Ionic determinants of functional reentry in a 2-D model of human atrial cells during simulated chronic atrial fibrillation

Recent studies suggest that atrial fibrillation (AF) is maintained by fibrillatory conduction emanating from a small number of high-frequency reentrant sources ( rotors). Our goal was to study the ionic correlates of a rotor during simulated chronic AF conditions. We utilized a two-dimensional (2-D), homogeneous, isotropic sheet (5 x 5 cm(2)) of human atrial cells to create a chronic AF substrate, which was able to sustain a stable rotor (dominant frequency similar to 5.7 Hz, rosette-like tip meander similar to 2.6 cm). Doubling the magnitude of the inward rectifier K+ current (I-K1) increased rotor frequency (similar to 8.4 Hz), and reduced tip meander (similar to 1.7 cm). This rotor stabilization was due to a shortening of the action potential duration and an enhanced cardiac excitability. The latter was caused by a hyperpolarization of the diastolic membrane potential, which increased the availability of the Na+ current (I-Na). The rotor was terminated by reducing the maximum conductance (by 90%) of the atrial-specific ultrarapid delayed rectifier K+ current (I-Kur), or the transient outward K+ current (I-to), but not the fast or slow delayed rectifier K+ currents (I-Kr/I-Ks). Importantly, blockade of I-Kur/I-to prolonged the atrial action potential at the plateau, but not at the terminal phase of repolarization, which led to random tip meander and wavebreak, resulting in rotor termination. Altering the recti. cation profile of I-K1 also slowed down or abolished reentrant activity. In combination, these simulation results provide novel insights into the ionic bases of a sustained rotor in a 2-D chronic AF substrate.