儿茶酚胺介导的多形性室速研究进展

儿茶酚胺介导的多形性室速是一种少见却严重的遗传性心律失常,表现为无器质性心脏病的个体在运动或激动时发生双向性、多形性室速导致发作性晕厥及进展为心室颤动导致猝死。心肌细胞肌浆网异常释放钙离子使细胞内钙离子超载引起的延迟后除极可能是儿茶酚胺介导的多形性室速发生的机制。目前已知的和儿茶酚胺介导的多形性室速相关的基因为常染色体显性遗传的RyR2（位于1q42.1-q43）和常染色体隐性遗传的CASQ2（位于1p13.3-p11）。治疗：β-阻断剂适用于所有临床症状的个体和可能有RyR2突变而没有心脏事件（晕厥）或运动试验诱发的室性心律失常等病史的个体。反复心脏骤停患者需植入式心律转复除颤器。每6至12个月随访以监测疗效。患者所有的一级亲属,都应予心脏评估。     

儿茶酚胺敏感性多形性室速的基因诊治进展

儿茶酚胺敏感性多形性室速(CPVT)是具有较高猝死风险的罕见单基因遗传病.已知多种CPVT基因突变可通过影响肌浆网钙通道蛋白RyR2的功能,破坏细胞内钙稳态,触发室性心律失常,而依靠腺相关病毒载体(AAVs)及CRISPR/Cas9技术进行基因层面的干预有望为CPVT的治疗提供新思路.本文就其遗传特征及基因干预等领域的研究现状作一总结.     

Molecular and electrophysiological bases of catecholaminergic polymorphic ventricular tachycardia

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is an inherited arrhythmogenic disorder characterized by adrenergically mediated polymorphic ventricular tachyarrhythmias. Genetic investigations have identified two variants of the disease: an autosomal dominant form associated with mutations in the gene encoding the cardiac ryanodine receptor (RyR2) and a recessive form associated with homozygous mutations in the gene encoding the cardiac isoform of calsequestrin (CASQ2). Functional characterization of mutations identified in the RyR2 and CASQ2 genes has demonstrated that CPVT are caused by derangements of the control of intracellular calcium. Investigations in a knock-in mouse model have shown that CPVT arrhythmias are initiated by delayed afterdepolarizations and triggered activity. In the present article, we review clinical and molecular understanding of CPVT and discuss the most recent approaches to develop novel therapeutic strategies for the disease.     

In vivo and in vitro effects of the pineal gland and melatonin on [Ca2++ Mg2+]-dependent ATPase in cardiac sarcolemma

Abstract: The possible diurnal variation in cardiac [Ca²⁺+ Mg²⁺]-dependent ATPase (Ca²⁺ pump) activity and the influence of pinealectomy and melatonin on this enzyme in rat heart have been studied. Lowest levels of cardiac sarcolemma] membrane [Ca²⁺+ Mg²⁺]-dependent ATPase activity were measured in late afternoon in rats kept under a 14:10 light:dark cycle. Late in the dark phase the enzyme activity began to increase with the rise continuing until 0900, 3 hr after light onset. These time-dependent changes in [Ca²⁺+ Mg²⁺]-dependent ATPase activity did not occur in either pinealectomized or light-exposed rats suggesting that melatonin, secreted from the pineal gland during the night, induces the change in [Ca²⁺+ Mg²⁺]-dependent ATPase activity. In vitro studies in which cardiac tissue was incubated in the presence of melatonin over a wide range of doses showed that this indole stimulated the Ca²⁺ pump. The half-maximal effect of melatonin was observed at a melatonin concentration of 28 ng/ml. These findings suggest that the daily change in [Ca²⁺+ Mg²⁺]-dependent ATPase activity in the sarcolemma of heart tissue is a result of the circadian rhythm in pineal melatonin production and secretion. These findings may be applicable to normal cardiac physiology.     

Genotypic heterogeneity and phenotypic mimicry among unrelated patients referred for catecholaminergic polymorphic ventricular tachycardia genetic testing

BACKGROUND: Mutations in the RyR2-encoded cardiac ryanodine receptor/calcium release channel and in CASQ2-encoded calsequestrin cause catecholaminergic polymorphic ventricular tachycardia (CPVT1 and CPVT2, respectively). OBJECTIVES: The purpose of this study was to evaluate the extent of genotypic and phenotypic heterogeneity among referrals for CPVT genetic testing. METHODS: Using denaturing high-performance liquid chromatography and DNA sequencing, mutational analysis of 23 RyR2 exons previously implicated in CPVT1, comprehensive analysis of all translated exons in CASQ2 (CPVT2), KCNQ1 (LQT1), KCNH2 (LQT2), SCN5A (LQT3), KCNE1 (LQT5), KCNE2 (LQT6), and KCNJ2 (Andersen-Tawil syndrome [ATS1], also annotated LQT7), and analysis of 10 ANK2 exons implicated in LQT4 were performed on genomic DNA from 11 unrelated patients (8 females) referred to Mayo Clinic's Sudden Death Genomics Laboratory explicitly for CPVT genetic testing. RESULTS: Overall, putative disease causing mutations were identified in 8 patients (72%). Only 4 patients (3 males) hosted CPVT1-associated RyR2 mutations: P164S, V186M, S3938R, and T4196A. Interestingly, 4 females instead possessed either ATS1- or LQT5-associated mutations. Mutations were absent in >400 reference alleles. CONCLUSION: Putative CPVT1-causing mutations in RyR2 were seen in <40% of unrelated patients referred with a diagnosis of CPVT and preferentially in males. Phenotypic mimicry is evident with the identification of ATS1- and LQT5-associated mutations in females displaying a normal QT interval and exercise-induced bidirectional VT, suggesting that observed exercise-induced polymorphic VT in patients may reflect disorders other than CPVT. Clinical consideration for either Andersen-Tawil syndrome or long QT syndrome and appropriate genetic testing may be warranted for individuals with RyR2 mutation-negative CPVT, particularly females.     

pH-Regulated Na+ Influx into the Mammalian Ventricular Myocyte: The Relative Role of Na+-H+ Exchange and Na+-HCO3− Co-Transport

In the heart, intracellular Na⁺ concentration (Na⁺_i) is a controller of intracellular Ca²⁺ signaling, and hence of key aspects of cell contractility and rhythm. Na⁺_i will be influenced by variation in Na⁺ influx. In the present work, we consider one source of Na⁺ influx, sarcolemmal acid extrusion. Acid extrusion is accomplished by sarcolemmal H⁺ and HCO₃⁻ transporters that import Na⁺ ions while exporting H⁺ or importing HCO₃⁻. The capacity of this system to import Na⁺ is enormous, up to four times the maximum capacity of the Na⁺-K⁺ ATPase to extrude Na⁺ ions from the cell. In this review we consider the role of Na⁺-H⁺ exchange (NHE) and Na⁺-HCO₃⁻co-transport (NBC) in mediating Na⁺ influx into cardiac myocytes. We consider, in particular, the role of NBC, as so little is known about Na⁺ influx through this transporter. We show that both proteins mediate significant Na⁺ influx and that although, in the ventricular myocyte, NBC-mediated Na⁺ influx is less than through NHE, the proportions may be altered under a variety of conditions, including exposure to catecholamines, membrane depolarization, and interference with activity of the enzyme, carbonic anhydrase.     

Flecainide inhibits arrhythmogenic Ca waves by open state block of ryanodine receptor Ca release channels and reduction of Ca spark mass

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is linked to mutations in the cardiac ryanodine receptor (RyR2) or calsequestrin. We recently found that the drug flecainide inhibits RyR2 channels and prevents CPVT in mice and humans. Here we compared the effects of flecainide and tetracaine, a known RyR2 inhibitor ineffective in CPVT myocytes, on arrhythmogenic Ca²⁺ waves and elementary sarcoplasmic reticulum (SR) Ca²⁺ release events, Ca²⁺ sparks. In ventricular myocytes isolated from a CPVT mouse model, flecainide significantly reduced spark amplitude and spark width, resulting in a 40% reduction in spark mass. Surprisingly, flecainide significantly increased spark frequency. As a result, flecainide had no significant effect on spark-mediated SR Ca²⁺ leak or SR Ca²⁺ content. In contrast, tetracaine decreased spark frequency and spark-mediated SR Ca²⁺ leak, resulting in a significantly increased SR Ca²⁺ content. Measurements in permeabilized rat ventricular myocytes confirmed the different effects of flecainide and tetracaine on spark frequency and Ca²⁺ waves. In lipid bilayers, flecainide inhibited RyR2 channels by open state block, whereas tetracaine primarily prolonged RyR2 closed times. The differential effects of flecainide and tetracaine on sparks and RyR2 gating can explain why flecainide, unlike tetracaine, does not change the balance of SR Ca²⁺ fluxes. We suggest that the smaller spark mass contributes to flecainide's antiarrhythmic action by reducing the probability of saltatory wave propagation between adjacent Ca²⁺ release units. Our results indicate that inhibition of the RyR2 open state provides a new therapeutic strategy to prevent diastolic Ca²⁺ waves resulting in triggered arrhythmias, such as CPVT.     

Obesity as a risk factor for sustained ventricular tachyarrhythmias in MADIT II patients

Background: Obesity, as defined by body mass index ≥30 kg/m², has been shown to be a risk factor for cardiovascular disease. However, data on the relationship between body mass index (BMI) and the risk of ventricular arrhythmias and sudden cardiac death are limited. The aim of this study was to evaluate the risk of ventricular tachyarrhythmias and sudden death by BMI in patients after myocardial infarction with severe left ventricular dysfunction.
Methods : The risk of appropriate defibrillator therapy for ventricular tachycardia or ventricular fibrillation (VT/VF) by BMI status was analyzed in 476 nondiabetic patients with left ventricular dysfunction who received an implantable cardioverter defibrillator (ICD) in the Multicenter Automatic Defibrillator Implantation Trial-II (MADIT II).
Results : Mean BMI was 27 ± 5 kg/m². Obese patients comprised 25% of the study population. After 2 years of follow-up, the cumulative rates of appropriate ICD therapy for VT/VF were 39% in obese and 24% in nonobese patients, respectively (P = 0.014). In multivariate analysis, there was a significant 64% increase in the risk for appropriate ICD therapy among obese patients as compared with nonobese patients, which was attributed mainly to an 86% increase in the risk of appropriate ICD shocks (P = 0.006). Consistent with these results, the risk of the combined endpoint of appropriate VT/VF therapy or sudden cardiac death (SCD) was also significantly increased among obese patients (Hazard Ratio 1.59; P = 0.01).
Conclusions : Our findings suggest that in nondiabetic patients with ischemic left ventricular dysfunction, a BMI ≥30 kg/m² is an independent risk factor for ventricular tachyarrhythmias.     

Role of L-Type Calcium Channel Window Current in Generating Current-Induced Early Afterdepolarizations

Ionic Mechanism of EADs. Introduction: Early afterdepolarizations (EADs) can give rise to triggered activity and thereby produce cardiac arrhythmias. We used the whole-cell patch clamp technique to examine the relationship between L-type Ca²⁺ channel window current and the generation of EADs in single ventricular myocytes isolated from guinea pig hearts.
Methods and Results: With a high concentration of EGTA in the internal solution and Na⁺-containing physiologic external solution, EADs were induced in unclamped cells by injecting intracellular depolarizing current pulses. During voltage clamp protocols designed to simulate action potentials interrupted by EADs, we recorded an inward shift in total current up to 0.7 pA/pF over 400 msec at test steps in the range of the take-off potential for EADs. Cd^2t (0.2 mM) blocked most of the inward shift of current during the test steps and abolished EADs. When the same voltage clamp protocol was used following perfusion with an Na⁺-free, K⁺-free external solution, the Cd²⁺-sensitive inward currents recorded during the test steps were similar to those obtained in physiologic external solution. The overlapping range of potentials for partial activation of the d and f variables of L-type Ca²⁺ current ("window" region) measured in Na⁺-free, K⁺-free external solution was virtually the same as the voltage range of the Cd²⁺–sensitive inward currents.
Conclusion: Our experiments suggest that: (1) EADs can arise under conditions of high EGTA buffering of intraccllular [Ca²⁺]; and (2) under these conditions, L-type Ca²⁺ channel window current plays a major role in the initiation of EADs.     

Na+/Ca2+ Exchanger Expression and Function in a Rabbit Model of Myocardial Infarction

Introduction: In general, sarcolemmal Na⁺/Ca²⁺ exchanger (NCX) protein and activity is increased in hearts with ventricular dysfunction. However, in a subset of studies, reduced activity of NCX has been reported. Left ventricular dysfunction (LVD) was induced in the rabbit eight weeks after an apical myocardial infarction.
Methods: Using single microelectrode voltage clamp to assess the NCX activity in isolated ventricular cells, a decrease in NCX activity by ∼30% was observed. Immunoblot analysis indicated increased NCX protein levels by ∼20% in the LVD group. The cause of this paradox is unknown. Overexpression of the protein sorcin increased the activity of NCX without affecting NCX protein levels.
Results: Sorcin protein (dimer) levels were significantly lower in the LVD group (0.67 ± 0.05 n = 15, P < 0.05) compared to sham (1.0 ± 0.16, n = 15). Sorcin monomer levels were not significantly different (sham: 1.0 ± 0.26, LVD: 0.83 ± 0.13). Mathematical modeling of NCX suggests that a reduction of NCX activity during diastole to that in LVD could be achieved by holding the diastolic membrane potential at −60 mV instead of −80 mV. Holding E_m at −60 mV decreased NCX-mediated Ca²⁺ efflux rates to values comparable to those seen in LVD and increased SR Ca²⁺ content and peak systolic [Ca²⁺] in sham and LVD cardiomyocytes.
Conclusions: In conclusion, reduced sorcin expression may be linked to the lower NCX activity in the rabbit model of LVD. Reduced NCX activity during diastole increases SR Ca²⁺ content and Ca²⁺ transient amplitude.     

Restitution of Ca2+ Release and Vulnerability to Arrhythmias

New information has recently been obtained along two essentially parallel lines of research: investigations into the fundamental mechanisms of Ca²⁺-induced Ca²⁺ release (CICR) in heart cells, and analyses of the factors that control the development of unstable rhythms such as repolarization alternans. These lines of research are starting to converge such that we can begin to understand unstable and potentially arrhythmogenic cardiac dynamics in terms of the underlying mechanisms governing not only membrane depolarization and repolarization but also the complex bidirectional interactions between electrical and Ca²⁺ signaling in heart cells. In this brief review, we discuss the progress that has recently been made in understanding the factors that control the beat-to-beat regulation of cardiac Ca²⁺ release and attempt to place these results within a larger context. In particular, we discuss factors that may contribute to unstable Ca²⁺ release and speculate about how instability in CICR may contribute to the development of arrhythmias under pathological conditions.     

Action Potential Duration, Rate of Stimulation, and Intracellular Sodium

In the first section of this short review the change of the cardiac action potential (APD) with the rate of stimulation under physiological conditions is described and mechanistically analyzed. A fast phase of adaptation is mainly caused by changes in gating characteristics of ionic currents, and rapid modulation of the Na⁺/Ca²⁺ exchanger. The slower phase is largely conditioned by incomplete recovery from inactivation of the late Na⁺ current (late I_Na) and changes in ion concentrations of [K⁺]_e, [Na⁺]_i, and [Ca²⁺]_i, which cause secondary changes in the permeation and the gating of ion channels and flux through transporters. In a second section, an analysis is presented of the rate dependence of APD in pathological conditions and its importance in the genesis of arrhythmias in hypertrophy, heart failure, congenital, and acquired LQT syndromes is summarized. The role of the late I_Na, Na⁺, and Ca²⁺ overload is emphasized. Special attention is given to the paradoxical transient lengthening of APD in LQT3 syndrome for the sudden increase in rate in this setting. The third section consists of a short commentary on Na⁺ and Ca²⁺ overload and drugs which block the late I_Na.     

A requirement for calcium in the caspase-independent killing of Burkitt lymphoma cell lines by Rituximab

The therapeutic monoclonal antibody rituximab has previously been shown to kill B cells in a caspase-independent manner. The signalling pathways underpinning this novel death pathway are unknown. The present study showed that rituximab treatment of Burkitt lymphoma cell lines induced a slow rise in intracellular calcium ([Ca²⁺]_i). This rise was only witnessed in cell lines that were killed by antibody, suggesting a critical role for Ca²⁺ in mediating rituximab-driven caspase-independent cell death. Inhibition of the two main intracellular store-located Ca²⁺ channels, i.e. the ryanodine and inositol-1,4,5-triphosphate receptor channels by dantrolene and xestospongen-c respectively did not prevent the rise in Ca²⁺ seen with rituximab or protect cells from subsequent death. In sharp contrast, inhibition of Ca²⁺ entry via plasma membrane channels with (2-aminoethoxy) diphenylborate or SKF-96365 or complete chelation of extracellular Ca²⁺ with ethyleneglycol bis (aminoethylether) tetra-acetate inhibited the rise in [Ca²⁺]_i and increased cell viability. Together, these data suggest that ligation of the CD20 receptor with rituximab allows a slow sustained influx of Ca²⁺ from the external environment that under certain conditions can lead to cell death.     

The L-type Ca2+ channel is involved in microvesicle-mediated glutamate exocytosis from rat pinealocytes

Abstract: Pinealocytes, parenchymal cells of the pineal gland, secrete glutamate through microvesicle-mediated exocytosis upon depolarization by KC1 in the presence of Ca²⁺, which is involved in a novel paracrine-like intercellular signal transduction mechanism in neuroendocrine organs. In the present study, we investigated whether or not the L-type Ca²⁺ channel is involved in the microvesicle-mediated glutamate secretion from cultured rat pinealocytes. Nifedipine, a specific antagonist of the L-type Ca²⁺ channel, inhibited the Ca²⁺-dependent glutamate exocytosis by 48% at 20 uM. Other L-type Ca²⁺ channel antagonists, such as nitrendipine, showed similar effects. 1,4-Dihydro-2,6-dimethyl-5-nitro-4[2-(trifluoromethyl)-phenyl]-3-pyridinecarboxylic acid methyl ester (BAY K8644), an agonist of the L-type Ca²⁺ channel, at 1 uM, on the other hand, stimulated the glutamate exocytosis about 1.6-fold. Consistently, these Ca²⁺ channel antagonists inhibited about 50% of the Ca²⁺ uptake, whereas BAY K8644 increased the uptake 5.3-fold. An antibody against the carboxyl-terminal region of the rabbit L-type Ca²⁺ channel recognized polypeptides of pinealocytes with apparent molecular masses of 250 and 270 kDa, respectively, and immunostained the plasma membrane region of the pinealocytes. These results strongly suggested that the entry of Ca²⁺ through L-type Ca²⁺ channel(s), at least in part, triggers microvesicle-mediated glutamate exocytosis in pinealocytes.     

Catecholaminergic Polymorphic Ventricular Tachycardia from Bedside to Bench and Beyond

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is a primary electrical myocardial disease characterized by exercise- and stress-related ventricular tachycardia manifested as syncope and sudden death. The disease has a heterogeneous genetic basis, with mutations in the cardiac Ryanodine Receptor channel (RyR2) gene accounting for an autosomal-dominant form (CPVT1) in approximately 50% and mutations in the cardiac calsequestrin gene (CASQ2) accounting for an autosomal-recessive form (CPVT2) in up to 2% of CPVT cases. Both RyR2 and calsequestrin are important participants in the cardiac cellular calcium homeostasis.We review the physiology of the cardiac calcium homeostasis, including the cardiac excitation contraction coupling and myocyte calcium cycling. The pathophysiology of cardiac arrhythmias related to myocyte calcium handling and the effects of different modulators are discussed.The putative derangements in myocyte calcium homeostasis responsible for CPVT, as well as the clinical manifestations and therapeutic options available, are described.     

Prevention of Ventricular Arrhythmia and Calcium Dysregulation in a Catecholaminergic Polymorphic Ventricular Tachycardia Mouse Model Carrying Calsequestrin‐2 Mutation

Arrhythmia Prevention in CPVT . Background: Catecholaminergic polymorphic ventricular tachycardia (CPVT) is a familial arrhythmic syndrome caused by mutations in genes encoding the calcium‐regulation proteins cardiac ryanodine receptor (RyR2) or calsequestrin‐2 (CASQ2). Mechanistic studies indicate that CPVT is mediated by diastolic Ca²⁺ overload and increased Ca²⁺ leak through the RyR2 channel, implying that treatment targeting these defects might be efficacious in CPVT. Method and results: CPVT mouse models that lack CASQ2 were treated with Ca²⁺‐channel inhibitors, β‐adrenergic inhibitors, or Mg²⁺. Treatment effects on ventricular arrhythmia, sarcoplasmic reticulum (SR) protein expression and Ca²⁺ transients of isolated myocytes were assessed. Each study agent reduced the frequency of stress‐induced ventricular arrhythmia in mutant mice. The Ca²⁺ channel blocker verapamil was most efficacious and completely prevented arrhythmia in 85% of mice. Verapamil significantly increased the SR Ca²⁺ content in mutant myocytes, diminished diastolic Ca²⁺ overload, increased systolic Ca²⁺ amplitude, and prevented Ca²⁺ oscillations in stressed mutant myocytes. Conclusions: Ca²⁺ channel inhibition by verapamil rectified abnormal calcium handling in CPVT myocytes and prevented ventricular arrhythmias. Verapamil‐induced partial normalization of SR Ca²⁺ content in mutant myocytes implicates CASQ2 as modulator of RyR2 activity, rather than or in addition to, Ca²⁺ buffer protein. Agents such as verapamil that attenuate cardiomyocyte calcium overload are appropriate for assessing clinical efficacy in human CPVT . (J Cardiovasc Electrophysiol, Vol. 22, pp. 316‐324, March 2011)     

Protective effect of melatonin on Ca2+ homeostasis and contractility in acute cholecystitis

Abstract:  Impaired Ca²⁺ homeostasis and smooth muscle contractility co-exist in acute cholecystitis (AC) leading to gallbladder dysfunction. There is no pharmacological treatment for this pathological condition. Our aim was to evaluate the effects of melatonin treatment on Ca²⁺ signaling pathways and contractility altered by cholecystitis. [Ca²⁺]_i was determined by epifluorescence microscopy in fura-2 loaded isolated gallbladder smooth muscle cells, and isometric tension was recorded from gallbladder muscle strips. Malondialdehyde (MDA) and reduced glutathione (GSH) contents were determined by spectrophotometry and cycloxygenase-2 (COX-2) expression was quantified by western blot. Melatonin was tested in two experimental groups, one of which underwent common bile duct ligation for 2 days and another that was later de-ligated for 2 days. Inflammation-induced impairment of Ca²⁺ responses to cholecystokinin and caffeine were recovered by melatonin treatment (30 mg/kg). This treatment also ameliorated the detrimental effects of AC on Ca²⁺ influx through both L-type and capacitative Ca²⁺ channels, and it was effective in preserving the pharmacological phenotype of these channels. Despite its effects on Ca²⁺ homeostasis, melatonin did not improve contractility. After de-ligation, Ca²⁺ influx and contractility were still impaired, but both were recovered by melatonin. These effects of melatonin were associated to a reduction of MDA levels, an increase in GSH content and a decrease in COX-2 expression. These findings indicate that melatonin restores Ca²⁺ homeostasis during AC and resolves inflammation. In addition, this indoleamine helps in the subsequent recovery of functionality.     

Mechanism underlying catecholaminergic polymorphic ventricular tachycardia and approaches to therapy

Catecholaminergic polymorphic ventricular tachycardia (CPVT) is an inherited arrhythmia syndrome characterized by VT induced by adrenergic stress in the absence of structural heart disease and high incidence of sudden cardiac death. The diagnosis is made based on reproducible ventricular tachyarrhythmias including bidirectional VT and polymorphic VT during exercise testings. Two causative genes of CPVT have been identified: RYR2, encoding the cardiac ryanodine receptor (RyR2) Ca²⁺ release channel, and CASQ2, encoding cardiac calsequestrin. A mutation in RYR2 or CASQ2 is identified in approximately 60% of patients with CPVT. Mutations in these two genes destabilize the RyR2 Ca²⁺ release channel complex in sarcoplasmic reticulum and result in spontaneous Ca²⁺ release through RyR2 channels leading to delayed after depolarization, triggered activity, and bidirectional/polymorphic VT. Implantable cardioverter defibrillators (ICDs) are recommended for prevention of sudden death in patients with CPVT.1. A.E. Epstein, J.P. DiMarco, K.A. Ellenbogen, et al., ACC/AHA/HRS 2008 Guidelines for Device-Based Therapy of Cardiac Rhythm Abnormalities: a report of the American College of Cardiology/American Heart Association Task Force on Practice Guidelines (Writing Committee to Revise the ACC/AHA/NASPE 2002 Guideline Update for Implantation of Cardiac Pacemakers and Antiarrhythmia Devices): developed in collaboration with the American Association for Thoracic Surgery and Society of Thoracic Surgeons. Circulation. 2008;117:e350 However, painful shocks can trigger further adrenergic stress and arrhythmias, and deaths have occurred despite appropriate ICD shocks. Treatment with β-adrenergic blockers reduces arrhythmia burden and mortality, but is not completely effective. The beneficial effects of Ca²⁺ channel blocker verapamil in combination with β-blocker have been reported, but the role of verapamil has not been well assessed. Because Ca²⁺ leakage through ryanodine channel is a common mechanism of CPVT, ryanodine channel block may have a therapeutic effect. We discovered that flecainide directly inhibits RyR2 channels and prevent CPVT. Left cardiac sympathetic denervation may be an effective alternative treatment in combination with ICD, especially for patients whose arrhythmias are not controlled by drug therapies.     

Ethanol and Cocaine Cause Additive Inhibitory Effects on the Calcium Transients and Contraction in Single Cardiomyocytes

The heart is a major locus for the toxic actions of cocaine and ethanol, each of which has been shown to interfere with excitation-contraction coupling in cardiac muscle cells. Because these drugs are frequently used in combination, the present study was designed to investigate how they interact to modify the Ca²⁺ transient and associated contraction in fura2-loaded cardiomyocytes. A high-speed imaging technique using a charge-coupled device as detector and short-term image store was used to measure cytosolic Ca²⁺ and contraction simultaneously from fluorescence images obtained during the contractile cycle. Ethanol (100 mM) and cocaine (50 μM) caused reversible reductions in Ca²⁺ transient amplitude of 24.3 ± 3.0% and 25.1 ± 3.6%, respectively. Neither agent modified basal Ca²⁺. Ethanol treatment decreased peak shortening by 44.3 ± 3.5%, whereas the contractile depression by cocaine was 31.4 ± 5.3%. The relatively greater effect of ethanol on contraction resulted from a Ca²⁺-independent component of ethanol action on contractility. When cardiomyocytes were exposed simultaneously to ethanol and cocaine, Ca²⁺ transient amplitude was reduced by 38.7 ± 3.0%, and peak contraction was decreased by 55.1 ± 3.5%. These values represent a significantly greater inhibition than observed with either drug alone (p < 0.02) and are compatible with additive effects of the two drugs acting at distinct loci within the excitation-contraction coupling pathway. Thus, simultaneous use of cocaine and ethanol leads to an enhanced depression of myocardial contractility, which is likely to contribute to the cardiotoxic actions of the combination of these two drugs.     

Effect of Glutathione on Inositol 1,4,5-Triphosphate-Induced Ca2+ Release in Permeabilized Hepatocytes from Control and Chronic Ethanol-Fed Rats

The effect of oxidized and reduced glutathione on inositol 1,4,5-trisphosphate (InsP3)-induced Ca²⁺ release from endoplasmic reticular Ca²⁺ stores was studied in digitonin-permeabilized hepatocytes from chronically ethanol-fed rats and pair-fed control animals. The fractional Ca²⁺ release induced by a subsaturating concentration of lnsP3 was significantly enhanced in cells from ethanol-fed rats in the absence of a change in maximal lnsP3-releasable Ca²⁺ pool size, and this difference was not affected by preincubation with reduced glutathione. Incubation with oxidized glutathione (1 mM) increased the efficacy of Ca²⁺ release by subsaturating concentrations of lnsP3 in both control preparations and in cells from ethanol-fed rats. The shift in the InsP3 dose-response curve was not significantly different between the two preparations. These findings suggest that the enhanced efficacy of InsP3-induced Ca²⁺ release in hepatocytes from ethanol-fed rats is not caused by the oxidation of protein-bound thiol groups on the lnsP3 receptor.