Amplitude Of Delayed Afterdepolarizations Research Articles

Cardiac Na/Ca Exchange Suppression: A Late‐Breaking Knockout Story Showing That There Is No Free Lunch

Cardiac Na/Ca Exchange Suppression: A Late‐Breaking Knockout Story Showing That There Is No Free Lunch

Acute Genetic Ablation of Cardiac Sodium-Calcium Exchange Suppresses Arrhythmogenic Delayed after Depolarizations

Sodium-calcium exchange (NCX) is the dominant calcium (Ca) efflux mechanism in cardiac myocytes. As an electrogenic transporter, NCX is thought to generate the inward current responsible for Ca-dependent arrhythmogenic delayed afterdepolarizations (DADs). To test this hypothesis, we patch-clamped ventricular myocytes isolated from our novel tamoxifen-inducible cardiac-specific NCX knockout (KO) mouse (αMHC-MerCreMer promotor). Tamoxifen (40mg/kg/day IP) was injected for 5 consecutive days in 10 to 12-week-old mice. 4 weeks after injection, we found a 96% decrease of NCX protein expression and an 87% reduction of caffeine-induced NCX current (p< 0.001), with 25% of cells showing no NCX current whatsoever. Sarcoplasmic reticulum Ca load was similar to control. Transient outward current (ITO) and Kv4.2 protein expression were significantly increased (by 60 and 34% respectively, p < 0.01) and ICa was reduced by 47% (p < 0.001). The action potential duration (APD) was shortened when measured at either 10 % (8.6 ± 0.7 ms in control vs 5.5 ± 0.5 ms in KO, p < 0.05) or 90 % (43.9 ± 4.9 ms in control vs 11.1 ± 0.9 ms in KO, p < 0.001) repolarization. In response to a pro-arrhythmic stimulation protocol, spontaneous APs triggered by DADs occurred in 95% of control cells, but only in 35% of NCX KO myocytes, even though we observed no change in the mean number of DADs per cell. Notably, DAD amplitude was significantly reduced in NCX KO mice: 14% of KO cells exhibited DADs with an amplitude ≥ 3mV versus 78 % in control cells. In the 25% of KO cells with no detectable DADs, there were no spontaneous APs. Most likely these were the cells with no NCX activity. Thus, acute genetic reduction/ablation of NCX suppresses DAD frequency/amplitude and subsequent spontaneous AP generation.

Afterdepolarizations and triggered activity as a mechanism for clinical arrhythmias.

Afterdepolarizations cause triggered arrhythmias. One kind occurs after repolarization is complete, delayed afterdepolarizations (DADs). Another occurs as an interruption in repolarization, early afterdepolarizations (EADs). Afterdepolarizations initiate arrhythmias when they depolarize membrane potential to threshold potential for triggering action potentials. DADs usually occur mostly when Ca2+ in the sarcoplasmic reticulum (SR) is elevated. The SR leaks some of the Ca2+ into the myoplasm through Ca2+ release channels controlled by ryanodine receptors (RyR2) during diastole. The Na+ -Ca2+ exchanger extrudes elevated diastolic Ca2+ from the cell in exchange for Na+ (1 Ca2+ for 3 Na+ ) generating inward current causing DADs. DAD amplitude increases with decreasing cycle length, causing triggered activity during an increase in heart rate or during programmed electrical stimulation (PES). Coupling interval of the first triggered impulse is directly related to initiating cycle length. EADs are associated with an increased action potential duration (APD) causing long QT (LQT). EADs are caused by net inward currents (ICaL , INCX ) as a consequence. Hundreds of mutations can cause congenital LQT by altering repolarizing ion channels. Acquired LQT results from drug interaction with repolarizing ion channels. EAD-triggered ventricular tachycardia is polymorphic and called "torsade de pointes." Effects of PES on EAD-triggered activity is related to effects of cycle length on APD. Shortening cycle length prevents EADs by accelerating repolarization. Typical PES protocols inhibit formation of EADs which can be therapeutic.

Estimating the probabilities of rare arrhythmic events in multiscale computational models of cardiac cells and tissue.

Ectopic heartbeats can trigger reentrant arrhythmias, leading to ventricular fibrillation and sudden cardiac death. Such events have been attributed to perturbed Ca2+ handling in cardiac myocytes leading to spontaneous Ca2+ release and delayed afterdepolarizations (DADs). However, the ways in which perturbation of specific molecular mechanisms alters the probability of ectopic beats is not understood. We present a multiscale model of cardiac tissue incorporating a biophysically detailed three-dimensional model of the ventricular myocyte. This model reproduces realistic Ca2+ waves and DADs driven by stochastic Ca2+ release channel (RyR) gating and is used to study mechanisms of DAD variability. In agreement with previous experimental and modeling studies, key factors influencing the distribution of DAD amplitude and timing include cytosolic and sarcoplasmic reticulum Ca2+ concentrations, inwardly rectifying potassium current (IK1) density, and gap junction conductance. The cardiac tissue model is used to investigate how random RyR gating gives rise to probabilistic triggered activity in a one-dimensional myocyte tissue model. A novel spatial-average filtering method for estimating the probability of extreme (i.e. rare, high-amplitude) stochastic events from a limited set of spontaneous Ca2+ release profiles is presented. These events occur when randomly organized clusters of cells exhibit synchronized, high amplitude Ca2+ release flux. It is shown how reduced IK1 density and gap junction coupling, as observed in heart failure, increase the probability of extreme DADs by multiple orders of magnitude. This method enables prediction of arrhythmia likelihood and its modulation by alterations of other cellular mechanisms.

Arrhythmogenic Delayed Afterdepolarizations Are Promoted by Severe Hypothermia But Not Therapeutic Hypothermia.

Severe hypothermia (SH) is known to be arrhythmogenic, but the effect of therapeutic hypothermia (TH) on arrhythmias is unclear. It is hypothesized that susceptibility to Ca-mediated arrhythmia triggers would be increased only by SH.Methods and Results:Spontaneous Ca release (SCR) and resultant delayed afterdepolarizations (DADs) were evaluated by optical mapping in canine wedge preparations during normothermia (N, 36℃), TH (32℃) or SH (28℃; n=8 each). The slope (amplitude/rise time) of multicellular SCR (mSCR) events, a determinant of triggered activity, was suppressed in TH (24.4±3.4%/s vs. N: 41.5±6.0%/s), but significantly higher in SH (96.3±8.1%/s) producing higher amplitude DADs in SH (35.7±1.6%) and smaller in TH (5.3±1.0% vs. N: 10.0±1.1%, all P<0.05). Triggered activity was only observed in SH. In isolated myocytes, sarcoplasmic reticulum (SR) Ca release kinetics slowed in a temperature-dependent manner, prolonging Ca transient rise time [33±3 (N) vs. 50±6 (TH) vs. 88±12 ms (SH), P<0.05], which can explain the decreased mSCR slope and DAD amplitude in TH. Although the SR Ca content was similar in TH and SH, Ca spark frequency was markedly increased only in SH, suggesting that increased ryanodine receptor open probability could explain the increased triggered activity during SH. Temperature dependence of Ca release can explain susceptibility to Ca-mediated arrhythmia triggers in SH. This may therefore explain the increased risk of lethal arrhythmia in SH, but not during TH.

Multiscale Determinants of Delayed Afterdepolarization Amplitude in Cardiac Tissue

Spontaneous calcium (Ca) waves in cardiac myocytes underlie delayed afterdepolarizations (DADs) that trigger cardiac arrhythmias. How these subcellular/cellular events overcome source-sink factors in cardiac tissue to generate DADs of sufficient amplitude to trigger action potentials is not fully understood. Here, we evaluate quantitatively how factors at the subcellular scale (number of Ca wave initiation sites), cellular scale (sarcoplasmic reticulum (SR) Ca load), and tissue scale (synchrony of Ca release in populations of myocytes) determine DAD features in cardiac tissue using a combined experimental and computational modeling approach. Isolated patch-clamped rabbit ventricular myocytes loaded with Fluo-4 to image intracellular Ca were rapidly paced during exposure to elevated extracellular Ca (2.7 mmol/L) and isoproterenol (0.25 μmol/L) to induce diastolic Ca waves and subthreshold DADs. As the number of paced beats increased from 1 to 5, SR Ca content (assessed with caffeine pulses) increased, the number of Ca wave initiation sites increased, integrated Ca transients and DADs became larger and shorter in duration, and the latency period to the onset of Ca waves shortened with reduced variance. In silico analysis using a computer model of ventricular tissue incorporating these experimental measurements revealed that whereas all of these factors promoted larger DADs with higher probability of generating triggered activity, the latency period variance and SR Ca load had the greatest influences. Therefore, incorporating quantitative experimental data into tissue level simulations reveals that increased intracellular Ca promotes DAD-mediated triggered activity in tissue predominantly by increasing both the synchrony (decreasing latency variance) of Ca waves in nearby myocytes and SR Ca load, whereas the number of Ca wave initiation sites per myocyte is less important.

Triggered activity in atrial myocytes is influenced by Na+/Ca2 + exchanger activity in genetically altered mice

AimsIn atrial fibrillation, increased function of the Na+/Ca2+-exchanger (NCX) is one among several electrical remodeling mechanisms. Methods/resultsUsing the patch-clamp- and Ca2+ imaging-methods, we investigated atrial myocytes from NCX-homozygous-overexpressor (OE)- and heterozygous-knockout (KO)-mice and their corresponding wildtypes (WTOE; WTKO). NCX mediated Ca2+ extrusion capacity was reduced in KO and increased in OE. There was no evidence for structural or molecular remodeling. During a proarrhythmic pacing-protocol, the number of low amplitude delayed afterdepolarizations (DADs) was unaltered in OE vs. WTOE and KO vs. WTKO. However, DADs triggered full spontaneous action potentials (sAP) significantly more often in OE vs. WTOE (ratio sAP/DAD: OE:0.18±0.05; WTOE:0.02±0.02; p<0.001). Using the same protocol, a DAD triggered an sAP by tendency less often in KO vs. WTKO (p=0.06) and significantly less often under a more aggressive proarrhythmic protocol (ratio sAP/DAD: KO:0.01±0.003; WT KO: 0.12±0.05; p=0.007). The DAD amplitude was increased in OE vs. WTOE and decreased in KO vs. WTKO. There were no differences in SR-Ca2+-load, the number of spontaneous Ca2+-release-events or IKACh/IK1. ConclusionsAtrial myocytes with increased NCX expression exhibited increased vulnerability towards sAPs while atriomyocytes with reduced NCX expression were protected. The underlying mechanism consists of a modification of the DAD-amplitude by the level of NCX-activity. Thus, although the number of spontaneous Ca2+-releases and therefore DADs is unaltered, the higher DAD-amplitude in OE made a transgression of the voltage-threshold of an sAP more likely. These findings indicate that the level of NCX activity could influence triggered activity in atrial myocytes independent of possible remodeling processes.

Determinants of the Diastolic Calcium-Voltage Coupling Gain for Delayed Afterdepolarization Mediated Triggered Activity

Spontaneous calcium (Ca) release causes delayed afterdepolarizations (DADs) which can result in triggered activity (TA) if the DAD amplitude reaches a critical threshold. It is known that increasing Na-Ca exchange (NCX) or deceasing IK1, which both occur in heart failure remodeling, increases the diastolic Ca-voltage coupling gain potentiating TA. However, the relative influence of NCX, IK1, and the Ca release amplitude on the Ca-voltage coupling gain and threshold for TA are not well understood. Using computer simulation and stability analysis, we show that elevation of the membrane voltage to the TA threshold is governed by non-linear dynamics, consistent with experiments in rabbit ventricular myocytes. This results in the Ca-voltage coupling gain being a nonlinear function of NCX, IK1, and peak Ca. For example, in the UCLA rabbit action potential model, a clamped submembrane Ca peak of 0.4 μM corresponds to a 3.5 mV subthreshold DAD (below the threshold for TA). For this DAD, the NCX conductance had to be increased by a factor of 4 to elicit TA, but decreasing IK1 conductance even to zero failed to elicit TA. For a clamped Ca peak of 0.6 μM corresponding to a 7.5 mV subthreshold DAD, NCX conductance had to be increased by a factor of 2 to elicit TA, and now reducing IK1 by a factor of 4 was able to cause TA. When the peak Ca was increased to 1.1 μM, TA occurred with the unchanged control conductance values of NCX and IK1. Analysis of the underlying nonlinear dynamics provides the theoretical basis for the differential and nonlinear dependence of TA threshold and Ca-voltage coupling gain on peak Ca, NCX, and IK1.

Abstract 17193: Inhibiting Late Sodium Current Attenuates Isoproterenol-Induced Transient Inward Current and Delayed Afterdepolarizations in Ventricular Myocytes

Introduction: The β-adrenergic agonist isoproterenol (ISO) is known to induce the arrhythmogenic transient inward current (I Ti ) and delayed afterdepolarization (DAD) via a stimulation of L-type Ca 2+ current. Recent studies found that ISO-induced DADs in cardiac tissues are inhibited by GS967, a selective blocker of the late Na + current (I NaL ). Thus, we hypothesize that I NaL contributes to the actions of ISO, and selective inhibition of this current will reduce ISO-induced I Ti and DADs. Methods: Transmembrane currents and action potentials of rabbit and guinea pig (GP) ventricular myocytes were recorded using the whole-cell patch-clamp technique. ISO (0.1 μM), GS967 (1 μM) and the Na + channel blocker tetrodotoxin (TTX, 3 μM) were used in the experiments. Results: In rabbit myocytes, application of ISO caused an increase in the amplitude of I NaL from -0.10±0.03 to -0.32±0.04 pA/pF (n = 17, p &lt; 0.05). The ISO-stimulated I NaL was inhibited by GS967 and TTX. In one series of experiments, ISO increased the I NaL from -0.14±0.04 to -0.35±0.06 pA/pF, and GS967 applied in the presence of ISO reduced the current to -0.14±0.03 pA/pF (n = 9, p &lt; 0.05). In another series of experiments, the amplitude of I NaL was increased by ISO from -0.17±0.08 to -0.41±0.09 pA/pF, and was decreased to -0.09±0.08 pA/pF when TTX was applied with ISO (n = 5, p &lt; 0.05). Application of ISO also induced I Ti and DADs. GS967 applied in the presence of ISO inhibited the amplitude of I Ti by 52±6%, from -1.79±0.30 to -0.87±0.16 pA/pF (n = 8, p &lt; 0.05). Consistent with the inhibition of I Ti , GS967 suppressed the amplitude of ISO-induced DADs by 56±12%, from 6.54±1.59 to 3.22±1.27 mV (n = 5, p &lt; 0.05). Similarly, in GP myocytes ISO-induced I Ti and DADs were decreased by GS967 from -1.14±0.21 to -0.73±0.16 pA/pF (n = 7, p &lt; 0.05) and from 7.16±0.59 to 4.67±0.24 mV (n = 5, p &lt; 0.05), respectively. Conclusions: An increased I NaL is likely to contribute to the proarrhythmic effects of ISO in cardiac myocytes. GS967 significantly attenuated ISO-induced I NaL , I Ti and DADs, suggesting that inhibiting this current could be an effective strategy to antagonize the arrhythmogenic actions of β-adrenergic stimulation.

Suppression of Early and Late Afterdepolarizations by Heterozygous Knockout of the Na+/Ca2+ Exchanger in a Murine Model.

The Na(+)/Ca(2+) exchanger (NCX) has been implied to cause arrhythmias. To date, information on the role of NCX in arrhythmogenesis derived from models with increased NCX expression, hypertrophy, and heart failure. Furthermore, the exact mechanism by which NCX exerts its potentially proarrhythmic effect, ie, by promoting early afterdepolarization (EAD) or delayed afterdepolarization (DAD) or both, is unknown. We investigated isolated cardiomyocytes from a murine model with heterozygous knockout of NCX (hetKO) using the patch clamp and Ca(2+) imaging techniques. Action potential duration was shorter in hetKO with IKtot not being increased. The rate of spontaneous Ca(2+) release events and the rate of DADs were unaltered; however, DADs had lower amplitude in hetKO. A DAD triggered a spontaneous action potential significantly less often in hetKO when compared with wild-type. The occurrence of EADs was also drastically reduced in hetKO. ICa activity was reduced in hetKO, an effect that was abolished in the presence of the Ca(2+) buffer BAPTA. Genetic suppression of NCX reduces both EADs and DADs. The following molecular mechanisms apply: (1) Although the absolute number of DADs is unaffected, an impaired translation of DADs into spontaneous action potentials results from a reduced DAD amplitude. (2) EADs are reduced in absolute number of occurrence, which is presumably a consequence of shortened action potential duration because of reduced NCX activity but also reduced ICa the latter possibly being caused by a direct modulation of Ca(2+)-dependent ICa inhibition by reduced NCX activity. This is the first study to demonstrate that genetic inhibition of NCX protects against afterdepolarizations and to investigate the underlying mechanisms.

Electrophysiological properties and mechanism of aging for the susceptibility of left atrium to arrhythmogenesis in rabbits

To explore the electrophysiological properties and mechanism of aging for the susceptibility of left atrium (LA) to arrhythmogenesis in rabbits. A total of 50 male New Zealand rabbits were divided into young (6 months, n = 25) and aged (26 months, n = 25) groups. According to comparisons before and after isoproterenol (ISO) dosing, young rabbits were further divided into young control and medication groups while aged ones into aged control and medication groups. Electrophysiological study was performed on perfused isolated left atrium and conventional microelectrodes were used for recording transmenbrane action potentials (tAP), resting membrane potential (RMP), amplitude of APs (APA), maximum upstroke velocity (Vmax), APs duration at 20%, at 50% or at 90% repolarization (APD20, APD50, APD90), atrial effective refractory periods (AERP) and atrial arrhythmia inducibility before and after ISO dosing. L-type calcium current (ICa-L) was measured via whole-cell patch clamp technique. And fluorescent intensity of intracellular Ca²⁺ was detected with Flup-3/AM loading by laser scanning confocal microscope in enzymatically dissociated single LA myocytes before and after dosing. Compared with major parameters of tAP in young control group, APDs and AERP significantly increased and APA, Vmax, RMP and AERP/APD90 obviously decreased in aged control group (all P < 0.05). Otherwise, the conduction of tAP was rapid and delayed afterdepolarizations (DADs) were induced in both young and aged medication groups after ISO dosing. However, as compared with major parameters of young medication group, APDs dramatically declined while APA, Vmax, RMP and DAD amplitude significantly increased. And triggered activity was induced in aged medication group (all P < 0.05). With voltage clamp model, ICa-L significantly declined in aged control group versus young control group. The peak current density of ICa-L dramatically decreased ((6.3 ± 0.8) pA/pF in aged control group vs (9.5 ± 1.3) pA/pF in young control group, P < 0.01) under 10 mV clamp voltage. And current-voltage (I-V) curve of ICa-L was greatly upgraded in aged control group on the top of all I-V curves without changing direction. The current density of ICa-L increased much more in aged medication group than that in young medication group ((15.9 ± 3.1) vs (11.3 ± 2.6) pA/pF, P < 0.01). And I-V curve of ICa-L shifted the bottom of all I-V curves in aged medication group. Fluorescent intensities of intracellular free Ca²⁺ became completely attenuated ((437.9 ± 21.7) µmol/L in aged control group vs (778.3 ± 19.5) µmol/L in young control group, P < 0.01). Under the effect of ISO administration, fluorescent intensities of intracellular free Ca²⁺ significantly increased in both young and aged medication groups. But [Ca²⁺]i were altered more in aged medication group than that in young medication group ((1 635.6 ± 51.6) vs (1 008.7 ± 34.5) µmol/L, P < 0.05). The electrophysiological characteristics are significantly altered with electrical remodeling before and after ISO dosing in aging LA in rabbits. And it may contribute to the vulnerability and pathophysiological basis for reentry and atrial arrhythmogenesis. The electrophysiological mechanism is probably attributed to the alterations of ICa-L and [Ca²⁺]i before and after ISO perfusion in aging LA myocytes.

Delayed afterdepolarizations generate both triggers and a vulnerable substrate promoting reentry in cardiac tissue

Delayed afterdepolarizations (DADs) have been well characterized as arrhythmia triggers, but their role in generating a tissue substrate vulnerable to reentry is not well understood. The purpose of this study was to test the hypothesis that random DADs can self-organize to generate both an arrhythmia trigger and a vulnerable substrate simultaneously in cardiac tissue as a result of gap junction coupling. Computer simulations in 1-dimensional cable and 2-dimensional tissue models were performed. The cellular DAD amplitude was varied by changing the strength of sarcoplasmic reticulum calcium release. Random DAD latency and amplitude in different cells were simulated using gaussian distributions. Depending on the strength of spontaneous sarcoplasmic reticulum calcium release and other conditions, random DADs in cardiac tissue resulted in the following behaviors: (1) triggered activity (TA); (2) a vulnerable tissue substrate causing unidirectional conduction block and reentry by inactivating sodium channels; (3) both triggers and a vulnerable substrate simultaneously by generating TA in regions next to regions with subthreshold DADs susceptible to unidirectional conduction block and reentry. The probability of the latter 2 behaviors was enhanced by reduced sodium channel availability, reduced gap junction coupling, increased tissue heterogeneity, and less synchronous DAD latency. DADs can self-organize in tissue to generate arrhythmia triggers, a vulnerable tissue substrate, and both simultaneously. Reduced sodium channel availability and gap junction coupling potentiate this mechanism of arrhythmias, which are relevant to a variety of heart disease conditions.

Multiscale Consequences of Spontaneous Calcium Release on Cardiac Delayed Afterdepolarizations

Spontaneous calcium (Ca) release in cardiac myocytes can produce delayed afterdepolarizations (DADs) that promote arrhythmias. In these studies, we combined experimental and computational approaches to investigate quantitatively how Ca release features at subcellular, cellular, and tissue scales influence DADs. Spontaneous Ca release events and corresponding DADs were measured experimentally from isolated rabbit ventricular myocytes exposed to elevated extracellular Ca (2.7 mM) and isoproterenol (0.25 μM), and were compared between lower and higher total intracellular Ca levels following 1 or 5 paced beats at 400 ms intervals, respectively. At the subcellular scale, Ca waves emerged simultaneously from an average of 1.36 (low) and 2.5 (high) sites per Ca release event. Consistent with the criticality-based theory of Ca wave emergence, Ca wave numbers were proportional to sarcoplasmic reticulum (SR) Ca levels that produced INCX with −71.1 pA/pF×ms (low) and −106.0 pA/pF×ms (high) integrated current densities upon rapid exposure to caffeine (10 mM). At the cellular scale, whole-cell calcium transients had peak amplitudes of 0.462 F/Fo (low) and 0.717 F/Fo (high) and full widths at half maximum (FWHMs) of 309.0 ms (low) and 180.0 ms (high). Resultant DADs had peak amplitudes of 1.41 mV (low) and 6.03 mV (high) and FWHMs of 302.3 ms (low) and 140.3 ms (high). Ca wave latencies in single myocytes, which determine Ca release synchrony in cardiac tissue, were 0.996 s (low) and 0.416 s (high). Computer simulation analyses implementing various combinations of the experimentally measured factors suggest that greater numbers of Ca waves, increased SR Ca release, and greater synchrony are associated with larger DAD amplitudes in cardiac tissue. Simulations also suggest that the subcellular number of Ca waves has the largest impact on DAD amplitude in tissue.

Abstract 214: Therapeutic Hypothermia Promotes Arrhythmogenic Calcium-Mediated Delayed Afterdepolarizations During Sympathetic Stimulation

Introduction: Most patients who undergo therapeutic hypothermia (TH) suffer cardiac arrest due to ischemia-induced ventricular fibrillation (VF). Ischemia and hypothermia both cause cellular calcium (Ca) overload, a substrate for Ca-mediated triggered arrhythmias. Previously, we showed that under non-ischemic conditions, TH suppresses Ca-mediated arrhythmias. However, the mechanistic relationship between hypothermia-induced Ca dysregulation and arrhythmias during ischemia, the most common clinical scenario, is unknown. We hypothesized that TH and ischemia would synergistically promote spontaneous Ca release and subsequent arrhythmogenic delayed afterdepolarizations (DADs). We examined this relationship during sympathetic stimulation, as occurs when epinephrine is given during resuscitation from VF. Methods and Results: Dual optical mapping of Ca transients and action potentials was performed in canine left ventricular wedge preparations during 10 minutes of no flow ischemia during normothermia (NT, 36°C, n=5) and TH (32°C, n=7) with and without beta-sympathetic stimulation (isoproterenol, 0.2uM, each n=5). Multicellular spontaneous Ca release events (mSCRs) and resultant DADs were induced by rapid pacing. During ischemia, TH increased mSCR amplitude (44±3%) vs. NT (26±4%, p&lt;0.04). In contrast, the slope (amplitude/rise time) of mSCRs, an important determinant of triggered activity (TA), and DAD amplitudes were similar in TH and NT (24±6 and 21%±2% respectively, both p=NS). Importantly, when sympathetic stimulation was present during ischemia, TH increased the amplitude mSCRs compared to NT (61±5 vs. 31±7%, respectively p&lt;.01) and the slope of mSCRs was 2-fold greater (p&lt;.01), resulting in greater amplitude of DADs (50±9 vs. 28±7%, p=.08) and TA was observed. Conclusions: During ischemia, TH is not arrhythmogenic unless sympathetic stimulation is introduced, where it promotes spontaneous Ca release and DADs. Moreover, these data suggest that when TH is initiated during ischemia, as in intra-arrest cooling, sympathetic stimulation during cardiac arrest may be proarrhythmic by increasing susceptibility to Ca-mediated triggered arrhythmias.

Ablation of the Androgen Receptor Gene Modulates Atrial Electrophysiology and Arrhythmogenesis With Calcium Protein Dysregulation

Androgen deficiency is important in the pathophysiology of atrial fibrillation. Androgen regulates cardiac electrophysiology and calcium (Ca(2+)) homeostasis. The purpose of this study is to evaluate whether androgen receptor knockout (ARKO) can modulate atrial electrophysiology and arrhythmogenesis with modulation of Ca(2+) homeostasis proteins. We used conventional microelectrodes to study the action potential (AP) in left atrium (LA) tissues prepared from wild-type (WT) and ARKO mice (aged 6-10 months) before and after the administration of isoproterenol, hypocalcemic/hypercalcemic solutions, and ouabain. Echocardiography and Western blots were used to evaluate the cardiac function and expression levels of ionic channel proteins in WT and ARKO LAs. ARKO LAs had larger LA diameter with decreased LA fractional shortening than did WT LAs. In the current study, we found that ARKO LAs had a lower negative resting membrane potential and a greater 90% AP duration (APD) than did WT LAs. Isoproterenol increased the incidence and amplitude of delayed afterdepolarizations (DADs) in ARKO LAs but not in WT LAs. Hypocalcemic solutions prolonged APD in WT and ARKO LAs but increased DAD amplitude only in ARKO LAs. Hypercalcemic solutions shortened APD in ARKO LAs but not in WT LAs. Ouabain increased DAD amplitude in ARKO LAs but not in WT LAs. ARKO LAs expressed higher amounts of Ca(2+)/calmodulin-dependent protein kinase II, Na(+)/Ca(2+) exchanger, and phosphorylated phospholamban (Ser-16/Thr-17 site) and less Cav1.2, Kir2.1, Kir3.1, and Kv7.1 than WT LAs. These observations indicate that ARKO alters atrial electrophysiology with increased atrial arrhythmogenesis.

Distinctive sodium and calcium regulation associated with sex differences in atrial electrophysiology of rabbits

BackgroundSex and sodium/calcium regulation play critical roles in cardiac electrophysiology and atrial arrhythmogenesis. We investigated whether sodium and calcium contributed to sex differences in atrial electrophysiology. MethodsWhole-cell patch clamp techniques and the indo-1 fluorometric ratio technique were used to investigate the ionic current and intracellular calcium in single isolated male and female rabbit myocytes from the left atrium posterior wall (LAPW) and right atrium (RA). ResultsFemale LAPW (n=95) and RA (n=49) myocytes had larger cell widths (15.1±0.4 vs. 13.8±0.4μm, p<0.05; 14.9±0.6 vs. 13.5±0.4μm, p<0.05) than male LAPW (n=142) and RA (n=57) myocytes. Male LAPW myocytes (n=26) had a higher incidence (57 vs. 16%, p<0.05) of delayed afterdepolarizations (DADs) than female LAPW myocytes (n=24) but there were similar incidences (20 vs. 20%, p>0.05) of DADs in male and female RA myocytes. The late sodium current, calcium transients, and sarcoplasmic reticulum calcium contents were larger in male than female LAPW myocytes but were similar in male and female RA myocytes. However, the ICa-L and nickel-sensitive sodium/calcium exchanger currents were similar between two groups. Different from those in female myocytes, ouabain (10μM) only induced repeated atrial beats (0 to 45%, p<0.05) in male myocytes (n=11). Moreover, ranolazine (3μM) perfusion (4.5±0.6 vs. 1min, p<0.05) was required to decrease the amplitude of DADs in male but not female LAPW myocytes. ConclusionsIncreased late sodium currents and calcium contents may contribute to higher arrhythmogenesis in male LAPW myocytes.

Abstract 30: Therapeutic Hypothermia Suppresses Arrhythmogenic, Calcium-Mediated Delayed Afterdepolarizations

Background : Therapeutic hypothermia (TH) is recommended for patients with return of spontaneous circulation after resuscitation from cardiac arrest. Hypothermia is known to cause derangements in cellular calcium (Ca) cycling, leading to Ca overload; however, the effect of TH on Ca-mediated arrhythmogenic substrates remains unclear. We hypothesized abnormal Ca cycling during TH would promote spontaneous Ca release and delayed afterdepolarizations (DADs). Methods and Results : Transmural dual optical mapping of Ca transients and action potentials was performed in canine wedge preparations during normothermia (36ºC; N group, n=5) or TH (32ºC; n=6). Multicellular spontaneous Ca release events (mSCR) and resultant DADs were induced by rapid pacing. TH significantly slowed the kinetics of Ca cycling compared to normal controls, significantly prolonging the time to peak (60.4 ± 5.7 vs. 40.1 ± 2.6 ms), duration (401.2 ± 13.8 vs. 305.8 ± 12.4 ms) and rate of decay (247.4 ± 29.2 vs. 146.0 ± 12.6 ms, all p&lt;0.05) of full Ca transients. The maximal mSCR amplitude was not significantly different in TH (21.1 ± 6.0%) compared to normals (28.4 ± 1.9%, p=NS). However, the slope (mSCR amplitude/time to peak) of mSCR activity, an important determinant of triggered activity, was significantly slower in TH (14.3 ± 3.7 %/s) compared to N (29.9 ± 3.9%/s, p&lt;0.05). Importantly, DAD amplitude was also significantly lower in TH (4.1 ± 2.2%) compared to N (11.1 ± 1.5%, p&lt;0.05). Conclusion : Even though therapeutic hypothermia did not decrease the magnitude of spontaneous calcium release, it did significantly reduce the slope, which was associated with significantly lower DAD amplitude. These results suggest that TH could be antiarrhythmic, in part, by slowing the kinetics of spontaneous calcium release and, thus, decreasing delayed afterdepolarizations, an important cause of triggered activity and arrhythmias.

Acceleration of Ca2+ Waves in Monocrotaline-Induced Right Ventricular Hypertrophy in the Rat

Triggered arrhythmias arise from delayed afterdepolarizations (DADs), with Ca(2+) waves playing an important role in their formation. In ventricular hypertrophy, however, it remains unclear how Ca(2+) waves change their propagation features and affect arrhythmogenesis. We addressed this important issue in a rat model of hypertrophy. Rats were given a subcutaneous injection of 60 mg/kg monocrotaline (MCT-rats) or solvent (Ctr-rats). After 4 weeks, MCT-rats showed high right ventricular (RV) pressure and RV hypertrophy. Trabeculae were dissected from 36 right ventricles. The force was measured using a silicon strain gauge and regional intracellular Ca(2+) ([Ca(2+)](i)) was determined using microinjected fura-2. Reproducible Ca(2+) waves were induced by stimulus trains (2 Hz, 7.5s). MCT-rats showed a higher diastolic [Ca(2+)](i) and faster and larger Ca(2+) waves (P<0.01). The velocity and amplitude of Ca(2+) waves were correlated with the diastolic [Ca(2+)](i) both in the Ctr- and MCT-rats. The velocity of Ca(2+) waves in the MCT-rats was larger at the given amplitude of Ca(2+) waves than that in the Ctr-rats (P < 0.01). The amplitude of DADs was correlated with the velocity and amplitude of Ca(2+) waves in the Ctr- and MCT-rats. The results suggest that an increase in diastolic [Ca(2+)](i) and an increase in Ca(2+) sensitivity of the sarcoplasmic reticulum Ca(2+) release channel accelerate Ca(2+) waves in ventricular hypertrophy, thereby causing arrhythmogenesis.

Variability in Timing of Spontaneous Calcium Release in the Intact Rat Heart Is Determined by the Time Course of Sarcoplasmic Reticulum Calcium Load

Abnormalities in intracellular calcium (Ca) cycling during Ca overload can cause triggered activity because spontaneous calcium release (SCR) activates sufficient Ca-sensitive inward currents to induce delayed afterdepolarizations (DADs). However, little is known about the mechanisms relating SCR and triggered activity on the tissue scale. Laser scanning confocal microscopy was used to measure the spatiotemporal properties of SCR within large myocyte populations in intact rat heart. Computer simulations were used to predict how these properties of SCR determine DAD magnitude. We measured the average and standard deviation of the latency distribution of SCR within a large population of myocytes in intact tissue. We found that as external [Ca] is increased, and with faster pacing rates, the average and SD of the latency distribution decreases substantially. This result demonstrates that the timing of SCR occurs with less variability as the sarcoplasmic reticulum (SR) Ca load is increased, causing more sites to release Ca within each cell. We then applied a mathematical model of subcellular Ca cycling to show that a decrease in SCR variability leads to a higher DAD amplitude and is dictated by the rate of SR Ca refilling following an action potential. Our results demonstrate that the variability of the timing of SCR in a population of cells in tissue decreases with SR load and is dictated by the time course of the SR Ca content.

Detection of patients with delayed afterdepolarization using cardiac magnetic field mapping at rest

Event Abstract Back to Event Detection of patients with delayed afterdepolarization using cardiac magnetic field mapping at rest Taysia Sosnytska1*, Leonyd Stadnyuk1 and Volodymyr Sosnytskyy2 1 P.L. Shupik Medical Academy for Postgraduate Education of UH Ministry, Ukraine 2 Glushkov Institute of Cybernetics, MCG Laboratory, Ukraine Objective. Human ventricular cells studies have shown that the Na+/Ca2+ exchange current (INa/Ca) contribute to the transient inward current (Iti). Iti is responsible for the proarrhythmic delayed afterdepolarizations (DADs). Although DADs are generally accepted to be Ca2+-dependent, the relationship between sarcopasmic reticulum Ca2+ release and DAD amplitude has not been measured, partly because the underlying Ca2+ -activated currents are hard to control. The overall objective of the present study was to demonstrate the efficacy of magnetocardiography (MCG) in the diagnosis of the Ca2+ overload in some area of the heart in patients presenting with DADs. Methods. Standard clinical examination (e.g. Tissue Doppler echocardiograms (TDE), 12-channel ECG, 24-h Holter monitoring) and MCG were performed in 24 patients (age 25±1.9) affected by DADs and 22 (age 27±1.9) healthy controls. MCG was recorded by means of 4x3–channel MCG system (Kiev Medical Group Ltd, Ukraine) in an unshielded setting. Current density vector (CDV) maps were plotted by the magnetic moments method of the “inverse problem solution”. MCG based indices, namely direction of the largest CDV, its deviation, shift and current density total sum during electric diastole interval were performed using an automated MCG analysis program. Results. Two MCG based parameters showed a significant difference between DADs patients and the control groups. We also found that verapamil, the prototype calcium-channel blocker, effects these parameters in patients. Conclusion. We conclude MCG is a promising procedure for a non-invasive testing and localization of an area of the heart which provides a substrate that favors electrical abnormalities and lately arrhythmias. Conference: Biomag 2010 - 17th International Conference on Biomagnetism , Dubrovnik, Croatia, 28 Mar - 1 Apr, 2010. Presentation Type: Poster Presentation Topic: MCG: Instrumentation, Modeling, Basic and Clinical Studies Citation: Sosnytska T, Stadnyuk L and Sosnytskyy V (2010). Detection of patients with delayed afterdepolarization using cardiac magnetic field mapping at rest. Front. Neurosci. Conference Abstract: Biomag 2010 - 17th International Conference on Biomagnetism . doi: 10.3389/conf.fnins.2010.06.00331 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 06 Apr 2010; Published Online: 06 Apr 2010. * Correspondence: Taysia Sosnytska, P.L. Shupik Medical Academy for Postgraduate Education of UH Ministry, Kiev, Ukraine, t_sosnytskaya@mail.ru Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Taysia Sosnytska Leonyd Stadnyuk Volodymyr Sosnytskyy Google Taysia Sosnytska Leonyd Stadnyuk Volodymyr Sosnytskyy Google Scholar Taysia Sosnytska Leonyd Stadnyuk Volodymyr Sosnytskyy PubMed Taysia Sosnytska Leonyd Stadnyuk Volodymyr Sosnytskyy Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.