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(Circulation. 2004;110:2299-2306.)
© 2004 American Heart Association, Inc.

Arrhythmia/Electrophysiology

Role of I_Kur in Controlling Action Potential Shape and Contractility in the Human Atrium

Influence of Chronic Atrial Fibrillation

Erich Wettwer, PhD^*; Ottó Hála, PhD^*; Torsten Christ, MD; Jürgen F. Heubach, MD; Dobromir Dobrev, MD, PhD; Michael Knaut, MD; András Varró, MD; Ursula Ravens, MD, PhD

From the Department of Pharmacology and Toxicology, Dresden University of Technology (E.W., T.C., J.F.H., D.D., U.R.), and the Cardiovascular Center (M.K.), Dresden, Germany; and the Department of Pharmacology and Pharmacotherapy, Medical University of Szeged, Szeged, Hungary (O.H., A.V.).

Correspondence to Dr Erich Wettwer, Fetscherstraße 74, 01307 Dresden, Germany. E-mail wettwer{at}rcs.urz.tu-dresden.de

Received November 4, 2003; de novo received April 5, 2004; revision received May 17, 2004; accepted May 21, 2004.

	Abstract

Top Abstract Introduction Methods Results Discussion Conclusions References

 
Background— The ultrarapid outward current I_Kur is a major repolarizing current in human atrium and a potential target for treating atrial arrhythmias. The effects of selective block of I_Kur by low concentrations of 4-aminopyridine or the biphenyl derivative AVE 0118 were investigated on right atrial action potentials (APs) in trabeculae from patients in sinus rhythm (SR) or chronic atrial fibrillation (AF).

Methods and Results— AP duration at 90% repolarization (APD₉₀) was shorter in AF than in SR (300±16 ms, n=6, versus 414±10 ms, n=15), whereas APD₂₀ was longer (35±9 ms in AF versus 5±2 ms in SR, P<0.05). 4-Aminopyridine (5 µmol/L) elevated the plateau to more positive potentials from –21±3 to –6±3 mV in SR and 0±3 to +12±3 mV in AF. 4-Aminopyridine reversibly shortened APD₉₀ from 414±10 to 350±10 ms in SR but prolonged APD₉₀ from 300±16 to 320±13 ms in AF. Similar results were obtained with AVE 0118 (6 µmol/L). Computer simulations of I_Kur block in human atrial APs predicted secondary increases in I_Ca,L and in the outward rectifiers I_Kr and I_Ks, with smaller changes in AF than SR. The indirect increase in I_Ca,L was supported by a positive inotropic effect of 4-aminopyridine without direct effects on I_Ca,L in atrial but not ventricular preparations. In accordance with the model predictions, block of I_Kr with E-4031 converted APD shortening effects of I_Kur block in SR into AP prolongation.

Conclusions— Whether inhibition of I_Kur prolongs or shortens APD depends on the disease status of the atria and is determined by the level of electrical remodeling.

Key Words: ion channels • potassium channel blockers • contraction • action potentials • fibrillation

	Introduction

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The cardiac repolarization process is regulated by several outward currents, of which the ultrarapid delayed rectifier potassium current (I_Kur) is thought to play a major role. This current is absent in the ventricles and hence represents a suitable target for selectively modulating action potentials (APs) in the atria.^1–3 The selective blocker of I_Kur 4-aminopyridine in low micromolar concentrations is expected to prolong the AP duration (APD). However, experimental results in human atrial preparations from patients with sinus rhythm (SR) are inconsistent with shortening of APD in multicellular trabeculae⁴ and prolongation of APD in isolated atrial myocytes.¹ Although block of I_Kur and the subsequent prolongation of the APD are expected to be beneficial in chronic atrial fibrillation (AF), the experimental proof is still lacking. Furthermore, AF itself induces shortening of APD and effective refractory period, known as electrical remodeling, that is associated with changes in expression and activity of the involved ion channels.⁵ Human I_Kur was found to be reduced by 50%, whereas other groups reported no changes.⁵

To predict the AP alterations associated with I_Kur block during chronic AF, Courtemanche et al² simulated these changes in a model of human APs in AF and demonstrated that the reduction of I_Kur was associated with prolongation of APD. However, this article only predicted the changes without experimental verification.

Here, we studied the effects of I_Kur block with 4-aminopyridine and with the new I_Kur blocker, the biphenyl derivative AVE 0118, on APs in trabeculae from SR and AF patients. The impact of I_Kur block on other atrial currents was predicted by simulating APs in a modified Luo-Rudy model, and the predictions were tested experimentally by recording APs or force of contraction (F_c) in trabeculae or by measuring membrane currents in isolated atrial myocytes.

	Methods

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Human Tissue Samples and Patient Characteristics
The study was approved by the ethics committee of the Dresden University of Technology (No. EK790799). Each patient gave written, informed consent.

Right atrial appendages were obtained from 34 patients with SR and 18 patients with chronic AF (AF >6 months, Table 1). Significant differences between the 2 groups were found for underlying heart disease and pulmonary hypertension. AF patients more often received digitalis and diuretics, and ß-blockers and lipid-lowering drugs were prescribed more frequently in SR patients. Ventricular trabeculae (n=10) originated from left and right ventricles from 2 explanted end-stage failing hearts.

View this table: [in this window] [in a new window]   TABLE 1. Characteristics of Patients in SR and Chronic AF

Electrophysiological Recordings
Atrial APs were recorded with conventional intracellular microelectrodes in right atrial trabeculae (microelectrode tip resistance, 10 to 25 M; 1-ms stimulus, 25% above threshold intensity). The bath solution contained (in mmol/L): NaCl 127, KCl 4.5, MgCl₂ 1.5, CaCl₂ 1.8, glucose 10, NaHCO₃ 22, and NaH₂PO₄ 0.42, equilibrated with O₂/CO₂ (95:5) at 35±2°C, pH 7.4. A computer-aided system (developed at the University of Szeged, Hungary) was used for generating stimuli and data acquisition of various AP parameters. After equilibration for 1 hour, control APs were recorded for 20 minutes before 4-aminopyridine or AVE 0118 was cumulatively added, with 20 minutes of recording between concentration increments.

Human myocytes were isolated from atrial appendages as previously described.⁶ I_Ca,L and I_K1 were measured with conventional voltage-clamp technique as previously described.^6,7 Mean cell capacitance was 101±6.5 pF (n=29).

Measurement of F_c
Pairs of right atrial or ventricular trabeculae were mounted in organ baths filled with 50 mL of buffer (composition in mmol/L: NaCl 126.7, KCl 5.4, CaCl₂ 1.8, MgCl₂ 1.05, NaHCO₃ 22.0, NaH₂PO₄ 0.42, EDTA 0.04, and glucose 5.0, equilibrated with O₂/CO₂ [95:5] at 37°C, pH 7.4).⁸ The bath solution contained 200 nmol/L (–)-propranolol to exclude effects mediated through ß-adrenoceptors. The preparations were paced (0.5 Hz, 5-ms stimulus, 10% above threshold intensity) and were stretched to 80% of the length associated with maximum developed force. After an equilibration period of 30 minutes, the effects of 4-aminopyridine or AVE 0118 on F_c were determined by cumulatively increasing concentrations every 20 minutes.

Action Potential Simulations
For simulating the shapes of human atrial APs, we modified mathematical models described in the literature.^9–11 The incorporated membrane currents and transporters and factors controlling intracellular ion concentrations are listed in the Appendix (see online-only Data Supplement), which also contains specific kinetic parameters, constants of ion currents, and the equations used for current calculations. Maximum current conductances of I_Ca,L, I_to,f, I_Kur, I_K1, and I_NaCaX were adapted to simulate the characteristic AP shapes. For AP simulation in chronic AF, original current data were taken from the literature and, when not available, amplitudes were extrapolated from changes in channel expression.

As an approximation, APs were assumed to be spatially uniform, ie, conduction within the preparation was neglected. Changes of membrane potential were calculated for space-clamp conditions as follows: d(V_m)/dt = –(I_ion,total+I_stim)/C_m, with I_ion,total = I_Na+I_Ca,T+ I_Ca,L+I_to,f+I_Kur+I_Kr+I_Ks+I_K1+I_K,Ach+I_K,Ca+I_b,Na+I_b,Ca+I_b,Cl+ I_Na,Kpump+ I_NaCaE and I_stim = stimulus current of –90 µA/µF applied 0.5 ms at beginning of each cycle, and C_m = membrane capacitance (specific C_m assumed to be 1 µF/cm²).

Numerical integration of d(V_m)/dt was performed according to a modified Euler method suggested by Rush and Larsen.¹² The length of the time steps was varied between 0.02 and 20 ms, depending on the extent of resultant voltage changes. Momentary values of gating variables were calculated from analytical solution of the related first-order differential equations^9–11,13: x(V_m, t_j) = x_SS(V_m)–[x_SS(V_m)– x_SS(V_m, t_i)] x exp[–dt_i/(V_m)], with dt_i = ith timestep (ie, t_j = t_i+dt_i) and x_SS (V_m) = voltage-dependent steady-state value, (V_m) = voltage-dependent time constant.

Simulated APs, currents, and ionic concentrations were allowed to stabilize for at least 200 cycles. The model was implemented in Turbo Pascal 6.0. For further details of the model, see Appendix.

Statistical Analysis
Differences between continuous data were compared by unpaired Student t test or 1-way ANOVA. Frequency data were analyzed with ² statistics. Data are mean±SEM. A value of P<0.05 was considered statistically significant.

	Results

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Effect of 4-Aminopyridine on AP Configuration in SR and AF
Resting membrane potentials were –75±1 mV in SR (n=15) and significantly more negative in AF (–80±3 mV, n=6, P<0.05). In accordance with our previous results,¹⁴ APD at 20% of repolarization (APD₂₀) was shorter in SR (5±2 ms) than in AF (35±9 ms, P<0.05), whereas APD₉₀ was longer in SR (414±10 ms) compared with AF (300±16 ms, P<0.05).

In the presence of 4-aminopyridine (5 µmol/L), the potentials of the notch and dome (SR) and of the plateau shoulder (AF) were shifted to more positive values, leading to a significant increase in APD₂₀ from 5±2 to 12±5 ms in SR (P=0.05) and from 35±9 to 78±5 ms in AF (P=0.002, Figure 1, A and B). The final repolarization phase was accelerated in both groups; however, APD₉₀ was shortened from 414±10 to 350±10 ms in SR (P<0.0001) but was prolonged from 300±16 to 320±13 ms in AF (P=0.006; Figure 1, C and D), suggesting that the directions of APD₉₀ changes are different in SR and AF. The effects of 4-aminopyridine were reversible on washout.

View larger version (31K): [in this window] [in a new window]   Figure 1. Effect of 4-aminopyridine (5 µmol/L) on AP shapes recorded in atrial trabeculae from patients in SR and AF. A and B, Original registrations of predrug control (C) and after 20 minutes of exposure to 4-aminopyridine (4-AP). Gray shaded area underlying plateau phase indicates time window of 20 to 80 ms after AP upstroke, during which plateau potential is averaged. C and D, APD20 and APD90 before (hatched columns) and after (black columns) exposure to 5 µmol/L 4-aminopyridine, *P<0.01. E and F, Plateau potential before (hatched columns) and after (filled columns) exposure to 5 µmol/L 4-aminopyridine. #P<0.01 vs control in SR and AF; *P<0.01 vs control in SR.

In SR preparations, the mean notch potential, ie, the minimum potential after the initial rapid repolarization phase, was shifted by 4-aminopyridine (5 µmol/L) from –32±4 to –22±5 mV (P<0.003), and the mean dome potential, ie, the most positive plateau potential before final repolarization, was shifted from –28±4 to –13±6 mV (P=0.001). Because APs in AF did not exhibit the typical notch as in SR, we introduced the parameter "plateau potential" for analysis of changes in this potential range. Plateau potential is defined as the mean amplitude within the time window of 20 to 80 ms after the upstroke of the AP (Figure 1, A and B). In SR, the plateau potential was shifted by 4-aminopyridine (5 µmol/L) from –21±3 to –6±3 mV (P<0.0001, Figure 1E). In AF, the plateau potential was more positive (0±3 mV, P=0.0001 versus SR) and was shifted by 4-aminopyridine to +12±3 mV (P=0.0012, Figure 1F).

The effect of 4-aminopyridine on the shift in notch-and-dome potentials was concentration-dependent (0.3 to 100 µmol/L, Figure 2, A and B). In SR, the notch potential became less negative and shifted from –23±2 to –4±2 mV with 100 µmol/L 4-aminopyridine (Figure 2, A and C, n=5, P<0.001), and the respective dome potential was changed from –18±1 to 0±2 mV (P<0.001). Nonlinear curve fitting resulted in EC₅₀ values of 15 µmol/L 4-aminopyridine for elevation of notch potential and 14 µmol/L for elevation of dome potential (Figure 2C). In AF, the effect of 4-aminopyridine on the plateau potential was also concentration-dependent, with an EC₅₀ value of 28 µmol/L in 1 experiment (Figure 2, B and D).

View larger version (21K): [in this window] [in a new window]   Figure 2. Concentration-dependence of 4-aminopyridine effect on notch-and-dome potential in trabeculae from SR patients and on plateau potential in AF patients. A, APs recorded with increasing concentrations of 4-aminopyridine in SR. B, Original recordings of APs in AF. C, Concentration-response curve in SR. Concentration-response curve was fitted to mean data values assuming bottom and top values of –22 and –3 mV and –18 and +2 mV for notch and dome potential, respectively (EC50=15 µmol/L for elevation of notch potential; EC50=14 µmol/L for elevation of dome potential, n=5). D, Concentration-response curve for 4-aminopyridine effect on plateau potential for same AF preparation as in B (EC50=28 µmol/L; assumed basal and top values were +7.8 and +15.6 mV, respectively).

Model Simulations of APs in SR and AF: Block of I_Kur
Inserting experimentally estimated parameters into the mathematical model of human atrial APs (see Appendix) provided the spike-and-dome shape typical for tissue from SR patients (compare Figures 1A and 3 ). Selective inhibition of I_Kur by 4-aminopyridine was simulated by setting the current to 20% of its regular value in SR (see Table 2). The model reproduced the observed shifts of notch-and-dome potentials to more positive values (Figure 3) and shortened APD₉₀. Block of I_Kur induced a series of indirect effects on other currents because of their voltage and time dependence during the course of a free-running action potential. The model predicted peak I_Ca,L to increase by 37% at the elevated plateau potential (Figure 3) and increases in the outward potassium currents I_Kr and I_Ks by 80% and 50%, respectively.

View larger version (18K): [in this window] [in a new window]   Figure 3. Simulation of SR (left) and AF (right) atrial APs and underlying membrane currents after block of IKur. SR (left, from top to bottom): simulated AP under control conditions (gmax of IKur, 100%, solid line) and after reducing IKur to 20% (dashed line). Reducing IKur is accompanied by an elevation of plateau and by AP shortening. Time courses of simulated ICa, IKr, IKs, Ito,f, IKur (IKur 100%, solid line, IKur 20%, dashed line), and IK1. Reduction of IKur induces a significant increase in ICa, IKr, and IKs amplitudes (see text). AF (right, from top to bottom): AF AP was simulated by changing gmax of contributing currents as listed in Table 2. Simulated AP under control conditions (solid line, gmax of IKur is 100% of AF and 48% of gmax in SR) and after reducing IKur to 20% (dashed line, gmax of IKur is 9.6% of gmax in SR). Reducing IKur is accompanied by an elevation of plateau and AP lengthening. Time courses of simulated ICa, IKr, IKs, Ito,f, IKur (solid line, IKur 100% of AF; dashed line, IKur 20% of AF), and IK1. In AF, reduction of IKur induces less increase in ICa, IKr, and IKs amplitudes than in SR (see text).

View this table: [in this window] [in a new window]   TABLE 2. Maximum Conductances, gmax, of the Main Currents Incorporated in the Mathematical Model for Action Potential Simulation

The characteristic triangular shape of APs in AF was reproduced by setting conductances of I_K1, I_Ca,L, I_to,f, I_Kur, and I_NaCaX to values of reported ion current densities or channel expression (Figures 1B and 3; see Reference ⁵ and Table 2). The effect of 4-aminopyridine was simulated by setting the maximum conductance of I_Kur to 20% of the AF control value and computing membrane currents in the course of the resulting AP. The simulation produced a shift of the plateau potential and an overall prolongation of APD₂₀ and APD₉₀ (Figure 3). The accompanying indirect changes of other currents were qualitatively similar to those in SR but were substantially smaller in size.

Experimental Verification of Model Predictions (I_Ca,L, I_Kr)
The model predicted an increase of I_Ca,L by block of I_Kur. 4-Aminopyridine did not modulate I_Ca,L in atrial myocytes (peak current density during clamp steps from –80 to +10 mV: 5.5±1.3 pA/pF before and 5.1±1.0 pA/pF after 100 µmol/L 4-aminopyridine, n=8 cells from 6 SR patients; P=NS). Similar results, although at lower basal I_Ca,L, were obtained in atrial myocytes from AF patients (data not shown). 4-Aminopyridine (100 µmol/L) had no effect on I_K1 (data not shown).

To test for indirect effects of block of I_Kur on Ca²⁺ entry via I_Ca,L, we measured F_c as an indirect indicator of I_Ca,L activity (Figure 4A). In SR trabeculae, 4-aminopyridine (100 µmol/L) enhanced F_c from 4.5±0.7 to 7.3±1.1 mN (n=8/6 [trabeculae/patients], P<0.05). The EC₅₀ for 4-aminopyridine obtained by curve fitting was 52 µmol/L (Figure 4B). With 1 mmol/L 4-aminopyridine, F_c increased maximally (8.8±1.3 mN), corresponding to 58±3% of the F_c developed by 8 mmol/L CaCl₂ (14.9±1.6 mN). As expected, basal F_c was lower (1.3±0.4 mN, n=9/5, P<0.001) and the effect of 4-aminopyridine on F_c was smaller in AF than in SR. F_c increased at 1 mmol/L 4-aminopyridine to 2.9±0.9 mN, corresponding to 28±8% of that obtained by 8 mmol/L CaCl₂ (10.9±1.6 mN, P=NS versus SR). The EC₅₀ value was 36 µmol/L. The positive inotropic effects were not affected by (–)-propranolol and hence were independent of enhanced neurotransmitter release.⁸ If the effect of 4-aminopyridine on F_c was a result of the blockade of I_Kur, it should be absent in ventricular preparations, in which I_Kur cannot be measured.¹⁵ Indeed, 4-aminopyridine (0.3 µmol/L to 1 mmol/L) did not affect F_c in ventricular trabeculae (Figure 4, C and D), excluding a direct effect of 4-aminopyridine on Ca²⁺ release or contractile machinery.

View larger version (23K): [in this window] [in a new window]   Figure 4. Effect of 4-aminopyridine on Fc in isolated human atrial and ventricular preparations. A, Original recordings of Fc with increasing concentrations of 4-aminopyridine (300 nmol/L to 1 mmol/L) for atrial trabeculae from an SR patient (top) and an AF patient (below). B, Concentration-dependent effects of 4-aminopyridine on Fc in atrial preparations from SR (open squares) and AF (open circles) patients (EC50=52 µmol/L, SR, and EC50=36 µmol/L, AF). Maximum contractile responses were tested with high extracellular Ca2+ (8 mmol/L) at end of each experiment. C, Original registrations of Fc in time-matched controls (TMC, top) and with 4-aminopyridine (0.3 µmol/L to 1 mmol/L) of human ventricular trabeculae (below). D, Concentration-dependent effects of 4-aminopyridine on Fc in human ventricular preparations.

According to the model prediction in SR, the shortening of APD₉₀ by block of I_Kur is expected to be induced by an increase in I_Kr. Block of I_Kr with E-4031 (1 µmol/L) prolonged APD₉₀, although the effect did not reach the level of statistical significance (P=0.3661, n=5); Figure 5, A and C). In the presence of E-4031, 4-aminopyridine (25 µmol/L) no longer shortened but rather prolonged APD₉₀ from 412±43 to 483±33 ms (n=5, P=0.2217; versus control, P=0.0249) and significantly elevated the notch and plateau potentials from –30±2 to –19±3 mV (n=5, P<0.05) and from –27±2 to –14±3 mV (n=5, P<0.001), respectively. These effects were reproduced in the model simulation by reducing the conductance of I_Kr by 95% and that of I_Kur by 90% (Figure 5B).

View larger version (23K): [in this window] [in a new window]   Figure 5. Effect of block of IKur by 4-aminopyridine in presence of IKr block with E-4031. A, Original AP registration in SR; C, control AP (solid line), 20 minutes after E-4031 (1 µmol/L, short dashes), and 20 minutes after 4-aminopyridine (25 µmol/L) in presence of E-4031 (long dashes). B, Simulated AP; C, control, after reducing gmax of IKr to 5% (dashed line) and gmax of IKur to 10% of control value. C, Means of APD90 (left), notch potential (middle), and plateau potential (right) for atrial trabeculae from SR patients (n=5, *P<0.05).

Effect of the I_Kur Blocker AVE 0118 on AP in SR and AF
The effects of I_Kur inhibition were verified with the new I_Kur blocker AVE 0118 (6 µmol/L). AVE 0118 revealed the typical changes in AP shape as found with low concentrations of 4-aminopyridine (5 µmol/L), ie, elevation of the plateau potential from –16.3±4.1 to –6.7±4.2 mV (n=9, P<0.01) in the presence of 6 µmol/L AVE 0118, and shortening of ADP₉₀ from 343±14 to 328±17 ms (P=0.066) in SR. In AF, the plateau amplitude increased from –2.4±3.6 to 8.8±3.6 mV (n=6, P<0.001), and APD₉₀ increased from 260±14 to 280±13 ms (P<0.05; Figure 6).

View larger version (11K): [in this window] [in a new window]   Figure 6. Effect of AVE 0118 (6 µmol/L) on AP shapes in atrial trabeculae from patients in SR and AF. A and B, Original registrations of predrug control (C) and after 20 minutes of exposure to AVE 0118.

	Discussion

Top Abstract Introduction Methods Results Discussion Conclusions References

 
The concept of block of I_Kur as a therapeutic target in AF is widely accepted,³ despite inconsistent experimental and theoretical evidence for changes in APD. Here, we report that selective block of I_Kur shortens APD of human atrial trabeculae from SR but prolongs APD in AF. Changes in APD in SR and AF were consistent with secondary current changes as predicted by a modified Luo-Rudy model and were verified experimentally.

Effects of Block of I_Kur on Shapes of Atrial APs in SR and AF
In low concentrations, 4-aminopyridine is a selective blocker of I_Kur. In earlier work, we found that 4-aminopyridine blocks I_Kur and I_to,f in human atrial myocytes with IC₅₀ values of 8 µmol/L and 1 mmol/L, respectively.¹⁵ Similar values were published by others (ie, 49 µmol/L for I_Kur and 1.9 mmol/L for I_to,f).¹

Block of repolarizing outward current is expected to prolong APD; however, we observed shortening in SR preparations instead. In the literature, shortening,⁴ prolongation,¹ and even no effect at all have been reported.² How can these inconsistencies be reconciled? The shape of the cardiac AP is the result of the balanced activity of several ion currents. Provided that 4-aminopyridine at the concentrations used is in fact selective for I_Kur block, APD shortening in SR must be associated with alterations in additional currents. In this context, the pronounced elevation of the action potential plateau may provide an important clue because enhanced amplitude of I_Ca,L at more positive potentials could activate repolarizing outward currents that would shorten APD.⁴

In AF, block of I_Kur resulted in the expected APD prolongation. The characteristic triangular AP shape in AF compared with the spike-and-dome configuration in SR is a result of electrical remodeling that comprises changes in several ion conductances. The densities of I_Ca,L and I_to,f are reduced by 70%^16,17 and 60%,^16,18,19 respectively, and inward rectifier I_K1 is increased by 100%.^14,16,18,20 From these considerations, it follows that selective block of I_Kur (or any other current) will perturb the balance of ion channel activation differently in AF and SR and may result in different patterns of secondary effects.

Computer Simulations: Indirect Effects of 4-Aminopyridine on I_Ca,L
To predict individual current changes secondary to selective block of I_Kur, we used a computer model of AP simulation. For this purpose, ion conductances in the Luo-Rudy model⁹ were set to the experimentally observed values reported recently for human AF. The model simulated the characteristic AP shapes. For selective block of I_Kur in SR APs, the model produced a more pronounced spike-and-dome configuration. The shift of the plateau to more positive potentials was associated with enhanced I_Ca,L activation in the model (although 4-aminopyridine had no direct effects on I_Ca,L) and is expected to increase systolic Ca²⁺ influx during a free-running AP. Indeed, 4-aminopyridine significantly increased F_c in atrial trabeculae in a concentration-dependent manner (see Figure 4) in SR and AF, respectively. Although the positive inotropic effect was much smaller in AF, it is considered to be clinically interesting, because atrial contractility is already impaired in AF²¹ and all other drugs currently used in the treatment of AF, eg, ß-adrenoceptor blockers or calcium channel blockers, have a negative inotropic effect.

Computer Simulations: Indirect Effects of 4-Aminopyridine on I_Kr
Secondary enhancement of outward currents as an explanation for APD shortening in SR by block of I_Kur was also predicted by the model. As the likely candidates, the model identified I_Kr and I_Ks. Because of its slow activation kinetics, I_Ks will not play a major role in the course of the short atrial APs. Conversely, the impact of I_Kr was examined experimentally: pharmacological block of I_Kr in trabeculae from SR unmasks the APD-prolonging effect of I_Kur block (see Figure 5). Therefore, we speculate that 4-aminopyridine prolongs APD in atrial myocytes, because I_Kr may be absent because of the enzymatic isolation procedure, as was shown for canine atrial myocytes.²²

The model simulation of the human atrial SR AP reproduces the experimentally observed APD shortening that is induced by I_Kur block. However, this experimental finding is reproduced only when I_Kr is set to sufficiently high values (Table 2). If low values are used (see, for instance, Nygren et al²³), APD either is prolonged²³ or remains unaffected.² Although the large conductance of I_Kr assumed in our model was not proved with current measurements, the APD-prolonging effect of I_Kr block by E-4031 (Figure 5) points to a prominent role of this current in atrial repolarization, supported also by current measurements of I_Kr in human atrium.²⁴ In the presence of I_Kr block, I_Kur inhibition further prolonged APD. We therefore assume that in multicellular trabeculae, I_Kr was fully available for repolarization. In conscious dogs, the I_Kr blocker ibutilide prolonged atrial monophasic APs and effective refractory period,²⁵ providing evidence for a prominent I_Kr contribution.

Because I_Ca,L is reduced in AF, block of I_Kur cannot produce much indirect increase in I_Ca,L, and therefore the plateau is only slightly elevated. Because of enhanced I_K1 in AF, rapid and strong induction of the repolarization process abbreviates the time window for activation of I_Kr and thus reduces its repolarizing potency. All interacting factors together will produce an AP-prolonging effect of I_Kur block, which possibly represents the antiarrhythmic potency of putative I_Kur blockers.

Study Limitations
The majority of patients donating tissue samples suffered from coronary artery disease. Therefore, AP measurements from SR trabeculae may not be considered to be fully representative of APs from trabeculae of a healthy population. In addition, the patients’ medications may have additional effects. The large variability in action potential shapes may represent this inhomogeneity of the material.

Early repolarization is controlled not only by I_Kur but also by I_to,f. Although low concentrations of 4-aminopyridine are selective for block of I_Kur, a contribution of I_to,f block cannot be excluded at larger concentrations. In the absence of selective blockers for I_to,f, model simulations are useful for estimating the consequences of I_to,f block on atrial AP. Like I_Kur block, reduced basal conductance of I_to,f shifted the notch potential to more positive values and abbreviated APD₉₀ in SR and only slightly prolonged APD₉₀ in AF. In contrast to I_Kur block, selective block of I_to,f did not elevate the dome potential or enhance the spike-and-dome configuration (data not shown). The rapid I_to,f inactivation most likely limits indirect enhancement of I_Kr if I_to,f is blocked.

	Conclusions

Top Abstract Introduction Methods Results Discussion Conclusions References

 
We hypothesize that I_Kur is a major determinant in controlling action potential shape and therefore also contractility in the human atrium. The antiarrhythmic potency of I_Kur inhibitors is determined largely by the level of electrical remodeling of the diseased atrium. Although the antiarrhythmic effectiveness of I_Kr blockers is significantly attenuated in chronic AF,²⁶ selective I_Kur blockers may be more potent in prolonging atrial refractory period in chronic AF than in SR and may therefore be more efficient in terminating AF than in maintaining SR.

	Acknowledgments

 
This work was supported by the Sächsisches Staatsministerium für Wissenschaft und Kunst (Az 4-7531.50-04-0370-01/2, -02/4, and -03/8) and by the Bundesministerium für Bildung und Forschung (Atrial Fibrillation Network, No 01GI0204). We acknowledge the donation of AVE 0118 by Aventis Pharma AG, Frankfurt, Germany, and thank Prof Dr H. Gögelein from Aventis Pharma AG for valuable comments on the manuscript.

	Footnotes

 
*The first 2 authors contributed equally to this work.

The online-only Data Supplement, which contains an Appendix, is available with this article at http://www.circulationaha.org.

	References

Top Abstract Introduction Methods Results Discussion Conclusions References

 

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