Vicious LQT induced by a combination of factors different from hERG inhibition

Abstract

Clinically, drug-induced torsades de pointes (TdP) are rare events, whereas the reduction of the human ether-à-go-go-related gene (hERG) current is common. In this study, we aimed to explore the specific factors that contribute to the deterioration of hERG inhibition into malignant ventricular arrhythmias. Cisapride, a drug removed from the market because it caused long QT (LQT) syndrome and torsade de pointes (TdP), was used to induce hERG inhibition. The effects of cisapride on the hERG current were evaluated using a whole-cell patch clamp. Based on the dose-response curve of cisapride, models of its effects at different doses (10, 100, and 1,000 nM) on guinea pig heart in vitro were established. The effects of cisapride on electrocardiogram (ECG) signals and QT interval changes in the guinea pigs were then comprehensively evaluated by multi-channel electrical mapping and high-resolution fluorescence mapping, and changes in the action potential were simultaneously detected. Cisapride dose-dependently inhibited the hERG current with a half inhibitory concentration (IC50) of 32.63 ± 3.71 nM. The complete hERG suppression by a high dose of cisapride (1,000 nM) prolonged the action potential duration (APD), but not early after depolarizations (EADs) and TdP occurred. With 1 μM cisapride and lower Mg²⁺/K⁺, the APD exhibited triangulation, dispersion, and instability. VT was induced in two of 12 guinea pig hearts. Furthermore, the combined administration of isoproterenol was not therapeutic and increased susceptibility to ventricular fibrillation (VF) development. hERG inhibition alone led to QT and ERP prolongation and exerted an anti-arrhythmic effect. However, after the combination with low concentrations of magnesium and potassium, the prolonged action potential became unstable, triangular, and dispersed, and VT was easy to induce. The combination of catecholamines shortened the APD, but triangulation and dispersion still existed. At this time, VF was easily induced and sustained.

Keywords: QT prolongation; TDP; cisapride; dispersion; instability; optical mapping; triangulation.

FIGURE 1

Cisapride dose-dependently inhibited the hERG current and increased APD and ERP. (A) Representative diagram of the concentration-dependent inhibition of the hERG channel by cisapride; (B) dose-response curve for the inhibition of the hERG channel by cisapride (n = 10); (C) representative graph of cisapride concentration-dependently increase APD; (D) APD and (E) ERP in the absence of different concentrations of cisapride (10, 100, and 1,000 nM) (n = 8).

FIGURE 2

Representative traces and statistical graph of induced VF/VT under different conditions. (A) High-frequency stimulation induced VT/VF with 1 μM Cis (n = 18); (B) VT/VF was induced by high-frequency stimulation of 1 μM Cis + low Mg²⁺/K⁺(n = 12); (C) high-frequency stimulation induced VT/VF with 1 μM Cis + low Mg²⁺/K⁺+ iso (n = 7); (D) statistical chart of VT/VF induction rate under different conditions.

FIGURE 3

Effects of different conditions on AP morphology and dispersion. (A) Effects of different conditions on AP morphology; (B) quantitative statistics of APD prolongation under different conditions; (C) quantitative statistics of AP triangulation under different conditions. The APD30/APD80 ratio was used to quantify the AP triangular morphology under different conditions. A smaller ratio was associated with more significant AP triangular morphology; (D) optical mapping of ventricular AP under different conditions; (E) quantitative statistics of ventricular AP dispersion under different conditions were expressed by APD (Q3)-APD (Q1). We found that under low-magnesium and -potassium conditions, the dispersion of AP was significantly increased by cisapride (*p < 0.05; **p < 0.01; and ***p < 0.001).

FIGURE 4

Instability. (A) Map of the APD90 difference (Dif) in the epicardial between the odd beats and the even beats under different inducing conditions. Dif = Odd-Even; (B) statistics of max dif. There was no significant change in instability when cisapride was given alone, but the instability was significantly altered by the combination of cisapride and low Mg²⁺/K⁺ or that with low Mg²⁺/K⁺ and iso (p-values were all less than 0.001); (C) action potential representative traces of eight consecutive beats were recorded; (D) instability of the action potential duration of eight beats APs in Figure C (■ APD30; ● APD50; ▲ APD70; ▼APD90). APDs fluctuated most significantly after the treatment with cisapride and low Mg²⁺/K⁺;€ EAD occurred only under cisapride combined with low Mg²⁺/K⁺ and iso. (E) EAD occurred only under cisapride with low Mg&K and ISO

FIGURE 5

Evolution of QT prolongation caused by hERG inhibition and other factors. HERG inhibition alone led to QT prolongation, along with ERP prolongation, and exerted anti-arrhythmic effects. However, when combined with the effects of low magnesium, low potassium, or low frequency, the prolonged action potential became unstable, triangular, and discrete, and it was easy to induce VT. Still, without the occurrence of VF, VT could not be sustained. When catecholamines or ischemia and hypoxia continued to be combined, the APD was shortened, but instability, triangulation, and dispersion still existed. At this time, VF was easily induced and could be sustained.