Measurements of cardiac ion channel subunits in the chronic atrioventricular block dog

β-Adrenoceptor blockade can augment the torsadogenic action of risperidone

Risperidone is a second-generation antipsychotic for treating schizophrenia and bipolar disorder. It can potently inhibit IKr, but is classified into conditional risk for torsade de pointes (TdP) by CredibleMeds®. Our previous studies using chronic atrioventricular block dogs showed that risperidone alone did not induce TdP, and that dl-sotalol (β-adrenoceptor blockade plus IKr inhibition) induced TdP three times more frequently than d-sotalol (IKr inhibition alone). Since risperidone can block α1-adrenoceptor and decrease blood pressure, the resulting reflex-mediated increase of sympathetic tone on β-adrenoceptor might protect the heart from its IKr inhibition-associated TdP. To validate this hypothesis, risperidone was administered to chronic atrioventricular block dogs after β-adrenoceptor blocker atenolol infusion with monitoring J-Tpeak and Tpeak-Tend, which are proarrhythmic surrogate markers of "substrate" and "trigger" toward TdP, respectively. Atenolol alone induced TdP in 1 out of 5 dogs; moreover, an additional infusion of risperidone induced TdP in 3 out of 4 dogs. Risperidone prolonged QT interval, J-Tpeak and Tpeak-Tend in animals that induced TdP. These findings indicate that β-adrenoceptor blockade can diminish repolarization reserve to augment risperidone's torsadogenic potential, thus advising caution when using β-adrenoceptor blockers in patients with IKr inhibition-linked labile repolarization.

Characterization of pathological remodeling in the chronic atrioventricular block cynomolgus monkey heart

We studied time course of pathological remodeling occurring in the cynomolgus monkey hearts against persistent atrioventricular block condition (n = 10). The atrioventricular block induced the ventricular and atrial dilation followed by the ventricular hypertrophy. Interstitial fibrosis in the ventricle was also observed along with gradual increases in the plasma angiotensin II and aldosterone concentrations. These adaptations were associated with the changes in gene expression profiling reflecting fibrosis and hypertrophy. Atrioventricular block reduced the ventricular rate and cardiac output, but the ejection fraction and stroke volume increased, whereas the cardiac output was gradually restored to its basal level. Systolic/diastolic blood pressure after the atrioventricular block was kept equal to or lower than that before the block, according with lack of increase in the plasma catecholamine levels. Chronic atrioventricular block gradually prolonged the QRS width and JT interval, leading to the QT interval prolongation in conscious state. 10 mg/kg of dl-sotalol hydrochloride induced torsade de pointes (TdP) in 6 out of 10 animals by 15 months. Animals showing longer QTcF under anesthesia after the atrioventricular block developed dl-sotalol-induced TdP earlier. No marked difference was observed in pharmacokinetics of dl-sotalol between 1 and 7 months after the atrioventricular block. Each TdP spontaneously terminated, reflecting a monkey's relatively small "effective size of the heart (=∛(left ventricular weight)/wavelength of reentry)". These fundamental knowledge will help better utilize the chronic atrioventricular block monkeys as an in vivo proarrhythmia model for detecting drug-induced TdP.

Severe Bradycardia Increases the Incidence and Severity of Torsade de Pointes Arrhythmias by Augmenting Preexistent Spatial Dispersion of Repolarization in the CAVB Dog Model

INTRODUCTION

Torsade de pointes arrhythmias (TdP) in the chronic atrioventricular block (CAVB) dog model result from proarrhythmic factors, which trigger TdP and/or reinforce the arrhythmic substrate. This study investigated electrophysiological and arrhythmogenic consequences of severe bradycardia for TdP.

METHODS

Dofetilide (25 μg/kg per 5 min) was administered to eight anesthetized, idioventricular rhythm (IVR) remodeled CAVB dogs in two serial experiments: once under 60 beats per minute (bpm), right ventricular apex paced (RVA60) conditions, once under more bradycardic IVR conditions. Recordings included surface electrocardiogram and short-term variability (STV) of repolarization from endocardial unipolar electrograms. TdP inducibility (three or more episodes within 10 min after start of dofetilide) and arrhythmic activity scores (AS) were established. Mapping experiments in 10 additional dogs determined the effect of lowering rate on STV and spatial dispersion of repolarization (SDR) in baseline.

RESULTS

IVR-tested animals had longer baseline RR-interval (1,403 ± 271 ms) and repolarization intervals than RVA60 animals. Dofetilide increased STV similarly under both rhythm strategies. Nevertheless, TdP inducibility and AS were higher under IVR conditions (6/8 and 37 ± 27 vs. 1/8 and 8 ± 12 in RVA60, respectively, both p < 0.05). Mapping: Pacing from high (128 ± 10 bpm) to middle (88 ± 10 bpm) to experimental rate (61 ± 3 bpm) increased all electrophysiological parameters, including interventricular dispersion, due to steeper left ventricular restitution curves, and intraventricular SDR: maximal cubic dispersion from 60 ± 14 (high) to 69 ± 17 (middle) to 84 ± 22 ms (p < 0.05 vs. high and middle rate).

CONCLUSION

In CAVB dogs, severe bradycardia increases the probability and severity of arrhythmic events by heterogeneously causing electrophysiological instability, which is mainly reflected in an increased spatial, and to a lesser extent temporal, dispersion of repolarization.

Reverse translational analysis of clinically reported, lamotrigine-induced cardiovascular adverse events using the halothane-anesthetized dogs

Lamotrigine has been used for patients with epilepsy and/or bipolar disorder, overdose of which induced the hypotension, elevation of the atrial pacing threshold, cardiac conduction delay, wide complex tachycardia, cardiac arrest and Brugada-like electrocardiographic pattern. To clarify how lamotrigine induces those cardiovascular adverse events, we simultaneously assessed its cardiohemodynamic and electrophysiological effects using the halothane-anesthetized dogs (n = 4). Lamotrigine was intravenously administered in doses of 0.1, 1 and 10 mg/kg/10 min under the monitoring of cardiovascular variables, possibly providing subtherapeutic to supratherapeutic plasma concentrations. The low or middle dose of lamotrigine did not alter any of the variables. The high dose significantly delayed the intra-atrial and intra-ventricular conductions in addition to the prolongation of ventricular effective refractory period, whereas no significant change was detected in the other variables. Lamotrigine by itself has relatively wide safety margin for cardiohemodynamics, indicating that clinically reported hypotension may not be induced through its direct action on the resistance arterioles or capacitance venules. The electrophysiological effects suggested that lamotrigine can inhibit Na⁺ channel in the in situ hearts. This finding may partly explain the onset mechanism of lamotrigine-associated cardiac adverse events in the clinical cases. In addition, elevation of J wave was induced in half of the animals, suggesting that lamotrigine may have some potential to unmask Brugada electrocardiographic genotype in susceptible patients.

Impacts of Surgically Performed Renal Denervation on the Cardiovascular and Electrophysiological Variables in the Chronic Atrioventricular Block Dogs - Comparison With Those of Amiodarone Treatment

BACKGROUND

In order to begin to precisely clarify the impact of renal denervation on the blood pressure, atrial fibrillation and ventricular tachyarrhythmias, in addition to proarrhythmic potential, its cardiovascular effects were assessed by using the chronic complete atrioventricular block dogs.

METHODS AND RESULTS

Cardiohemodynamic and electrophysiological effects, together with neurohumoral factors and/or electrolytes, were assessed before and 4 weeks after either renal denervation (n=5) or amiodarone treatment (n=6). Amiodarone hydrochloride was given orally to the animals every day in a dose of 200 mg/day for the first 7 days followed by 100 mg/day for the following 21 days. The renal denervation decreased the systolic pressure, idioventricular rate, prolonged ventricular effective refractory period, and slightly suppressed the adrenergic tone and the renin-angiotensin-aldosterone system, but hardly affected the atrial effective refractory period and terminal repolarization period. Amiodarone prolonged the atrial effective refractory period, whereas no significant change was detected in the other variables.

CONCLUSIONS

Surgically performed renal denervation may possess the anti-ventricular tachyarrhythmic rather than anti-atrial fibrillatory potentials, and it also modestly decreased the blood pressure. Thus, currently obtained information may be used as guidance for better understanding the utility and limitation of renal denervation against various types of cardiovascular diseases. (Circ J 2016; 80: 1556-1563).

Overexpression of microRNA-1 causes atrioventricular block in rodents

The present study was designed to investigate whether microRNAs (miRNAs) are involved in atrioventricular block (AVB) in the setting of myocardial ischemia (MI). A cardiac-specific miR-1 transgenic (Tg) mouse model was successfully established for the first time in this study using microinjection. miR-1 level was measured by real-time qRT-PCR. Whole-cell patch clamp was employed to record L-type calcium current (I_Ca,L) and inward rectifier K⁺ current (I_K1). Expression of connexin 43 (Cx43) protein was determined by western blot analysis. Alternations of [Ca²⁺]_i was detected by laser scanning confocal microscopy in ventricular myocytes. The incidence of AVB was higher in miR-1 Tg mice than that in wild-type (WT) mice. The normalized peak current amplitude of I_Ca,L was lower in ventricular myocytes from miR-1 Tg mice as compared with WT mice. Similarly, the current density of I_K1 was decreased in miR-1 Tg mice than that in WT mice. Compared with WT mice, miR-1 Tg mice exhibited a significant decrease of the systolic [Ca²⁺]_i in ventricular myocytes but a prominent increase of the resting [Ca²⁺]_i. Moreover, Cx43 protein was downregulated in miR-1 Tg mice compared to that in WT mice. Administration of LNA-modified antimiR-1 reversed all the above changes. miR-1 overexpression may contribute to the increased susceptibility of the heart to AVB, which provides us novel insights into the molecular mechanisms underlying ischemic cardiac arrhythmias.