Cellular and subcellular alternans in the canine left ventricle

Discordant Ca2+ release in cardiac myocytes: characterization and susceptibility to pharmacological RyR2 modulation

Systolic Ca²⁺ transients are shaped by the concerted summation of Ca²⁺ sparks across cardiomyocytes. At high pacing rates, alterations of excitation-contraction coupling manifest as pro-arrhythmic Ca²⁺ alternans that can be classified as concordant or discordant. Discordance is ascribed to out-of-phase alternation of local Ca²⁺ release across the cell, although the triggers and consequences of this phenomenon remain unclear. Rat ventricular cardiomyocytes were paced at increasing rates. A discordance index (SD of local alternans ratios) was developed to quantify discordance in confocal recordings of Ca²⁺ transients. Index values were significantly increased by rapid pacing, and negatively correlated with Ca²⁺ transient amplitude change, indicating that discordance is an important contributor to the negative Ca²⁺ transient-frequency relationship. In addition, the largest local calcium transient in two consecutive transients was measured to build a potential "best release" profile, which quantitatively confirmed discordance-induced Ca²⁺ release impairment (DICRI). Diastolic Ca²⁺ homeostasis was also observed to be disrupted by discordance, as late Ca²⁺ release events elicited instability of resting Ca²⁺ levels. Finally, the effects of two RyR2 inhibitors (VK-II-86 and dantrolene) were tested. While both compounds inhibited Ca²⁺ wave generation, only VK-II-86 augmented subcellular discordance. Discordant Ca²⁺ release is a quantifiable phenomenon, sensitive to pacing frequency, and impairs both systolic and diastolic Ca²⁺ homeostasis. Interestingly, RyR2 inhibition can induce discordance, which should be considered when evaluating pharmacological RyR2 modulators for clinical use.

Inducing Ito,f and phase 1 repolarization of the cardiac action potential with a Kv4.3/KChIP2.1 bicistronic transgene

The fast transient outward potassium current (I_to,f) plays a key role in phase 1 repolarization of the human cardiac action potential (AP) and its reduction in heart failure (HF) contributes to the loss of contractility. Therefore, restoring I_to,f might be beneficial for treating HF. The coding sequence of a P2A peptide was cloned, in frame, between Kv4.3 and KChIP2.1 genes and ribosomal skipping was confirmed by Western blotting. Typical I_to,f properties with slowed inactivation and accelerated recovery from inactivation due to the association of KChIP2.1 with Kv4.3 was seen in transfected HEK293 cells. Both bicistronic components trafficked to the plasmamembrane and in adenovirus transduced rabbit cardiomyocytes both t-tubular and sarcolemmal construct labelling appeared. The resulting current was similar to I_to,f seen in human ventricular cardiomyocytes and was 50% blocked at ~0.8 mmol/l 4-aminopyridine and increased ~30% by 5 μmol/l NS5806 (an I_to,f agonist). Variation in the density of the expressed I_to,f, in rabbit cardiomyocytes recapitulated typical species-dependent variations in AP morphology. Simultaneous voltage recording and intracellular Ca²⁺ imaging showed that modification of phase 1 to a non-failing human phenotype improved the rate of rise and magnitude of the Ca²⁺ transient. I_to,f expression also reduced AP triangulation but did not affect I_Ca,L and I_Na magnitudes. This raises the possibility for a new gene-based therapeutic approach to HF based on selective phase 1 modification.

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Action potential phase 1 depends on fast transient outward current (I_to,f).

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Construction of a bicistronic transgene for Kv4.3 and KChIP2.1 with P2A separator

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Expressed bicistronic Kv4.3/KChIP2.1 proteins traffic to the cell surface membrane

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Viral transduction with Kv4.3/KChIP2.1 increases I_to,f in cardiomyocytes.

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Kv4.3/KChIP2.1 transgene expression increased AP phase 1 and EC coupling

Lehnart S, Mongillo M. Multiphoton Imaging of Ca2+ Instability in Acute Myocardial Slices from a RyR2R2474S Murine Model of Catecholaminergic Polymorphic Ventricular Tachycardia

Catecholaminergic Polymorphic Ventricular Tachycardia (CPVT) is a familial stress-induced arrhythmia syndrome, mostly caused by mutations in Ryanodine receptor 2 (RyR2), the sarcoplasmic reticulum (SR) Ca²⁺ release channel in cardiomyocytes. Pathogenetic mutations lead to gain of function in the channel, causing arrhythmias by promoting diastolic spontaneous Ca²⁺ release (SCR) from the SR and delayed afterdepolarizations. While the study of Ca²⁺ dynamics in single cells from murine CPVT models has increased our understanding of the disease pathogenesis, questions remain on the mechanisms triggering the lethal arrhythmias at tissue level. Here, we combined subcellular analysis of Ca²⁺ signals in isolated cardiomyocytes and in acute thick ventricular slices of RyR2^R2474S knock-in mice, electrically paced at different rates (1–5 Hz), to identify arrhythmogenic Ca²⁺ dynamics, from the sub- to the multicellular perspective. In both models, RyR2^R2474S cardiomyocytes had increased propensity to develop SCR upon adrenergic stimulation, which manifested, in the slices, with Ca²⁺ alternans and synchronous Ca²⁺ release events in neighboring cardiomyocytes. Analysis of Ca²⁺ dynamics in multiple cells in the tissue suggests that SCRs beget SCRs in contiguous cells, overcoming the protective electrotonic myocardial coupling, and potentially generating arrhythmia triggering foci. We suggest that intercellular interactions may underscore arrhythmic propensity in CPVT hearts with ‘leaky’ RyR2.

Ca2+-CaM Dependent Inactivation of RyR2 Underlies Ca2+ Alternans in Intact Heart

RATIONALE

Ca²⁺ alternans plays an essential role in cardiac alternans that can lead to ventricular fibrillation, but the mechanism underlying Ca²⁺ alternans remains undefined. Increasing evidence suggests that Ca²⁺ alternans results from alternations in the inactivation of cardiac RyR2 (ryanodine receptor 2). However, what inactivates RyR2 and how RyR2 inactivation leads to Ca²⁺ alternans are unknown.

OBJECTIVE

To determine the role of CaM (calmodulin) on Ca²⁺ alternans in intact working mouse hearts.

METHODS AND RESULTS

We used an in vivo local gene delivery approach to alter CaM function by directly injecting adenoviruses expressing CaM-wild type, a loss-of-function CaM mutation, CaM (1-4), and a gain-of-function mutation, CaM-M37Q, into the anterior wall of the left ventricle of RyR2 wild type or mutant mouse hearts. We monitored Ca²⁺ transients in ventricular myocytes near the adenovirus-injection sites in Langendorff-perfused intact working hearts using confocal Ca²⁺ imaging. We found that CaM-wild type and CaM-M37Q promoted Ca²⁺ alternans and prolonged Ca²⁺ transient recovery in intact RyR2 wild type and mutant hearts, whereas CaM (1-4) exerted opposite effects. Altered CaM function also affected the recovery from inactivation of the L-type Ca²⁺ current but had no significant impact on sarcoplasmic reticulum Ca²⁺ content. Furthermore, we developed a novel numerical myocyte model of Ca²⁺ alternans that incorporates Ca²⁺-CaM-dependent regulation of RyR2 and the L-type Ca²⁺ channel. Remarkably, the new model recapitulates the impact on Ca²⁺ alternans of altered CaM and RyR2 functions under 9 different experimental conditions. Our simulations reveal that diastolic cytosolic Ca²⁺ elevation as a result of rapid pacing triggers Ca²⁺-CaM dependent inactivation of RyR2. The resultant RyR2 inactivation diminishes sarcoplasmic reticulum Ca²⁺ release, which, in turn, reduces diastolic cytosolic Ca²⁺, leading to alternations in diastolic cytosolic Ca²⁺, RyR2 inactivation, and sarcoplasmic reticulum Ca²⁺ release (ie, Ca²⁺ alternans).

CONCLUSIONS

Our results demonstrate that inactivation of RyR2 by Ca²⁺-CaM is a major determinant of Ca²⁺ alternans, making Ca²⁺-CaM dependent regulation of RyR2 an important therapeutic target for cardiac alternans.

Susceptibility to Ventricular Arrhythmias Resulting from Mutations in FKBP1B, PXDNL, and SCN9A Evaluated in hiPSC Cardiomyocytes

Background

We report an inherited cardiac arrhythmia syndrome consisting of Brugada and Early Repolarization Syndrome associated with variants in SCN9A, PXDNL, and FKBP1B. The proband inherited the 3 mutations and exhibited palpitations and arrhythmia-mediated syncope, whereas the parents and sister, who carried one or two of the mutations, were asymptomatic.

Methods and Results

We assessed the functional impact of these mutations in induced pluripotent stem cell cardiomyocytes (hiPSC-CMs) derived from the proband and an unaffected family member. Current and voltage clamp recordings, as well as confocal microscopy analysis of Ca²⁺ transients, were evaluated in hiPSC-CMs from the proband and compared these results with hiPSC-CMs from undiseased controls. Genetic analysis using next-generation DNA sequencing revealed heterozygous mutations in SCN9A, PXDNL, and FKBP1B in the proband. The proband displayed right bundle branch block and exhibited episodes of syncope. The father carried a mutation in FKBP1B, whereas the mother and sister carried the SCN9A mutation. None of the 3 family members screened developed cardiac events. Action potential recordings from control hiPSC-CM showed spontaneous activity and a low upstroke velocity. In contrast, the hiPSC-CM from the proband showed irregular spontaneous activity. Confocal microscopy of the hiPSC-CM of the proband revealed low fluorescence intensity Ca²⁺ transients that were episodic in nature. Patch-clamp measurements in hiPSC-CM showed no difference in I_Na but reduced I_Ca in the proband compared with control. Coexpression of PXDNL-R391Q with SCN5A-WT displayed lower I_Na density compared to PXDNL-WT. In addition, coexpression of PXDNL-R391Q with KCND3-WT displayed significantly higher I_to density compared to PXDNL-WT.

Conclusion

SCN9A, PXDNL, and FKBP1B variants appeared to alter spontaneous activity in hiPSC-CM. Only the proband carrying all 3 mutations displayed the ERS/BrS phenotype, whereas one nor two mutations alone did not produce the clinical phenotype. Our results suggest a polygenic cause of the BrS/ERS arrhythmic phenotype due to mutations in these three gene variants caused a very significant loss of function of I_Na and I_Ca and gain of function of I_to.

Transverse cardiac slicing and optical imaging for analysis of transmural gradients in membrane potential and Ca2+ transients in murine heart

Key points

A robust cardiac slicing approach was developed for optical mapping of transmural gradients in transmembrane potential (V_m) and intracellular Ca²⁺ transient (CaT) of murine heart.

Significant transmural gradients in V_m and CaT were observed in the left ventricle.

Frequency‐dependent action potentials and CaT alternans were observed in all ventricular regions with rapid pacing, with significantly greater incidence in the endocardium than epicardium.

The observations demonstrate the feasibility of our new approach to cardiac slicing for systematic analysis of intrinsic transmural and regional gradients in V_m and CaT.

Abstract

Transmural and regional gradients in membrane potential and Ca²⁺ transient in the murine heart are largely unexplored. Here, we developed and validated a robust approach which combines transverse ultra‐thin cardiac slices and high resolution optical mapping to enable systematic analysis of transmural and regional gradients in transmembrane potential (V_m) and intracellular Ca²⁺ transient (CaT) across the entire murine ventricles. The voltage dye RH237 or Ca²⁺ dye Rhod‐2 AM were loaded through the coronary circulation using a Langendorff perfusion system. Short‐axis slices (300 μm thick) were prepared from the entire ventricles (from the apex to the base) by using a high‐precision vibratome. Action potentials (APs) and CaTs were recorded with optical mapping during steady‐state baseline and rapid pacing. Significant transmural gradients in V_m and CaT were observed in the left ventricle, with longer AP duration (APD₅₀ and APD₇₅) and CaT duration (CaTD₅₀ and CaTD₇₅) in the endocardium compared with that in the epicardium. No significant regional gradients were observed along the apico‐basal axis of the left ventricle. Interventricular gradients were detected with significantly shorter APD₅₀, APD₇₅ and CaTD₅₀ in the right ventricle compared with left ventricle and ventricular septum. During rapid pacing, AP and CaT alternans were observed in most ventricular regions, with significantly greater incidence in the endocardium in comparison with epicardium. In conclusion, these observations demonstrate the feasibility of our new approach to cardiac slicing for systematic analysis of intrinsic transmural and regional gradients in V_m and CaT in murine ventricular tissue.

A robust cardiac slicing approach was developed for optical mapping of transmural gradients in transmembrane potential (V_m) and intracellular Ca²⁺ transient (CaT) of murine heart.

Significant transmural gradients in V_m and CaT were observed in the left ventricle.

Frequency‐dependent action potentials and CaT alternans were observed in all ventricular regions with rapid pacing, with significantly greater incidence in the endocardium than epicardium.

The observations demonstrate the feasibility of our new approach to cardiac slicing for systematic analysis of intrinsic transmural and regional gradients in V_m and CaT.

The cardiac ryanodine receptor, but not sarcoplasmic reticulum Ca2+-ATPase, is a major determinant of Ca2+ alternans in intact mouse hearts

Sarcoplasmic reticulum (SR) Ca²⁺ cycling is governed by the cardiac ryanodine receptor (RyR2) and SR Ca²⁺-ATPase (SERCA2a). Abnormal SR Ca²⁺ cycling is thought to be the primary cause of Ca²⁺ alternans that can elicit ventricular arrhythmias and sudden cardiac arrest. Although alterations in either RyR2 or SERCA2a function are expected to affect SR Ca²⁺ cycling, whether and to what extent altered RyR2 or SERCA2a function affects Ca²⁺ alternans is unclear. Here, we employed a gain-of-function RyR2 variant (R4496C) and the phospholamban-knockout (PLB-KO) mouse model to assess the effect of genetically enhanced RyR2 or SERCA2a function on Ca²⁺ alternans. Confocal Ca²⁺ imaging revealed that RyR2-R4496C shortened SR Ca²⁺ release refractoriness and markedly suppressed rapid pacing-induced Ca²⁺ alternans. Interestingly, despite enhancing RyR2 function, intact RyR2-R4496C hearts exhibited no detectable spontaneous SR Ca²⁺ release events during pacing. Unlike for RyR2, enhancing SERCA2a function by ablating PLB exerted a relatively minor effect on Ca²⁺ alternans in intact hearts expressing RyR2 WT or a loss-of-function RyR2 variant, E4872Q, that promotes Ca²⁺ alternans. Furthermore, partial SERCA2a inhibition with 3 μm 2,5-di-tert-butylhydroquinone (tBHQ) also had little impact on Ca²⁺ alternans, whereas strong SERCA2a inhibition with 10 μm tBHQ markedly reduced the amplitude of Ca²⁺ transients and suppressed Ca²⁺ alternans in intact hearts. Our results demonstrate that enhanced RyR2 function suppresses Ca²⁺ alternans in the absence of spontaneous Ca²⁺ release and that RyR2, but not SERCA2a, is a key determinant of Ca²⁺ alternans in intact working hearts, making RyR2 an important therapeutic target for cardiac alternans.

Transmural cellular heterogeneity in myocardial electromechanics

Myocardial heterogeneity is an attribute of the normal heart. We have developed integrative models of cardiomyocytes from the subendocardial (ENDO) and subepicardial (EPI) ventricular regions that take into account experimental data on specific regional features of intracellular electromechanical coupling in the guinea pig heart. The models adequately simulate experimental data on the differences in the action potential and contraction between the ENDO and EPI cells. The modeling results predict that heterogeneity in the parameters of calcium handling and myofilament mechanics in isolated ENDO and EPI cardiomyocytes are essential to produce the differences in Ca2+ transients and contraction profiles via cooperative mechanisms of mechano-calcium-electric feedback and may further slightly modulate transmural differences in the electrical properties between the cells. Simulation results predict that ENDO cells have greater sensitivity to changes in the mechanical load than EPI cells. These data are important for understanding the behavior of cardiomyocytes in the intact heart.

Species-Dependent Mechanisms of Cardiac Arrhythmia: A Cellular Focus

Although ventricular arrhythmia remains a leading cause of morbidity and mortality, available antiarrhythmic drugs have limited efficacy. Disappointing progress in the development of novel, clinically relevant antiarrhythmic agents may partly be attributed to discrepancies between humans and animal models used in preclinical testing. However, such differences are at present difficult to predict, requiring improved understanding of arrhythmia mechanisms across species. To this end, we presently review interspecies similarities and differences in fundamental cardiomyocyte electrophysiology and current understanding of the mechanisms underlying the generation of afterdepolarizations and reentry. We specifically highlight patent shortcomings in small rodents to reproduce cellular and tissue-level arrhythmia substrate believed to be critical in human ventricle. Despite greater ease of translation from larger animal models, discrepancies remain and interpretation can be complicated by incomplete knowledge of human ventricular physiology due to low availability of explanted tissue. We therefore point to the benefits of mathematical modeling as a translational bridge to understanding and treating human arrhythmia.

A dual potassium channel activator improves repolarization reserve and normalizes ventricular action potentials

BACKGROUND

A loss of repolarization reserve due to downregulation of K(+) currents has been observed in cultured ventricular myocytes. A similar reduction of K(+) currents is well documented under numerous pathophysiological conditions. We examined the extent of K(+) current downregulation in cultured canine cardiac myocytes and determined whether a dual K(+) current activator can normalize K(+) currents and restore action potential (AP) configuration.

METHODS AND RESULTS

Ventricular myocytes were isolated and cultured for up to 48 h. Current and voltage clamp recordings were made using patch electrodes. Application of NS3623 to coronary-perfused left ventricular wedges resulted in increased phase 1 magnitude, epicardial AP notch and J wave amplitude. Patch clamp measurements of IKr and Ito revealed an increase in the magnitude of both currents. Culturing of Mid ventricular cells resulted in a significant decrease in Ito and IKr density. NS3623 increased Ito from 16.4 ± 2.23 to 31.8 ± 4.5 pA/pF, and IKr from 0.28 ± 0.06 to 0.47 ± 0.09 pA/pF after 2 days in culture. AP recordings from 2 day cultured cells exhibited a reduced phase 1 repolarization, AP prolongation, and early afterdepolarizations (EADs). NS3623 restored the AP notch and was able to suppress EADs.

CONCLUSIONS

NS3623 is a dual Ito and IKr activator. Application of this compound to cells with a reduced repolarization reserve resulted in an increase in these currents and a shortening of AP duration, increase in phase 1 repolarization and suppression of EADs. Our results suggest a potential benefit of K(+) current activators under conditions of reduced repolarization reserve including heart failure.

Cardiac alternans and intracellular calcium cycling

Cardiac alternans refers to a condition in which there is a periodic beat-to-beat oscillation in electrical activity and the strength of cardiac muscle contraction at a constant heart rate. Clinically, cardiac alternans occurs in settings that are typical for cardiac arrhythmias and has been causally linked to these conditions. At the cellular level, alternans is defined as beat-to-beat alternations in contraction amplitude (mechanical alternans), action potential duration (APD; electrical or APD alternans) and Ca(2+) transient amplitude (Ca(2+) alternans). The cause of alternans is multifactorial; however, alternans always originate from disturbances of the bidirectional coupling between membrane voltage (Vm ) and intracellular calcium ([Ca(2+) ]i ). Bidirectional coupling refers to the fact that, in cardiac cells, Vm depolarization and the generation of action potentials cause the elevation of [Ca(2+) ]i that is required for contraction (a process referred to as excitation-contraction coupling); conversely, changes of [Ca(2+) ]i control Vm because important membrane currents are Ca(2+) dependent. Evidence is mounting that alternans is ultimately caused by disturbances of cellular Ca(2+) signalling. Herein we review how two key factors of cardiac cellular Ca(2+) cycling, namely the release of Ca(2+) from internal stores and the capability of clearing the cytosol from Ca(2+) after each beat, determine the conditions under which alternans occurs. The contributions from key Ca(2+) -handling proteins (i.e. surface membrane channels, ion pumps and transporters and internal Ca(2+) release channels) are discussed.

Cellular mechanism of cardiac alternans: an unresolved chicken or egg problem

T-wave alternans, a specific form of cardiac alternans, has been associated with the increased susceptibility to cardiac arrhythmias and sudden cardiac death (SCD). Plenty of evidence has related cardiac alternans at the tissue level to the instability of voltage kinetics or Ca(2+) handling dynamics at the cellular level. However, to date, none of the existing experiments could identify the exact cellular mechanism of cardiac alternans due to the bi-directional coupling between voltage kinetics and Ca(2+) handling dynamics. Either of these systems could be the origin of alternans and the other follows as a secondary change, therefore making the cellular mechanism of alternans a difficult chicken or egg problem. In this context, theoretical analysis combined with experimental techniques provides a possibility to explore this problem. In this review, we will summarize the experimental and theoretical advances in understanding the cellular mechanism of alternans. We focus on the roles of action potential duration (APD) restitution and Ca(2+) handling dynamics in the genesis of alternans and show how the theoretical analysis combined with experimental techniques has provided us a new insight into the cellular mechanism of alternans. We also discuss the possible reasons of increased propensity for alternans in heart failure (HF) and the new possible therapeutic targets. Finally, according to the level of electrophysiological recording techniques and theoretical strategies, we list some critical experimental or theoretical challenges which may help to determine the origin of alternans and to find more effective therapeutic targets in the future.

Formation of spatially discordant alternans due to fluctuations and diffusion of calcium

Spatially discordant alternans (SDA) of action potential duration (APD) is a phenomenon where different regions of cardiac tissue exhibit an alternating sequence of APD that are out-of-phase. SDA is arrhythmogenic since it can induce spatial heterogeneity of refractoriness, which can cause wavebreak and reentry. However, the underlying mechanisms for the formation of SDA are not completely understood. In this paper, we present a novel mechanism for the formation of SDA in the case where the cellular instability leading to alternans is caused by intracellular calcium (Ca) cycling, and where Ca transient and APD alternans are electromechanically concordant. In particular, we show that SDA is formed when rapidly paced cardiac tissue develops alternans over many beats due to Ca accumulation in the sarcoplasmic reticulum (SR). The mechanism presented here relies on the observation that Ca cycling fluctuations dictate Ca alternans phase since the amplitude of Ca alternans is small during the early stages of pacing. Thus, different regions of a cardiac myocyte will typically develop Ca alternans which are opposite in phase at the early stages of pacing. These subcellular patterns then gradually coarsen due to interactions with membrane voltage to form steady state SDA of voltage and Ca on the tissue scale. This mechanism for SDA is distinct from well-known mechanisms that rely on conduction velocity restitution, and a Turing-like mechanism known to apply only in the case where APD and Ca alternans are electromechanically discordant. Furthermore, we argue that this mechanism is robust, and is likely to underlie a wide range of experimentally observed patterns of SDA.

Effects of gender, ejection fraction and weight on cardiac force development in patients undergoing cardiac surgery--an experimental examination

Background

It has long been recognized that differences exist between men and women in the impact of risc factors, symptoms, development and outcome of special diseases like the cardiovascular disease. Gender determines the cardiac baseline parameters like the number of cardiac myocyte, size and demand and may suggest differences in myofilament function among genders, which might be pronounced under pathological conditions. Does gender impact and maybe impair the contractile apparatus? Are the differences more prominent when other factors like weight, age, ejection fraction are added?

Therefore we performed a study on 36 patients (21 male, 15 female) undergoing aortic valve replacement (AVR) or aortocoronary bypass operation (CABG) to examine the influence of gender, ejection fraction, surgical procedure and body mass index (BMI) on cardiac force development.

Methods

Tissue was obtained from the right auricle and was stored in a special solution to prevent any stretching of the fibers. We used the skinned muscle fiber model and single muscle stripes, which were mounted on the “muscle machine” and exposed to a gradual increase of calcium concentration calculated by an attached computer program.

Results

1.) In general female fibers show more force than male fibers: 3.9 mN vs. 2.0 mN (p = 0.03) 2.) Female fibers undergoing AVR achieved more force than those undergoing CABG operation: 5.7 mN vs. 2.8 mN (p = 0.02) as well as male fibers with AVR showed more force values compared to those undergoing CABG: 2.0 mN vs. 0.5 mN (p = 0.01). 3.) Male and female fibers of patients with EF > 55% developed significantly more force than from those with less ejection fraction than 30%: p = 0.002 for the male fibers (1.6 vs. 2.8 mN) and p = 0.04 for the female fibers (5.7 vs. 2.8 mN). 4.) Patients with a BMI between 18 till 25 develop significant more force than those with a BMI > 30: Females 5.1 vs. 2.6 mN; p 0.03, Males 3.8 vs. 0.8 mN; p 0.04).

Conclusion

Our data suggest that female patients undergoing AVR or CABG develop significantly more force than male fibers. Additionally we could image the clinical impression of negative impact of overweight and obesity as well as low ejection fraction on cardiac function on level of the myofilaments and observed a reduced force capacity, which is more prominent in male fibers.

Developmental changes in expression and biophysics of ion channels in the canine ventricle

BACKGROUND

Developmental changes in the electrical characteristics of the ventricular myocardium are not well defined. This study examines the contribution of inwardly rectifying K(+) current (IK1), transient outward K(+) current (Ito), delayed rectifier K(+) currents (IKr and IKs) and sodium channel current (INa) to repolarization in the canine neonate myocardium.

METHODS

Single myocytes isolated from the left ventricle of 2-3week old canine neonate hearts were studied using patch-clamp techniques.

RESULTS

Neonate cells were ~6-fold smaller than those of adults (28.8±8.8 vs. 176±6.7pF). IK1 was larger in neonate myocytes and displayed a substantial inward component and an outward component with negative slope conductance, peaking at -60mV (4.13 pA/pF). IKr tail currents (at -40mV), were small (<20pA). IKs could not be detected, even after exposure to isoproterenol (100nM). Ito was also absent in the neonate, consistent with the absence of a phase 1 in the action potential. Peak INa, late INa and ICa were smaller in the neonate compared with adults. KCND3, KCNIP2 and KCNQ1 mRNA expression was half, while KCNH2 was equal and KCNJ2 was greater in the neonate when compared with adults.

CONCLUSIONS

Two major repolarizing K(+) currents (IKs and Ito) present in adult ventricular cells are absent in the 2week old neonate. Peak and late INa are significantly smaller in the neonate. Our results suggest that the absence of these two currents in the neonate heart may increase the susceptibility to arrhythmias under certain long QT conditions.

Effects of pacing site and stimulation history on alternans dynamics and the development of complex spatiotemporal patterns in cardiac tissue

Alternans of action potential duration has been associated with T wave alternans and the development of arrhythmias because it produces large gradients of repolarization. However, little is known about alternans dynamics in large mammalian hearts. Using optical mapping to record electrical activations simultaneously from the epicardium and endocardium of 9 canine right ventricles, we demonstrate novel arrhythmogenic complex spatiotemporal dynamics. (i) Alternans predominantly develops first on the endocardium. (ii) The postulated simple progression from normal rhythm to concordant to discordant alternans is not always observed; concordant alternans can develop from discordant alternans as the pacing period is decreased. (iii) In contrast to smaller tissue preparations, multiple stationary nodal lines may exist and need not be perpendicular to the pacing site or to each other. (iv) Alternans has fully three-dimensional dynamics and the epicardium and endocardium can show significantly different dynamics: multiple nodal surfaces can be transmural or intramural and can form concave/convex surfaces resulting in islands of discordant alternans. (v) The complex spatiotemporal patterns observed during alternans are very sensitive to both the site of stimulation and the stimulation history. Alternans in canine ventricles not only exhibit larger amplitudes and persist for longer cycle length regimes compared to those found in smaller mammalian hearts, but also show novel dynamics not previously described that enhance dispersion and show high sensitivity to initial conditions. This indicates some underlying predisposition to chaos and can help to guide the design of new drugs and devices controlling and preventing arrhythmic events.

Non-linear dynamics of cardiac alternans: subcellular to tissue-level mechanisms of arrhythmia

Cardiac repolarization alternans is a rhythm disturbance of the heart in which rapid stimulation elicits a beat-to-beat alternation in the duration of action potentials and magnitude of intracellular calcium transients in individual cardiac myocytes. Although this phenomenon has been identified as a potential precursor to dangerous reentrant arrhythmias and sudden cardiac death, significant uncertainty remains regarding its mechanism and no clinically practical means of halting its occurrence or progression currently exists. Cardiac alternans has well-characterized tissue, cellular, and subcellular manifestations, the mechanisms and interplay of which are an active area of research.

Nonlinear dynamics in cardiology

The dynamics of many cardiac arrhythmias, as well as the nature of transitions between different heart rhythms, have long been considered evidence of nonlinear phenomena playing a direct role in cardiac arrhythmogenesis. In most types of cardiac disease, the pathology develops slowly and gradually, often over many years. In contrast, arrhythmias often occur suddenly. In nonlinear systems, sudden changes in qualitative dynamics can, counterintuitively, result from a gradual change in a system parameter-this is known as a bifurcation. Here, we review how nonlinearities in cardiac electrophysiology influence normal and abnormal rhythms and how bifurcations change the dynamics. In particular, we focus on the many recent developments in computational modeling at the cellular level that are focused on intracellular calcium dynamics. We discuss two areas where recent experimental and modeling work has suggested the importance of nonlinearities in calcium dynamics: repolarization alternans and pacemaker cell automaticity.

Physiological consequences of transient outward K+ current activation during heart failure in the canine left ventricle

BACKGROUND

Remodeling of ion channel expression is well established in heart failure (HF). We determined the extent to which I(to) is reduced in tachypacing-induced HF and assessed the ability of an I(to) activator (NS5806) to recover this current.

METHOD AND RESULTS

Whole-cell patch clamp was used to record I(to) in epicardial (Epi) ventricular myocytes. Epi- and endocardial action potentials were recorded from left ventricular wedge preparations. Right ventricular tachypacing-induced heart failure reduced I(to) density in Epi myocytes (Control=22.1±1.9pA/pF vs 16.1±1.4 after 2weeks and 10.7±1.4pA/pF after 5 weeks, +50mV). Current decay as well as recovery of I(to) from inactivation progressively slowed with the development of heart failure. Reduction of I(to) density was paralleled by a reduction in phase 1 magnitude, epicardial action potential notch and J wave amplitude recorded from coronary-perfused left ventricular wedge preparations. NS5806 increased I(to) (at +50mV) from 16.1±1.4 to 23.9±2.1pA/pF (p<0.05) at 2weeks and from 10.7±1.4 to 14.4±1.9pA/pF (p<0.05) in 5 weeks tachypaced dogs. NS5806 increased both fast and slow phases of I(to) recovery in 2 and 5-week HF cells and restored the action potential notch and J wave in wedge preparations from HF dogs.

CONCLUSIONS

The I(to) agonist NS5806 increases the rate of recovery and density of I(to), thus reversing the HF-induced reduction in these parameters. In wedge preparations from HF dogs, NS5806 restored the spike-and-dome morphology of the Epi action potential providing proof of principal that some aspects of electrical remodelling during HF can be pharmacologically reversed.

Crescendo in depolarization and repolarization heterogeneity heralds development of ventricular tachycardia in hospitalized patients with decompensated heart failure

BACKGROUND

A critical need exists for reliable warning markers of in-hospital life-threatening arrhythmias. We used a new quantitative method to track interlead heterogeneity of depolarization and repolarization to detect premonitory changes before ventricular tachycardia (VT) in hospitalized patients with acute decompensated heart failure.

METHODS AND RESULTS

Ambulatory ECGs (leads V(1), V(5), and aVF) recorded before initiation of drug therapy from patients enrolled in the PRECEDENT (Prospective Randomized Evaluation of Cardiac Ectopy with Dobutamine or Nesiritide Therapy) trial were analyzed. R-wave heterogeneity (RWH) and T-wave heterogeneity (TWH) were assessed by second central moment analysis and T-wave alternans (TWA) by modified moving average analysis. Of 44 patients studied, 22 had experienced episodes of VT (≥4 beats at heart rates >100 beats/min) following ≥120 minutes of stable sinus rhythm, and 22 were age- and sex-matched patients without VT. TWA increased from 18.6±2.1 μV (baseline, mean±SEM) to 27.9±4.6 μV in lead V(5) at 15 to 30 minutes before VT (P<0.05) and remained elevated until the arrhythmia occurred. TWA results in leads V(1) and aVF were similar. RWH and TWH were elevated from 164.1±33.1 and 134.5±20.6 μV (baseline) to 299.8±54.5 and 239.2±37.0 μV at 30 to 45 minutes before VT (P<0.05), respectively, preceding the crescendo in TWA by 15 minutes. Matched patients without VT did not display elevated RWH (185.5±29.4 μV) or TWH (157.1±27.2 μV) during the 24-hour period.

CONCLUSIONS

This investigation is the first clinical demonstration of the potential utility of tracking depolarization and repolarization heterogeneity to detect crescendos in electrical instability that could forewarn of impending nonsustained VT. Clinical Trial Registration- URL: http://www.clinicaltrials.gov. Unique identifier: NCT00270400.

Extracellular proton depression of peak and late Na⁺ current in the canine left ventricle

Cardiac ischemia reduces excitability in ventricular tissue. Acidosis (one component of ischemia) affects a number of ion currents. We examined the effects of extracellular acidosis (pH 6.6) on peak and late Na(+) current (I(Na)) in canine ventricular cells. Epicardial and endocardial myocytes were isolated, and patch-clamp techniques were used to record I(Na). Action potential recordings from left ventricular wedges exposed to acidic Tyrode solution showed a widening of the QRS complex, indicating slowing of transmural conduction. In myocytes, exposure to acidic conditions resulted in a 17.3 ± 0.9% reduction in upstroke velocity. Analysis of fast I(Na) showed that current density was similar in epicardial and endocardial cells at normal pH (68.1 ± 7.0 vs. 63.2 ± 7.1 pA/pF, respectively). Extracellular acidosis reduced the fast I(Na) magnitude by 22.7% in epicardial cells and 23.1% in endocardial cells. In addition, a significant slowing of the decay (time constant) of fast I(Na) was observed at pH 6.6. Acidosis did not affect steady-state inactivation of I(Na) or recovery from inactivation. Analysis of late I(Na) during a 500-ms pulse showed that the acidosis significantly reduced late I(Na) at 250 and 500 ms into the pulse. Using action potential clamp techniques, application of an epicardial waveform resulted in a larger late I(Na) compared with when an endocardial waveform was applied to the same cell. Acidosis caused a greater decrease in late I(Na) when an epicardial waveform was applied. These results suggest acidosis reduces both peak and late I(Na) in both cell types and contributes to the depression in cardiac excitability observed under ischemic conditions.

Effect of resistive barrier location on the relationship between T-wave alternans and cellular repolarization alternans: a 1-D modeling study

Structural inhomogeneities in cardiac tissue have been associated with increased cellular repolarization alternans in animal experiments and increased T-wave alternans (TWA) in clinical studies. However, the effect of structural inhomogeneities on the relationship between cellular alternans and TWA has not been thoroughly investigated. We created 1-dimensional multicellular fiber models with and without a resistive barrier in various fiber regions and paced each model to induce cellular alternans. The models demonstrate that a resistive barrier in one fiber region substantially alters cellular repolarization alternans throughout the fiber. A midmyocardial or subepicardial barrier increase both TWA amplitude and maximum cellular alternans magnitude, relative to a fiber without a barrier. In addition, a direct relationship exists between TWA amplitude and maximum cellular alternans magnitude, which was highly dependent on barrier location. These results suggest that the position of a structural inhomogeneity within the myocardium may have substantial effects on dynamic repolarization instability and arrhythmogenicity.

Understanding cardiac alternans: a piecewise linear modeling framework

Cardiac alternans is a beat-to-beat alternation in action potential duration (APD) and intracellular calcium (Ca(2+)) cycling seen in cardiac myocytes under rapid pacing that is believed to be a precursor to fibrillation. The cellular mechanisms of these rhythms and the coupling between cellular Ca(2+) and voltage dynamics have been extensively studied leading to the development of a class of physiologically detailed models. These have been shown numerically to reproduce many of the features of myocyte response to pacing, including alternans, and have been analyzed mathematically using various approximation techniques that allow for the formulation of a low dimensional map to describe the evolution of APDs. The seminal work by Shiferaw and Karma is of particular interest in this regard [Shiferaw, Y. and Karma, A., "Turing instability mediated by voltage and calcium diffusion in paced cardiac cells," Proc. Natl. Acad. Sci. U.S.A. 103, 5670-5675 (2006)]. Here, we establish that the key dynamical behaviors of the Shiferaw-Karma model are arranged around a set of switches. These are shown to be the main elements for organizing the nonlinear behavior of the model. Exploiting this observation, we show that a piecewise linear caricature of the Shiferaw-Karma model, with a set of appropriate switching manifolds, can be constructed that preserves the physiological interpretation of the original model while being amenable to a systematic mathematical analysis. In illustration of this point, we formulate the dynamics of Ca(2+) cycling (in response to pacing) and compute the properties of periodic orbits in terms of a stroboscopic map that can be constructed without approximation. Using this, we show that alternans emerge via a period-doubling instability and track this bifurcation in terms of physiologically important parameters. We also show that when coupled to a spatially extended model for Ca(2+) transport, the model supports spatially varying patterns of alternans. We analyze the onset of this instability with a generalization of the master stability approach to accommodate the nonsmooth nature of our system.

The role of mitochondria for the regulation of cardiac alternans

Electro-mechanical and Ca alternans is a beat-to-beat alternation of action potential duration, contraction strength and Ca transient amplitude observed in cardiac myocytes at regular stimulation frequency. Ca alternans is a multifactorial process that is causally linked to cardiac arrhythmias. At the cellular level, conditions that increase fractional release from the sarcoplasmic reticulum or reduce diastolic Ca sequestration favor the occurrence of alternans. Mitochondria play a significant role in cardiac excitation–contraction coupling and Ca signaling by providing the energy for contraction and ATP-dependent processes and possibly by serving as Ca buffering organelles. Here we tested the hypothesis that impairment of mitochondrial function generates conditions that favor the occurrence of Ca alternans. Alternans were elicited by electrical pacing (>1 Hz) in single cat atrial myocytes and intracellular Ca ([Ca]_i) was measured with the fluorescent Ca indicator Indo-1. The degree of alternans was quantified as the alternans ratio (AR = 1 − S/L, where S/L is the ratio of the small to the large amplitude of a pair of alternating Ca transients). Dissipation of mitochondrial membrane potential (with FCCP) as well as inhibition of mitochondrial F₁/F₀-ATP synthase (oligomycin), electron transport chain (rotenone, antimycin, CN⁻), Ca-dependent dehydrogenases and mitochondrial Ca uptake or extrusion, all enhanced AR and lowered the threshold for the occurrence of Ca alternans. The data indicate that impairment of mitochondrial function adversely affects cardiac Ca cycling leading to proarrhythmic Ca alternans.

Feedback-control induced pattern formation in cardiac myocytes: a mathematical modeling study

Cardiac alternans is a dangerous rhythm disturbance of the heart, in which rapid stimulation elicits a beat-to-beat alternation in the action potential duration (APD) and calcium (Ca) transient amplitude of individual myocytes. Recently, "subcellular alternans", in which the Ca transients of adjacent regions within individual myocytes alternate out-of-phase, has been observed. A previous theoretical study suggested that subcellular alternans may result during static pacing from a Turing-type symmetry breaking instability, but this was only predicted in a subset of cardiac myocytes (with negative Ca to voltage (Ca-->V(m)) coupling) and has never been directly verified experimentally. A recent experimental study, however, showed that subcellular alternans is dynamically induced in the remaining subset of myocytes during pacing with a simple feedback control algorithm ("alternans control"). Here we show that alternans control pacing changes the effective coupling between the APD and the Ca transient (V(m)-->Ca coupling), such that subcellular alternans is predicted to occur by a Turing instability in cells with positive Ca-->V(m) coupling. In addition to strengthening the understanding of the proposed mechanism for subcellular alternans formation, this work (in concert with previous theoretical and experimental results) illuminates subcellular alternans as a striking example of a biological Turing instability in which the diffusing morphogens can be clearly identified.

Transmural heterogeneity of repolarization and Ca2+ handling in a model of mouse ventricular tissue

Mouse hearts have a diversity of action potentials (APs) generated by the cardiac myocytes from different regions. Recent evidence shows that cells from the epicardial and endocardial regions of the mouse ventricle have a diversity in Ca(2+) handling properties as well as K(+) current expression. To examine the mechanisms of AP generation, propagation, and stability in transmurally heterogeneous tissue, we developed a comprehensive model of the mouse cardiac cells from the epicardial and endocardial regions of the heart. Our computer model simulates the following differences between epicardial and endocardial myocytes: 1) AP duration is longer in endocardial and shorter in epicardial myocytes, 2) diastolic and systolic intracellular Ca(2+) concentration and intracellular Ca(2+) concentration transients are higher in paced endocardial and lower in epicardial myocytes, 3) Ca(2+) release rate is about two times larger in endocardial than in epicardial myocytes, and 4) Na(+)/Ca(2+) exchanger rate is greater in epicardial than in endocardial myocytes. Isolated epicardial cells showed a higher threshold for stability of AP generation but more complex patterns of AP duration at fast pacing rates. AP propagation velocities in the model of two-dimensional tissue are close to those measured experimentally. Simulations show that heterogeneity of repolarization and Ca(2+) handling are sustained across the mouse ventricular wall. Stability analysis of AP propagation in the two-dimensional model showed the generation of Ca(2+) alternans and more complex transmurally heterogeneous irregular structures of repolarization and intracellular Ca(2+) transients at fast pacing rates.

Differential effects of the transient outward K(+) current activator NS5806 in the canine left ventricle

To examine the electrophysiological and molecular properties of the transient outward current (I(to)) in canine left ventricle using a novel I(to) activator, NS5806, I(to) was measured in isolated epicardial (Epi), midmyocardial (Mid) and endocardial (Endo) cells using whole-cell patch-clamp techniques. NS5806 activation of K(v)4.3 current was also studied in CHO-K1 cells and Xenopus laevis oocytes. In CHO-K1 cells co-transfected with K(v)4.3 and KChIP2, NS5806 (10 microM) caused a 35% increase in current amplitude and a marked slowing of current decay with tau increasing from 7.0+/-0.4 to 10.2+/-0.3 ms. In the absence of KChIP2, current decay was unaffected by NS5806. In ventricular myocytes, NS5806 increased I(to) density by 80%, 82%, and 16% in Epi, Mid, and Endo myocytes, respectively (at +40 mV) and shifted steady-state inactivation to negative potentials. NS5806 also significantly slowed decay of I(to), increasing total charge to 227%, 192% and 83% of control in Epi, Mid and Endo cells, respectively (+40 mV, p<0.05). Quantification of K(v)4.3 and KChIP2 mRNA in the 3 ventricular cell types revealed that levels of K(v)4.3 message was uniform but those of KChIP2 were significantly greater in Epi and Mid cells. The KChIP2 gradient was confirmed at the protein level by Western blot. Our results suggest that NS5806 augments I(to) by increasing current density and slowing decay and that both depend on the presence of KChIP2. I(to) and its augmentation by NS5806 are greatest in Epi and Mid cells because KChIP2 levels are highest in these cell types.

Dynamical mechanism for subcellular alternans in cardiac myocytes

RATIONALE

Cardiac repolarization alternans is an arrhythmogenic rhythm disturbance, manifested in individual myocytes as a beat-to-beat alternation of action potential durations and intracellular calcium transient magnitudes. Recent experimental studies have reported "subcellular alternans," in which distinct regions of an individual cell are seen to have counterphase calcium alternations, but the mechanism by which this occurs is not well understood. Although previous theoretical work has proposed a possible dynamical mechanism for subcellular alternans formation, no direct evidence for this mechanism has been reported in vitro. Rather, experimental studies have generally invoked fixed subcellular heterogeneities in calcium-cycling characteristics as the mechanism of subcellular alternans formation.

OBJECTIVE

In this study, we have generalized the previously proposed dynamical mechanism to predict a simple pacing algorithm by which subcellular alternans can be induced in isolated cardiac myocytes in the presence or absence of fixed subcellular heterogeneity. We aimed to verify this hypothesis using computational modeling and to confirm it experimentally in isolated cardiac myocytes. Furthermore, we hypothesized that this dynamical mechanism may account for previous reports of subcellular alternans seen in statically paced, intact tissue.

METHODS AND RESULTS

Using a physiologically realistic computational model of a cardiac myocyte, we show that our predicted pacing algorithm induces subcellular alternans in a manner consistent with theoretical predictions. We then use a combination of real-time electrophysiology and fluorescent calcium imaging to implement this protocol experimentally and show that it robustly induces subcellular alternans in isolated guinea pig ventricular myocytes. Finally, we use computational modeling to demonstrate that subcellular alternans can indeed be dynamically induced during static pacing of 1D fibers of myocytes during tissue-level spatially discordant alternans.

CONCLUSION

Here we provide the first direct experimental evidence that subcellular alternans can be dynamically induced in cardiac myocytes. This proposed mechanism may contribute to subcellular alternans formation in the intact heart.

Accelerated inactivation of the L-type calcium current due to a mutation in CACNB2b underlies Brugada syndrome

Recent studies have demonstrated an association between mutations in CACNA1c or CACNB2b and Brugada syndrome (BrS). Previously described mutations all caused a loss of function secondary to a reduction of peak calcium current (I(Ca)). We describe a novel CACNB2b mutation associated with BrS in which loss of function is caused by accelerated inactivation of I(Ca). The proband, a 32 year old male, displayed a Type I ST segment elevation in two right precordial ECG leads following a procainamide challenge. EP study was positive with induction of polymorphic VT/VF. Interrogation of implanted ICD revealed brief episodes of very rapid ventricular tachycardia. He was also diagnosed with vasovagal syncope. Genomic DNA was isolated from lymphocytes. All exons and intron borders of 15 ion channel genes were amplified and sequenced. The only mutation uncovered was a missense mutation (T11I) in CACNB2b. We expressed WT or T11I CACNB2b in TSA201 cells co-transfected with WT CACNA1c and CACNA2d. Patch clamp analysis showed no significant difference between WT and T11I in peak I(Ca) density, steady-state inactivation or recovery from inactivation. However, both fast and slow decays of I(Ca) were significantly faster in mutant channels between 0 and + 20 mV. Action potential voltage clamp experiments showed that total charge was reduced by almost half compared to WT. We report the first BrS mutation in CaCNB2b resulting in accelerated inactivation of L-type calcium channel current. Our results suggest that the faster current decay results in a loss-of-function responsible for the Brugada phenotype

Identification of Ikr kinetics and drug binding in native myocytes

Determining the effect of a compound on I_Kr is a standard screen for drug safety. Often the effect is described using a single IC₅₀ value, which is unable to capture complex effects of a drug. Using verapamil as an example, we present a method for using recordings from native myocytes at several drug doses along with qualitative features of I_Kr from published studies of HERG current to estimate parameters in a mathematical model of the drug effect on I_Kr. I_Kr was recorded from canine left ventricular myocytes using ruptured patch techniques. A voltage command protocol was used to record tail currents at voltages from −70 to −20 mV, following activating pulses over a wide range of voltages and pulse durations. Model equations were taken from a published I_Kr Markov model and the drug was modeled as binding to the open state. Parameters were estimated using a combined global and local optimization algorithm based on collected data with two additional constraints on I_KrI–V relation and I_Kr inactivation. The method produced models that quantitatively reproduce both the control I_Kr kinetics and dose dependent changes in the current. In addition, the model exhibited use and rate dependence. The results suggest that: (1) the technique proposed here has the practical potential to develop data-driven models that quantitatively reproduce channel behavior in native myocytes; (2) the method can capture important drug effects that cannot be reproduced by the IC₅₀ method. Although the method was developed for I_Kr, the same strategy can be applied to other ion channels, once appropriate channel-specific voltage protocols and qualitative features are identified.

Multiple defects in intracellular calcium cycling in whole failing rat heart

BACKGROUND

A number of defects in excitation-contraction coupling have been identified in failing mammalian hearts. The goal of this study was to measure the defects in intracellular Ca(2+) cycling in left ventricular epicardial myocytes of the whole heart in an animal model of congestive heart failure (CHF).

METHODS AND RESULTS

Intracellular Ca(2+) transients were measured using confocal microscopy in whole rat hearts from age-matched Wistar-Kyoto control rats and spontaneously hypertensive rats at approximately 23 months of age. Basal Ca(2+) transients in myocytes in spontaneously hypertensive rats were smaller in amplitude and longer in duration than Wistar-Kyoto control rats. There was also greater variability in transient characteristics associated with duration between myocytes of CHF than Wistar-Kyoto controls. Approximately 21% of CHF myocytes demonstrated spontaneous Ca(2+) waves compared with very little of this activity in Wistar-Kyoto control rats. A separate population of spontaneously hypertensive rat myocytes showed Ca(2+) waves that were triggered during pacing and were absent at rest (triggered waves). Rapid pacing protocols caused Ca(2+) alternans to develop at slower heart rates in CHF.

CONCLUSIONS

Epicardial cells demonstrate both serious defects and greater cell-to-cell variability in Ca(2+) cycling in CHF. The defects in Ca(2+) cycling include both spontaneous and triggered waves of Ca(2+) release, which promote triggered activity. The slowing of Ca(2+) repriming in the sarcoplasmic reticulum is probably responsible for the increased vulnerability to Ca(2+) alternans in CHF. Our results suggest that defective Ca(2+) cycling could contribute both to reduced cardiac output in CHF and to the establishment of repolarization gradients, thus creating the substrate for reentrant arrhythmias.

Acidosis and ischemia increase cellular Ca2+ transient alternans and repolarization alternans susceptibility in the intact rat heart

Cardiac cellular Ca(2+) transient (CaT) alternans and electrocardiographic T-wave alternans (TWA) often develop in myocardial ischemia, but the mechanisms for this relationship have not been elucidated. Acidosis is a major component of ischemia, but there is no direct evidence linking acidosis-induced cellular CaT alternans to ischemia-induced CaT alternans and TWA in whole heart. We used laser-scanning confocal microscopy to measure intracellular Ca(2+) (Ca(i)(2+)) cycling in individual myocytes of fluo-4 AM-loaded rat hearts and simultaneously recorded pseudo-ECGs to investigate changes in CaTs and late-phase repolarization, respectively, during baseline and rapid pacing under control and either globally acidic or globally ischemic conditions. Acidosis (hypercapnia; pH 6.6) increased diastolic Ca(i)(2+) levels, prolonged CaT duration, and shifted to slower heart rates both the development of pacing-induced acidosis-induced CaT alternans (both concordant and discordant) and of repolarization alternans (RPA, a measure of TWA in rat ECGs). The magnitudes of these shifts were equivalent for both CaT alternans and RPA, suggesting a close association between them. Nearly identical results were found in low-flow global ischemia. Additionally, ischemic preconditioning reduced the increased propensity for CaT alternans and RPA development and was mimicked by preconditioning by acidosis alone. Our results demonstrate that global acidosis or ischemia modifies Ca(i)(2+) cycling in myocytes such that the diastolic Ca(i)(2+) rises and the cellular CaT duration is prolonged, causing spatially concordant as well as spatially discordant cellular CaT alternans to develop at slower heart rates than in controls. Since RPA also developed at slower heart rates, our results suggest that acidosis is a major contributor to CaT alternans, which underlies the proarrhythmic state induced by myocardial ischemia and therefore may play a role in its modulation and prevention.

The effects of membrane potential, SR Ca2+ content and RyR responsiveness on systolic Ca2+ alternans in rat ventricular myocytes

Previous work has shown that small depolarizing pulses produce a beat to beat alternation in the amplitude of the systolic Ca(2+) transient in ventricular myocytes. The aim of the present work was to investigate the role of changes of SR Ca(2+) content and L-type Ca(2+) current in this alternans. As the amplitude of the depolarizing pulse was increased from 10 to 30 mV the magnitude of alternans decreased. Confocal linescan studies showed that this was accompanied by an increase in the number of sites from which Ca(2+) waves propagated. A sudden decrease in the depolarisation amplitude resulted in three classes of behaviour: (1) a gradual decrease in Ca(2+) transient amplitude before alternans developed accompanied by a loss of SR Ca(2+), (2) a gradual increase in Ca(2+) transient amplitude before alternans accompanied by a gain of SR Ca(2+), and (3) immediate development of alternans with no change of SR content. We conclude that alternans develops if the combination of decreased opening of L-type channels and change of SR Ca(2+) content results in spatially fragmented release from the SR as long as there is sufficient Ca(2+) in the SR to sustain wave propagation. Potentiation of the opening of the ryanodine receptor (RyR) by low concentrations of caffeine (100 microm) abolished alternans for a few pulses but the alternans then redeveloped once SR Ca(2+) content fell to the new threshold for wave propagation. Finally we show evidence that inhibiting L-type Ca(2+) current with 200 mum Cd(2+) produces alternans by means of a similar fragmentation of the Ca(2+) release profile and propagation of mini-waves of Ca(2+) release.

A transient outward potassium current activator recapitulates the electrocardiographic manifestations of Brugada syndrome

AIMS

Transient outward potassium current (I(to)) is thought to be central to the pathogenesis of the Brugada syndrome (BrS). However, an I((to)) activator has not been available with which to validate this hypothesis. Here, we provide a direct test of the hypothesis using a novel I(to) activator, NS5806.

METHODS AND RESULTS

Isolated canine ventricular myocytes and coronary-perfused wedge preparations were used. Whole-cell patch-clamp studies showed that NS5806 (10 microM) increased peak I(to) at +40 mV by 79 +/- 4% (24.5 +/- 2.2 to 43.6 +/- 3.4 pA/pF, n = 7) and slowed the time constant of inactivation from 12.6 +/- 3.2 to 20.3 +/- 2.9 ms (n = 7). The total charge carried by I(to) increased by 186% (from 363.9 +/- 40.0 to 1042.0 +/- 103.5 pA x ms/pF, n = 7). In ventricular wedge preparations, NS5806 increased phase 1 and notch amplitude of the action potential in the epicardium, but not in the endocardium, and accentuated the ECG J-wave, leading to the development of phase 2 re-entry and polymorphic ventricular tachycardia (n = 9). Although sodium and calcium channel blockers are capable of inducing BrS only in right ventricular (RV) wedge preparations, the I(to) activator was able to induce the phenotype in wedges from both ventricles. NS5806 induced BrS in 4/6 right and 2/10 left ventricular wedge preparations.

CONCLUSION

The I(to) activator NS5806 recapitulates the electrographic and arrhythmic manifestation of BrS, providing evidence in support of its pivotal role in the genesis of the disease. Our findings also suggest that a genetic defect leading to a gain of function of I(to) could explain variants of BrS, in which ST-segment elevation or J-waves are evident in both right and left ECG leads.

Heart failure enhances susceptibility to arrhythmogenic cardiac alternans

BACKGROUND

Although heart failure (HF) is closely associated with susceptibility to sudden cardiac death (SCD), the mechanisms linking contractile dysfunction to cardiac electrical instability are poorly understood. Cardiac alternans has also been closely associated with SCD, and has been linked to a mechanism for amplifying electrical heterogeneities in the heart. However, previous studies have focused on alternans in normal rather than failing myocardium.

OBJECTIVE

This study sought to investigate the hypothesis that HF enhances susceptibility to arrhythmogenic cardiac alternans.

METHODS

High-resolution transmural optical mapping was performed in canine wedge preparations from normal (n = 8) and HF (n = 8) hearts produced by rapid ventricular pacing.

RESULTS

HF significantly (P < .004) lowered the heart rate (HR) threshold for action potential duration alternans (APD-ALT) from 236 +/- 25 beats/min to 185 +/- 25 beats/min. In dual optical mapping of action potentials and intracellular Ca experiments (n = 16), HF lowered the HR threshold for Ca-ALT (beat-to-beat alternations of cellular Ca cycling) from 238 +/- 35 to 177 +/- 26 beats/min (P < .005). Importantly: (1) Ca-ALT always either developed at slower HR or simultaneously with APD-ALT in the same cells, and (2) the magnitude of Ca-ALT and APD-ALT were closely correlated (P < .05). HF similarly lowered the HR threshold for Ca-ALT in isolated myocytes under nonalternating action potential clamp, indicating that HF enhances susceptibility to cellular alternans independent of HF-associated changes in repolarization. Importantly, HF significantly (P < .02) lowered the HR threshold for spatially discordant arrhythmogenic alternans (different regions of cells alternating in opposite phase, DIS-ALT). Ventricular fibrillation (VF) was induced in 88% of HF preparations, but only 12% of normal preparations (P < .003) and was uniformly preceded by development of DIS-ALT.

CONCLUSION

Heart failure increases the susceptibility to arrhythmogenic cardiac alternans, which arises from HF-induced impairment in calcium cycling.

Characteristics of intracellular Ca2+ cycling in intact rat heart: a comparison of sex differences

Males and females show distinct differences in action potential waveform, ion channel expression patterns, and ECG characteristics. However, it is not known how sex-based differences in Ca2+ cycling might contribute to these differences in electrophysiological activity. The goal of this study was to investigate the differences in cellular Ca2+ transients in males and females and to examine how these might contribute to electrophysiological function. Ca2+ transients were measured in individual myocytes within microscopic regions of the fluo-4 AM-loaded left ventricular epicardium of intact rat heart of both sexes (3 to 5 mo old). Pacing protocols were used to measure transient characteristics at a basic cycle length of 500 ms and during 10-s trains of rapid pacing delivered to the left ventricular apex. Ca2+ transients were smaller in magnitude and longer in duration in females than in males. More importantly, the variability in Ca2+ transient characteristics between myocytes in a microscopic recording site (heterogeneity index) was greater for females than males for characteristics related to transient duration. The rate sensitivity of Ca2+ alternans development in individual myocytes was greater in females than in males, but there was also a greater heterogeneity in cellular responses to the rate dependence of alternans development in females. The longer Ca2+ transients in females were also associated with slower restitution, which was likely to be responsible for the development of Ca2+ and repolarization alternans at slower heart rates. These results demonstrate that there are distinct differences in cellular Ca2+ cycling in male and female rat hearts. Not only is there slower reuptake of Ca2+ in female rats, but there is greater local variability in Ca2+ cycling at the microscopic level. These sex-based differences in Ca2+ cycling could contribute to differences in ECG morphology and in arrhythmia sensitivity in males and females.

Simulated ischemia-induced preconditioning of isolated ventricular myocytes from young adult and aged Fischer-344 rat hearts

The impact of ischemic preconditioning (IPC) on contraction, Ca(2+) homeostasis, and cell survival was compared in isolated ventricular myocytes from young adult ( approximately 3 mo) and aged ( approximately 24 mo) male Fischer-344 rats. Myocytes were field stimulated at 4 Hz (37 degrees C). Contraction (edge detector) and intracellular Ca(2+) (fura-2) were measured simultaneously. Viability was assessed with trypan blue. All cells were exposed to 30 min of simulated ischemia followed by reperfusion. Some cells were preconditioned by exposure to 5 min of simulated ischemia before prolonged ischemia. Pretreatment with IPC abolished postischemic contractile depression, inhibited diastolic contracture, and increased Ca(2+) transient amplitudes in reperfusion in young adult and aged cells. IPC did not affect the modest rise in diastolic Ca(2+) in ischemia in young adult myocytes. However, IPC abolished the marked rise in diastolic Ca(2+) observed in ischemia and early reperfusion in aged myocytes. IPC also suppressed mechanical alternans in ischemia in aged cells, but younger myocytes showed little evidence of mechanical alternans whether or not cells were preconditioned. IPC markedly improved cell viability in reperfusion in young adult but not aged cells. These results suggest that IPC augments the recovery of contractile function in reperfusion by increasing Ca(2+) transient amplitudes in ventricular myocytes from young adult and aged rats. IPC reduced diastolic Ca(2+) accumulation in ischemia in aged myocytes, which may diminish the severity of mechanical alternans in aged cells. Nonetheless, the efficacy of IPC is compromised in aging, as IPC did not improve survival of aged myocytes exposed to ischemia and reperfusion.

Cellular mechanisms of arrhythmogenic cardiac alternans

Despite the strong association between mechanical dysfunction of the heart and sudden death due to arrhythmias, the causal relationship is not well understood. Cardiac alternans has been linked to arrhythmogenesis and can be mediated by intracellular calcium handling. Given the integral role intracellular calcium plays in contractile function, calcium-mediated alternans may represent an important mechanistic link between mechanical dysfunction and electrical instability. This relationship, however, is not well understood due to complex feedback between membrane currents, intracellular calcium, and contraction. This manuscript describes the cellular mechanisms of cardiac alternans. Through several pathways, calcium transient alternans is coupled to repolarization alternans that can form a substrate for reentrant excitation. Abnormal intracellular calcium cycling, either impaired release or impaired reuptake of sarcoplasmic reticulum calcium, is a cellular mechanism of calcium transient alternans. Thus, cardiac alternans is an important mechanistic link between mechanical dysfunction and sudden cardiac death.