Cardiac potassium channel subtypes: new roles in repolarization and arrhythmia

About 10 distinct potassium channels in the heart are involved in shaping the action potential. Some of the K+ channels are primarily responsible for early repolarization, whereas others drive late repolarization and still others are open throughout the cardiac cycle. Three main K+ channels drive the late repolarization of the ventricle with some redundancy, and in atria this repolarization reserve is supplemented by the fairly atrial-specific KV1.5, Kir3, KCa, and K2P channels. The role of the latter two subtypes in atria is currently being clarified, and several findings indicate that they could constitute targets for new pharmacological treatment of atrial fibrillation. The interplay between the different K+ channel subtypes in both atria and ventricle is dynamic, and a significant up- and downregulation occurs in disease states such as atrial fibrillation or heart failure. The underlying posttranscriptional and posttranslational remodeling of the individual K+ channels changes their activity and significance relative to each other, and they must be viewed together to understand their role in keeping a stable heart rhythm, also under menacing conditions like attacks of reentry arrhythmia.

Cardiac ion channels and mechanisms for protection against atrial fibrillation

Atrial fibrillation (AF) is recognised as the most common sustained cardiac arrhythmia in clinical practice. Ongoing drug development is aiming at obtaining atrial specific effects in order to prevent pro-arrhythmic, devastating ventricular effects. In principle, this is possible due to a different ion channel composition in the atria and ventricles. The present text will review the aetiology of arrhythmias with focus on AF and include a description of cardiac ion channels. Channels that constitute potentially atria-selective targets will be described in details. Specific focus is addressed to the recent discovery that Ca(2+)-activated small conductance K(+) channels (SK channels) are important for the repolarisation of atrial action potentials. Finally, an overview of current pharmacological treatment of AF is included.

Inhibition of Small-Conductance Ca 2+ -Activated K + Channels Terminates and Protects Against Atrial Fibrillation

Background—

Recently, evidence has emerged that small-conductance Ca 2+ -activated K + (SK) channels are predominantly expressed in the atria in a number of species including human. In rat, guinea pig, and rabbit ex vivo and in vivo models of atrial fibrillation (AF), we used 3 different SK channel inhibitors, UCL1684, N -(pyridin-2-yl)-4-(pyridin-2-yl)thiazol-2-amine (ICA), and NS8593, to assess the hypothesis that pharmacological inhibition of SK channels is antiarrhythmic.

Methods and Results—

In isolated, perfused guinea pig hearts, AF could be induced in all control hearts (n=7) with a combination of 1 μmol/L acetylcholine combined with electric stimulation. Pretreatment with 3 μmol/L NS8593, which had no effect on QT interval, prolonged the atrial effective refractory period by 37.1±7.7% ( P <0.001) and prevented acetylcholine-induced AF ( P <0.001, n=7). After AF induction, perfusion with NS8593 (10 μmol/L), UCL1684 (1 μmol/L), or ICA (1 μmol/L) terminated AF in all hearts, comparable to 10 μmol/L amiodarone. In isolated, perfused rat hearts, AF was induced with electric stimulation; 10 μmol/L NS8593 terminated AF and prevented reinduction of AF in all hearts (n=6, P <0.001). In all hearts, AF could be reinduced after washing. In isolated, perfused rabbit hearts, AF was induced with 10 μmol/L acetylcholine and burst pacing; 10 μmol/L NS8593 terminated AF and prevented reinduction of AF in all hearts (n=6, P <0.001). After washing, AF could be reinduced in 75% of the hearts (n=4, P =0.06). In an in vivo rat model of acute AF induced by burst pacing, injection of 5 mg/kg of either NS8593 or amiodarone shortened AF duration significantly to (23.2±20.0%, P <0.001, n=5, and 26.2±17.9%, P <0.001, n=5, respectively) as compared with injection of vehicle (96.3±33.2%, n=5).

Conclusions—

Inhibition of SK channels prolongs atrial effective refractory period without affecting QT interval and prevents and terminates AF ex vivo and in vivo, thus offering a promising new therapeutic opportunity in the treatment of AF.