in voltage-clamp and current-clamp mode in rabbit left ventricular myocytes (LVMs) and left atrial myocytes (LAMs), respectively. Icariin significantly shortened action potential durations (APDs) at 50 and 90% repolarization (APD₅₀ and APD₉₀) and reduced AP amplitude (APA) and the maximum upstroke velocity (V_max) of APs in LAMs and LVMs; however, icariin had no effect on resting membrane potential (RMP) in these cells. Icariin decreased the rate-dependence of the APD and completely abolished anemonia toxin II (ATX-II)-induced early afterdepolarizations (EADs). Moreover, icariin significantly suppressed delayed afterdepolarizations (DADs) and triggered activities (TAs) elicited by isoproterenol (ISO, 1 μM) and high extracellular calcium concentrations ([Ca²⁺]_o, 3.6 mM) in LVMs. Icariin also decreased I_NaT in a concentration-dependent manner in LAMs and LVMs, with IC₅₀ values of 12.28 ± 0.29 μM (n = 8 cells/4 rabbits) and 11.83 ± 0.92 μM (n = 10 cells/6 rabbits; p > 0.05 vs. LAMs), respectively, and reversed ATX-II-induced I_NaL in a concentration-dependent manner in LVMs. Furthermore, icariin attenuated I_CaL in a dose-dependent manner in LVMs. The corresponding IC₅₀ value was 4.78 ± 0.89 μM (n = 8 cells/4 rabbits), indicating that the aforementioned current in LVMs was 2.8-fold more sensitive to icariin than I_CaL in LAMs (13.43 ± 2.73 μM; n = 9 cells/5 rabbits). Icariin induced leftward shifts in the steady-state inactivation curves of I_NaT and I_CaL in LAMs and LVMs but did not have a significant effect on their activation processes. Moreover, icariin had no effects on I_K1 and I_Kr in LVMs or I_to and I_Kur in LAMs. These results revealed for the first time that icariin is a multichannel blocker that affects I_NaT, I_NaL and I_CaL in the myocardium and that the drug had significant inhibitory effects on aconitine-induced arrhythmias in whole rabbits. Therefore, icariin has potential as a class I and IV antiarrhythmic drug.

Andrographolide inhibits arrhythmias and is cardioprotective in rabbits

Andrographolide has a protective effect on the cardiovascular system. To study its cardic-electrophysiological effects, action potentials and voltage-gated Na⁺ (I_Na), Ca²⁺ (I_CaL), and K⁺ (I_K1, I_Kr, I_to and I_Kur) currents were recorded using whole-cell patch clamp and current clamp techniques. Additionally, the effects of andrographolide on aconitine-induced arrhythmias were assessed on electrocardiograms in vivo. We found that andrographolide shortened action potential duration and reduced maximum upstroke velocity in rabbit left ventricular and left atrial myocytes. Andrographolide attenuated rate-dependence of action potential duration, and reduced or abolished delayed afterdepolarizations and triggered activities induced by isoproterenol (1 μM) and high calcium ([Ca²⁺]_o=3.6 mM) in left ventricular myocytes. Andrographolide also concentration-dependently inhibited I_Na and I_CaL, but had no effect on I_to, I_Kur, I_K1, or I_Kr in rabbit left ventricular and left atrial myocytes. Andrographolide treatment increased the time and dosage thresholds of aconitine-induced arrhythmias, and reduced arrhythmia incidence and mortality in rabbits. Our results indicate that andrographolide inhibits cellular arrhythmias (delayed afterdepolarizations and triggered activities) and aconitine-induced arrhythmias in vivo, and these effects result from I_Na and I_CaL inhibition. Andrographolide may be useful as a class I and IV antiarrhythmic therapeutic.

Engineering prokaryotic channels for control of mammalian tissue excitability

The ability to directly enhance electrical excitability of human cells is hampered by the lack of methods to efficiently overexpress large mammalian voltage-gated sodium channels (VGSC). Here we describe the use of small prokaryotic sodium channels (BacNa_v) to create de novo excitable human tissues and augment impaired action potential conduction in vitro. Lentiviral co-expression of specific BacNa_v orthologues, an inward-rectifying potassium channel, and connexin-43 in primary human fibroblasts from the heart, skin or brain yields actively conducting cells with customizable electrophysiological phenotypes. Engineered fibroblasts ('E-Fibs') retain stable functional properties following extensive subculture or differentiation into myofibroblasts and rescue conduction slowing in an in vitro model of cardiac interstitial fibrosis. Co-expression of engineered BacNa_v with endogenous mammalian VGSCs enhances action potential conduction and prevents conduction failure during depolarization by elevated extracellular K⁺, decoupling or ischaemia. These studies establish the utility of engineered BacNa_v channels for induction, control and recovery of mammalian tissue excitability.

Islands of spatially discordant APD alternans underlie arrhythmogenesis by promoting electrotonic dyssynchrony in models of fibrotic rat ventricular myocardium

Fibrosis and altered gap junctional coupling are key features of ventricular remodelling and are associated with abnormal electrical impulse generation and propagation. Such abnormalities predispose to reentrant electrical activity in the heart. In the absence of tissue heterogeneity, high-frequency impulse generation can also induce dynamic electrical instabilities leading to reentrant arrhythmias. However, because of the complexity and stochastic nature of such arrhythmias, the combined effects of tissue heterogeneity and dynamical instabilities in these arrhythmias have not been explored in detail. Here, arrhythmogenesis was studied using in vitro and in silico monolayer models of neonatal rat ventricular tissue with 30% randomly distributed cardiac myofibroblasts and systematically lowered intercellular coupling achieved in vitro through graded knockdown of connexin43 expression. Arrhythmia incidence and complexity increased with decreasing intercellular coupling efficiency. This coincided with the onset of a specialized type of spatially discordant action potential duration alternans characterized by island-like areas of opposite alternans phase, which positively correlated with the degree of connexinx43 knockdown and arrhythmia complexity. At higher myofibroblast densities, more of these islands were formed and reentrant arrhythmias were more easily induced. This is the first study exploring the combinatorial effects of myocardial fibrosis and dynamic electrical instabilities on reentrant arrhythmia initiation and complexity.

Semaphorin 3a transfection into the left stellate ganglion reduces susceptibility to ventricular arrhythmias after myocardial infarction in rats

AIMS

Myocardial infarction (MI) induces neural remodelling of the left stellate ganglion (LSG), which may contribute to ischaemia-induced arrhythmias. The neural chemorepellent Semaphorin 3a (Sema3a) has been identified as a negative regulator of sympathetic innervation in the LSG and heart. We previously reported that overexpression of Sema3a in the border zone could reduce the arrhythmogenic effects of cardiac sympathetic hyperinnervation post-MI. This study investigated whether Sema3a overexpression within the LSG confers an antiarrhythmic effect after MI through decreasing extra- and intra-cardiac neural remodelling.

METHODS AND RESULTS

Sprague-Dawley rats were subjected to MI, and randomly allocated to intra-LSG microinjection of either phosphate-buffered saline (PBS), adenovirus encoding green fluorescent protein (AdGFP), or adenovirus encoding Sema3a (AdSema3a). Sham-operated rats served as controls. Two weeks after infarction, MI-induced nerve sprouting and sympathetic hyperinnervation in the LSG and myocardium were significantly attenuated by intra-LSG injection with AdSema3a, as assessed by immunohistochemistry and western blot analysis of growth-associated protein 43 and tyrosine hydroxylase. This was also confirmed by sympathetic nerve function changes assessed by cardiac norepinephrine content. Additionally, intra-LSG injection with AdSema3a alleviated MI-induced accumulation of dephosphorylated connexin 43 in the infarct border zone. Furthermore, Sema3a overexpression in the LSG reduced the incidence of inducible ventricular tachyarrhythmia by programmed electrical stimulation post-MI, and arrhythmia scores were significantly lower in the AdSema3a group than in the PBS and AdGFP groups.

CONCLUSION

Semaphorin 3a overexpression in the LSG ameliorates the inducibility of ventricular arrhythmias after MI, mainly through attenuation of neural remodelling within the cardiac-neuraxis.

Stem cells can form gap junctions with cardiac myocytes and exert pro-arrhythmic effects

Stem cell therapy has been suggested to be a promising option for regeneration of injured myocardium, for example following a myocardial infarction. For clinical use cell-based therapies have to be safe and applicable and are aimed to renovate the architecture of the heart. Yet for functional and coordinated activity synchronized with the host myocardium stem cells have to be capable of forming electrical connections with resident cardiomyocytes. In this paper we discuss whether stem cells are capable of establishing functional electrotonic connections with cardiomyocytes and whether these may generate a risk for arrhythmias. Application of stem cells in the clinical setting with outcomes concerning arrhythmogenic safety and future perspectives will also briefly be touched upon.

Mitochondrial instability during regional ischemia-reperfusion underlies arrhythmias in monolayers of cardiomyocytes

Regional depolarization of the mitochondrial network can alter cellular electrical excitability and increase the propensity for reentry, in part, through the opening of sarcolemmal KATP channels. Mitochondrial inner membrane potential (ΔΨm) instability or oscillation can be induced in myocytes by exposure to reactive oxygen species (ROS), laser excitation, or glutathione depletion, and is thought to be a major factor in arrhythmogenesis during ischemia-reperfusion. Nevertheless, the correlation between ΔΨm recovery kinetics and reperfusion-induced arrhythmias has been difficult to demonstrate experimentally. Here, we investigate the relationship between subcellular changes in ΔΨm, cellular glutathione redox potential, electrical excitability, and wave propagation during coverslip-induced ischemia-reperfusion (IR) in neonatal rat ventricular myocyte (NRVM) monolayers. Ischemia led to decreased action potential amplitude and duration followed by electrical inexcitability after ~15min of ischemia. ΔΨm depolarization occurred in two phases during ischemia: in phase 1 (<30min ischemia), mitochondrial clusters within individual NRVMs depolarized, while phase 2 ΔΨm depolarization (30-60min) was characterized by global functional collapse of the mitochondrial network across the whole ischemic region of the monolayer, typically involving a propagating metabolic wave. Oxidation of the glutathione (GSSG:GSH) redox potential occurred during ischemia, followed by recovery upon reperfusion (i.e., lifting the coverslip). ΔΨm recovered in the mitochondria of individual myocytes quite rapidly upon reperfusion (<5min), but was highly unstable, characterized by subcellular oscillations or flickering of clusters of mitochondria in NRVMs across the reperfused region. Electrical excitability also recovered in a heterogeneous manner, providing an arrhythmogenic substrate which led to formation of sustained reentry. Treatment with 4'-chlorodiazepam, a peripheral benzodiazepine receptor ligand, prevented ΔΨm oscillation, improved GSH recovery rate, and prevented reentry during reperfusion, indicating that stabilization of mitochondrial network dynamics is important for preventing post-ischemic arrhythmias. This article is part of a Special Issue entitled "Mitochondria: From Basic Mitochondrial Biology to Cardiovascular Disease".

Optical imaging of arrhythmias in the cardiomyocyte monolayer

In recent years, cultured cardiac cell monolayers have become a contemporary experimental preparation for the study of fundamental mechanisms that underlie normal and pathologic electrophysiology at the tissue level. Ion channels and gap junctions in the cardiomyocyte monolayer may be modulated using drugs that suppress or enhance certain channels/junctions, or by genetic silencing or overexpression. The cardiomyocyte monolayer is particularly well suited for studies of functional electrophysiologic properties of mixtures of cardiac and noncardiac cells (eg, myofibroblasts), which otherwise would be difficult to investigate. Optical mapping of monolayers has provided insight into mechanisms that can set the stage for arrhythmias, such as unidirectional conduction block, gap junction uncoupling, ischemia, alternans, and anisotropy, and continues to enhance our understanding of basic electrophysiologic mechanisms.

Ratiometric imaging of calcium during ischemia-reperfusion injury in isolated mouse hearts using Fura-2

Background

We present an easily implementable method for measuring Fura-2 fluorescence from isolated mouse hearts using a commercially available switching light source and CCD camera. After calibration, it provides a good estimate of intracellular [Ca²⁺] with both high spatial and temporal resolutions, permitting study of changes in dispersion of diastolic [Ca²⁺], Ca²⁺ transient dynamics, and conduction velocities in mouse hearts. In a proof-of-principle study, we imaged isolated Langendorff-perfused mouse hearts with reversible regional myocardial infarctions.

Methods

Isolated mouse hearts were perfused in the Landendorff-mode and loaded with Fura-2. Hearts were then paced rapidly and subjected to 15 minutes of regional ischemia by ligation of the left anterior descending coronary artery, following which the ligation was removed to allow reperfusion for 15 minutes. Fura-2 fluorescence was recorded at regular intervals using a high-speed CCD camera. The two wavelengths of excitation light were interleaved at a rate of 1 KHz with a computer controlled switching light source to illuminate the heart.

Results

Fura-2 produced consistent Ca²⁺ transients from different hearts. Ligating the coronary artery rapidly generated a well defined region with a dramatic rise in diastolic Ca²⁺ without a significant change in transient amplitude; Ca²⁺ handling normalized during reperfusion. Conduction velocity was reduced by around 50% during ischemia, and did not recover significantly when monitored for 15 minutes following reperfusion.

Conclusions

Our method of imaging Fura-2 from isolated whole hearts is capable of detecting pathological changes in intracellular Ca²⁺ levels in cardiac tissue. The persistent change in the conduction velocities indicates that changes to tissue connectivity rather than altered intracellular Ca²⁺ handling may be underlying the electrical instabilities commonly seen in patients following a myocardial infarction.

Emergence of spiral wave activity in a mechanically heterogeneous reaction-diffusion-mechanics system

We perform a numerical study of emergent spiral wave activity in a two-dimensional reaction-diffusion-mechanics medium with a regional inhomogeneity in active and passive mechanical properties. We find that self-sustaining spiral wave activity emerges for a wide range of mechanical parameters of the inhomogeneity via five mechanisms. We classify these mechanisms, relate them to parameters of the inhomogeneity, and discuss how these results can be applied to understand the onset of cardiac arrhythmias due to regional mechanical heterogeneity.

Optical imaging of voltage and calcium in cardiac cells & tissues

Cardiac optical mapping has proven to be a powerful technology for studying cardiovascular function and disease. The development and scientific impact of this methodology are well-documented. Because of its relevance in cardiac research, this imaging technology advances at a rapid pace. Here, we review technological and scientific developments during the past several years and look toward the future. First, we explore key components of a modern optical mapping set-up, focusing on: (1) new camera technologies; (2) powerful light-emitting-diodes (from ultraviolet to red) for illumination; (3) improved optical filter technology; (4) new synthetic and optogenetic fluorescent probes; (5) optical mapping with motion and contraction; (6) new multiparametric optical mapping techniques; and (7) photon scattering effects in thick tissue preparations. We then look at recent optical mapping studies in single cells, cardiomyocyte monolayers, atria, and whole hearts. Finally, we briefly look into the possible future roles of optical mapping in the development of regenerative cardiac research, cardiac cell therapies, and molecular genetic advances.

Regional increase in extracellular potassium can be arrhythmogenic due to nonuniform muscle contraction in rat ventricular muscle

In the ischemic myocardium, extracellular potassium ([K(+)](o)) increases to ≥20 mmol/l. To determine how lethal arrhythmias occur during ischemia, we investigated whether the increased spatial pattern of [K(+)](o), i.e., a regional or a global increase, affects the incidence of arrhythmias. Force, sarcomere length, membrane potential, and nonuniform intracellular Ca(2+) ([Ca(2+)](i)) were measured in rat ventricular trabeculae. A "regional" or "global" increase in [K(+)](o) was produced by exposing a restricted region of muscle to a jet of 30 mmol/l KCl or by superfusing trabeculae with a solution containing 30 mmol/l KCl, respectively. The increase in [Ca(2+)](i) (Ca(CW)) during Ca(2+) waves was measured (24°C, 3.0 mmol/l [Ca(2+)](o)). A regional increase in [K(+)](o) caused nonuniform [Ca(2+)](i) and contraction. In the presence of isoproterenol, the regional increase in [K(+)](o) induced sustained arrhythmias in 10 of 14 trabeculae, whereas the global increase did not induce such arrhythmias. During sustained arrhythmias, Ca(2+) surged within the jet-exposed region. In the absence of isoproterenol, the regional increase in [K(+)](o) increased Ca(CW), whereas the global increase decreased it. This increase in Ca(CW) with the regional increase in [K(+)](o) was not suppressed by 100 μmol/l streptomycin, whereas it was suppressed by 1) a combination of 10 μmol/l cilnidipine and 3 μmol/l SEA0400; 2) 20 mmol/l 2,3-butanedione monoxime; and 3) 10 μmol/l blebbistatin. A regional but not a global increase in [K(+)](o) induces sustained arrhythmias, probably due to nonuniform excitation-contraction coupling. The same mechanism may underlie arrhythmias during ischemia.

Nutrient restriction preserves calcium cycling and mitochondrial function in cardiac myocytes during ischemia and reperfusion

Nutrient restriction (NR) prolongs longevity via enhanced mitochondrial function. We tested the hypothesis that NR enhances resistance to ischemia/reperfusion (IR) arrhythmias via preserved calcium (Ca) cycling and mitochondrial function. We examined the protective effects of NR on regional IR in cultured neonatal rat ventricular myocyte monolayers. Optical mapping of intracellular Ca and mitochondrial membrane potential Δψ(m) was performed using Rhod 2-AM and TMRE, respectively. Regional ischemia was mimicked by covering a portion of monolayer with a glass coverslip until loss of Ca propagation, and reperfusion was mimicked by removing the coverslip. NR was mimicked by culture in serum- and glucose-free medium for 24 h. Relative to controls, NR monolayers: (1) sustained Ca oscillations during longer periods of ischemia (19.2 ± 1.8 min vs 10.4 ± 1.4 min, p<0.001); (2) had attenuated increases in Ca transient duration (CaD) and time decay constant (Tau) during ischemia; (3) had preserved conduction velocity (CV) during early reperfusion, leading to protection against reperfusion arrhythmias; (4) had minimal "rebound" decreased CaD and Tau during reperfusion; and (5) had no depolarization of Δψ(m) during IR. NR attenuates IR arrhythmias via (1) stable calcium cycling and (2) prevention of Δψ(m) depolarization during IR. Enhanced mitochondrial resistance to IR arrhythmias may play a role in NR-induced longevity prolongation.

Single-detector simultaneous optical mapping of V(m) and [Ca(2+)](i) in cardiac monolayers

Simultaneous mapping of transmembrane voltage (V(m)) and intracellular Ca(2+) concentration (Ca(i)) has been used for studies of normal and abnormal impulse propagation in cardiac tissues. Existing dual mapping systems typically utilize one excitation and two emission bandwidths, requiring two photodetectors with precise pixel registration. In this study we describe a novel, single-detector mapping system that utilizes two excitation and one emission band for the simultaneous recording of action potentials and calcium transients in monolayers of neonatal rat cardiomyocytes. Cells stained with the Ca(2+)-sensitive dye X-Rhod-1 and the voltage-sensitive dye Di-4-ANEPPS were illuminated by a programmable, multicolor LED matrix. Blue and green LED pulses were flashed 180° out of phase at a rate of 488.3 Hz using a custom-built dual bandpass excitation filter that transmitted blue (482 ± 6 nm) and green (577 ± 31 nm) light. A long-pass emission filter (>605 nm) and a 504-channel photodiode array were used to record combined signals from cardiomyocytes. Green excitation yielded Ca(i) transients without significant crosstalk from V(m). Crosstalk present in V(m) signals obtained with blue excitation was removed by subtracting an appropriately scaled version of the Ca(i) transient. This method was applied to study delay between onsets of action potentials and Ca(i) transients in anisotropic cardiac monolayers.

Stabilizing Microtubules Decreases Myocardial Ischaemia-Reperfusion Injury

Microtubules are important components of the cytoskeleton that forms the backbone of myocardial architecture, sustaining its form and size. This study investigated whether stabilizing microtubules with paclitaxel (0.1 or 1 μM) could decrease myocardial ischaemia-reperfusion injury, and reduce myocardial infarct size and the incidence of ischaemic ventricular arrhythmia. Isolated rat hearts were used, with arrhythmia induced by regional ischaemia and myocardial infarcts induced by ischaemia-reperfusion. In these ex vivo rat models, paclitaxel decreased myocardial ischaemia-reperfusion injury, significantly reducing the incidence and severity of ischaemic ventricular arrhythmia and significantly decreasing infarct size.

Functional consequences of abnormal Cx43 expression in the heart

The major gap junction protein expressed in the heart, connexin43 (Cx43), is highly remodeled in the diseased heart. Usually, Cx43 is down-regulated and heterogeneously redistributed to the lateral sides of cardiomyocytes. Reverse remodeling of the impaired Cx43 expression could restore normal cardiac function and normalize electrical stability. In this review, the reduced and heterogeneous Cx43 expression in the heart will be addressed in hypertrophic, dilated and ischemic cardiomyopathy together with its functional consequences of conduction velocity slowing, dispersed impulse conduction, its interaction with fibrosis and propensity to generate arrhythmias. Finally, different therapies are discussed. Treatments aimed to improve the Cx43 expression levels show new potentially anti-arrhythmic therapies during heart failure, but those in the context of acute ischemia can be anti-arrhythmogenic at the cost of larger infarct sizes. This article is part of a Special Issue entitled: The Communicating junctions, composition, structure and characteristics.

Taxol, a microtubule stabilizer, prevents ischemic ventricular arrhythmias in rats

Microtubule integrity is important in cardio-protection, and microtubule disruption has been implicated in the response to ischemia in cardiac myocytes. However, the effects of Taxol, a common microtubule stabilizer, are still unknown in ischemic ventricular arrhythmias. The arrhythmia model was established in isolated rat hearts by regional ischemia, and myocardial infarction model by ischemia/reperfusion. Microtubule structure was immunohistochemically measured. The potential mechanisms were studied by measuring reactive oxygen species (ROS), activities of oxidative enzymes, intracellular calcium concentration ([Ca(2+) ](i) ) and Ca(2+) transients by using fluorometric determination, spectrophotometric assays and Fura-2-AM and Fluo-3-AM, respectively. The expression and activity of sarcoplasmic reticulum Ca(2+)-ATPase (SERCA2a) was also examined using real-time polymerase chain reaction, Western blot and pyruvate/Nicotinamide adenine dinucleotide-coupled reaction. Our data showed that Taxol (0.1, 0.3 and 1 μM) effectively reduced the number of ventricular premature beats and the incidence and duration of ventricular tachycardia. The infarct size was also significantly reduced by Taxol (1 μM). At the same time, Taxol preserved the microtubule structure, increased the activity of mitochondrial electron transport chain complexes I and III, reduced ROS levels, decreased the rise in [Ca(2+)](i) and preserved the amplitude and decay times of Ca(2+) transients during ischemia. In addition, SERCA2a activity was preserved by Taxol during ischemia. In summary, Taxol prevents ischemic ventricular arrhythmias likely through ameliorating abnormal calcium homeostasis and decreasing the level of ROS. This study presents evidence that Taxol may be a potential novel therapy for ischemic ventricular arrhythmias.

Spontaneous high-frequency action potential

Action potential, which is the foundation of physiology and electrophysiology, is most vital in physiological research. This work starts by detecting cardiac electrophysiology (tachyarrhythmias), combined with all spontaneous discharge phenomena in vivo such as wound currents and spontaneous neuropathic pain, elaborates from generation, induction, initiation, to all of the features of spontaneous high-frequency action potential-SSL action potential mechanism, i.e., connecting-end hyperpolarization initiates spontaneous depolarization and action potential in somatic membrane. This work resolves the conundrums of in vivo spontaneous discharge in tachyarrhythmias, wounds, denervation supersensitivity, neurogenic pain (hyperalgesia and allodynia), epileptic discharge and diabetic pain in pathophysiological and clinical researches that have puzzled people for a hundred years.

Increased intracellular calcium concentration causes electrical turbulence in guinea pig ventricular myocytes

Dysregulation of intracellular Ca(2+) homeostasis is associated with various pathological conditions and arrhythmogenesis of the heart. The objective of this study was to investigate the effects of an acute increase in intracellular Ca(2+) concentration ([Ca(2+)](i)) on the electrophysiology of ventricular myocytes by mimicking intracellular Ca(2+) overload. The [Ca(2+)](i) was clamped to either a controlled (65-100 nmol L(-1)) or increased (1 μmol L(-1)) level. The transmembrane action potentials and ionic currents were recorded using whole-cell patch clamp techniques. We found that the acute increase in [Ca(2+)](i) shortened the action potential duration, reduced the action potential amplitude, maximum depolarization velocity and resting membrane potential, caused delayed after-depolarizations (DADs), and triggered activity-compared with these parameters in the control. The increased [Ca(2+)](i) augmented late I (Na) in a time-dependent manner, reduced I (CaL) and I (K1), and increased I (Kr) but not I (Ks). The results of this study can be used to explain calcium overload-induced ventricular arrhythmias.

Anisotropic conduction block and reentry in neonatal rat ventricular myocyte monolayers

Anisotropy can lead to unidirectional conduction block that initiates reentry. We analyzed the mechanisms in patterned anisotropic neonatal rat ventricular myocyte monolayers. Voltage and intracellular Ca (Ca(i)) were optically mapped under the following conditions: extrastimulus (S1S2) testing and/or tetrodotoxin (TTX) to suppress Na current availability; heptanol to reduce gap junction conductance; and incremental rapid pacing. In anisotropic monolayers paced at 2 Hz, conduction velocity (CV) was faster longitudinally than transversely, with an anisotropy ratio [AR = CV(L)/CV(T), where CV(L) and CV(T) are CV in the longitudinal and transverse directions, respectively], averaging 2.1 ± 0.8. Interventions decreasing Na current availability, such as S1S2 pacing and TTX, slowed CV(L) and CV(T) proportionately, without changing the AR. Conduction block preferentially occurred longitudinal to fiber direction, commonly initiating reentry. Interventions that decreased gap junction conductance, such as heptanol, decreased CV(T) more than CV(L), increasing the AR and causing preferential transverse conduction block and reentry. Rapid pacing resembled the latter, increasing the AR and promoting transverse conduction block and reentry, which was prevented by the Ca(i) chelator 1,2-bis oaminophenoxy ethane-N,N,N',N'-tetraacetic acid (BAPTA). In contrast to isotropic and uniformly anisotropic monolayers, in which reentrant rotors drifted and self-terminated, bidirectional anisotropy (i.e., an abrupt change in fiber direction exceeding 45°) caused reentry to anchor near the zone of fiber direction change in 77% of monolayers. In anisotropic monolayers, unidirectional conduction block initiating reentry can occur longitudinal or transverse to fiber direction, depending on whether the experimental intervention reduces Na current availability or decreases gap junction conductance, agreeing with theoretical predictions.

Characterizing functional stem cell-cardiomyocyte interactions

Despite the progress in traditional pharmacological and organ transplantation therapies, heart failure still afflicts 5.3 million Americans. Since June 2000, stem cell-based approaches for the prevention and treatment of heart failure have been pursued in clinics with great excitement; however, the exact mechanisms of how transplanted cells improve heart function remain elusive. One of the main difficulties in answering these questions is the limited ability to directly access and study interactions between implanted cells and host cardiomyocytes in situ. With the growing number of candidate cell types for potential clinical use, it is becoming increasingly more important to establish standardized, well-controlled in vitro and in situ assays to compare the efficacy and safety of different stem cells in cardiac repair. This article describes recent innovative methodologies to characterize direct functional interactions between stem cells and cardiomyocytes, aimed to facilitate the rational design of future cell-based therapies for heart disease.

Right-to-left ventricular diastolic delay in chronic thromboembolic pulmonary hypertension is associated with activation delay and action potential prolongation in right ventricle

BACKGROUND

Delayed left ventricle (LV)-to-right ventricle (RV) peak shortening results in cardiac output reduction in patients with chronic thromboembolic hypertension (CTEPH) and other types of pulmonary arterial hypertension. Why the synchrony between LV and RV is lost is unknown. We hypothesized that RV electrophysiological remodeling, notably, conduction slowing and action potential prolongation, contribute to this loss in synchrony.

METHODS AND RESULTS

We conducted epicardial mapping during pulmonary endarterectomy in 26 patients with CTEPH and compared these findings with clinical, hemodynamic, and echocardiographic variables. We consecutively placed a multielectrode grid on the epicardium of the RV free wall and LV lateral wall. These regions corresponded to RV and LV areas where echocardiographic Doppler sample volumes were placed to measure RV-to-LV diastolic interventricular delay. RV and LV epicardial action potential duration was assessed by measuring activation-recovery interval. Onset of diastolic relaxation of RV free wall with respect to LV lateral wall (diastolic interventricular delay) was delayed by 38+/-31 ms in patients with CTEPH versus -12+/-13 ms in control subjects (P<0.001), because, in patients with CTEPH, RV completed electric activation later than LV (65+/-20 versus 44+/-7 ms, P<0.001) and epicardial action potential duration, as assessed by activation-recovery interval measurement, was longer in RV free wall than in LV lateral wall (253+/-29 versus 240+/-22 ms, P<0.001).

CONCLUSIONS

Additive effects of electrophysiological changes in RV, notably, conduction slowing and action potential prolongation, assessed by epicardial activation-recovery interval, contribute to diastolic interventricular delay in patients with CTEPH.