Increased Arrhythmia Susceptibility Research Articles

Arrhythmia susceptibility in a rat model of acute atrial dilation

Atrial fibrillation (AF) is the most common cardiac arrhythmia, associated with an increased risk of stroke and heart failure. Acute AF occurs in response to sudden increases of atrial hemodynamic load, leading to atrial stretch. The mechanisms of stretch-induced AF were investigated in large mammals with controversial results. We optimized an approach to monitor rat atrial electrical activity using a red-shifted voltage sensitive dye (VSD). The methodology includes cauterization of the main ventricular coronary arteries, allowing improved atrial staining by the VSD and appropriate atrial perfusion for long experiments. Next, we developed a rat model of acute biatrial dilation (ABD) through the insertion of latex balloons into both atria, which could be inflated with controlled volumes. A chronic model of atrial dilation (spontaneous hypertensive rats; SHR) was used for comparison. ABD was performed on atria from healthy Wistar-Kyoto (WKY) rats (WKY-ABD). The atria were characterized in terms of arrhythmias susceptibility, action potential duration and conduction velocity. The occurrence of arrhythmias in WKY-ABD was significantly higher compared to non-dilated WKY atria. In WKY-ABD we found a reduction of conduction velocity, similar to that observed in SHR atria, while action potential duration was unchanged. Low-dose caffeine was used to introduce a drop of CV in WKY atria (WKY-caff), quantitatively similar to the one observed after ABD, but no increased arrhythmia susceptibility was observed with caffeine only. In conclusion, CV decrease is not sufficient to promote arrhythmias; enlargement of atrial surface is essential to create a substrate for acute reentry-based arrhythmias.

Cellular Stress p38MAPK Activation Decrease NaV1.5 Current Density and Contributes to the Develop of Arrhythmia in Elderly

Rationale: During progressive aging, atrial and ventricular conduction velocities are slower which has been associated with combined NaV1.5 and Cx43 functional disruption and fibrosis, consequently resulting in greater vulnerability of aged hearts to develop arrhythmias. The p38MAPK proteins are members of the mitogen-activated protein kinase family. They are expressed in the heart and activated during stress responses. However, it is unknown whether p38MAPK regulates functional expression of cardiac NaV1.5 channels or if p38MAPK hyperactivation increases arrhythmia susceptibility in aged hearts. Hypothesis: The human cardiac NaV1.5 channel exhibits a p38MAPK-dependent modulation and in aged hearts p38MAPK hyperactivation contributes to development of arrhythmia susceptibility in elderly. Methods and Results: Using a multidisciplinary approach based on biochemical techniques, together with electrophysiological and cell imaging experiments the data show that with aging, stress-associated p38MAPK activation increases, hearts present cellular electrophysiological changes, and old mice had increased susceptibility to develop arrhythmias. Moreover, activation of p38MAPK decrease INa current density in transiently transfected HEK-293T cells with NaV1.5 and dV/dtmax in hiPSC-CMs. Furthermore, p38MAPK activation does not change voltage dependence of activation or inactivation or recovery from inactivation of NaV1.5 channel neither affected total NaV1.5 protein expression. Finally, pretreatment with a specific inhibitor of p38MAPK prevented the INa and dV/dtmax decrease in HEK-293T cells and hiPSC-CMs, respectively. Conclusions Altogether the results raise the possibility that p38MAPK signaling contributes to age-dependent NaV1.5 channel dysregulation, and consequently increases the susceptibility to develop arrhythmias. It may be possible to modulate p38MAPK activity to treat or prevent arrhythmias in the elderly population.

Increased arrhythmia susceptibility in type 2 diabetic mice related to dysregulation of ventricular sympathetic innervation.

Patients with type 2 diabetes mellitus (T2DM) have a greater risk of developing life-threatening cardiac arrhythmias. Because the underlying mechanisms and potential influence of diabetic autonomic neuropathy are not well understood, we aimed to assess the relevance of a dysregulation in cardiac autonomic tone. Ventricular arrhythmia susceptibility was increased in Langendorff-perfused hearts isolated from mice with T2DM (db/db). Membrane properties and synaptic transmission were similar at cardiac postganglionic parasympathetic neurons from diabetic and control mice; however, a greater asynchronous neurotransmitter release was present at sympathetic postganglionic neurons from the stellate ganglia of db/db mice. Western blot analysis showed a reduction of tyrosine hydroxylase (TH) from the ventricles of db/db mice, which was confirmed with confocal imaging as a heterogeneous loss of TH-immunoreactivity from the left ventricular wall but not the apex. In vivo stimulation of cardiac parasympathetic (vagus) or cardiac sympathetic (stellate ganglion) nerves induced similar changes in heart rate in control and db/db mice, and the kinetics of pacing-induced Ca2+ transients (recorded from isolated cardiomyocytes) were similar in control and db/db cells. Antagonism of cardiac muscarinic receptors did not affect the frequency or severity of arrhythmias in db/db mice, but sympathetic blockade with propranolol completely inhibited arrhythmogenicity. Collectively, these findings suggest that the increased ventricular arrhythmia susceptibility of type 2 diabetic mouse hearts is due to dysregulation of the sympathetic ventricular control.NEW & NOTEWORTHY Patients with type 2 diabetes mellitus have greater risk of suffering from sudden cardiac death. We found that the increased ventricular arrhythmia susceptibility in type 2 diabetic mouse hearts is due to cardiac sympathetic dysfunction. Sympathetic dysregulation is indicated by an increased asynchronous release at stellate ganglia, a heterogeneous loss of tyrosine hydroxylase from the ventricular wall but not apex, and inhibition of ventricular arrhythmias in db/db mice after β-sympathetic blockade.

Cardiac rhythm disorders in obstructive sleep apnea.

Obstructive sleep apnea (OSA) is common among patients with cardiac rhythm disorders. OSA may contribute to arrhythmias due to acute mechanisms, such as generation of negative intrathoracic pressure during futile efforts to breath, intermittent hypoxia, and surges in sympathetic activity. In addition, OSA may lead to heart remodeling and increases arrhythmia susceptibility. Atrial distension and remodeling, that has been shown to be associated with OSA, is a well-known anatomical substrate for atrial fibrillation (AF). AF is the arrhythmia most commonly described in patients with OSA. Several observational studies have shown that the treatment of OSA with continuous positive airway pressure (CPAP) reduces recurrence of AF after electrical cardioversion and catheter ablation. There is also evidence that nocturnal hypoxemia, a hallmark of OSA, predicts sudden cardiac death (SCD) independently of well-established cardiovascular risk factors. Among patients with an implantable cardiac defibrillator, those with OSA have a higher risk of receiving treatment for life-threatening arrhythmias. Nocturnal hypoxemia may also increase vagal tone, which increases susceptibility to bradycardic and conduction rhythm disorders that have also been described in patients with OSA. In conclusion, there are several biological pathways linking OSA and increased cardiac arrhythmogenesis propensity. However, the independent association is derived from observational studies and the direction of the association still needs clarification due to the lack of large clinical trials. This review focuses on the current scientific evidence linking OSA to cardiac rhythm disorders and point out future directions.

Abstract 394: The Role Of Camkii-dependent Late Sodium Current In Ischemia/reperfusion-associated Arrhythmogenesis

Ischemic heart disease is the leading cause of death in the world. Importantly, CaMKII activity is upregulated during ischemia/reperfusion (I/R). However, the mechanisms by which CaMKII induces arrhythmias remains incompletely understood. We tested the hypothesis that CaMKII-dependent upregulation of the late Na+ current (INa,L) via direct phosphorylation of Nav1.5 at Ser571 is responsible for increased arrhythmia susceptibility during I/R. The role of CaMKII-dependent regulation of INa,L was explored using mice harboring Nav channel variants Nav1.5-Ser571E (S571E, phosphomimetic) and Nav1.5-Ser571A (S571A, phosphorylation incompetent). Isolated, perfused WT, S571E and S571A hearts were subjected to global ischemia (15 min) followed by reperfusion (15 min). Immunoblots demonstrated a significant increase in phosphorylated Nav1.5 at Ser571 in WT hearts during I/R compared to baseline (2.2±0.3 vs 1.0±0.3, respectively; P<0.05, n=5). Optical mapping using the voltage-sensitive dye di-4-ANEPPs (4 μM) revealed a significant increase in action potential duration in WT and S571E mice compared to S571A during I/R (WT: 55.2±0.6 ms*, S571E: 70.0±2.3 ms*, S571A: 46.0±2.9 ms; *P<0.05 vs. S571A, n=5/genotype). Furthermore, spontaneous arrhythmia incidence was increased in WT and S571E hearts (WT: 0.60±0.06 events/min*, S571E: 0.81±0.14 events/min*, S571A : 0.23±0.03 events/min; *P<0.05 vs. S571A, n=5/genotype). Interestingly, arrhythmia inducibility using an S1-S2 protocol was also increased in WT and S571E hearts (WT: 80%*, S571E: 100%*, S571A: 0%; *P<0.05 vs. S571A, n=5/genotype). The Na+ channel blocker mexiletine (10 μM) normalized APD and decreased arrhythmia inducibility in WT hearts during I/R (0% arrhythmia incidence in WT hearts treated with mexiletine). These data indicate a central role for CaMKII-dependent phosphorylation of Nav1.5 and augmentation of INa,L in the development of arrhythmogenesis during I/R.

CaMKII-dependent late Na+ current increases electrical dispersion and arrhythmia in ischemia-reperfusion.

The mechanisms underlying Ca2+/calmodulin-dependent protein kinase II (CaMKII)-induced arrhythmias in ischemia-reperfusion (I/R) are not fully understood. We tested the hypothesis that CaMKII increases late Na+ current ( INa,L) via phosphorylation of Nav1.5 at Ser571 during I/R, thereby increasing arrhythmia susceptibility. To test our hypothesis, we studied isolated, Langendorff-perfused hearts from wild-type (WT) mice and mice expressing Nav channel variants Nav1.5-Ser571E (S571E) and Nav1.5-Ser571A (S571A). WT hearts showed a significant increase in the levels of phosphorylated CaMKII and Nav1.5 at Ser571 [p-Nav1.5(S571)] after 15 min of global ischemia (just before the onset of reperfusion). Optical mapping experiments revealed an increase in action potential duration (APD) and APD dispersion without changes in conduction velocity during I/R in WT and S571E compared with S571A hearts. At the same time, WT and S571E hearts showed an increase in spontaneous arrhythmia events (e.g., premature ventricular contractions) and an increase in the inducibility of reentrant arrhythmias during reperfusion. Pretreatment of WT hearts with the Na+ channel blocker mexiletine (10 μM) normalized APD dispersion and reduced arrhythmia susceptibility during I/R. We conclude that CaMKII-dependent phosphorylation of Nav1.5 is a crucial driver for increased INa,L, arrhythmia triggers, and substrate during I/R. Selective targeting of this CaMKII-dependent pathway may have therapeutic potential for reducing arrhythmias in the setting of I/R. NEW & NOTEWORTHY Ca2+/calmodulin-dependent protein kinase II (CaMKII) phosphorylation of Nav1.5 at Ser571 leads to a prolongation of action potential duration (APD), increased APD dispersion, and increased arrhythmia susceptibility after ischemia-reperfusion in isolated mouse hearts. Genetic ablation of the CaMKII-dependent phosphorylation site Ser571 on Nav1.5 or low-dose mexiletine (to inhibit late Na+ current) reduced APD dispersion, arrhythmia triggers, and ventricular tachycardia inducibility.

Variability in pulmonary vein electrophysiology and fibrosis determines arrhythmia susceptibility and dynamics.

Success rates for catheter ablation of persistent atrial fibrillation patients are currently low; however, there is a subset of patients for whom electrical isolation of the pulmonary veins alone is a successful treatment strategy. It is difficult to identify these patients because there are a multitude of factors affecting arrhythmia susceptibility and maintenance, and the individual contributions of these factors are difficult to determine clinically. We hypothesised that the combination of pulmonary vein (PV) electrophysiology and atrial body fibrosis determine driver location and effectiveness of pulmonary vein isolation (PVI). We used bilayer biatrial computer models based on patient geometries to investigate the effects of PV properties and atrial fibrosis on arrhythmia inducibility, maintenance mechanisms, and the outcome of PVI. Short PV action potential duration (APD) increased arrhythmia susceptibility, while longer PV APD was found to be protective. Arrhythmia inducibility increased with slower conduction velocity (CV) at the LA/PV junction, but not for cases with homogeneous CV changes or slower CV at the distal PV. Phase singularity (PS) density in the PV region for cases with PV fibrosis was increased. Arrhythmia dynamics depend on both PV properties and fibrosis distribution, varying from meandering rotors to PV reentry (in cases with baseline or long APD), to stable rotors at regions of high fibrosis density. Measurement of fibrosis and PV properties may indicate patient specific susceptibility to AF initiation and maintenance. PV PS density before PVI was higher for cases in which AF terminated or converted to a macroreentry; thus, high PV PS density may indicate likelihood of PVI success.

Endocardial TRPC-6 Channels Act as Atrial Mechanosensors and Load-Dependent Modulators of Endocardial/Myocardial Cross-Talk

Mechanoelectrical feedback may increase arrhythmia susceptibility, but the molecular mechanisms are incompletely understood. This study showed that mechanical stretch altered the localization, protein levels, and function of the cation-selective transient receptor potential channel (TRPC)-6 in atrial endocardial cells in humans, pigs, and mice. In endocardial/myocardial cross-talk studies, addition of media from porcine atrial endocardium (AE) cells altered the calcium (Ca2+) transient characteristics of human-induced pluripotent stemcell-derived cardiomyocytes. These changes did not occur with media from stretched AE cells. Our datasuggested that endocardial TRPC-6-dependent paracrine signaling may modulate myocardial Ca2+ homeostasisunder basal conditions and protect against stretch-induced atrial arrhythmias.

216 Identifying a novel role for pmca1 (atp2b1) in heart rhythm instability

Arrhythmias continue to be a leading cause of death and disability across the world, and genetics are one of the mechanisms that are known to increase susceptibility. By identifying new...

Role of suppression of the inward rectifier current in terminal action potential repolarization in the failing heart

The failing heart exhibits an increased arrhythmia susceptibility that is often attributed to action potential (AP) prolongation due to significant ion channel remodeling. The inwardly rectifying K+ current (IK1) has been reported to be reduced, but its contribution to shaping the AP waveform and cell excitability in the failing heart remains unclear. The purpose of this study was to define the effect of IK1 suppression on the cardiac AP and excitability in the normal and failing hearts. We used electrophysiological and pharmacological approaches to investigate IK1 function in a swine tachy-pacing model of heart failure (HF). Terminal repolarization of the AP (TRAP; the time constant of the exponential fit to terminal repolarization) was markedly prolonged in both myocytes and arterially perfused wedges from animals with HF. TRAP was increased by 54.1% in HF myocytes (P< .001) and 26.2% in HF wedges (P = .014). The increase in TRAP was recapitulated by the potent and specific IK1 inhibitor, PA-6 (pentamidine analog 6), indicating that IK1 is the primary determinant of the final phase of repolarization. Moreover, we find that IK1 suppression reduced the ratio of effective refractory period to AP duration at 90% of repolarization, permitting re-excitation before full repolarization, reduction of AP upstroke velocity, and likely promotion of slow conduction. Using an objective measure of terminal repolarization, we conclude that IK1 is the major determinant of the terminal repolarization time course. Moreover, suppression of IK1 prolongs repolarization and reduces postrepolarization refractoriness without marked effects on the overall AP duration. Collectively, these findings demonstrate how IK1 suppression may contribute to arrhythmogenesis in the failing heart.

Four weeks of exposure to environmentally relevant levels of second hand smoke decreases heart rate variability—but not baroreflex sensitivity—in mice

BACKGROUNDSecond hand smoke (SHS), a source of environmental air pollution exposure, is a significant risk factor for cardiovascular morbidity and mortality. Impaired autonomic function, as indexed by decreased heart rate variability (HRV), has been implicated as one of the underlying mechanisms of increased arrhythmia susceptibility that often precedes sudden cardiac death. We previously showed, in mice, that short term (3 days) exposure to a high level of SHS (30 mg/m3, 6 hr/d) reduced HRV that persisted after this acute window of SHS exposure. However, we have limited information on the time course of SHS‐induced cardiovascular dysfunction at more environmentally relevant concentrations. Here, we sought to test the hypothesis that an environmentally relevant level of SHS decreases HRV and baroreflex sensitivity (BRS), while increasing blood pressure (BP) and BP variability (BPV).METHODSAdult male C57BL/6J mice were implanted with BP/ECG telemetry devices. Two weeks after recovering from surgery, mice were randomly assigned to either filtered air (FA, n=10 for HRV, n=5 for BPV and BRS) or SHS (n=6) groups. SHS exposure level was similar to that of a “smoky bar” (3 mg/m3) for 6hr/d, 5d/wk for 4wk. Weekly, 36 hours of continuous BP and ECG recordings were performed after five days of exposure. Data were divided into three 12‐hour periods: the dark cycle immediate after the 5th day of exposure and the following light and dark cycles. Standard time domain HRV parameters were determined from normal‐to‐normal R wave intervals (SDNN, CV% and rMSSD). BRS was determined with the sequence method using systolic BP.RESULTSSHS exposure for four weeks did not change BP and HR in either dark or light cycle. SHS significantly decreased short term HRV (rMSSD) during the light cycle in as few as two weeks of exposure (28% lower than FA group, p&lt;0.05). However, there was no significant change in overall HRV (SDNN and CV%) after four weeks of SHS exposure. These data suggest that the initial change in cardiac autonomic regulation is likely mediated by reduced vagal regulation. There was a trend, but not statistically significant effect, for an increase in BPV (12% higher than FA group) after four weeks of SHS exposure; however, no change in BRS was observed after four weeks of SHS exposure. These data suggest that changes in cardiac regulation precede changes in BP regulation, and that the cardiac vagal limb of the autonomic nervous system is particularly sensitive to SHS exposure.CONCLUSIONSWe conclude that this environmentally relevant dose of SHS can impair cardiovascular regulation in a time‐dependent manner, further emphasizing the importance of regulating and limiting exposure to SHS.Support or Funding InformationR01 ES025229; T32 HL086350

Electrophysiology of Hypokalemia and Hyperkalemia.

Electrophysiology of Hypokalemia and Hyperkalemia.

Conditional knockout of Fgf13 in murine hearts increases arrhythmia susceptibility and reveals novel ion channel modulatory roles

The intracellular fibroblast growth factors (iFGF/FHFs) bind directly to cardiac voltage gated Na+ channels, and modulate their function. Mutations that affect iFGF/FHF-Na+ channel interaction are associated with arrhythmia syndromes. Although suspected to modulate other ionic currents, such as Ca2+ channels based on acute knockdown experiments in isolated cardiomyocytes, the in vivo consequences of iFGF/FHF gene ablation on cardiac electrical activity are still unknown. We generated inducible, cardiomyocyte-restricted Fgf13 knockout mice to determine the resultant effects of Fgf13 gene ablation. Patch clamp recordings from ventricular myocytes isolated from Fgf13 knockout mice showed a ~25% reduction in peak Na+ channel current density and a hyperpolarizing shift in steady-state inactivation. Electrocardiograms on Fgf13 knockout mice showed a prolonged QRS duration. The Na+ channel blocker flecainide further prolonged QRS duration and triggered ventricular tachyarrhythmias only in Fgf13 knockout mice, suggesting that arrhythmia vulnerability resulted, at least in part, from a loss of functioning Na+ channels. Consistent with these effects on Na+ channels, action potentials in Fgf13 knockout mice, compared to Cre control mice, exhibited slower upstrokes and reduced amplitude, but unexpectedly had longer durations. We investigated candidate sources of the prolonged action potential durations in myocytes from Fgf13 knockout mice and found a reduction of the transient outward K+ current (Ito). Fgf13 knockout did not alter whole-cell protein levels of Kv4.2 and Kv4.3, the Ito pore-forming subunits, but did decrease Kv4.2 and Kv4.3 at the sarcolemma. No changes were seen in the sustained outward K+ current or voltage-gated Ca2+ current, other candidate contributors to the increased action potential duration. These results implicate that FGF13 is a critical cardiac Na+ channel modulator and Fgf13 knockout mice have increased arrhythmia susceptibility in the setting of Na+ channel blockade. The unanticipated effect on Ito revealed new FGF13 properties and the unexpected lack of an effect on voltage-gated Ca2+ channels highlight potential compensatory changes in vivo not readily revealed with acute Fgf13 knockdown in cultured cardiomyocytes.

Molecular Mechanisms of Sympathetic Remodeling and Arrhythmias.

A variety of pathological conditions can increase risk for development of ventricular arrhythmias in humans including diabetes mellitus, obesity, myocardial infarction (MI), and heart failure. Many of these diseases involve global disruption of the autonomic nervous system, including increased sympathetic drive and parasympathetic withdrawal, but another common factor among these disorders is sympathetic dysfunction within the heart. Treatments that target cardiac sympathetic transmission, including β-blockers and ganglionectomy, prolong life and decrease arrhythmias.1–5 Sympathetic control of the heart under normal conditions occurs primarily via norepinephrine acting on β adrenergic receptors (β-AR) to stimulate increases in heart rate (chronotropy), conduction velocity (dromotropy), and contractility (inotropy). These positive effects of sympathetic stimulation allow myocytes to meet increased cardiac demands during stress or exercise, serving to maintain homeostasis. The nervous system adapts to changing conditions, however, and sympathetic neurons undergo structural and functional alterations in response to injury and disease. There are at least 4 types of sympathetic remodeling that occur during conditions of increased arrhythmia susceptibility: hyperinnervation (increased nerve density), denervation (decreased nerve density), altered neurotransmitter or neuropeptide production and increased neuronal excitability. Rubart and Zipes6 proposed a model to explain how these diverse changes in sympathetic transmission might contribute to arrhythmia generation, suggesting that inappropriate heterogeneity of norepinephrine release within the heart leads to differential electric remodeling of cardiac myocytes and predisposes the heart to electric instability. Many studies now support the hypothesis that heterogeneity of noradrenergic transmission increases the risk of arrhythmia and have identified some of the mechanisms that underlie neuronal remodeling. This review will examine the mechanisms of sympathetic remodeling and will connect neural changes to increased arrhythmia susceptibility. We will focus on ventricular arrhythmias because atrial arrhythmias were reviewed recently.7 Regional hyperinnervation was the first type of neural remodeling linked to arrhythmia generation …

Myocardial Infarction Causes Transient Cholinergic Transdifferentiation of Cardiac Sympathetic Nerves via gp130.

Sympathetic neurons normally make norepinephrine (NE), which increases heart rate and the contractility of cardiac myocytes. We found that, after myocardial infarction, the sympathetic neurons innervating the heart begin to make acetylcholine (ACh), which slows heart rate and decreases contractility. Several lines of evidence confirmed that the source of ACh was sympathetic nerves rather than parasympathetic nerves that are the normal source of ACh in the heart. Global application of NE with or without ACh to ex vivo hearts showed that ACh partially reversed the NE-stimulated decrease in cardiac action potential duration and increase in myocyte calcium transients. That suggests that sympathetic co-release of ACh and NE may impair adaptation to high heart rates and increase arrhythmia susceptibility.

11 PMCA1 and arrhythmias: does deletion of PMCA1 lead to heart rhythm instability?

Arrhythmic events are a common consequence of cardiomyopathies, continuing to be a leading cause of death and disability. By identifying mechanisms which increase arrhythmia susceptibility, therapeutic advancements can be made. Genetics are known to influence arrhythmia development during heart failure and one gene linked to several features of heart failure is the plasma membrane calcium ATPase 1 (PMCA1). Therefore, along with its role in hypertension, we believe PMCA1 may also influence heart rhythm stability and consequently the development of arrhythmias. To determine the role of PMCA1 in cardiac electrical activity, electrocardiograms (ECG) were performed on cardiomyocyte-specific knockout mice (PMCA1CKO) and mice heterozygous for PMCA1 (PMCA1Ht). Complete cardiomyocyte deletion of PMCA1 resulted in abnormal heart rhythms related to cardiac repolarisation dysfunction, prolonged QT and JT intervals (p = 2+ channel (p =

Increased Regional Pannexin1 (Px1) Channel Expression linked to Focal Ischemia is Pro‐Arrhythmic During Stress

Mice with the hypertrophic cardiomyopathy‐linked TnT‐I79N mutation develop arrhythmias during physiological stress. In isolated hearts these arrhythmias are linked to regional ischemia (no decrease in global coronary flow) which causes regional hypoxia and cellular energy deprivation and results in regional conduction velocity slowing and increased arrhythmia susceptibility (84.7% vs 14.3% in control).To test if regional hypoxia/ischemia also occurred spontaneously in vivo in TnT‐I79N mice, we measured mRNA expression of genes up‐regulated by hypoxia (GLUT1, TUBB3, Px1) and found increased expression (0.3fold, 3fold, 10fold respectively). To determine the spatial distribution of Px1‐hypoxia marker we first subjected Px1 ‐βGal reporter mice to coronary ligation and demonstrate that Xgal staining is limited to the perfusion bed of the ligated artery. Then we crossed the TnT‐I79N with the Px1 reporter mice and found that Xgal staining increased in a spatially non‐uniform “patchy” distribution, consistent with focal ischemia. Px1 channels have been implicated in the regulation of vascular perfusion during hypoxia. Strikingly, ablation of Px1 in TnT‐I79N mice reduced the arrhythmia incidence in isolated hearts (to 36.4%) and prevented the focal energy deprivation during stress.Although the physiological role for the increase in Px1 after ischemia remains unclear, Px1 contributes to the increased arrhythmia susceptibility in TnT‐I79N mice. Hence, our data suggest that Px1 may be involved in the regulation of local coronary perfusion and is maladaptive during stress.

Targeting protein tyrosine phosphatase σ after myocardial infarction restores cardiac sympathetic innervation and prevents arrhythmias.

Millions of people suffer a myocardial infarction (MI) every year, and those who survive have increased risk of arrhythmias and sudden cardiac death. Recent clinical studies have identified sympathetic denervation as a predictor of increased arrhythmia susceptibility. Chondroitin sulfate proteoglycans present in the cardiac scar after MI prevent sympathetic reinnervation by binding the neuronal protein tyrosine phosphatase receptor σ (PTPσ). Here we show that the absence of PTPσ, or pharmacologic modulation of PTPσ by the novel intracellular sigma peptide (ISP) beginning 3 days after injury, restores sympathetic innervation to the scar and markedly reduces arrhythmia susceptibility. Using optical mapping we observe increased dispersion of action potential duration, supersensitivity to β-adrenergic receptor stimulation and Ca2+ mishandling following MI. Sympathetic reinnervation prevents these changes and renders hearts remarkably resistant to induced arrhythmias.

Adverse remodeling of the electrophysiological response to ischemia-reperfusion in human heart failure is associated with remodeling of metabolic gene expression.

Ventricular arrhythmias occur more frequently in heart failure during episodes of ischemia-reperfusion although the mechanisms underlying this in humans are unclear. We assessed, in explanted human hearts, the remodeled electrophysiological response to acute ischemia-reperfusion in heart failure and its potential causes, including the remodeling of metabolic gene expression. We optically mapped coronary-perfused left ventricular wedge preparations from 6 human end-stage failing hearts (F) and 6 donor hearts rejected for transplantation (D). Preparations were subjected to 30 minutes of global ischemia, followed by 30 minutes of reperfusion. Failing hearts had exaggerated electrophysiological responses to ischemia-reperfusion, with greater action potential duration shortening (P<0.001 at 8-minute ischemia; P=0.001 at 12-minute ischemia) and greater conduction slowing during ischemia, delayed recovery of electric excitability after reperfusion (F, 4.8±1.8 versus D, 1.0±0 minutes; P<0.05), and incomplete restoration of action potential duration and conduction velocity early after reperfusion. Expression of 46 metabolic genes was probed using custom-designed TaqMan arrays, using extracted RNA from 15 failing and 9 donor hearts. Ten genes important in cardiac metabolism were downregulated in heart failure, with SLC27A4 and KCNJ11 significantly downregulated at a false discovery rate of 0%. We demonstrate, for the first time in human hearts, that the electrophysiological response to ischemia-reperfusion in heart failure is accelerated during ischemia with slower recovery after reperfusion. This can enhance spatial conduction and repolarization gradients across the ischemic border and increase arrhythmia susceptibility. This adverse response was associated with downregulation of expression of cardiac metabolic genes.

Cardiac myofilament Caa2+ sensitization disturbs regulation of coronary flow and leads to focal myocardial ischemia and arrhythmias during stress (1079.25)

Myofilament Ca2+ sensitization has been linked to increased arrhythmia susceptibility, but the underlying mechanism is not completely understood. Mice with the Ca2+ sensitizing TnT‐I79N mutation develop arrhythmias during stress, despite the absence of an overt anatomical substrate. Stress by fast pacing of isolated perfused TnT‐I79N hearts for 16 min elicited arrhythmias (63%) and caused the formation of hypoxic regions (14.1% of total tissue in I79N vs 2.6% in control, Hypoxyprobe) associated with focal cellular energy depletion (Connexin isoforms P0 and pT172‐AMP kinase both increased). The cause for the local energetic supply‐and‐demand mismatch is unclear and we tested if insufficient perfusion is at least in part responsible. Global coronary flow was not decreased in isolated TnT‐I79N hearts. To test if flow distribution was altered, we infused fluorescent 15 µm microspheres and compared the fraction of microspheres inside hypoxic areas with the fraction of hypoxic tissue. Microspheres inside hypoxic regions were drastically reduced, suggesting regional ischemia. These results were reproduced in control hearts treated with the Ca2+ sensitizing drug EMD57033. Next we tested if hypoxia also occurs naturally in TnT‐I79N mice in vivo. Despite expecting low sensitivity due to localized effects, the mRNA expression levels of genes up‐regulated by hypoxia (GLUT1, TUBB3) were increased (0.3fold, 3fold).We conclude that myofilament Ca2+ sensitization disturbs regulation of coronary flow and leads to focal ischemia and arrhythmias during stress. This suggests that myofilament properties of cardiomyocytes importantly contribute to local regulation of coronary flow.Grant Funding Source: supported by AHA 10SDG2640109 and NIH HL71670