In silico investigation of a KCNQ1 mutation associated with familial atrial fibrillation.

Mutations in transmembrane domains of the KCNQ1 subunit of the I(Ks) potassium channel have been associated with familial atrial fibrillation. We have investigated mechanisms by which the S1 domain S140G KCNQ1 mutation influences atrial arrhythmia risk and, additionally, whether it can affect ventricular electrophysiology. In perforated-patch recordings, S140G-KCNQ1+KCNE1 exhibited leftward-shifted activation, slowed deactivation and marked residual current. In human atrial action potential (AP) simulations, AP duration and refractoriness were shortened and rate-dependence flattened. Simulated I(Ks) but not I(Kr) block offset AP shortening produced by the mutation. In atrial tissue simulations, temporal vulnerability to re-entry was little affected by the S140G mutation. Spatial vulnerability was markedly increased, leading to more stable and stationary spiral wave re-entry in 2D stimulations, which was offset by I(Ks) block, and to scroll waves in 3D simulations. These changes account for vulnerability to AF with this mutation. Ventricular AP clamp experiments indicate a propensity for increased ventricular I(Ks) with the S140G KCNQ1 mutation and ventricular AP simulations showed model-dependent ventricular AP abbreviation.

Pharmacological inhibition of the hERG potassium channel is modulated by extracellular but not intracellular acidosis.

INTRODUCTION: Human ether-à-go-go related gene (hERG) is responsible for channels that mediate the rapid delayed rectifier K(+) channel current (I(Kr) ), which participates in repolarization of the ventricles and is a target for some antiarrhythmic drugs. Acidosis occurs in the heart in some pathological situations and can modify the function and responses to drugs of ion channels. The aim of this study was to determine the effects of extracellular and intracellular acidosis on the potency of hERG channel current (I(hERG)) inhibition by the antiarrhythmic agents dofetilide, flecainide, and amiodarone at 37 °C. METHODS AND RESULTS: Whole-cell patch-clamp recordings of I(hERG) were made at 37 °C from hERG-expressing Human Embryonic Kidney (HEK293) cells. Half-maximal inhibitory concentration (IC(50)) values for I(hERG) tail inhibition at -40 mV following depolarizing commands to +20 mV were significantly higher at external pH 6.3 than at pH 7.4 for both flecainide and dofetilide, but not for amiodarone. Lowering pipette pH from 7.2 to 6.3 altered neither I(hERG) kinetics nor the extent of observed I(hERG) blockade by any of these drugs. CONCLUSION: Conditions leading to localized extracellular acidosis may facilitate heterogeneity of action of dofetilide and flecainide, but not amiodarone via modification of hERG channel blockade. Such effects depend on the external pH change rather than intracellular acidification.

Enhanced inhibitory effect of acidosis on hERG potassium channels that incorporate the hERG1b isoform.

Extracellular acidosis occurs in the heart during myocardial ischemia and can lead to dangerous arrhythmias. Potassium channels encoded by hERG (human ether-à-go-go-related gene) mediate the cardiac rapid delayed rectifier K+ current (IKr), and impaired hERG function can exacerbate arrhythmia risk. Nearly all electrophysiological investigations of hERG have centred on the hERG1a isoform, although native IKr channels may be comprised of hERG1a and hERG1b, which has a unique shorter N-terminus. This study has characterised for the first time the effects of extracellular acidosis (an extracellular pH decrease from 7.4 to 6.3) on hERG channels incorporating the hERG1b isoform. Acidosis inhibited hERG1b current amplitude to a significantly greater extent than that of hERG1a, with intermediate effects on coexpressed hERG1a/1b. IhERG tail deactivation was accelerated by acidosis for both isoforms. hERG1a/1b activation was positively voltage-shifted by acidosis, and the fully-activated current-voltage relation was reduced in amplitude and right-shifted (by ∼10 mV). Peak IhERG1a/1b during both ventricular and atrial action potentials was both suppressed and positively voltage-shifted by acidosis. Differential expression of hERG isoforms may contribute to regional differences in IKr in the heart. Therefore inhibitory effects of acidosis on IKr could also differ regionally, depending on the relative expression levels of hERG1a and 1b, thereby increasing dispersion of repolarization and arrhythmia risk.

Action potential clamp and mefloquine sensitivity of recombinant 'I KS' channels incorporating the V307L KCNQ1 mutation.

The slow delayed rectifier potassium current, 'I(Ks)', contributes to repolarisation of cardiac ventricular action potentials and thereby to the duration of the QT interval of the electrocardiogram. Mutations to I(Ks) channel subunits occur in clinically significant cardiac repolarisation disorders. The short QT syndrome (SQTS) is associated with accelerated ventricular repolarisation and with an increased risk of arrhythmia and sudden death. The SQT2 variant of the SQTS has been linked to a gain-of-function amino-acid substitution (V307L) in the KCNQ1-encoded I(Ks) channel alpha-subunit. This study reports the first action potential (AP) voltage-clamp comparison between wild-type (WT) and V307L KCNQ1 (co-expressed with KCNE1 to recapitulate I(Ks)) and identifies an effective pharmacological inhibitor of recombinant 'I(Ks)' channels incorporating the V307L KCNQ1 mutation. Perforated-patch voltage-clamp recordings at 37 degrees C of whole-cell current carried by co-expressed KCNQ1 and KCNE1 showed a marked (-36 mV) shift in half-maximal activation for V307L compared to WT KCNQ1; a significant slowing of current deactivation was also observed. Under AP clamp, peak repolarising current was significantly augmented for V307L KCNQ1 compared to WT KCNQ1 for both ventricular and atrial AP commands, consistent with an ability of the V307L mutation to increase repolarising I(Ks) in both regions. The quinoline agent mefloquine inhibited WT KCNQ1+KCNE1 with an IC(50) of 3.4 muM compared to 3.3 muM for V307L KCNQ1+KCNE1 (P >0.05). This establishes mefloquine as an effective inhibitor of recombinant 'I(Ks)' channels incorporating this SQT2 KCNQ1 mutation.

The S140G KCNQ1 atrial fibrillation mutation affects 'I(KS)' profile during both atrial and ventricular action potentials.

KCNQ1 is responsible for the pore-forming subunit of channels that mediate the cardiac 'IKs' potassium channel current. The S140G KCNQ1 gain-of-function mutation is responsible for a form of heritable atrial fibrillation. Here the action potential (AP) voltage clamp technique was used to elucidate the effect of S140G KCNQ1 on the profile of recombinant I(Ks) during atrial and ventricular APs applied to KCNQ1+KCNE1 expressing CHO cells, at 37°C. Under conventional voltage clamp the S140G KCNQ1 mutation shifted voltage-dependent activation by ≈-62 mV, with a marked instantaneous current component evident on membrane depolarisation. Under atrial AP clamp, cells expressing wild-type (WT) KCNQ1 exhibited modest outward currents during atrial repolarisation, whilst those expressing S140G KCNQ1 exhibited a marked instantaneous outward current and peak repolarising current >4-fold that for WT KCNQ1. Under ventricular AP clamp, both WT and mutant KCNQ1 conditions showed greater peak repolarising current than when an atrial AP command was used and the S140G mutation resulted in peak repolarising current that was >3-fold that for WT KCNQ1. We conclude that the S140G KCNQ1 mutation would be predicted to augment substantially repolarising current both early and throughout atrial APs and, in principle, also to influence markedly ventricular AP repolarisation.