Atrial tachycardia induces remodelling of muscarinic receptors and their coupled potassium currents in canine left atrial and pulmonary vein cardiomyocytes

BACKGROUND AND PURPOSE

Both parasympathetic tone and atrial tachycardia (AT) remodelling of ion channels play important roles in atrial fibrillation (AF) pathophysiology. Different muscarinic cholinergic receptor (mAChR) subtypes (M2, M3, M4) in atrial cardiomyocytes are coupled to distinct K+-currents (called IKM2, IKM3, IKM4, respectively). Pulmonary veins (PVs) are important in AF and differential cholinergic current responses are a potential underlying mechanism. This study investigated AT-induced remodelling of mAChR subtypes and K+-currents in left-atrial (LA) and PV cardiomyocytes.

EXPERIMENTAL APPROACH

Receptor expression was assayed by western blot. IKM2, IKM3 and IKM4 were recorded with whole-cell patch-clamp in LA and PV cardiomyocytes of nonpaced control dogs and dogs after 7 days of AT-pacing (400 bpm).

KEY RESULTS

Current densities of IKM2, IKM3 and IKM4 were significantly reduced by AT-pacing in LA and PV cardiomyocytes. PV cardiomyocyte current-voltage relations were similar to LA for all three cholinergic currents, both in control and AT remodelling. Membrane-protein expression levels corresponding to M2, M3 and M4 subtypes were decreased significantly (by about 50%) after AT pacing. Agonist concentration-response relations for all three currents were unaffected by AT pacing.

CONCLUSIONS AND IMPLICATIONS

AT downregulated all three mAChR-coupled K+-current subtypes, along with corresponding mAChR protein expression. These changes in cholinergic receptor-coupled function may play a role in AF pathophysiology. Cholinergic receptor-coupled K+-currents in PV cardiomyocytes were similar to those in LA under control and AT-pacing conditions, suggesting that differential cholinergic current properties do not explain the role of PVs in AF.

Atrial cell action potential parameter fitting using genetic algorithms

Understanding of the considerable variation in action potential (AP) shape throughout the heart is necessary to explain normal and pathological cardiac function. Existing mathematical models reproduce typical APs, but not all measured APs, as fitting the sets of non-linear equations is a tedious process. The study describes the integration of a pre-existing mathematical model of an atrial cell AP with a genetic algorithm to provide an automated tool to generate APs for arbitrary cells by fitting ionic channel conductances. Using the Nygren model as the base, the technique was first verified by starting with random values and fitting the Nygren model to itself with an error of only 0.03%. The Courtemanche model, which has a different morphology from that of the Nygren model, was successfully fitted. The AP duration restitution curve generated by the fit matched that of the target model very well. Finally, experimentally recorded APs were reproduced. To match AP duration restitution behaviour properly, it was necessary simultaneously to fit over several stimulation frequencies. Also, fitting of the upstroke was better if the stimulating current pulse replicated that found in situ as opposed to a rectangular pulse. In conclusion, the modelled parameters were successfully able to reproduce any given atrial AP. This tool can be useful for determining parameters in new AP models, reproducing specific APs, as well as determining the locus of drug action by examining changes in conductance values.

Beta-blockers as antiarrhythmic agents

Drugs that suppress beta-adrenergic signaling by competitively inhibiting agonist binding to beta-adrenergic receptors ("beta-blockers") have important antiarrhythmic properties. They differ from most other antiarrhythmic agents by not directly modifying ion channel function; rather, they prevent the arrhythmia-promoting actions of beta-adrenergic stimulation. beta-Blockers are particularly useful in preventing sudden death due to ventricular tachyarrhythmias associated with acute myocardial ischemia, congenital long QT syndrome, and congestive heart failure. They are also quite valuable in controlling the ventricular rate in patients with atrial fibrillation. This chapter reviews the properties of beta-adrenoceptor signaling, the basic mechanisms of cardiac arrhythmias on which beta-blockers act, the ion channel mediators of beta-adrenergic responses, the evidence for clinical antiarrhythmic indications for beta-blocker therapy and the specific pharmacodynamic and pharmacokinetic properties of beta-blockers that differentiate the various agents of this class.

Of circles and spirals: Bridging the gap between the leading circle and spiral wave concepts of cardiac reentry

The “leading circle model” was the first detailed attempt at understanding the mechanisms of functional reentry, and remains a widely-used notion in cardiac electrophysiology. The “spiral wave” concept was developed more recently as a result of modern theoretical analysis and is the basis for consideration of reentry mechanisms in present biophysical theory. The goal of this paper is to present these models in a way that is comprehensible to both the biophysical and electrophysiology communities, with the idea of helping clinical and experimental electrophysiologists to understand better the spiral wave concept and of helping biophysicists to understand why the leading circle concept is so attractive and widely used by electrophysiologists. To this end, the main properties of the leading circle and spiral wave models of reentry are presented. Their basic assumptions and determinants are discussed and the predictions of the two concepts with respect to pharmacological responses of arrhythmias are reviewed. A major difference between them lies in the predicted responses to Na+-channel blockade, for which the spiral wave paradigm appears more closely to correspond to the results of clinical and experimental observations. The basis of this difference is explored in the context of the fundamental properties of the models.

Properties, Expression and Potential Roles of Cardiac K + Channel Accessory Subunits: MinK, MiRPs, KChIP, and KChAP

Over the past 10 years, cDNAs encoding a wide range of pore-forming K(+)-channel alpha-subunits have been cloned and found to result in currents with many properties of endogenous cardiac K(+) channels upon homomeric expression in heterologous systems. However, a variety of remaining discrepancies have led to a search for other subunits that might be involved in the formation of native channels. Over the past few years, a series of accessory subunits has been discovered that modify current properties upon coexpression with alpha-subunits. One of these, the minimal K(+)-channel subunit minK, is essential for formation of the cardiac slow delayed-rectifier K(+) current, I(Ks), and may also interact in functionally important ways with other alpha-subunits. Another, the K(+)-channel interacting protein KChIP appears critical in formation of native transient outward current (I(to)) channels. The roles of 2 other accessory subunits, the minK-related peptide MiRP and the K(+)-channel accessory protein, KChAP, remain unclear. This article reviews the available knowledge regarding the accessory subunits minK, MiRP, KChIP and KChAP, dealing with their structure, effects on currents carried by coexpressed alpha-subunits, expression in cardiac tissues and potential physiological function. On the basis of the available information, we attempt to assess the potential involvement of these accessory K(+)-channel subunits in cardiac pathophysiology and in developing new therapeutic approaches.

Amiodarone reduces procedures and costs related to atrial fibrillation in a controlled clinical trial

BACKGROUND

Atrial fibrillation is the most common sustained cardiac arrhythmia, and engenders significant health care costs. The impact of various treatment options for atrial fibrillation on hospital costs has not been evaluated in a randomized trial.

METHODS

We analysed 1-year follow-up data on 392 patients randomized to low dose amiodarone (200 mg. day(-1)) or alternative first-line therapy (sotalol or propafenone) in a multicentre trial (Canadian Trial of Atrial Fibrillation, CTAF).

RESULTS

Patients in the amiodarone group had fewer electrical cardioversions (65 vs 109 for patients in the sotalol/propafenone group, P<0.0001), and pacemaker insertions (4 vs 11, P=0.07). The average amiodarone patient spent fewer days in hospital (0.47 vs 0.97, P=0.01), and incurred lower costs ($532 vs $898, P=0.03), for admissions where atrial fibrillation was the admitting diagnosis. Average total hospital costs per patient for all admissions, as well as average combined hospital and physician costs per patient, showed wide variations within the treatment arms and were not significantly different between groups.

CONCLUSION

For patients in whom antiarrhythmic drug therapy is indicated, low dose amiodarone significantly reduces atrial fibrillation-related costs by reducing the number of atrial fibrillation-related procedures.

Phospholipid metabolite 1-palmitoyl-lysophosphatidylcholine enhances human ether-a-go-go-related gene (HERG) K(+) channel function

BACKGROUND

Lysophosphatidylcholine (LPC), a naturally occurring phospholipid metabolite, accumulates in the ischemic heart and causes extracellular K(+) accumulation and action potential shortening. LPC has been incriminated as a biochemical trigger of lethal cardiac arrhythmias, but the underlying mechanisms remain poorly understood.

METHODS AND RESULTS

We studied the effect of 1-palmitoyl-LPC (Pal-LPC) on currents resulting from human ether-a-go-go-related gene (HERG) expression in human embryonic kidney (HEK) cells using whole-cell patch-clamp techniques. Bath application of Pal-LPC consistently and reversibly increased HERG current (I(HERG)). The effects of Pal-LPC were apparent as early as 3 minutes after application of the drug, reached maximum within 10 minutes, and were reversible on washout. Pal-LPC increased I(HERG) at voltages between -20 and +30 mV, with greater effects at stronger depolarization. However, Pal-LPC did not affect the voltage-dependence of I(HERG) activation. In contrast, Pal-LPC significantly shifted the inactivation curve toward more positive potentials, causing a mean 20.0+/-2.2 mV shift in half-inactivation voltage relative to control.

CONCLUSIONS

Our results indicate that apart from being a well-recognized target for drug inhibition, I(HERG) can also be enhanced by natural substances. An increase in I(HERG) by Pal-LPC may contribute to K(+) loss, abnormal electrophysiology, and arrhythmia occurrence in the ischemic heart.

Effects of angiotensin-converting enzyme inhibition on the development of the atrial fibrillation substrate in dogs with ventricular tachypacing-induced congestive heart failure

BACKGROUND

Atrial structural remodeling creates a substrate for atrial fibrillation (AF), but the underlying signal transduction mechanisms are unknown. This study assessed the effects of ACE inhibition on arrhythmogenic atrial remodeling and associated mitogen-activated protein kinase (MAPK) changes in a dog model of congestive heart failure (CHF).

METHODS AND RESULTS

Dogs were subjected to various durations of ventricular tachypacing (VTP, 220 to 240 bpm) in the presence or absence of oral enalapril 2 mg. kg(-1). d(-1). VTP for 5 weeks induced CHF, local atrial conduction slowing, and interstitial fibrosis and prolonged atrial burst pacing-induced AF. Atrial angiotensin II concentrations and MAPK expression were increased by tachypacing, with substantial changes in phosphorylated forms of c-Jun N-terminal kinase (JNK), extracellular signal-regulated kinase (ERK), and p38-kinase. Enalapril significantly reduced tachypacing-induced changes in atrial angiotensin II concentrations and ERK expression. Enalapril also attenuated the effects of CHF on atrial conduction (conduction heterogeneity index reduced from 3.1+/-0.4 to 1.9+/-0.2 ms/mm, P<0.05), atrial fibrosis (from 11.9+/-1.1% to 7.5+/-0.4%, P<0.01), and mean AF duration (from 651+/-164 to 218+/-75 seconds, P<0.05). Vasodilator therapy of a separate group of VTP dogs with hydralazine and isosorbide mononitrate did not alter CHF-induced fibrosis or AF promotion.

CONCLUSIONS

CHF-induced increases in angiotensin II content and MAPK activation contribute to arrhythmogenic atrial structural remodeling. ACE inhibition interferes with signal transduction leading to the AF substrate in CHF and may represent a useful new component to AF therapy.

Coronary artery endothelial protection after local delivery of 17beta-estradiol during balloon angioplasty in a porcine model: a potential new pharmacologic approach to improve endothelial function

OBJECTIVES

The goal of this research was to study the effect of locally delivered 17beta-estradiol (17beta-E) during angioplasty on endothelial function after percutaneous transluminal coronary angioplasty (PTCA) at four weeks.

BACKGROUND

The endothelium plays a major role in the structural and functional integrity of coronary arteries and is damaged by PTCA.

METHODS

Juvenile swine were subjected to PTCA, after which each artery was randomly-assigned to 600-microg 17beta-E delivered locally, an equal volume of vehicle (V) or PTCA alone. After four weeks, the improvement in endothelial function was assessed by angiography using intracoronary acetylcholine (Ach) infusion and by immunohistochemistry.

RESULTS

At 10(-5) mol/l and 10(-4) mol/l Ach, significant vasoconstriction was noted in arteries treated with PTCA alone (p < 0.01 and p < 0.0001, respectively) and with PTCA plus V (p < 0.02 and p < 0.001, respectively). No significant vasoconstrictive response to Ach was observed in arteries treated with PTCA plus 17beta-E. Immunohistochemistry of vessels four weeks after PTCA revealed enhanced re-endothelialization (p < 0.0005) and endothelial nitric-oxide synthase (eNOS) expression (p < 0.0005) in PTCA plus 17beta-E-treated arteries compared with the other two treatment groups. Arteries treated with 17beta-E showed significantly lower neointima formation, which correlated inversely with the extent of re-endothelialization and eNOS expression.

CONCLUSIONS

Locally delivered 17beta-E significantly enhances re-endothelialization and endothelial function after PTCA, possibly by improving the expression of eNOS. Since endothelial dysfunction can promote both restenosis and coronary spasm, local 17beta-E administration is a promising new approach to improve long-term results after PTCA.

Remodeling of atrial dimensions and emptying function in canine models of atrial fibrillation

OBJECTIVES

Atrial tachycardia-induced remodeling (ATR) and ventricular tachypacing-induced heart failure (HF) create experimental substrates for atrial fibrillation (AF), and both have been reported to produce atrial dilation and hypocontractility. The relative importance of changes in atrial size and contractility in the two models is unknown. This study compared changes in atrial dimensions and emptying in ATR versus HF dog models and related them to AF promotion.

METHODS

In ATR dogs (n=11), the right atrium (RA) was paced at 400/min for 42 days. In HF dogs (n=10), the right ventricle was paced at 240 bpm for 2 weeks, followed by 3 weeks at 220 bpm. Transthoracic echocardiography was performed at baseline and weekly thereafter. At a terminal electrophysiological study, RA effective refractory period (ERP) was recorded and AF induced repeatedly by atrial burst pacing to measure mean AF duration (DAF).

RESULTS

Left atrial (LA) systolic area increased by 10.0% in ATR versus 48.2% in HF dogs (P=0.008), with significant time-dependent changes in HF (P=0.0001), but not ATR (P=0.16). LA diastolic area increased over time in both groups (P=0.004, 0.0001 for ATR and HF respectively), but increases were much larger in CHF (80.2%) compared to ATR (24.2%, P=0.0002). Similar findings were obtained for RA. Fractional area shortening (FAS) decreased by 19.4% (ATR) versus 41.8% (HF, P=0.007) in LA and 13.7% (ATR) versus 33.7% (HF, P=0.03) in RA. RA ERP correlated with DAF in ATR dogs (r=-0.79, P<0.001), but not in HF dogs (r=0.20, P=NS). DAF and diastolic areas of RA and LA were highly correlated (r=0.71, 0.77; P<0.01 for each) in HF dogs, but not in ATR dogs (r=-0.18, 0.29; P=NS).

CONCLUSIONS

Remodeling of atrial size and emptying function is much greater in HF than in ATR. Whereas in ATR, electrophysiological remodeling is of prime importance in AF promotion, structural remodeling (as reflected in changes in atrial size and contraction) appears much more important in HF-induced AF.

Ionic remodeling of cardiac Purkinje cells by congestive heart failure

BACKGROUND

Cardiac Purkinje cells (PCs) are important for the generation of triggered arrhythmias, particularly in association with abnormal repolarization. The effects of congestive heart failure (CHF) on the ionic properties of PCs are unknown.

METHODS AND RESULTS

PCs were isolated from false tendons of control dogs and dogs with ventricular tachypacing-induced CHF. CHF PCs were hypertrophied (capacitance, mean+/-SEM, 149+/-4 pF, n=130; versus 128+/-3 pF, n=150, control; P<0.001). Transient outward current density was reduced in CHF PCs without change in voltage dependence or kinetics. CHF also reduced inward-rectifier current density, with no change in form of the current-voltage relationship. Densities of L- and T-type calcium, rapid and slow delayed rectifier, and Na(+)-Ca(2+) exchange currents were unaltered by CHF, but L-type calcium current inactivation was slowed at positive potentials. Purkinje fiber action potentials from CHF dogs showed decreased phase 1 amplitudes and elevated plateau voltages and demonstrated twice as much prolongation on exposure to the rapid delayed rectifier blocker E-4031 as control Purkinje fibers.

CONCLUSIONS

CHF causes remodeling of important K(+) and Ca(2+) currents in cardiac PCs, decreasing repolarization reserve and causing an exaggerated repolarization delay in response to a class III drug. These results have important potential implications regarding ventricular arrhythmogenesis, particularly related to triggered activity in PCs, in patients with CHF.

Arrhythmogenic ionic remodeling: adaptive responses with maladaptive consequences

Compensatory changes in ion transport mechanisms occur in response to a variety of cardiac disease processes. Recent work has demonstrated that these adaptive responses can produce the arrhythmogenic substrate for a variety of important cardiac rhythm disorders. Two important paradigms are atrial tachycardia-induced remodeling and ionic remodeling caused by congestive heart failure. Atrial tachycardia promotes cellular Ca(2)+ loading and downregulates a variety of ion channels, particularly L-type Ca(2)+ channels, thereby promoting the occurrence and maintenance of atrial fibrillation. Congestive heart failure alters the expression and function of a variety of membrane transport processes, including several K(+)-channels and key Ca(2)+-transport systems, favoring the occurrence of arrhythmogenic afterdepolarizations. An improved understanding of the mechanisms and consequences of arrhythmogenic ionic remodeling promises to lead to novel and improved therapeutic approaches.

Oxidative preconditioning and apoptosis in L-cells. Roles of protein kinase B and mitogen-activated protein kinases

Oxidative stress can cause significant cell death by apoptosis. We performed studies in L-cells to explore whether prior exposure to oxidative stress ("oxidative preconditioning") can protect the cell against the apoptotic consequences of subsequent oxidative insults and to establish the mediators in the preconditioning signaling cascade. Cells were preconditioned with three 5-min exposures to H(2)O(2), followed by 10-h recovery and subsequent exposure to 600 microm H(2)O(2) for 10 h. A single 10-h exposure to H(2)O(2) induced substantial apoptotic cell death (approximately 90%), as determined by enzyme-linked immunosorbent assay, TUNEL (terminal deoxyribonucleotide transferase-mediated dUTP nick end labeling), and Annexin V methods, but apoptosis was largely prevented in preconditioned cells. The degree of cytoprotection depended on the strength of preconditioning or H(2)O(2) concentration (20 approximately 600 microm). Transient increases in mitogen-activated protein kinase (MAPK), p38, and JNK/SAPK activities and sustained protein kinase B (Akt) activation, accompanied by drastically reduced caspase 3 activity, were seen after preconditioning. The expression levels of these kinases were unaltered. Inhibitors of p38 (SB203580) and phosphoinositide 3-kinase (PI3K, LY294002) pathways abolished the protection provided by preconditioning. We conclude that oxidative preconditioning protects cells against apoptosis and that this effect involves MAPK and PI3K/Akt pathways. This system may be important in regulating apoptotic cell death in development and disease states.

Potential ionic mechanism for repolarization differences between canine right and left atrium

Experimental and clinical evidence suggests a critical role for the left atrium (LA) in atrial fibrillation (AF). In animal models, repolarization is faster in the LA than in the right atrium (RA), leading to more stable reentry circuits with a shorter intrinsic period in the LA. The ionic mechanisms underlying LA-RA repolarization differences are unknown. Therefore, we evaluated ionic currents and action potentials (APs) with the whole-cell patch clamp in isolated canine atrial myocytes. The density of the rapid delayed rectifier current (I(Kr)) was greater in the LA (eg, 1.83+/-0.10 pA/pF at +20 mV) than in the RA (1.15+/-0.07 pA/pF, P<0.01; n=16 cells per group). The slow and ultrarapid delayed rectifier, the inward rectifier, L-type Ca(2+), and transient outward K(+) currents were all comparable in the LA and RA. There were no differences in kinetic or voltage-dependent properties of currents in LA versus RA. Western blots of ether-a-go-go-related gene (ERG) protein in three RA and corresponding LA regions showed significantly greater ERG expression in LA. AP duration (APD) was shorter in the LA versus RA in both isolated cells and multicellular preparations, and the effective refractory period (ERP) was shorter in the LA compared with the RA in vivo. Dofetilide had significantly larger APD- and ERP-increasing effects in the LA compared with RA, and LA-RA repolarization differences were eliminated by exposure to dofetilide. We conclude that LA myocytes have larger I(Kr) than do RA myocytes, contributing importantly to the shorter APD and ERP in LA. The larger LA I(Kr) may participate in the ability of the LA to act as a "driver region" for AF, with potentially important implications for understanding AF mechanisms and antiarrhythmic therapy.

Method for simultaneous epicardial and endocardial mapping of in vivo canine heart: application to atrial conduction properties and arrhythmia mechanisms

INTRODUCTION

It has been suggested that the three-dimensional structure of the atria may be crucial in arrhythmogenesis; however, previous in vivo atrial activation mapping studies have been limited to either endocardial or epicardial approaches.

METHODS AND RESULTS

To investigate the role of endocardial and epicardial structures and their interaction in atrial conduction and arrhythmias, we used five epicardial plaques and two intra-atrial balloon arrays to record a total of 368 unipolar electrograms from the entire epicardial and endocardial surface of both atria. During regular 1:1 pacing from the right atrial appendage, right atrial endocardial activation spread considerably faster than epicardial (total activation time 45+/-12 msec vs 60+/-19 msec, respectively [mean +/- SD]; P < 0.05), pointing to preferential conduction over structures like the crista terminalis and pectinate muscles. No such differences were noted in the left atrium. Transseptal spread occurred via discrete anterior and posterior pathways, causing separate breakthroughs in anterior and posterior atrial regions, respectively. Dissociation between septal pathways played a role in reentry during vagal atrial fibrillation. In 2 of 4 dogs with atrial fibrillation associated with congestive heart failure, single macroreentrant circuits involving endocardial and epicardial components were revealed during the arrhythmia.

CONCLUSION

We conclude that activation mapping using simultaneous recording from both epicardial and endocardial surfaces provides potentially important insights into the mechanisms of atrial conduction and arrhythmogenesis.

Expression of multiple subtypes of muscarinic receptors and cellular distribution in the human heart

Five isoforms of the muscarinic acetylcholine receptor (mAChR) have been identified by molecular cloning and designated m(1)-m(5), of which four correspond to the functional subtypes M(1), M(2), M(3), and M(4) in primary tissues. The presence of M(5) receptors in tissues remains uncertain. The present study was designed to explore the diversity and cellular distribution of various mAChR subtypes in human hearts. Competition binding of [N-methyl-(3)H]-scopolamine methyl chloride with various mAChR antagonists yielded data consistent with the presence of multiple subtypes (M(1)/M(2)/M(3)/M(5)) of mAChRs in both human atrial (HA) and ventricular (HV) tissues. Expression of mRNAs encoding all five subtypes was readily detected by reverse transcription-polymerase chain reaction in both HA and HV samples. Immunoblotting with subtype-specific antibodies confirmed the presence of M(1), M(2), M(3), and M(5), but not M(4), proteins in membrane preparations from both HA and HV. The protein levels of M(1) and M(2) were comparable between HA and HV. Although the density of M(3) appeared approximately 10-fold higher in HV than HA, that of M(5) was approximately 5 times lower in HV than in HA. Positive immunostaining of single ventricular myocytes by M(1), M(2), M(3), and M(5) antibodies, respectively, was consistently detected. Under confocal microscopy, M(5) showed characteristic localization to the intercalated discs, whereas other subtypes were more evenly distributed throughout the surface membrane. Our results provide the first molecular evidence for the presence of multiple subtypes of mAChR, including endogenous M(5) receptors, in human hearts and suggest that different subtypes have different tissue distributions and cellular localization.

The class III antiarrhythmic drugs dofetilide and sotalol prevent AF induction by atrial premature complexes at doses that fail to terminate AF

BACKGROUND

Clinical trials suggest that sotalol and dofetilide are much more effective in preventing atrial fibrillation (AF) than in terminating it. This study evaluated potential mechanisms of discordant sotalol and dofetilide effects on AF termination vs. prevention.

METHODS

We applied 240-electrode epicardial mapping and programmed stimulation in a vagotonic dog model of AF before and after dofetilide or sotalol.

RESULTS

Under control conditions, sustained AF could be induced by single S(2) extrastimuli that caused unidirectional block and macroreentry. Sotalol (2 mg/kg) and dofetilide (0.04 mg/kg) failed to terminate AF in any dog, but prevented AF induction by S(2) stimuli in 19/22 (86%) and 4/5 (80%) of animals, respectively. With sotalol and dofetilide, unidirectional block still occurred, but wavefront reentry failed. The prevention of S(2)-induced reentry was related to large increases in the effective refractory period (ERP) at a BCL of 1000 ms, leading to ERPs that exceeded the conduction delay following S(2). Reverse use-dependent effects resulted in smaller ERP increases at BCLs closer to the AF cycle length. Although the number of zones of reactivation per cycle during sustained AF were decreased by sotalol and dofetilide, the changes were small and insufficient to terminate AF.

CONCLUSIONS

Sotalol and dofetilide prevent AF initiation by premature depolarizations at doses that fail to terminate vagotonic AF, by increasing ERP at the basic cycle length beyond the associated conduction delay that leads to reentry.

Effects of verapamil on atrial fibrillation and its electrophysiological determinants in dogs

BACKGROUND

Atrial tachycardia-induced remodeling promotes the occurrence and maintenance of atrial fibrillation (AF) and decreases L-type Ca(2+) current. There is also a clinical suggestion that acute L-type Ca(2) channel blockade can promote AF, consistent with an AF promoting effect of Ca(2+) channel inhibition.

METHODS

To evaluate the potential mechanisms of AF promotion by Ca(2+) channel blockers, we administered verapamil to morphine-chloralose anesthetized dogs. Diltiazem was used as a comparison drug and autonomic blockade with atropine and nadolol was applied in some experiments. Epicardial mapping with 240 epicardial electrodes was used to evaluate activation during AF.

RESULTS

Verapamil caused AF promotion in six dogs, increasing mean duration of AF induced by burst pacing, from 8+/-4 s (mean+/-S.E.) to 95+/-39 s (P<0.01 vs. control) at a loading dose of 0.1 mg/kg and 228+/-101 s (P<0.0005 vs. control) at a dose of 0.2 mg/kg. Underlying electrophysiological mechanisms were studied in detail in five additional dogs under control conditions and in the presence of the higher dose of verapamil. In these experiments, verapamil shortened mean effective refractory period (ERP) from 122+/-5 to 114+/-4 ms (P<0.02) at a cycle length of 300 ms, decreased ERP heterogeneity (from 15+/-1 to 10+/-1%, P<0.05), heterogeneously accelerated atrial conduction and decreased the cycle length of AF (94+/-4 to 84+/-3 ms, P<0.005). Diltiazem did not affect ERP, AF cycle length or AF duration, but produced conduction acceleration similar to that caused by verapamil (n=5). In the presence of autonomic blockade, verapamil failed to promote AF and increased, rather than decreasing, refractoriness. Neither verapamil nor diltiazem affected atrial conduction in the presence of autonomic blockade. Epicardial mapping suggested that verapamil promoted AF by increasing the number of simultaneous wavefronts reflected by separate zones of reactivation in each cycle.

CONCLUSIONS

Verapamil promotes AF in normal dogs by promoting multiple circuit reentry, an effect dependent on intact autonomic tone and not shared by diltiazem.

Differential efficacy of L- and T-type calcium channel blockers in preventing tachycardia-induced atrial remodeling in dogs

BACKGROUND

Tachycardia-induced remodeling likely plays an important role in atrial fibrillation (AF) maintenance and recurrence after cardioversion, and Ca(2+) overload may be an important mediator. This study was designed to evaluate the relative efficacies of selective T-type (mibefradil) and L-type (diltiazem) Ca(2+)-channel blockers in preventing tachycardia-induced atrial remodeling.

METHODS

Dogs were given daily doses of mibefradil (100 mg), diltiazem (240 mg) or placebo in a blinded fashion, beginning 4 days before and continuing through a 7-day period of atrial pacing at 400 bpm. An electrophysiological study was then performed to assess changes in refractoriness, refractoriness heterogeneity and AF duration.

RESULTS

Mean duration of burst-pacing induced AF was similar in placebo (567+/-203 s) and diltiazem-treated (963+/-280 s, P=NS) animals, but was much less in mibefradil-treated dogs (3.6+/-0.9 s, P<0.002) and non-paced controls (6.6+/-2.7 s). In contrast to mibefradil, diltiazem did not alter tachycardia-induced refractoriness abbreviation or heterogeneity. To exclude inadequate dosing as an explanation for diltiazem's inefficacy, we studied an additional group of dogs treated with 720 mg/day of diltiazem, and again noted no protective effect. Acute intravenous administration of diltiazem to control dogs failed to alter atrial refractoriness or AF duration, excluding a masking of remodeling suppression by offsetting profibrillatory effects of the drug.

CONCLUSIONS

Whereas the selective T-type Ca(2+)-channel blocker mibefradil protects against atrial remodeling caused by 7-day atrial tachycardia, the selective L-type blocker diltiazem is without effect. These findings are potentially important for understanding the mechanisms and prevention of clinically-relevant atrial-tachycardia-induced remodeling.

Slow delayed rectifier current and repolarization in canine cardiac Purkinje cells

Although cardiac Purkinje cells (PCs) are believed to be the source of early afterdepolarizations generating ventricular tachyarrhythmias in long Q-T syndromes (LQTS), the ionic determinants of PC repolarization are incompletely known. To evaluate the role of the slow delayed rectifier current (I(Ks)) in PC repolarization, we studied PCs from canine ventricular false tendons with whole cell patch clamp (37 degrees C). Typical I(Ks) voltage- and time-dependent properties were noted. Isoproterenol enhanced I(Ks) in a concentration-dependent fashion (EC(50) approximately 30 nM), negatively shifted I(Ks) activation voltage dependence, and accelerated I(Ks) activation. Block of I(Ks) with 293B did not alter PC action potential duration (APD) in the absence of isoproterenol; however, in the presence of isoproterenol, 293B significantly prolonged APD. We conclude that, without beta-adrenergic stimulation, I(Ks) contributes little to PC repolarization; however, beta-adrenergic stimulation increases the contribution of I(Ks) by increasing current amplitude, accelerating I(Ks) activation, and shifting activation voltage toward the PC plateau voltage range. I(Ks) may therefore provide an important "braking" function to limit PC APD prolongation in the presence of beta-adrenergic stimulation.

Intracellular calcium changes and tachycardia-induced contractile dysfunction in canine atrial myocytes

OBJECTIVES

Indirect evidence suggests a role for Ca(2+)-overload in electrical and mechanical alterations caused by atrial tachycardia. The present study assessed the alterations in cellular [Ca(2+)] and contractile function caused by rapid atrial cellular activation.

METHODS

Intracellular Ca(2+) transients (CaT) and cell shortening (CS) were measured by microfluorometry (Indo-1 AM) and video edge-detection in isolated, field-stimulated canine atrial myocytes (37 degrees C).

RESULTS

Abrupt increases in frequency (0.3-3 Hz) caused rapid increases in diastolic [Ca(2+)]i (DCa) that were maintained during rapid-pacing for up to 50 min. When short-term (3-min) rapid-pacing was imposed, CaT and CS increased initially upon returning to 0.3 Hz, but then declined rapidly to 64+/-5 and 49+/-7%, respectively, of pre-tachycardia values, returning to control after approximately 15 min. Post-tachycardia CaT and CS reductions were prevented by decreasing [Ca(2+)]o during tachycardia to prevent Ca(2+)-overload. CS reductions correlated with indices of Ca(2+) loading during tachycardia. Restoration of CaT to normal during post-tachycardia contractile dysfunction (by increasing [Ca(2+)]o) returned CS to normal, indicating that reduced Ca(2+) release, not reduced myofilament Ca(2+)-sensitivity, caused post-tachycardia contractile failure. Estimation of sarcoplasmic-reticulum Ca(2+)-stores (caffeine-induced Ca(2+)-release) confirmed tachycardia-induced Ca(2+)-loading and suggested that reduced Ca(2+)-stores decreased Ca(2+)-release post-tachycardia.

CONCLUSIONS

Atrial tachycardia increases cellular Ca(2+)-loading, leading to post-tachycardia abnormalities in Ca(2+)-handling that produce contractile dysfunction. These findings are the first direct evidence for the frequently-postulated role of Ca(2+)-overload in tachycardia-induced abnormalities of atrial function.

Mathematical analysis of canine atrial action potentials: rate, regional factors, and electrical remodeling

Dogs have been used extensively to study atrial arrhythmias, but there are no published mathematical models of the canine atrial action potential (AP). To obtain insights into the ionic mechanisms governing canine atrial AP properties, we incorporated formulations of K(+), Na(+), Ca(2+), and Cl(-) currents, based on measurements in canine atrial myocytes, into a mathematical model of the AP. The rate-dependent behavior of model APs corresponded to experimental measurements and pointed to a central role for L-type Ca(2+) current inactivation in rate adaptation. Incorporating previously described regional ionic current variations into the model largely reproduced AP forms characteristic of the corresponding right atrial regions (appendage, pectinate muscle, crista terminalis, and atrioventricular ring). When ionic alterations induced by tachycardia-dependent remodeling were incorporated, the model reproduced qualitatively the AP features constituting the cellular substrate for atrial fibrillation. We conclude that this ionic model of the canine atrial AP agrees well with experimental measurements and gives potential insights into mechanisms underlying functionally important electrophysiological phenomena in canine atrium.

Basic mechanisms of atrial fibrillation--very new insights into very old ideas

Atrial fibrillation (AF) was recognized and studied extensively in the early twentieth century, but many fundamental aspects of the arrhythmia were poorly understood until quite recently. It is now recognized that AF can be initiated by a variety of mechanisms that share the ability to cause extremely rapid, irregular atrial electrical activity. Once initiated, AF causes alterations in atrial electrical properties (electrical remodeling), including both rapid functional changes and slower alterations in ion channel gene expression, which promote the maintenance of AF and facilitate reinitiation of the arrhythmia should it terminate. Electrical remodeling decreases the atrial refractory period in a heterogeneous way, thus decreasing the size and stability of potential functional atrial reentry waves and promoting multiple-circuit reentry. Whatever the initial cause of AF, electrical remodeling is likely to be a final common pathway that ultimately supervenes. Recent advances in understanding ion channel function, regulation, and remodeling at the molecular level have allowed for a much more detailed appreciation of the basic determinants of AF. Improvements in the clinical management of AF will inevitably follow the recent advances in our understanding of its detailed pathophysiology.