Targeting atrioventricular differences in ion channel properties for terminating acute atrial fibrillation in pigs

Abstract

Aims: The goal was to terminate atrial fibrillation (AF) by targeting atrioventricular differences in ionic properties.

Methods and results: Optical mapping was used to record electrical activity during carbachol (0.25-0.5 μM)-induced AF in pig hearts. The atrial-specific current, I(Kur), was blocked with 100 μM 4-aminopyridine (4-AP) or with 0.5 μM DPO-1. Hearts in AF and ventricular fibrillation (VF) were also subjected to increasing levels of extracellular K(+) ([K(+)](o): 6-12 mM), compared with controls (4 mM). We hypothesized that due to the more negative steady-state half inactivation voltage for the atrial Na(+) current, I(Na), compared with the ventricle, AF would terminate before VF in hyperkalaemia. Mathematical models were used to interpret experimental findings. The I(Kur) block did not terminate AF in a majority of experiments (6/9 with 4-AP and 3/4 with DPO-1). AF terminated in mild hyperkalaemia ([K(+)](o) ≤ 10.0 mM; N = 8). In contrast, only two of five VF episodes terminated at the maximum ([K(+)](o): 12 mM [K(+)](o)). The I(Kur) block did not terminate a simulated rotor in cholinergic AF because its contribution to repolarization was dwarfed by the large magnitude of the acetylcholine-activated K(+) current (I(K,ACh)). Simulations showed that the lower availability of the atrial Na(+) current at depolarized potentials, and a smaller atrial tissue size compared with the ventricle, could partly explain the earlier termination of AF compared with VF during hyperkalaemia.

Conclusion: I(Kur) is an ineffective anti-arrhythmic drug target in cholinergic AF. Manipulating Na(+) current 'availability' might represent a viable anti-arrhythmic strategy in AF.

Figure 1

Experiments in acute atrial fibrillation and in the presence of I_Kur blockade. (A) Representative dominant frequency maps and single-pixel recordings during acute atrial fibrillation in the left atrium and the right atrium. (B) Plot of the maximum dominant frequency (DF_max) in acute atrial fibrillation experiments in pig hearts vs. time. Time before the introduction of the drug to selectively block I_Kur (100 μM 4-AP) is assigned negative values.

Figure 2

Experiments in acute atrial fibrillation and ventricular fibrillation and in the presence of I_Kur blockade (via DPO-1). (A) Representative pig optical atrial action potential in control (solid) and in the presence of 0.5 μM DPO-1 (dashed) at 300 ms pacing. (B) Representative pig optical ventricular action potential in control (solid) and in the presence of 0.5 μM DPO-1 (dashed) at 300 ms pacing. (C) Average atrial action potential durations at 50% (APD₅₀) and 75% (APD₇₅) repolarization in control and in the presence of 0.5, 1.0, and 5.0 μM DPO-1. (D) Average ventricular action potential durations at 50% (APD₅₀) and 75% (APD₇₅) repolarization in control and in the presence of 0.5 and 1.0 μM DPO-1. (E) Plot of DF_max in acute atrial fibrillation experiments in control and in the presence of 0.5 μM DPO-1. (F) Plot of DF_max during ventricular fibrillation in control and in the presence of 0.5 μM DPO-1. (G) Plot of DF_max in acute atrial fibrillation experiments in control and in the presence of 1.0 and 5.0 μM DPO-1. (H) Plot of DF_max during ventricular fibrillation in control and in the presence of 1.0 and 5.0 μM DPO-1.

Figure 3

Experiments in acute atrial and ventricular fibrillation simultaneously during hyperkalaemia (6–12 mM [K⁺]_o). (A) Representative plots of the dominant frequency maps in acute atrial (in the left atrium) and ventricular fibrillation (in the left ventricle) in the same heart that was perfused at different concentrations of [K⁺]_o at 4, 6, 8, 10, and 12 mM, for 10 min at each concentration. DF_max of atrial fibrillation changed substantially during perfusion of 8 mM [K⁺]_o; hence, three maps at the initial, middle, and final stages of the 10 min perfusion period are shown. Atrial fibrillation converted to atrial tachycardia (ATach) in the final stages and subsequently terminated during transition from 8 to 10 mM [K⁺]_o. In this experiment, ventricular fibrillation did not terminate even at 12 mM [K⁺]_o. (B) Comparison of average DF_max values during atrial and ventricular fibrillation at different concentrations of [K⁺]_o.

Figure 4

New numerical formulation of I_Kur. (A) Plots of model time constants for the fast and slow time constants of the inactivation for I_Kur in the human atrial mathematical model (Courtemanche), adjusted according to data for inactivation in the pig atrium. (B) This plot shows the frequency dependence of I_Kur; this current is reduced in magnitude in response to pulses at higher frequencies. (C) Plot of the model action potential at 1 Hz under control conditions (to mimic the pig atrial action potential) and the plot of the action potential when I_Kur was completely blocked at 1 Hz.

Figure 5

Simulations of I_Kur block on rotor. (A) Left two panels depict representative snapshots of a rotor under control conditions, and when I_Kur was blocked completely, in carbachol-induced atrial fibrillation; right panels show respective dominant frequency maps. (B) Snapshot of spatial distribution of I_Kur and I_K,ACh currents in a rotor during atrial fibrillation. (C) Top two left panels depict a dominant frequency map and a rotor snapshot when atrial fibrillation was induced in the absence of carbachol. The rotor tip trajectory before I_Kur block is plotted in blue. Bottom two left panels depict rotor snapshots in simulated atrial fibrillation (in the absence of I_K,ACh) when I_Kur was blocked. The two snapshots are given at t = 3117 ms and just before termination of re-entry at t = 3177 ms. The rightmost panel depicts rotor tip meander before and after I_Kur block.

Figure 6

Effect of hyperkalaemia on simulated rotor. (A) The left panel depicts differences between the steady-state inactivation properties of I_Na in the atrium (red) and ventricle (blue). The inactivation is measured as the product of the fast (h) and the slow (j) sodium channel inactivation gates: hj. The right panel depicts how steady-state inactivation properties of I_Na vary as a function of increases in [K⁺]_o. (B) The left panels show snapshots of the simulated rotor (with atrial model steady-state Na⁺ inactivation) at different values of [K⁺]_o. The right panel depicts the trajectory of the rotor tip at different values of [K⁺]_o. The rotor is seen to terminate at 6.2 mM [K⁺]_o. (C) The top panel shows snapshots of the simulated rotor in a 3 × 3 cm tissue (with ventricular model steady-state Na⁺ inactivation) at different values of [K⁺]_o. The rotor is seen to terminate at 10.6 mM [K⁺]_o. The bottom panel depicts snapshots of the simulated rotor, but now in a 5 × 5 cm tissue (with ventricular I_Na) at different values of [K⁺]_o. The rotor is now seen to terminate at 11.4 mM [K⁺]_o.