The Effect of High Extracellular Potassium on IKr Inhibition by Anti-Arrhythmic Agents

Abstract

Background: Hyperkalemia is a potentially life-threatening disorder frequently occurring in hospitalized patients. The ischemic myocardium releases potassium into the extracellular space which can cause regional hyperkalemia. These changes may modify the effects of anti-arrhythmic drugs acting on the rapid component of the delayed rectifier potassium current (IKr). We evaluated the influence of increased extracellular potassium concentration [K⁺]_e on IKr inhibition by amiodarone, azimilide, dofetilide, quinidine and sotalol. Methods and Results: Experiments were performed at room temperature. IKr current was studied by using HERG gene expressed in Xenopus oocytes as a model of cardiac IKr. Two-electrode voltage clamp technique was employed. The recording bath solutions contained either 5 or 10 mmol/l KCl. Amiodarone, azimilide, dofetilide, quinidine and sotalol all produced a dose-dependent inhibition of HERG current. At 5 mmol/l [K⁺]_e, the IC₅₀ was 37.0 ± 12.5 µ M for amiodarone, 5.8 ± 0.4 µ M for azimilide, 1.5 ± 0. 2 µ M for dofetilide, 9.1 ± 1.5 µ M for quinidine, and 5.1 ± 0.8 m M for sotalol. Raising the extracellular potassium to 10 mmol/l, HERG block by azimilide, dofetilide, quinidine and sotalol was significantly decreased, while the block by amiodarone was unchanged. The differences in the percentage current block produced by 3 µ M drugs at 5 and 10 mmol/l [K⁺]_e were: –0.9% for amiodarone, 13.8% for quinidine, 20.5% for azimilide, and 16.2% for dofetilide. The differences in percentage block between 5 and 10 mmol/l [K⁺]_e by sotalol 10 and 30 m M were 7.1 and 5.6%. At 10 mmol/l [K⁺]_e, the IC₅₀ was increased for azimilide, dofetilide, quinidine and sotalol but not for amiodarone; the IC₅₀ was 24.7 ± 7.4 µ M for amiodarone, 29.3 ± 3.9 µ M for azimilide, 2.7 ± 0.2 µ M for dofetilide, 27.6 ± 4.0 µ M for quinidine, and 7.2 ± 1.7 m M for sotalol. Conclusion: Inhibition of IKr by azimilide, quinidine, dofetilide and sotalol was diminished by increasing [K⁺]_e, while the inhibition by amiodarone was unchanged at normal and high [K⁺]_e. The differential effects of azimilide, dofetilide, quinidine and sotalol at normal and high [K⁺]_e could be pro-arrhythmic by favoring re-entry arrhythmias. These results further support the unique electrophysiological effect of amiodarone.