Regional gap junction inhibition increases defibrillation thresholds

Muscone attenuates susceptibility to ventricular arrhythmia by inhibiting NLRP3 inflammasome activation in rats after myocardial infarction

Fibrosis and abnormal expression of connexin 43 (Cx43) in the ventricle play vital roles in ventricular arrhythmias (VAs) after myocardial infarction (MI). Muscone, an active monomer of heart-protecting musk pill, has various biological activities, but its effect on susceptibility to VAs in rats with MI has not been determined. In the present study, we investigated the effects of muscone on ventricular inflammation, fibrosis, Cx43 expression, and the occurrence of VAs after MI. An MI model was established by ligating the proximal left anterior descending coronary artery. Then, the MI model rats were administered muscone (2 mg/kg/day) or vehicle (saline)via intragastric injection for 14 days. Cardiac function was evaluated by echocardiography, and an in vivo electrophysiological study was performed on Day 14. Cardiac inflammation, fibrosis, and Cx43 expression were determined by histochemical analysis and western blot analysis. Our results indicated that muscone treatment significantly improved cardiac function and inhibited ventricular inflammation, fibrosis, and nucleotide-binding oligomerization domain, leucine-rich repeat, and pyrin domain containing 3 (NLRP3) inflammasome activation. Electrocardiogrphy and electrophysiology studies showed that muscone shortened the QRS interval, QT interval, QTc interval, and action potential duration; prolonged the effective refractory period; and reduced susceptibility to VAs in rats after MI. Furthermore, Cx43 expression in the BZ was increased by muscone treatment, and this change was coupled by inhibition of the NLRP3/IL-1β/p38 MAPK pathway. Taken together, our results demonstrated that muscone reduces susceptibility to VA, mainly by decreasing ventricular inflammation and fibrosis, and attenuates abnormal Cx43 expression by inhibiting NLRP3 inflammasome activation after myocardial infarction in rats.

Melatonin treatment improves ventricular conduction via upregulation of Nav1.5 channel proteins and sodium current in the normal rat heart

Melatonin treatment was reported to reduce the risk of cardiac arrhythmias, and crucial for this antiarrhythmic action was the effect of melatonin on activation spread. The aim of the present study was evaluation of the mechanisms of this activation enhancement. Experiments were performed in a total of 123 control and melatonin-treated (10 mg/kg, daily, for 7 days) male Wistar rats. In epicardial mapping studies (64 leads, interlead distance 0.5 mm) in the anesthetized animals, activation times (ATs) were determined in each lead as dV/dt minimum during QRS complex under sinus rhythm. Epicardial pacing was performed to measure conduction velocity (CV) across the mapped area. Average left ventricular ATs were shorter in the treated animals as compared to the controls, whereas the minimal epicardial ATs indicating the duration of activation propagation via the ventricular conduction system did not differ between the groups. CV was higher in the treated groups indicating that melatonin affected conduction via contractile myocardium The area of Cx43-derived fluorescence, as well as the expression of Cx43 protein, was similar in ventricles in the control and melatonin-treated groups. Expression of Gja1 gene transcripts encoding Cx43, was increased in the last group. An uncoupling agent octanol modified myocardial conduction properties (time of activation, action potential upstroke velocity, passive electrotonic phase duration) similarly in both groups. On the other hand, the expression of both Scn5a gene transcripts encoding Nav1.5 proteins, as well as peak density of transmembrane sodium current were increased in the ventricular myocytes from the melatonin-treated animals. Thus, a week-long melatonin treatment caused the increase of conduction velocity via enhancement of sodium channel proteins expression and increase of sodium current in the ventricular myocytes.

Heterogeneities in Ventricular Conduction Following Treatment with Heptanol: A Multi-Electrode Array Study in Langendorff-Perfused Mouse Hearts

Background: Previous studies have associated slowed ventricular conduction with the arrhythmogenesis mediated by the gap junction and sodium channel inhibitor heptanol in mouse hearts. However, they did not study the propagation patterns that might contribute to the arrhythmic substrate. This study used a multi-electrode array mapping technique to further investigate different conduction abnormalities in Langendorff-perfused mouse hearts exposed to 0.1 or 2 mM heptanol. Methods: Recordings were made from the left ventricular epicardium using multi-electrode arrays in spontaneously beating hearts during right ventricular 8 Hz pacing or S1S2 pacing. Results: In spontaneously beating hearts, heptanol at 0.1 and 2 mM significantly reduced the heart rate from 314 ± 25 to 189 ± 24 and 157 ± 7 bpm, respectively (ANOVA, p < 0.05 and p < 0.001). During regular 8 Hz pacing, the mean LATs were increased by 0.1 and 2 mM heptanol from 7.1 ± 2.2 ms to 19.9 ± 5.0 ms (p < 0.05) and 18.4 ± 5.7 ms (p < 0.05). The standard deviation of the mean LATs was increased from 2.5 ± 0.8 ms to 10.3 ± 4.0 ms and 8.0 ± 2.5 ms (p < 0.05), and the median of phase differences was increased from 1.7 ± 1.1 ms to 13.9 ± 7.8 ms and 12.1 ± 5.0 ms by 0.1 and 2 mM heptanol (p < 0.05). P5 took a value of 0.2 ± 0.1 ms and was not significantly altered by heptanol at 0.1 or 2 mM (1.1 ± 0.9 ms and 0.9 ± 0.5 ms, p > 0.05). P50 was increased from 7.3 ± 2.7 ms to 24.0 ± 12.0 ms by 0.1 mM heptanol and then to 22.5 ± 7.5 ms by 2 mM heptanol (p < 0.05). P95 was increased from 1.7 ± 1.1 ms to 13.9 ± 7.8 ms by 0.1 mM heptanol and to 12.1 ± 5.0 ms by 2 mM heptanol (p < 0.05). These changes led to increases in the absolute inhomogeneity in conduction (P5−95) from 7.1 ± 2.6 ms to 31.4 ± 11.3 ms, 2 mM: 21.6 ± 7.2 ms, respectively (p < 0.05). The inhomogeneity index (P5−95/P50) was significantly reduced from 3.7 ± 1.2 to 3.1 ± 0.8 by 0.1 mM and then to 3.3 ± 0.9 by 2 mM heptanol (p < 0.05). Conclusion: Increased activation latencies, reduced CVs, and the increased inhomogeneity index of conduction were associated with both spontaneous and induced ventricular arrhythmias.

Ventricular Arrhythmias in First Acute Myocardial Infarction: Epidemiology, Mechanisms, and Interventions in Large Animal Models

Ventricular arrhythmia and subsequent sudden cardiac death (SCD) due to acute myocardial infarction (AMI) is one of the most frequent causes of death in humans. Lethal ventricular arrhythmias like ventricular fibrillation (VF) prior to hospitalization have been reported to occur in more than 10% of all AMI cases and survival in these patients is poor. Identification of risk factors and mechanisms for VF following AMI as well as implementing new risk stratification models and therapeutic approaches is therefore an important step to reduce mortality in people with high cardiovascular risk. Studying spontaneous VF following AMI in humans is challenging as it often occurs unexpectedly in a low risk subgroup. Large animal models of AMI can help to bridge this knowledge gap and are utilized to investigate occurrence of arrhythmias, involved mechanisms and therapeutic options. Comparable anatomy and physiology allow for this translational approach. Through experimental focus, using state-of-the-art technologies, including refined electrical mapping equipment and novel pharmacological investigations, valuable insights into arrhythmia mechanisms and possible interventions for arrhythmia-induced SCD during the early phase of AMI are now beginning to emerge. This review describes large experimental animal models of AMI with focus on first AMI-associated ventricular arrhythmias. In this context, epidemiology of first AMI, arrhythmogenic mechanisms and various potential therapeutic pharmacological targets will be discussed.

Ultrasound improves the outcomes of cardiopulmonary resuscitation in rats by stimulating the cholinergic anti‑inflammatory pathway

The present study investigated the effects of the ultrasound (US), a noninvasive technique, on ischemia-reperfusion injury (IRI) following cardiopulmonary resuscitation (CPR). The animals used in the present study were randomized into five groups (n=8 per group) as follows: i) The CPR group, where the rats underwent 6 min of untreated ventricular fibrillation (VF) followed by CPR and defibrillation; ii) the US group, in which the treatment was identical to the CPR group with the exception that rats were exposed to US treatment 24 h prior to CPR; iii) the MLA group, in which the treatment was identical to the US group with the exception that the α7 nicotinic acetylcholine receptor (α7nAChR) antagonist MLA (4 mg/kg) was administered 30 min prior to US and VF respectively; iv) the GTS group, in which the treatment was identical to the CPR group with the exception that the α7nAChR agonist GTS-21 (4 mg/kg) was injected 30 min prior to VF; and v) the SHAM group, in which the rats were exposed to surgical preparation without CPR and US application. At 1 day prior to CPR, the US treatment was administered to the left kidney by US pulses (contrast general mode with 9 MHz) with a bursting mechanical index of 0.72 for 2 min. Following treatment of the left kidney, the right kidney was exposed to identical US treatment for an additional 2 min. The results demonstrated that US preconditioning decreased the number of defibrillations required and shortened the duration of CPR. US also suppressed tumor necrosis factor-α and interleukin-6 levels following resuscitation (P<0.05), and a significantly longer overall survival time was observed in the US-treated animals (P<0.01). In addition, US attenuated neuronal injury and promoted the expression of α7nAChR in hippocampal neurons (P<0.05). However, the protective effects of US were abolished by MLA and imitated by GTS-21. The results of the present study demonstrated that prior exposure to US may improve animal outcomes following CPR, and the protective effects of US may be dependent on the cholinergic anti-inflammatory pathway (CAP) via α7nAChR.

Xuezhikang improves the outcomes of cardiopulmonary resuscitation in rats by suppressing the inflammation response through TLR4/NF-κB pathway

BACKGROUND/AIMS

Xuezhikang (XZK), a red yeast rice extract with lipid-lowering effect, contains a family of naturally statins, such as lovastatin. In recent years, its effect beyond the regulation of lipids has also been received increasing attention. Therefore, the purpose of this study was to explore the protective effects and possible molecular mechanisms of XZK on brain injury after cardiac arrest (CA) and cardiopulmonary resuscitation (CPR), and to investigate whether it has a dose-dependent effect and the difference with lovastatin.

METHODS

Rats were treated with low-dose XZK (XZK-L, 20 mg/kg/d), high-dose XZK (XZK-H, 200 mg/kg/d) and lovastatin by gavage once daily for 2 weeks before CA. The levels of TNF-α, IL-6 and IL-1β were evaluated at 1, 4, and 72 h post-CA/CPR. The survival rate, neurological deficit score (NDS), and expression of TLR4, phosphorylated NF-κB and TNF-α in hippocampal tissues were evaluated at 72 h post-CA/CPR.

RESULTS

CA/CPR induced a significant increase in serum TNF-α, IL-6 and IL-1β, as well as increased expressions of TLR4, phosphorylated NF-κB and TNF-α in the hippocampus. Both low-dose and high-dose XZK treatment inhibited the expression of these inflammatory cytokines. In addition, it reduced the number of defibrillations and shortened the duration of CPR required for return of spontaneous circulation (ROSC). XZK treatment also improved neurological function and 72-hour survival rate in rats. However, high-dose XZK was superior to lovastatin in the suppression of IL-1β mRNA level and TNF-α protein level in hippocampal tissue after CPR. There were no significant differences observed among high-dose XZK, low-dose XZK and lovastatin groups in other respects.

CONCLUSION

These results indicated that XZK had a protective effect against brain injury post-CA/CPR. The mechanisms underlying the protective effects of XZK may be related to the suppressing of CA/CPR-induced inflammatory response through the inhibiting TLR4/NF-κB signaling pathway.

Effects of Rotigaptide, a Gap Junction Modifier, on Defibrillation Energy and Resuscitation From Cardiac Arrest in Rabbits

The gap junction modifier Rotigaptide (ZP123), which promotes cellular coupling, was hypothesized to decrease defibrillation thresholds during prolonged ventricular fibrillation (VF). Thirty-two New Zealand white rabbits were randomized to receive saline (control, n = 16) or Rotigaptide (n = 16). Following 4 min of untreated VF, biphasic defibrillation shocks were applied through chest wall patches, starting either at 300 volts (V) (n = 16) or 500 V (n = 16), with 200 V increasing steps to 900 V in case of shock failure. Rotigaptide significantly decreased defibrillation voltage requirements (average cumulative voltage of all shocks: 1206 ± 709 V in control group vs. 844 ± 546 V in treated group, P = .002). Rotigaptide had no effect on heart rate, QRS duration, QT interval, ventricular effective refractory period, monophasic action potential duration or on connexin 43 density using immunofluorescence. Rotigaptide improves the ability to defibrillate after untreated VF.

Ventricular arrhythmogenesis following slowed conduction in heptanol-treated, Langendorff-perfused mouse hearts

Arrhythmogenic effects of slowed action potential conduction produced by the gap junction and sodium-channel inhibitor heptanol (0.1-2 mM) were explored in Langendorff-perfused mouse hearts. Monophasic action potential recordings showed that 2 mM heptanol induced ventricular tachycardia in the absence of triggered activity arising from early or after-depolarizations during regular 8 Hz pacing and programmed electrical stimulation (PES). It also increased activation latencies and ventricular effective refractory periods (VERPs), but did not alter action potential duration (APD), thereby reducing local critical intervals for re-excitation given by APD(90) - VERP. Bipolar electrogram recordings showed that 2 mM heptanol increased electrogram duration (EGD) and ratios of EGDs obtained at the longest to those obtained at the shortest S1S2 intervals studied during PES, suggesting increased dispersion of conduction velocities. These findings show, for the first time in the mouse heart, that slowed conduction induces reversible arrhythmogenic effects despite repolarization abnormalities expected to reduce arrhythmogenicity.

Pharmacological modulation of connexin-formed channels in cardiac pathophysiology

Coordinated electrical activity in the heart is supported by gap junction channels located at the intercalated discs of cardiomyocytes. Impaired gap junctional communication between neighbouring cardiomyocytes contributes to the development of re-entry arrhythmias after myocardial ischaemia. Current antiarrhythmic therapy is hampered by a lack of efficiency and side effects, creating the need for a new generation of drugs. In this review, we focus on compounds that increase gap junctional communication, thereby increasing the conduction velocity and decreasing the risk of arrhythmias. Some of these compounds also inhibit connexin 43 (Cx43) hemichannels, thereby limiting adenosine triphosphate loss and volume overload following ischaemia/reperfusion, thus potentially increasing the survival of cardiomyocytes. The compounds discussed in this review are: (i) antiarrythmic peptide (AAP), AAP10, ZP123; (ii) GAP-134; (iii) RXP-E; and (vi) the Cx mimetic peptides Gap 26 and Gap 27. None of these compounds have effects on Na(+) , Ca(2+) and K(+) channels, and therefore have no proarrhythmic activity associated with currently available antiarrhythmic drugs. GAP-134, RXP-E, Gap 26 and Gap 27 are pharmalogical agents with a favorable clinical safety profile, as already confirmed in phase I clinical trials for GAP-134. These agents show an excellent promise for treatment of arrhythmias in patients with ischaemic cardiomyopathy.

Effect of ZP123, a gap junction modifier, on prolonged ventricular fibrillation in swine

OBJECTIVES

It was the aim of this study to investigate the effect of ZP123 on prolonged ventricular fibrillation (VF) in swine.

METHODS

VF was electrically induced in 20 pigs. The animals randomly received either ZP123 or saline control infusion before VF. After 8 min of untreated VF, cardiopulmonary resuscitation and biphasic defibrillation shocks were applied. VF mean frequency (VF(mf)) and mean amplitude (VF(ma)), hemodynamics, outcome of defibrillation and the rate of return of spontaneous circulation (ROSC) were analyzed.

RESULTS

Compared with the control group, VF(mf) was higher but VF(ma) lower during the 8 min of VF in the drug group (11.8 ± 2.1 vs. 10.4 ± 2.0 Hz and 0.24 ± 0.10 vs. 0.31 ± 0.16 mV, respectively; p < 0.05). Hemodynamic variables in the 2 groups were comparable (p > 0.05). The defibrillation threshold was lower and the rate of successful defibrillation was higher in the drug group compared with the control group (92.2 ± 26.4 vs. 133.3 ± 28.9 J and 90 vs. 30%, respectively; p < 0.05). The rate of ROSC was not different between the 2 groups (40 vs. 30%; p > 0.05).

CONCLUSION

In prolonged VF, ZP123 could decrease the defibrillation threshold and improve the rate of successful defibrillation. However, it could not improve the rate of ROSC - which may be due to its side effect of decreasing VF(ma).

Myocardial Ca2+ handling and cell-to-cell coupling, key factors in prevention of sudden cardiac deathThis article is one of a selection of papers published in a special issue on Advances in Cardiovascular Research

Using whole-heart preparations, we tested our hypothesis that Ca2+ handling is closely related to cell-to-cell coupling at the gap junctions and that both are critical for the development and particularly the termination of ventricular fibrillation (VF) and hence the prevention of sudden arrhythmic death. Intracellular free calcium concentration ([Ca2+]i), ECG, and left ventricular pressure were continuously monitored in isolated guinea pig hearts before and during development of low K+-induced sustained VF and during its conversion into sinus rhythm facilitated by stobadine. We also examined myocardial ultrastructure to detect cell-to-cell coupling alterations. We demonstrated that VF occurrence was preceded by a 55.9% ± 6.2% increase in diastolic [Ca2+]i, which was associated with subcellular alterations indicating Ca2+ overload of the cardiomyocytes and disorders in coupling among the cells. Moreover, VF itself further increased [Ca2+]i by 58.2% ± 3.4% and deteriorated subcellular and cell-to-cell coupling abnormalities that were heterogeneously distributed throughout the myocardium. In contrast, termination of VF and its conversion into sinus rhythm was marked by restoration of basal [Ca2+]i, resulting in recovery of intercellular coupling linked with synchronous contraction. Furthermore, we have shown that hearts exhibiting lower SERCA2a (sarcoplasmic reticulum Ca2+-ATPase) activity and abnormal intercellular coupling (as in older guinea pigs) are more prone to develop Ca2+ overload associated with cell-to-cell uncoupling than hearts with higher SERCA2a activity (as in young guinea pigs). Consequently, young animals are better able to terminate VF spontaneously. These findings indicate the crucial role of Ca2+ handling in relation to cell-to-cell coupling in both the occurrence and termination of malignant arrhythmia.

Pharmacological modulation of cardiac gap junctions to enhance cardiac conduction: evidence supporting a novel target for antiarrhythmic therapy

BACKGROUND

Disease-induced alterations of cardiac gap junctions lead to intercellular uncoupling, which is an important mechanism of arrhythmogenesis. Therefore, drugs that selectively open gap junctions potentially offer a novel strategy for antiarrhythmic therapy. Because the peptide ZP123 was found to increase conductance between paired myocytes, we hypothesized that ZP123 would suppress acidosis-induced gap junction closure in the intact heart.

METHODS AND RESULTS

High-resolution optical mapping was used to measure conduction velocity (CV) and action potential duration from ventricular epicardium of Langendorff-perfused guinea pig hearts at baseline (pH 7.4) and during 45 minutes of perfusion with acidotic (pH 6.0) Tyrode's solution with (n=8) and without (control, n=7) ZP123 (80 nmol/L). Acidosis produced conduction slowing transverse (29.1+/-0.1 to 16.8+/-0.2 cm/s, P<0.0001) and longitudinal (47.2+/-2.4 to 33.2+/-4.8 cm/s, P<0.0001) to cardiac fibers. Importantly, ZP123 inhibited conduction slowing during acidosis by approximately 60%. The peak effect of ZP123 was achieved after 16 minutes of acidosis, consistent with inhibition of uncoupling. ZP123 did not affect Na+ current in isolated myocytes, additionally affirming that preservation of CV was attributable to the compound's action on gap junctions. ZP123 had no effect on CV in the absence of acidosis, suggesting that drug activity targets gap junctions under metabolic stress. Action potential duration heterogeneity was significantly reduced by ZP123 (6.7+/-0.8 ms) compared with controls (9.7+/-3.1 ms, P<0.05), presumably by enhancing cell-to-cell coupling.

CONCLUSIONS

These data suggest that ZP123 significantly attenuates gap junction closure during acidosis. Preservation of intercellular coupling diminished CV slowing and heterogeneous repolarization, eliminating arrhythmogenic substrates.