Hypokalemia promotes late phase 3 early afterdepolarization and recurrent ventricular fibrillation during isoproterenol infusion in Langendorff perfused rabbit ventricles. Heart Rhythm. 2014;11:697-706. [PMID: 24378768 DOI: 10.1016/j.hrthm.2013.12.032]

Dissecting the roles of calcium cycling and its coupling with voltage in the genesis of early afterdepolarizations in cardiac myocyte models

Early afterdepolarizations (EADs) are abnormal depolarizations during the plateau phase of the action potential, which are known to be associated with lethal arrhythmias in the heart. There are two major hypotheses for EAD genesis based on experimental observations, i.e., the voltage (Vm)-driven and intracellular calcium (Ca)-driven mechanisms. In ventricular myocytes, Ca and Vm are bidirectionally coupled, which can affect each other's dynamics and result in new dynamics, however, the roles of Ca cycling and its coupling with Vm in the genesis of EADs have not been well understood. In this study, we use an action potential model that is capable of independent Vm and Ca oscillations to investigate the roles of Vm and Ca coupling in EAD genesis. Four different mechanisms of EADs are identified, which are either driven by Vm oscillations or Ca oscillations alone, or oscillations caused by their interactions. We also use 5 other ventricular action potential models to assess these EAD mechanisms and show that EADs in these models are mainly Vm-driven. These mechanistic insights from our simulations provide a theoretical base for understanding experimentally observed EADs and EAD-related arrhythmogenesis.

Rapid changes of mRNA expressions of cardiac ion channels affected by Torsadogenic drugs influence susceptibility of rat hearts to arrhythmias induced by Beta-Adrenergic stimulation

Drug-induced long QT syndrome (LQTS) and Torsades de Pointes (TdP) are serious concerns in drug development. Although rats are a useful scientific tool, their hearts, unlike larger species, usually do not respond to torsadogenic drugs. Consequently, their resistance to drug-induced arrhythmias is poorly understood. Here, we challenged rats with rapid delayed rectifier current (Ikr)-inhibiting antibiotic clarithromycin (CLA), loop diuretic furosemide (FUR) or their combination (CLA + FUR), and examined functional and molecular abnormalities after stimulation with isoproterenol. Clarithromycin and furosemide were administered orally at 12-h intervals for 7 days. To evaluate electrical instability, electrocardiography (ECG) was recorded either in vivo or ex vivo using the Langendorff-perfused heart method under basal conditions and subsequently under beta-adrenergic stimulation. Gene expression was measured using real-time quantitative PCR in left ventricular tissue. Indeed, FUR and CLA + FUR rats exhibited hypokalemia. CLA and CLA + FUR treatment resulted in drug-induced LQTS and even an episode of TdP in one CLA + FUR rat. The combined treatment dysregulated gene expression of several ion channels subunits, including KCNQ1, calcium channels and Na+/K + -ATPase subunits, while both monotherapies had no impact. The rat with recorded TdP exhibited differences in the expression of ion channel genes compared to the rest of rats within the CLA + FUR group. The ECG changes were not detected in isolated perfused hearts. Hence, we report rapid orchestration of ion channel reprogramming of hearts with QT prolongation induced by simultaneous administration of clarithromycin and furosemide in rats, which may account for their ability to avoid arrhythmias triggered by beta-adrenergic stimulation.

KairoSight-3.0: A validated optical mapping software to characterize cardiac electrophysiology, excitation-contraction coupling, and alternans

Background

Cardiac optical mapping is an imaging technique that measures fluorescent signals across a cardiac preparation. Dual optical imaging of voltage-sensitive and calcium-sensitive probes allows for simultaneous recordings of cardiac action potentials and intracellular calcium transients with high spatiotemporal resolution. The analysis of these complex optical datasets is both time intensive and technically challenging; as such, we have developed a software package for semi-automated image processing and analysis. Herein, we report an updated version of our software package (KairoSight-3.0) with features to enhance the characterization of cardiac parameters using optical signals.

Methods

To test software validity and applicability, we used Langendorff-perfused heart preparations to record transmembrane voltage and intracellular calcium signals from the epicardial surface. Isolated hearts from guinea pigs and rats were loaded with a potentiometric dye (RH237) and/or calcium indicator dye (Rhod-2AM) and fluorescent signals were acquired. We used Python 3.8.5 programming language to develop the KairoSight-3.0 software. Cardiac maps were validated with a user-specified manual mapping approach.

Results

Manual maps of action potential duration (30 or 80 % repolarization), calcium transient duration (30 or 80 % reuptake), action potential and calcium transient alternans were constituted to validate the accuracy of software-generated maps. Manual and software maps had high accuracy, with >97 % of manual and software values falling within 10 ms of each other and >75 % within 5 ms for action potential duration and calcium transient duration measurements (n = 1000-2000 pixels). Further, our software package includes additional measurement tools to analyze signal-to-noise ratio, conduction velocity, action potential and calcium transient alternans, and action potential-calcium transient coupling time to produce physiologically meaningful optical maps.

Conclusions

KairoSight-3.0 has enhanced capabilities to perform measurements of cardiac electrophysiology, calcium handling, alternans, and the excitation-contraction coupling with satisfactory accuracy.

KairoSight-3.0 : A Validated Optical Mapping Software to Characterize Cardiac Electrophysiology, Excitation-Contraction Coupling, and Alternans

Background

Cardiac optical mapping is an imaging technique that measures fluorescent signals across a cardiac preparation. Dual optical mapping of voltage-sensitive and calcium-sensitive probes allow for simultaneous recordings of cardiac action potentials and intracellular calcium transients with high spatiotemporal resolution. The analysis of these complex optical datasets is both time intensive and technically challenging; as such, we have developed a software package for semi-automated image processing and analysis. Herein, we report an updated version of our software package ( KairoSight-3 . 0 ) with features to enhance characterization of cardiac parameters using optical signals.

Methods

To test software validity and applicability, we used Langendorff-perfused heart preparations to record transmembrane voltage and intracellular calcium signals from the epicardial surface. Isolated hearts from guinea pigs and rats were loaded with a potentiometric dye (RH237) and/or calcium indicator dye (Rhod-2AM) and fluorescent signals were acquired. We used Python 3.8.5 programming language to develop the KairoSight-3 . 0 software. Cardiac maps were validated with a user-specified manual mapping approach.

Results

Manual maps of action potential duration (30 or 80% repolarization), calcium transient duration (30 or 80% reuptake), action potential and calcium transient alternans were constituted to validate the accuracy of software-generated maps. Manual and software maps had high accuracy, with >97% of manual and software values falling within 10 ms of each other and >75% within 5 ms for action potential duration and calcium transient duration measurements (n=1000-2000 pixels). Further, our software package includes additional cardiac metric measurement tools to analyze signal-to-noise ratio, conduction velocity, action potential and calcium transient alternans, and action potential-calcium transient coupling time to produce physiologically meaningful optical maps.

Conclusions

KairoSight-3 . 0 has enhanced capabilities to perform measurements of cardiac electrophysiology, calcium handling, and the excitation-contraction coupling with satisfactory accuracy.

Graphical Abstract Demonstrating Experimental and Data Analysis Workflow

Created with Biorender.com.

Gene therapy targeting protein trafficking regulator MOG1 in mouse models of Brugada syndrome, arrhythmias, and mild cardiomyopathy

Brugada syndrome (BrS) is a fatal arrhythmia that causes an estimated 4% of all sudden death in high-incidence areas. SCN5A encodes cardiac sodium channel Na_V1.5 and causes 25 to 30% of BrS cases. Here, we report generation of a knock-in (KI) mouse model of BrS (Scn5a^G1746R/+). Heterozygous KI mice recapitulated some of the clinical features of BrS, including an ST segment abnormality (a prominent J wave) on electrocardiograms and development of spontaneous ventricular tachyarrhythmias (VTs), seizures, and sudden death. VTs were caused by shortened cardiac action potential duration and late phase 3 early afterdepolarizations associated with reduced sodium current density (I_Na) and increased Kcnd3 and Cacna1c expression. We developed a gene therapy using adeno-associated virus serotype 9 (AAV9) vector-mediated MOG1 delivery for up-regulation of MOG1, a chaperone that binds to Na_V1.5 and traffics it to the cell surface. MOG1 was chosen for gene therapy because the large size of the SCN5A coding sequence (6048 base pairs) exceeds the packaging capacity of AAV vectors. AAV9-MOG1 gene therapy increased cell surface expression of Na_V1.5 and ventricular I_Na, reversed up-regulation of Kcnd3 and Cacna1c expression, normalized cardiac action potential abnormalities, abolished J waves, and blocked VT in Scn5a^G1746R/+ mice. Gene therapy also rescued the phenotypes of cardiac arrhythmias and contractile dysfunction in heterozygous humanized KI mice with SCN5A mutation p.D1275N. Using a small chaperone protein may have broad implications for targeting disease-causing genes exceeding the size capacity of AAV vectors.

Mechanisms of phase-3 early afterdepolarizations and triggered activities in ventricular myocyte models

Early afterdepolarizations (EADs) are abnormal depolarizations during the repolarizing phase of the action potential, which are associated with cardiac arrhythmogenesis. EADs are classified into phase-2 and phase-3 EADs. Phase-2 EADs occur during phase 2 of the action potential, with takeoff potentials typically above -40 mV. Phase-3 EADs occur during phase 3 of the action potential, with takeoff potential between -70 and -50 mV. Since the amplitude of phase-3 EADs can be as large as that of a regular action potential, they are also called triggered activities (TAs). This also makes phase-3 EADs and TAs much more arrhythmogenic than phase-2 EADs since they can propagate easily in tissue. Although phase-2 EADs have been widely observed, phase-3 EADs and TAs have been rarely demonstrated in isolated ventricular myocytes. Here we carry out computer simulations of three widely used ventricular action potential models to investigate the mechanisms of phase-3 EADs and TAs. We show that when the T-type Ca2+ current (ICa,T ) is absent (e.g., in normal ventricular myocytes), besides the requirement of increasing inward currents and reducing outward currents as for phase-2 EADs, the occurrence of phase-3 EADs and TAs requires a substantially large increase of the L-type Ca2+ current and the slow component of the delayed rectifier K+ current. The presence of ICa,T (e.g., in neonatal and failing ventricular myocytes) can greatly reduce the thresholds of these two currents for phase-3 EADs and TAs. This implies that ICa,T may play an important role in arrhythmogenesis in cardiac diseases.

Model Systems for Addressing Mechanism of Arrhythmogenesis in Cardiac Repair

PURPOSE OF REVIEW

Cardiac cell-based therapy represents a promising approach for cardiac repair. However, one of the main challenges is cardiac arrhythmias associated with stem cell transplantation. The current review summarizes the recent progress in model systems for addressing mechanisms of arrhythmogenesis in cardiac repair.

RECENT FINDINGS

Animal models have been extensively developed for mechanistic studies of cardiac arrhythmogenesis. Advances in human induced pluripotent stem cells (hiPSCs), patient-specific disease models, tissue engineering, and gene editing have greatly enhanced our ability to probe the mechanistic bases of cardiac arrhythmias. Additionally, recent development in multiscale computational studies and machine learning provides yet another powerful tool to quantitatively decipher the mechanisms of cardiac arrhythmias. Advancing efforts towards the integrations of experimental and computational studies are critical to gain insights into novel mitigation strategies for cardiac arrhythmias in cell-based therapy.

Arrhythmogenic Mechanisms in Hypokalaemia: Insights From Pre-clinical Models

Potassium is the predominant intracellular cation, with its extracellular concentrations maintained between 3. 5 and 5 mM. Among the different potassium disorders, hypokalaemia is a common clinical condition that increases the risk of life-threatening ventricular arrhythmias. This review aims to consolidate pre-clinical findings on the electrophysiological mechanisms underlying hypokalaemia-induced arrhythmogenicity. Both triggers and substrates are required for the induction and maintenance of ventricular arrhythmias. Triggered activity can arise from either early afterdepolarizations (EADs) or delayed afterdepolarizations (DADs). Action potential duration (APD) prolongation can predispose to EADs, whereas intracellular Ca²⁺ overload can cause both EADs and DADs. Substrates on the other hand can either be static or dynamic. Static substrates include action potential triangulation, non-uniform APD prolongation, abnormal transmural repolarization gradients, reduced conduction velocity (CV), shortened effective refractory period (ERP), reduced excitation wavelength (CV × ERP) and increased critical intervals for re-excitation (APD-ERP). In contrast, dynamic substrates comprise increased amplitude of APD alternans, steeper APD restitution gradients, transient reversal of transmural repolarization gradients and impaired depolarization-repolarization coupling. The following review article will summarize the molecular mechanisms that generate these electrophysiological abnormalities and subsequent arrhythmogenesis.

Atrial natriuretic peptide accelerates onset and dynamics of ventricular fibrillation during hypokalemia in isolated rabbit hearts

BACKGROUND

Atrial natriuretic peptide (ANP), which is released by the heart in response to acute cardiac stretch, possesses cardiac electrophysiological properties that include modulation of ion channel function and repolarization. However, data regarding whether ANP can directly modulate electrical instability or arrhythmias are largely lacking.

OBJECTIVE

This study sought to determine whether ANP modifies onset or electrophysiological characteristics of ventricular fibrillation (VF) induced by severe hypokalemia in an isolated heart model.

METHODS

Langendorff-perfused rabbit hearts in the absence and presence of 10 nM ANP (n = 9 in each group) were subjected to a low potassium (K⁺) perfusate (1.2 mM K⁺). Left ventricular (LV) epicardial monophasic action potential (MAP) and pressure were monitored continuously. Incidence and time to onset of VF and dominant frequency during VF determined by spectral analysis were evaluated.

RESULTS

ANP did not alter ventricular repolarization (MAP duration) or LV pressure during perfusion with physiologic, K⁺-containing solution. Within the first 30 s after low K⁺ perfusion, ANP accelerated the onset of beat-to-beat repolarization alternans (100% vs. 33% in ANP-treated vs. non-treated hearts, p < 0.01). During low K⁺ perfusion, the incidence of VF did not differ between ANP-treated and non-treated hearts (8 of 9 [89%] in each group). However, VF occurred sooner (3.75 ± 0.33 vs. 5.78 ± 0.70 min, P < 0.05) and immediately after VF onset, peak dominant frequency was higher (24.1 ± 7.3 vs. 14.2 ± 2.3 Hz, P = 0.01) in ANP-treated than in non-treated hearts.

CONCLUSIONS

ANP accelerates initiation of VF and increases maximum dominant frequency during VF in isolated hearts subjected to severe hypokalemia.

Animal models of arrhythmia: classic electrophysiology to genetically modified large animals

Arrhythmias are common and contribute substantially to cardiovascular morbidity and mortality. The underlying pathophysiology of arrhythmias is complex and remains incompletely understood, which explains why mostly only symptomatic therapy is available. The evaluation of the complex interplay between various cell types in the heart, including cardiomyocytes from the conduction system and the working myocardium, fibroblasts and cardiac immune cells, remains a major challenge in arrhythmia research because it can be investigated only in vivo. Various animal species have been used, and several disease models have been developed to study arrhythmias. Although every species is useful and might be ideal to study a specific hypothesis, we suggest a practical trio of animal models for future use: mice for genetic investigations, mechanistic evaluations or early studies to identify potential drug targets; rabbits for studies on ion channel function, repolarization or re-entrant arrhythmias; and pigs for preclinical translational studies to validate previous findings. In this Review, we provide a comprehensive overview of different models and currently used species for arrhythmia research, discuss their advantages and disadvantages and provide guidance for researchers who are considering performing in vivo studies.

Determinants of early afterdepolarization properties in ventricular myocyte models

Early afterdepolarizations (EADs) are spontaneous depolarizations during the repolarization phase of an action potential in cardiac myocytes. It is widely known that EADs are promoted by increasing inward currents and/or decreasing outward currents, a condition called reduced repolarization reserve. Recent studies based on bifurcation theories show that EADs are caused by a dual Hopf-homoclinic bifurcation, bringing in further mechanistic insights into the genesis and dynamics of EADs. In this study, we investigated the EAD properties, such as the EAD amplitude, the inter-EAD interval, and the latency of the first EAD, and their major determinants. We first made predictions based on the bifurcation theory and then validated them in physiologically more detailed action potential models. These properties were investigated by varying one parameter at a time or using parameter sets randomly drawn from assigned intervals. The theoretical and simulation results were compared with experimental data from the literature. Our major findings are that the EAD amplitude and takeoff potential exhibit a negative linear correlation; the inter-EAD interval is insensitive to the maximum ionic current conductance but mainly determined by the kinetics of I_Ca,L and the dual Hopf-homoclinic bifurcation; and both inter-EAD interval and latency vary largely from model to model. Most of the model results generally agree with experimental observations in isolated ventricular myocytes. However, a major discrepancy between modeling results and experimental observations is that the inter-EAD intervals observed in experiments are mainly between 200 and 500 ms, irrespective of species, while those of the mathematical models exhibit a much wider range with some models exhibiting inter-EAD intervals less than 100 ms. Our simulations show that the cause of this discrepancy is likely due to the difference in I_Ca,L recovery properties in different mathematical models, which needs to be addressed in future action potential model development.

Early afterdepolarizations (EADs) are abnormal depolarizations during the plateau phase of action potential in cardiac myocytes, arising from a dual Hopf-homoclinic bifurcation. The same bifurcations are also responsible for certain types of bursting behaviors in other cell types, such as beta cells and neuronal cells. EADs are known to play important role in the genesis of lethal arrhythmias and have been widely studied in both experiments and computer models. However, a detailed comparison between the properties of EADs observed in experiments and those from mathematical models have not been carried out. In this study, we performed theoretical analyses and computer simulations of different ventricular action potential models as well as different species to investigate the properties of EADs and compared these properties to those observed in experiments. While the EAD properties in the action potential models capture many of the EAD properties seen in experiments, the inter-EAD intervals in the computer models differ a lot from model to model, and some of them show very large discrepancy with those observed in experiments. This discrepancy needs to be addressed in future cardiac action potential model development.

Sex-specific activation of SK current by isoproterenol facilitates action potential triangulation and arrhythmogenesis in rabbit ventricles

KEY POINTS

It is unknown if a sex difference exists in cardiac apamin-sensitive small conductance Ca²⁺ -activated K⁺ (SK) current (I_KAS ). There is no sex difference in I_KAS in the basal condition. However, there is larger I_KAS in female rabbit ventricles than in male during isoproterenol infusion. I_KAS activation by isoproterenol leads to action potential triangulation in females, indicating its abundant activation at early phases of repolarization. I_KAS activation in females induces negative Ca²⁺ -voltage coupling and promotes electromechanically discordant phase 2 repolarization alternans. I_KAS is important in the mechanisms of ventricular fibrillation in females during sympathetic stimulation.

ABSTRACT

Sex has a large influence on cardiac electrophysiological properties. Whether sex differences exist in apamin-sensitive small conductance Ca²⁺ -activated K⁺ (SK) current (I_KAS ) remains unknown. We performed optical mapping, transmembrane potential, patch clamp, western blot and immunostaining in 62 normal rabbit ventricles, including 32 females and 30 males. I_KAS blockade by apamin only minimally prolonged action potential (AP) duration (APD) in the basal condition for both sexes, but significantly prolonged APD in the presence of isoproterenol in females. Apamin prolonged APD at the level of 25% repolarization (APD₂₅ ) more prominently than APD at the level of 80% repolarization (APD₈₀ ), consequently reversing isoproterenol-induced AP triangulation in females. In comparison, apamin prolonged APD to a significantly lesser extent in males and failed to restore the AP plateau during isoproterenol infusion. I_KAS in males did not respond to the L-type calcium current agonist BayK8644, but was amplified by the casein kinase 2 (CK2) inhibitor 4,5,6,7-tetrabromobenzotriazole. In addition, whole-cell outward I_KAS densities in ventricular cardiomyocytes were significantly larger in females than in males. SK channel subtype 2 (SK2) protein expression was higher and the CK2/SK2 ratio was lower in females than in males. I_KAS activation in females induced negative intracellular Ca²⁺ -voltage coupling, promoted electromechanically discordant phase 2 repolarization alternans and facilitated ventricular fibrillation (VF). Apamin eliminated the negative Ca²⁺ -voltage coupling, attenuated alternans and reduced VF inducibility, phase singularities and dominant frequencies in females, but not in males. We conclude that β-adrenergic stimulation activates ventricular I_KAS in females to a much greater extent than in males. I_KAS activation plays an important role in ventricular arrhythmogenesis in females during sympathetic stimulation.

Development of hypokalemia in patients with chronic hepatitis B: Causes and nursing care

AIM: To investigate the causes of hypokalemia in patients with chronic hepatitis B and summarize the methods of care.

METHODS: Clinical data for 60 chronic hepatitis B patients with hypokalemia treated from January 2012 to December 2013 at Yanchi County People's Hospital were analyzed retrospectively. The methods of care were discussed.

RESULTS: Main causes of hypokalemia in patients with chronic hepatitis B were drug-induced pseudo-aldosteronism (70.0%) and excessive chronic loss (30.0%). Potassium content was 3.1 mmol/L ± 0.5 mmol/L before nursing care and 4.3 mmol/L ± 1.1 mmol/L after nursing, and the difference was statistically significant (P < 0.05). Before nursing care, patients' chronic hepatitis B and hypokalemia-related knowledge scores were 63.6 points ± 4.8 points and 55.3 points ± 7.4 points, respectively; after nursing care, these scores were significantly elevated to 84.6 points ± 5.1 points and 80.4 points ± 5.5 points, respectively (P < 0.05). Nursing care also significantly improved SAS and SDS scores (55.3 points ± 6.3 points vs 23.3 points ± 2.7 points, 54.8 min ± 6.5 min vs 24.3 points ± 2.3 points, P < 0.05).

CONCLUSION: Hypokalemia in chronic hepatitis B occurs mainly due to drug-induced pseudo-aldosteronism and chronic loss. Nursing care and treatment should be strengthened to reduce the occurrence of hypokalemia in chronic hepatitis B.

Atrial-selective targeting of arrhythmogenic phase-3 early afterdepolarizations in human myocytes

BACKGROUND

We have previously shown that non-equilibrium Na(+) current (INa) reactivation drives isoproterenol-induced phase-3 early afterdepolarizations (EADs) in mouse ventricular myocytes. In these cells, EAD initiation occurs secondary to potentiated sarcoplasmic reticulum Ca(2+) release and enhanced Na(+)/Ca(2+) exchange (NCX). This can be abolished by tetrodotoxin-blockade of INa, but not ranolazine, which selectively inhibits ventricular late INa.

AIM

Since repolarization of human atrial myocytes is similar to mouse ventricular myocytes in that it is relatively rapid and potently modulated by Ca(2+), we investigated whether similar mechanisms can evoke EADs in human atrium. Indeed, phase-3 EADs have been shown to re-initiate atrial fibrillation (AF) during autonomic stimulation, which is a well-recognized initiator of AF.

METHODS

We integrated a Markov model of INa gating in our human atrial myocyte model. To simulate experimental results, we rapidly paced this cell model at 10Hz in the presence of 0.1μM acetylcholine and 1μM isoproterenol, and assessed EAD occurrence upon return to sinus rhythm (1Hz).

RESULTS

Cellular Ca(2+) loading during fast pacing results in a transient period of hypercontractility after return to sinus rhythm. Here, fast repolarization and enhanced NCX facilitate INa reactivation via the canonical gating mode (i.e., not late INa burst mode), which drives EAD initiation. Simulating ranolazine administration reduces atrial peak INa and leads to faster repolarization, during which INa fails to reactivate and EADs are prevented.

CONCLUSIONS

Non-equilibrium INa reactivation can critically contribute to arrhythmias, specifically in human atrial myocytes. Ranolazine might be beneficial in this context by blocking peak (not late) atrial INa.

Mechanisms of ventricular arrhythmias: from molecular fluctuations to electrical turbulence

Ventricular arrhythmias have complex causes and mechanisms. Despite extensive investigation involving many clinical, experimental, and computational studies, effective biological therapeutics are still very limited. In this article, we review our current understanding of the mechanisms of ventricular arrhythmias by summarizing the state of knowledge spanning from the molecular scale to electrical wave behavior at the tissue and organ scales and how the complex nonlinear interactions integrate into the dynamics of arrhythmias in the heart. We discuss the challenges that we face in synthesizing these dynamics to develop safe and effective novel therapeutic approaches.