Monophasic Action Research Articles

Antiarrhythmic drug effects on premature beats are partly determined by prior cardiac activation frequency in perfused guinea‐pig heart

What is the central question of this study? Can antiarrhythmic drug effects on repolarization, conduction time and excitation wavelength in premature beats be determined by prior cardiac activation frequency? What is the main finding and its importance? In premature beats induced after a series of cardiac activations at a slow rate, antiarrhythmics prolong repolarization but evoke little or no conduction delay, thus increasing the excitation wavelength, which indicates an antiarrhythmic effect. Fast prior activation rate attenuates prolongation of repolarization, while amplifying the conduction delay induced by drugs, which translates into the reduced excitation wavelength, indicating proarrhythmia. These findings suggest that a sudden increase in heart rate can shape adverse pharmacological profiles in patients with ventricular ectopy. Antiarrhythmic drugs used to treat atrial fibrillation can occasionally induce ventricular tachyarrhythmia, which is typically precipitated by a premature ectopic beat through a mechanism related, in part, to the shortening of the excitation wavelength (EW). The arrhythmia is likely to occur when a drug induces a reduction, rather than an increase, in the EW of ectopic beats. In this study, I examined whether the arrhythmic drug profile is shaped by the increased cardiac activation rate before ectopic excitation. Ventricular monophasic action potential durations, conduction times and EW values were assessed during programmed stimulations applied at long (S1 -S1 [basic drive cycle length] =550ms) and short (S1 -S1 =200ms) cycle lengths in perfused guinea-pig hearts. The premature activations were induced with extrastimulus application immediately upon termination of the refractory period. With dofetilide, a classIII antiarrhythmic agent, a prolongation in action potential duration and the resulting increase in the EW obtained at S1 -S1 =550ms were significantly attenuated at S1 -S1 =200ms, in both the regular (S1 ) and the premature (S2 ) beats. With classI antiarrhythmic agents (quinidine, procainamide and flecainide), fast S1 -S1 pacing was found to attenuate the drug-induced increase in action potential duration, while amplifying drug-induced conduction slowing, in both S1 and S2 beats. As a result, although the EW was increased (quinidine and procainamide) or not changed (flecainide) at the long S1 -S1 intervals, it was invariably reduced by these agents at the short S1 -S1 intervals. These findings indicate that the increased heart rate before ectopic activation shapes the arrhythmic profiles by facilitating drug-induced EW reduction.

Study of the time-relationship of the mechano-electrical interaction in an animal model of tetralogy of Fallot: implications for the risk assessment of ventricular arrhythmias.

The long-term outcome of tetralogy of Fallot (TOF) is determined by progressive right ventricular (RV) dysfunction through pulmonary regurgitation (PR) and the risk of malignant arrhythmia. Although mechano-electrical coupling in TOF is well-known, its time effect on the inducibility of arrhythmia remains ill-defined. The goal of this study was to investigate the mechano-electrical properties at different times in animals with chronic PR. PR was induced by a transannular patch with limited RV scarring in infant pigs. Haemodynamic assessment included biventricular pressure-volume loops after 3 (n = 8) and 6 months (n = 7) compared to controls (n = 5). The electrophysiological study included endocardial monophasic action potential registration, intraventricular conduction velocity and induction of ventricular arrhythmia by burst pacing. Progressive RV dilation was achieved at 6 months (RV end-diastolic volume 143 ± 13 ml/m2-RV end-systolic volume 96 ± 7 ml/m2; P < 0.001), in association with depressed RV contractility (preload recruitable stroke work-slope: 19 ± 1 and 11 ± 3 Mw.ml-1.s-1 for control and 6 m; P < 0.001) and left ventricular contractility (preload recruitable stroke work-slope: 60 ± 13 and 40 ± 11 Mw.ml-1.s-1 for control and 6 m; P = 0.005). Concomitant to QRS prolongation, monophasic action potential90-duration and dispersion at the RV and left ventricle were increased at 6 months. Intraventricular conduction was delayed only in the RV at 6 months (1.8 ± 0.2 and 2.4 ± 0.6 m/s for group 6M and the control group; P = 0.035). Sustained ventricular arrhythmias were not inducible. In animals yielding the sequelae of a contemporary operation for TOF, mechano-electrical alterations are progressive and affect predominantly the RV after midterm exposure of PR. Because ventricular arrhythmias were not inducible despite significant RV dilation, the data suggest that the haemodynamic RV deterioration effectively precedes the risk of inducing sustained arrhythmia after TOF repair and opens a window for renewed stratification of contemporary risk factors of ventricular arrhythmias in patients operated on with currently used pulmonary valve- and RV-related techniques.

Sensitivity of inhalation anesthetics isoflurane and sevoflurane for the drug-induced QT-interval prolongation in guinea pigs

We investigated effects of isoflurane and sevoflurane on sparfloxacin-induced QT-interval prolongation in guinea pigs under the monitoring of electrocardiogram and monophasic action potential (MAP), which was compared with those of halothane or non-inhaled anesthetics ketamine/xylazine. Intravenous administration of sparfloxacin at 3 and 10 mg/kg prolonged the QT interval and MAP duration together with bradycardic action under 4 different anesthetic conditions. The order of extent of prolongation of corrected QT interval after the administration of sparfloxacin was isoflurane ≈ sevoflurane ≈ halothane >> ketamine/xylazine, whereas that of the MAP90 at a pacing cycle length of 300 ms was halothane ≥ isoflurane ≈ sevoflurane >> ketamine/xylazine. These results suggest that isoflurane and sevoflurane as well as halothane could sensitize the heart to sparfloxacin-induced QT interval prolongation in guinea pigs.

Altered adrenergic response in myocytes bordering a chronic myocardial infarction underlies in vivo triggered activity and repolarization instability.

Key points Ventricular arrhythmias are a major complication after myocardial infarction (MI), associated with sympathetic activation. The structurally heterogeneous peri‐infarct zone is a known substrate, but the functional role of the myocytes is less well known.Recordings of monophasic action potentials in vivo reveal that the peri‐infarct zone is a source of delayed afterdepolarizations (DADs) and has a high beat‐to‐beat variability of repolarization (BVR) during adrenergic stimulation (isoproterenol, ISO).Myocytes isolated from the peri‐infarct region have more DADs and spontaneous action potentials, with spontaneous Ca2+ release, under ISO. These myocytes also have reduced repolarization reserve and increased BVR. Other properties of post‐MI remodelling are present in both peri‐infarct and remote myocytes.These data highlight the importance of altered myocyte adrenergic responses in the peri‐infarct region as source and substrate of post‐MI arrhythmias. Ventricular arrhythmias are a major early complication after myocardial infarction (MI). The heterogeneous peri‐infarct zone forms a substrate for re‐entry while arrhythmia initiation is often associated with sympathetic activation. We studied the mechanisms triggering these post‐MI arrhythmias in vivo and their relation to regional myocyte remodelling. In pigs with chronic MI (6 weeks), in vivo monophasic action potentials were simultaneously recorded in the peri‐infarct and remote regions during adrenergic stimulation with isoproterenol (isoprenaline; ISO). Sham animals served as controls. During infusion of ISO in vivo, the incidence of delayed afterdepolarizations (DADs) and beat‐to‐beat variability of repolarization (BVR) was higher in the peri‐infarct than in the remote region. Myocytes isolated from the peri‐infarct region, in comparison to myocytes from the remote region, had more DADs, associated with spontaneous Ca2+ release, and a higher incidence of spontaneous action potentials (APs) when exposed to ISO (9.99 ± 4.2 vs. 0.16 ± 0.05 APs/min, p = 0.004); these were suppressed by CaMKII inhibition. Peri‐infarct myocytes also had reduced repolarization reserve and increased BVR (26 ± 10 ms vs. 9 ± 7 ms, P < 0.001), correlating with DAD activity. In contrast to these regional distinctions under ISO, alterations in Ca2+ handling at baseline and myocyte hypertrophy were present throughout the left ventricle (LV). Expression of some of the related genes was, however, different between the regions. In conclusion, altered myocyte adrenergic responses in the peri‐infarct but not the remote region provide a source of triggered activity in vivo and of repolarization instability amplifying the substrate for re‐entry. These findings stimulate further exploration of region‐specific therapies targeting myocytes and autonomic modulation.

Precise safety pharmacology studies of lapatinib for onco-cardiology assessed using in vivo canine models

Cancer chemotherapies have improved prognosis in cancer patients, resulting in a large and rapidly increasing number of cancer survivors. “Onco-cardiology” or “cardio-oncology” is a new discipline for addressing the unanticipated cardiac side effects of newly developed cancer drugs. Lapatinib, a tyrosine kinase inhibitor suppressing the epidermal growth factor receptor and ErbB2, has been used in advanced or metastatic breast cancer treatment. Reportedly, lapatinib has induced cardiovascular adverse events including QT-interval prolongation and heart failure. However, they have not been predicted by preclinical studies. Hence, a new method to assess the tyrosine kinase inhibitor-induced adverse effects needs to be established. Here, we intravenously administered lapatinib to halothane-anaesthetised dogs, evaluating cardiohemodynamic, electrophysiological, and echocardiographic profiles for pharmacological safety assessments. We intravenously administered lapatinib to chronic atrioventricular block beagle dogs to assess its proarrhythmic potential. The therapeutic concentration of lapatinib significantly increased total peripheral vascular resistance, QT, QTc, monophasic action potential (MAP)90(sinus), MAP90(CL400), effective refractory period, and plasma concentration of cardiac troponin I (cTnI), suggesting that lapatinib prolonged the ventricular repolarization without inducing lethal ventricular arrhythmia. Careful monitoring of plasma cTnI concentration and an electrocardiogram could be supportive biomarkers, predicting the onset of lapatinib-induced cardiovascular adverse events.

Restitution metrics in Brugada syndrome: a systematic review and meta-analysis

Brugada syndrome (BrS) is an ion channelopathy that predisposes affected subjects to ventricular tachycardia/fibrillation (VT/VF) and sudden cardiac death. Restitution analysis has been examined in BrS patients but not all studies have reported significant differences between BrS patients and controls. Therefore, we conducted a systematic review and meta-analysis to investigate the different restitution indices used in BrS. PubMed and Embase were searched until April 7, 2019, identifying 20 and 27 studies. A total of ten studies involving 178 BrS (mean age 38years old, 63% male) and 102 controls (mean age 31years old, 42% male) were included in this systematic review. Pacing was carried out at the right ventricular outflow tract (RVOT)/right ventricular apex (RPA) (n = 4), RPA (n = 4), or right atrium (RA) (n = 1). Basic cycle lengths of 400 (n = 4), 500 (n = 2), 600 (n = 6) and 750ms (n = 1) were used. Recording methods include electrograms (n = 4), monophasic action potentials (n = 5), and electrocardiograms (n = 1). Signals were obtained from the RVOT (n = 8), RVA (n = 3), RA (n = 1), or the body surface (n = 1). The maximum restitution slope for endocardial repolarization at the RVOT was 0.87 for BrS patients (n = 5; 95% confidence interval [CI] 0.68-1.07) compared with 0.74 in control subjects (n = 4; 95% CI 0.42-1.06), with a significant mean difference of 0.40 (n = 4; 95% CI 0.11-0.69; P = 0.007). Steeper endocardial repolarization restitution slopes are found in BrS patients compared with controls at baseline. Restitution analysis can provide important information for risk stratification in BrS.

Therapeutic observation on superficial needling with different frequencies for intractable facial paralysis

Objective: To obverse the therapeutic effect of superficial needling with different frequencies for intractable facial paralysis. Methods: A total of 120 patients with intractable peripheral facial paralysis were allocated into a superficial needling with high frequency group (150 times/min), a moderate frequency group (100 times/min) and a low frequency group (50 times/min) according to the random number table method. The Toronto facial grading system (TFGS) was used to evaluate facial nerve functions before treatment and after 2 weeks and 4 weeks of treatment respectively. The electromyography (EMG) test of the mandibular branch of facial nerve was used to compare the motor nerve conduction velocity (MCV), sensory nerve conduction velocity (SCV) and monophasic action potential (MAP) among different groups, and was done before treatment and after 4 weeks of treatment. The clinical efficacy was also compared. Results: After 2 weeks and 4 weeks of treatment, the changes of TFGS scores in the three groups all showed statistical significance (all P 0.05). The total effective rate was 65.0%, 80.0% and 95.0% in the high frequency group, moderate frequency group and low frequency group respectively, and the between-group differences showed statistical significance (P<0.05). Conclusion: Compared with the superficial needling with high and moderate frequencies, superficial needling with low frequency can produce more significant clinical efficacy for intractable facial paralysis.

Endothelin receptors in the brain modulate autonomic responses and arrhythmogenesis during acute myocardial infarction in rats

AimsEndothelin has been implicated in various processes in the brain, including the modulation of sympathetic responses. The present study examined the pathophysiologic role of brain endothelin-receptors in the setting of acute myocardial infarction, characterized by high incidence of ventricular tachyarrhythmias. Main methodsWe investigated the effects of intracerebroventricular administration of antagonists of endothelin-receptors ETA, ETB, or both, during a 24 h-observation period post-coronary ligation in (n = 70) rats. Continuous recording was performed via implanted telemetry transmitters, followed by arrhythmia-analysis and calculation of autonomic indices derived from heart rate variability. The regional myocardial electrophysiologic properties were assessed by monophasic action potentials and multi-electrode recordings. Key findingsSympathetic-activity was decreased and vagal-activity was enhanced after intracerebroventricular ETA-receptor blockade, thus attenuating regional myocardial repolarization inhomogeneity. As a result, the incidence of ventricular tachyarrhythmias was markedly lower in this group. Such effects were also observed after intracerebroventricular blockade of ETB-, or both, ETA- and ETB-receptors, although to a lesser extent. SignificanceETA-receptors in the brain modulate sympathetic and vagal responses and alter arrhythmogenesis during evolving myocardial necrosis in rats. These findings provide insights into arrhythmogenic mechanisms during acute myocardial infarction and call for further investigation on the role of endothelin in the central autonomic network.

P6591Arrhythmogenic effects of non-steroidal anti-inflammatory drugs (NSAID)

Abstract Background Clinical and experimental data suggest potential deleterious effects of non-steroidal anti-inflammatory drugs (NSAID) in cardiac disease. The aim of the present study was to characterize potential direct arrhythmogenic effects of ibuprofen or indometacin on cardiac electrophysiology. Methods and results In 13 isolated rabbit hearts, indometacin was administered in increasing concentrations (10, 30 and 50μM) after obtaining baseline data. In further 13 hearts, ibuprofen was infused (10, 30 and 50μM). Eight endo- and epicardial monophasic action potentials and a simultaneously recorded 12-lead ECG showed a significant decrease of QT-interval in indometacin- (50μM: −60ms, p&lt;0.05) and ibuprofen-treated hearts (50μM: −81ms, p&lt;0.01). This was accompanied by a similar decrease of action potential duration (APD). Effective refractory period (ERP) was also reduced by indometacin (−24ms, p&lt;0.05) and ibuprofen (−22ms, p&lt;0.05) while spatial dispersion of repolarisation remained stable with both drugs. Programmed ventricular stimulation and burst stimulation were employed for provocation of ventricular arrhythmias. In the indomethacin group 7 episodes of ventricular arrhythmias were inducible at baseline. The incidence was increased to 22 (10μM), 37 (30μM) and 51 (50μM) with higher concentrations of indometacin. In the ibuprofen group 15 episodes were inducible at baseline. After infusion of 10μM ibuprofen, also 15 episodes of ventricular arrhythmias occurred while 34 episodes were inducible with 30μM ibuprofen. Conclusion Infusion of the NSAID ibuprofen and indometacin resulted in a significant abbreviation of myocardial repolarization and a shortening of ventricular refractory periods. This resulted in an enhanced inducibility of ventricular arrhythmias and may contribute to the proposed deleterious effects of NSAID in the presence of cardiac disease.

P1905Propofol suppresses atrial fibrillation in an experimental whole-heart model

Abstract Background Many clinical studies suggest an impact of propofol on the occurrence of atrial arrhythmias. Purpose Since experimental data is sparse, purpose of this study was to investigate the influence of propofol on atrial electrophysiology and the vulnerability to atrial fibrillation in a sensitive whole-heart model of atrial fibrillation. Methods 11 hearts of New Zealand white rabbits were isolated and Langendorff-perfused. Two catheters were placed on both atria to record atrial monophasic action potentials. Hearts were stimulated at three different cycle lengths (350ms, 250ms, 150ms), thereby obtaining cycle-length dependent atrial action potential durations (aAPD90) and effective refractory periods (aERP). Atrial post-repolarization (aPRR) refractoriness was defined as the difference between aERP and aAPD90. To increase the occurrence of atrial fibrillation, hearts were perfused with a combination of acetylcholine (ACh, 1μM) and isoproterenol (Iso, 1μM) after obtaining baseline data. Vulnerability to atrial fibrillation was tested by a standardized protocol consisting of several trains of burst pacing. Results Acetylcholine/isoproterenol infusion reduced aAPD90 (−3ms), aERP (−16ms) and aPRR (−14ms). Additional treatment with propofol prolonged aAPD90 (+5ms), aERP (+24ms) and aPRR (+20ms). Under baseline conditions, 13 episodes of atrial fibrillation lasting longer than 1s occurred. Additional treatment with acetylcholine/isoproterenol provoked more episodes of atrial fibrillation (33 episodes, p&lt;0.05). Infusion of propofol on top of acetylcholine/isoproterenol significantly suppressed atrial fibrillation (2 episodes, p&lt;0.05). Conclusion In this study, propofol prolonged aAPD90 and significantly reduced the occurrence of atrial fibrillation. Prolongation of effective refractory periods and especially of post-repolarization refractoriness protects the atrial myocardium against premature excitation and thereby prevents arrhythmias.

Sevoflurane Alleviates Reperfusion Arrhythmia by Ameliorating TDR and MAPD90 in Isolated Rat Hearts after Ischemia-Reperfusion.

Objective To investigate the effect of sevoflurane on the monophasic action potentials (MAPs) in isolated rat hearts after ischemia-reperfusion. Methods Twenty-four healthy SD male rats, weighing 280–320 g, were randomly divided into three groups after successful preparation of a Langendorff isolated heart perfusion model with a stabilization period perfusion of 15 min with Krebs–Henseleit (K–H) fluid (n = 8): the control group (group A, continuously perfused with K–H fluid for 105 min), the ischemia-reperfusion group (group B, continuously perfused with K–H fluid for 15 min, and then exposed to 60 min of global ischemia induced by Thomas solution followed by 30 min of reperfusion), and the sevoflurane group (group C, K–H fluid contained 1.0 MAC sevoflurane, and other procedures were same as in group B). Heart rate (HR) and MAPs including time course (MAPD50 or MAPD90) of the epicardium (Epi) and endocardium (Endo) were recorded at the time of balance perfusion for 15 min (T0), continuous perfusion for 15 min (T1), reperfusion for 15 min/continuous perfusion for 105 min (T2), and reperfusion for 30 min/continuous perfusion for 120 min (T3), and the transmural dispersion of repolarization (TDR) was calculated. The incidence of arrhythmia, time for restoration of spontaneous heart beat, and duration of arrhythmia were recorded during the period of reperfusion. Results HR in group B and group C was lower at T2 and T3 than that in group A, while that in group B was significantly lower than that in group A at T2 and T3, and HR in group C was higher than that in group B at T2 and T3 (P < 0.05). There was no difference of TDR in each group at T0 and T1 (P > 0.05), while TDR in group B was increased at T2 and T3 compared with that in group C and group A (P < 0.05). TDR in group C was decreased compared with that in group B at T2 and T3 (P < 0.05), while there was no such difference between group C and group A (P > 0.05). The time for restoration of spontaneous heart beat and duration of arrhythmia in group C were shorter than those in group B (P < 0.05), while cardiac arrhythmia scores in group B were higher than those in group C (P < 0.05). There was no difference of MAPD50 in each group (P > 0.05). The MAPD90 in group B was much longer than that in other groups at T2 and T3 (P < 0.05), while there was no such difference between group C and group A (P > 0.05). The prolonged MAPD90 at T2 and T3 in group B strikingly differed from that at T0 and T1 (P < 0.05). Nevertheless, there was no such difference in other groups at different time points (P > 0.05). Conclusion Sevoflurane alleviates reperfusion arrhythmia induced by myocardial ischemia-reperfusion through the shortening of MPAD90 in isolated rat hearts.

Pro-Arrhythmic Ventricular Remodeling Is Associated With Increased Respiratory and Low-Frequency Oscillations of Monophasic Action Potential Duration in the Chronic Atrioventricular Block Dog Model.

In addition to beat-to-beat fluctuations, action potential duration (APD) oscillates at (1) a respiratory frequency and (2) a low frequency (LF) (<0.1 Hz), probably caused by bursts of sympathetic nervous system discharge. This study investigates whether ventricular remodeling in the chronic AV block (CAVB) dog alters these oscillations of APD and whether this has consequences for arrhythmogenesis. We performed a retrospective analysis of 39 dog experiments in sinus rhythm (SR), acute AV block (AAVB), and after 2 weeks of chronic AV block. Spectral analysis of left ventricular monophasic action potential duration (LV MAPD) was done to quantify respiratory frequency (RF) power and LF power. Dofetilide (0.025 mg/kg in 5 min) was infused to test for inducibility of Torsade de Pointes (TdP) arrhythmias. RF power was significantly increased at CAVB compared to AAVB and SR (log[RF] of −1.13 ± 1.62 at CAVB vs. log[RF] of −2.82 ± 1.24 and −3.29 ± 1.29 at SR and AAVB, respectively, p < 0.001). LF power was already significantly increased at AAVB and increased even further at CAVB (−3.91 ± 0.70 at SR vs. −2.52 ± 0.85 at AAVB and −1.14 ± 1.62 at CAVB, p < 0.001). In addition, LF power was significantly larger in inducible CAVB dogs (log[LF] −0.6 ± 1.54 in inducible dogs vs. −2.56 ± 0.43 in non-inducible dogs, p < 0.001). In conclusion, ventricular remodeling in the CAVB dog results in augmentation of respiratory and low-frequency (LF) oscillations of LV MAPD. Furthermore, TdP-inducible CAVB dogs show increased LF power.

Understanding the cardiac toxicity of the anthropogenic pollutant phenanthrene on the freshwater indicator species, the brown trout (Salmo trutta): From whole heart to cardiomyocytes

Freshwater systems are faced with a myriad of stressors including geomorphological alterations, nutrient overloading and pollution. Previous studies in marine fish showed polyaromatic hydrocarbons (PAHs) to be cardiotoxic. However, the cardiotoxicity of anthropogenic pollutants in freshwater fishes is unclear and has not been examined across multiple levels of cardiac organization. Here we investigated the effect of phenanthrene (Phe), a pervasive anthropogenic pollutant on a sentinel freshwater species, the brown trout (Salmo trutta). We first examined the electrical activity of the whole heart and found prolongation (∼8.6%) of the QT interval (time between ventricular depolarization and repolarization) of the electrocardiogram (ECG) and prolongation (∼13.2%) of the monophasic action potential duration (MAPD) following ascending doses of Phe. At the tissue level, Phe significantly reduced trabecular force generation by ∼24% at concentration 15 μM and above, suggesting Phe reduces cellular calcium cycling. This finding was supported by florescent microscopy showing a reduction (∼39%) in the intracellular calcium transient amplitude following Phe exposure in isolated brown trout ventricular myocytes. Single-cell electrophysiology was used to reveal the mechanism underlying contractile and electrical dysfunction following Phe exposure. A Phe-dependent reduction (∼38%) in the L-type Ca2+ current accounts, at least in part, for the lowered Ca2+ transient and force production. Prolongation of the MAPD and QT interval was explained by a reduction (∼70%) in the repolarising delayed rectifier K+ current following Phe exposure. Taken together, our study shows a direct impact of Phe across multiple levels of cardiac organization in a key freshwater salmonid.

Arrhythmia development during inhibition of small-conductance calcium-activated potassium channels in acute myocardial infarction in a porcine model.

Acute myocardial infarction (AMI) is associated with intracellular Ca2+ build-up. In healthy ventricles, small conductance Ca2+-activated K+ (SK) channels are present but do not participate in repolarization. However, SK current is increased in chronic myocardial infarction and heart failure, and recently, SK channel inhibition was demonstrated to reduce arrhythmias in AMI rats. Hence, we hypothesized that SK channel inhibitors (NS8593 and AP14145) could reduce arrhythmia development during AMI in a porcine model. Twenty-seven pigs were randomized 1:1:1 to control, NS8593, or AP14145. Haemodynamic and electrophysiological parameters [electrocardiogram (ECG) and monophasic action potentials (MAP)] were continuously recorded. A balloon was placed in the mid-left anterior descending artery, blinded to treatment. Infusion lasted from 10 min before occlusion until 30 min after. Occlusion was maintained for 1 h, followed by 2 h of reperfusion. Upon occlusion, cardiac output dropped similarly in all groups, while blood pressure remained stable. Heart rate decreased in the NS8593 and AP14145 groups. QRS duration increased upon occlusion in all groups but more prominently in AP14145-treated pigs. Inhibition of SK channels did not affect QT interval. Infarct MAP duration shortened comparably in all groups. Ventricular fibrillation developed in 4/9 control-, 4/9 AP14145-, and 2/9 NS8593-treated pigs. Ventricular tachycardia was rarely observed in either group, whereas ventricular extrasystoles occurred comparably in all groups. Inhibition of SK channels was neither beneficial nor detrimental to ventricular arrhythmia development in the setting of AMI in this porcine model.

Theoretical Modeling and Experimental Detection of the Extracellular Phasic Impedance Modulation in Rabbit Hearts.

Theoretical cardiac electrophysiology focuses on the dynamics of the membrane and sarcoplasmic reticulum ion currents; however, passive (e.g., membrane capacitance) and quasi-active (response to small signals) properties of the cardiac sarcolemma, which are quantified by impedance, are also important in determining the behavior of cardiac tissue. Theoretically, impedance varies in the different phases of a cardiac cycle. Our goal in this study was to numerically predict and experimentally validate these phasic changes. We calculated the expected impedance signal using analytic methods (for generic ionic models) and numerical computation (for a rabbit ventricular ionic model). Cardiac impedance is dependent on the phase of the action potential, with multiple deflections caused by a sequential activation and inactivation of various membrane channels. The two main channels shaping the impedance signal are the sodium channel causing a sharp and transient drop at the onset of action potential and the inward rectifying potassium channel causing an increase in impedance during the plateau phase. This dip and dome pattern was confirmed in an ex-vivo rabbit heart model using high-frequency sampling through a monophasic action potential electrode. The hearts were immobilized using a myosin-inhibitor to minimize motion artifacts. We observed phasic impedance changes in three out of four hearts with a dome amplitude of 2 − 4Ω. Measurement of phasic impedance modulation using an extracellular electrode is feasible and provides a non-invasive way to gain insight into the state of cardiac cells and membrane ionic channels. The observed impedance recordings are consistent with the dip and dome pattern predicted analytically.

Myocardial Monophasic Action Potential Recorded by Suction Electrode for Ionic Current Studies in Zebrafish.

The study of myocardial transmembrane ion currents is fundamental to understand frequent pathologies such as arrhythmias and ischemia. Conventional electrocardiography (ECG) is not able to record ion currents, while the use of intracellular microelectrodes in a beating heart has technical limitations. Myocardial monophasic action potentials (MAPs) recorded with suction electrodes allow the evaluation of ionic currents similar to those recorded by intracellular glass microelectrodes. The technique is based on the fact that suction, through a small diameter tube, on the myocardial cell, induces an opening at the membrane, connecting the intracellular media to the electrode by a saline bridge. The electrophysiology of zebrafish heart is remarkably similar to the human; however, in situ evaluation of MAPs has not been yet explored. In this study, we aimed to establish a myocardial MAP recording technique for adult zebrafish. Male adult wild-type zebrafish were anesthetized and 50% of the beating ventricle was exposed. A glass hematocrit capillary tube (1.1 mm inner diameter) was used as a suction electrode connected to a 3-way stopcock valve, which is also connected to a syringe containing a chloride-coated silver wire for signal recording. Gentle suction was exerted by a syringe filled with ringer and connected to the 3-way stopcock valve. Two needles were used for ground (tail) and indifferent (abdomen) electrodes. Without suction, the system can record conventional ECG, but applying suction MAPs are registered and show typical morphology with phase 0-4 sequence. MAP amplitude and duration values show low variability. Ischemia and/or lidocaine-induced Na+ channel blocking dramatically reduced MAP amplitude. These results strongly suggest that the suction electrode technique is a promising method to record myocardial ion currents in situ in zebrafish.

Dose-Dependent Effects of Ranolazine on Reentrant Ventricular Arrhythmias Induced After Subacute Myocardial Infarction in Rabbits.

Ranolazine has been found to prevent ventricular arrhythmias (VAs) during acute myocardial infarction (AMI). This study aimed to investigate its efficacy on VAs induced several days post-MI. For this purpose, 13 anesthetized rabbits underwent coronary artery ligation. Ten of these animals that survived AMI were reanesthetized 3 to 7 days later for electrophysiologic testing. An endocardial monophasic action potential combination catheter was placed in the right ventricle for simultaneous pacing and recording. Monophasic action potential duration, ventricular effective refractory period (VERP), and VAs induced by programmed stimulation were assessed. Measurements were performed during control pacing, and following an intravenous infusion of either a low-dose ranolazine (2.4 mg/kg, R1) or a higher dose ranolazine (4.8 mg/kg cumulative dose, R2). During control stimulation, 2 animals developed primary ventricular fibrillation (VF), 6 sustained ventricular tachycardia (sVT), and 2 nonsustained VT (nsVT). R1 did not prevent the appearance of VAs in any of the experiments; in contrast, it aggravated nsVT into sVT and complicated sVT termination in 2 of 6 animals. Sustained ventricular tachycardia cycle length and VERP were only slightly decreased after R1 (112 ± 5 vs 110 ± 6 ms and 101 ± 11 vs 98 ± 10 ms, respectively). R2 suppressed inducibility of control nsVT, VF, and sVT in 2 animals. In 4 animals with still inducible sVT, R2 significantly prolonged VT cycle length by 150 ± 23 ms (P < .01), and VERP by 120 ± 7 ms (P < .001) versus control. In conclusion, R2 exerted antiarrhythmic efficacy against subacute-MI VAs, whereas R1 rather aggravated than prevented these arrhythmias. Ventricular effective refractory period prolongation could partially explain the antiarrhythmic action of R2 in this rabbit model.

Flexible and precise control of cardiac rhythm with blue light

Optogenetics is an innovative method for precise control of biological function, which makes light manipulation displays more advantages than electric energy because of contactless spatial flexibility and cell-to-cell synchronous communication. The aim of this study was to perform different illumination modes with blue laser to investigate optical control of the mice hearts. In this study, we transfected the light sensitive protein ChR2(H134R) into mouse hearts, which were illuminated with a 473 nm laser on the Langendorff apparatus. We recorded all the signals of electrograms (EGs), epicardium monophasic action potential (MAPs) and light output signals to analyze myocardial electrical activity. EGs and MAP showed that ChR2 expression in the heart can be flexibly controlled by blue light across different illumination sites with corresponding triggered ectopic rhythm. Illumination intensity, pulse duration, and impulse frequency were associated with the light capture rate. Continuous illumination with the threshold intensity on the left ventricle had little influence on sinus rhythm and ventricular electrophysiology. Our results support that flexible control of the cardiac rhythm with optogenetics provides an innovative approach to cardiac research and therapy.

Effect of sevoflurane on electrophysiological stability of isolated rat hearts subjected to hypothermic perfusion

Objective To evaluate the effect of sevoflurane on the electrophysiological stability of isolated rat hearts subjected to hypothermic perfusion. Methods Clean-grade healthy adult male Sprague-Dawley rats, weighing 280-360 g, were heparinized and anesthetized with pentobarbital sodium.Their hearts were excised and perfused in a Langendorff apparatus with K-H solution saturated with 95% O2-5% CO2 at 37 ℃.Twenty-four Langendorff-perfused hearts were divided into 4 groups (n=6 each) using a random number table method: control group (C group), sevoflurane group (S group), 32 ℃ hypothermia group (H group) and 32 ℃ hypothermia plus sevoflurane group (HS group). After 15 min of equilibration, the isolated hearts were continuously perfused for 30 min with K-H solution at 37 ℃, with K-H solution containing 2.3% sevoflurane at 37 ℃, with K-H solution at 32 ℃, and with K-H solution containing 2.3% sevoflurane at 32 ℃ in C, S, H and HS groups, respectively.Heart rate and monophasic action potential in three layers of the left ventricular anterior wall were recorded at 15 min of equilibration (T0) and 30 of continuous perfusion (T2), the transmural dispersion of repolarization (TDR) were calculated, and the occurrence of arrhythmia was observed. Results Compared with C and S groups, the heart rate was significantly decreased and TDR was enlarged at T1, and the incidence of arrhythmia was increased in H and HS groups (P<0.05). Compared with H group, TDR was significantly reduced at T1, and the incidence of arrhythmia was decreased in HS group (P<0.05). Conclusion Sevoflurane can improve the electrophysiological instability of isolated rat hearts subjected to hypothermic perfusion, and thus decrease the development of arrhythmia. Key words: Anesthetics, inhalation; Hypothermia; Cardiac electrophysiology

Upregulation of MMP-9 and CaMKII prompts cardiac electrophysiological changes that predispose denervated transplanted hearts to arrhythmogenesis after prolonged cold ischemic storage

BackgroundAlthough the importance of the potential mechanisms of arrhythmogenesis, including cardiac electrophysiology dysfunction and gap junction electrical uncoupling by Cx43 degradation under pathological conditions, has been widely and thoroughly established, the correlations between these mechanisms and biomarkers of cold ischemic insult have not yet been confirmed. Limited evidence suggests that these biomarkers are significantly associated with cardiac dysrhythmia. Thus, in this study, we analyzed such correlations in denervated transplanted hearts. Methods and resultsRat hearts were divided into two groups according to a random number table. The hearts in the first group were stored in histidine-tryptophan-ketoglutarate (HTK) solution for 6 h to produce cold ischemic insult (insult group, n = 8), while the hearts in the second group were left untreated (control group, n = 8). Ventricular arrhythmias (VAs) were successively observed in 8/8 hearts stored in HTK solution and 0/8 untreated hearts (P < 0.05). The analysis of the electrophysiological parameters by programmed electrical stimulation (PES) and monophasic action potential (MAP) recording demonstrated that compared with the control hearts, the insult hearts had increased transmural dispersion of repolarization (TDR) and effective refractory periods (ERPs) (P < 0.001), decreased conduction velocities (CVs) and excitation wavelengths (CV × ERP, λ) (P < 0.05), and longer times between peak MAP values and 90% repolarization (MAPD90) in the epicardium (P < 0.05). However, no significant difference in endocardial MAPD90 was observed between the two groups of hearts. The investigation of biomarkers and Cx43 proteins by a western blot analysis showed that the matrix metalloproteinase-9 (MMP-9) and Ca2+/calmodulin-dependent protein kinase II (CaMKII) levels were strongly elevated and that the Cx43 protein levels were markedly reduced in the insult hearts. ConclusionMMP-9 overexpression mediates gap junction (GJ) electrical uncoupling induced by Cx43 decreases, while CaMKII upregulation induces transmural dispersion of repolarization (TDR) increase, both of which increase the risk of ventricular arrhythmias (VAs) in denervated transplanted hearts after prolonged cold ischemic storage.