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Cardiovascular Research 2003 60(2):298-306; doi:10.1016/S0008-6363(03)00543-1
© 2003 by European Society of Cardiology
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Copyright © 2003, European Society of Cardiology

Atrial effects of the novel K+-channel-blocker AVE0118 in anesthetized pigs

Klaus J Wirth*, Tobias Paehler, Bjoern Rosenstein, Karsten Knobloch, Thomas Maier, Jennifer Frenzel, Joachim Brendel, Andreas E Busch and Markus Bleich

Aventis Pharma Deutschland GmbH, DG Cardiovascular Diseases, Industriepark Höchst, Building H826, 65926 Frankfurt am Main, Germany

*Corresponding author. Tel.: +49-69-305-4274; fax: +49-69-305-81252. Email address: klaus.wirth{at}aventis.com

Received 5 May 2003; revised 28 July 2003; accepted 31 July 2003


   Abstract
Top
 Abstract
1. Introduction
 2. Material and methods
 3. Results
 4. Discussion
 5. Conclusion
 References
 
Objectives: AVE0118 is a novel blocker of the K+ channels Kv1.5 and Kv4.3 which are the molecular basis for the human cardiac ultrarapid delayed rectifier potassium current (IKur) and the transient outward current (Ito). The objective of this study was to investigate the effect of AVE0118 on atrial refractoriness (ERP), left atrial vulnerability (LAV) and on left atrial monophasic action potentials (MAP) in pentobarbital anesthetized pigs in comparison to the selective IKr blocker dofetilide in order to assess the therapeutic potential of the novel K+ channel blocker for atrial fibrillation. Methods: Atrial ERP was determined with the S1–S2-stimulus method in the free walls of left and right atrium at 240, 300 and 400 ms basic cycle length (BCL). The inducibility of mostly nonsustained atrial tachyarrhythmias by the premature S2 extrastimulus, which is very high in the left pig atrium and referred to as LAV, was evaluated before and after drugs. Left atrial epicardial MAP was recorded to study the influence of the potassium channel blockers on the time course of repolarization. Left ventricular epicardial MAP, ERP and QT interval were measured to investigate a possible effect of AVE0118 on ventricular repolarization. Results: ERPs determined at 240, 300 and 400 ms BCL were significantly shorter in the left vs. right atrium (99±3, 106±4 and 113±3 ms vs. 133±4 ms, 142±4 and 149±5, respectively; p<0.001; n = 21). AVE0118 administered i.v. dose-dependently prolonged the atrial ERP independent from rate and inhibited LAV (100% at 0.5 and 1 mg/kg) while having no effect at all on the corrected QT (QTc) interval. At 1 mg/kg (n = 5) AVE0118 prolonged left vs. right atrial ERP by 49.6±4.1 ms vs. 37.7±9.7 ms (means±SEM of changes at 240, 300, and 400 ms BCL), respectively, corresponding to a relative increase of 53.2±6.2% vs. 27.6±6.8% (p<0.05 for percent increase of left vs. right atrial ERP). In a separate group of pigs (n = 5) AVE0118 had no effect on left ventricular ERP at 333, 400 and 500 ms BCL and no effect on MAP duration and QT at 600 ms BCL. After 1 mg/kg of AVE0118 the atrial MAP was significantly prolonged already at 10% repolarization (P<0.05; n = 7) reaching the maximum at 40% repolarization. In contrast to AVE0118 the effect of dofetilide (10 µg/kg) on atrial MAP started to become significant only at 60% repolarization (n = 6) with a maximum increase at 90%. Dofetilide, which prolonged the QTc interval by 16.9% (P<0.001), had a significantly stronger effect on right (34.7±5 ms) vs. left atrial ERP (23.5±7 ms) at 300 ms BCL, respectively, but did not significantly inhibit LAV (14%; n = 6). Conclusion: The novel K+ channel blocker AVE0118 prolonged atrial ERP and showed strong atrial antiarrhythmic efficacy with no apparent effect on ventricular repolarization in pigs in vivo.

KEYWORDS Atrial fibrillation; Effective refractory period; Kv1.5; Kv4.3; Potassium channel blocker; Dofetilide; Atrial vulnerability; QT-prolongation; Pigs


   1. Introduction
Top
 Abstract
 1. Introduction
2. Material and methods
 3. Results
 4. Discussion
 5. Conclusion
 References
 
Treatment of atrial fibrillation/flutter with available potassium channel blockers (class III antiarrhythmic agents which mainly block the delayed rectifier current, IKr) is associated with ventricular proarrhythmia. Prolongation of ventricular repolarization leads to early afterdepolarizations from which torsades de pointes arrhythmias can evolve [1]. Therefore, blockade of a cardiac current of exclusive relevance in the atria is highly desirable as it is expected to be devoid of ventricular proarrhythmic effects. The ultrarapid delayed rectifier potassium current (IKur) seems an ideal atrial antiarrhythmic target since it is found to contribute to action potential repolarization in the atrium but not the ventricle [2,3]. The molecular correlate of the human cardiac ultrarapid delayed rectifier potassium current seems to be the potassium channel Kv1.5 [2,4–6]. The aim of this project was to find potent inhibitors of the Kv1.5 channel for the development of drugs against atrial fibrillation. AVE0118 has been identified and characterized as a potent Kv1.5 blocker/IKur blocker. Apart from IKur blockade the compound has relevant inhibitory effects on Kv4.3/Ito and IKACh, but, most important, it is devoid of an inhibitory effect on IKr and IK1 at presumed therapeutic concentrations [7]. Since, in contrast to IKur, Kv4.3/Ito is also present in the ventricles and also inhibited by AVE0118 the question of an effect of the novel blocker on ventricular repolarization was of particular importance.

In this study we investigated the effect of AVE0118 in vivo on pig heart electrophysiology. Apart from the question of an atrial selective effect of AVE01118, comparison of its atrial effects to those of the most potent and selective IKr blocker dofetilide was of particular interest. In the atria we studied the effect of the compounds on left and right atrial ERP, MAP and on LAV, which we have previously introduced as a new atrial arrhythmia model [8,9]. The pig left atrium, which has significantly shorter ERP than the right atrium, is highly vulnerable to premature S2-stimuli (mostly brief, self-limiting runs of flutter or fibrillation). This is in contrast to the right atrium, where such susceptibility for tachyarrhythmias hardly exists. To judge the effect of AVE0118 on ventricular repolarization we determined ventricular ERP, MAP and QT interval.


   2. Material and methods
Top
 Abstract
 1. Introduction
 2. Material and methods
3. Results
 4. Discussion
 5. Conclusion
 References
 
The investigation conforms with the Guide for the Care and Use of Laboratory Animals published by the US National Institutes of Health (NIH Publication No. 85-13, revised 1996). Male castrated pigs (25–32 kg) of the German Landrace were anaesthetized with pentobarbital exactly as described previously [9]. After left thoracotomy the lung was retracted, the pericardium incised and the heart suspended in a pericardial cradle. Bipolar body surface ECG was recorded using subcutaneous needle electrodes in the classical lead II or lead III arrangement. The QT-interval was corrected for heart rate with Bazett's formula.

2.1. Atrial ERP measurements
ERP measurements at different basic cycle lengths (BCL; 240/300/400 ms) were performed as described previously [9]. Briefly, atrial responses to the pacing procedure were visualized as MAP from the left and right atrium. The MAP PacingTM catheter (EP Technologies, Model 1675, Boston Scientific, 92257 La Garenne-Colombes Cedex, France) was used for stimulation and registration. A conditioning train of 10 basic stimuli (S1) at twice-diastolic pacing threshold was followed by a diastolic extrastimulus (S2, pulse duration 1 ms) starting about 30 ms above the expected ERP with a 5-ms decrement (UHS 20, Universal heart stimulator, Biotronik, 12359 Berlin, Germany). The longest coupling interval unable to elicit a propagated atrial response was taken as the atrial ERP.

2.2. Left atrial vulnerability (LAV)
During the ERP-measurement procedure the S2-extrastimulus, which followed the 10 conditioning S1 stimuli during the ERP-measurement procedure, frequently triggered runs of atrial tachycardia in the left, but not in the right atrium. Whether or not a run of S2-triggered atrial tachyarrhythmia (at least four extra beats) occurred during the ERP-measurement procedure at a given BCL was noted. The occurrences of triggered tachyarrhythmias were summed up for the three BCLs tested over three time points during a 30-min period before or after drug application and served as a measure of atrial vulnerability. Thus, the maximum occurrence of induced atrial tachyarrhythmia was nine (3 BCL x 3 time points) corresponding to an arrhythmia incidence of 100%.

2.3. MAP recording in the left atrial free wall for the analysis of the influence of the potassium channel blockers on the time course of repolarization
MAP electrode and position of the electrode were the same as used for atrial ERP measurements (see above). The MAP pacing catheter was covered by a sponge and fixed epicardially in the middle of the left atrial free wall in an approximately perpendicular position by a holding device [10]. The contact pressure proved to be of critical importance. Therefore, a particular effort had to be made to obtain atrial MAP of sufficient quality for a detailed analysis of the influence of the potassium channel blockers on the different phases of repolarization. Drugs were administered only if a regular baseline and amplitude of the MAPs could be maintained for at least 10 min. The positioning procedure was repeated if changes of the shape of the MAP occurred during the 10-min interval. Small negative deflections or notches seen just before the upstroke of the MAP were accepted. After 10 min of stable MAP shape vehicle or drug was given i.v. The negative chronotropic effects of dofetilide made it necessary to keep the heart rate at a constant level by pacing the right atrium from a region nearby the sinus node. The heart rate was kept 10 beats above its initial rate by stimulation at twice-diastolic threshold. Left atrial MAP duration was evaluated from 10% to 90% repolarization (MAPD10 to MAPD90; at a 10% step) at different times after drug injection. To provide the highest quality of data, MAP analysis and ERP measurements were made in separate groups of pigs, and we limited the MAP investigation to the left atrium with its controlled epicardial access.

2.4. Effect of drugs on ventricular repolarization
In a separate group of five pigs, the effect of AVE0118 on left ventricular repolarization was investigated. The MAP pacing catheter was positioned on the epicardium of the left ventricle (as otherwise described for the left atrium) for the MAP evaluation and ERP measurements. MAP duration at 90% repolarization and QT were measured at 600 ms BCL (with left atrial pacing). Left ventricular effective refractory periods for AVE0118 were determined as described above for the left atrium. In order to avoid hemodynamic instability in the anesthetized pigs BCLs used for ventricular stimulation (333, 400 and 500 ms) were lower than in the atria.

2.5. Drugs
AVE0118 and dofetilde were synthesized in the Department of Medicinal Chemistry of Aventis Pharma Deutschland. AVE0118 was dissolved in 0.5 ml DMSO, 2.5 ml of polyethyleneglykol (PEG) 400 (Riedel-de Haen, Seelze, Germany) was added, and the solution was administered i.v. over 5 min. Vehicle as a control was injected at least 30 min before the drug. Doses between 0.1 and 1 mg/kg were administered. Due to limitations in solubility higher doses than 1 mg/kg were not given. Dofetilide was given at the clinically used dose for acute cardioversion (10 µg/kg), with saline as a vehicle over an i.v. infusion period of 5 min.

2.6. Statistical analysis
Data were presented as means±SEM. The longest ERP at each pacing rate after drug administration was taken and expressed as absolute or percent increase from vehicle control. Interatrial dispersion of the ERP was calculated as the difference between the left and right atrial ERP. One- or two-way ANOVA for repeated measures followed by Student's t-test was used for the calculation of statistically significant differences between left and right atrial ERP prolongations at the three basic cycle lengths, the inhibition of LAV, MAP duration and the difference between right and left atrial ERP (dispersion of the ERP). A value of p<0.05 was accepted as significant.


   3. Results
Top
 Abstract
 1. Introduction
 2. Material and methods
 3. Results
4. Discussion
 5. Conclusion
 References
 
3.1. Basal atrial ERPs
ERPs determined at 400, 300 and 240 ms BCL were clearly shorter in the left vs. right atrium (113±3, 106±4 and 99±3 ms vs. 149±5, 142±4 and 133±4 ms, respectively; p<0.001; n = 21) as previously reported [9]. Original registrations in Fig. 1 show this difference with longer right atrial MAPs.


Figure 1
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  		Fig. 1 MAPs from the free walls of left (LA) and right atrium (RA) at sinus rhythm in an anaesthetized pig. The right atrial MAP is longer than the left atrial MP.

 
3.2. Effect of drugs on atrial ERPs
Vehicle had no effect on atrial refractoriness. AVE0118 caused a dose-dependent rise in atrial ERPs in both atria. Baseline ERP and ERP after 0.3 (n = 3), 0.5 (n = 5) and 1 mg/kg (n = 5) for the three BCLs of the left atrium are shown in Fig. 2. Table 1 shows the absolute values before and after different doses of AVE0118. Between 240 and 400 ms BCL there is a similar increase in the ERP with no evidence for a rate-dependent effect. For better comparison of the effects on the left and right atrium, the mean increase in milliseconds of the three BCLs at doses of 0.1 to 1 mg/kg i.v. was calculated and shown in Fig. 3. Although there was only a trend for stronger left compared to right absolute ERP prolongations at 0.5 and 1 mg/kg the relative increases (in percent of baseline ERP) in the left atrial ERP were significantly stronger (p<0.05) because the baseline left atrial ERP was much shorter. Dofetilide, 10 µg/kg (n = 6), like other IKr blockers [8], had a significantly stronger effect (P<0.05) on right than on left atrial ERP (Fig. 4). A trend for a negative rate-dependent effect can be seen in the right atrium.


Figure 2
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  		Fig. 2 Effect of AVE0118 on left atrial ERP in anesthetized pigs at 240, 300 and 400 ms BCL. Differences are significant for all values. Open symbols show baseline values, closed symbols values after AVE0118, 0.3 (circles, n = 3), 0.5 (squares, n = 5), 1 mg/kg (triangles, n = 5).

 

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  		Table 1 Absolute ERP values at three BCL before and after different doses of AVE0118 in pig left and right atrium

 

Figure 3
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  		Fig. 3 Comparison of the effect of AVE0118 at rising doses on left and right atrial ERP. Filled columns (left atrium); open columns (right atrium). Each column shows the means of the increases at 240, 300 and 400 ms BCL (n = 3–5 pigs). Upper panels show the absolute prolongation of atrial ERP in milliseconds, lower panel gives the percent increase from baseline. *p<0.05 for difference of left vs. right atrial ERP increase.

 

Figure 4
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  		Fig. 4 Effect of dofetilide, 10 µg/kg on left and right atrial ERP in pigs at 240, 300 and 400 ms BCL (n = 6). Left atrium filled bars, right atrium open bars. Upper panel gives absolute ERP prolongation in milliseconds, lower panel shows percent increase of ERP from baseline. Dofetilide has a significantly stronger effect on the right compared to left atrium (*p<0.05; n = 6).

 
Since dofetilide had a stronger effect on the right atrium, which already had a much longer ERP at baseline compared with the left atrium as shown above, the difference between left and right atrial ERP increased significantly (p<0.05), while after AVE0118 this difference tended to decrease (Table 2).


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  		Table 2 Effect of AVE0118 and dofetilide on the interatrial difference of the atrial ERP (ms) calculated as the difference between right and left atrial ERP at three different BCLs (240, 300 and 400 ms)

 
3.3. Left atrial vulnerability
As previously reported [8,9] in pigs the extrastimulus S2 following the regular 10 S1 conditioning stimuli during the S1–S2–ERP measurement procedure frequently elicits mostly nonsustained and brief runs of atrial fibrillation/flutter as shown in an original registration (Fig. 5). Fig. 6 shows the dose-dependent inhibition of these arrhythmias by AVE0118 in comparison to dofetilide, 10 µg/kg. At 0.5 and 1 mg/kg AVE0118 abolished the arrhythmias. The incidence of S2-induced left atrial tachyarrhythmias was 94±6%, 85±10%, 60±19% and 85±10% before AVE0118 and 33±32%, 11±11%, 0±0% and 0±0% at 0.1, 0.3, 0.5 and 1.0 mg/kg, respectively. The selective IKr blocker dofetilide did not significantly inhibit the incidence of atrial tachyarrhythmias induced by the S2-extrastimulus (64±15% before vs. 56±14% after drug corresponding to 14% inhibition; n = 6).


Figure 5
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  		Fig. 5 Typical example of the left atrial arrhythmias that are elicited by a premature beat (S2) during the S1–S2–ERP measurement procedure (LAV). Upper panel: S2 after 10 conditioning S1 stimuli (black line) induces a run of nonsustained tachycardia (dashed line). Lower panel shows the absence of S2-induced arrhythmias after AVE0118, 0.5 mg/kg i.v.

 

Figure 6
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  		Fig. 6 Inhibition of LAV in pigs at different doses of AVE0118 in comparison with dofetilide. Columns show % inhibition of LAV. *p<0.05; n = 3 for 0.1 and 0.3 mg/kg and n = 5 for 0.5 and 1 mg/kg AVE0118; n = 6 for dofetilide.

 
3.4. Effect of drugs on left atrial monophasic action potential (MAP)
AVE0118 (1 mg/kg; n = 7) and dofetilide (10 µg/kg; n = 6) both prolonged the left atrial MAP. Typical original registrations for AVE0118 and dofetilide are shown in Fig 7. Fig. 8 shows the MAP prolongation at different degrees of repolarization 6 min after drug injection where the maximum effect was reached. AVE0118 started to prolong the MAP already at 10% of repolarization and its effect was already maximal at 40% repolarization. At 80% and 90% repolarization the prolongation induced by AVE0118 tended to decrease again (Figs. 7 and 8Go). In contrast, after dofetilide the first significant prolongation was observed at 60% repolarization and the prolongation reached its maximum at 90% of repolarization.


Figure 7
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  		Fig. 7 Typical original registrations of left atrial MAPs before and after administration of AVE0118 (superimposed), 1 mg/kg (upper panel) and dofetilide, 10 µg/kg (lower panel). Shorter MAPs in each panel are baseline MAPs.

 

Figure 8
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  		Fig. 8 Increase in MAP duration in milliseconds by AVE0118, 1 mg/kg (n = 7) and dofetilide 10 µg/kg (n = 6) at different degrees of repolarization. AVE0118 (close squares); dofetilide (open circles). Means±SEM. *p<0.05 for the first value that reached statistical significance.

 
3.5. Effect of drugs on ventricular repolarization
AVE0118 did not prolong the QT or QTc interval indicating that it is devoid of a meaningful effect on ventricular repolarization. In the pigs, in which atrial ERP measurements were performed, QTc was 426±15 ms before and 423±12 ms after 1 mg/kg AVE0118 10 min after injection. Dofetilide, 10 µg/kg, showed the expected prolongation of the QTc-interval (16.9% increase; from 456±15 ms to 533±12 ms; p<0.001) as previously reported by us [8,9].

In the separate group of pigs, in which the effect on ventricular ERP and MAP duration and QT was studied (n = 5), repolarization was not changed by the vehicle or AVE0118, 1 mg/kg. MAPD90 and QT at 600 ms BCL were 305±11 and 352±12 ms before and 311±11 and 352±9 ms after vehicle, respectively. MAPD90 and QT at 600 ms BCL were 300±12 and 348±15 before and 303±13 and 355±17 ms after 1 mg/kg AVE0118, respectively. At the three BCLs tested, 333, 400 and 500 ms, the left ventricular epicardial ERPs were 219±12, 235±14 and 254±14 ms before and 221±12, 237±11 and 251±11 ms after AVE0118, respectively. Vehicle had no effect on ERP. During the measurements with AVE0118 and vehicle, local capture (MAP potential on the left ventricular epicardium) was always followed by a left ventricular contraction as judged by the left ventricular pressure curve and the QRS complex.


   4. Discussion
Top
 Abstract
 1. Introduction
 2. Material and methods
 3. Results
 4. Discussion
5. Conclusion
 References
 
Our results in anesthetized pigs show that AVE0118 is a potent and atrium selective antiarrhythmic compound with no apparent effect on ventricular repolarization. It prolongs atrial refractoriness dose-dependently and abolishes the mostly nonsustained runs of atrial fibrillation/flutter induced by a premature extrastimulus (S2), defined as LAV. The high atrial antiarrhythmic efficacy occurs in the absence of a noticeable effect on ventricular repolarization as indicated by measurement of the QT-interval, left ventricular MAP duration at 90% repolarization and left ventricular ERP. The data are in agreement with in vitro data [7]. In different in vitro tests AVE0118 inhibited Ito, IKur and IKACh to a relevant extent but not IKr or IK1. Since AVE0118 did not prolong ventricular repolarization at strong atrial ERP prolongation it is expected to be devoid of the typical ventricular proarrhythmic effects of IKr-blockers (early afterdepolarizations leading to torsades de pointes arrhythmias). The selective IKr blocker dofetilide (10 µg/kg) prolonged the QTc interval to a similar extent as (right) atrial ERP. It is of scientific interest to note that Ito blockade by AVE0118 obviously does not prolong ventricular repolarization in pigs.

Previously we have compared IKr blockers with two other novel IKur blockers and we have already shown superior atrial actions of the latter with no effect on the QT-interval [8]. The advantage of AVE0118 over those previously published IKur blockers is that AVE0118 is suited for clinical development and meanwhile in phase I clinical studies.

The MAP technique delivers potentials of sufficient quality for an evaluation of the time course of repolarization (not for voltage) in the ventricles, but atrial MAPs were of lower quality most likely because the critical pressure for depolarizing the area underneath the electrode tip is more difficult to maintain in the thin atrial wall with the low atrial pressure. The contact pressure proved to be of critical importance, and we finally were able to record high-quality atrial MAPs. It could be shown that both dofetilide and AVE0118 prolonged the left atrial MAP, but AVE0118 acted in a completely different manner to reshape the action potential. AVE0118 started to prolong the left atrial MAP already significantly at 10% of repolarization and its effect was already maximal at 40% repolarization. At 80% and 90% repolarization the prolongation induced by AVE0118 tended to decrease again. This might be explained by a compensatory activation of IKr after inhibition of IKur as suggested by a mathematical model for the human atrial action potential [11]. In contrast, after dofetilide the first significant increase was at 60% repolarization and the effect was maximal at 90% of repolarization. The differences between the drugs clearly reflect their ion channel target profile; AVE0118 inhibits the rapidly activating currents Ito and IKur while dofetilide inhibits the IKr, which activates more slowly during the action potential.

As previously observed with two IKur blockers in pigs, AVE0118 had a stronger effect on left atrial ERP, while dofetilide, as previously reported for azimilide, ibutilide and sotalol, showed the opposite profile and prolonged right atrial ERP stronger than left atrial ERP [8]. Although the difference in absolute prolongation between the left and right atrium was not significant with AVE0118 the percent increases were significant, because baseline left atrial ERP was much shorter. AVE0118, like the two IKur blockers published previously [8], showed strong atrial antiarrhythmic efficacy (inhibition of LAV) even in the low dose range. Prolongation of left atrial ERP by dofetilide to a similar extent was nearly without effect on LAV. We have attributed the weak antiarrhythmic effect of IKr blockade to the fact that the dispersion of the ERP between the left and right atrium increased after IKr blockade [8,9]. IKr blockade had a stronger effect on the right atrial ERP, which was already longer at baseline compared to the left atrial ERP. AVE0118 demonstrated strong and unique efficacy against LAV at even modest increases of the S1 ERP compared to the low or modest efficacy of all other antiarrhythmic drugs tested so far [8,9]. This suggests that at least one of the currents inhibited by AVE0118 becomes strongly arrhythmogenic in the situation of a premature beat and is involved in the development of LAV. This is linked to the question why AVE0118 is so effective whereas IKr blockade is less effective against this type of arrhythmia. The altered repolarization as a consequence of a shortened diastolic interval is referred to as restitution. It reflects the time-dependent kinetics of the membrane currents [12]. As pointed out premature excitation (via the S2 stimulus) frequently induces left atrial tachyarrhythmias in the pig. Whether a tachyarrhythmia follows the S2-induced action potential may not depend on the duration and refractoriness of the S1-induced action potential with its relatively large preceding diastolic interval, but on the duration of the S2 induced action potential, which has a very brief preceding diastolic interval. Attempts to determine the refractoriness of the S2-induced action potential via a further S3 stimulus failed because of the strong arrhythmogenic effect of the procedure. The slow inactivation of the IKur [6] may play an important role. It can be assumed that in a premature excitation the IKur is increased because of its slow inactivation. This may lead to early repolarization and, consequently, to reentrant arrhythmias. Blockade of IKur would then prevent early repolarization explaining the high efficacy against atrial vulnerability. In addition, an accelerated repolarization of a premature action potential would diminish the contribution of the IKr because its major contribution to repolarization is later in the action potential. This is reflected in the late effect of the selective IKr blocker dofetilide on atrial MAP in the pig (Figs. 6 and 7)Go and might explain the weak efficacy of IKr blockade. Taken together, the contribution of IKur to repolarization in a premature action potential may increase at the expense of the contribution of IKr. High efficacy of IKur blockade and low efficacy of IKr blockade against the arrhythmogenic effect of a premature beat seem to be linked.

The interatrial differences demonstrated in our pigs—shorter left atrial ERPs and action potentials, stronger left atrial ERP prolongation by Ito/IKur blockade, existence of a high LAV (and its absence in the right atrium) and its nearly exclusive inhibition by Ito/IKur blockers—point to interatrial differences in the expression or function of the atrial ion channels. A unifying explanation could be a higher IKur and/or Ito vs. IKr ratio in the pig left atrium. Heterogeneity of restitution kinetics as a consequence of a stronger left atrial IKur with its slow inactivation may strongly alter the spatial pattern of repolarization during premature stimulation. This would further increase the preexisting baseline interatrial ERP differences (as measured at 240, 300 and 400 ms BCL), and thereby explain the high extent of S2-induced left atrial arrhythmias.

Expression of Kv1.5 mRNA and protein are only one part of the complex machinery that finally results in in vivo function and spatial distribution of IKur. β-subunit composition and autonomic regulation are additional factors [2,3], and it is unclear at present why there is ventricular expression of Kv1.5 without function (no measurable ventricular IKur) [13]. On that background a molecular approach to answer the question of interatrial differences in IKur would have been beyond the scope of this study.

An alternative explanation for interatrial differences in ion channels is a shorter left atrial calcium current. It would also lead to shorter left atrial action potentials and to stronger left atrial action potential prolongation by IKur blockade (according to a mathematical model of the human atrial action potential for the calculation of the prolongation conferred by IKur blockade in case of a reduced calcium current Ref. [11]).

The concept of left atrial arrhythmias induced by a premature stimulus (S2) seems to be of particular relevance since in human atrial fibrillation the left atrium receiving ectopic activity mainly from the pulmonary veins is thought to play an important role in the initiation of this tachyarrhythmia [14,15]. Therefore it is important to find out how the left atrium responds to premature stimulation and whether a drug is able to inhibit its vulnerability. Although our LAV model tests whether an artificial premature beat is able to elicit mostly nonsustained atrial fibrillation/flutter, it is easy to imagine that frequent ectopic beats could cause a nearly sustained atrial tachyarrhythmia that would give rise to atrial tachycardic electrical remodeling [16,17]. This would result in a further shortening of the action potential mainly through downregulation of the calcium current and induce persistent atrial fibrillation. Apart from downregulation of the calcium current, changes in K+ channels have also been found in chronic atrial fibrillation as a consequence of electrical remodeling ([18] for review). Ito seems to be diminished [17]; transcription of Kv4.3 decreased [19]. Concerning Kv1.5 mRNA, protein, or IKur conflicting results were published; decreases [20,21] or no changes [19,22] were reported. A possible limitation of this study with AVE0118 is that ERP was measured in anesthetized pigs in sinus rhythm (in the non-remodeled state). Assuming only decreases in Ito and IKur in chronic atrial fibrillation (not other ion currents) one could expect that blockade of these channels would less effectively translate into ERP prolongation in chronic atrial fibrillation compared to sinus rhythm. However, the overall relevance of the respective ion conductances in the remodeled state still depends on their fractional contribution to the whole cell conductance. Taking this into account Ito and IKur blockade in the remodeled atrium could be even more effective on ERP [11]. In fact, this could be observed in the remodeled atrium of chronically fibrillating goats [23]. AVE0118 showed an even stronger effect on the atrial ERP compared with sinus rhythm. The preserved or even stronger effect of AVE0118 in atrial fibrillation is in clear contrast to IKr blockers, which, in the remodeled state, lose most of the ERP-prolonging effect they have in sinus rhythm [23].


   5. Conclusion
Top
 Abstract
 1. Introduction
 2. Material and methods
 3. Results
 4. Discussion
 5. Conclusion
References
 
In pigs AVE0118 prolonged the atrial ERP and showed strong atrial antiarrhythmic efficacy with no apparent effect on ventricular repolarization. Apart from the lack of effect on ventricular repolarization, the atrial efficacy profile of AVE0118 is remarkably different from that of IKr blockade. Based on these properties AVE0118 is a promising new type of atrial selective antiarrhythmic drug.


   Notes
 
Time for primary review 14 days


   References
Top
 Abstract
 1. Introduction
 2. Material and methods
 3. Results
 4. Discussion
 5. Conclusion
 References
 

  1. Torp-Pedersen C., Moller M., Bloch-Thomsen P.E., et al. Dofetilide in patients with congestive heart failure and left ventricular dysfunction. N. Engl. J. Med. (1999) 341:857–865.[Abstract/Free Full Text]
  2. Wang Z., Fermini B., Nattel S. Sustained depolarization-induced outward current in human atrial myocytes: evidence for a novel delayed rectifier K+ current similar to Kv1.5 cloned channel currents. Circ. Res. (1993) 73:1061–1076.[Abstract/Free Full Text]
  3. Li G.R., Feng J., Wang Z., Fermini B., Nattel S. Adrenergic modulation of ultrarapid delayed rectifier K+ current in human atrial myocytes. Circ. Res. (1996) 78:903–915.[Abstract/Free Full Text]
  4. Fedida D., Wible B., Wang Z., et al. Identity of a novel delayed rectifier current form human heart with a cloned K+ channel current. Circ. Res. (1993) 73:210–216.[Abstract]
  5. Feng J., Wible B., Li G.R., Wang Z., Nattel S. Antisense oligodeoxynucleotides directed against Kv1.5 mRNA specifically inhibit ultrarapid delayed rectifier K+ current in cultured adult human atrial myocytes. Circ. Res. (1997) 80:572–579.[Abstract/Free Full Text]
  6. Feng J., Xu D., Wang Z., Nattel S. Ultrarapid delayed rectifier current inactivation in human atrial myocytes: properties and consequences. Am. J. Physiol. (1998) 275:H1717–H1725.[Web of Science][Medline]
  7. Goegelein H., Brendel J., Wirth K., et al. AVE0118: a novel inhibitor of atrial K+ channels in atrial myocytes and in CHO cells. Naunyn-Schmiedeberg's Arch. Pharmacol. (2003) 367(Suppl. 1):R68. [abs. 259].[CrossRef]
  8. Knobloch K., Brendel J., Peukert S., Rosenstein B., Busch A.E., Wirth K.J. Electrophysiological and antiarrhythmic effects of the novel IKur channel blockers, S9947 and S20951, on left vs. right pig atrium in vivo in comparison with the IKr blockers dofetilide, azimilide, D,L-sotalol and ibutilide. Naunyn-Schmiedeberg's Arch. Pharmacol. (2002) 366:482–487.[CrossRef][Web of Science][Medline]
  9. Wirth K.J., Knobloch K. Differential effects of dofetilide, amiodarone, and class Ic drugs on left and right atrial refractoriness and left atrial vulnerability in pigs. Naunyn-Schmiedeberg's Arch. Pharmacol. (2001) 363:166–174.[CrossRef][Web of Science][Medline]
  10. Yuan S., Blomstrom-Lundqvist C., Olsson S.B. Monophasic action potentials: concepts to practical applications. J. Cardiovasc. Electrophysiol. (1994) 5:287–308.[Web of Science][Medline]
  11. Courtemanche M., Ramirez R.J., Nattel S. Ionic targets for drug therapy and atrial fibrillation-induced electrical remodeling: insights from a mathematical model. Cardiovasc. Res. (1999) 42:477–489.[Abstract/Free Full Text]
  12. Laurita K.R., Pastore J.M., Rosenbaum D.S. Cardiac electrophysiology. From cell to bedside, 3rd ed. Zipes D.P., Jalife J., eds. (2000) Philadelphia: W.B. Saunders. 239–248.
  13. Mays D.J., Foose J.M., Philipson L.H., Tamkun M.M. Localization of the Kv1.5 K+ channel protein in explanted cardiac tissue. J. Clin. Invest. (1995) 96:282–292.[Web of Science][Medline]
  14. Bennet M.A., Pentecost B.L. The pattern of onset and spontaneous cessation of atrial fibrillation in man. Circulation (1970) 41:981–988.[Abstract/Free Full Text]
  15. Haissaguerre M., Jais P., Shah D.C., et al. Spontaneous initiation of atrial fibrillation by ectopic beats origination in the pulmonary veins. N. Engl. J. Med. (1998) 339:659–665.[Abstract/Free Full Text]
  16. Wijffels M.C., Kirchhof C.J., Dorland R., Power J., Allessie M.A. Electrical remodeling due to atrial fibrillation in chronically instrumented conscious goats: roles of neurohumoral changes, ischemia, atrial stretch, and high rate of electrical activation. Circulation (1997) 96:3710–3720.[Abstract/Free Full Text]
  17. Yue L., Melnyk P., Gaspo R., Wang Z., Nattel S. Molecular mechanisms underlying ionic remodeling in a dog model of atrial fibrillation. Circ. Res. (1997) 361:776–784.
  18. Bosch R.F., Nattel S. Cellular electrophysiology of atrial fibrillation. Cardiovasc. Res. (2002) 54:259–269.[Free Full Text]
  19. Grammer J.B., Bosch R.F., Kühlkamp V., Seipel L. Molecular remodelling of Kv4.3 potassium channels in human atrial fibrillation. J. Cardiovasc. Electrophysiol. (2000) 11:626–633.[Web of Science][Medline]
  20. Van Wagoner D.R., Pond A.L., McCarthy P.M., Trimmer J.S., Nerbonne J.M. Outward K+ current and densities and Kv1.5 expression are reduced in chronic human atrial fibrillation. Circ. Res. (1997) 80:772–781.[Abstract/Free Full Text]
  21. Brundel B.J.J.M., Van Gelder I.C., Henning R.H., et al. Ion channel remodeling is related to intraoperative atrial effective refractory periods in patients with paroxysmal and persistent atrial fibrillation. Circulation (2001) 103:684–690.[Abstract/Free Full Text]
  22. Bosch R.F., Zeng X.R., Grammer J.B., Popovic K., Mewis C., Kühlkamp V. Ionic mechanisms of electrical remodeling in human atrial fibrillation. Cardiovasc. Res. (1999) 44:121–131.[Abstract/Free Full Text]
  23. Blaauw Y., Tieleman R.G., Allessie M. Acute reversal of atrial electrical remodeling by a IKur/Ito-blocker (AVE0118) in the goat. Circulation (2002) 106:II-155.

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