Slow Pacing Rates Research Articles

Enhanced Depolarization Drive in Failing Rabbit Ventricular Myocytes: Calcium-Dependent and β-Adrenergic Effects on Late Sodium, L-Type Calcium, and Sodium-Calcium Exchange Currents.

Heart failure (HF) is characterized by electrophysiological remodeling resulting in increased risk of cardiac arrhythmias. Previous reports suggest that elevated inward ionic currents in HF promote action potential (AP) prolongation, increased short-term variability of AP repolarization, and delayed afterdepolarizations. However, the underlying changes in late Na+ current (INaL), L-type Ca2+ current, and NCX (Na+/Ca2+ exchanger) current are often measured in nonphysiological conditions (square-pulse voltage clamp, slow pacing rates, exogenous Ca2+ buffers). We measured the major inward currents and their Ca2+- and β-adrenergic dependence under physiological AP clamp in rabbit ventricular myocytes in chronic pressure/volume overload-induced HF (versus age-matched control). AP duration and short-term variability of AP repolarization were increased in HF, and importantly, inhibition of INaL decreased both parameters to the control level. INaL was slightly increased in HF versus control even when intracellular Ca2+ was strongly buffered. But under physiological AP clamp with normal Ca2+ cycling, INaL was markedly upregulated in HF versus control (dependent largely on CaMKII [Ca2+/calmodulin-dependent protein kinase II] activity). β-Adrenergic stimulation (often elevated in HF) further enhanced INaL. L-type Ca2+ current was decreased in HF when Ca2+ was buffered, but CaMKII-mediated Ca2+-dependent facilitation upregulated physiological L-type Ca2+ current to the control level. Furthermore, L-type Ca2+ current response to β-adrenergic stimulation was significantly attenuated in HF. Inward NCX current was upregulated at phase 3 of AP in HF when assessed by combining experimental data and computational modeling. Our results suggest that CaMKII-dependent upregulation of INaL in HF significantly contributes to AP prolongation and increased short-term variability of AP repolarization, which may lead to increased arrhythmia propensity, and is further exacerbated by adrenergic stress.

Exposure to Secondhand Smoke and Arrhythmogenic Cardiac Alternans in a Mouse Model.

Background:Epidemiological evidence suggests that a majority of deaths attributed to secondhand smoke (SHS) exposure are cardiovascular related. However, to our knowledge, the impact of SHS on cardiac electrophysiology, handling, and arrhythmia risk has not been studied.Objectives:The purpose of this study was to investigate the impact of an environmentally relevant concentration of SHS on cardiac electrophysiology and indicators of arrhythmia.Methods:Male C57BL/6 mice were exposed to SHS [total suspended particles (THS): , nicotine: , carbon monoxide: , or filtered air (FA) for 4, 8, or 12 wk (]. Hearts were excised and Langendorff perfused for dual optical mapping with voltage- and -sensitive dyes.Results:At slow pacing rates, SHS exposure did not alter baseline electrophysiological parameters. With increasing pacing frequency, action potential duration (APD), and intracellular alternans magnitude progressively increased in all groups. At 4 and 8 wk, there were no statistical differences in APD or alternans magnitude between SHS and FA groups. At 12 wk, both APD and alternans magnitude were significantly increased in the SHS compared to FA group (). SHS exposure did not impact the time constant of transient decay () at any exposure time point. At 12 wk exposure, the recovery of transient amplitude with premature stimuli was slightly (but nonsignificantly) delayed in SHS compared to FA hearts, suggesting that release via ryanodine receptors may be impaired.Conclusions:In male mice, chronic exposure to SHS at levels relevant to social situations in humans increased their susceptibility to cardiac alternans, a known precursor to ventricular arrhythmia. https://doi.org/10.1289/EHP3664

Restitution characteristics of His bundle and working myocardium in isolated rabbit hearts.

The Purkinje system (PS) and the His bundle have been recently implicated as an important driver of the rapid activation rate after 1–2 minutes of ventricular fibrillation (VF). It is unknown whether activations during VF propagate through the His-Purkinje system to other portions of the the working myocardium (WM). Little is known about restitution characteristic differences between the His bundle and working myocardium at short cycle lengths. In this study, rabbit hearts (n = 9) were isolated, Langendorff-perfused, and electromechanically uncoupled with blebbistatin (10 μM). Pacing pulses were delivered directly to the His bundle. By using standard glass microelectrodes, action potentials duration (APD) from the His bundle and WM were obtained simultaneously over a wide range of stimulation cycle lengths (CL). The global F-test indicated that the two restitution curves of the His bundle and the WM are statistically significantly different (P<0.05). Also, the APD of the His bundle was significantly shorter than that of WM throughout the whole pacing course (P<0.001). The CL at which alternans developed in the His bundle vs. the WM were shorter for the His bundle (134.2±13.1ms vs. 148.3±13.3ms, P<0.01) and 2:1 block developed at a shorter CL in the His bundle than in WM (130.0±10.0 vs. 145.6±14.2ms, P<0.01). The His bundle APD was significantly shorter than that of WM under both slow and rapid pacing rates, which suggest that there may be an excitable gap during VF and that the His bundle may conduct wavefronts from one bundle branch to the other at short cycle lengths and during VF.

Revealing the Concealed Nature of Long-QT Type 3 Syndrome.

Gain-of-function mutations in the voltage-gated sodium channel (Nav1.5) are associated with the long-QT-3 (LQT3) syndrome. Nav1.5 is densely expressed at the intercalated disk, and narrow intercellular separation can modulate cell-to-cell coupling via extracellular electric fields and depletion of local sodium ion nanodomains. Models predict that significantly decreasing intercellular cleft widths slows conduction because of reduced sodium current driving force, termed "self-attenuation." We tested the novel hypothesis that self-attenuation can "mask" the LQT3 phenotype by reducing the driving force and late sodium current that produces early afterdepolarizations (EADs). Acute interstitial edema was used to increase intercellular cleft width in isolated guinea pig heart experiments. In a drug-induced LQT3 model, acute interstitial edema exacerbated action potential duration prolongation and produced EADs, in particular, at slow pacing rates. In a computational cardiac tissue model incorporating extracellular electric field coupling, intercellular cleft sodium nanodomains, and LQT3-associated mutant channels, myocytes produced EADs for wide intercellular clefts, whereas for narrow clefts, EADs were suppressed. For both wide and narrow clefts, mutant channels were incompletely inactivated. However, for narrow clefts, late sodium current was reduced via self-attenuation, a protective negative feedback mechanism, masking EADs. We demonstrated a novel mechanism leading to the concealing and revealing of EADs in LQT3 models. Simulations predict that this mechanism may operate independent of the specific mutation, suggesting that future therapies could target intercellular cleft separation as a compliment or alternative to sodium channels.

The Contribution of Ionic Currents to Rate-Dependent Action Potential Duration and Pattern of Reentry in a Mathematical Model of Human Atrial Fibrillation.

Persistent atrial fibrillation (PeAF) in humans is characterized by shortening of action potential duration (APD) and attenuation of APD rate-adaptation. However, the quantitative influences of particular ionic current alterations on rate-dependent APD changes, and effects on patterns of reentry in atrial tissue, have not been systematically investigated. Using mathematical models of human atrial cells and tissue and performing parameter sensitivity analysis, we evaluated the quantitative contributions to action potential (AP) shortening and APD rate-adaptation of ionic current remodeling seen with PeAF. Ionic remodeling in PeAF was simulated by reducing L-type Ca2+ channel current (ICaL), increasing inward rectifier K+ current (IK1) and modulating five other ionic currents. Parameter sensitivity analysis, which quantified how each ionic current influenced APD in control and PeAF conditions, identified interesting results, including a negative effect of Na+/Ca2+ exchange on APD only in the PeAF condition. At high pacing rate (2 Hz), electrical remodeling in IK1 alone accounts for the APD reduction of PeAF, but at slow pacing rate (0.5 Hz) both electrical remodeling in ICaL alone (-70%) and IK1 alone (+100%) contribute equally to the APD reduction. Furthermore, AP rate-adaptation was affected by IKur in control and by INaCa in the PeAF condition. In a 2D tissue model, a large reduction (-70%) of ICaL becomes a dominant factor leading to a stable spiral wave in PeAF. Our study provides a quantitative and unifying understanding of the roles of ionic current remodeling in determining rate-dependent APD changes at the cellular level and spatial reentry patterns in tissue.

Atrial Early Afterdepolarization: An Emerging Property of the Fibrotic Atria?

Introduction: Early afterdepolarizations (EADs) interrupt and delay repolarization of the cardiac action potential. In the ventricular myocardium, EADs are triggers of torsades de pointes, ventricular tachycardia and fibrillation; in the pulmonary vein sleeves of the left atrium, EADs may contribute to the initiation of atrial fibrillation (AF). While EADs can be readily induced in isolated ventricular and pulmonary vein myocytes, EADs are not readily observed in (non-pulmonary-vein) atrial myocytes. Since fibrosis can promote AF at sites remote from the pulmonary veins, we investigated how fibroblast coupling to atrial myocytes affects the emergence of EADs and AF.Methods and Results: When patch-clamped myocytes from young rabbit hearts were exposed to hypokalemia (2.7mmol/L) and isoproterenol (1-2 μmol/L), EADs readily occurred in ventricular myocytes at slow pacing rates but not in atrial myocytes. However, when those patch-clamped myocytes were coupled to virtual fibroblasts using the dynamic clamp technique, the resultant gap junction current promoted EADs in ventricular myocytes at faster pacing rates and in atrial myocytes at both slow and fast pacing rates. While the early Ito-like gap junction current component was important for EAD genesis in ventricular myocytes, the late gap junction current component was critical for EAD genesis in atrial myocytes. In intact rabbit atria exposed to the same stressors and studied with optical mapping of voltage, spontaneous EADs inducing atrial fibrillation arose from non-pulmonary-vein areas when atrial fibrosis exceeded 10% on subsequent trichrome staining.Conclusion: In response to stressors that prolong repolarization, ventricular myocytes readily develop EADs whereas atrial myocytes are resistant. When coupled to fibroblasts, however, atrial myocytes also become susceptible to EADs. Our findings highlight the differences between ventricular and atrial myocytes and provide a potential mechanism linking atrial fibrosis to the emergence of non-pulmonary-vein triggers of AF.

Atria are More Sensitive Than Ventricles to GS-458967-Induced Inhibition of Late Sodium Current.

The differential response of atrial and ventricular cells to late sodium channel current (late INa) inhibition has not been thoroughly investigated. The aim of the present study was to compare the atrioventricular differences in electrophysiological actions of GS-458967, a potent late INa blocker. Canine coronary-perfused atrial and ventricular preparations and isolated ventricular myocytes were used. Transmembrane action potentials were recorded using standard microelectrode recording techniques. In coronary-perfused preparations paced at a cycle length (CL) of 500 ms, GS-458967 (100-300 nmol/L) significantly abbreviated action potential duration at 50% to 90% (APD50-90) in atria but not in the ventricles. GS-458967 (≥100 nmol/L) prolonged the effective refractory period (ERP) in atria due to the development of postrepolarization refractoriness (PRR) but did not alter ERP in the ventricles. The maximum rate of rise in the action potential upstroke (Vmax) was significantly reduced at concentrations ≥100 nmol/L in atria but not in the ventricles (CL = 300 ms). At slower pacing rates (CL = 2000 ms) and higher concentrations, GS-458967 (100-1000 nmol/L) still failed to abbreviate ventricular APD. However, when APD was prolonged by the rapidly activating delayed rectifier potassium channel blocker E-4031 (1 µmol/L), addition of 1 μmol/L GS-458967 abbreviated APD in the ventricles at slow rates. In contrast, GS-458967 (300 nmol/L) consistently abbreviated APD in untreated isolated ventricular myocytes. In canine coronary-perfused preparations, GS-458967 abbreviates APD, induces PRR, and reduces Vmax in atria but has no significant effect on these parameters in the ventricles, indicating an atrial-selective effect of GS-458967 on both peak and late INa-mediated parameters. In multicellular preparations, GS-458967 abbreviated ventricular APD only under long QT conditions, suggesting a pathology-specific action of GS-458967 in canine ventricular myocardium.

Dependency of Ca2+ Alternans on Ion Channel Localization in Human Atrial Cells

Introduction: Localization of ion channels to t-tubules (TTs) comprises a critical part of excitation-contraction coupling in cardiomyocytes. Recent studies have shown that TT organization varies greatly among atrial cells, which have different ion channel distribution than ventricular cells. Ca2+ transient (CaT) alternans are known to occur in atrial cells with fewer TTs, but the ion channels which contribute to this are unknown. Hypothesis: Decreased localization of L-type Ca2+ channels (LTCCs) and ryanodine receptors (RyRs) to TTs in atrial cells results in increased propensity for arrhythmogenic CaT alternans. Methods: Using a human atrial cell model from Grandi et al. incorporating chronic AF remodeling, the effects of ion channel redistribution were evaluated by moving 0-100% of TT ionic currents to the sub-sarcolemmal compartment (SL). Ryanodine receptor (RyR) redistribution was also tested by moving 0-100% of RyR release from the TTs to the cytosol. In 500 virtual cells, a total of 12 different ion channels were redistributed stochastically and independently. The effect of each ion channel on alternans threshold cycle length (CL) was evaluated by regression analysis. Results: The alternans threshold CL in the original model was 431 ms. Redistribution of LTCCs from TTs to the SL had the greatest effect on alternans threshold CL (regression coefficient of −0.95), with alternans occurring at slower pacing rates as the fraction of TT-localized LTCCs decreased due to steepening of the SR Ca2+ release slope. Redistribution of RyRs or Na+/Ca2+ exchangers had a smaller and opposite effect on alternans threshold CL (regression coefficients of 0.16 and 0.072, respectively), with decreased TT localization of these channels causing alternans to only occur at faster pacing rates. Conclusions: Decreased localization of LTCCs to TTs in atrial cells promotes arrhythmogenic CaT alternans at slower pacing rates.

Disrupted calcium release as a mechanism for atrial alternans associated with human atrial fibrillation.

Atrial fibrillation (AF) is the most common cardiac arrhythmia, but our knowledge of the arrhythmogenic substrate is incomplete. Alternans, the beat-to-beat alternation in the shape of cardiac electrical signals, typically occurs at fast heart rates and leads to arrhythmia. However, atrial alternans have been observed at slower pacing rates in AF patients than in controls, suggesting that increased vulnerability to arrhythmia in AF patients may be due to the proarrythmic influence of alternans at these slower rates. As such, alternans may present a useful therapeutic target for the treatment and prevention of AF, but the mechanism underlying alternans occurrence in AF patients at heart rates near rest is unknown. The goal of this study was to determine how cellular changes that occur in human AF affect the appearance of alternans at heart rates near rest. To achieve this, we developed a computational model of human atrial tissue incorporating electrophysiological remodeling associated with chronic AF (cAF) and performed parameter sensitivity analysis of ionic model parameters to determine which cellular changes led to alternans. Of the 20 parameters tested, only decreasing the ryanodine receptor (RyR) inactivation rate constant (kiCa) produced action potential duration (APD) alternans seen clinically at slower pacing rates. Using single-cell clamps of voltage, fluxes, and state variables, we determined that alternans onset was Ca2+-driven rather than voltage-driven and occurred as a result of decreased RyR inactivation which led to increased steepness of the sarcoplasmic reticulum (SR) Ca2+ release slope. Iterated map analysis revealed that because SR Ca2+ uptake efficiency was much higher in control atrial cells than in cAF cells, drastic reductions in kiCa were required to produce alternans at comparable pacing rates in control atrial cells. These findings suggest that RyR kinetics may play a critical role in altered Ca2+ homeostasis which drives proarrhythmic APD alternans in patients with AF.

Abstract 15280: Comparisons of Action Potentials and Ionic Currents Between Neonatal Rat Ventricular Myocytes and Adult Zebrafish Ventricular Myocytes

Introduction: Zebrafish heart is established as a model to investigate cardiac electrical abnormalities. However, electrical properties of adult zebrafish cardiomyocytes are not sufficiently characterized. Hypothesis: In this study, by comparing the electrical properties between neonatal rat ventricular myocytes (NRVMs) and adult zebrafish ventricular myocytes (AZVMs), we intended to characterize the action potential (AP), action current (AC) and the properties of Na + current ( I Na ) in AZVMs. Methods: We used patch-clamp technique to characterize the electrical properties, including AP, AC and I Na , in cultured NRVMs and freshly isolated AZVMs. Results: NVRMs showed larger AP amplitude (119±6 vs. 79±4mV, p&lt;.05) but shorter AP duration (APD 90 , 136±11 vs. 213±19 ms, p&lt;.05) than those of AZVMs. The AP duration exhibited marked frequency-dependent alterations in AZVMs. Under the slow pacing rate, early after-depolarizations (EAD) emerged under slow pacing rate with 0.05 Hz. In cell-attached voltage-clamp recordings made from AZVMs, ACs could be elicited by +10 mV steps. As the depolarization step increased to +70 mV, the latency for appearance of ACs was progressively reduced from &gt;123 ms to 9.8 ms. The presence of spontaneous ACs was monitored in spontaneously beating NRVMs and AZVMs. The AC amplitude in NRVMs was larger compared to that in AZVMs (17.3±2.1 vs. 11.6±1.1 pA, p&lt;.05), although firing frequency of AC in NRVMs is higher than in AZVMs (1.13±0.09 vs. 0.38±0.03 Hz, p&lt;.05). The lowering effect of ranolazine, a I Na antagonist, on firing frequency was significantly larger in NRVMs (1.13±0.09 to 0.31±0.02 Hz, p&lt;.05) than in AZVMs (0.38±0.03 to 0.27±0.02 Hz). There was a hyperpolarizing shift of peak I Na in AZVM compared to NRVM. Conclusions: Our results demonstrated major differences in the cellular electrical behavior between AZVMs and NRVMs.

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

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. 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. 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.

The Transient Outward Current Ito Promotes Early Afterdepolarizations

The transient outward current (Ito) plays important roles in action potential (AP) morphology and arrhythmogenesis in cardiac diseases, such as ischemia and the Brugada syndrome. It is well accepted that early afterdepolarizations (EADs) occur under conditions of reduced repolarization reserve, which can result from either increased inward currents or reduced outward currents. Here we show the novel finding that Ito, an outward current, promotes EADs in rabbit ventricular myocytes, raising the question: how does an outward current promote EADs? To answer this question, we carried out experimental studies in isolated rabbit ventricular myocytes, theoretical analysis, and computer simulations. In myocyte experiments, exposure to 0.2-1 mM H2O2 at slow pacing rates induced EADs, which were eliminated by selectively blocking Ito with 2 mM 4-aminopyridine. Pre-treating myocytes with 4-aminopyridine prolonged AP, but likewise prevented H2O2-induced EADs. Voltage-clamp experiments showed that besides promoting late ICa,L and late INa, H2O2 also increased the maximum conductance, slowed the inactivation and accelerated the recovery from inactivation of Ito. When the cells were clamped with AP morphologies corresponding to the absence and presence of Ito, Ito significantly enhanced the Ca current, promoting its reactivation as the mechanism induced EADs. In a computer model of the rabbit ventricular AP, we also showed that the presence of Ito promoted EADs. The rate of Ito inactivation played a critical role: if too fast, no EADs occurred, and if too slow, AP duration became too short and no EADs occurred either. The underlying dynamical mechanisms were revealed by bifurcation theory of EADs previously developed by our group (Tran et al, Phys. Rev. Lett. 2009; 102:258103).

IKs restricts excessive beat-to-beat variability of repolarization during beta-adrenergic receptor stimulation

In vivo studies have suggested that increased beat-to-beat variability of ventricular repolarization duration (BVR) is a better predictor of drug-induced torsades de pointes than repolarization prolongation alone. Cellular BVR and its dynamics before proarrhythmic events are poorly understood. We investigated differential responses of BVR in single myocytes during I Ks blockade versus I Kr blockade and late- I Na augmentation, under the influence of β-adrenergic receptor stimulation. Transmembrane action potentials were recorded from isolated canine left-ventricular midmyocytes at various pacing rates. I Ks was blocked by HMR1556, I Kr by dofetilide. Late I Na was augmented by sea anemone toxin-II. Isoproterenol was added for β-adrenergic receptor stimulation. BAPTA-AM buffered intracellular Ca 2+. SEA0400 partially inhibited the Na +–Ca 2+ exchanger. BVR was quantified as variability of action-potential duration at 90% repolarization: Σ(|APD90; i + 1 minus APD90; i|) / [nbeats × √ 2] for 30 consecutive action potentials. Baseline BVR was significantly increased by I Kr blockade and late- I Na augmentation, especially at slow pacing rates. β-adrenergic stimulation restabilized these BVR changes. In contrast, I Ks blockade caused very little change in repolarization when compared to baseline conditions, but predisposed the myocyte to increased BVR during β-adrenergic stimulation, especially at fast rates. BAPTA-AM and SEA0400 reduced this excessive BVR and eliminated early afterdepolarizations. In conclusion, β-adrenergic receptor stimulation exaggerates BVR during I Ks blockade, indicating a BVR-stabilizing role of β-adrenergic-sensitive I Ks. Loss of I Ks plus overriding of Ca 2+-dependent membrane currents, including inward Na +–Ca 2 + exchange current, conspire to proarrhythmic BVR under these conditions.

Ranolazine Antagonizes The Effects Of Anemone Toxin-II On Intracellular Ca2+ Cycling In Whole Heart

The late sodium current (INa,L) is pathologically enhanced in several cardiac disease states, including ischemia, causing increased intracellular Na+ and Ca2+ loading and cellular dysfunction. Ranolazine (RAN) is a blocker of INa,L and this mechanism is thought to underlie its effectiveness at reversing many of the cellular effects of ischemia. The goal of this study was to determine if RAN antagonizes the effect of anemone toxin II (ATX-II), an INa,L enhancer known to increase Na+ influx, to alter intracellular Ca2+ cycling in individual myocytes of intact heart. Langendorff-perfused rat hearts were loaded with fluo-4AM (15μM) and placed in a chamber on the stage of a confocal microscope (contractions abolished with cytochalasin-D and blebbistatin). ATX-II (1nM) prolonged the early phase[j1] of basal Ca2+ transients (CaTs) in cells of hearts paced at a rate of 2 Hz[j2]. ATX-II slowed the rate of recovery of cellular CaTs (i.e., restitution) and promoted the development of CaT alternans at slower pacing rates. RAN (10μM) partially reversed the effects of ATX on restitution and alternans, shifting both to shorter intervals[j3] . In addition, pre-treatment with RAN reduced the effects of subsequent exposure to ATX-II on both restitution and alternans development and blunted the ATX-induced changes in basal CaTs. These effects are consistent with an action of RAN to block INa,L, reducing Na+ influx and resulting intracellular Ca2+ accumulation, and therefore suggest RAN treatment may reverse the effects of Ca2+ accumulation that occur in response to disease states in which INa,L is enhanced (such as ischemia). Consequently, RAN may also reduce the arrhythmias that might result from repolarization gradients established by Ca2+ alternans and the resulting action potential duration alternans.

Simulations of propagated mouse ventricular action potentials: effects of molecular heterogeneity

The molecular heterogeneity of repolarizing currents produces significant spatial heterogeneity and/or dispersion of repolarization in many mammalian cardiac tissues. Transgenic mice are prominent experimental models for the study of the molecular basis of repolarization and arrhythmias. However, it is debated whether the small mouse heart can sustain physiologically relevant heterogeneity of repolarization. We used a comprehensive model of the mouse action potential (AP) to predict how small a region of the cardiac tissue can maintain spatial gradients of repolarization due to differential expression of channels. Our simulations of a one-dimensional multicellular ring or cable predict that substantial gradients in repolarization and intracellular Ca(2+) concentration transients can be maintained through heterogeneity of expression of K(+) channels in distances of approximately 10 cells that are sufficient to block propagation. The abruptness of expression gradients and the site of stimulation can cause Ca(2+) transient oscillations and affect the stability of Ca(2+) dynamics and AP propagation. Two different mechanisms of instability of AP propagation in one-dimensional cable occur at fast pacing rates. Transitions from periodic activity to alternans or to irregular behavior were observed. Abrupt gradients of channel expression can cause alternans at slower pacing rates than gradual changes. Our simulations demonstrate the importance of incorporating realistic Ca(2+) dynamics and current densities into models of propagated AP. They also emphasize that microscopic aspects of tissue organization are important for predicting large-scale propagation phenomena. Finally, our results predict that the mouse heart should be able to sustain substantial molecularly based heterogeneity of repolarization.

Arrhythmogenic consequences of Na + channel mutations in the transmurally heterogeneous mammalian left ventricle: Analysis of the I1768V SCN5A mutation

Congenital mutations in the cardiac Na+ channel (encoded by SCN5A) underlie long QT syndrome type 3. The sea anemone peptide toxin ATX-II mimics the slowed inactivation kinetics characteristic of many long QT type 3 (LQT3) mutations. However, the I1768V SCN5A mutation is associated with faster recovery kinetics, for which there exists no known pharmacologic equivalent. The purpose of this study was to investigate the proarrhythmic consequences of the I1768V SCN5A mutation in a transmurally heterogeneous canine left ventricular wedge. We hypothesized that amplification of intrinsic electrical heterogeneities may contribute to abnormal repolarization patterns. We developed a multiscale computational model of the canine ventricular wedge preparation that accounts for a comprehensive set of ionic currents (including transmural heterogeneities of the voltage-dependent transient outward current I(Kv43), the late sodium current I(NaL), the slowly activating delayed rectifier current I(Ks), and the sarco[endo]plasmic reticulum Ca2+-ATPase pump SERCA) and includes mechanistic descriptions of intracellular Ca2+ cycling, the effects of ATX-II, and the I1768V mutation. Experimentally observed QT intervals and rate-dependent transmural gradients in action potential duration were recapitulated in our simulations, both with and without ATX-II. With the I1768V SCN5A mutation, the model predicted endocardial early afterdepolarizations that triggered epicardial beats and R-on-T extrasystoles. Brief episodes of polymorphic, followed by sustained monomorphic, ventricular tachycardia were observed at slow pacing rates. Importantly, these arrhythmias are driven by recurring reactivation of Na+ channels localized to the endocardium. Our findings suggest that an increase in sustained inward Na+ current arising from the I1768V SCN5A mutation leads to clinically relevant arrhythmias in the transmurally heterogeneous canine left ventricular myocardium. This novel approach for simulating transmural heterogeneity provides new insight into the development of rhythm disturbances in the mammalian left ventricle.

Action Potential Duration Dispersion and Alternans in Simulated Heterogeneous Cardiac Tissue with a Structural Barrier

Structural barriers to wave propagation in cardiac tissue are associated with a decreased threshold for repolarization alternans both experimentally and clinically. Using computer simulations, we investigated the effects of a structural barrier on the onset of spatially concordant and discordant alternans. We used two-dimensional tissue geometry with heterogeneity in selected potassium conductances to mimic known apex-base gradients. Although we found that the actual onset of alternans was similar with and without the structural barrier, the increase in alternans magnitude with faster pacing was steeper with the barrier—giving the appearance of an earlier alternans onset in its presence. This is consistent with both experimental structural barrier findings and the clinical observation of T-wave alternans occurring at slower pacing rates in patients with structural heart disease. In ionically homogeneous tissue, discordant alternans induced by the presence of the structural barrier arose at intermediate pacing rates due to a source-sink mismatch behind the barrier. In heterogeneous tissue, discordant alternans occurred during fast pacing due to a barrier-induced decoupling of tissue with different restitution properties. Our results demonstrate a causal relationship between the presence of a structural barrier and increased alternans magnitude and action potential duration dispersion, which may contribute to why patients with structural heart disease are at higher risk for ventricular tachyarrhythmias.

Intracellular sodium determines frequency-dependent alterations in contractility in hypertrophied feline ventricular myocytes

Hypertrophy and failure (H/F) in humans and large mammals are characterized by a change from a positive developed force-frequency relationship (+FFR) in normal myocardium to a flattened or negative developed force-frequency relationship (-FFR) in disease. Altered Ca(2+) homeostasis underlies this process, but the role of intracellular Na(+) concentration ([Na(+)](i)) in H/F and frequency-dependent contractility reserve is unclear. We hypothesized that altered [Na(+)](i) is central to the -FFR response in H/F feline myocytes. Aortic constriction caused left ventricular hypertrophy (LVH). We found that as pacing rate was increased, contraction magnitude was maintained in isolated control myocytes (CM) but decreased in LVH myocytes (LVH-M). Quiescent LVH-M had higher [Na(+)](i) than CM (LVH-M 13.3 +/- 0.3 vs. CM 8.9 +/- 0.2 mmol/l; P < 0.001) with 0.5-Hz pacing (LVH-M 14.9 +/- 0.5 vs. CM 10.8 +/- 0.4 mmol/l; P < 0.001) but were not different at 2.5 Hz (17.0 +/- 0.7 vs. control 16.0 +/- 0.7 mmol/l; not significant). [Na(+)](i) was altered by patch pipette dialysis to define the effect of [Na(+)](i) on contraction magnitude and action potential (AP) wave shape at slow and fast pacing rates. Using AP clamp, we showed that LVH-M require increased [Na(+)](i) and long diastolic intervals to maintain normal shortening. Finally, we determined the voltage dependence of contraction for Ca(2+) current (I(Ca))-triggered and Na(+)/Ca(2+) exchanger-mediated contractions and showed that there is a greater [Na(+)](i) dependence of contractility in LVH-M. These data show that increased [Na(+)](i) is essential for maintaining contractility at slow heart rates but contributes to small contractions at fast rates unless rate-dependent AP shortening is prevented, suggesting that altered [Na(+)](i) regulation is a critical contributor to abnormal contractility in disease.

Abnormal frequency-dependent responses represent the pathophysiologic signature of contractile failure in human myocardium

Background. – The normal increase in isometric developed force (DF) with faster pacing rates, known as the positive force–frequency response/relationship (FFR), is altered in failing myocardium, as shown by its negative response to increased pacing. The objective of this study was to determine if increasing Ca 2+ influx with L-type Ca 2+ channel (L-CaCh) agonists: BayK 8644 (BayK) and FPL 64176 (FPL) or increased extracellular Ca 2+ could increase contractility and normalize the FFR in failing myocardium. Methods. – Isometric DF was measured in right ventricular trabeculae from failing ( n = 28) and non-failing ( n = 12) human hearts at various stimulation frequencies (0.5–2.5 Hz) before and after bath application of BayK (250 nM), FPL (100 nM), or high Ca 2+ (7.0 mM). Post-rest (PR) experiments were also conducted on several trabeculae. Results. – In trabeculae from failing hearts, the DF decreased with an increase in pacing. Addition of L-CaCh agonists increased DF to similar levels in trabeculae from both failing and non-failing hearts at slow pacing rates, but did not alter the negative FFR in the failing group. During increased rest intervals, the amount of PR potentiation was diminished in trabeculae from failing hearts as compared to the non-failing preparations. Conclusion. – This study demonstrates that the abnormal FFR observed in trabeculae from failing hearts is a reliable physiologic signature of the cardiomyopathic state even when DF, at slow stimulation frequencies, is relatively high. These studies further demonstrate that the impaired FFR is not due to an inability to further increase contractility. Rather, our findings suggest that the abnormal FFR and blunted PR potentiation alike are a reflection of an altered functional balance between Ca 2+ re-uptake and Ca 2+ extrusion.

Probing the contribution of IKs to canine ventricular repolarization: key role for beta-adrenergic receptor stimulation.

In large mammals and humans, the contribution of IKs to ventricular repolarization is still incompletely understood. In vivo and cellular electrophysiological experiments were conducted to study IKs in canine ventricular repolarization. In conscious dogs, administration of the selective IKs blocker HMR 1556 (3, 10, or 30 mg/kg PO) caused substantial dose-dependent QT prolongations with broad-based T waves. In isolated ventricular myocytes under baseline conditions, however, IKs block (chromanols HMR 1556 and 293B) did not significantly prolong action potential duration (APD) at fast or slow steady-state pacing rates. This was because of the limited activation of IKs in the voltage and time domains of the AP, although at seconds-long depolarizations, the current was substantial. Isoproterenol increased and accelerated IKs activation to promote APD95 shortening. This shortening was importantly reversed by HMR 1556 and 293B. Quantitatively similar effects were obtained in ventricular-tissue preparations. Finally, when cellular repolarization was impaired by IKr block, IKs block exaggerated repolarization instability with further prolongation of APD. Ventricular repolarization in conscious dogs is importantly dependent on IKs. IKs function becomes prominent during beta-adrenergic receptor stimulation, when it promotes AP shortening by increased activation, and during IKr block, when it limits repolarization instability by time-dependent activation. Unstimulated IKs does not contribute to cellular APD at baseline. These data highlight the importance of the synergism between an intact basal IKs and the sympathetic nervous system in vivo.