The calcium/calmodulin/kinase system and arrhythmogenic afterdepolarizations in bradycardia-related acquired long-QT syndrome

BACKGROUND

Sustained bradycardia is associated with long-QT syndrome in human beings and causes spontaneous torsades de pointes in rabbits with chronic atrioventricular block (CAVB), at least partly by downregulating delayed-rectifier K(+)-current to cause action potential (AP) prolongation. We addressed the importance of altered Ca(2+) handling, studying underlying mechanisms and consequences.

METHODS AND RESULTS

We measured ventricular cardiomyocyte [Ca(2+)](i) (Indo1-AM), L-type Ca(2+)-current (I(CaL)) and APs (whole-cell perforated-patch), and Ca(2+)-handling protein expression (immunoblot). CAVB increased AP duration, cell shortening, systolic [Ca(2+)](i) transients, and caffeine-induced [Ca(2+)](i) release, and CAVB cells showed spontaneous early afterdepolarizations (EADs). I(CaL) density was unaffected by CAVB, but inactivation was shifted to more positive voltages, increasing the activation-inactivation overlap zone for I(CaL) window current. Ca(2+)-calmodulin-dependent protein kinase-II (CaMKII) autophosphorylation was enhanced in CAVB, indicating CaMKII activation. CAVB also enhanced CaMKII-dependent phospholamban-phosphorylation and accelerated [Ca(2+)](i)-transient decay, consistent with phosphorylation-induced reductions in phospholamban inhibition of sarcoplasmic reticulum (SR) Ca(2+)-ATPase as a contributor to enhanced SR Ca(2+) loading. The CaMKII-inhibitor KN93 reversed CAVB-induced changes in caffeine-releasable [Ca(2+)](i) and I(CaL) inactivation voltage and suppressed CAVB-induced EADs. Similarly, the calmodulin inhibitor W7 suppressed CAVB-induced I(CaL) inactivation voltage shifts and EADs, and a specific CaMKII inhibitory peptide prevented I(CaL) inactivation voltage shifts. The SR Ca(2+)-uptake inhibitor thapsigargin and the SR Ca(2+) release inhibitor ryanodine also suppressed CAVB-induced EADs, consistent with an important role for SR Ca(2+) loading and release in arrhythmogenesis. AP-duration changes reached a maximum after 1 week of bradypacing, but peak alterations in CaMKII and [Ca(2+)](i) required 2 weeks, paralleling the EAD time course.

CONCLUSIONS

CAVB-induced remodeling enhances [Ca(2+)](i) load and activates the Ca(2+)-calmodulin-CaMKII system, producing [Ca(2+)](i)-handling abnormalities that contribute importantly to CAVB-induced arrhythmogenic afterdepolarizations.

Ion channel subunit expression changes in cardiac Purkinje fibers: a potential role in conduction abnormalities associated with congestive heart failure

Purkinje fibers (PFs) play key roles in cardiac conduction and arrhythmogenesis. Congestive heart failure (CHF) causes well-characterized atrial and ventricular ion channel subunit expression changes, but effects on PF ion channel subunits are unknown. This study assessed changes in PF ion channel subunit expression (real-time PCR, immunoblot, immunohistochemistry), action potential properties, and conduction in dogs with ventricular tachypacing-induced CHF. CHF downregulated mRNA expression of subunits involved in action potential propagation (Nav1.5, by 56%; connexin [Cx]40, 66%; Cx43, 56%) and repolarization (Kv4.3, 43%, Kv3.4, 46%). No significant changes occurred in KChIP2, KvLQT1, ERG, or Kir3.1/3.4 mRNA. At the protein level, downregulation was seen for Nav1.5 (by 38%), Kv4.3 (42%), Kv3.4 (57%), Kir2.1 (26%), Cx40 (53%), and Cx43 (30%). Cx43 dephosphorylation was indicated by decreased larger molecular mass bands (pan-Cx43 antibody) and a 57% decrease in Ser368-phosphorylated Cx43 (phospho-specific antibody). Immunohistochemistry revealed reduced Cx40, Cx43, and phospho-Cx43 expression at intercalated disks. Action potential changes were consistent with observed decreases in ion channel subunits: CHF decreased phase 1 slope (by 56%), overshoot (by 32%), and phase 0 dV/dt(max) (by 35%). Impulse propagation was slowed in PF false tendons: conduction velocity decreased significantly from 2.2+/-0.1 m/s (control) to 1.5+/-0.1 m/s (CHF). His-Purkinje conduction also slowed in vivo, with HV interval increasing from 35.5+/-1.2 (control) to 49.3+/-3.4 ms (CHF). These results indicate important effects of CHF on PF ion channel subunit expression. Alterations in subunits governing conduction properties may be particularly important, because CHF-induced impairments in Purkinje tissue conduction, which this study is the first to describe, could contribute significantly to dyssynchronous ventricular activation, a major determinant of prognosis in CHF-patients.

Funny Current Downregulation and Sinus Node Dysfunction Associated With Atrial Tachyarrhythmia

Background— Sinoatrial node (SAN) dysfunction is frequently associated with atrial tachyarrhythmias (ATs). Abnormalities in SAN pacemaker function after termination of ATs can cause syncope and require pacemaker implantation, but underlying mechanisms remain poorly understood. This study examined the hypothesis that ATs impair SAN function by altering ion channel expression.

Methods and Results— SAN tissues were obtained from 28 control dogs and 31 dogs with 7-day atrial tachypacing (400 bpm). Ionic currents were measured from single SAN cells with whole-cell patch-clamp techniques. Atrial tachypacing increased SAN recovery time in vivo by ≈70% ( P <0.01), a change which reflects impaired SAN function. In dogs that underwent atrial tachypacing, SAN mRNA expression (real-time reverse-transcription polymerase chain reaction) was reduced for hyperpolarization-activated cyclic nucleotide-gated subunits (HCN2 and HCN4) by >50% ( P <0.01) and for the β-subunit minK by ≈42% ( P <0.05). SAN transcript expression for the rapid delayed-rectifier ( I Kr ) α-subunit ERG, the slow delayed-rectifier ( I Ks ) α-subunit KvLQT1, the β-subunit MiRP1, the L-type ( I CaL ) and T-type ( I CaT ) Ca 2+ -current subunits Cav1.2 and Cav3.1, and the gap-junction subunit connexin 43 (were unaffected by atrial tachypacing. Atrial tachypacing reduced densities of the HCN-related funny current ( I f ) and I Ks by ≈48% ( P <0.001) and ≈34% ( P <0.01), respectively, with no change in voltage dependence or kinetics. I Kr , I CaL , and I CaT were unaffected. SAN cells lacked Ba 2+ -sensitive inward-rectifier currents, irrespective of AT. SAN action potential simulations that incorporated AT-induced alterations in I f accounted for slowing of periodicity, with no additional contribution from changes in I Ks .

Conclusions— AT downregulates SAN HCN2/4 and minK subunit expression, along with the corresponding currents I f and I Ks . Tachycardia-induced remodeling of SAN ion channel expression, particularly for the “pacemaker” subunit I f , may contribute to the clinically significant association between SAN dysfunction and supraventricular tachyarrhythmias.

Prevention of atrial fibrillation: report from a national heart, lung, and blood institute workshop

The National Heart, Lung, and Blood Institute convened an expert panel April 28 to 29, 2008, to identify gaps and recommend research strategies to prevent atrial fibrillation (AF). The panel reviewed the existing basic scientific, epidemiological, and clinical literature about AF and identified opportunities to advance AF prevention research. After discussion, the panel proposed the following recommendations: (1) enhance understanding of the epidemiology of AF in the population by systematically and longitudinally investigating symptomatic and asymptomatic AF in cohort studies; (2) improve detection of AF by evaluating the ability of existing and emerging methods and technologies to detect AF; (3) improve noninvasive modalities for identifying key components of cardiovascular remodeling that promote AF, including genetic, fibrotic, autonomic, structural, and electrical remodeling markers; (4) develop additional animal models reflective of the pathophysiology of human AF; (5) conduct secondary analyses of already-completed clinical trials to enhance knowledge of potentially effective methods to prevent AF and routinely include AF as an outcome in ongoing and future cardiovascular studies; and (6) conduct clinical studies focused on secondary prevention of AF recurrence, which would inform future primary prevention investigations.

Transcriptional profiling of ion channel genes in Brugada syndrome and other right ventricular arrhythmogenic diseases

AIMS

Brugada syndrome is an inherited sudden-death arrhythmia syndrome. Na(+)-current dysfunction is central, but mutations in the SCN5A gene (encoding the cardiac Na(+)-channel Nav1.5) are present in only 20% of probands. This study addressed the possibility that Brugada patients display specific expression patterns for ion-channels regulating cardiac conduction, excitability, and repolarization.

METHODS AND RESULTS

Transcriptional profiling was performed on right-ventricular endomyocardial biopsies from 10 unrelated Brugada probands, 11 non-diseased organ-donors, seven heart-transplant recipients, 10 with arrhythmogenic right-ventricular cardiomyopathy, and nine with idiopathic right-ventricular outflow-tract tachycardia. Brugada patients showed distinct clustering differences vs. the two control and two other ventricular-tachyarrhythmia groups, including 14 of 77 genes encoding important ion-channel/ion-transporter subunits. Nav1.5 and K(+)-channels Kv4.3 and Kir3.4 were more weakly expressed, whereas the Na(+)-channel Nav2.1 and the K(+)-channel TWIK1 were more strongly expressed, in Brugada syndrome. Differences were also seen in Ca(2+)-homeostasis transcripts, including stronger expression of RYR2 and NCX1. The molecular profile of Brugada patients with SCN5A mutations did not differ from Brugada patients without SCN5A mutations.

CONCLUSION

Brugada patients exhibit a common ion-channel molecular expression signature, irrespective of the culprit gene. This finding has potentially important implications for our understanding of the pathophysiology of Brugada syndrome, with possible therapeutic and diagnostic consequences.

Absence of protective effect of renin-angiotensin system inhibitors on atrial fibrillation development: insights from the Canadian Trial of Atrial Fibrillation (CTAF)

BACKGROUND

Antiarrhythmic agents have modest efficacy in preventing atrial fibrillation (AF) recurrence. Although retrospective analyses have suggested a preventive effect of inhibitors of the renin-angiotensin system (RAS) on AF development in patients with congestive heart failure or hypertension, the value of these agents has not been evaluated in patients with AF but without a high prevalence of hypertension or heart failure.

METHODS AND RESULTS

A retrospective analysis of the Canadian Trial of Atrial Fibrillation (CTAF) was conducted. CTAF demonstrated the superiority of amiodarone (A) over sotalol or propafenone (SP) in maintaining sinus rhythm in patients with AF. Of the 403 patients randomly assigned in CTAF, 11.7% of the A group and 12.7% of the SP group were receiving a RAS inhibitor at baseline. By multivariate analysis (including all the risk factors known to be associated with AF available in the database), the use of RAS blockers in addition to antiarrhythmic agents was not associated with additional benefit against AF development. There was a recurrence of AF in 59 patients (38.3%) and 14 patients (29.8%) of groups A and A-RAS, respectively, while 93 patients (61.6%) and 32 patients (62.8%) of the SP and SP-RAS groups, respectively, experienced recurrent AF.

CONCLUSION

Blocking the RAS did not provide additional benefit against AF recurrence in CTAF patients treated with an antiarrhythmic drug. These results underscore the need for randomized clinical trials to clearly define the role of RAS inhibitors in treating AF.

Remodelling of cardiac repolarization: how homeostatic responses can lead to arrhythmogenesis

Cardiac action potentials (APs) are driven by ionic currents flowing through specific channels and exchangers across cardiomyocyte membranes. Once initiated by rapid Na(+) entry during phase 0, the AP time course is determined by the balance between inward depolarizing currents, carried mainly by Na(+) and Ca(2+), and outward repolarizing currents carried mainly by K(+). K(+) currents play a major role in repolarization. The loss of a K(+) current can impair repolarization, but there is a redundancy of K(+) currents so that when one K(+) current is dysfunctional, other K(+) currents increase to compensate, a phenomenon called 'repolarization reserve'. Repolarization reserve protects repolarization under conditions that increase inward current or reduce outward current, threatening the balance that governs AP duration. This protection comes at the expense of reduced repolarization reserve, potentially resulting in unexpectedly large AP prolongation and arrhythmogenesis, when an additional repolarization-suppressing intervention is superimposed. The critical role of appropriate repolarization is such that cardiac rhythm stability can be impaired with either abnormally slow or excessively rapid repolarization. In cardiac disease states such as heart failure and atrial fibrillation (AF), changes in ion channel properties appear as part of an adaptive response to maintain function in the face of disease-related stress on the cardiovascular system. However, if the stress is maintained the adaptive ion channel changes may themselves lead to dysfunction, in particular cardiac arrhythmias. The present article reviews ionic remodelling of cardiac repolarization, and focuses on how potentially adaptive repolarization changes with congestive heart failure and AF can have arrhythmogenic consequences.

Differential interactions of Na+ channel toxins with T-type Ca2+ channels

Two types of voltage-dependent Ca²⁺ channels have been identified in heart: high (I_CaL) and low (I_CaT) voltage-activated Ca²⁺ channels. In guinea pig ventricular myocytes, low voltage–activated inward current consists of I_CaT and a tetrodotoxin (TTX)-sensitive I_Ca component (I_Ca(TTX)). In this study, we reexamined the nature of low-threshold I_Ca in dog atrium, as well as whether it is affected by Na⁺ channel toxins. Ca²⁺ currents were recorded using the whole-cell patch clamp technique. In the absence of external Na⁺, a transient inward current activated near −50 mV, peaked at −30 mV, and reversed around +40 mV (HP = −90 mV). It was unaffected by 30 μM TTX or micromolar concentrations of external Na⁺, but was inhibited by 50 μM Ni²⁺ (by ∼90%) or 5 μM mibefradil (by ∼50%), consistent with the reported properties of I_CaT. Addition of 30 μM TTX in the presence of Ni²⁺ increased the current approximately fourfold (41% of control), and shifted the dose–response curve of Ni²⁺ block to the right (IC₅₀ from 7.6 to 30 μM). Saxitoxin (STX) at 1 μM abolished the current left in 50 μM Ni²⁺. In the absence of Ni²⁺, STX potently blocked I_CaT (EC₅₀ = 185 nM) and modestly reduced I_CaL (EC₅₀ = 1.6 μM). While TTX produced no direct effect on I_CaT elicited by expression of hCa_V3.1 and hCa_V3.2 in HEK-293 cells, it significantly attenuated the block of this current by Ni²⁺ (IC₅₀ increased to 550 μM Ni²⁺ for Ca_V3.1 and 15 μM Ni²⁺ for Ca_V3.2); in contrast, 30 μM TTX directly inhibited hCa_V3.3-induced I_CaT and the addition of 750 μM Ni²⁺ to the TTX-containing medium led to greater block of the current that was not significantly different than that produced by Ni²⁺ alone. 1 μM STX directly inhibited Ca_V3.1-, Ca_V3.2-, and Ca_V3.3-mediated I_CaT but did not enhance the ability of Ni²⁺ to block these currents. These findings provide important new implications for our understanding of structure–function relationships of I_CaT in heart, and further extend the hypothesis of a parallel evolution of Na⁺ and Ca²⁺ channels from an ancestor with common structural motifs.

Feedback Remodeling of Cardiac Potassium Current Expression

Background— Inhibition of individual K + currents causes functionally based compensatory increases in other K + currents that minimize changes in action potential duration, a phenomenon known as repolarization reserve. The possibility that sustained K + channel inhibition may induce remodeling of ion current expression has not been tested. Accordingly, we assessed the effects of sustained inhibition of one K + current on various other cardiac ionic currents.

Methods and Results— Adult canine left ventricular cardiomyocytes were incubated in primary culture and paced at a physiological rate (1 Hz) for 24 hours in the presence or absence of the highly selective rapid delayed-rectifier K + current (I Kr ) blocker dofetilide (5 nmol/L). Sustained dofetilide exposure led to shortened action potential duration and increased repolarization reserve (manifested as a reduced action potential duration–prolonging response to I Kr blockade). These repolarization changes were accompanied by increased slow delayed-rectifier (I Ks ) density, whereas I Kr , transient-outward (I to ), inward-rectifier (I K1 ), L-type Ca 2+ (I CaL ), and late Na + current remained unchanged. The mRNA expression corresponding to KvLQT1 and minK (real-time polymerase chain reaction) was unchanged, but their protein expression (Western blot) was increased, suggesting posttranscriptional regulation. To analyze possible mechanisms, we quantified the muscle-specific microRNA subtypes miR -133a and miR -133b, which can posttranscriptionally regulate and repress KvLQT1 protein expression without affecting mRNA expression. The expression levels of miR -133a and miR -133b were significantly decreased in cells cultured in dofetilide compared with control, possibly accounting for KvLQT1 protein upregulation.

Conclusions— Sustained reductions in I Kr may lead to compensatory upregulation of I Ks through posttranscriptional upregulation of underlying subunits, likely mediated (at least partly) by microRNA changes. These results suggest that feedback control of ion channel expression may influence repolarization reserve.

Cellular signaling underlying atrial tachycardia remodeling of L-type calcium current

Atrial tachycardia (AT) downregulates L-type Ca(2+) current (I(CaL)) and causes atrial fibrillation-promoting electric remodeling. This study assessed potential underlying signal transduction. Cultured adult canine atrial cardiomyocytes were paced at 0, 1, or 3 Hz (P0, P1, P3) for up to 24 hours. Cellular tachypacing (P3) mimicked effects of in vivo AT: decreased I(CaL) and transient outward current (I(to)), unchanged I(CaT), I(Kr), and I(Ks), and reduced action potential duration (APD). I(CaL) was unchanged in P3 at 2 and 8 hours but decreased by 55+/-6% at 24 hours. Tachypacing caused Ca(2+)(i) accumulation in P3 cells at 2 to 8 hours, but, by 24 hours, Ca(2+)i returned to baseline. Ca(v)1.2 mRNA expression was not altered at 2 hours but decreased significantly at 8 and 24 hours (32+/-4% and 48+/-4%, respectively) and protein expression was decreased (47+/-8%) at 24 hours only. Suppressing Ca(2+)(i) increases during tachypacing with the I(CaL) blocker nimodipine or the Ca(2+) chelator BAPTA-AM prevented I(CaL) downregulation. Calcineurin activity increased in P3 at 2 and 8 hours, respectively, returning to baseline at 24 hours. Nuclear factor of activated T cells (NFAT) nuclear translocation was enhanced in P3 cells. Ca(2+)-dependent signaling was probed with inhibitors of Ca(2+)/calmodulin (W-7), calcineurin (FK-506), and NFAT (INCA6): each prevented I(CaL) downregulation. Significant APD reductions ( approximately 30%) at 24 hours in P3 cells were prevented by nimodipine, BAPTA-AM, W-7, or FK-506. Thus, rapid atrial cardiomyocyte activation causes Ca(2+) loading, which activates the Ca(2+)-dependent calmodulin-calcineurin-NFAT system to cause transcriptional downregulation of I(CaL), restoring Ca(2+)i to normal at the cost of APD reduction. These studies elucidate for the first time the molecular feedback mechanisms underlying arrhythmogenic AT remodeling.

Mechanisms underlying rate-dependent remodeling of transient outward potassium current in canine ventricular myocytes

Transient outward K+ current (I to) downregulation following sustained tachycardia in vivo is usually attributed to tachycardiomyopathy. This study assessed potential direct rate regulation of cardiac I(to) and underlying mechanisms. Cultured adult canine left ventricular cardiomyocytes (37 degrees C) were paced continuously at 1 or 3 Hz for 24 hours. I to was recorded with whole-cell patch clamp. The 3-Hz pacing reduced I to by 44% (P<0.01). Kv4.3 mRNA and protein expression were significantly reduced (by approximately 30% and approximately 40%, respectively) in 3-Hz paced cells relative to 1-Hz cells, but KChIP2 expression was unchanged. Prevention of Ca2+ loading with nimodipine or calmodulin inhibition with W-7, A-7, or W-13 eliminated 3-Hz pacing-induced I to downregulation, whereas downregulation was preserved in the presence of valsartan. Inhibition of Ca2+/calmodulin-dependent protein kinase (CaMK)II with KN93, or calcineurin with cyclosporin A, also prevented I to downregulation. CaMKII-mediated phospholamban phosphorylation at threonine 17 was increased in 3-Hz paced cells, compatible with enhanced CaMKII activity, with functional significance suggested by acceleration of the Ca2+i transient decay time constant (Indo 1-acetoxymethyl ester microfluorescence). Total phospholamban expression was unchanged, as was expression of Na+/Ca2+ exchange and sarcoplasmic reticulum Ca2+-ATPase proteins. Nuclear localization of the calcineurin-regulated nuclear factor of activated T cells (NFAT)c3 was increased in 3-Hz paced cells compared to 1-Hz (immunohistochemistry, immunoblot). INCA-6 inhibition of NFAT prevented I to reduction in 3-Hz paced cells. Calcineurin activity increased after 6 hours of 3-Hz pacing. CaMKII inhibition prevented calcineurin activation and NFATc3 nuclear translocation with 3-Hz pacing. We conclude that tachycardia downregulates I to expression, with the Ca2+/calmodulin-dependent CaMKII and calcineurin/NFAT systems playing key Ca2+-sensing and signal-transducing roles in rate-dependent I to control.

Mechanisms of atrial fibrillation termination by rapidly unbinding Na+ channel blockers: insights from mathematical models and experimental correlates

Atrial fibrillation (AF) is the most common sustained clinical arrhythmia and is a problem of growing proportions. Recent studies have increased interest in fast-unbinding Na(+) channel blockers like vernakalant (RSD1235) and ranolazine for AF therapy, but the mechanism of efficacy is poorly understood. To study how fast-unbinding I(Na) blockers affect AF, we developed realistic mathematical models of state-dependent Na(+) channel block, using a lidocaine model as a prototype, and studied the effects on simulated cholinergic AF in two- and three-dimensional atrial substrates. We then compared the results with in vivo effects of lidocaine on vagotonic AF in dogs. Lidocaine action was modeled with the Hondeghem-Katzung modulated-receptor theory and maximum affinity for activated Na(+) channels. Lidocaine produced frequency-dependent Na(+) channel blocking and conduction slowing effects and terminated AF in both two- and three-dimensional models with concentration-dependent efficacy (maximum approximately 89% at 60 microM). AF termination was not related to increases in wavelength, which tended to decrease with the drug, but rather to decreased source Na(+) current in the face of large ACh-sensitive K(+) current-related sinks, leading to the destabilization of primary generator rotors and a great reduction in wavebreak, which caused primary rotor annihilations in the absence of secondary rotors to resume generator activity. Lidocaine also reduced the variability and maximum values of the dominant frequency distribution during AF. Qualitatively similar results were obtained in vivo for lidocaine effects on vagal AF in dogs, with an efficacy of 86% at 2 mg/kg iv, as well as with simulations using the guarded-receptor model of lidocaine action. These results provide new insights into the mechanisms by which rapidly unbinding class I antiarrhythmic agents, a class including several novel compounds of considerable promise, terminate AF.

RELATIVE CONTRIBUTIONS OF FIBROSIS AND CONNEXIN CHANGES TOTHE ATRIAL FIBRILLATION SUBSTRATE DURING THE DEVELOPMENT AND REVERSAL OF CONGESTIVEHEART FAILURE

Background: Connexin alterations occur in various atrial fibrillation (AF) paradigms, but their functional significance remains unclear. No data are available regarding the effects of CHF on atrial connexin expression and phosphorylation. We therefore analyzed connexin changes and their contribution to the AF substrate during the development and reversal ofCHF. Methods and Results: Dogs were allocated to three groups: CHF induced by 2-week ventricular tachypacing (CHF, n=15); CHF dogs allowed to recover for 4 weeks after 2-week tachypacing (REC, n=15) and non-paced shams (CTL, n=11). Left ventricular end-diastolic pressure increased with CHF (14.5±1.0*** vs.3.7±0.7, ***P < 0.001 vs. CTL) and normalized upon CHF recovery (5.1±1.0^†††, ^††† P < 0.001 vs. CHF). Real-time PCR and Western-blot analyses revealed connexin43 (Cx43) and connexin40 (Cx40) mRNA and protein expression to be unchanged by CHF and REC. However, CHF caused Cx43 dephosphorylation(by ~73%***) and increased Cx40/Cx43 protein ratio (by ~35%***), with both alterations completely reversing in REC. Immunofluorescent confocal microscopy confirmed connexin protein trends, with a reduction in phosphorylated Cx43 (by ~68%*** in CHF) that returned to control in REC. CHF caused conduction abnormalities (phasedelay-range and heterogeneity index, both P < 0.01) and burst pacing-induced AF prolongation (CTL 22±7s, CHF 1100±171s***, REC 884±220s***) which persisted in the recovery period, along with residual fibrosis (CTL 3.6±0.7%, CHF 14.7±1.5%***, REC13.3±2.3%***). Fibrosis physically interrupted muscle bundle continuity and anionically-based action potential model of canine atrium showed that fibrosiswas able to account for the observed conduction abnormalities. Conclusions: CHF causes connexin-dephosphorylation and Cx40/Cx43ratio increases. With CHF reversal, atrial connexin alterations recover completely, but tissue fibrosis, conduction abnormalities and a substrate forAF remain with fibrosis accounting for conduction abnormalities. Thus, althougha trial connexin changes occur with CHF, they are not essential for conduction disturbances and AF promotion, which appear rather to be related primarily tofibrotic interruption of muscle-bundle continuity.

Rhythm control versus rate control for atrial fibrillation and heart failure

BACKGROUND

It is common practice to restore and maintain sinus rhythm in patients with atrial fibrillation and heart failure. This approach is based in part on data indicating that atrial fibrillation is a predictor of death in patients with heart failure and suggesting that the suppression of atrial fibrillation may favorably affect the outcome. However, the benefits and risks of this approach have not been adequately studied.

METHODS

We conducted a multicenter, randomized trial comparing the maintenance of sinus rhythm (rhythm control) with control of the ventricular rate (rate control) in patients with a left ventricular ejection fraction of 35% or less, symptoms of congestive heart failure, and a history of atrial fibrillation. The primary outcome was the time to death from cardiovascular causes.

RESULTS

A total of 1376 patients were enrolled (682 in the rhythm-control group and 694 in the rate-control group) and were followed for a mean of 37 months. Of these patients, 182 (27%) in the rhythm-control group died from cardiovascular causes, as compared with 175 (25%) in the rate-control group (hazard ratio in the rhythm-control group, 1.06; 95% confidence interval, 0.86 to 1.30; P=0.59 by the log-rank test). Secondary outcomes were similar in the two groups, including death from any cause (32% in the rhythm-control group and 33% in the rate-control group), stroke (3% and 4%, respectively), worsening heart failure (28% and 31%), and the composite of death from cardiovascular causes, stroke, or worsening heart failure (43% and 46%). There were also no significant differences favoring either strategy in any predefined subgroup.

CONCLUSIONS

In patients with atrial fibrillation and congestive heart failure, a routine strategy of rhythm control does not reduce the rate of death from cardiovascular causes, as compared with a rate-control strategy. (ClinicalTrials.gov number, NCT00597077.)

Calcium-Handling Abnormalities Underlying Atrial Arrhythmogenesis and Contractile Dysfunction in Dogs With Congestive Heart Failure

Background— Congestive heart failure (CHF) is a common cause of atrial fibrillation. Focal sources of unknown mechanism have been described in CHF-related atrial fibrillation. The authors hypothesized that abnormal calcium (Ca 2+ ) handling contributes to the CHF-related atrial arrhythmogenic substrate.

Methods and Results— CHF was induced in dogs by ventricular tachypacing (240 bpm �2 weeks). Cellular Ca 2+ -handling properties and expression/phosphorylation status of key Ca 2+ handling and myofilament proteins were assessed in control and CHF atria. CHF decreased cell shortening but increased left atrial diastolic intracellular Ca 2+ concentration ([Ca 2+ ] i ), [Ca 2+ ] i transient amplitude, and sarcoplasmic reticulum (SR) Ca 2+ load (caffeine-induced [Ca 2+ ] i release). SR Ca 2+ overload was associated with spontaneous Ca 2+ transient events and triggered ectopic activity, which was suppressed by the inhibition of SR Ca 2+ release (ryanodine) or Na + /Ca 2+ exchange. Mechanisms underlying abnormal SR Ca 2+ handling were then studied. CHF increased atrial action potential duration and action potential voltage clamp showed that CHF-like action potentials enhance Ca 2+ i loading. CHF increased calmodulin-dependent protein kinase II phosphorylation of phospholamban by 120%, potentially enhancing SR Ca 2+ uptake by reducing phospholamban inhibition of SR Ca 2+ ATPase, but it did not affect phosphorylation of SR Ca 2+ -release channels (RyR2). Total RyR2 and calsequestrin (main SR Ca 2+ -binding protein) expression were significantly reduced, by 65% and 15%, potentially contributing to SR dysfunction. CHF decreased expression of total and protein kinase A–phosphorylated myosin-binding protein C (a key contractile filament regulator) by 27% and 74%, potentially accounting for decreased contractility despite increased Ca 2+ transients. Complex phosphorylation changes were explained by enhanced calmodulin-dependent protein kinase IIδ expression and function and type-1 protein-phosphatase activity but downregulated regulatory protein kinase A subunits.

Conclusions— CHF causes profound changes in Ca 2+ -handling and -regulatory proteins that produce atrial fibrillation–promoting atrial cardiomyocyte Ca 2+ -handling abnormalities, arrhythmogenic triggered activity, and contractile dysfunction.