Recent evidence indicates that the voltage clock (cyclic activation and deactivation of membrane ion channels) and Ca2+ clocks (rhythmic spontaneous sarcoplasmic reticulum Ca2+ release) jointly regulate sinoatrial node (SAN) automaticity. However, the relative importance of the voltage clock and Ca2+ clock for pacemaking was not revealed in sick sinus syndrome. Previously, we mapped the intracellular calcium (Cai) and membrane potentials of the normal intact SAN simultaneously using optical mapping in Langendorff-perfused canine right atrium. We demonstrated that the sinus rate increased and the leading pacemaker shifted to the superior SAN with robust late diastolic Cai elevation (LDCAE) during β-adrenergic stimulation. We also showed that the LDCAE was caused by spontaneous diastolic sarcoplasmic reticulum (SR) Ca2+ release and was closely related to heart rate changes. In contrast, in pacing induced canine atrial fibrillation and SAN dysfunction models, Ca2+ clock of SAN was unresponsiveness to β-adrenergic stimulation and caffeine. Ryanodine receptor 2 (RyR2) in SAN was down-regulated. Using the prolonged low dose isoproterenol together with funny current block, we produced a tachybradycardia model. In this model, chronically elevated sympathetic tone results in abnormal pacemaking hierarchy in the right atrium, including suppression of the superior SAN and enhanced pacemaking from ectopic sites. Finally, if the LDCAE was too small to trigger an action potential, then it induced only delayed afterdepolarization (DAD)-like diastolic depolarization (DD). The failure of DAD-like DD to consistently trigger a sinus beat is a novel mechanism of atrial arrhythmogenesis. We conclude that dysfunction of both the Ca2+ clock and the voltage clock are important in sick sinus syndrome.
Calcium; sinoatrial node; sarcoplasmic reticulum; sick sinus syndrome
Carvedilol and its analogues suppress delayed afterdepolarizations (DADs) and catecholaminergic polymorphic ventricular tachycardias by direct action on the cardiac ryanodine receptor (RyR2).
We tested a hypothesis that carvedilol analogue may also prevent triggered activities (TAs) through the suppression of early afterdepolarizations (EADs).
Intracellular Ca2+ and membrane voltage were simultaneously recorded using optical mapping technique in Langendorff-perfused mouse and rabbit hearts to study the effect of carvedilol analogue, VK-II-36 that does not have significant beta-blocking effects.
Spontaneous intracellular Ca2+ elevations (SCaEs) during diastole was induced by rapid ventricular pacing and isoproterenol infusion in intact rabbit ventricles. Systolic and diastolic SCaEs were simultaneously noted in Langendorff-perfused RyR2 R4496+/− mouse hearts after creating atrioventricular block. VK-II-36 effectively suppressed SCaEs and eliminated TAs observed in both mouse and rabbit ventricles. We tested the effect of VK-II-36 on EADs using a rabbit model of acquired long QT syndrome in which phase-2 and phase-3 EADs were observed in association with systolic SCaEs. VK-II-36 abolished the systolic SCaEs and phase-2 EADs, and greatly decreased the dispersion of repolarization and the amplitude of phase-3 EADs. VK-II-36 completely prevented EAD-mediated TAs in all ventricles studied.
A carvedilol analogue, VK-II-36 inhibits ventricular tachyarrhythmias in intact mouse and rabbit ventricles by suppression of SCaEs, independent of beta-blocking activity. The RyR2 may be a potential target for treating focal ventricular arrhythmias triggered by either EADs or DADs.
afterdepolarization; intracellular calcium; long-QT syndrome
To elucidate the mechanism of late-phase 3 early after depolarization (EAD) in ventricular arrhythmogenesis, we hypothesized that intracellular calcium (Cai) overloading and action potential duration (APD) shortening may promote late phase 3 EAD and triggered activity, leading to development of ventricular fibrillation (VF).
Methods and Results
In isolated rabbit hearts, we performed microelectrode recording and simultaneous dual optical mapping of transmembrane potential (Vm) and Cai transient on left ventricular endocardium. An IKATP channel opener, pinacidil, was used to abbreviate action potential duration (APD). Rapid-pacing was then performed. Upon abrupt cessation of rapid pacing with cycle lengths of 60–200 ms, there were APD90 prolongation and the corresponding Cai overloading in the first post-pacing beats. The duration of Cai transient recovered to 50% (DCaT50) and 90% (DCaT90) in the first post-pacing beats was significantly longer than baseline. Abnormal Cai elevation coupled with shortened APD produced late-phase 3 EAD induced triggered activity and VF. In additional 6 preparations, the heart tissues were treated with BAPTA-AM, a calcium chelator. BAPTA-AM significantly reduced the maximal Cai amplitude (26.4±3.5% of the control; p<0.001) and the duration of Cai transients in the mapped region, preventing the development of EAD and triggered activity that initiated VF.
IKATP channel activation along with Cai overloading are associated with the development of late phase 3 EAD and VF. Because acute myocardial ischemia activates the IKATP channel, late phase 3 EADs may be a mechanism for VF initiation during acute myocardial ischemia.
ventricular fibrillation; early afterdepolarization; triggered activity; calcium; APD shortening
Apamin sensitive potassium current (IKAS), carried by the type 2 small conductance Ca2+-activated potassium (SK2) channels, plays an important role in post-shock action potential duration (APD) shortening and recurrent spontaneous ventricular fibrillation (VF) in failing ventricles.
To test the hypothesis that amiodarone inhibits IKAS in human embryonic kidney 293 (HEK-293) cells.
We used the patch-clamp technique to study IKAS in HEK-293 cells transiently expressing human SK2 before and after amiodarone administration.
Amiodarone inhibited IKAS in a dose-dependent manner (IC50, 2.67±0.25 µM with 1 µM intrapipette Ca2+). Maximal inhibition was observed with 50 µM amiodarone which inhibited 85.6±3.1% of IKAS induced with 1 µM intrapipette Ca2+ (n = 3). IKAS inhibition by amiodarone was not voltage-dependent, but was Ca2+-dependent: 30 µM amiodarone inhibited 81.5±1.9% of IKAS induced with 1 µM Ca2+ (n = 4), and 16.4±4.9% with 250 nM Ca2+ (n = 5). Desethylamiodarone, a major metabolite of amiodarone, also exerts voltage-independent but Ca2+ dependent inhibition of IKAS.
Both amiodarone and desethylamiodarone inhibit IKAS at therapeutic concentrations. The inhibition is independent of time and voltage, but is dependent on the intracellular Ca2+ concentration. SK2 current inhibition may in part underlie amiodarone's effects in preventing electrical storm in failing ventricles.
Myocardial infarction (MI) results in cardiac nerve sprouting in the myocardium. Whether or not similar neural remodeling occurs in the stellate ganglia (SG) is unknown. We aimed to test the hypothesis that MI induces bilateral SG nerve sprouting.
Acute MI was created by coronary artery ligation in rabbits (n=12). Serum nerve growth factor (NGF) level was measured by enzyme-linked immunosorbent assay (ELISA). The hearts and bilateral SGs were harvested for immunohistochemistry after 1 week in 6 rabbits, and after 1 month in 6 rabbits. Immunostaining for tyrosine hydroxylase (TH), growth-associated protein 43 (GAP43), cholineacetyltransferase (ChAT) and synaptophysin (SYN) was performed to determine the magnitude of nerve sprouting. Tissues from 6 normal rabbits were used as controls. Nerve density was determined by computerized morphometry.
MI results in increased serum NGF levels at 1 week (1519.8±632.2 ng/ml) that persists to 1 month (1361.2±176.3 ng/ml) as compared to controls (89.6±34.9 ng/ml), (p=.0002, and , p=.0001, respectively). Immunostaining demonstrated nerve sprouting and hyperinnervation in both SGs after MI. The nerve densities (µm2/ganglion cell) in SG 1 week after MI, 1 month after MI and in control groups, respectively, were: GAP43, 278±96, 225±39 and 149±57 (p=.01); SYN, 244±152, 268±115 and 102±60 (p=.02); TH, 233±71, 180±50 and 135±68 (p=.047); ChAT, 244±100, 208±46 and 130±41 µm2/cell (p=.01).
MI increases serum NGF levels and induces nerve sprouting and hyperinnervation in bilateral SGs for at least 1 month after MI. The hyperinnervation includes both postganglionic adrenergic axons and preganglionic cholinergic axons in the SG.
Myocardial Infarction; Ventricular Arrhythmia; Autonomic Nervous System; Stellate Ganglion; Nerve Sprouting; Sudden Cardiac Death
Bidirectional ventricular tachycardia; Andersen-Tawil Syndrome; Alternans
Calcium transient triggered firing (CTTF) is induced by large intracellular calcium (Cai) transient and short action potential duration (APD). We hypothesized that CTTF underlies the mechanisms of early afterdepolarization (EAD) and spontaneous recurrent atrial fibrillation (AF) in transgenic (Tx) mice with overexpression of transforming growth factor β1 (TGF-β1).
Methods and Results
MHC-TGFcys33ser Tx mice develop atrial fibrosis because of elevated levels of TGF-β1. We studied membrane potential and Cai transients of isolated superfused atria from Tx and wild-type (Wt) littermates. Short APD and persistently elevated Cai transients promoted spontaneous repetitive EADs, triggered activity and spontaneous AF after cessation of burst pacing in Tx but not Wt atria (39% vs. 0%, P=0.008). We were able to map optically 4 episodes of spontaneous AF re-initiation. All first and second beats of spontaneous AF originated from the right atrium (4/4, 100%), which is more severely fibrotic than the left atrium. Ryanodine and thapsigargin inhibited spontaneous re-initiation of AF in all 7 Tx atria tested. Western blotting showed no significant changes of calsequestrin or sarco/endoplasmic reticulum Ca2+-ATPase 2a.
Spontaneous AF may occur in the Tx atrium because of CTTF, characterized by APD shortening, prolonged Cai transient, EAD and triggered activity. Inhibition of Ca2+ release from the sarcoplasmic reticulum suppressed spontaneous AF. Our results indicate that CTTF is an important arrhythmogenic mechanism in TGF-β1 Tx atria.
Arrhythmia; Atrial fibrillation; Ca2+ triggers; Intracellular calcium; Optical mapping; Transgenic mice models
Purpose of review
The autonomic nerve system is a potentially potent modulator of the initiation and perpetuation of atrial fibrillation (AF). This review will briefly summarize the neural mechanisms of AF.
Complex interactions exist between the sympathetic and parasympathetic nervous system on the atrial electrophysiologic properties. Direct autonomic recordings in canine models demonstrated simultaneous sympathovagal discharges are the most common triggers of paroxysmal atrial tachycardia and paroxysmal AF. Also, intrinsic cardiac autonomic nerve can serve as a sole triggering factor for the initiation of AF. Modulation of autonomic nervous system (ANS) by electrical stimulation has been tried as a treatment strategy clinically and experimentally. Recent studies showed that autonomic nervous system modulation can suppress the stellate ganglion nerve activity and reduce the incidence of paroxysmal atrial tachyarrhythmias in ambulatory dogs.
The autonomic nerve system influences the initiation and perpetuation of AF. Scientific advances toward a better understanding of the complex interrelationships of the various components of the ANS will hopefully lead to improvement of treatments for this common arrhythmia.
Autonomic nerve; atrial fibrillation; vagal nerve stimulation
Arrhythmogenic right ventricular cardiomyopathy (ARVC) is an inheritable myocardial disorder associated with fibrofatty replacement of myocardium and ventricular arrhythmia. A subset of ARVC is categorized as Naxos disease, which is characterized by ARVC and a cutaneous disorder. A homozygous loss-of-function mutation of the Plakoglobin (Jup) gene, which encodes a major component of the desmosome and the adherens junction, had been identified in Naxos patients, although the underlying mechanism remained elusive. We generated Jup mutant mice by ablating Jup in cardiomyocytes. Jup mutant mice largely recapitulated the clinical manifestation of human ARVC: ventricular dilation and aneurysm, cardiac fibrosis, cardiac dysfunction and spontaneous ventricular arrhythmias. Ultra-structural analyses revealed that desmosomes were absent in Jup mutant myocardia, whereas adherens junctions and gap junctions were preserved. We found that ventricular arrhythmias were associated with progressive cardiomyopathy and fibrosis in Jup mutant hearts. Massive cell death contributed to the cardiomyocyte dropout in Jup mutant hearts. Despite the increase of β-catenin at adherens junctions in Jup mutant cardiomyoicytes, the Wnt/β-catenin-mediated signaling was not altered. Transforming growth factor-beta-mediated signaling was found significantly elevated in Jup mutant cardiomyocytes at the early stage of cardiomyopathy, suggesting an important pathogenic pathway for Jup-related ARVC. These findings have provided further insights for the pathogenesis of ARVC and potential therapeutic interventions.
We hypothesize that unresponsiveness of superior sinoatrial node (SAN) to sympathetic stimulation is strongly associated with the development of symptomatic bradycardia in patients with atrial fibrillation (AF).
Methods and Results
We performed 3-dimensional endocardial mapping in healthy control (Group 1, n=10) and in patients with AF without (Group 2, n=57) or with (Group 3, n=15) symptomatic bradycardia at baseline and during isoproterenol infusion. Corrected SAN recovery time was abnormal in 0%, 11% and 36% of groups 1, 2 and 3, respectively (p=0.02). At baseline, 90%, 26% and 7% (p<0.001) of the patients had multicentric SAN activation patterns. For groups 1, 2 and 3, respectively, the median distance from the superior vena cava-right atrial junction to the most cranial earliest activation site (EAS) was 5.0 (25–75 percentile range, 3.5–21.3) mm, 10.0 (4–20) mm and 17.5 (12–34) mm at baseline (p=0.01), and was 4.0 (0–5) mm, 5.0 (1–10) mm and 15.0 (5.4–33.3) mm during isoproterenol infusion (p=0.01), suggesting upward shift of EAS during isoproterenol infusion. However, while the EAS during isoproterenol infusion was at the upper 1/3 of crista terminalis in 100% of Group 1 and 78% of Group 2 patients, only 20% of the groups 3 patients moved EAS to that region (p < 0.001).
Superior SAN serves as the EAS during sympathetic stimulation in normal patients and in most patients with AF without symptomatic bradycardia. In contrast, unresponsiveness of superior SAN to sympathetic stimulation is a characteristic finding in patients with AF and symptomatic bradycardia.
sinoatrial node; nervous system; sympathetic; atrial fibrillation; sick sinus syndrome; pacemakers
Anodal stimulation hyperpolarizes cell membrane and increases intracellular Ca2+ (Cai) transient. This study tested the hypothesis that The maximum slope of Cai decline (–(dCai/dt)max) corresponds to the timing of anodal dip on the strength-interval curve and the initiation of repetitive responses and ventricular fibrillation (VF) after a premature stimulus (S2).
Methods and Results
We simultaneously mapped membrane potential (Vm) and Cai in 23 rabbit ventricles. A dip was observed on the anodal strength-interval curve. During the anodal dip, ventricles were captured by anodal break excitation directly under the S2 electrode. The Cai following anodal stimuli is larger than that following cathodal stimuli. The S1-S2 intervals of the anodal dip (203 ± 10 ms) coincided with the -(dCai/dt)max (199 ± 10 ms, p=NS). BAPTA-AM (n=3), INCX inhibition by low extracellular Na+ (n=3), and combined ryanodine and thapsigargin infusion (n=2) eliminated the anodal supernormality. Strong S2 during the relative refractory period (n=5) induced 29 repetitive responses and 10 VF episodes. The interval between S2 and the first non-driven beat was coincidental with the time of -(dCai/dt)max.
Larger Cai transient and INCX activation induced by anodal stimulation produces anodal supernormality. Time of maximum INCX activation is coincidental to the induction of non- driven beats from the Cai sinkhole after a strong premature stimulation.
anodal dip; anodal stimulus; intracellular calcium; Na+-Ca2+ exchanger current
FK506 binding protein 12 (FKBP12) is a known cis-trans peptidyl prolyl isomerase and highly expressed in the heart. Its role in regulating postnatal cardiac function remains largely unknown.
Methods and Results
We generated FKBP12 overexpressing transgenic (αMyHC-FKBP12) mice and cardiomyocyte-restricted FKBP12 conditional knockout (FKBP12f/f/αMyHC-Cre) mice, and analyzed their cardiac electrophysiology in vivo and in vitro. A high incidence (38%) of sudden death was found in αMyHC-FKBP12 mice. Surface and ambulatory ECGs documented cardiac conduction defects, which were further confirmed by electrical measurements and optical mapping in Langendorff-perfused hearts. αMyHC-FKBP12 hearts had slower action potential upstrokes, and longer action potential durations. Whole-cell patch-clamp analyses demonstrated an ~80% reduction in peak density of the tetrodotoxin-resistant, voltage-gated sodium current, INa, in αMyHC-FKBP12 ventricular cardiomyocytes, a slower recovery of INa from inactivation, shifts of steady-state activation and inactivation curves of INa to more depolarized potentials, and augmentation of late INa, suggesting that the arrhythmogenic phenotype of αMyHC-FKBP12 mice is due to abnormal INa. Ventricular cardiomyocytes isolated from FKBP12f/f/αMyHC-Cre hearts showed faster action potential upstrokes and a more than 2-fold increase in peak INa density. Dialysis of exogenous recombinant FKBP12 protein into FKBP12-deficient cardiomyocytes promptly recapitulated alterations in INa seen in αMyHC-FKBP12 myocytes.
FKBP12 is a critical regulator of INa and is important to cardiac arrhythmogenic physiology. FKPB12-mediated dysregulation of INa may underlie clinical arrhythmias associated with FK506 administration.
proteins; ion channels; conduction; heart block; long-QT syndrome
We tested the hypothesis that heart failure (HF) results in right atrial ganglionated plexus (RAGP) denervation which contributes to sinoatrial node dysfunction.
HF is associated with sinoatrial node dysfunction. However, the detailed mechanisms remain unclear.
We recorded nerve activity (NA) from the RAGP, right stellate ganglion (SG) and right vagal nerve in 7 ambulatory dogs at baseline and after pacing-induced HF. We also determined the effects of RAGP stimulation in isolated normal and HF canine right atria (RA).
Nerve activities in both the SG and vagal were significantly higher in HF than at baseline. The relationship between 1-min integrated nerve activities of vagal and RAGP showed either a positive linear correlation (Group 1, n=4) or an L-shaped correlation (Group 2, n=3). In all dogs, a reduced heart rate was observed when vagal-NA was associated with simultaneously increased RAGP-NA. On the other hand, when vagal-NA was not associated with increased RAGP-NA, the heart rate was not reduced. The induction of HF significantly decreased RAGP-NA in all dogs (P<0.05). Stimulating the superior RAGP in isolated right atrium significantly reduced the sinus rate in normal but not the HF hearts. Immunohistochemical staining revealed lower densities of tyrosine hydroxylase and choline acetyltransferase -positive nerve tissues in HF RAGP than normal (P<0.001 and P=0.001, respectively).
The RAGP nerve activity is essential for the vagal nerve to counterbalance the stellate ganglion in sinus rate control. In heart failure, RAGP denervation and decreased RAGP nerve activity contribute to the sinus node dysfunction.
heart failure; nervous system; autonomic; sinoatrial node
Fibrillation-defibrillation episodes in failing ventricles may be followed by action potential duration (APD) shortening and recurrent spontaneous ventricular fibrillation (SVF).
We hypothesized that activation of apamin-sensitive small-conductance Ca2+-activated K+ (SK) channels are responsible for the postshock APD shortening in failing ventricles.
Methods and Results
A rabbit model of tachycardia-induced heart failure was used. Simultaneous optical mapping of intracellular Ca2+ and membrane potential (Vm) was performed in failing and non-failing ventricles. Three failing ventricles developed SVF (SVF group), 9 did not (no-SVF group). None of the 10 non-failing ventricles developed SVF. Increased pacing rate and duration augmented the magnitude of APD shortening. Apamin (1 μmol/L) eliminated recurrent SVF, increased postshock APD80 in SVF group from 126±5 ms to 153±4 ms (p<0.05), in no-SVF group from147±2 ms to 162±3 ms (p<0.05) but did not change of APD80 in non-failing group. Whole cell patch-clamp studies at 36°C showed that the apamin-sensitive K+ current (IKAS) density was significantly larger in the failing than in the normal ventricular epicardial myocytes, and epicardial IKAS density is significantly higher than midmyocardial and endocardial myocytes. Steady-state Ca2+ response of IKAS was leftward-shifted in the failing cells compared with the normal control cells, indicating increased Ca2+ sensitivity of IKAS in failing ventricles. The Kd was 232 ± 5 nM for failing myocytes and 553 ± 78 nM for normal myocytes (p = 0.002).
Heart failure heterogeneously increases the sensitivity of IKAS to intracellular Ca2+, leading to upregulation of IKAS, postshock APD shortening and recurrent SVF.
arrhythmia; intracellular calcium; ion channels; ventricular fibrillation
Recent evidence indicates that spontaneous sarcoplasmic reticulum Ca release and Na-Ca exchanger current activation contribute to the sinoatrial node (SAN) automaticity. These findings suggest that SAN activity may share mechanisms that underlie both automaticity and triggered activity. The aim of this study is to test the hypothesis that spontaneous, non-voltage gated, intracellular Ca (Cai) elevation may induce delayed afterdepolarization (DAD) in intact SAN during isoproterenol infusion.
Methods and Results
We simultaneously mapped Cai and membrane potential in 31 isolated Langendorff-perfused canine right atriums (RA). Isoproterenol increased heart rate and late diastolic Cai elevation (LDCAE) of the superior SAN, leading to consistent SAN automaticity in all 31 RAs. However, DAD-like diastolic depolarizations (DD) were transiently observed in 4 RAs during isoproterenol infusion. These DAD-like DDs were preceded by LDCAE, but did not trigger a full action potential. The LDCAE preceding DAD-like DDs had smaller amplitude (0.41 ± 0.08 AU vs. 0.48 ± 0.07 AU, p=0.001) and less steep slopes (3.7 ± 1.3 AU/s vs. 4.8 ± 1.4 AU/s, p=0.001) than that of sinus beats. The coupling interval of DAD-like DDs was longer than that of the preceding normal beats (407 ± 48 ms vs. 371 ± 44 ms, p=0.002).
The isoproterenol-induced LDCAE of superior SAN induced a full action potential in most cases. However, if the LDCAE was too small to trigger an action potential, then it induces only DAD-like DD. The failure of DAD-like DD to consistently trigger a sinus beat is a novel mechanism of atrial arrhythmogenesis.
calcium dynamics; sympathetic nervous system; triggered activity; sinoatrial node; afterdepolarization
Whether autonomic nerve activity is important in the development of pacing-induced sustained atrial fibrillation (AF) is unclear.
We tested the hypothesis that patterns of baseline autonomic nerve activities are important in the development of pacing-induced sustained AF.
We implanted radiotransmitters in 12 ambulatory dogs to record left stellate ganglion nerve activity (SGNA) and vagal nerve activity (VNA). Sustained (>48 hrs) AF was induced with intermittent rapid atrial pacing.
At baseline (before pacing), the one-min integrated nerve activity between SGNA and VNA demonstrated either a single linear relationship with excellent correlation (Group 1, N=3, r=0.816±0.105) or non-linear relationships with poor correlation (Group 2, N=9, r=0.316±0.162, P<0.05 compared with Group 1). Group 1 dogs had higher VNA (97.0±11.5 mV-s) compared to Group 2 (33.4±21.7 mV-s, P<0.001). Group 1 dogs had more frequent sympathovagal coactivation episodes than Group 2 (50±19/d vs. 15±6/d, P<0.05) and more paroxysmal atrial tachycardia (PAT; 5±1/d vs. 2±1/d, P<0.05) at baseline. Sustained AF occurred after 16±4 d (range 13–20 d) of pacing in Group 1 and after 46±18 d (range 23–72 d) of pacing in Group 2 (P<0.05). In the week before the development of sustained AF, the VNA of Group 2 dogs had significantly (P<0.05) increased compared to baseline.
Ambulatory dogs with good linear sympathovagal correlation and higher vagal tone at baseline have more PAT episodes at baseline and faster induction of sustained AF by rapid pacing. Rapid atrial pacing increased the VNA of the remaining dogs before the induction of sustained AF.
Autonomic nervous system; Nerve recording; Atrial fibrillation; Arrhythmia; Atrial pacing
Both phase-2 and phase-3 early afterdepolarizations (EADs) occur in long QT syndromes, but their respective roles in generating arrhythmias in intact cardiac tissue are incompletely understood.
Methods and Results
Intracellular Ca (Cai) and membrane voltage (Vm) were optically mapped in a quasi 2-dimensional model of cryoablated Langendorff-perfused rabbit ventricles (n = 16). E-4031 (an IKr blocker) combined with reduced extracellular K ([K+]o) and Mg ([Mg2+]o) prolonged action potential duration (APD) heterogeneously and induced phase-2 and phase-3 EADs. While phase-2 EADs were Cai-dependent, phase-3 EADs were not. The origins of 47 triggered activity (TA) episodes were attributed to phase-2 EADs in 12 episodes (26%) and phase-3 EADs in 35 episodes (74%). When phase-2 EADs accompanied phase-3 EADs, they accentuated APD heterogeneity, creating a large Vm gradient across the boundary between long and short APD regions from which TA emerged. The amplitude of phase-3 EADs correlated with the Vm gradient (r = 0.898, P < 0.001). Computer simulation studies showed that coupling of cells with heterogeneous repolarization could extrinsically generate phase-3 EADs via electrotonic current flow. Alternatively, reduced IK1 caused by low [K+]o could generate intrinsic phase-3 EADs capable of inducing TA at the boundary zone.
Phase-3 EADs can be extrinsic due to electrotonic current across steep repolarization gradients, or intrinsic due to low IK1, and do not require spontaneous sarcoplasmic reticulum Ca release. Reduction of IK1 by low [K+]o strongly promotes ventricular arrhythmias mediated by phase-3 EADs in acquired long QT syndrome due to IKr blockade.
action potentials; calcium; depolarization; long-QT syndrome; torsade de pointe
Little is known about the relationship between intrinsic cardiac nerve activity (ICNA) and spontaneous arrhythmias in ambulatory animals.
Methods and Results
We implanted radiotransmitters to record extrinsic cardiac nerve activity (ECNA, including stellate ganglion nerve activity, SGNA; vagal nerve activity, VNA) and ICNA (including superior left ganglionated plexi nerve activity, SLGPNA; ligament of Marshall nerve activity, LOMNA) in 6 ambulatory dogs. Intermittent rapid left atrial pacing was performed to induce paroxysmal atrial fibrillation (PAF) or atrial tachycardia (PAT). The vast majority (94%) of LOMNA were preceded or co-activated with ECNA (SGNA or VNA), whereas 6% of episodes were activated alone without concomitant SGNA or VNA. PAF and PAT were invariably (100%) preceded (<5 s) by ICNA. Most of PAT events (89%) were preceded by ICNA and sympathovagal co-activation, whereas 11% were preceded by ICNA and SGNA-only activation. Most of PAF events were preceded only by ICNA (72%); the remaining 28% by ECNA and ICNA together. Complex fractionated atrial electrograms (CFAEs) were observed during ICNA discharges that preceded the onset of PAT and PAF. Immunostaining confirmed the presence of both adrenergic and cholinergic nerve at ICNA sites.
There is a significant temporal relationship between ECNA and ICNA. However, ICNA can also activate alone. All PAT and PAF episodes were invariably preceded by ICNA. These findings suggest that ICNA (either alone or in collaboration with ECNA) is an invariable trigger of paroxysmal atrial tachyarrhythmias. ICNA might contaminate local atrial electrograms, resulting in CFAE-like activity.
nervous system; autonomic; atrium; arrhythmia; ligament of Marshall
Effects of d,l-sotalol at therapeutic concentrations (≤10 mg/L) on wavefront dynamics during ventricular fibrillation (VF) and electrophysiological heterogeneity remain unclear.
Methods and Results
By using an optical mapping system, epicardial activation patterns of VF were studied in 6 Langendorff-perfused rabbit hearts at baseline, during 10 mg/L d,l-sotalol infusion, and after washout. An additional 4 hearts, action potential duration (APD), conduction velocity, and wavelength (WL) restitutions were determined. During d,l-sotalol infusion, VF was terminated in 3 of the 6 hearts. Only one experienced a transient ventricular tachycardia (VT). d,l-Sotalol reduced the number of phase singularities (i.e., wavebreak) during VF (p<0.05). d,l-Sotalol also increased the occurrence frequency (p<0.05) and life span (p<0.05) of epicardial reentry during VF. These reentries were non-stationary in nature and did not anchor on anatomical structures. Restitution data showed that d,l-Sotalol flattened APD restitution. Furthermore, APD dispersion and spatial heterogeneity of restitutions were not enhanced by d,l-sotalol.
d,l-Sotalol at therapeutic concentrations decreased wavebreak and facilitated the occurrence of long-lasting non-stationary reentry during VF. However, VT rarely occurred. The related mechanisms include: (1) APD restitution flattening without enhancement of spatial heterogeneity of electrophysiological properties causes wavefront organization, and (2) WL prolongation prevents a steady anchoring of reentry.
d,l-sotalol; reentry; ventricular fibrillation
A beat-to-beat variation in the electric wave propagation morphology in myocardium is referred to as cardiac alternans and it has been linked to the onset of life threatening arrhythmias and sudden cardiac death. Experimental studies have demonstrated that alternans can be annihilated by the feedback modulation of the basic pacing interval in a small piece of cardiac tissue. In this work, we study the capability of feedback control to suppress alternans both spatially and temporally in an extracted rabbit heart and in a cable of cardiac cells. This work demonstrates real-time control of cardiac alternans in an extracted rabbit heart and provides an analysis of the control methodology applied in the case of a one-dimensional (1D) cable of cardiac cells. The real-time system control is realized through feedback by proportional perturbation of the basic pacing cycle length (PCL). The measurements of the electric wave propagation are obtained by optical mapping of fluorescent dye from the surface of the heart and are fed into a custom-designed software that provides the control action signal that perturbs the basic pacing cycle length. In addition, a novel pacing protocol that avoids conduction block is applied. A numerical analysis, complementary to the experimental study is also carried out, by the ionic model of a 1D cable of cardiac cells under a self-referencing feedback protocol, which is identical to the one applied in the experimental study. Further, the amplitude of alternans linear parabolic PDE that is associated with the 1D ionic cardiac cell cable model under full state feedback control is analyzed. We provide an analysis of the amplitude of alternans parabolic PDE which admits a standard evolutionary form in a well defined functional space. Standard modal decomposition techniques are used in the analysis and the controller synthesis is carried out through pole-placement. State and output feedback controller realizations are developed and the important issue of measurement noise in the controller implementation is addressed. The analysis of stabilization of the amplitude of alternans PDE is in agreement with the experimental results and numerical results produced by the ionic 1D cable of cardiac cells model. Finally, a discussion is provided in light of these results in order to use control to suppress alternans in the human myocardium.
Cardiac Alternans; Action Potential Duration (APD); Proportional Perturbation Feedback (PPF); Dissipative Parabolic PDEs; State/Output Feedback Control
ablation; arrhythmia; electrophysiology; tachyarrhythmias; tachycardia
To test the hypothesis that rhythmic spontaneous sarcoplasmic reticulum (SR) calcium (Ca) release (the “Ca clock”) plays an important role in atrioventricular junction (AVJ) automaticity.
The AVJ is a primary backup pacemaker to the sinoatrial node. The mechanisms of acceleration of AVJ intrinsic rate during sympathetic stimulation are unclear.
We simultaneously mapped transmembrane potential (Vm) and intracellular Ca (Cai) in Langendorff-perfused canine AVJ preparations that did not contain sinoatrial node (N=10).
Baseline AVJ rate was 37.5 ± 4.0 bpm. The wavefront from leading pacemaker site propagated first through the slow pathway, then the fast pathway and atria. There was no late diastolic Ca elevation (LDCAE) at baseline. Isoproterenol up to 3 μmol/L increased heart rate to 100 ± 6.8 bpm, concomitant with the appearance of LDCAE that preceded the phase 0 of action potential by 97.3 ± 35.2 ms and preceded the onset of late diastolic depolarization by 23.5 ± 3.5 ms. Caffeine also produced LDCAE and AVJ acceleration. The maximal slope of LDCAE and diastolic depolarization always co-localized with the leading pacemaker sites. Ryanodine markedly slowed the rate of spontaneous AVJ rhythm. Isoproterenol did not induce LDCAE in the presence of ryanodine. The If blocker ZD 7288 did not prevent LDCAE or AVJ acceleration induced by isoproterenol (N=2).
Isoproterenol and caffeine induced LDCAE and accelerated intrinsic AVJ rhythm. Consistent co-localization of the maximum LDCAE and the leading pacemaker sites indicates that Ca clock is important to the intrinsic AVJ rate acceleration during sympathetic stimulation.
Automaticity; sympathetic stimulation; optical mapping; electrophysiology; arrhythmia
Historically, milestones in science are usually associated with methodological breakthroughs. Likewise, the advent of electrocardiography, microelectrode recordings and more recently optical mapping have ushered in new periods of significance of advancement in elucidating basic mechanisms in cardiac electrophysiology. As with any novel technique, however, data interpretation is challenging and should be approached with caution, as it cannot be simply extrapolated from previously used methodologies and with experience and time eventually becomes validated. A good example of this is the use of optical mapping in the sinoatrial node (SAN): when microelectrode and optical recordings are obtained from the same site in myocardium, significantly different results may be noted with respect to signal morphology and as a result have to be interpreted by a different set of principles. Given the rapid spread of the use of optical mapping, careful evaluation must be made in terms of methodology with respect to interpretation of data gathered by optical sensors from fluorescent potential-sensitive dyes. Different interpretations of experimental data may lead to different mechanistic conclusions. This review attempts to address the origin and interpretation of the “double component” morphology in the optical action potentials obtained from the SAN region. One view is that these two components represent distinctive signals from the sinoatrial node and atrial cells, and can be fully separated with signal processing. A second view is that the first component preceding the phase 0 activation represents the membrane currents and intracellular calcium transients induced diastolic depolarization from the SAN. While the consensus from both groups is that ionic mechanisms, namely the joint action of the membrane and calcium automaticity, are important in the SAN function, it is unresolved whether the double-component originates from the recording methodology or represents the underlying physiology. This overview aims to advance a common understanding of the basic principles of optical mapping in complex three-dimensional anatomical structures.
sinoatrial node; optical mapping; calcium; sinoatrial exit block
Recent evidence indicates that spontaneous sarcoplasmic reticulum (SR) Ca release underlies the mechanism of sinoatrial node (SAN) acceleration during β-stimulation, indicating the importance of Ca clock in SAN automaticity. Whether or not the same mechanism applies to atrial ectopic pacemakers (AEP) remains unclear.
The purpose of this study was to assess the mechanism of AEP.
We simultaneously mapped intracellular calcium (Cai) and membrane potential in 12 isolated canine right atria. The late diastolic Cai elevation (LDCAE) was used to detect the Ca clock activity. Pharmacological interventions with isoproterenol (ISO), ryanodine, and ZD7288, a blocker of the If membrane current, were performed.
Ryanodine, which inhibits SR Ca release, reduced LDCAE in SAN, resulting in an inferior shift of the pacemaking site. Cycle length increased significantly in a dose-dependent way. In the presence of 3-10 μmol/L of ryanodine, ISO infusion consistently induces AEPs from the lower crista terminalis. All ectopic beats continuing over 30 seconds were located at the lower crista terminalis. These AEPs were resistant to ryanodine treatment even at high doses. Subsequent blockade of If inhibited the AEP and resulted in profound bradycardia.
Spontaneous SR Ca release underlies ISO-induced increase of superior SAN activity. As compared with SAN, the AEP is less dependent on the Ca clock and more dependent on the membrane clock for its automaticity. AEPs outside the SAN can effectively serve as backup pacemakers when the Ca clock functionality is reduced.
calcium; sinoatrial node; sarcoplasmic reticulum; If current; ectopic beat; Ca clock