The Small Conductance Calcium-Activated Potassium Channel Inhibitors NS8593 and UCL1684 Prevent the Development of Atrial Fibrillation Through Atrial-Selective Inhibition of Sodium Channel Activity.

The mechanisms underlying atrial-selective prolongation of effective refractory period (ERP) and suppression of atrial fibrillation (AF) by NS8593 and UCL1684, small conductance calcium-activated potassium (SK) channel blockers, are poorly defined. The purpose of the study was to confirm the effectiveness of these agents to suppress AF and to probe the underlying mechanisms. Transmembrane action potentials and pseudoelectrocardiograms were recorded from canine isolated coronary-perfused canine atrial and ventricular wedge preparations. Patch clamp techniques were used to record sodium channel current (INa) in atrial and ventricular myocytes and human embryonic kidney cells. In both atria and ventricles, NS8593 (3-10 µM) and UCL1684 (0.5 µM) did not significantly alter action potential duration, suggesting little to no SK channel inhibition. Both agents caused atrial-selective: (1) prolongation of ERP secondary to development of postrepolarization refractoriness, (2) reduction of Vmax, and (3) increase of diastolic threshold of excitation (all are sodium-mediated parameters). NS8593 and UCL1684 significantly reduced INa density in human embryonic kidney cells as well as in atrial but not in ventricular myocytes at physiologically relevant holding potentials. NS8593 caused a shift of steady-state inactivation to negative potentials in atrial but not ventricular cells. NS8593 and UCL1684 prevented induction of acetylcholine-mediated AF in 6/6 and 8/8 preparations, respectively. This anti-AF effect was associated with strong rate-dependent depression of excitability. The SK channel blockers, NS8593 and UCL1684, are effective in preventing the development of AF due to potent atrial-selective inhibition of INa, causing atrial-selective prolongation of ERP secondary to induction of postrepolarization refractoriness.

Results of a curtailed randomized controlled trial, evaluating the efficacy and safety of azimilide in patients with implantable cardioverter-defibrillators: The SHIELD-2 trial.

BACKGROUND: Frequent hospital attendances in patients with implantable cardioverter-defibrillators (ICDs) result in significant morbidity and health care costs. Current drugs to reduce ICD shocks and hospital visits have limited efficacy and considerable toxicity. We evaluated the efficacy and safety of azimilide, a novel oral class III antiarrhythmic, for use in ICD patients. METHODS: A total of 240 patients were enrolled in a prospective, randomized, double-blind, placebo-controlled trial to evaluate the effect of oral azimilide 75 mg daily in ICD patients with previously documented ventricular tachycardia or ventricular fibrillation, and a left ventricular ejection fraction ≤40%. The primary outcome metric was the adjudicated time-to-first unplanned cardiovascular (CV) hospitalization, or CV emergency department (ED) visit, or CV death. The trial was prematurely discontinued due to withdrawal of study sponsorship. RESULTS: Azimilide demonstrated numerical but statistically nonsignificant reductions in the primary composite outcome (odds ratio [OR] 0.79, 95% CI 0.44-1.44), unplanned CV hospitalizations (OR 0.75, 95% CI 0.41-1.38), ED visits (OR 0.68, 95% CI 0.35-1.31), and all-cause shocks (OR 0.58, 95% CI 0.32-1.05). The incidence of adverse events was lower in the azimilide group. Neutropenia was not observed (absolute neutrophil count <1000 µ/L), and there was one possible torsade de pointes case that led to a successful ICD discharge. CONCLUSION: The SHIELD-2 trial was statistically underpowered due to early trial termination and did not meet its primary objective. Despite this limitation, azimilide showed promise as a safe and effective drug in reducing all-cause shocks, unplanned hospitalizations, and ED visits in ICD patients.

Perspective on Antiarrhythmic Drug Combinations.

Physicians use multiple drugs in combination to treat hypertension, heart failure, diabetes mellitus, angina, hyperlipidemia, and many other cardiovascular conditions and risk factors. However, administering antiarrhythmic drugs (AAD) in combination is rarely discussed. Yet, the possibility of increasing efficacy and/or tolerance and/or safety of AADs (by adding mechanisms, offsetting adverse mechanisms, and/or using lower doses) exists. Unfortunately, this topic has not been reviewed in any contemporary cardiac literature of which we are aware, although information regarding AAD combinations has been published. In conclusion, and accordingly, this review discusses the possibility of using AAD combinations for both ventricular arrhythmias and atrial fibrillation, in which the rationale for such combinations, considerations for such combinations, and supporting literature are covered.

Mechanisms underlying the antiarrhythmic effect of ARumenamide-787 in experimental models of the J wave syndromes and hypothermia.

BACKGROUND: Brugada (BrS) and early repolarization syndromes (ERS), the so-called J wave syndromes (JWS), are associated with life-threatening ventricular arrhythmias. Pharmacologic approaches to therapy are currently limited. In this study, we examine the effects of ARumenamide-787 (AR-787) to suppress the electrocardiographic and arrhythmic manifestations of JWS and hypothermia. METHODS: We studied the effects of AR-787 on INa and IKr in HEK-293 cells stably expressing the α- and ß1-subunits of the cardiac (NaV1.5) sodium channel and hERG channel, respectively. In addition, we studied its effect on Ito, INa and ICa in dissociated canine ventricular myocytes along with action potentials and ECG from coronary-perfused right (RV) and left (LV) ventricular wedge preparations. The Ito agonist, NS5806 (5-10 µM), ICa blocker, verapamil (2.5 µM), and INa blocker, ajmaline (2.5 µM), were used to mimic the genetic defects associated with JWS and to induce the electrocardiographic and arrhythmic manifestations of JWS (prominent J waves/ST segment elevation, phase 2 reentry and polymorphic VT/VF) in canine ventricular wedge preparations. RESULTS: AR-787 (1, 10 and 50 µM) exerted pleiotropic effects on cardiac ion channels. The predominant effect was inhibition of the transient outward current (Ito) and enhancement of the sodium channel current (INa), with lesser effects to inhibit IKr and augment calcium channel current (ICa). AR-787 diminished the electrocardiographic J wave and prevented and/or suppressed all arrhythmic activity in canine RV and LV experimental models of BrS, ERS and hypothermia. CONCLUSIONS: Our findings point to AR-787 as promising candidate for the pharmacologic treatment of JWS and hypothermia.

A carvedilol analogue, VK-II-86, prevents hypokalaemia-induced ventricular arrhythmia through novel multi-channel effects.

BACKGROUND AND PURPOSE: QT prolongation and intracellular Ca2+ loading with diastolic Ca2+ release via ryanodine receptors (RyR2) are the predominant mechanisms underlying hypokalaemia-induced ventricular arrhythmia. We investigated the antiarrhythmic actions of two RyR2 inhibitors: dantrolene and VK-II-86, a carvedilol analogue lacking antagonist activity at ß-adrenoceptors, in hypokalaemia. EXPERIMENTAL APPROACH: Surface ECG and ventricular action potentials (APs) were recorded from whole-heart murine Langendorff preparations. Ventricular arrhythmia incidence was compared in hearts perfused with low [K+ ], and those pretreated with dantrolene or VK-II-86. Whole-cell patch clamping was used in murine and canine ventricular cardiomyocytes to study effects of dantrolene and VK-II-86 on AP parameters in low [K+ ] and effects of VK-II-86 on the inward rectifier current (IK1 ), late sodium current (INa_L ) and the L-type Ca2+ current (ICa ). Effects of VK-II-86 on IKr were investigated in transfected HEK-293 cells. A fluorogenic probe quantified the effects of VK-II-86 on oxidative stress in hypokalaemia. KEY RESULTS: Dantrolene reduced the incidence of ventricular arrhythmias induced by low [K+ ] in explanted murine hearts by 94%, whereas VK-II-86 prevented all arrhythmias. VK-II-86 prevented hypokalaemia-induced AP prolongation and depolarization but did not alter AP parameters in normokalaemia. Hypokalaemia was associated with decreased IK1 and IKr , and increased INa-L , and ICa . VK-II-86 prevented all hypokalaemia-induced changes in ion channel activity and oxidative stress. CONCLUSIONS AND IMPLICATIONS: VK-II-86 prevents hypokalaemia-induced arrhythmogenesis by normalizing calcium homeostasis and repolarization reserve. VK-II-86 may provide an effective treatment in hypokalaemia and other arrhythmias caused by delayed repolarization or Ca2+ overload.

Acacetin suppresses the electrocardiographic and arrhythmic manifestations of the J wave syndromes.

BACKGROUND: J wave syndromes (JWS), including Brugada (BrS) and early repolarization syndromes (ERS), are associated with increased risk for life-threatening ventricular arrhythmias. Pharmacologic approaches to therapy are currently very limited. Here, we evaluate the effects of the natural flavone acacetin. METHODS: The effects of acacetin on action potential (AP) morphology and transient outward current (Ito) were first studied in isolated canine RV epicardial myocytes using whole-cell patch clamp techniques. Acacetin's effects on transmembrane APs, unipolar electrograms and transmural ECGs were then studied in isolated coronary-perfused canine RV and LV wedge preparations as well as in whole-heart, Langendorff-perfused preparations from which we recorded a 12 lead ECG and unipolar electrograms. Using floating glass microelectrodes we also recorded transmembrane APs from the RVOT of the whole-heart model. The Ito agonist NS5806, sodium channel blocker ajmaline, calcium channel blocker verapamil or hypothermia (32°C) were used to pharmacologically mimic the genetic defects and conditions associated with JWS, thus eliciting prominent J waves and provoking VT/VF. RESULTS: Acacetin (5-10 µM) reduced Ito density, AP notch and J wave area and totally suppressed the electrocardiographic and arrhythmic manifestation of both BrS and ERS, regardless of the experimental model used. In wedge and whole-heart models of JWS, increasing Ito with NS5806, decreasing INa or ICa (with ajmaline or verapamil) or hypothermia all resulted in accentuation of epicardial AP notch and ECG J waves, resulting in characteristic BrS and ERS phenotypes. Phase 2-reentrant extrasystoles originating from the RVOT triggered VT/VF. The J waves in leads V1 and V2 were never associated with a delay of RVOT activation and always coincided with the appearance of the AP notch recorded from RVOT epicardium. All repolarization defects giving rise to VT/VF in the BrS and ERS models were reversed by acacetin, resulting in total suppression of VT/VF. CONCLUSIONS: We present experimental models of BrS and ERS capable of recapitulating all of the ECG and arrhythmic manifestations of the JWS. Our findings provide definitive support for the repolarization but not the depolarization hypothesis proposed to underlie BrS and point to acacetin as a promising new pharmacologic treatment for JWS.

Mexiletine Prevents Recurrent Torsades de Pointes in Acquired Long QT Syndrome Refractory to Conventional Measures.

OBJECTIVES: The purpose of this study was to examine the role of mexiletine, a late sodium current (INa-L) blocker, in acute termination of torsades de pointes (TdP) refractory to conventional therapy in acquired long QT syndromes (LQTS). BACKGROUND: Long QT interval can predispose to TdP and is therefore associated with significant mortality. Currently, there is no available pharmacotherapy to target directly the ionic basis of most LQTS for the acute termination of TdP. Earlier evidence highlighted the role of INa-L in the pathophysiology of long QT and TdP, particularly in patients with congenital LQTS. METHODS: Twelve patients with TdP caused by acquired LQTS were treated with mexiletine after failure of conventional treatment including discontinuation of QT-prolonging drugs, intravenous administration of magnesium, and correction of serum electrolyte abnormalities. RESULTS: No recurrence of TdP occurred within 2 h after initiation of treatment with mexiletine in all 12 patients. Macro T-wave alternans accompanied by QT prolongation, an electrocardiographic precursor of TdP that was seen in 3 patients, was also abolished by mexiletine. Treatment with mexiletine shortened the QTc interval from 599 ± 27 ms to 514 ± 16 ms (p = 0.001). The interval from the peak to the end of the T-wave (Tp-e interval) decreased from 145 ± 18 ms to 106 ± 9 ms (p = 0.005). The Tp-e/QT ratio decreased from 0.27 ± 0.02 to 0.23 ± 0.018 (p = 0.01). Mexiletine had no significant effect on QRS complex duration. CONCLUSIONS: INa-L blockade with mexiletine may be an effective treatment approach to terminate refractory TdP from several acquired causes of LQTS.

Heterogeneous distribution of INa-L determines interregional differences in rate adaptation of repolarization.

BACKGROUND: The QT interval, an index of ventricular repolarization, is rate dependent. Exaggerated rate dependence of repolarization is arrhythmogenic. Whether there are differences in rate-dependent repolarization among different heart chambers is unknown. OBJECTIVE: The purpose of this study was to test our hypothesis that heterogeneous distribution in late sodium current (INa-L) is responsible for interregional differences in rate adaptation of repolarization. METHODS: Action potential duration (APD), QT intervals, and their rate adaptation were studied in isolated arterially perfused ventricular and atrial wedge preparations. APD and INa-L were recorded in isolated rabbit myocytes using the microelectrode and the whole-cell patch clamp technique, respectively. RESULTS: There were distinct interregional differences in repolarization and its rate adaptations among different cardiac chambers: the left ventricle exhibited the longest QT and APD with the greatest rate dependence, followed by the right ventricle, whereas the atria had the shortest APD without significant rate dependence. Interestingly, INa-L densities distributed heterogeneously and matched to interregional APDs and rate dependences as follows: left ventricle > right ventricle > left/right atria. Both dofetilide (an IKr blocker) and anemone toxin (a specific INa-L enhancer) amplified QT/APD rate adaptation preferentially in the left ventricle and therefore exacerbated interregional dispersion of repolarization. On the other hand, the atria exhibited blunted responses to both QT-prolonging agents. Mexiletine with an INa-L blockade effect blunted QT/APD rate adaptation preferentially in the ventricles and significantly reduced bradycardia-dependent interregional dispersion of repolarization. CONCLUSION: Our results demonstrate that heterogeneous distribution of INa-L contributes importantly to the interregional heterogeneity of repolarization in response to rate changes and APD/QT-prolonging agents.

Should Physicians Continue to Recommend Fish Oil for Patients with Atrial Fibrillation?

Many physicians recommend the use of fish oil or omega-3 polyunsaturated fatty acids (n-3 PUFA) in atrial fibrillation (AF) patients. N-3 PUFA have demonstrated anti-fibrillatory properties in several animal studies, however, data regarding their efficacy in preventing AF in humans have been mixed. This article critically reviews studies that have investigated the use of n-3 PUFA for the secondary prevention of paroxysmal and persistent AF and the primary prevention of post-operative AF. We conclude that n-3 PUFA should no longer be recommended for use in any of these AF subtypes until more data are available.