Effects of zacopride, a moderate IK1 channel agonist, on triggered arrhythmia and contractility in human ventricular myocardium

Abstract

Ventricular tachycardia is the leading cause of sudden arrhythmic death in the U.S. Recently, the moderate I_K1 channel activator, zacopride, was shown to suppress triggered ventricular tachycardia in rats. Nonetheless, concerns were raised about the possibility of pro-arrhythmic activity after I_K1 channel stimulation based on the promising anti-arrhythmic strategy of I_K1 blockade in other animal models. Therefore, the goal of the current study was to investigate the ex-vivo effects of zacopride on triggered arrhythmia and contractility in ventricular human myocardium in order to validate data that was solely obtained from animal models. Application of 100nmol/L isoproterenol and 0.5mmol/L caffeine led to triggered arrhythmia in isolated cardiac muscles from non-failing and end-stage failing hearts. However, the occurrence of arrhythmia in muscles of non-failing hearts was markedly higher than those of end-stage failing hearts. Interestingly, zacopride eliminated the ex-vivo triggered arrhythmia in these muscles of non-failing and failing hearts in a concentration-dependent manner, with an effective IC₅₀ in the range of 28-40μmol/L. Conversely, in the absence of isoproterenol/caffeine, zacopride led to a negative inotropic effect in a concentration-dependent manner. Reduced cardiac contraction was clearly observed at high zacopride concentration of 200μmol/L, along with the occurrence of contractile alternans in muscles of non-failing and failing hearts. Zacopride shows promising antiarrhythmic effects against triggered arrhythmia in human ventricular myocardium. However, in the absence of Ca²⁺ overload/arrhythmia, zacopride, albeit at high concentrations, decreases the force of contraction and increases the likelihood of occurrence of contractile alternans, which may predispose the heart to contractile dysfunction and/or arrhythmia. Overall, our results represent a key step in translating this drug from the benchtop to the bedside in the research area.

Keywords: Contractility; Human heart; I(K1) channel; Triggered arrhythmia; Zacopride.

Figure 1

Percentage of arrhythmia occurrence in ventricular trabeculae isolated from non-failing [RV: right ventricle (8 out of 10) & LV: left ventricle (5 out of 8)] and failing [RV (4 out of 12) & LV (3 out of 12)] human hearts.

Figure 2

Original representative recordings of twitches (normalized to cross-sectional area) in non-failing human myocardium at basal status, in presence of Iso + Caff [isoproterenol (100 nmol/L)/caffeine (0.5 mmol/L)] and zacopride effective antiarrhythmic concentrations when arrhythmias in the form of extra systolic contractions with either high (A & B) or low (C & D) amplitudes are triggered in trabeculae isolated from both RV, right ventricle (heart # 415217 & 514489) and LV, left ventricle (heart # 219852 & 514489) of non-failing hearts, respectively. Original representative recordings of twitches (normalized to cross-sectional area) in failing human myocardium at basal status, in presence of Iso + Caff and zacopride effective antiarrhythmic concentrations when arrhythmias in the form of extra systolic contractions with either high (E & F) or low (G & H) amplitudes are triggered in trabeculae isolated from both RV (heart # 233587 & 645444) and LV (heart # 728878 & 645444) of failing hearts, respectively. IC₅₀: median inhibitory concentrations of zacopride in the RV and LV of non-failing (I) and failing (J) human hearts.

Figure 3

The effect of zacopride on Iso/Caff [isoproterenol (100 nmol/L)/caffeine (0.5 mmol/L)]-induced changes in contractility of ventricular trabeculae isolated from the RV, right ventricle of failing human hearts, which are not susceptible to Iso/Caff-provoked triggered arrhythmia including: Fdev, peak isometric developed force (A); TTP, time to peak force (B); and RT50, time from peak force to 50% relaxation (C). Original representative recordings of twitches (normalized to cross-sectional area) in the RV of failing human myocardium (heart # 611422) that is not susceptible to Iso/Caff-provoked triggered arrhythmia at basal status, in presence of Iso/Caff and zacopride concentrations (5 – 100 µmol/L) (D). The effect of zacopride on Iso/Caff-induced changes in contractility of ventricular trabeculae isolated from the LV, left ventricle of failing human hearts, which are not susceptible to Iso/Caff-provoked triggered arrhythmia including: Fdev (E); TTP, time to peak force (F); and RT50 (G). Original representative recordings of twitches (normalized to cross-sectional area) in the LV of failing human myocardium (heart # 611422) that are not susceptible to Iso/Caff-provoked triggered arrhythmia at basal status, in presence of Iso/Caff and zacopride concentrations (5 – 100 µmol/L) (H). *A significant change compared to Iso/Caff, P ≤ 0.05. n = 4 – 9 based on time-dependent increase in zacopride concentration.

Figure 4

The effect of zacopride on contractile parameters of ventricular trabeculae isolated from the RV, right ventricle of non-failing human hearts, including: Fdev, peak isometric developed force (A); TTP, time to peak force (B); and RT50, time from peak force to 50% relaxation (C). Superimposed original representative recordings of twitches (normalized to cross-sectional area) in the RV of non-failing human myocardium (heart # 958987) in the presence of zacopride (5 – 200 µmol/L) showing a concentration-dependent decrease in the Fdev and occurrence of contractile alternans at the highest applied concentration of 200 µmol/L (D). The effect of zacopride on contractile parameters of ventricular trabeculae isolated from the LV, left ventricle of non-failing human hearts, including, Fdev (E), TTP (F) and RT50 (G). Superimposed original representative recordings of twitches (normalized to cross-sectional area) in the LV of non-failing human myocardium (heart # 958987) in the presence of zacopride (5 – 200 µmol/L) showing a concentration-dependent decrease in the Fdev (H). *A significant change compared to basal (zacopride concentration: 0 µmol/L), P ≤ 0.05 and n = 4 – 5.

Figure 5

The effect of zacopride on contractile parameters of ventricular trabeculae isolated from the RV, right ventricle of failing human hearts, including: Fdev, peak isometric developed force (A); TTP, time to peak force (B); and RT50, time from peak force to 50% relaxation (C). Superimposed original representative recordings of twitches (normalized to cross-sectional area) in the RV of failing human myocardium (heart # 904475) in the presence of zacopride (5 – 200 µmol/L) showing a concentration-dependent decrease in the Fdev and occurrence of contractile alternans at the highest applied concentration of 200 µmol/L (D). The effect of zacopride on contractile parameters of ventricular trabeculae isolated from the LV, left ventricle of failing human hearts, including, Fdev (E), TTP (F) and RT50 (G). Superimposed original representative recordings of twitches (normalized to cross-sectional area) in the LV of non-failing human myocardium (heart # 820447) in the presence of zacopride (5 – 200 µmol/L) showing a concentration-dependent decrease in the Fdev and occurrence of contractile alternans at the highest applied concentration of 200 µmol/L (H). n = 5 – 7.

Figure 6

Percentage of the occurrence of contractile alternans in ventricular trabeculae isolated from non-filing [RV, right ventricle (3 out of 5) & LV, left ventricle (0 out of 4)] and failing [RV (5 out of 7) & LV (2 out of 6)] human hearts.