Effects of SEA0400 on Arrhythmogenicity in a Langendorff-Perfused 1-Month Myocardial Infarction Rabbit Model

Antiarrhythmic and Inotropic Effects of Selective Na+/Ca2+ Exchanger Inhibition: What Can We Learn from the Pharmacological Studies?

Life-long stable heart function requires a critical balance of intracellular Ca²⁺. Several ion channels and pumps cooperate in a complex machinery that controls the influx, release, and efflux of Ca²⁺. Probably one of the most interesting and most complex players of this crosstalk is the Na⁺/Ca²⁺ exchanger, which represents the main Ca²⁺ efflux mechanism; however, under some circumstances, it can also bring Ca²⁺ into the cell. Therefore, the inhibition of the Na⁺/Ca²⁺ exchanger has emerged as one of the most promising possible pharmacological targets to increase Ca²⁺ levels, to decrease arrhythmogenic depolarizations, and to reduce excessive Ca²⁺ influx. In line with this, as a response to increasing demand, several more or less selective Na⁺/Ca²⁺ exchanger inhibitor compounds have been developed. In the past 20 years, several results have been published regarding the effect of Na⁺/Ca²⁺ exchanger inhibition under various circumstances, e.g., species, inhibitor compounds, and experimental conditions; however, the results are often controversial. Does selective Na⁺/Ca²⁺ exchanger inhibition have any future in clinical pharmacological practice? In this review, the experimental results of Na⁺/Ca²⁺ exchanger inhibition are summarized focusing on the data obtained by novel highly selective inhibitors.

Sacubitril/Valsartan Therapy Ameliorates Ventricular Tachyarrhythmia Inducibility in a Rabbit Myocardial Infarction Model

BACKGROUND

Coronary artery disease is the most common cause of heart failure (HF) in developed countries. The aim of this study was to elucidate the mechanisms of reduction of arrhythmias after sacubitril/valsartan (LCZ696) therapy in a myocardial infarction (MI)-HF rabbit model.

METHODS AND RESULTS

Chronic MI in rabbits with HF were divided into 3 groups: placebo control, valsartan 30 mg/day and LCZ696 60 mg/day. After 4 weeks of therapy, an electrophysiologic study and a dual voltage-calcium optical mapping study were performed. The LCZ696 group had significantly better left ventricular ejection fraction and lower ventricular tachyarrhythmia inducibility than the valsartan and placebo groups. The most common ventricular tachyarrhythmia pattern was 1 or 2 ectopic beats originating from the peri-infarct areas, followed by re-entrant beats surrounding phase singularity points. Compared to the valsartan and placebo groups, the LCZ696 group had significantly shorter action-potential duration, shorter intracellular calcium tau constant, faster conduction velocity, and shorter pacing cycle length to induce arrhythmogenic alternans. LCZ696 therapy reduced the phosphorylated calmodulin-dependent protein kinase II (CaMKII-p) expression.

CONCLUSIONS

In a rabbit model with chronic MI and HF, LCZ696 therapy ameliorated postinfarct heart function impairment and electrophysiologic remodeling and altered CaMKII-p expression, leading to reduced ventricular tachyarrhythmia inducibility.

Roles of impaired intracellular calcium cycling in arrhythmogenicity of diabetic mouse model

BACKGROUND

Diabetes mellitus is associated an increased risk of ventricular arrhythmias (VAs), but the underlying electrophysiological mechanisms are not fully explored. This study was aimed to test whether dynamic factors and Ca_i handling play roles in arrhythmogenesis of a diabetic animal model.

METHODS

We used 26 db/db type 2 diabetes mice and 28 control mice in this study. VA inducibility was evaluated in vivo under isoflurane general anesthesia. The intracellular Ca²⁺ (Ca_i ) and membrane voltage (V_m ) signals of the Langendorff-perfused mouse hearts were simultaneously recorded using the optical mapping technique. Action potential duration (APD), Ca_i dynamics conduction velocity (CV), and arrhythmogenic alternans were analyzed. Western blot was conducted to examine expressions of calcium handling and associated ion channels proteins.

RESULTS

The diabetic db/db mice showed significantly increased VA inducibility and severity. Longer APD and Ca_i transient duration and slower Ca_i decay and CV in the db/db mice than these in the control ones were observed. Dynamic pacing showed increased incidence of spatially discordant alternans leading to more VA inducibility in the db/db mice. Western blot analyses revealed increased phosphorylated-Ca²⁺ /calmodulin-dependent protein kinase II protein expression and decreased ryanodine receptor protein expression, which probably underlay the molecular mechanisms of enhanced arrhythmogenicity in db/db mice.

CONCLUSIONS

The type 2 diabetic mouse hearts show impaired repolarization, Ca_i handling homeostasis, and cardiac conduction reserve, leading to vulnerability of spatially discordant alternans development and induction of VA. Altered Ca_i -handling protein expressions probably underlie the molecular mechanisms of arrhythmogenicity in the type 2 diabetes animal model.

Role of sarcoplasmic reticulum calcium in development of secondary calcium rise and early afterdepolarizations in long QT syndrome rabbit model

Background

L-type calcium current reactivation plays an important role in development of early afterdepolarizations (EADs) and torsades de pointes (TdP). Secondary intracellular calcium (Ca_i) rise is associated with initiation of EADs.

Objective

To test whether inhibition of sarcoplasmic reticulum (SR) Ca²⁺ cycling suppresses secondary Ca_i rise and genesis of EADs.

Methods

Langendorff perfusion and dual voltage and Ca_i optical mapping were conducted in 10 rabbit hearts. Atrioventricular block (AVB) was created by radiofrequency ablation. After baseline studies, E4031, SR Ca²⁺ cycling inhibitors (ryanodine plus thapsigargin) and nifedipine were then administrated subsequently, and the protocols were repeated.

Results

At baseline, there was no spontaneous or pacing-induced TdP. After E4031 administration, action potential duration (APD) was significantly prolonged and the amplitude of secondary Ca_i rise was enhanced, and 7 (70%) rabbits developed spontaneous or pacing-induced TdP. In the presence of ryanodine plus thapsigargin, TdP inducibility was significantly reduced (2 hearts, 20%, p = 0.03). Although APD was significantly prolonged (from 298 ± 30 ms to 457 ± 75 ms at pacing cycle length of 1000 m, p = 0.007) by ryanodine plus thapsigargin, the secondary Ca_i rise was suppressed (from 8.8 ± 2.6% to 1.2 ± 0.9%, p = 0.02). Nifedipine inhibited TdP inducibility in all rabbit hearts.

Conclusion

In this AVB and long QT rabbit model, inhibition of SR Ca²⁺ cycyling reduces the inducibility of TdP. The mechanism might be suppression of secondary Ca_i rise and genesis of EADs.

Paradoxical effects of KB-R7943 on arrhythmogenicity in a chronic myocardial infarction rabbit model

BACKGROUND

Na(+)/Ca(2+) exchanger blockade has been reported to be anti-arrhythmic in different models. The effects of KB-R7943, a Na(+)/Ca(2+) exchanger blocker, on arrhythmogenesis in hearts with chronic myocardial infarction (MI) remain unclear.

METHODS

Dual voltage and intracellular Ca(2+) (Cai) optical mapping was performed in nine rabbit hearts with chronic MI and four control hearts. Electrophysiology studies including inducibility of ventricular tachyarrhythmias, ventricular fibrillation dominant frequency, action potential, Cai alternans, Cai decay, and conduction velocity were performed. The same protocol was repeated in the presence of KB-R7943 (0.5, 1, and 5μM) after the baseline studies.

RESULTS

KB-R7943 was effective in suppressing afterdepolarizations and spontaneous ventricular tachyarrhythmias in hearts with chronic MI. Surprisingly, KB-R7943 increased the inducibility of ventricular tachyarrhythmias in a dose-dependent manner (11%, 11%, 22%, and 56% at baseline and with 0.5, 1, and 5μM KB-R7943, respectively, p=0.02). Optical mapping analysis revealed that the underlying mechanisms of the induced ventricular tachyarrhythmias were probably spatially discordant alternans with wave breaks and rotors. Further analysis showed that KB-R7943 significantly enhanced both action potential (p=0.033) and Cai (p=0.001) alternans, prolonged Cai decay (tau value) in a dose-dependent manner (p=0.004), and caused heterogeneous conduction delay especially at peri-infarct zones during rapid burst pacing. In contrast, KB-R7943 had insignificant effects in control hearts.

CONCLUSIONS

In this chronic MI rabbit model, KB-R7943 has contrasting effects on arrhythmogenesis, suppressing afterdepolarizations and spontaneous ventricular tachyarrhythmias, but enhancing the inducibility of tachyarrhythmias. The mechanism is probably the enhanced spatially discordant alternans because of prolonged Cai decay and heterogeneous conduction delay.