Exploring the mechanism of action of herbal compounding in the treatment of myasthenia gravis based on network pharmacology.

Myasthenia gravis is a major disease in the context of an ageing society, and the discovery of effective herbal compound and herbal active ingredients is a highly promising direction for the treatment of myasthenia gravis. In this study, we selected shujiao, dried ginger and ginseng from the compound ingredients through a network pathology approach. The three ingredients were used to obtain drug targets in Traditional Chinese Medicine Systems Pharmacology (TCMSP), HERB and BATMAN-TCM data and intersected with the disease targets of myasthenia gravis. The resulting regulatory network maps were then used to identify core genes through the String database, and finally the core genes were molecularly aligned with the corresponding active ingredients using Autodock vina software. The 'herbal-component-target' regulatory network of the Chinese herbal formulae was constructed, which is important for finding the potential molecular mechanism for the treatment of myasthenia gravis. It will provide a theoretical basis for the therapeutic and clinical research of myasthenia gravis.

Size Matters: Ryanodine Receptor Cluster Size Heterogeneity Potentiates Calcium Waves.

Ryanodine receptors (RyRs) mediate calcium (Ca)-induced Ca release and intracellular Ca homeostasis. In a cardiac myocyte, RyRs group into clusters of variable size from a few to several hundred RyRs, creating a spatially nonuniform intracellular distribution. It is unclear how heterogeneity of RyR cluster size alters spontaneous sarcoplasmic reticulum (SR) Ca releases (Ca sparks) and arrhythmogenic Ca waves. Here, we tested the impact of heterogeneous RyR cluster size on the initiation of Ca waves. Experimentally, we measured RyR cluster sizes at Ca spark sites in rat ventricular myocytes and further tested functional impacts using a physiologically detailed computational model with spatial and stochastic intracellular Ca dynamics. We found that the spark frequency and amplitude increase nonlinearly with the size of RyR clusters. Larger RyR clusters have lower SR Ca release threshold for local Ca spark initiation and exhibit steeper SR Ca release versus SR Ca load relationship. However, larger RyR clusters tend to lower SR Ca load because of the higher Ca leak rate. Conversely, smaller clusters have a higher threshold and a lower leak, which tends to increase SR Ca load. At the myocyte level, homogeneously large or small RyR clusters limit Ca waves (because of low load for large clusters but low excitability for small clusters). Mixtures of large and small RyR clusters potentiates Ca waves because the enhanced SR Ca load driven by smaller clusters enables Ca wave initiation and propagation from larger RyR clusters. Our study suggests that a spatially heterogeneous distribution of RyR cluster size under pathological conditions may potentiate Ca waves and thus afterdepolarizations and triggered arrhythmias.

Size Matters: Ryanodine Receptor Cluster Size Affects Arrhythmogenic Sarcoplasmic Reticulum Calcium Release.

BACKGROUND: Ryanodine receptors (RyR) mediate sarcoplasmic reticulum calcium (Ca2+) release and influence myocyte Ca2+ homeostasis and arrhythmias. In cardiac myocytes, RyRs are found in clusters of various sizes and shapes, and RyR cluster size may critically influence normal and arrhythmogenic Ca2+ spark and wave formation. However, the actual RyR cluster sizes at specific Ca2+ spark sites have never been measured in the physiological setting. METHODS AND RESULTS: Here we measured RyR cluster size and Ca2+ sparks simultaneously to assess how RyR cluster size influences Ca2+ sparks and sarcoplasmic reticulum Ca2+ leak. For small RyR cluster sizes (<50), Ca2+ spark frequency is very low but then increases dramatically at larger cluster sizes. In contrast, Ca2+ spark amplitude is nearly maximal even at relatively small RyR cluster size (≈10) and changes little at larger cluster size. These properties agreed with computational simulations of RyR gating within clusters. CONCLUSIONS: Our study explains how this combination of properties may limit arrhythmogenic Ca2+ sparks and wave propagation (at many junctions) while preserving the efficacy and spatial synchronization of Ca2+-induced Ca2+-release during normal excitation-contraction coupling. However, variations in RyR cluster size among individual junctions and RyR sensitivity could exacerbate heterogeneity of local sarcoplasmic reticulum Ca2+ release and arrhythmogenesis under pathological conditions.

Slow [Na]i Changes and Positive Feedback Between Membrane Potential and [Ca]i Underlie Intermittent Early Afterdepolarizations and Arrhythmias.

BACKGROUND: Most cardiac arrhythmias occur intermittently. As a cellular precursor of lethal cardiac arrhythmias, early afterdepolarizations (EADs) during action potentials(APs) have been extensively investigated, and mechanisms for the occurrence of EADs on a beat-to-beat basis have been proposed. However, no previous study explains slow fluctuations in EADs, which may underlie intermittency of EAD trains and consequent arrhythmias. We hypothesize that the feedback of intracellular calcium and sodium concentrations ([Na](i) and [Ca](i)) that influence membrane voltage (V) can explain EAD intermittency. METHODS AND RESULTS: AP recordings in rabbit ventricular myocytes revealed intermittent EADs, with slow fluctuations between runs of APs with EADs present or absent. We then used dynamical systems analysis and detailed mathematical models of rabbit ventricular myocytes that replicate the observed behavior and investigated the underlying mechanism. We found that a dominance of inward Na-Ca exchanger current (I(NCX)) over Ca-dependent inactivation of L-type Ca current (I(CaL)) forms a positive feedback between [Ca](i) and V, thus resulting in 2 stable AP states, with and without EADs (ie, bistability). Slow changes in [Na](i) determine the transition between these 2 states, forming a bistable on-off switch of EADs. Tissue simulations showed that this bistable switch of cellular EADs provided both a trigger and a functional substrate for intermittent arrhythmias in homogeneous tissues. CONCLUSIONS: Our study demonstrates that the interaction among V, [Ca](i), and [Na](i) causes slow on-off switching (or bistability) of AP duration in cardiac myocytes and EAD-mediated arrhythmias and suggests a novel possible mechanism for intermittency of cardiac arrhythmias.

Targeting the late component of the cardiac L-type Ca2+ current to suppress early afterdepolarizations.

Early afterdepolarizations (EADs) associated with prolongation of the cardiac action potential (AP) can create heterogeneity of repolarization and premature extrasystoles, triggering focal and reentrant arrhythmias. Because the L-type Ca(2+) current (ICa,L) plays a key role in both AP prolongation and EAD formation, L-type Ca(2+) channels (LTCCs) represent a promising therapeutic target to normalize AP duration (APD) and suppress EADs and their arrhythmogenic consequences. We used the dynamic-clamp technique to systematically explore how the biophysical properties of LTCCs could be modified to normalize APD and suppress EADs without impairing excitation-contraction coupling. Isolated rabbit ventricular myocytes were first exposed to H2O2 or moderate hypokalemia to induce EADs, after which their endogenous ICa,L was replaced by a virtual ICa,L with tunable parameters, in dynamic-clamp mode. We probed the sensitivity of EADs to changes in the (a) amplitude of the noninactivating pedestal current; (b) slope of voltage-dependent activation; (c) slope of voltage-dependent inactivation; (d) time constant of voltage-dependent activation; and (e) time constant of voltage-dependent inactivation. We found that reducing the amplitude of the noninactivating pedestal component of ICa,L effectively suppressed both H2O2- and hypokalemia-induced EADs and restored APD. These results, together with our previous work, demonstrate the potential of this hybrid experimental-computational approach to guide drug discovery or gene therapy strategies by identifying and targeting selective properties of LTCC.

Repolarization reserve evolves dynamically during the cardiac action potential: effects of transient outward currents on early afterdepolarizations.

BACKGROUND: Transient outward K currents (Ito) have been reported both to suppress and to facilitate early afterdepolarizations (EADs) when repolarization reserve is reduced. Here, we used the dynamic clamp technique to analyze how Ito accounts for these paradoxical effects on EADs by influencing the dynamic evolution of repolarization reserve during the action potential. METHODS AND RESULTS: Isolated patch-clamped rabbit ventricular myocytes were exposed to either oxidative stress (H2O2) or hypokalemia to induce bradycardia-dependent EADs at a long pacing cycle length of 6 s, when native rabbit Ito is substantial. EADs disappeared when the pacing cycle length was shortened to 1 s, when Ito becomes negligible because of incomplete recovery from inactivation. During 6-s pacing cycle length, EADs were blocked by the Ito blocker 4-aminopyridine, but reappeared when a virtual current with appropriate Ito-like properties was reintroduced using the dynamic clamp (n=141 trials). During 1-s pacing cycle length in the absence of 4-aminopyridine, adding a virtual Ito-like current (n=1113 trials) caused EADs to reappear over a wide range of Ito conductance (0.005-0.15 nS/pF), particularly when inactivation kinetics were slow (τinact≥20 ms) and the pedestal (noninactivating component) was small (<25% of peak Ito). Faster inactivation or larger pedestals tended to suppress EADs. CONCLUSIONS: Repolarization reserve evolves dynamically during the cardiac action potential. Whereas sufficiently large Ito can suppress EADs, a wide range of intermediate Ito properties can promote EADs by influencing the temporal evolution of other currents affecting late repolarization reserve. These findings raise caution in targeting Ito as an antiarrhythmic strategy.

Arrhythmogenic transient dynamics in cardiac myocytes.

Cardiac action potential alternans and early afterdepolarizations (EADs) are linked to cardiac arrhythmias. Periodic action potentials (period 1) in healthy conditions bifurcate to other states such as period 2 or chaos when alternans or EADs occur in pathological conditions. The mechanisms of alternans and EADs have been extensively studied under steady-state conditions, but lethal arrhythmias often occur during the transition between steady states. Why arrhythmias tend to develop during the transition is unclear. We used low-dimensional mathematical models to analyze dynamical mechanisms of transient alternans and EADs. We show that depending on the route from one state to another, action potential alternans and EADs may occur during the transition between two periodic steady states. The route taken depends on the time course of external perturbations or intrinsic signaling, such as ß-adrenergic stimulation, which regulate cardiac calcium and potassium currents with differential kinetics.

How does ß-adrenergic signalling affect the transitions from ventricular tachycardia to ventricular fibrillation?

AIMS: Ventricular tachycardia (VT) and fibrillation (VF) are the most lethal cardiac arrhythmias. The degeneration of VT into VF is associated with the breakup of a spiral wave of the action potential in cardiac tissue. ß-Adrenergic (ßAR) signalling potentiates the L-type Ca current (ICaL) faster than the slow delayed rectifier potassium current (IKs), which transiently prolongs the action potential duration (APD) and promotes early after depolarizations. In this study, we aimed at investigating how ßAR signalling affects the transition from VT to VF. METHODS AND RESULTS: We used a physiologically detailed computer model of the rabbit ventricular myocyte in a two-dimensional tissue to determine how spiral waves respond to ßAR activation following administration of isoproterenol. A simplified mathematical model was also used to investigate the underlying dynamics. We found that the spatiotemporal behaviour of spiral waves strongly depends on the kinetics of ßAR activation. When ßAR activation is rapid, a stable spiral wave turns into small fragments and its electrocardiogram reveals the transition from VT to VF. This is due to the transiently steepened APD restitution induced by the faster activation of ICaL vs. IKs upon sudden ßAR activation. The spiral wave may also disappear if its transient wavelength is too large to be supported by the tissue size upon sudden strong ßAR activation that prolongs APD transiently. When ßAR activation is gradual, a stable spiral wave remains such, because of more limited increase in both APD and slope of APD restitution due to more contemporaneous ICaL and IKs activation. CONCLUSION: Changes in APD restitution during ßAR activation revealed a novel transient spiral wave dynamics; this spatiotemporal characteristic strongly depends on the protocol of isoproterenol application.

ß-adrenergic stimulation activates early afterdepolarizations transiently via kinetic mismatch of PKA targets.

Sympathetic stimulation regulates cardiac excitation-contraction coupling in hearts but can also trigger ventricular arrhythmias caused by early afterdepolarizations (EADs) in pathological conditions. Isoproterenol (ISO) stimulation can transiently cause EADs which could result from differential kinetics of L-type Ca current (ICaL) vs. delayed rectifier potassium current (IKs) effects, but multiple PKA targets complicate mechanistic analysis. Utilizing a biophysically detailed model integrating Ca and ß-adrenergic signaling, we investigate how different phosphorylation kinetics and targets influence ß-adrenergic-induced transient EADs. We found that: 1) The faster time course of ICaL vs. IKs increases recapitulates experimentally observed ISO-induced transient EADs (which are due to ICaL reactivation). These EADs disappear at steady state ISO and do not occur during more gradual ISO application. 2) This ICaL vs. IKs kinetic mismatch with ISO can also induce transient EADs due to spontaneous sarcoplasmic reticulum (SR) Ca release and Na/Ca exchange current. The increased ICaL, SR Ca uptake and action potential duration (APD) raise SR Ca to cause spontaneous SR Ca release, but eventual IKs activation and APD shortening abolish these EADs. 3) Phospholemman (PLM) phosphorylation decreases both types of EADs by increasing outward Na/K-ATPase current (INaK) for ICaL-mediated EADs, and reducing intracellular Na and Ca loading for SR Ca-release-mediated EADs. Slowing PLM phosphorylation kinetics abolishes this protective effect. 4) Blocking phospholamban (PLB) phosphorylation has little effect on ICaL-mediated transient EADs, but abolishes SR Ca-release-mediated transient EADs by limiting SR Ca loading. 5) RyR phosphorylation has little effect on either transient EAD type. Our study emphasizes the importance of understanding non-steady state kinetics of several systems in mediating ß-adrenergic-induced EADs and arrhythmias.

Ca²âº waves in the heart.

Ca(2+) waves were probably first observed in the early 1940s. Since then Ca(2+) waves have captured the attention of an eclectic mixture of mathematicians, neuroscientists, muscle physiologists, developmental biologists, and clinical cardiologists. This review discusses the current state of mathematical models of Ca(2+) waves, the normal physiological functions Ca(2+) waves might serve in cardiac cells, as well as how the spatial arrangement of Ca(2+) release channels shape Ca(2+) waves, and we introduce the idea of Ca(2+) phase waves that might provide a useful framework for understanding triggered arrhythmias.

Synchronization of early afterdepolarizations and arrhythmogenesis in heterogeneous cardiac tissue models.

Early afterdepolarizations (EADs) are linked to both triggered arrhythmias and reentrant arrhythmias by causing premature ventricular complexes (PVCs), focal excitations, or heterogeneous tissue substrates for reentry formation. However, a critical number of cells that synchronously exhibit EADs are needed to result in arrhythmia triggers and substrates in tissue. In this study, we use mathematical modeling and computer simulations to investigate EAD synchronization and arrhythmia induction in tissue models with random cell-to-cell variations. Our major observations are as follows. Random cell-to-cell variations in action potential duration without EAD presence do not cause large dispersion of refractoriness in well-coupled tissue. In the presence of phase-2 EADs, the cells may synchronously exhibit the same number of EADs or no EADs with a very small dispersion of refractoriness, or synchronize regionally to result in large dispersion of refractoriness. In the presence of phase-3 EADs, regional synchronization leads to propagating EADs, forming PVCs in tissue. Interestingly, even though the uncoupled cells exhibit either no EAD or only a single EAD, when these cells are coupled to form a tissue, more than one PVC can occur. When the PVCs occur at different locations and time, multifocal arrhythmias are triggered, with the foci shifting in space and time in an irregular manner. The focal arrhythmias either spontaneously terminate or degenerate into reentrant arrhythmias due to heterogeneities and spatiotemporal chaotic dynamics of the foci.

Role of the transient outward potassium current in the genesis of early afterdepolarizations in cardiac cells.

AIMS: The transient outward potassium current (I(to)) plays important roles in action potential (AP) morphology and dynamics; however, its role in the genesis of early afterdepolarizations (EADs) is not well understood. We aimed to study the effects and mechanisms of I(to) on EAD genesis in cardiac cells using combined experimental and computational approaches. METHODS AND RESULTS: We first carried out patch-clamp experiments in isolated rabbit ventricular myocytes exposed to H(2)O(2) (0.2 or 1 mM), in which EADs were induced at a slow pacing rate. EADs were eliminated by either increasing the pacing rate or blocking I(to) with 2 mM 4-aminopyridine. In addition to enhancing the L-type calcium current (I(Ca,L)) and the late sodium current, H(2)O(2) also increased the conductance, slowed inactivation, and accelerated recovery from the inactivation of I(to). Computer simulations showed that I(to) promoted EADs under the condition of reduced repolarization reserve, consistent with the experimental observations. However, EADs were only promoted in the intermediate ranges of the I(to) conductance and the inactivation time constant. The underlying mechanism is that I(to) lowers the AP plateau voltage into the range at which the time-dependent potassium current (namely I(Ks)) activation is further slowed and I(Ca,L) is available for reactivation, leading to voltage oscillations to manifest EADs. Further experimental studies in cardiac cells of other species validated the theoretical predictions. CONCLUSION: In cardiac cells, I(to), with a proper conductance and inactivation speed, potentiates EADs by setting the AP plateau into the voltage range where I(Ca,L) reactivation is facilitated and I(Ks) activation is slowed.

Arrhythmogenic consequences of myofibroblast-myocyte coupling.

AIMS: Fibrosis is known to promote cardiac arrhythmias by disrupting myocardial structure. Given recent evidence that myofibroblasts form gap junctions with myocytes at least in co-cultures, we investigated whether myofibroblast-myocyte coupling can promote arrhythmia triggers, such as early afterdepolarizations (EADs), by directly influencing myocyte electrophysiology. METHODS AND RESULTS: Using the dynamic voltage clamp technique, patch-clamped adult rabbit ventricular myocytes were electrotonically coupled to one or multiple virtual fibroblasts or myofibroblasts programmed with eight combinations of capacitance, membrane resistance, resting membrane potential, and gap junction coupling resistance, spanning physiologically realistic ranges. Myocytes were exposed to oxidative (1 mmol/L H(2)O(2)) or ionic (2.7 mmol/L hypokalaemia) stress to induce bradycardia-dependent EADs. In the absence of myofibroblast-myocyte coupling, EADs developed during slow pacing (6 s), but were completely suppressed by faster pacing (1 s). However, in the presence of myofibroblast-myocyte coupling, EADs could no longer be suppressed by rapid pacing, especially when myofibroblast resting membrane potential was depolarized (-25 mV). Analysis of the myofibroblast-myocyte virtual gap junction currents revealed two components: an early transient-outward I(to)-like current and a late sustained current. Selective elimination of the I(to)-like component prevented EADs, whereas selective elimination of the late component did not. CONCLUSION: Coupling of myocytes to myofibroblasts promotes EAD formation as a result of a mismatch in early vs. late repolarization reserve caused by the I(to)-like component of the gap junction current. These cellular and ionic mechanisms may contribute to the pro-arrhythmic risk in fibrotic hearts.

Shaping a new Ca²âº conductance to suppress early afterdepolarizations in cardiac myocytes.

Sudden cardiac death (SCD) due to ventricular fibrillation (VF) is a major world-wide health problem. A common trigger of VF involves abnormal repolarization of the cardiac action potential causing early afterdepolarizations (EADs). Here we used a hybrid biological-computational approach to investigate the dependence of EADs on the biophysical properties of the L-type Ca(2+) current (I(Ca,L)) and to explore how modifications of these properties could be designed to suppress EADs. EADs were induced in isolated rabbit ventricular myocytes by exposure to 600 µmol l(-1) H(2)O(2) (oxidative stress) or lowering the external [K(+)] from 5.4 to 2.0-2.7 mmol l(-1) (hypokalaemia). The role of I(Ca,L) in EAD formation was directly assessed using the dynamic clamp technique: the paced myocyte's V(m) was input to a myocyte model with tunable biophysical parameters, which computed a virtual I(Ca,L), which was injected into the myocyte in real time. This virtual current replaced the endogenous I(Ca,L), which was suppressed with nifedipine. Injecting a current with the biophysical properties of the native I(Ca,L) restored EAD occurrence in myocytes challenged by H(2)O(2) or hypokalaemia. A mere 5 mV depolarizing shift in the voltage dependence of activation or a hyperpolarizing shift in the steady-state inactivation curve completely abolished EADs in myocytes while maintaining a normal Ca(i) transient. We propose that modifying the biophysical properties of I(Ca,L) has potential as a powerful therapeutic strategy for suppressing EADs and EAD-mediated arrhythmias.

Suppression of re-entrant and multifocal ventricular fibrillation by the late sodium current blocker ranolazine.

OBJECTIVES: The purpose of this study was to test the hypothesis that the late Na current blocker ranolazine suppresses re-entrant and multifocal ventricular fibrillation (VF). BACKGROUND: VF can be caused by either re-entrant or focal mechanism. METHODS: Simultaneous voltage and intracellular Ca(+)² optical mapping of the left ventricular epicardial surface along with microelectrode recordings was performed in 24 isolated-perfused aged rat hearts. Re-entrant VF was induced by rapid pacing and multifocal VF by exposure to oxidative stress with 0.1 mM hydrogen peroxide (H2O2). RESULTS: Rapid pacing induced sustained VF in 7 of 8 aged rat hearts, characterized by 2 to 4 broad propagating wavefronts. Ranolazine significantly (p < 0.05) reduced the maximum slope of action potential duration restitution curve and converted sustained to nonsustained VF lasting 24 ± 8 s in all 7 hearts. Exposure to H2O2 initiated early afterdepolarization (EAD)-mediated triggered activity that led to sustained VF in 8 out of 8 aged hearts. VF was characterized by multiple foci, appearing at an average of 6.8 ± 3.2 every 100 ms, which remained confined to a small area averaging 2.8 ± 0.85 mm² and became extinct after a mean of 43 ± 16 ms. Ranolazine prevented (when given before H2O2) and suppressed H2O2-mediated EADs by reducing the number of foci, causing VF to terminate in 8 out of 8 hearts. Simulations in 2-dimensional tissue with EAD-mediated multifocal VF showed progressive reduction in the number of foci and VF termination by blocking the late Na current. CONCLUSIONS: Late Na current blockade with ranolazine is effective at suppressing both pacing-induced re-entrant VF and EAD-mediated multifocal VF.

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.

Genesis of phase 3 early afterdepolarizations and triggered activity in acquired long-QT syndrome.

BACKGROUND: Both phase 2 and phase 3 early afterdepolarizations (EADs) occur in long-QT syndromes, but their respective roles in generating arrhythmias in intact cardiac tissue are incompletely understood. METHODS AND RESULTS: Intracellular Ca (Ca(i)) and membrane voltage (V(m)) were optically mapped in a quasi 2-dimensional model of cryoablated Langendorff-perfused rabbit ventricles (n=16). E-4031 (an I(Kr) blocker) combined with reduced extracellular K ([K(+)](o)) and Mg ([Mg(2+)](o)) prolonged action potential duration heterogeneously and induced phase 2 and phase 3 EADs. Whereas phase 2 EADs were Ca(i)-dependent, phase 3 EADs were not. The origins of 47 triggered activity episodes were attributed to phase 2 EADs in 12 episodes (26%) and phase 3 EADs in 35 episodes (74%). When phase 2 EADs accompanied phase 3 EADs, they accentuated action potential duration heterogeneity, creating a large V(m) gradient across the boundary between long and short action potential duration regions from which triggered activity emerged. The amplitude of phase 3 EADs correlated with the V(m) gradient (r=0.898, P<0.001). Computer simulation studies showed that coupling of cells with heterogeneous repolarization could extrinsically generate phase 3 EADs via electrotonic current flow. Alternatively, reduced I(K1) caused by low [K(+)](o) could generate intrinsic phase 3 EADs capable of inducing triggered activity at the boundary zone. CONCLUSIONS: Phase 3 EADs can be extrinsic as the result of electrotonic current across steep repolarization gradients or intrinsic as the result of low I(K1) and do not require spontaneous sarcoplasmic reticulum Ca release. Reduction of I(K1) by low [K(+)](o) strongly promotes ventricular arrhythmias mediated by phase 3 EADs in acquired long-QT syndrome caused by I(Kr) blockade.

So little source, so much sink: requirements for afterdepolarizations to propagate in tissue.

How early (EADs) and delayed afterdepolarizations (DADs) overcome electrotonic source-sink mismatches in tissue to trigger premature ventricular complexes remains incompletely understood. To study this question, we used a rabbit ventricular action potential model to simulate tissues in which a central area of contiguous myocytes susceptible to EADs or DADs was surrounded by unsusceptible tissue. In 1D tissue with normal longitudinal conduction velocity (0.55 m/s), the numbers of contiguous susceptible myocytes required for an EAD and a barely suprathreshold DAD to trigger a propagating action potential were 70 and 80, respectively. In 2D tissue, these numbers increased to 6940 and 7854, and in 3D tissue to 696,910 and 817,280. These numbers were significantly decreased by reduced gap junction conductance, simulated fibrosis, reduced repolarization reserve and heart failure electrical remodeling. In conclusion, the source-sink mismatch in well-coupled cardiac tissue powerfully protects the heart from arrhythmias due to sporadic afterdepolarizations. Structural and electrophysiological remodeling decrease these numbers significantly but still require synchronization mechanisms for EADs and DADs to overcome the robust protective effects of source-sink mismatch.

T-wave alternans and arrhythmogenesis in cardiac diseases.

T-wave alternans, a manifestation of repolarization alternans at the cellular level, is associated with lethal cardiac arrhythmias and sudden cardiac death. At the cellular level, several mechanisms can produce repolarization alternans, including: 1) electrical restitution resulting from collective ion channel recovery, which usually occurs at fast heart rates but can also occur at normal heart rates when action potential is prolonged resulting in a short diastolic interval; 2) the transient outward current, which tends to occur at normal or slow heart rates; 3) the dynamics of early afterdepolarizations, which tends to occur during bradycardia; and 4) intracellular calcium cycling alternans through its interaction with membrane voltage. In this review, we summarize the cellular mechanisms of alternans arising from these different mechanisms, and discuss their roles in arrhythmogenesis in the setting of cardiac disease.

Effects of fibroblast-myocyte coupling on cardiac conduction and vulnerability to reentry: A computational study.

BACKGROUND: Recent experimental studies have documented that functional gap junctions form between fibroblasts and myocytes, raising the possibility that fibroblasts play roles in cardiac electrophysiology that extend beyond acting as passive electrical insulators. OBJECTIVE: The purpose of this study was to use computational models to investigate how fibroblasts may affect cardiac conduction and vulnerability to reentry under different fibroblast-myocyte coupling conditions and tissue structures. METHODS: Computational models of two-dimensional tissue with fibroblast-myocyte coupling were developed and numerically simulated. Myocytes were modeled by the phase I of the Luo-Rudy model, and fibroblasts were modeled by a passive model. RESULTS: Besides slowing conduction by cardiomyocyte decoupling and electrotonic loading, fibroblast coupling to myocytes elevates myocyte resting membrane potential, causing conduction velocity to first increase and then decrease as fibroblast content increases, until conduction failure occurs. Fibroblast-myocyte coupling can also enhance conduction by connecting uncoupled myocytes. These competing effects of fibroblasts on conduction give rise to different conduction patterns under different fibroblast-myocyte coupling conditions and tissue structures. Elevation of myocyte resting potential due to fibroblast-myocyte coupling slows sodium channel recovery, which extends postrepolarization refractoriness. Owing to this prolongation of the myocyte refractory period, reentry was more readily induced by a premature stimulation in heterogeneous tissue models when fibroblasts were electrotonically coupled to myocytes compared with uncoupled fibroblasts acting as pure passive electrical insulators. CONCLUSIONS: Fibroblasts affect cardiac conduction by acting as obstacles or by creating electrotonic loading and elevating myocyte resting potential. Functional fibroblast-myocyte coupling prolongs the myocyte refractory period, which may facilitate induction of reentry in cardiac tissue with fibrosis.