Beta2-adrenergic receptor antagonists protect against ventricular fibrillation: in vivo and in vitro evidence for enhanced sensitivity to beta2-adrenergic stimulation in animals susceptible to sudden death

Palmitoylation regulates norepinephrine transporter trafficking and expression and is potentially involved in the pathogenesis of postural orthostatic tachycardia syndrome

Postural orthostatic tachycardia syndrome (POTS) is an adrenergic signaling disorder characterized by excessive plasma norepinephrine, postural tachycardia, and syncope. The norepinephrine transporter (NET) modulates adrenergic homeostasis via reuptake of extracellular catecholamines and is implicated in the pathogenesis of adrenergic and neurological disorders. Previous research has outlined that NET activity and trafficking is modulated via reversible post-translational modifications like phosphorylation and ubiquitylation. S-palmitoylation, or the addition of a 16-carbon saturated fatty acid, is another post-translational modification responsible for numerous biological mechanisms. In this study, we reveal that NET is dynamically palmitoylated and inhibition of this modification with the palmitoyl acyltransferase (DHHC) inhibitor, 2-bromopalmitate (2BP), results in decreased NET palmitoylation within 90 min of treatment. This result was followed closely with a reduction in transport capacity, cell surface, and total cellular NET expression after 120 min of treatment. Increasing 2BP concentrations and treatment time revealed a nearly complete loss of total NET protein. Co-expression with individual DHHCs revealed a single DHHC enzyme, DHHC1, promoted WT hNET palmitoylation and elevated NET protein levels. The POTS associated NET mutant, A457P, exhibits dramatically decreased transport capacity and cell surface levels which we have confirmed in the current study. In an attempt to recover A457P NET expression we co-expressed the A457P variant with DHHC1 to drive expression as seen with the WT protein but instead saw an increase in NET N-terminal immuno-detectable fragments. Further investigation of A457P NET palmitoylation and surface expression is necessary, but our preliminary novel findings reveal palmitoylation as a mechanism of NET regulation and suggest that dysregulation of this process may contribute to the pathogenesis of POTS.

Regulation of heart rate following genetic deletion of the ß1 adrenergic receptor in larval zebrafish

Aim

Although zebrafish are gaining popularity as biomedical models of cardiovascular disease, our understanding of their cardiac control mechanisms is fragmentary. Our goal was to clarify the controversial role of the ß1‐adrenergic receptor (AR) in the regulation of heart rate in zebrafish.

Methods

CRISPR‐Cas9 was used to delete the adrb1 gene in zebrafish allowing us to generate a stable adrb1^−/− line. Larval heart rates were measured during pharmacological protocols and with exposure to hypercapnia. Expression of the five zebrafish adrb genes were measured in larval zebrafish hearts using qPCR.

Results

Compared with genetically matched wild‐types (adrb1^+/+), adrb1^−/− larvae exhibited ~20 beats min⁻¹ lower heart rate, measured from 2 to 21 days post‐fertilization (dpf). Nevertheless, adrb1^−/− larvae exhibited preserved positive chronotropic responses to pharmacological treatment with AR agonists (adrenaline, noradrenaline, isoproterenol), which were blocked by propranolol (general ß‐AR antagonist). Regardless of genotype, larvae exhibited similar increases in heart rate in response to hypercapnia (1% CO₂) at 5 dpf, but tachycardia was blunted in adrb1^−/− larvae at 6 dpf. adrb1 gene expression was abolished in the hearts of adrb1^−/− larvae, confirming successful knockout. While gene expression of adrb2a and adrb3a was unchanged, adrb2b and adrb3b mRNA levels increased in adrb1^−/− larval hearts.

Conclusion

Despite adrb1 contributing to the setting of resting heart rate in larvae, it is not strictly essential for zebrafish, as we generated a viable and breeding adrb1^−/− line. The chronotropic effects of adrenergic stimulation persist in adrb1^−/− zebrafish, likely due to the upregulation of other ß‐AR subtypes.

High-resolution structure-function mapping of intact hearts reveals altered sympathetic control of infarct border zones

Remodeling of injured sympathetic nerves on the heart after myocardial infarction (MI) contributes to adverse outcomes such as sudden arrhythmic death, yet the underlying structural mechanisms are poorly understood. We sought to examine microstructural changes on the heart after MI and to directly link these changes with electrical dysfunction. We developed a high-resolution pipeline for anatomically precise alignment of electrical maps with structural myofiber and nerve-fiber maps created by customized computer vision algorithms. Using this integrative approach in a mouse model, we identified distinct structure-function correlates to objectively delineate the infarct border zone, a known source of arrhythmias after MI. During tyramine-induced sympathetic nerve activation, we demonstrated regional patterns of altered electrical conduction aligned directly with altered neuroeffector junction distribution, pointing to potential neural substrates for cardiac arrhythmia. This study establishes a synergistic framework for examining structure-function relationships after MI with microscopic precision that has potential to advance understanding of arrhythmogenic mechanisms.

Clinical management of electrical storm: a current overview

The number of patients affected by electrical storm has been continuously increasing in emergency departments. Patients are often affected by multiple comorbidities requiring multidisciplinary interventions to achieve a clinical stability. Careful reprogramming of cardiac devices, correction of electrolyte imbalance, knowledge of underlying heart disease and antiarrhythmic drugs in the acute phase play a crucial role. The aim of this review is to provide a comprehensive overview of pharmacological treatment, latest transcatheter ablation techniques and advanced management of patients with electrical storm.

Comparing Drugs for Out-of-hospital, Shock-refractory Cardiac Arrest: Systematic Review and Network Meta-analysis of Randomized Controlled Trials

INTRODUCTION

The benefit of medications used in out-of-hospital, shock-refractory cardiac arrest remains controversial. This study aims to compare the treatment outcomes of medications for out-of-hospital, shock-refractory ventricular fibrillation (VF) or pulseless ventricular tachycardia (pVT).

METHODS

The inclusion criteria were randomized controlled trials of participants older than eight years old who had atraumatic, out-of-hospital, shock-refractory VF/pVT in which at least one studied group received a medication. We conducted a database search on October 28, 2019, that included PubMed, Scopus, Web of Science, CINAHL Complete, and Cochrane CENTRAL. Citations of relevant meta-analyses were also searched. We performed frequentist network meta-analysis (NMA) to combine the comparisons. The outcomes were analyzed by using odds ratios (OR) and compared to placebo. The primary outcome was survival to hospital discharge. The secondary outcomes included the return of spontaneous circulation (ROSC), survival to hospital admission, and the neurological outcome at discharge. We ranked all outcomes using surface under the cumulative ranking score.

RESULTS

We included 18 studies with 6,582 participants. The NMA of 20 comparisons included 12 medications and placebo. Only norepinephrine showed a significant increase of ROSC (OR = 8.91, 95% confidence interval [CI], 1.88-42.29). Amiodarone significantly improved survival to hospital admission (OR = 1.53, 95% CI, 1.01-2.32). The ROSC and survival-to-hospital admission data were significantly heterogeneous with the I2 of 55.1% and 59.1%, respectively. This NMA satisfied the assumption of transitivity.

CONCLUSION

No medication was associated with improved survival to hospital discharge from out-of-hospital, shock-refractory cardiac arrest. For the secondary outcomes, norepinephrine was associated with improved ROSC and amiodarone was associated with an increased likelihood of survival to hospital admission in the NMA.

Expert consensus on acute management of ventricular arrhythmias - VT network Austria

The Arrhythmia Working Group of the Austrian Society of Cardiology (ÖKG) has set the goal of systematically structuring and organizing the acute care of patients with ventricular arrhythmias (VA), i.e. ventricular tachycardia (VT) or ventricular fibrillation (VF) in Austria. Within a consensus paper, national recommendations on the basic diagnostic work-up of VA (12-lead ECG, medical history, family history, laboratory analyses, echocardiography, search for reversible causes, ICD interrogation), as well as further medical treatment and therapeutic measures (indication of coronary angiography, ablation therapy) are established. Since acute ablation of VT is indicated in the current ESC guidelines as a class IB indication for scar-associated incessant VT or electrical storm (ES; ≥ 3 ICD therapies in 24 h) as well as for ischemic cardiomyopathy (iCMP) with recurrent ICD shocks, organizational measures must be taken to ensure that these guidelines can be implemented. Therefore, a VT network will be established covering all areas in Austria, consisting of primary and secondary VT centers. Organizational aspects of an acute VT network are defined and should subsequently be implemented by the participating hospitals. All electrophysiologic centers in Austria that deal with VT ablation are to be integrated into the network in the medium-term. Centers that co-operate in the network are divided into primary and secondary VT centers according to predefined criteria.

[Electrical storm : Recognition and management]

The electrical storm (ES) defined as ≥3 sustained episodes of ventricular tachycardia within a 24 h period comprises a wide spectrum of clinical entities. Mostly patients suffer from severe heart insufficiency and comorbidities making an immediate transfer into a heart center with multidisciplinary expertise in the treatment of ES mandatory. As these patients are often traumatized by ongoing tachycardia despite multiple cardioversions, early deep sedation and β‑blockade to break the vicious circle of sympathico-adrenergic hyperactivation is very effective. Multiple ICD discharges suggesting the diagnosis of ES are inadequate in one third of cases. Pharmacological suppression, frequency control or ablation of supraventricular tachycardias (SVT) help in most cases. In some cases "oversensing" demands optimization of ICD programming. Even so not all adequate ICD discharges, however, are necessary. Since every ICD discharge worsens the patient's prognosis, any kind of ICD discharge should be prevented as far as hemodynamically feasible. After clinical stabilization of the patient with simultaneous acquisition of ECG and testing for reversible causes of ES, ES should be terminated by external or internal cardioversion followed by urgent but elective therapy. Some cases of ES, however, may require immediate escalation of therapy with emergency ablation or revascularization sometimes with circulatory support systems. If ES still persists, a further step in escalation may be taken by cardiac sympathetic denervation. Due to the poor prognosis of patients after ES, close monitoring of the patient, preferably with telemedicine, is indicated.

Propranolol Versus Metoprolol for Treatment of Electrical Storm in Patients With Implantable Cardioverter-Defibrillator

BACKGROUND

Electrical storm (ES), characterized by unrelenting recurrences of ventricular arrhythmias, is observed in approximately 30% of patients with implantable cardioverter-defibrillators (ICDs) and is associated with high mortality rates.

OBJECTIVES

Sympathetic blockade with β-blockers, usually in combination with intravenous (IV) amiodarone, have proved highly effective in the suppression of ES. In this study, we compared the efficacy of a nonselective β-blocker (propranolol) versus a β₁-selective blocker (metoprolol) in the management of ES.

METHODS

Between 2011 and 2016, 60 ICD patients (45 men, mean age 65.0 ± 8.5 years) with ES developed within 24 h from admission were randomly assigned to therapy with either propranolol (160 mg/24 h, Group A) or metoprolol (200 mg/24 h, Group B), combined with IV amiodarone for 48 h.

RESULTS

Patients under propranolol therapy in comparison with metoprolol-treated individuals presented a 2.67 times decreased incidence rate (incidence rate ratio: 0.375; 95% confidence interval: 0.207 to 0.678; p = 0.001) of ventricular arrhythmic events (tachycardia or fibrillation) and a 2.34 times decreased rate of ICD discharges (incidence rate ratio: 0.428; 95% CI: 0.227 to 0.892; p = 0.004) during the intensive care unit (ICU) stay, after adjusting for age, sex, ejection fraction, New York Heart Association functional class, heart failure type, arrhythmia type, and arrhythmic events before ICU admission. At the end of the first 24-h treatment period, 27 of 30 (90.0%) patients in group A, while only 16 of 30 (53.3%) patients in group B were free of arrhythmic events (p = 0.03). The termination of arrhythmic events was 77.5% less likely in Group B compared with Group A (hazard ratio: 0.225; 95% CI: 0.112 to 0.453; p < 0.001). Time to arrhythmia termination and length of hospital stay were significantly shorter in the propranolol group (p < 0.05 for both).

CONCLUSIONS

The combination of IV amiodarone and oral propranolol is safe, effective, and superior to the combination of IV amiodarone and oral metoprolol in the management of ES in ICD patients.

Anti-arrhythmic drug therapy in implantable cardioverter-defibrillator recipients

Implantable cardioverter-defibrillators (ICDs) have revolutionized the primary and secondary prevention of patients with ventricular arrhythmias. However, the adverse effects of appropriate or inappropriate shocks may require the adjunctive use of anti-arrhythmic drugs (AADs). Beta blockers are the cornerstone of pharmacological primary and secondary prevention of ventricular arrhythmias. In addition to their established efficacy at reducing the incidence of ventricular arrhythmias, beta-blockers are safe with few side effects. Amiodarone is superior to beta blockers and sotalol for the prevention of ventricular arrhythmia recurrence. However, long-term amiodarone use is associated with significant side effects that limit its utility. Sotalol and mexiletine are the main alternatives to amiodarone with a better side effect profile though they are less efficacious at preventing ventricular arrhythmia recurrence. Dofetilide, azimilide and ranolazine are emerging as therapeutic options for secondary prevention; more studies are needed to assess efficacy and safety in comparison to currently used agents. Beta blockers and amiodarone are the mainstay of therapy in patients experiencing electrical storm; their use reduces the frequency of ventricular arrhythmias and ICD intervention as well as affording time until catheter ablation can be considered.

Exercise training reduces ventricular arrhythmias through restoring calcium handling and sympathetic tone in myocardial infarction mice

Exercise can improve morbidity and mortality in heart failure patients; however, the underlying mechanisms remain to be fully investigated. Thus, we investigated the effects of exercise on cardiac function and ventricular arrhythmias in myocardial infarction (MI) induced heart failure mice. Wild-type male mice underwent sham-operation or permanent left coronary artery ligation to induce MI. MI mice were divided into a sedentary (MI-Sed) and two intervention groups: MI-Ex (underwent 6-week treadmill exercise training) and MI-βb (oral bisoprolol treatment (1 mg/kg/d) without exercise). Cardiac function and structure were assessed by echocardiography and histology. Exercise capacity and cardiopulmonary function was accepted as oxygen consumption at peak exercise (peak VO2 ). Autonomic nervous system function and the incidence of spontaneous ventricular arrhythmia were evaluated via telemetry recording. mRNA and protein expressions in the left ventricle (LV) were investigated by real-time PCR and Western blotting. There were no differences in survival rate, MI size, cardiac function and structure, while exercise training improved peak VO2 . Compared with MI-Sed, MI-Ex, and MI-βb showed decreased sympathetic tone and lower incidence of spontaneous ventricular arrhythmia. By Western blot, the hyperphosphorylation of CaMKII and RyR2 were restored by exercise and β-blocker treatment. Furthermore, elevated expression of miR-1 and decreased expression of its target protein PP2A were recovered by exercise and β-blocker treatment. Continuous intensive exercise training can suppress ventricular arrhythmias in subacute to chronic phase of MI through restoring autonomic imbalance and impaired calcium handling, similarly to that for β-blockers.

In the Wrong Place with the Wrong SNP: The Association Between Stressful Neighborhoods and Cardiac Arrest Within Beta-2-adrenergic Receptor Variants

BACKGROUND

Sudden cardiac arrest has been linked independently both to stressful neighborhood conditions and to polymorphisms in the ADRB2 gene. The ADRB2 gene mediates sympathetic activation in response to stress. Therefore, if neighborhood conditions cause cardiac arrest through the stress pathway, the ADRB2 variant may modify the association between neighborhood conditions, such as socioeconomic deprivation and incidence of cardiac arrest.

METHODS

The Cardiac Arrest Blood Study Repository is a population-based repository of specimens and other data from adult cardiac arrest patients residing in King County, Washington. Cases (n = 1,539) were 25- to 100-year-old individuals of European descent who experienced out-of-hospital cardiac arrest from 1988 to 2004. Interactions between neighborhood conditions and the ADRB2 genotype on cardiac arrest risk were assessed in a case-only study design. Gene-environment independence was assessed in blood samples obtained from King County residents initially contacted by random-digit dialing.

RESULTS

Fewer than 4% of study subjects resided in socioeconomically deprived neighborhoods. Nonetheless, the case-only analysis indicated the presence of supramultiplicative interaction of socioeconomic deprivation and the homozygous Gln27Glu variant (case-only odds ratio: 1.8 [95% confidence interval: 1.0, 2.9]). Interactions between population density and the homozygous Gln27Glu variant were weaker (case-only odds ratio: 1.2 [95% confidence interval: 0.97, 1.5]).

CONCLUSIONS

Findings support a supramultiplicative interaction between the Gln27Glu ADRB2 variant and socioeconomic deprivation among individuals of European descent. This result is consistent with the hypothesis that the elevation in cardiac arrest risk associated with socioeconomic deprivation operates through the stress pathway.

Electrical Storm in Patients with Implantable Cardioverter-defibrillators: A Practical Overview

Electrical storm (ES) is an increasingly common medical emergency characterized by clustered episodes of sustained ventricular arrhythmias (VAs) that lead to repeated appropriate implantable cardioverter-defibrillator (ICD) therapies. A diagnosis of ES can be made with the occurrence of three or more sustained episodes of VAs, or of three or more appropriate ICD therapies within 24 hours in patients with implanted devices. ES is associated with poor outcomes in patients with structural heart disease, particularly those with severe left ventricular dysfunction. In large clinical trials involving patients with ICDs for primary and secondary prevention, ES appears to be a predictor of cardiac death, with notably higher rates of mortality soon after the event. ES management is challenging and requires special medical attention with accurate patient risk stratification and a multidisciplinary approach that includes the use of pharmacologic therapies such as antiarrhythmic drugs (AADs) and interventional approaches like catheter ablation, surgical ablation, or sympathetic neuromodulation. Initial management involves determining and addressing the underlying ischemia, any electrolyte imbalances, and/or other causative factors. Hemodynamic support needs to be considered in high-risk patients with unstable VAs or those with severe comorbidities such as low left ventricular ejection fraction, advanced New York Heart Association class, and/or chronic pulmonary disease. Following the acute phase of ES, treatment should shift towards maximizing therapeutic efforts to address heart failure, performing revascularization, and preventing subsequent VAs. In the present manuscript, we offer an overview of the most relevant clinical aspects of ES with regard to novel therapeutic strategies.

Antiarrhythmic Action ofβ-Blockers: Potential Mechanisms

Sympathetic nervous system overactivity has been linked to ventricular tachyarrhythmias and sudden death. It has been hypothesized that the extent and nature of the arrhythmogenic effect of sympathetic stimulation depends on the underlying myocardial substrate, the mechanism of the arrhythmia, and the integrated effects of sympathetic stimulation in the particular individual circumstance. Multiple direct and indirect mechanisms of adrenergic action on the heart may benefit from the known antiarrhythmic actions of β-blocker therapy and other interventions that decrease sympathetic tone. The antiarrhythmic mechanism of β-blockade (and possibly α-blockade) will depend on the specific mechanism of the individual arrhythmia and will differ for those arrhythmias caused by tachycardia and ischemia, those caused by reentry and promoted by decreased conduction velocity and shortened refractoriness, and those caused by early or delayed afterdepolarizations, usually in the context of prolonged action potential duration. Antagonism of cardiac adrenergic activity by β-blockade in particular is the best-established drug therapy to prevent ventricular arrhythmias.

Glycyrrhizic Acid Reduces Heart Rate and Blood Pressure by a Dual Mechanism

Beta adrenergic receptors are crucial for their role in rhythmic contraction of heart along with their role in the pathological conditions such as tachycardia and high risk of heart failure. Studies report that the levels of beta-1 adrenergic receptor tend to decrease by 50%, whereas, the levels of beta-2 adrenergic receptor remains constant during the risk of heart failure. Beta blockers—the antagonistic molecules for beta-adrenergic receptors, function by slowing the heart rate, which thereby allows the left ventricle to fill completely during tachycardia incidents and hence helps in blood pumping capacity of heart and reducing the risk of heart failure. In the present study, we investigate the potential of glycyrrhizic acid (GA) as a possible principal drug molecule for cardiac arrhythmias owing to its ability to induce reduction in the heart rate and blood pressure. We use in vitro and in silico approach to study GA′s effect on beta adrenergic receptor along with an in vivo study to examine its effect on heart rate and blood pressure. Additionally, we explore GA′s proficiency in eliciting an increase in the plasma levels of vasoactive intestinal peptide, which by dilating the blood vessel consequently, can be a crucial aid during the occurrence of a potential heart attack. Therefore, we propose GA as a potential principal drug molecule via its potential in modulating heart rate and blood pressure.

Cardiac Sympathetic Nerve Sprouting and Susceptibility to Ventricular Arrhythmias after Myocardial Infarction

Ventricular arrhythmogenesis is thought to be a common cause of sudden cardiac death following myocardial infarction (MI). Nerve remodeling as a result of MI is known to be an important genesis of life-threatening arrhythmias. It is hypothesized that neural modulation might serve as a therapeutic option of malignant arrhythmias. In fact, left stellectomy or β-blocker therapy is shown to be effective in the prevention of ventricular tachyarrhythmias (VT), ventricular fibrillation (VF), and sudden cardiac death (SCD) after MI both in patients and in animal models. Results from decades of research already evidenced a positive relationship between abnormal nerve density and ventricular arrhythmias after MI. In this review, we summarized the molecular mechanisms involved in cardiac sympathetic rejuvenation and mechanisms related to sympathetic hyperinnervation and arrhythmogenesis after MI and analyzed the potential therapeutic implications of nerve sprouting modification for ventricular arrhythmias and SCD control.

[Electrical storm in the emergency room: clinical pathways]

In patients with structural heart disease, occurrence of an electrical storm (ES) is associated with increased mortality acutely and during medium term follow-up. Depending on the underlying heart disease and baseline type of arrhythmia, different clinical pathways have to be followed to reach sustained freedom from ventricular arrhythmia recurrences. Trigger elimination, sympathetic blockade (initially using betablockers and sedation), antiarrhythmic therapy with amiodarone and catheter ablation, treatment of heart failure and invasive hemodynamic support are cornerstones of the treatment. We present an algorithm which may help to organize an optimized treatment for each ES patient, implementing invasive treatment options like coronary angioplasty, catheter ablation and invasive circulatory support. Further studies are necessary to evaluate medium term outcome of such a structured therapy.

Myocardial electrotonic response to submaximal exercise in dogs with healed myocardial infarctions: evidence for β-adrenoceptor mediated enhanced coupling during exercise testing

INTRODUCTION

Autonomic neural activation during cardiac stress testing is an established risk-stratification tool in post-myocardial infarction (MI) patients. However, autonomic activation can also modulate myocardial electrotonic coupling, a known factor to contribute to the genesis of arrhythmias. The present study tested the hypothesis that exercise-induced autonomic neural activation modulates electrotonic coupling (as measured by myocardial electrical impedance, MEI) in post-MI animals shown to be susceptible or resistant to ventricular fibrillation (VF).

METHODS

Dogs (n = 25) with healed MI instrumented for MEI measurements were trained to run on a treadmill and classified based on their susceptibility to VF (12 susceptible, 9 resistant). MEI and ECGs were recorded during 6-stage exercise tests (18 min/test; peak: 6.4 km/h @ 16%) performed under control conditions, and following complete β-adrenoceptor (β-AR) blockade (propranolol); MEI was also measured at rest during escalating β-AR stimulation (isoproterenol) or overdrive-pacing.

RESULTS

Exercise progressively increased heart rate (HR) and reduced heart rate variability (HRV). In parallel, MEI decreased gradually (enhanced electrotonic coupling) with exercise; at peak exercise, MEI was reduced by 5.3 ± 0.4% (or -23 ± 1.8Ω, P < 0.001). Notably, exercise-mediated electrotonic changes were linearly predicted by the degree of autonomic activation, as indicated by changes in either HR or in HRV (P < 0.001). Indeed, β-AR blockade attenuated the MEI response to exercise while direct β-AR stimulation (at rest) triggered MEI decreases comparable to those observed during exercise; ventricular pacing had no significant effects on MEI. Finally, animals prone to VF had a significantly larger MEI response to exercise.

CONCLUSIONS

These data suggest that β-AR activation during exercise can acutely enhance electrotonic coupling in the myocardium, particularly in dogs susceptible to ischemia-induced VF.

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 Ca(2+) mishandling following MI. Sympathetic reinnervation prevents these changes and renders hearts remarkably resistant to induced arrhythmias.

Optimal antiarrhythmic drug therapy for electrical storm

Electrical storm, defined as 3 or more separate episodes of ventricular tachycardia or ventricular fibrillation within 24 hours, carries significant morbidity and mortality. These unstable ventricular arrhythmias have been described with a variety of conditions including ischemic heart disease, structural heart disease, and genetic conditions. While implantable cardioverter defibrillator implantation and ablation may be indicated and required, antiarrhythmic medication remains an important adjunctive therapy for these persons.

β2 adrenergic receptor activation governs cardiac repolarization and arrhythmogenesis in a guinea pig model of heart failure

β2-AR activation increases the risk of sudden cardiac death (SCD) in heart failure (HF) patients. Non-selective β-AR blockers have greater benefits on survival than selective β1-AR blockers in chronic HF patients, indicating that β2-AR activation contributes to SCD in HF. This study investigated the role of β2-AR activation on repolarization and ventricular arrhythmia (VA) in the experimental HF model. The guinea pig HF was induced by descending aortic banding. The effective refractoriness period (ERP), corrected QT (QTc) and the incidence of VA were examined using Langendorff and programmed electrical stimulation. Ikr and APD were recorded by the whole cell patch clamp. Selective β2-AR agonist salbutamol significantly increased the incidence of VA, prolonged QTc and shortened ERP. These effects could be prevented by the selective β2-AR antagonist, ICI118551. Salbutamol prolonged APD90 and reduced Ikr in guinea pig HF myocytes. The antagonists of cAMP (Rp-cAMP) and PKA (KT5720) attenuated Ikr inhibition and APD prolongation induced by salbutamol. However, the antagonists of Gi protein (PTX) and PDE III (amrinone) showed opposite effects. This study indicates that β2-AR activation increases the incidence of VA in the experimental HF model via activation of Gs/cAMP/PKA and/or inhibition of Gi/PDE pathways.

Aerobic exercise inhibits sympathetic nerve sprouting and restores β-adrenergic receptor balance in rats with myocardial infarction

Background

Cardiac sympathetic nerve sprouting and the dysregulation of β-adrenergic receptor (β-AR) play a critical role in the deterioration of cardiac function after myocardial infarction (MI). Growing evidence indicates that exercise provides protection against MI. The aims of this study were to investigate whether aerobic exercise following MI could inhibit sympathetic nerve sprouting and restore the balance of β3-AR/β1-AR.

Methods

Male Sprague-Dawley rats were divided into three groups: sham-operated control group (SC), MI group (MI), and MI with aerobic exercise group (ME). The rats in ME group were assigned to 8 weeks of exercise protocol (16 m/min, 50 min/d, 5 d/wk). The expression of nerve growth factor (NGF), the sympathetic nerve marker-tyrosine hydroxylase (TH), the nerve sprouting marker-growth associated protein 43 (GAP43), and β1- and β2-AR expression in the peri-infarct area of the left ventricle (LV) were measured by Western blot and immunohistochemistry, while β3-AR expression was determined by Western blot and immunofluorescence. Endothelial nitric oxide synthase (NOS2), phospho-NOS2 (p-NOS2), and neuronal nitric oxide synthase (NOS1) were measured by Western blot.

Results

MI increased LV end-diastolic pressure (LVEDP), and decreased LV systolic pressure (LVSP). Compared with the MI group, aerobic exercise significantly decreased LVEDP and increased LVSP. The protein expression of TH, GAP43 and NGF was significantly increased after MI, which was normalized by exercise. Compared with the SC group, the ratios of β2-AR/β1-AR and β3-AR/β1-AR were elevated in the MI group, and the protein expression of β3-AR and NOS1 increased after MI. Compared with the MI group, the ratios of β2-AR/β1-AR and β3-AR/β1-AR were normalized in the ME group, while the protein expression of β3-AR and NOS1 significantly increased, and NOS2 was activated by exercise.

Conclusions

Aerobic exercise inhibits cardiac sympathetic nerve sprouting, restores β3-AR/β1-AR balance and increases β3-AR expression through the activation of NOS2 and NOS1 after myocardial infarction.

[Electrical storm in ICD patients: prevention and treatment]

Medical progress and demographic changes cause a continuous increase in patients with implantable cardioverter-defibrillators (ICD). Up to one third of patients with ICDs for secondary prevention and half of the patients with previous electrical storm (ES) will suffer from (further) ESs. When multiple ICD shocks are reported by patients (ICD storm), appropriate, inappropriate and phantom shocks have to be distinguished. Reported shocks without clinical correlates (phantom) often affect patients suffering from posttraumatic stress syndrome after an ICD storm. Approximately one third of all ICD shocks are inappropriate, most often due to supraventricular tachycardia with fast atrioventricular (AV) nodal conduction or lead failure. Within 10 years after implantation lead failure can be detected in up to 20 % of cases and approximately one third of these failures are only seen after inappropriate ICD shocks. Furthermore, inappropriate shocks are due to oversensing of far field atrial electrograms, T-waves, diaphragmatic potentials and electrical noise.Appropriate ICD shocks can rarely also be stimulated by the proarrhythmogenicity of lead implantation or ICD programming. Modifications of the waiting period to therapy, time to detection, detection window, antitachycardia pacing (ATP) stimulation and supraventricular discrimination algorithms may minimize ICD shocks. Some stimulation algorithms may improve the hemodynamic stability during ES. In addition to ventricular ablation, blockade of the sympathetic autonomic nervous system and antiarrhythmic treatment are the main pillars of ES treatment. The best ES prevention, however, is optimized heart failure treatment, especially when a cardiac resynchronization with defibrillator (CRT-D) system is implanted.

The relation of autonomic function to physical fitness in patients suffering from alcohol dependence

BACKGROUND

Reduced cardio-vascular health has been found in patients suffering from alcohol dependence. Low cardio-respiratory fitness is an independent predictor of cardio-vascular disease.

METHODS

We investigated physical fitness in 22 alcohol-dependent patients 10 days after acute alcohol withdrawal and compared results with matched controls. The standardized 6-min walk test (6 MWT) was used to analyze the relationship of autonomic dysfunction and physical fitness. Ventilatory indices and gas exchanges were assessed using a portable spiroergometric system while heart rate recordings were obtained separately. We calculated walking distance, indices of heart rate variability and efficiency parameters of heart rate and breathing. In addition, levels of exhaled carbon monoxide were measured in all participants to account for differences in smoking behaviour. Multivariate analyses of variance (MANOVA) were performed to investigate differences between patients and controls with regard to autonomic and efficiency parameters.

RESULTS

Patients walked a significantly shorter distance in comparison to healthy subjects during the 6 MWT. Significantly decreased heart rate variability was observed before and after the test in patients when compared to controls, while no such difference was observed during exercise. The efficiency parameters indicated significantly reduced efficiency in physiological regulation when the obtained parameters were normalized to the distance.

DISCUSSION

The 6 MWT is an easily applied instrument to measure physical fitness in alcohol dependent patients. It can also be used during exercise interventions. Reduced physical fitness, as observed in our study, might partly be caused by autonomic dysfunction, leading to less efficient regulation of physiological processes during exercise.

Deletion of the β2-adrenergic receptor prevents the development of cardiomyopathy in mice

Beta adrenergic receptor (β-AR) subtypes act through diverse signaling cascades to modulate cardiac function and remodeling. Previous in vitro studies suggest that β1-AR signaling is cardiotoxic whereas β2-AR signaling is cardioprotective, and may be the case during ischemia/reperfusion in vivo. The objective of this study was to assess whether β2-ARs also play a cardioprotective role in the pathogenesis of non-ischemic forms of cardiomyopathy. To dissect the role of β1 vs β2-ARs in modulating MLP (Muscle LIM Protein) cardiomyopathy, we crossbred MLP-/- with β1-/- or β2-/- mice. Deletion of the β2-AR improved survival, cardiac function, exercise capacity and myocyte shortening; by contrast haploinsufficency of the β1-AR reduced survival. Pathologic changes in Ca(2+) handling were reversed in the absence of β2-ARs: peak Ca(2+) and SR Ca(2+) were decreased in MLP-/- and β1+/-/MLP-/- but restored in β2-/-MLP-/-. These changes were associated with reversal of alterations in troponin I and phospholamban phosphorylation. Gi inhibition increased peak and baseline Ca(2+), recapitulating changes observed in the β2-/-/MLP-/-. The L-type Ca(2+) blocker verapamil significantly decreased cardiac function in β2-/-MLP-/- vs WT. We next tested if the protective effects of β2-AR ablation were unique to the MLP model using TAC-induced heart failure. Similar to MLP, β2-/- mice demonstrated delayed progression of heart failure with restoration of myocyte shortening and peak Ca(2+) and Ca(2+) release. Deletion of β2-ARs prevents the development of MLP-/- cardiomyopathy via positive modulation of Ca(2+) due to removal of inhibitory Gi signaling and increased phosphorylation of troponin I and phospholamban. Similar effects were seen after TAC. Unlike previous models where β2-ARs were found to be cardioprotective, in these two models, β2-AR signaling appears to be deleterious, potentially through negative regulation of Ca(2+) dynamics.

Norepinephrine transporter variant A457P knock-in mice display key features of human postural orthostatic tachycardia syndrome

Postural orthostatic tachycardia syndrome (POTS) is a common autonomic disorder of largely unknown etiology that presents with sustained tachycardia on standing, syncope and elevated norepinephrine spillover. Some individuals with POTS experience anxiety, depression and cognitive dysfunction. Previously, we identified a mutation, A457P, in the norepinephrine (NE; also known as noradrenaline) transporter (NET; encoded by SLC6A2) in POTS patients. NET is expressed at presynaptic sites in NE neurons and plays a crucial role in regulating NE signaling and homeostasis through NE reuptake into noradrenergic nerve terminals. Our in vitro studies demonstrate that A457P reduces both NET surface trafficking and NE transport and exerts a dominant-negative impact on wild-type NET proteins. Here we report the generation and characterization of NET A457P mice, demonstrating the ability of A457P to drive the POTS phenotype and behaviors that are consistent with reported comorbidities. Mice carrying one A457P allele (NET(+/P)) exhibited reduced brain and sympathetic NE transport levels compared with wild-type (NET(+/+)) mice, whereas transport activity in mice carrying two A457P alleles (NET(P/P)) was nearly abolished. NET(+/P) and NET(P/P) mice exhibited elevations in plasma and urine NE levels, reduced 3,4-dihydroxyphenylglycol (DHPG), and reduced DHPG:NE ratios, consistent with a decrease in sympathetic nerve terminal NE reuptake. Radiotelemetry in unanesthetized mice revealed tachycardia in NET(+/P) mice without a change in blood pressure or baroreceptor sensitivity, consistent with studies of human NET A457P carriers. NET(+/P) mice also demonstrated behavioral changes consistent with CNS NET dysfunction. Our findings support that NET dysfunction is sufficient to produce a POTS phenotype and introduces the first genetic model suitable for more detailed mechanistic studies of the disorder and its comorbidities.

Pathophysiological relevance of the cardiac β2-adrenergic receptor and its potential as a therapeutic target to improve cardiac function

β-adrenoceptors are members of the G protein-coupled receptor superfamily which play a key role in the regulation of myocardial function. Their activation increases cardiac performance but can also induce deleterious effects such as cardiac arrhythmias or myocardial apoptosis. In fact, inhibition of β-adrenoceptors exerts a protective effect in patients with sympathetic over-stimulation during heart failure. Although β(2)-adrenoceptor is not the predominant subtype in the heart, it seems to importantly contribute to the cardiac effects of adrenergic stimulation; however, the mechanism by which this occurs is not fully understood. This review summarizes the current knowledge on the role of β(2)-adrenoceptors in the regulation of cardiac contractility, metabolism, cardiomyocyte survival and cardiac arrhythmias. In addition, therapeutic considerations relating to stimulation of the β(2)-adrenoceptor such as an increase in cardiac contractility with low arrythmogenic effect, protection of the myocardium again apoptosis or positive regulation of heart metabolism are discussed.

Endurance exercise training normalizes repolarization and calcium-handling abnormalities, preventing ventricular fibrillation in a model of sudden cardiac death

The risk of sudden cardiac death is increased following myocardial infarction. Exercise training reduces arrhythmia susceptibility, but the mechanism is unknown. We used a canine model of sudden cardiac death (healed infarction, with ventricular tachyarrhythmias induced by an exercise plus ischemia test, VF+); we previously reported that endurance exercise training was antiarrhythmic in this model (Billman GE. Am J Physiol Heart Circ Physiol 297: H1171-H1193, 2009). A total of 41 VF+ animals were studied, after random assignment to 10 wk of endurance exercise training (EET; n = 21) or a matched sedentary period (n = 20). Following (>1 wk) the final attempted arrhythmia induction, isolated myocytes were used to test the hypotheses that the endurance exercise-induced antiarrhythmic effects resulted from normalization of cellular electrophysiology and/or normalization of calcium handling. EET prevented VF and shortened in vivo repolarization (P < 0.05). EET normalized action potential duration and variability compared with the sedentary group. EET resulted in a further decrement in transient outward current compared with the sedentary VF+ group (P < 0.05). Sedentary VF+ dogs had a significant reduction in repolarizing K(+) current, which was restored by exercise training (P < 0.05). Compared with controls, myocytes from the sedentary VF+ group displayed calcium alternans, increased calcium spark frequency, and increased phosphorylation of S2814 on ryanodine receptor 2. These abnormalities in intracellular calcium handling were attenuated by exercise training (P < 0.05). Exercise training prevented ischemically induced VF, in association with a combination of beneficial effects on cellular electrophysiology and calcium handling.

Phosphoinositide 3-kinase γ protects against catecholamine-induced ventricular arrhythmia through protein kinase A-mediated regulation of distinct phosphodiesterases

BACKGROUND

Phosphoinositide 3-kinase γ (PI3Kγ) signaling engaged by β-adrenergic receptors is pivotal in the regulation of myocardial contractility and remodeling. However, the role of PI3Kγ in catecholamine-induced arrhythmia is currently unknown.

METHODS AND RESULTS

Mice lacking PI3Kγ (PI3Kγ(-/-)) showed runs of premature ventricular contractions on adrenergic stimulation that could be rescued by a selective β(2)-adrenergic receptor blocker and developed sustained ventricular tachycardia after transverse aortic constriction. Consistently, fluorescence resonance energy transfer probes revealed abnormal cAMP accumulation after β(2)-adrenergic receptor activation in PI3Kγ(-/-) cardiomyocytes that depended on the loss of the scaffold but not of the catalytic activity of PI3Kγ. Downstream from β-adrenergic receptors, PI3Kγ was found to participate in multiprotein complexes linking protein kinase A to the activation of phosphodiesterase (PDE) 3A, PDE4A, and PDE4B but not of PDE4D. These PI3Kγ-regulated PDEs lowered cAMP and limited protein kinase A-mediated phosphorylation of L-type calcium channel (Ca(v)1.2) and phospholamban. In PI3Kγ(-/-) cardiomyocytes, Ca(v)1.2 and phospholamban were hyperphosphorylated, leading to increased Ca(2+) spark occurrence and amplitude on adrenergic stimulation. Furthermore, PI3Kγ(-/-) cardiomyocytes showed spontaneous Ca(2+) release events and developed arrhythmic calcium transients.

CONCLUSIONS

PI3Kγ coordinates the coincident signaling of the major cardiac PDE3 and PDE4 isoforms, thus orchestrating a feedback loop that prevents calcium-dependent ventricular arrhythmia.

Functional antagonism of β-adrenoceptor subtypes in the catecholamine-induced automatism in rat myocardium

BACKGROUND AND PURPOSE

Myocardial automatism and arrhythmias may ensue during strong sympathetic stimulation. We sought to investigate the relevant types of adrenoceptor, as well as the role of phosphodiesterase (PDE) activity, in the production of catecholaminergic automatism in atrial and ventricular rat myocardium.

EXPERIMENTAL APPROACH

The effects of adrenoceptor agonists on the rate of spontaneous contractions (automatic response) and the amplitude of electrically evoked contractions (inotropic response) were determined in left atria and ventricular myocytes isolated from Wistar rats.

KEY RESULTS

Catecholaminergic automatism was Ca(2+) -dependent, as it required a functional sarcoplasmic reticulum to be exhibited. Although both α- and β-adrenoceptor activation caused inotropic stimulation, only β(1) -adrenoceptors seemed to mediate the induction of spontaneous activity. Catecholaminergic automatism was enhanced and suppressed by β(2) -adrenoceptor blockade and stimulation respectively. Inhibition of either PDE3 or PDE4 (by milrinone and rolipram, respectively) potentiated the automatic response of myocytes to catecholamines. However, only rolipram abolished the attenuation of automatism produced by β(2) -adrenoceptor stimulation.

CONCLUSIONS AND IMPLICATIONS

α- and β(2) -adrenoceptors do not seem to be involved in the mediation of catecholaminergic stimulation of spontaneous activity in atrial and ventricular myocardium. However, a functional antagonism of β(1) - and β(2) -adrenoceptor activation was identified, the former mediating catecholaminergic myocardial automatism and the latter attenuating this effect. Results suggest that hydrolysis of cAMP by PDE4 is involved in the protective effect mediated by β(2) -adrenoceptor stimulation.

The increase in rat ventricular automaticity induced by salbutamol is mediated through β(1)- but not β(2)-adrenoceptors: role of phosphodiesterases

AIMS

While β(2)-adrenoceptor (AR) agonists are useful bronchodilators, they also produce cardiac arrhythmias. These agents are not fully selective and also activate β(1)-AR, but the involvement of β(1)-AR and β(2)-AR in the observed pro-arrhythmic effect has not been established. We studied the effect of β(1)-AR and β(2)-AR activation on ventricular automaticity and the role of phosphodiesterases (PDE) in regulating this effect.

MAIN METHODS

Experiments were performed in the spontaneously beating isolated right ventricle of the rat heart. We also measured cAMP production in this tissue.

KEY FINDINGS

The β(2)-AR agonist salbutamol (1-100 μM) produced a concentration-dependent increase in ventricular automaticity that was not affected by 50nM of the β(2)-AR antagonist ICI 118551. This effect was enhanced by the non-selective PDE inhibitor theophylline (100 μM) and by the selective PDE4 inhibitors rolipram (1 μM) and Ro 201724 (2 μM), but not modified by the selective PDE3 inhibitors cilostamide (0.3 μM) or milrinone (0.2 μM). The effects of salbutamol alone and in the presence of either theophylline or rolipram were virtually abolished by 0.1 μM β(1)-AR antagonist CGP 20712A. Salbutamol (10 μM) increased the cAMP concentration, and this effect was abolished by CGP 20712A (0.1 μM) but enhanced by theophylline (100 μM) or rolipram (1 μM). Cilostamide (0.3 μM) failed to modify the effect of salbutamol on cAMP concentration.

SIGNIFICANCE

These results indicate that the increase of ventricular automaticity elicited by salbutamol was exclusively mediated through β(1)-AR and enhanced by non-selective PDE inhibition with theophylline or selective PDE4 inhibition. However, PDE3 did not appear to regulate this effect.

Intense exercise training induces adaptation in expression and responsiveness of cardiac β-adrenoceptors in diabetic rats

Background

Informations about the effects of intense exercise training on diabetes-induced myocardial dysfunctions are lacking. We have examined the effects of intense exercise training on the cardiac function of diabetic rats, especially focusing on the Langendorff β-adrenergic responsiveness and on the β-adrenoceptors protein expression.

Methods

Control or Streptozotocin induced-diabetic male Wistar rats were randomly assigned to sedentary or trained groups. The training program consisted of 8 weeks running on a treadmill (10° incline, up to 25 m/min, 60 min/day) and was considered to be intense for diabetic rats.

Results

This intense exercise training amplified the in vivo diabetes-induced bradycardia. It had no effect on Langendorff basal cardiac contraction and relaxation performances in control and diabetic rats. In diabetic rats, it accentuated the Langendorff reduced responsiveness to β-adrenergic stimulation. It did not blunt the diabetes-induced decrease of β1-adrenoceptors protein expression, displayed a significant decrease in the β2-adrenoceptors protein expression and normalized the β3-adrenoceptors protein expression.

Conclusions

Intense exercise training accentuated the decrease in the myocardial responsiveness to β-adrenergic stimulation induced by diabetes. This defect stems principally from the β2-adrenoceptors protein expression reduction. Thus, these results demonstrate that intense exercise training induces specific effects on the β-adrenergic system in diabetes.

Infarction-induced cytokines cause local depletion of tyrosine hydroxylase in cardiac sympathetic nerves

Myocardial infarction causes a heterogeneity of noradrenergic transmission that contributes to the development of ventricular arrhythmias and sudden cardiac death. Ischaemia-induced alterations in sympathetic transmission include regional variations in cardiac noradrenaline (NA) and in tyrosine hydroxylase, the rate-limiting enzyme in NA synthesis. Inflammatory cytokines that act through gp130 are elevated in the heart after myocardial infarction. These cytokines decrease expression of tyrosine hydroxylase in sympathetic neurons, and indirect evidence suggests that they contribute to the local depletion of tyrosine hydroxylase in the damaged left ventricle. However, gp130 cytokines are also important for the survival of cardiac myocytes following damage to the heart. To examine the effect of cytokines on tyrosine hydroxylase and NA content in cardiac nerves we used gp130(DBH-Cre/lox) mice, which have a deletion of the gp130 receptor in neurons expressing dopamine beta-hydroxylase. The absence of neuronal gp130 prevented the loss of tyrosine hydroxylase in cardiac sympathetic nerves innervating the left ventricle 1 week after ischaemia-reperfusion compared with wild-type C57BL/6J mice. Surprisingly, restoration of tyrosine hydroxylase in the damaged ventricle did not return neuronal NA content to normal levels. Noradrenaline uptake into cardiac nerves was significantly lower in gp130 knockout mice, contributing to the lack of neuronal NA stores. There were no significant differences in left ventricular peak systolic pressure, dP/dt(max) or dP/dt(min) between the two genotypes after myocardial infarction, but ganglionic blockade revealed differences in autonomic tone between the genotypes. Stimulation of the heart with dobutamine or release of endogenous NA with tyramine generated similar responses in both genotypes. Thus, the removal of gp130 from sympathetic neurons prevents the post-infarct depletion of tyrosine hydroxylase in the left ventricle, but does not alter NA content or cardiac function.

Cardiac autonomic neural remodeling and susceptibility to sudden cardiac death: effect of endurance exercise training

Sudden cardiac death resulting from ventricular tachyarrhythmias remains the leading cause of death in industrially developed countries, accounting for between 300,000 and 500,000 deaths each year in the United States. Yet, despite the enormity of this problem, both the identification of factors contributing to ventricular fibrillation as well as the development of safe and effective antiarrhythmic agents remain elusive. Subnormal cardiac parasympathetic regulation coupled with an elevated cardiac sympathetic activation may allow for the formation of malignant ventricular arrhythmias. In particular, myocardial infarction can reduce cardiac parasympathetic regulation and alter beta-adrenoceptor subtype expression enhancing beta(2)-adrenoceptor sensitivity that can lead to intracellular calcium dysregulation and arrhythmias. As such, myocardial infarction can induce a remodeling of cardiac autonomic regulation that may be required to maintain cardiac pump function. If alterations in cardiac autonomic regulation play an important role in the genesis of life-threatening arrhythmias, then one would predict that interventions designed to either augment parasympathetic activity and/or reduce cardiac adrenergic activity would also protect against ventricular fibrillation. Recently, studies using a canine model of sudden death demonstrate that endurance exercise training (treadmill running) enhanced cardiac parasympathetic regulation (increased heart rate variability), restored a more normal beta-adrenoceptor balance (i.e., reduced beta(2)-adrenoceptor sensitivity and expression), and protected against ventricular fibrillation induced by acute myocardial ischemia. Thus exercise training may reverse the autonomic neural remodeling induced by myocardial infarction and thereby enhance the electrical stability of the heart in individuals shown to be at an increased risk for sudden cardiac death.

Absence of gp130 in dopamine beta-hydroxylase-expressing neurons leads to autonomic imbalance and increased reperfusion arrhythmias

Inflammatory cytokines that act through glycoprotein (gp)130 are elevated in the heart after myocardial infarction and in heart failure. These cytokines are potent regulators of neurotransmitter and neuropeptide production in sympathetic neurons but are also important for the survival of cardiac myocytes after damage to the heart. To examine the effect of gp130 cytokines on cardiac nerves, we used gp130(DBH-Cre/lox) mice, which have a selective deletion of the gp130 cytokine receptor in neurons expressing dopamine beta-hydroxylase (DBH). Basal sympathetic parameters, including norepinephrine (NE) content, tyrosine hydroxylase expression, NE transporter expression, and sympathetic innervation density, appeared normal in gp130(DBH-Cre/lox) compared with wild-type mice. Likewise, basal cardiovascular parameters measured under isoflurane anesthesia were similar in both genotypes, including mean arterial pressure, left ventricular peak systolic pressure, dP/dt(max), and dP/dt(min). However, pharmacological interventions revealed an autonomic imbalance in gp130(DBH-Cre/lox) mice that was correlated with an increased incidence of premature ventricular complexes after reperfusion. Stimulation of NE release with tyramine and infusion of the beta-agonist dobutamine revealed blunted adrenergic transmission that correlated with decreased beta-receptor expression in gp130(DBH-Cre/lox) hearts. Due to the developmental expression of the DBH-Cre transgene in parasympathetic ganglia, gp130 was eliminated. Cholinergic transmission was impaired in gp130(DBH-Cre/lox) hearts due to decreased parasympathetic drive, but tyrosine hydroxylase immunohistochemistry in the brain stem revealed that catecholaminergic nuclei appeared grossly normal. Thus, the apparently normal basal parameters in gp130(DBH-Cre/lox) mice mask an autonomic imbalance that includes alterations in sympathetic and parasympathetic transmission.

Epilepsy and sudden death: personal reflections and call for global action

To solve the mystery of sudden unexpected death in persons with epilepsy (SUDEP), a global focus is needed to identify persons at risk, develop treatment regimens, and prevent its occurrence. A world wide network of professionals must focus on basic scientific research programs and clinical and epidemiology studies. Team work among different multidisciplinary professionals in clinical settings and within and among laboratories should address the global issues of SUDEP. If the correct term 'SUDEP' is used on autopsy reports and if verbal autopsies postmortem are conducted when needed, the true incidence of SUDEP may be found to be much higher than previously thought and the market for new antiepileptics and other drugs to prevent SUDEP will be larger. Symposia should discuss new data and lessons learned from the last 20 to 30 years to be applied by scientists and clinicians worldwide to gain a better understanding of SUDEP. 'Think out of the box' when evaluating an established animal model with potential for modification(s) to study mechanism(s) of SUDEP. Multiple relevant animal models are needed to understand the pathophysiology of SUDEP, hypothesize about effective treatments, develop small pilot studies in persons with epilepsy, and conduct confirmatory large-scale clinical trials. The fields of pharmacology, clinical pharmacology, and cardiology have much to offer as we work to improve compliance, develop new antiepileptic drugs, and apply different categories of drugs to resolve the mystery of SUDEP. Ambulatory simultaneous EKG and EEG telemetry monitoring of patients at risk for sudden death will help identify cardiac vs. brain epileptogenic triggers for treatment to decrease risk of SUDEP. Respiratory function monitoring is also needed. Academic fellowships and competitions for medical students, postdoctoral fellows, residents and faculty will attract medical and graduate trainees to work on SUDEP. Grant funding is essential to move the SUDEP knowledge base forward. Leaders must solve the global mystery of SUDEP using a leadership philosophy foundation that provides innovative vision and approaches for SUDEP research and teaching programs. The interaction of teaching and research is essential: while a student is learning how to conduct research he must simultaneously learn to become a teacher. Medical and graduate leaders must provide vision and a fertile environment to teach students of today to become the self learners and leaders of tomorrow to find solutions for SUDEP.

The potential beneficial effects of beta adrenergic blockade in the treatment of ventricular fibrillation

Cardiac arrest remains a major medical emergency in Western societies, with ventricular fibrillation being the initial rhythm in a significant proportion of cases. Adrenaline is generally accepted to improve the resuscitation outcome, since it improves coronary and cerebral blood flow during cardiopulmonary resuscitation, but several detrimental effects have been associated with its use, most of which are thought to be mediated by its beta adrenergic properties. Several animal studies suggest that beta adrenergic blockade during resuscitation, is associated with increased rates of resuscitation and improved post-resuscitation myocardial function. This article reviews the presence and function of beta-adrenoceptor subtypes in the intact and diseased human myocardium, as well as the differences observed in beta(1)- and beta(2) adrenoceptor subtypes in different species.

Reduced cardio-respiratory coupling in acute alcohol withdrawal

BACKGROUND

Chronic alcoholism represents a risk factor for cardiac arrhythmias. One underlying mechanism is a sympathetically dominated autonomic imbalance. This is especially apparent during acute withdrawal from alcohol. Since linear analysis of heart rate variability may not be entirely adequate to detect such autonomic dysfunction in acute alcohol withdrawal, we applied novel non-linear parameters and measures for cardio-respiratory coupling.

METHODS

20 patients suffering from acute alcohol withdrawal syndrome and 20 controls were included. For patients, heart rate and respiration were recorded on admission, after medication and at discharge. From these data, complexity measures (symbolic dynamics, approximate entropy) of heart rate modulation and respiration as well as parameters for cardio-respiratory coupling (coherence, cross-approximate entropy) which relate to vagal function were calculated.

RESULTS

Heart rate modulation was significantly less complex in patients acutely admitted for alcohol withdrawal. Furthermore, coupling between beat-to-beat (RR) intervals and respiration time series was significantly diminished. Of the parameters assessed, cross-approximate entropy showed a trend for correlation with symptom severity.

CONCLUSION

These data indicate diminished vagal function in acute alcohol withdrawal. Applying the methods described thus allows a sensitive detection of vagal neuropathy in this disease.

Beat-to-beat QT interval variability is primarily affected by the autonomic nervous system

BACKGROUND

Beat-to-beat QT interval variability is associated with life-threatening arrhythmias and sudden death, however, its precious mechanism and the autonomic modulation on it remains unclear. The purpose of this study was to determine the effect of drugs that modulate the autonomic nervous system on beat-to-beat QT interval.

METHOD

RR and QT intervals were determined for 512 consecutive beats during fixed atrial pacing with and without propranolol and automatic blockade (propranolol plus atropine) in 11 patients without structural heart disease. Studied parameters included: RR, QTpeak (QRS onset to the peak of T wave), QTend (QRS onset to the end of T wave) interval, standard deviation (SD) of the RR, QTpeak, and QTend (RR-SD, QTpeak-SD, and QTend-SD), coefficients of variation (RR- CV, QTpeak-CV, and QTend-CV) from time domain analysis, total power (TP; RR-TP, QTpeak-TP, and QTend-TP), and power spectral density of the low-frequency band (LF; RR-LF, QTpeak-LF, and QTend-LF) and the high-frequency band (HF; RR-HF, QTpeak-HF and QTend-HF).

RESULTS

Administration of propranolol and infusion of atropine resulted in the reduction of SD, CV, TP, and HF of the QTend interval when compared to controlled atrial pacing (3.7 +/- 0.6 and 3.5 +/- 0.5 vs 4.8 +/- 1.4 ms, 0.9 +/- 0.1 and 0.9 +/- 0.1 vs 1.2 +/- 0.3%, 7.0 +/- 2.2 and 7.0 +/- 2.2 vs 13.4 +/- 8.1 ms(2), 4.2 +/- 1.4 and 4.2 +/- 1.2 vs 8.4 +/- 4.9 ms(2), respectively). Administration of propranolol and atropine did not affect RR interval or QTpeak interval indices during controlled atrial pacing.

CONCLUSIONS

Beat-to-beat QT interval variability is affected by drugs that modulate the autonomic nervous system.

Arrhythmogenic effects of beta2-adrenergic stimulation in the failing heart are attributable to enhanced sarcoplasmic reticulum Ca load

Ventricular tachycardia in heart failure (HF) can initiate by nonreentrant mechanisms such as delayed afterdepolarizations. In an arrhythmogenic rabbit model of HF, we have shown that isoproterenol induces ventricular tachycardia in vivo and aftercontractions and transient inward currents in HF myocytes. To determine whether beta(2)-adrenergic receptor (beta(2)-AR) stimulation contributes, we performed in vivo drug infusion, in vitro myocyte and biochemical studies. Intravenous zinterol (2.5 microg/kg) led to ventricular arrhythmias, including ventricular tachycardia up to 13 beats long in 4 of 6 HF rabbits (versus 0 of 5 controls, P<0.01), an effect blocked by beta(2)-AR antagonist ICI-118,551 (0.2 mg/kg). In field-stimulated myocytes (0.5 to 4 Hz, 37 degrees C), beta(2)-AR stimulation (1 micromol/L zinterol+300 nmol/L beta(1)-AR antagonist CGP-29712A) induced aftercontractions and Ca aftertransients in 88% of HF versus 0% of control myocytes (P<0.01). beta(2)-AR stimulation in HF (but not control) myocytes increased Ca transient amplitude (by 29%), sarcoplasmic reticulum (SR) Ca load (by 28%), the rate of [Ca](i) decline (by 28%; n=12, all P<0.05), and phospholamban phosphorylation at Ser16, but Ca current was unchanged. All of these effects in HF myocytes were blocked by ICI-118,551 (100 nmol/L). Although total beta-AR expression was reduced by 47% in HF rabbit left ventricle, beta(2)-AR number was unchanged, indicating more potent beta(2)-AR-dependent SR Ca uptake and arrhythmogenesis in HF. Human HF myocytes showed similar beta(2)-AR-induced aftercontractions, aftertransients, and enhanced Ca transient amplitude, SR Ca load and twitch [Ca](i) decline rate. Thus, beta(2)-AR stimulation is arrhythmogenic in HF, mediated by SR Ca overload-induced spontaneous SR Ca release and aftercontractions.

Post-myocardial infarction beta-blocker therapy: the bradycardia conundrum. Rationale and design for the Pacemaker & beta-blocker therapy post-MI (PACE-MI) trial

Multiple clinical trials have demonstrated beta-blockers improving survival after myocardial infarction (MI). Patients with "bradycardia-related" contraindications to beta-blockers, such as those with asymptomatic bradycardia or AV conduction abnormalities, have been excluded from clinical trials of beta-blockers and continue to be excluded from post-MI beta-blocker therapy in routine clinical practice. These patients tend to be elderly and have a high 1-year mortality. If beta-blockers provide benefit to the post-MI patient independent of their heart rate-lowering effect, then these patients could benefit substantially from initiation of beta-blocker therapy. However, in this particular group of patients, beta-blockers can be safely initiated only if more severe or significant bradycardia can be prevented by pacemaker implantation. It is unclear whether adverse effects related to pacemaker implantation could also negate some or all of the hypothesized benefit of beta-blocker therapy. Although beta-blockers are particularly effective in the elderly, the benefit of beta-blocker therapy in patients with bradycardia-related contraindications to beta-blockers has not been established. The PACE-MI trial is a randomized controlled trial that will address whether beta-blocker therapy enabled by pacemaker implantation is superior to no beta-blocker and no pacemaker therapy after MI in patients with rhythm contraindications to beta-blockers or in those who have developed symptomatic bradycardia due to beta-blockers. The trial will randomize 1124 patients to standard therapy (not to include beta-blockers as patients must have a contraindication to be enrolled) or standard therapy plus pacemaker implantation and beta-blocker. The primary end point is the composite end point of total mortality plus nonfatal reinfarction.

Exercise training normalizes beta-adrenoceptor expression in dogs susceptible to ventricular fibrillation

Previous studies demonstrated an enhanced beta(2)-adrenoceptor (AR) responsiveness in animals susceptible to ventricular fibrillation (VF) that was eliminated by exercise training. The present study investigated the effects of endurance exercise training on beta(1)-AR and beta(2)-AR expression in dogs susceptible to VF. Myocardial ischemia was induced by a 2-min occlusion of the left circumflex artery during the last minute of exercise in dogs with healed infarctions: 20 had VF [susceptible (S)] and 13 did not [resistant (R)]. These dogs were randomly assigned to either 10-wk exercise training [treadmill running; n = 9 (S) or 8 (R)] or an equivalent sedentary period [n = 11 (S) or 5 (R)]. Left ventricular tissue beta-AR protein and mRNA were quantified by Western blot analysis and RT-PCR, respectively. Because beta(2)-ARs are located in caveolae, caveolin-3 was also quantified. beta(1)-AR gene expression decreased ( approximately 5-fold), beta(2)-AR gene expression was not changed, and the ratio of beta(2)-AR to beta(1)-AR gene expression was significantly increased in susceptible compared with resistant dogs. beta(1)-AR protein decreased ( approximately 50%) and beta(2)-AR protein increased (400%) in noncaveolar fractions of the cell membrane in susceptible dogs. Exercise training returned beta(1)-AR gene expression to levels seen in resistant animals but did not alter beta(2)-AR protein levels in susceptible dogs. These data suggest that beta(1)-AR gene expression was decreased in susceptible dogs compared with resistant dogs and, further, that exercise training improves beta(1)-AR gene expression, thereby restoring a more normal beta-AR balance.