Mark E. Anderson

Calmodulin Kinase II and Arrhythmias in a Mouse Model of Cardiac Hypertrophy

Background—Calmodulin kinase (CaMK) II is linked to arrhythmia mechanisms in cellular models where repolarization is prolonged. CaMKII upregulation and prolonged repolarization are general features of cardiomyopathy, but the role of CaMKII in arrhythmias in cardiomyopathy is unknown. Methods and Results—We studied a mouse model of cardiac hypertrophy attributable to transgenic (TG) overexpression of a constitutively active form of CaMKIV that also has increased endogenous CaMKII activity. ECG-telemetered TG mice had significantly more arrhythmias than wild-type (WT) littermate controls at baseline, and arrhythmias were additionally increased by isoproterenol. Arrhythmias were significantly suppressed by an inhibitory agent targeting endogenous CaMKII. TG mice had longer QT intervals and action potential durations than WT mice, and TG cardiomyocytes had frequent early afterdepolarizations (EADs), a hypothesized mechanism for triggering arrhythmias. EADs were absent in WT cells before and after isoproterenol, whereas EAD frequency was unaffected by isoproterenol in TG mice. L-type Ca2+ channels (LTTCs) can activate EADs, and LTCC opening probability (Po) was significantly higher in TG than WT cardiomyocytes before and after isoproterenol. A CaMKII inhibitory peptide equalized TG and WT LTCC Po and eliminated EADs, whereas a peptide antagonist of the Na+/Ca2+ exchanger current, also hypothesized to support EADs, was ineffective. Conclusions—These findings support the hypothesis that CaMKII is a proarrhythmic signaling molecule in cardiac hypertrophy in vivo. Cellular studies point to EADs as a triggering mechanism for arrhythmias but suggest that the increase in arrhythmias after &bgr;-adrenergic stimulation is independent of enhanced EAD frequency.

Systemic Administration of Calmodulin Antagonist W-7 or Protein Kinase A Inhibitor H-8 Prevents Torsade de Pointes in Rabbits

BACKGROUNDnThe ventricular arrhythmia torsade de pointes (TdP) occurs after QT interval prolongation and is associated with sudden cardiac death. The afterdepolarizations that initiate TdP are facilitated by protein kinase A and the multifunctional Ca(2+)/calmodulin-dependent protein kinase II (CaM kinase).nnnMETHODS AND RESULTSnIn this study, we evaluated the feasibility of suppression of TdP through systemic therapy with kinase inhibitory agents in an established animal model. Under control conditions, TdP was inducible in 6 of 8 rabbits. CaM kinase blockade with the calmodulin antagonist W-7 reduced TdP in a dose-dependent fashion (4 of 7 inducible at 25 micromol/kg and 1 of 7 inducible at 50 micromol/kg). Increased intracellular Ca(2+) has been implicated in the genesis of afterdepolarizations, but pretreatment with high-dose W-7 did not prevent TdP in response to the L-type Ca(2+) channel agonist BAY K 8644 (300 nmol/kg), suggesting that CaM kinase-independent activation of L-type Ca(2+) current was not affected by W-7. Compared with control animals, W-7 reduced TdP inducibility without shortening the QT interval, increasing heart rate, or reducing the blood pressure. The protein kinase A antagonist H-8 also caused a dose-dependent reduction in TdP inducibility (5 of 6 at 1 micromol/kg, 4 of 6 at 5 micromol/kg, and 0 of 6 at 10 micromol/kg), but unlike W-7, H-8 did so by shortening the QT interval.nnnCONCLUSIONSnThese findings show that the acute systemic application of W-7 and H-8 is hemodynamically tolerated and indicate that kinase inhibition may be a viable antiarrhythmic strategy.

Frequency of late recurrence of intra-atrial reentry tachycardia after radiofrequency catheter ablation in patients with congenital heart disease

Forty-seven catheter ablation procedures for intra-atrial reentry tachycardia were performed in 40 patients with palliated congenital heart disease. The acute success rate was 87% and the recurrence rate was 34% during an average follow-up of 36 months. Of those patients who had recurrence, 88% did so within 1 year of ablation. Of the 23 patients who were free of recurrence 1 year after ablation, 21 (91%) remain free from recurrence at an average of 45 months (median 39; range 15 to 88) after ablation.

Pause-dependent polymorphic ventricular tachycardia during long-term treatment with dofetilide: a placebo-controlled, implantable cardioverter-defibrillator-based evaluation.

OBJECTIVESnTo compare the incidence of pause-dependent polymorphic ventricular tachycardia (PVT) in patients with implantable cardioverter-defibrillators (ICDs) randomly assigned to the QT-prolonging antiarrhythmic dofetilide or placebo.nnnBACKGROUNDnDrug-related torsade de pointes (TdP) is usually recognized within days of initiating therapy, but its incidence during long-term therapy is unknown.nnnMETHODSnWe assessed the frequency of TdP and ICD electrograms compatible with TdP in a multicenter study that randomized ICD patients to placebo (n = 87) or dofetilide (n = 87). As reported elsewhere, the number of patients with a primary trial end point (ICD intervention for VT or ventricular fibrillation) was similar in the two groups. For this analysis, a qualifying event was TdP (on electrocardiogram) or an intracardiac electrogram showing pause-dependent PVT.nnnRESULTSnA total of 620 electrograms obtained in 131 patients were analyzed blindly by prospectively defined criteria for episodes of pause-dependent polymorphic VT. These were identified in 15/87 (17%) patients receiving dofetilide and 5/87 (6%) patients on placebo (p < 0.05). Five of these episodes were early (<3 days), all of which were TdP on dofetilide. There were 15 late events, 10 on dofetilide and five on placebo (p = 0.29). The median time to a late event was 22 days (range 6 to 107 days) for dofetilide and 99 days (range 34 to 207 days) for placebo.nnnCONCLUSIONSnPause-dependent PVT was more common among patients receiving dofetilide, although total VT incidence was similar in the two groups. These data suggest that in ICD patients either long-term dofetilide therapy is associated with an increased risk of TdP or the drug alters VT morphology.

Progress toward the prevention and treatment of atrial fibrillation: A summary of the Heart Rhythm Society Research Forum on the Treatment and Prevention of Atrial Fibrillation, Washington, DC, December 9-10, 2013

The Heart Rhythm Society convened a research symposium on December 9–10, 2013, in Washington, DC, that focused on the prevention of atrial fibrillation (AF) as well as AF-related stroke and morbidity. Attendees sought to summarize advances in understanding AF since a 2008 National Institutes of Health (NIH) conference on this topic1 and to identify continued knowledge gaps and current research priorities. The research symposium also sought to identify key deficiencies and opportunities in research infrastructure, operations, and methodologies. The committee sought to identify both basic research targets and how clinical AF research could be improved in the current health care environment. This whitepaper summarizes our deliberations in an effort to accelerate progress toward preventing AF and its consequences. Although largely focused on primary prevention of AF, the paper also addresses some aspects of secondary prevention of recurrent AF due to the continuum of risk factors that contribute to arrhythmogenesis, permissive left atrial (LA) substrates, and the emergence of AF.

Calmodulin kinase is functionally targeted to the action potential plateau for regulation of L‐type Ca2+ current in rabbit cardiomyocytes

L‐type Ca2+ current (ICa−L) triggers Ca2+ release from the sarcoplasmic reticulum (SR) and both SR and ICa−L are potential sources of intracellular Ca2+ (Ca2+i) for feedback regulation of ICa−L. Ca2+i bound to calmodulin (Ca2+–CaM) can inhibit ICa−L, while Ca2+–CaM can also activate Ca2+–CaM‐dependent protein kinase II (CaMK) to increase ICa. However, it is not known whether ICa−L or the SR is the primary source of Ca2+ for ICa−L regulation. The L‐type Ca2+ channel C terminus is implicated as a critical transduction element for ICa−L responses to Ca2+–CaM and CaMK, and the C terminus undergoes voltage‐dependent steric changes, suggesting that Ca2+i control of ICa−L may also be regulated by cell membrane potential. We developed conditions to separately test the relationship of Ca2+–CaM and CaMK to ICa−L and SR Ca2+i release during voltage clamp conditions modelled upon time and voltage domains relevant to the cardiac action potential. Here we show that CaMK increases ICa−L after brief positive conditioning pulses, whereas Ca2+–CaM reduces ICa−L over a broad range of positive and negative conditioning potentials. SR Ca2+ release was required for both Ca2+–CaM and CaMK ICa−L responses after strongly positive conditioning pulses (+10 and +40 mV), while Ca2+i from ICa−L was sufficient for Ca2+–CaM during weaker depolarizations. These findings show that ICa−L responses to CaMK are voltage dependent and suggest a new model of L‐type Ca2+ channel regulation where voltage‐dependent changes control ICa−L responses to Ca2+–CaM and CaMK signalling.

Oxidant stress promotes disease by activating CaMKII

CaMKII is activated by oxidation of methionine residues residing in the regulatory domain. Oxidized CaMKII (ox-CaMKII) is now thought to participate in cardiovascular and pulmonary diseases and cancer. This invited review summarizes current evidence for the role of ox-CaMKII in disease, considers critical knowledge gaps and suggests new areas for inquiry.

Calmodulin and the philosopher's stone: Changing Ca2+ into arrhythmias.

Calmodulin and the Philosophers Stone. It has been recognized for some time that intracellular Ca2+ ([Ca2+]i) can contribute to the genesis of cardiac arrhythmias, but understanding of the molecular signaling machinery that links disordered [Ca2+]i to arrhythmias has been lacking. Exciting recent work has focused on the ubiquitous intracellular Ca2+–binding protein calmodulin. Calmodulin is a molecular sensor that can translate increases in [Ca2+]I into modulatory signals for ion channels and activate other Ca2+–dependent signaling molecules. This article will examine the implications of these recent findings for arrhythmogenesis and the development of new antiarrhythmic therapies.

Functional Similarity Between Electrograms Recorded from an Implantable Cardioverter Defibrillator Emulator and the Surface Electrocardiogram

MAZUR, A., et al.: Functional Similarity Between Electrograms Recorded from an Implantable Cardioverter Defibrillator Emulator and the Surface Electrocardiogram. Clinical use of stored electrogram (EGM) configurations currently used in ICDs is limited. The hypothesis that EGMs recorded from electrodes on the ICD surface may improve diagnostic capabilities of the device was tested in the present study. The Buttons on Active Can Emulator (BACE), an ICD‐sized device containing four button electrodes, was temporarily placed into a subcutaneous or submuscular left pectoral pocket in 16 patients during ICD implantation. Simultaneous recordings were obtained from the ECG lead II, bipolar EGMs using BACE electrodes, and a bipolar atrial EGM during sinus rhythm (SR), ventricular pacing (VP) at cycle lengths of 500 and 400 ms, and VT. Visible P waves were present in all patients during SR (n = 15), in 5 (33%) of 15 patients during VP, and none of the patients during VT (n = 4) using BACE EGMs and lead II. P and QRS amplitudes and the P:QRS ratio during SR in BACE EGMs were significantly lower than those in lead II. BACE EGMs showed prominent changes in QRS morphology and duration during VP and VT compared to SR, and the magnitude of QRS prolongation during VP was similar to that in lead II. Measurements of PR, QRS, and QT duration during SR showed good agreement between BACE EGMs and lead II. In conclusion, EGMs recorded from electrodes embedded on the ICD housing may potentially improve visual discrimination between supraventricular and ventricular arrhythmias. They also may be useful as a surrogate of the ECG for analysis and monitoring of different components of P‐QRS‐T complex.

Molecular and cellular neurocardiology: development, and cellular and molecular adaptations to heart disease

The nervous system and cardiovascular system develop in concert and are functionally interconnected in both health and disease. This white paper focuses on the cellular and molecular mechanisms that underlie neural–cardiac interactions during development, during normal physiological function in the mature system, and during pathological remodelling in cardiovascular disease. The content on each subject was contributed by experts, and we hope that this will provide a useful resource for newcomers to neurocardiology as well as aficionados.