Source-sink Mismatch Research Articles

Abstract 24057: Induced Pacemaker Spheroids As A Model To Reverse-Engineer The Native Sinoatrial Node

Background: The sinoatrial node (SAN) has intricate architecture, which facilitates the spontaneous action potentials generated from the SAN to pace and drive the neighboring myocardium. We sought to create an engineered SAN that recapitulates the native SAN’s ability to overcome source-sink mismatch. We hypothesized spheroids consisting of induced pacemaker cells (iPM) can pace and drive surrounding quiescent myocardium. Methods: The iPMs were created by singular expression of a transcription factor, TBX18, to neonatal rat ventricular myocytes as we have previously demonstrated. The iPM or GFP-spheroids (control) were created by subjecting the NRVMs to the AggreWell™ plate at 1000 cells/well, and cultured for three days in suspension. Spheroids consisted of 90% NRVM and 10% cardiac fibroblast. Results: iPM-spheroids demonstrated spontaneous pacing at 145±26 bpm compared to 66±1 bpm (p=6.5770E-9) of control, GFP-spheroids. When one iPM-spheroid was surrounded by a monolayer of quiescent NRVMs (iPM:NRVM=1:100), the iPM-spheroids were able to pace and drive the quiescent ventricular myocardium at a capture rate of 48±7%. In contrast, the random activity of control spheroids captured the myocardium at 7±4% (p=0.0078). A monolayer of TBX18 cells (1:4=TBX18:NRVM) failed to pace and drive the neighboring sheet of ventricular myocytes. iPM-spheroids had a 17-fold increase in SAN-specific gap junction, Cx45, transcripts (p=0.0001) and a 2-fold decrease in myocardial gap junction, Cx43 (p=0.0030), compared to GFP-spheroids. The iPM-spheroids have superior viability compared to control GFP-spheroids, 87±1% and 72±5% respectively (p= 0.0463). TUNEL staining confirmed apoptotic fibroblast in the periphery. When cultured for >2 weeks, iPM-spheroids demonstrated small α-sarcomeric actinin positive cells organized as a mesh in the core, similar to the pacemaker cells in the native SAN. Conclusion: iPM-spheroids can pace and drive surrounding quiescent myocardium, overcoming the source-sink mismatch. The iPM-spheroids are viable in long-term and exhibit native SAN-like pacemaker cell organization. These data provide an in vitro platform on which the design principles of native SAN could be tested.

Mechanisms Underlying the Emergence of Post-acidosis Arrhythmia at the Tissue Level: A Theoretical Study.

Acidosis has complex electrophysiological effects, which are associated with a high recurrence of ventricular arrhythmias. Through multi-scale cardiac computer modeling, this study investigated the mechanisms underlying the emergence of post-acidosis arrhythmia at the tissue level. In simulations, ten Tusscher-Panfilov ventricular model was modified to incorporate various data on acidosis-induced alterations of cellular electrophysiology and intercellular electrical coupling. The single cell models were incorporated into multicellular one-dimensional (1D) fiber and 2D sheet tissue models. Electrophysiological effects were quantified as changes of action potential profile, sink-source interactions of fiber tissue, and the vulnerability of tissue to the genesis of unidirectional conduction that led to initiation of re-entry. It was shown that acidosis-induced sarcoplasmic reticulum (SR) calcium load contributed to delayed afterdepolarizations (DADs) in single cells. These DADs may be synchronized to overcome the source-sink mismatch arising from intercellular electrotonic coupling, and produce a premature ventricular complex (PVC) at the tissue level. The PVC conduction can be unidirectionally blocked in the transmural ventricular wall with altered electrical heterogeneity, resulting in the genesis of re-entry. In conclusion, altered source-sink interactions and electrical heterogeneity due to acidosis-induced cellular electrophysiological alterations may increase susceptibility to post-acidosis ventricular arrhythmias.

The control of cardiac ventricular excitability by autonomic pathways

Central to the genesis of ventricular cardiac arrhythmia are variations in determinants of excitability. These involve individual ionic channels and transporters in cardiac myocytes but also tissue factors such as variable conduction of the excitation wave, fibrosis and source-sink mismatch. It is also known that in certain diseases and particularly the channelopathies critical events occur with specific stressors. For example, in hereditary long QT syndrome due to mutations in KCNQ1 arrhythmic episodes are provoked by exercise and in particular swimming. Thus not only is the static substrate important but also how this is modified by dynamic signalling events associated with common physiological responses. In this review, we examine the regulation of ventricular excitability by signalling pathways from a cellular and tissue perspective in an effort to identify key processes, effectors and potential therapeutic approaches. We specifically focus on the autonomic nervous system and related signalling pathways.

Optogenetic manipulation of anatomical re-entry by light-guided generation of a reversible local conduction block.

Anatomical re-entry is an important mechanism of ventricular tachycardia, characterized by circular electrical propagation in a fixed pathway. It's current investigative and therapeutic approaches are non-biological, rather unspecific (drugs), traumatizing (electrical shocks), or irreversible (ablation). Optogenetics is a new biological technique that allows reversible modulation of electrical function with unmatched spatiotemporal precision using light-gated ion channels. We therefore investigated optogenetic manipulation of anatomical re-entry in ventricular cardiac tissue. Transverse, 150-μm-thick ventricular slices, obtained from neonatal rat hearts, were genetically modified with lentiviral vectors encoding Ca2+-translocating channelrhodopsin (CatCh), a light-gated depolarizing ion channel, or enhanced yellow fluorescent protein (eYFP) as control. Stable anatomical re-entry was induced in both experimental groups. Activation of CatCh was precisely controlled by 470-nm patterned illumination, while the effects on anatomical re-entry were studied by optical voltage mapping. Regional illumination in the pathway of anatomical re-entry resulted in termination of arrhythmic activity only in CatCh-expressing slices by establishing a local and reversible, depolarization-induced conduction block in the illuminated area. Systematic adjustment of the size of the light-exposed area in the re-entrant pathway revealed that re-entry could be terminated by either wave collision or extinction, depending on the depth (transmurality) of illumination. In silico studies implicated source-sink mismatches at the site of subtransmural conduction block as an important factor in re-entry termination. Anatomical re-entry in ventricular tissue can be manipulated by optogenetic induction of a local and reversible conduction block in the re-entrant pathway, allowing effective re-entry termination. These results provide distinctively new mechanistic insight into re-entry termination and a novel perspective for cardiac arrhythmia management.

Stochastic spontaneous calcium release events trigger premature ventricular complexes by overcoming electrotonic load.

Premature ventricular complexes (PVCs) due to spontaneous calcium (Ca) release (SCR) events at the cell level can precipitate ventricular arrhythmias. However, the mechanistic link between SCRs and PVC formation remains incompletely understood. The aim of this study was to investigate the conditions under which delayed afterdepolarizations resulting from stochastic subcellular SCR events can overcome electrotonic source-sink mismatch, leading to PVC initiation. A stochastic subcellular-scale mathematical model of SCR was incorporated in a realistic model of the rabbit ventricles and Purkinje system (PS). Elevated levels of diastolic sarcoplasmic reticulum Ca(2+) (CaSR) were imposed until triggered activity was observed, allowing us to compile statistics on probability, timing, and location of PVCs. At CaSR≥ 1500 µmol/L PVCs originated in the PS. When SCR was incapacitated in the PS, PVCs also emerged in the ventricles, but at a higher CaSR (≥1550 µmol/L) and with longer waiting times. For each model configuration tested, the probability of PVC occurrence increased from 0 to 100% within a well-defined critical CaSR range; this transition was much more abrupt in organ-scale models (∼50 µmol/L CaSR range) than in the tissue strand (∼100 µmol/L) or single-cell (∼450 µmol/L) models. Among PVCs originating in the PS, ∼68% were located near Purkinje-ventricular junctions (<1 mm). SCR events overcome source-sink mismatch to trigger PVCs at a critical CaSR threshold. Above this threshold, PVCs emerge due to increased probability and reduced variability in timing of SCR events, leading to significant diastolic depolarization. Sites of lower electronic load, such as the PS, are preferential locations for triggering.

A leap(frog) forward in understanding focal arrhythmia.

A leap(frog) forward in understanding focal arrhythmia.

Synchronous systolic subcellular Ca2+-elevations underlie ventricular arrhythmia in drug-induced long QT type 2.

Repolarization delay is a common clinical problem, which can promote ventricular arrhythmias. In myocytes, abnormal sarcoplasmic reticulum Ca(2+)-release is proposed as the mechanism that causes early afterdepolarizations, the cellular equivalent of ectopic-activity in drug-induced long-QT syndrome. A crucial missing link is how such a stochastic process can overcome the source-sink mismatch to depolarize sufficient ventricular tissue to initiate arrhythmias. Optical maps of action potentials and Ca(2+)-transients from Langendorff rabbit hearts were measured at low (150×150 μm(2)/pixel) and high (1.5×1.5 μm(2)/pixel) resolution before and during arrhythmias. Drug-induced long QT type 2, elicited with dofetilide inhibition of IKr (the rapid component of rectifying K+ current), produced spontaneous Ca(2+)-elevations during diastole and systole, before the onset of arrhythmias. Diastolic Ca(2+-)waves appeared randomly, propagated within individual myocytes, were out-of-phase with adjacent myocytes, and often died-out. Systolic secondary Ca(2+-)elevations were synchronous within individual myocytes, appeared 188±30 ms after the action potential-upstroke, occurred during high cytosolic Ca(2+) (40%-60% of peak-Ca(2+)-transients), appeared first in small islands (0.5×0.5 mm(2)) that enlarged and spread throughout the epicardium. Synchronous systolic Ca(2+-)elevations preceded voltage-depolarizations (9.2±5 ms; n=5) and produced pronounced Spatial Heterogeneities of Ca(2+)-transient-durations and action potential-durations. Early afterdepolarizations originating from sites with the steepest gradients of membrane-potential propagated and initiated arrhythmias. Interestingly, more complex subcellular Ca(2+)-dynamics (multiple chaotic Ca(2+)-waves) occurred during arrhythmias. K201, a ryanodine receptor stabilizer, eliminated Ca(2+)-elevations and arrhythmias. The results indicate that systolic and diastolic Ca(2+)-elevations emanate from sarcoplasmic reticulum Ca(2+)-release and systolic Ca(2+)-elevations are synchronous because of high cytosolic and luminal-sarcoplasmic reticulum Ca(2+), which overcomes source-sink mismatch to trigger arrhythmias in intact hearts.

Remodeling of cardiac passive electrical properties and susceptibility to ventricular and atrial arrhythmias.

Coordinated electrical activation of the heart is essential for the maintenance of a regular cardiac rhythm and effective contractions. Action potentials spread from one cell to the next via gap junction channels. Because of the elongated shape of cardiomyocytes, longitudinal resistivity is lower than transverse resistivity causing electrical anisotropy. Moreover, non-uniformity is created by clustering of gap junction channels at cell poles and by non-excitable structures such as collagenous strands, vessels or fibroblasts. Structural changes in cardiac disease often affect passive electrical properties by increasing non-uniformity and altering anisotropy. This disturbs normal electrical impulse propagation and is, consequently, a substrate for arrhythmia. However, to investigate how these structural changes lead to arrhythmias remains a challenge. One important mechanism, which may both cause and prevent arrhythmia, is the mismatch between current sources and sinks. Propagation of the electrical impulse requires a sufficient source of depolarizing current. In the case of a mismatch, the activated tissue (source) is not able to deliver enough depolarizing current to trigger an action potential in the non-activated tissue (sink). This eventually leads to conduction block. It has been suggested that in this situation a balanced geometrical distribution of gap junctions and reduced gap junction conductance may allow successful propagation. In contrast, source-sink mismatch can prevent spontaneous arrhythmogenic activity in a small number of cells from spreading over the ventricle, especially if gap junction conductance is enhanced. Beside gap junctions, cell geometry and non-cellular structures strongly modulate arrhythmogenic mechanisms. The present review elucidates these and other implications of passive electrical properties for cardiac rhythm and arrhythmogenesis.

Sodium Current Reduction Unmasks a Structure-Dependent Substrate for Arrhythmogenesis in the Normal Ventricles

BackgroundOrgan-scale arrhythmogenic consequences of source-sink mismatch caused by impaired excitability remain unknown, hindering the understanding of pathophysiology in disease states like Brugada syndrome and ischemia.ObjectiveWe sought to determine whether sodium current (INa) reduction in the structurally normal heart unmasks a regionally heterogeneous substrate for the induction of sustained arrhythmia by premature ventricular contractions (PVCs).MethodsWe conducted simulations in rabbit ventricular computer models with 930 unique combinations of PVC location (10 sites) and coupling interval (250–400 ms), INa reduction (30 or 40% of normal levels), and post-PVC sinus rhythm (arrested or persistent). Geometric characteristics and source-sink mismatch were quantitatively analyzed by calculating ventricular wall thickness and a newly formulated 3D safety factor (SF), respectively.ResultsReducing INa to 30% of its normal level created a substrate for sustained arrhythmia induction by establishing large regions of critical source-sink mismatch (SF<1) for ectopic wavefronts propagating from thin to thick tissue. In the same simulations but with 40% of normal INa, PVCs did not induce reentry because the volume of tissue with SF<1 was >95% smaller. Likewise, when post-PVC sinus activations were persistent instead of arrested, no ectopic excitations initiated sustained reentry because sinus activation breakthroughs engulfed the excitable gap.ConclusionOur new SF formulation can quantify ectopic wavefront propagation robustness in geometrically complex 3D tissue with impaired excitability. This novel methodology was applied to show that INa reduction precipitates source-sink mismatch, creating a potent substrate for sustained arrhythmia induction by PVCs originating near regions of ventricular wall expansion, such as the RV outflow tract.

Localising re-entry in atrial fibrillation: Anatomical clues to the substrate of rotors

Atrial fibrillation (AF) is the most common sustained cardiac arrhythmia in humans. Although AF prevalence is lower in Asian populations ( 1%) than in Caucasians, the absolute numbers of patients in Asia suggest that the overall disease burden attributable to AF may eclipse that found in Western countries [1]. To combat this surging regional AF epidemic, new strategies resting on a more detailed understanding of the AF pathophysiology will be needed. A fundamental challenge in the field is that arrhythmia dynamics during AF remain unclear, despite over a century of research [2]. AF has been hypothesised to involve a combination of local drivers, including re-entrant circuits known as rotors and focal sources, or due to multiple simultaneous re-entrant wavelets, with a related postulated role for endo-epicardial wavefront dissociation [3]. It is clear that AF arises in the context of electrophysiological and structural remodelling occurring in the context of clinical risk factors including aging, heart failure, valvular heart disease, sinus node disease, hypertension, obesity and obstructive sleep apnoea. These factors are known to modulate local conduction velocity, refractoriness and heterogeneity to create the critical substrate for AF arrhythmogenesis. Recently, increasing attention has been paid to the role of detailed muscular architecture of the atrium in providing the substrate for AF re-entrant circuits. The complex interlacing myoarchitecture of the atrium, whose detailed structure was first outlined by Papez [4], may provide a functional substrate for AF, even in the absence of disease. Specific attention has been focused on the histoanatomy of the pulmonary vein (PV)–posterior left atrium (LA) junction. Klos et al. identified source–sink mismatch at the PV–LA junction as a possible mechanism for wavebreak at the onset of AF in the sheep heart [5]. Simulation studies have further delineated the 3-dimensional nature of myofiber orientation as a critical player. In humans, the PV–LA junction and posterior LA have similarly been determined as locations for conduction delay and block, providing an intrinsic histologically pre-determined substrate for re-entry [6]. It is in this context, that the recent study of Matsuyama et al. sheds further light on the relationships between histoanatomy and atrial arrhythmogenesis [7]. This investigation used high-density optical mapping to study atrial flutter/atrial fibrillation in isolated rat hearts [7]. Atrial arrhythmia was induced in 15 of 19 hearts tested by programmed stimulation from the right atrium [7]. Atrial arrhythmia in this model was predominantly initiated by a common re-entrant mechanism, with conduction slowing and block at the left atrial roof, with rapid antegrade conduction along the coronary sinus, providing the basis for a re-entry on the posterior wall of the LA. For the first time, these findings were correlated with the intrinsic histology of the regions [7]. Myocyte density was reduced and fibrosis increased on the roof and posterior LA compared to the CS [7]. Myocytes in the LA roof had a broader distribution of cut lengths than in the CS, with more lateralised expression on

Principles of Cardiac Electric Propagation and Their Implications for Re-entrant Arrhythmias

The study of clinical electrophysiology essentially comprises examining how electric excitation develops and spreads through the millions of cells that constitute the heart. Given the enormous number of cells in a human heart, there is an extremely large number of possible ways that the heart can behave. We encounter rhythms across the spectrum from the organized and orderly behavior of sinus rhythm through repetitive continuous excitation (via reentry) in structurally defined circuits like atrial flutter and, finally, the complex, dynamic, and disorganized behavior of fibrillation. Despite these myriad possibilities, one can apply a basic understanding of the principles of propagation to predict how cardiac tissue will behave under varied circumstances and in response to various manipulations. In this article, we review the principles of propagation and how these can be used to understand reentry of all degrees of complexity. We use these principles to explain the mechanisms by which antiarrhythmic medications and ablation can terminate and prevent reentry. This article is not intended to be an exhaustive description of the physiology of cardiac propagation, rather, it is meant to capture the essence of propagation with sufficient detail to provide an intuitive feel for the interplay of the physiological features relevant to propagation. The figures and videos used in this article were created using a computational model of cardiac propagation (VisibleEP LLC, Colchester, VT). It is a hybrid between a physics-based and cellular automaton model. The model incorporates the fundamental features of propagation without modeling individual ion channels.1 The model manifests several relevant emergent properties, for example, electrotonic interactions, restitution of action potential duration, and conduction velocity as well as source–sink balance–dependent propagation. ### Cell Excitation A cell becomes excited when the balance of inward and outward currents passes a critical point after which inward currents exceed outward and an action potential ensues. …

Bradycardia alters Ca2+dynamics enhancing dispersion of repolarization and arrhythmia risk

Bradycardia prolongs action potential (AP) durations (APD adaptation), enhances dispersion of repolarization (DOR), and promotes tachyarrhythmias. Yet, the mechanisms responsible for enhanced DOR and tachyarrhythmias remain largely unexplored. Ca(2+) transients and APs were measured optically from Langendorff rabbit hearts at high (150 × 150 μm(2)) or low (1.5 × 1.5 cm(2)) magnification while pacing at a physiological (120 beats/min) or a slow heart rate (SHR = 50 beats/min). Western blots and pharmacological interventions were used to elucidate the regional effects of bradycardia. As a result, bradycardia (SHR 50 beats/min) increased APDs gradually (time constant τf→s = 48 ± 9.2 s) and caused a secondary Ca(2+) release (SCR) from the sarcoplasmic reticulum during AP plateaus, occurring at the base on average of 184.4 ± 9.7 ms after the Ca(2+) transient upstroke. In subcellular imaging, SCRs were temporally synchronous and spatially homogeneous within myocytes. In diastole, SHR elicited variable asynchronous sarcoplasmic reticulum Ca(2+) release events leading to subcellular Ca(2+) waves, detectable only at high magnification. SCR was regionally heterogeneous, correlated with APD prolongation (P < 0.01, n = 5), enhanced DOR (r = 0.9277 ± 0.03, n = 7), and was gradually reversed by pacing at 120 beats/min along with APD shortening (P < 0.05, n = 5). A stabilizer of leaky ryanodine receptors (RyR2), 3-(4-benzylcyclohexyl)-1-(7-methoxy-2,3-dihydrobenzo[f][1,4]thiazepin-4(5H)-yl)propan-1-one (K201; 1 μM), suppressed SCR and reduced APD at the base, thereby reducing DOR (P < 0.02, n = 5). Ventricular ectopy induced by bradycardia (n = 5/15) was suppressed by K201. Western blot analysis revealed spatial differences of voltage-gated L-type Ca(2+) channel protein (Cav1.2α), Na(+)-Ca(2+) exchange (NCX1), voltage-gated Na(+) channel (Nav1.5), and rabbit ether-a-go-go-related (rERG) protein [but not RyR2 or sarcoplasmic reticulum Ca(2+) ATPase 2a] that correlate with the SCR distribution and explain the molecular basis for SCR heterogeneities. In conclusion, acute bradycardia elicits synchronized subcellular SCRs of sufficient magnitude to overcome the source-sink mismatch and to promote afterdepolarizations.

Slow Calcium-Depolarization-Calcium waves may initiate fast local depolarization waves in ventricular tissue

Intercellular calcium waves in cardiac myocytes are a well-recognized, if incompletely understood, phenomenon. In a variety of preparations, investigators have reported multi-cellular calcium waves or triggered propagated contractions, but the mechanisms of propagation and pathological importance of these events remain unclear. Here, we review existing experimental data and present a computational approach to investigate the mechanisms of multi-cellular calcium wave propagation.Over the past 50 years, the standard modeling paradigm for excitable cardiac tissue has seen increasingly detailed models of the dynamics of individual cells coupled in tissue solely by intercellular and interstitial current flow. Although very successful, this modeling regime has been unable to capture two important phenomena: 1) the slow intercellular calcium waves observed experimentally, and 2) how intercellular calcium events resulting in delayed after depolarizations at the cellular level could overcome a source-sink mismatch to initiate depolarization waves in tissue. In this paper, we introduce a mathematical model with subcellular spatial resolution, in which we allow both inter- and intracellular current flow and calcium diffusion. In simulations of coupled cells employing this model, we observe: a) slow inter-cellular calcium waves propagating at about 0.1 mm/s, b) faster Calcium-Depolarization-Calcium (CDC) waves, traveling at about 1 mm/s, and c) CDC-waves that can set off fast depolarization-waves (50 cm/s) in tissue with varying gap-junction conductivity.

110 Local β-adrenergic stimulation overcomes source-sink mismatch to generate focal arrhythmia

Backgroundβ-Adrenergic receptor (β-AR) stimulation produces sarcoplasmic reticulum (SR) Ca2+ overload and delayed after-depolarisations (DADs) in isolated ventricular myocytes. However, in the intact heart, strong electrotonic coupling means that depolarisation from...

Local β-Adrenergic Stimulation Overcomes Source-Sink Mismatch to Generate Focal Arrhythmia

β-Adrenergic receptor stimulation produces sarcoplasmic reticulum Ca(2+) overload and delayed afterdepolarizations in isolated ventricular myocytes. How delayed afterdepolarizations are synchronized to overcome the source-sink mismatch and produce focal arrhythmia in the intact heart remains unknown. To determine whether local β-adrenergic receptor stimulation produces spatiotemporal synchronization of delayed afterdepolarizations and to examine the effects of tissue geometry and cell-cell coupling on the induction of focal arrhythmia. Simultaneous optical mapping of transmembrane potential and Ca(2+) transients was performed in normal rabbit hearts during subepicardial injections (50 μL) of norepinephrine (NE) or control (normal Tyrode's solution). Local NE produced premature ventricular complexes (PVCs) from the injection site that were dose-dependent (low-dose [30-60 μmol/L], 0.45±0.62 PVCs per injection; high-dose [125-250 μmol/L], 1.33±1.46 PVCs per injection; P<0.0001) and were inhibited by propranolol. NE-induced PVCs exhibited abnormal voltage-Ca(2+) delay at the initiation site and were inhibited by either sarcoplasmic/endoplasmic reticulum Ca(2+)-ATPase inhibition or reduced perfusate [Ca(2+)], which indicates a Ca(2+)-mediated mechanism. NE-induced PVCs were more common at right ventricular than at left ventricular sites (1.48±1.50 versus 0.55±0.89, P<0.01), and this was unchanged after chemical ablation of endocardial Purkinje fibers, which suggests that source-sink interactions may contribute to the greater propensity to right ventricular PVCs. Partial gap junction uncoupling with carbenoxolone (25 μmol/L) increased focal activity (2.18±1.43 versus 1.33±1.46 PVCs per injection, P<0.05), which further supports source-sink balance as a critical mediator of Ca(2+)-induced PVCs. These data provide the first experimental demonstration that localized β-adrenergic receptor stimulation produces spatiotemporal synchronization of sarcoplasmic reticulum Ca(2+) overload and release in the intact heart and highlight the critical nature of source-sink balance in initiating focal arrhythmias.

Action Potential Duration Maps in Homogeneous Cardiac Ventricular Tissue Slices are Independent of Changes in Stimulation Site

Cardiac slices are an increasingly popular model system for cardiac electrophysiology research, as they combine ease of handling with patho-physiologically relevant cell-type representation and distribution. The revival of the cardiac slice technique, first established in the 1980's1 has been linked to improved techniques for preparation and monitoring. It now offers a well-posed target for computational modelling of cardiac structure-function interrelations.This study used slices from the New Zealand white rabbits (female, ∼1kg, n=12), cut tangentially to the left ventricular wall surface (350um thick, ∼1x2cm large), offering a simplified ‘pseudo-2D’ experimental model. Slices, loaded with Di-4-ANBDQPQ (voltage sensitive dye; 10uM) to optically monitor action potential duration (APD), were stimulated at four different sites with concentric-bipolar point-electrodes, using a biphasic rectangular pulse 50% above activation threshold and, for comparison, with field stimulation.APD at 50% and 80% repolarization (APD50 and APD80, respectively), was used to establish APD-maps in each slice. Locally-resolved average APD patterns were obtained for pacing frequencies of 1-4Hz. In addition, at each slice location the largest (and smallest) APD-values were selected to create a maximum (and minimum) APD-map. This illustrated the variability-range per slice which, as an example for APD80 at 2Hz, was 196.37ms±6.59ms (maximum; mean±SD), 187.00ms±6.57ms (minimum), and 191.69ms±5.44ms (average). Interestingly, even though the spread of activation differed significantly for the different stimulus sites, resulting APD-maps were comparable for all pacing rates in homogeneous slices (i.e. in the absence of pronounced source-sink mismatches).Thus, APD heterogeneity in cardiac tissue slices shows a reproducible intrinsic pattern that prevails regardless of acute changes in stimulation site and frequency.Claycomb. J Biol Chem. 1976/251:P6082-P6089.

Oxidized CaMKII: a “heart stopper” for the sinus node?

Each normal heart beat is triggered by an electrical impulse emitted from a group of specialized cardiomyocytes that together form the sinoatrial node (SAN). In this issue of the JCI, Swaminathan and colleagues demonstrate a new molecular mechanism that can disrupt the normal beating of the heart: angiotensin II - typically found in increased levels in heart failure and hypertension - oxidizes and activates Ca2+/calmodulin-dependent kinase II via NADPH oxidase activation, causing SAN cell death. The loss of SAN cells produces an electrical imbalance termed the "source-sink mismatch," which may contribute to the SAN dysfunction that affects millions of people later in life and complicates a number of heart diseases.

Arrhythmogenesis in Brugada Syndrome: Role of Ventricular Structure

Brugada syndrome is a genetic disorder that results in decreased expression of cardiac Na+ channels, leading to abnormal electrical activity with onset in the right ventricle (RV). Given that INa is reduced globally, it remains unclear why the RV is more susceptible to arrhythmogenesis. This study tests the hypothesis that differences in geometry between left ventricle (LV) and RV promote altered conduction patterns under reduced INa. Using a 3D rabbit ventricular model that incorporates realistic geometry, simulations were performed where INa maximal conductance was varied from 60 to 100% of the normal level. Reentry was induced via diastolic stimulation of the LV or RV epicardium. Sustained reentry developed only at the 60% INa level for RV stimulation, while no reentry was induced for the LV regardless of INa levels. Activation map (figure) for the 60% INa model shows conduction block at the RV insertion region, where the safety factor (the ratio of total intracellular current vs. current needed to excite the cell) was zero. Propagation from the thin RV has insufficient current to excite tissue in the septum/LV, resulting in source-sink mismatch and block, thus predisposing the RV to arrhythmogenesis under reduced INa.

Early afterdepolarizations and cardiac arrhythmias

Early afterdepolarizations (EADs) are an important cause of lethal ventricular arrhythmias in long QT syndromes and heart failure, but the mechanisms by which EADs at the cellular scale cause arrhythmias such as polymorphic ventricular tachycardia (PVT) and torsades de pointes (TdP) at the tissue scale are not well understood. Here we summarize recent progress in this area, discussing (1) the ionic basis of EADs, (2) evidence that deterministic chaos underlies the irregular behavior of EADs, (3) mechanisms by which chaotic EADs synchronize in large numbers of coupled cells in tissue to overcome source-sink mismatches, (4) how this synchronization process allows EADs to initiate triggers and generate mixed focal reentrant ventricular arrhythmias underlying PVT and TdP, and (5) therapeutic implications.

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.