sea-0400 and Arrhythmias--Cardiac

In search of better antiarrhythmic therapy, targeting the Na/Ca exchanger is an option to be explored. The rationale is that increased activity of the Na/Ca exchanger has been implicated in arrhythmogenesis in a number of conditions. The evidence is strong for triggered arrhythmias related to Ca2+ overload, due to increased Na+ load or during adrenergic stimulation; the Na/Ca exchanger may be important in triggered arrhythmias in heart failure and in atrial fibrillation. There is also evidence for a less direct role of the Na/Ca exchanger in contributing to remodelling processes. In this chapter, we review this evidence and discuss the consequences of inhibition of Na/Ca exchange in the perspective of its physiological role in Ca2+ homeostasis. We summarize the current data on the use of available blockers of Na/Ca exchange and propose a framework for further study and development of such drugs. Very selective agents have great potential as tools for further study of the role the Na/Ca exchanger plays in arrhythmogenesis. For therapy, they may have their specific indications, but they carry the risk of increasing Ca2+ load of the cell. Agents with a broader action that includes Ca2+ channel block may have advantages in other conditions, e.g. with Ca2+ overload. Additional actions such as block of K+ channels, which may be unwanted in e.g. heart failure, may be used to advantage as well.

Topics: Action Potentials; Aniline Compounds; Animals; Anti-Arrhythmia Agents; Arrhythmias, Cardiac; Calcium; Cardiomegaly; Heart; Heart Failure; Humans; Myocardium; Phenyl Ethers; Sodium-Calcium Exchanger; Thiourea

Chagas disease (CD) is a neglected disease that induces heart failure and arrhythmias in approximately 30% of patients during the chronic phase of the disease. Despite major efforts to understand the cellular pathophysiology of CD there are still relevant open questions to be addressed. In the present investigation we aimed to evaluate the contribution of the Na+/Ca2+ exchanger (NCX) in the electrical remodeling of isolated cardiomyocytes from an experimental murine model of chronic CD.. Male C57BL/6 mice were infected with Colombian strain of Trypanosoma cruzi. Experiments were conducted in isolated left ventricular cardiomyocytes from mice 180-200 days post-infection and with age-matched controls. Whole-cell patch-clamp technique was used to measure cellular excitability and Real-time PCR for parasite detection. In current-clamp experiments, we found that action potential (AP) repolarization was prolonged in cardiomyocytes from chagasic mice paced at 0.2 and 1 Hz. After-depolarizations, both subthreshold and with spontaneous APs events, were more evident in the chronic phase of experimental CD. In voltage-clamp experiments, pause-induced spontaneous activity with the presence of diastolic transient inward current was enhanced in chagasic cardiomyocytes. AP waveform disturbances and diastolic transient inward current were largely attenuated in chagasic cardiomyocytes exposed to Ni2+ or SEA0400.. The present study is the first to describe NCX as a cellular arrhythmogenic substrate in chagasic cardiomyocytes. Our data suggest that NCX could be relevant to further understanding of arrhythmogenesis in the chronic phase of experimental CD and blocking NCX may be a new therapeutic strategy to treat arrhythmias in this condition.

Topics: Action Potentials; Aniline Compounds; Animals; Arrhythmias, Cardiac; Calcium; Chagas Cardiomyopathy; Electrophysiological Phenomena; Gene Expression Regulation; Male; Mice; Mice, Inbred C57BL; Myocytes, Cardiac; Neglected Diseases; Nickel; Patch-Clamp Techniques; Phenyl Ethers; Sarcoplasmic Reticulum; Sodium-Calcium Exchanger

During ischaemia/reperfusion, the rise in [Na(+)](i), induced by simultaneous depression of the Na(+)/K(+)-ATPase and activation of the Na(+)/H(+) exchanger (NHE), shifts the Na(+)/Ca(2+) exchanger (NCX) into reverse transport mode, resulting in Ca(2+)(i)overload, which is a critical factor in enhancing the liability to cardiac arrhythmias. The inhibition of NHE, and recently NCX has been suggested to effectively protect the heart from reperfusion-induced arrhythmias. In this study, we investigated and compared the efficacy of individual or the simultaneous inhibition of the NHE and NCX against reperfusion-induced arrhythmias in Langendorff-perfused rat hearts by applying a commonly used regional ischaemia-reperfusion protocol. The NHE and NCX were inhibited by cariporide and SEA0400 or the novel, more selective ORM-10103, respectively. Arrhythmia diagrams calculated for the reperfusion period were analysed for the incidence and duration of extrasystoles (ESs), ventricular tachycardia (VT) and ventricular fibrillation (VF). NHE inhibition by cariporide was highly efficient in reducing the recorded reperfusion-induced arrhythmias. Following the application of SEA0400 or ORM-10103, the number and duration of arrhythmic periods were efficiently or moderately decreased. While both NCX inhibitors effectively reduced ESs, the most frequently triggered arrhythmias, they exerted limited or no effect on VTs and VFs. Of the NCX inhibitors, ORM-10103 was more effective. Surprisingly, the simultaneous inhibition of the NCX and NHE failed to significantly improve the antiarrhythmic efficacy reached by NCX blockade alone. In conclusion, although principal simultaneous NHE+NCX inhibition should be highly effective against all types of the recorded reperfusion-induced arrhythmias, NCX inhibitors, alone or in combination with cariporide, seem to be moderately suitable to provide satisfactory cardioprotection - at least in the present arrhythmia model. Since ORM-10103 and SEA0400 are known to effectively inhibit after-depolarisations, it is suggested that their efficacy and that of other NCX inhibitors may be higher and more pronounced in the predominantly Ca(2+)(i)-dependent triggered arrhythmias.

Topics: Aniline Compounds; Animals; Anti-Arrhythmia Agents; Arrhythmias, Cardiac; Benzopyrans; Calcium; Cardiotonic Agents; Drug Therapy, Combination; Guanidines; Male; Myocardial Reperfusion; Myocardium; Myocytes, Cardiac; Phenyl Ethers; Pyridines; Rats; Rats, Sprague-Dawley; Sodium-Calcium Exchanger; Sodium-Hydrogen Exchangers; Sulfones; Tachycardia, Ventricular; Ventricular Fibrillation

Augmented Na(+) /Ca(2+) exchanger (NCX) activity may play a crucial role in cardiac arrhythmogenesis; however, data regarding the anti-arrhythmic efficacy of NCX inhibition are debatable. Feasible explanations could be the unsatisfactory selectivity of NCX inhibitors and/or the dependence of the experimental model on the degree of Ca(2+) i overload. Hence, we used NCX inhibitors SEA0400 and the more selective ORM10103 to evaluate the efficacy of NCX inhibition against arrhythmogenic Ca(2+) i rise in conditions when [Ca(2+) ]i was augmented via activation of the late sodium current (INaL ) or inhibition of the Na(+) /K(+) pump.. Action potentials (APs) were recorded from canine papillary muscles and Purkinje fibres by microelectrodes. NCX current (INCX ) was determined in ventricular cardiomyocytes utilizing the whole-cell patch clamp technique. Ca(2+) i transients (CaTs) were monitored with a Ca(2+) -sensitive fluorescent dye, Fluo-4.. Enhanced INaL increased the Ca(2+) load and AP duration (APD). SEA0400 and ORM10103 suppressed INCX and prevented/reversed the anemone toxin II (ATX-II)-induced [Ca(2+) ]i rise without influencing APD, CaT or cell shortening, or affecting the ATX-II-induced increased APD. ORM10103 significantly decreased the number of strophanthidin-induced spontaneous diastolic Ca(2+) release events; however, SEA0400 failed to restrict the veratridine-induced augmentation in Purkinje-ventricle APD dispersion.. Selective NCX inhibition - presumably by blocking rev INCX (reverse mode NCX current) - is effective against arrhythmogenesis caused by [Na(+) ]i -induced [Ca(2+) ]i elevation, without influencing the AP waveform. Therefore, selective INCX inhibition, by significantly reducing the arrhythmogenic trigger activity caused by the perturbed Ca(2+) i handling, should be considered as a promising anti-arrhythmic therapeutic strategy.

Topics: Action Potentials; Aniline Compounds; Animals; Arrhythmias, Cardiac; Benzopyrans; Calcium; Cnidarian Venoms; Dogs; Hypercalcemia; Myocytes, Cardiac; Papillary Muscles; Patch-Clamp Techniques; Phenyl Ethers; Purkinje Fibers; Pyridines; Sodium-Calcium Exchanger

L-type calcium channel (LTCC) and Na(+)/Ca(2+) exchanger (NCX) have been implicated in repolarization-dependent arrhythmias, but also modulate calcium and contractility. Although LTCC inhibition is negative inotropic, NCX inhibition has the opposite effect. Combined block may, therefore, offer an advantage for hemodynamics and antiarrhythmic efficiency, particularly in diseased hearts. In a model of proarrhythmia, the dog with chronic atrioventricular block, we investigated whether combined inhibition of NCX and LTCC with SEA-0400 is effective against dofetilide-induced torsade de pointes arrhythmias (TdP), while maintaining calcium homeostasis and hemodynamics.. Left ventricular pressure (LVP) and ECG were monitored during infusion of SEA-0400 and verapamil in anesthetized dogs. Different doses were tested against dofetilide-induced TdP in chronic atrioventricular block dogs. In ventricular myocytes, effects of SEA-0400 were tested on action potentials, calcium transients, and early afterdepolarizations. In cardiomyocytes, SEA-0400 (1 μmol/L) blocked 66±3% of outward NCX, 50±2% of inward NCX, and 33±9% of LTCC current. SEA-0400 had no effect on systolic calcium, but slowed relaxation, despite action potential shortening, and increased diastolic calcium. SEA-0400 stabilized dofetilide-induced lability of repolarization and suppressed early afterdepolarizations. In vivo, SEA-0400 (0.4 and 0.8 mg/kg) had no effect on left ventricular pressure and suppressed dofetilide-induced TdPs dose dependently. Verapamil (0.3 mg/kg) also inhibited TdP, but caused a 15±8% drop of left ventricular pressure. A lower dose of verapamil without effects on left ventricular pressure (0.06 mg/kg) was not antiarrhythmic.. In chronic atrioventricular block dogs, SEA-0400 treatment is effective against TdP. Unlike specific inhibition of LTCC, combined NCX and LTCC inhibition has no negative effects on cardiac hemodynamics.

Topics: Action Potentials; Aniline Compounds; Animals; Anti-Arrhythmia Agents; Arrhythmias, Cardiac; Calcium Channels, L-Type; Disease Models, Animal; Dogs; Electrocardiography; Heart Ventricles; Myocytes, Cardiac; Phenyl Ethers; Sodium-Calcium Exchanger; Ventricular Function; Ventricular Pressure

The effects of SEA0400, a Na(+) /Ca(2+) exchanger (NCX) blocker, on dynamic factors and arrhythmogenic alternans in 1-month myocardial infarction (MI) hearts remain unknown.. Simultaneous voltage and intracellular Ca(2+) (Cai ) optical mapping was performed in 12 rabbit hearts with MI for 1 month and six normal rabbit hearts as control. Western-blot studies were performed in both groups in an additional six hearts for each. Action potential duration (APD) restitution was constructed and arrhythmogenic alternans was induced by dynamic pacing. SEA0400 (0.03, 3 μM) was administered after baseline studies.. SEA0400 suppressed pacing-induced ventricular premature beats in a concentration-dependent manner. SEA0400 at 0.03 μM steepened APD restitution slopes and enhanced spatially discordant alternans (SDA), which became insignificant at 3 μM. The VF inducibility was seven of nine at baseline, nine of nine at 0.03 μM SEA0400, and five of nine at 3 μM SEA0400 (P = NS). Significant upregulation of NCX in the remote but not periinfarct zone and less degree downregulation of DHP1α in the remote versus periinfarct zone may play a role in enhancing SDA induction by SEA0400 in 1-month MI hearts.. In 1-month MI hearts, SEA0400 suppresses pacing-induced ventricular premature beats, but also is proarrhythmic by steepening APD restitution and enhancing SDA via NCX inhibition. Heterogeneous upregulation of NCX and downregulation of DHP1α may contribute to SDA augmentation by SEA0400 in this model. The insignificant effect of SEA0400 on VF inducibility suggests that suppression of both reentry and triggered activity is required to suppress VF induction in this model.

Topics: Aniline Compounds; Animals; Anti-Arrhythmia Agents; Arrhythmias, Cardiac; Heart Rate; Myocardial Infarction; Perfusion; Phenyl Ethers; Rabbits; Sodium-Calcium Exchanger; Treatment Outcome

Computer simulations have predicted that the balance of various electrogenic sarcolemmal ion currents may control the amplitude and phase of beat-to-beat alternans of membrane potential (V(m)). However, experimental evidence for the mechanism by which alternans of calcium transients produces alternation of V(m) (V(m)-ALT) is lacking.. To provide experimental evidence that Ca-to-V(m) coupling during alternans is determined by the balanced influence of 2 Ca-sensitive electrogenic sarcolemmal ionic currents: I(NCX) and I(Ca).. V(m)-ALT and Ca-ALT were measured simultaneously from isolated guinea pig myocytes (n = 41) by using perforated patch and Indo-1(AM) fluorescence, respectively. There were 3 study groups: (1) control, (2) I(NCX) predominance created by adenoviral-induced NCX overexpression, and (3) I(Ca) predominance created by I(NCX) inhibition (SEA-0400) or enhanced I(Ca) (As(2)O(3)). During alternans, 14 of 14 control myocytes demonstrated positive Ca-to-V(m) coupling, consistent with I(NCX), but not I(Ca), as the major electrogenic current in modulating action potential duration. Positive Ca-to-V(m) coupling was maintained during I(NCX) predominance in 8 of 8 experiments with concurrent increase in Ca-to-V(m) gain (P <.05), reaffirming the role of increased forward-mode electrogenic I(NCX). Conversely, I(Ca) predominance produced negative Ca-to-V(m) coupling in 14 of 19 myocytes (P < .05) and decreased Ca-to-V(m) gain compared with control (P <.05). Furthermore, computer simulation demonstrated that Ca-to-V(m) coupling changes from negative to positive because of a shift from I(Ca) to I(NCX) predominance with increasing pacing rate.. These data provide the first direct experimental evidence that coupling in phase and magnitude of Ca-ALT to V(m)-ALT is strongly determined by the relative balance of the prominence of I(NCX) vs I(Ca) currents.

Topics: Analysis of Variance; Aniline Compounds; Animals; Arrhythmias, Cardiac; Calcium; Guinea Pigs; Heart Conduction System; Membrane Potentials; Myocytes, Cardiac; Patch-Clamp Techniques; Phenyl Ethers; Sarcoplasmic Reticulum; Sodium-Calcium Exchanger

In vivo studies have suggested that increased beat-to-beat variability of ventricular repolarization duration (BVR) is a better predictor of drug-induced torsades de pointes than repolarization prolongation alone. Cellular BVR and its dynamics before proarrhythmic events are poorly understood. We investigated differential responses of BVR in single myocytes during I(Ks) blockade versus I(Kr) blockade and late-I(Na) augmentation, under the influence of beta-adrenergic receptor stimulation. Transmembrane action potentials were recorded from isolated canine left-ventricular midmyocytes at various pacing rates. I(Ks) was blocked by HMR1556, I(Kr) by dofetilide. Late I(Na) was augmented by sea anemone toxin-II. Isoproterenol was added for beta-adrenergic receptor stimulation. BAPTA-AM buffered intracellular Ca(2+). SEA0400 partially inhibited the Na(+)-Ca(2+) exchanger. BVR was quantified as variability of action-potential duration at 90% repolarization: Sigma(|APD90; i+1 minus APD90; i|)/[nbeatsx radical2] for 30 consecutive action potentials. Baseline BVR was significantly increased by I(Kr) blockade and late-I(Na) augmentation, especially at slow pacing rates. beta-adrenergic stimulation restabilized these BVR changes. In contrast, I(Ks) blockade caused very little change in repolarization when compared to baseline conditions, but predisposed the myocyte to increased BVR during beta-adrenergic stimulation, especially at fast rates. BAPTA-AM and SEA0400 reduced this excessive BVR and eliminated early afterdepolarizations. In conclusion, beta-adrenergic receptor stimulation exaggerates BVR during I(Ks) blockade, indicating a BVR-stabilizing role of beta-adrenergic-sensitive I(Ks). Loss of I(Ks) plus overriding of Ca(2+)-dependent membrane currents, including inward Na(+)-Ca(2)(+) exchange current, conspire to proarrhythmic BVR under these conditions.

Topics: Action Potentials; Aniline Compounds; Animals; Arrhythmias, Cardiac; Calcium; Cells, Cultured; Dogs; Egtazic Acid; Female; Heart Ventricles; Ion Channels; Isoproterenol; Myocytes, Cardiac; Phenethylamines; Phenyl Ethers; Potassium; Potassium Channel Blockers; Potassium Channels; Receptors, Adrenergic, beta; Sodium; Sodium-Calcium Exchanger; Sulfonamides

It has been previously reported that the transgenic mouse expressing the dominant negative mutant of the neuron restrictive silencing factor (dnNRSF) in the heart died from lethal arrhythmia, so the present study aimed to clarify the electrophysiological alteration of the ventricular myocyte isolated from the dnNRSF mouse.. The action potential (AP) and membrane currents were recorded using the whole-cell patch-clamp method. Intracellular Ca(2+) was measured with Indo-1AM. The AP of dnNRSF myocytes exhibited reduction of resting membrane potential, prolongation of AP duration, and frequent early afterdepolarization (EAD). The EAD was completely inhibited by SEA0400, a specific blocker of the Na(+)-Ca(2+) exchanger (NCX). The most notable alteration of membrane current was a reduction in the inward rectifier K(+) current (I(K1)) density. In addition to re-expression of fetal type cardiac ion channels, a Na(+)-permeable, late inward current was observed in a small population of dnNRSF myocytes. The diastolic intracellular Ca(2+) concentration was also raised in dnNRSF myocytes, and spontaneous Ca(2+) oscillation was induced by β-adrenergic stimulation.. In dnNRSF myocytes, the "repolarization reserve" of the AP was significantly reduced by specific alterations in membrane currents. Under these conditions, the amplitude of EAD generated by the inward NCX current might be enlarged, thereby increasing the cells' vulnerability to ventricular arrhythmia.

Topics: Action Potentials; Aniline Compounds; Animals; Arrhythmias, Cardiac; Calcium; Heart Ventricles; Mice; Mice, Mutant Strains; Mutation; Myocytes, Cardiac; Phenyl Ethers; Potassium; Repressor Proteins; Sodium-Calcium Exchanger

Intracellular Ca(2+) waves (CaWs) of cardiomyocytes are spontaneous events of Ca(2+) release from the sarcoplasmic reticulum that are regarded as an important substrate for triggered arrhythmias and delayed afterdepolarizations. However, little is known regarding whether or how CaWs within the heart actually produce arrhythmogenic membrane oscillation because of the lack of data confirming direct correlation between CaWs and membrane potentials (V(m)) in the heart. On the hypothesis that CaWs evoke arrhythmogenic oscillatory depolarization when they emerge synchronously and intensively in the heart, we conducted simultaneous fluorescence recording of intracellular Ca(2+) ([Ca(2+)](i)) dynamics and V(m) of ventricular myocytes on subepicardial surfaces of Langendorff-perfused rat hearts using in situ dual-view, rapid-scanning confocal microscopy. In intact hearts loaded with fluo4/acetoxymethyl ester and RH237 under perfusion with cytochalasin D at room temperature, individual myocytes exhibited Ca(2+) transients and action potentials uniformly on ventricular excitation, whereas low-K(+)-perfused (2.4 mmol/L) hearts exhibited CaWs sporadically between Ca(2+) transients without discernible membrane depolarization. Further [Ca(2+)](i) loading of the heart, produced by rapid pacing and addition of isoproterenol, evoked triggered activity and subsequent oscillatory V(m), which are caused by burst emergence of CaWs in individual myocytes. Such arrhythmogenic membrane oscillation was abolished by ryanodine or the Na(+)-Ca(2+) exchanger inhibitor SEA0400, indicating an essential role of CaWs and resultant Na(+)-Ca(2+) exchanger-mediated depolarization in triggered activity. In summary, we demonstrate a mechanistic link between intracellular CaWs and arrhythmogenic oscillatory depolarizations in the heart. Our findings provide a cellular perspective on abnormal [Ca(2+)](i) handling in the genesis of triggered arrhythmias in the heart.

Topics: Action Potentials; Aniline Compounds; Animals; Arrhythmias, Cardiac; Calcium; Calcium Signaling; Cardiotonic Agents; Heart; In Vitro Techniques; Isoproterenol; Microscopy, Confocal; Myocytes, Cardiac; Periodicity; Phenyl Ethers; Potassium; Rats; Rats, Wistar; Sarcoplasmic Reticulum; Sodium-Calcium Exchanger

Involvement of the Na+/Ca2+ exchanger in ouabain-induced inotropy and arrhythmogenesis was examined with a specific inhibitor, SEA0400. In right ventricular papillary muscle isolated from guinea-pig ventricle, 1 microM SEA0400, which specifically inhibits the Na+/Ca2+ exchanger by 80%, reduced the ouabain (1 microM)-induced positive inotropy by 40%, but had no effect on the inotropy induced by 100 microM isobutyl methylxantine. SEA0400 significantly inhibited the contracture induced by low Na+ solution. In HEK293 cells expressing the Na+/Ca2+ exchanger, 1 microM ouabain induced an increase in intracellular Ca2+, which was inhibited by SEA0400. The arrhythmic contractions induced by 3 microM ouabain were significantly reduced by SEA0400. These results provide pharmacological evidence that the Na+/Ca2+ exchanger is involved in ouabain-induced inotropy and arrhythmogenesis.

Topics: 1-Methyl-3-isobutylxanthine; Aniline Compounds; Animals; Anti-Arrhythmia Agents; Arrhythmias, Cardiac; Calcium; Cardiotonic Agents; Cell Line; Cytoplasm; Enzyme Inhibitors; Guinea Pigs; In Vitro Techniques; Myocardial Contraction; Ouabain; Papillary Muscles; Phenyl Ethers; Phosphodiesterase Inhibitors; Sodium-Calcium Exchanger

Sphingosine-1-phosphate (S1P) has been considered to play an important role in ischemia/reperfusion (I/R) injury. We used SEW2871 (SEW), a novel receptor-selective agonist for S1P1, to elucidate the role of S1P1 in myocardial I/R. Isolated perfused rat hearts exposed to S1P (1 and 10 mM) or SEW (1 and 0.1 mM) were subjected to 30 minutes of global no-flow ischemia and 2 hours of reperfusion. S1P at 1 and 10 mM significantly reduced infarct size and CK release compared with vehicle-control. The effect of 0.1 microM SEW on infarct size was modest. After I/R, S1P at both doses and SEW at 0.1 microM improved developed pressure (LVDP). SEW at 1 mM significantly prolonged the duration of ventricular tachycardia and ventricular fibrillation, leading to irreversible reperfusion tachyarrhythmias in 60% of the hearts. This is the first demonstration of the critical role of the S1P1 receptor in I/R injury.

Topics: Aniline Compounds; Animals; Arrhythmias, Cardiac; Dose-Response Relationship, Drug; Heart; In Vitro Techniques; Male; Models, Biological; Molecular Structure; Myocardial Infarction; Myocardial Reperfusion Injury; Oxadiazoles; Perfusion; Phenyl Ethers; Rats; Rats, Sprague-Dawley; Receptors, Lysosphingolipid; Sodium-Calcium Exchanger; Thiophenes; Time Factors; Ventricular Function, Left

SEA0400 (2-[4-[(2,5-difluorophenyl)methoxy]phenoxy]-5-ethoxyaniline), a novel and selective inhibitor of Na+/Ca2+ exchanger, was investigated for its possible antiarrhythmic effects on arrhythmias of Ca2+ overload induced by coronary ligation/reperfusion and by digitalis in the dog. SEA0400 (1.0 mg/kg) did not change the hemodynamics but slightly prolonged the QRS duration (P<0.05). Pre-ischemic administration (10 min before coronary occlusion) of SEA0400 (1.0 mg/kg) and post-ischemic administration (1 min before reperfusion) of SEA0400 (0.3, 1.0 and 3.0 mg/kg) had no effects on the incidence of ventricular fibrillation induced by coronary ligation/reperfusion. On the other hand, SEA0400 (3.0 mg/kg) decreased the arrhythmic ratio in the digitalis arrhythmias (P<0.01). However, atrioventricular block and cardiac standstill were induced in two digitalized dogs. In conclusion, SEA0400 has no significant antiarrhythmic effect on arrhythmias induced by coronary ligation/reperfusion, but has an obvious suppressing effect on tachyarrhythmias induced by digitalis in in vivo canine models.

Topics: Aniline Compounds; Animals; Anti-Arrhythmia Agents; Arrhythmias, Cardiac; Dogs; Dose-Response Relationship, Drug; Female; Heart Ventricles; Phenyl Ethers; Sodium-Calcium Exchanger

The sodium-calcium exchange (NCX) plays a pivotal role in regulating contractility and electrical activity in the heart. However, the effects of NCX blockers on ventricular arrhythmias are still controversial. We examined the effects of KB-R7943 (KBR) and SEA0400 (SEA), two NCX blockers, on aconitine-induced arrhythmias in guinea pigs using the ECG recordings and the current-clamp method. Using Luo's and Rudy's computer model (1991 Circ Res 68:1501-1526) for ventricular myocytes, we simulated abnormal membrane activity produced by NCX inhibition. In the whole-animal model, KBR in a dose range of 1 to 30 mg/kg (intravenous) suppressed aconitine-induced arrhythmias dose-dependently, but 10 mg/kg of SEA did not suppress these arrhythmias. There was a difference in isolated ventricular myocytes also. KBR (10 microM) suppressed abnormal electrical activity induced by aconitine, but SEA (100 microM) did not show such effects. KBR (10 microM) significantly changed the shape of the action potential configurations (action potential duration at 50% repolarization), but SEA (1-100 microM) did not change these configurations. In the computer simulation study, the aconitine-induced abnormal electrical activity was mimicked by a negative shift of the kinetics of Na+ channels, and this was followed by additional suppression of NCX activity by 90% (mimicking the effect of NCX inhibitors), which enhanced abnormal membrane activity. Our results indicate that the inhibition of aconitine-induced arrhythmias by KBR, not by SEA, might result from a mechanism other than the inhibition of NCX, and thus the involvement of the NCX system plays an insignificant role in the aconitine-induced arrhythmias.

Topics: Aconitine; Action Potentials; Aniline Compounds; Animals; Anti-Arrhythmia Agents; Arrhythmias, Cardiac; Computer Simulation; Disease Models, Animal; Electrophysiology; Guinea Pigs; Heart Ventricles; Kinetics; Muscle Cells; Phenyl Ethers; Sodium Channels; Sodium-Calcium Exchanger; Thiourea

The sodium-calcium exchanger (NCX) was considered to play an important role in arrhythmogenesis under certain conditions such as heart failure or calcium overload. In the present study, the effect of SEA-0400, a selective inhibitor of the NCX, was investigated on early and delayed afterdepolarizations in canine ventricular papillary muscles and Purkinje fibres by applying conventional microelectrode techniques at 37 degrees C. The amplitude of both early and delayed afterdepolarizations was markedly decreased by 1 microM SEA-0400 from 26.6+/-2.5 to 14.8+/-1.8 mV (n=9, P<0.05) and from 12.5+/-1.7 to 5.9+/-1.4 mV (n=3, P<0.05), respectively. In enzymatically isolated canine ventricular myocytes, SEA-0400 did not change significantly the L-type calcium current and the intracellular calcium transient, studied using the whole-cell configuration of the patch-clamp technique and Fura-2 ratiometric fluorometry. It is concluded that, through the reduction of calcium overload, specific inhibition of the NCX current by SEA-0400 may abolish triggered arrhythmias.

Topics: Aniline Compounds; Animals; Arrhythmias, Cardiac; Calcium; Calcium Channels, L-Type; Cardiotonic Agents; Dogs; Electrocardiography; Female; Heart; Male; Microelectrodes; Myocytes, Cardiac; Phenyl Ethers; Sodium-Calcium Exchanger; Strophanthins

The Na(+)/Ca(2+) exchanger (NCX) is involved in myocardial ischemia-reperfusion injuries. We examined the effects of 2-[4-[(2,5-difluorophenyl)methoxy]phenoxy]-5-ethoxyaniline (SEA0400), a potent and selective inhibitor of NCX, on myocardial ischemia-reperfusion injury models. In canine cardiac sarcolemmal vesicles and rat cardiomyocytes, SEA0400 potently inhibited the Na(+)-dependent 45Ca(2+) uptake with an IC(50) value of 90 and 92 nM, compared with 2-[2-[4-(4-nitrobenzyloxy)phenyl]isothiourea (KB-R7943, 7.0 and 9.5 microM), respectively. In rat cardiomyocytes, SEA0400 (1 and 3 microM) attenuated the Ca(2+) paradox-induced cell death. In isolated rat Langendorff hearts, SEA0400 (0.3 and 1 microM) improved the cardiac dysfunction induced by low-pressure perfusion followed by normal perfusion. In anesthetized rats, SEA0400 (0.3 and 1 mg/kg, i.v.) reduced the incidence of ventricular fibrillation and mortality induced by occlusion of the left anterior descending coronary artery followed by reperfusion. These results suggest that SEA0400 is a most potent NCX inhibitor in the heart and that it has protective effects against myocardial ischemia-reperfusion injuries.

Topics: Aniline Compounds; Animals; Anti-Arrhythmia Agents; Arrhythmias, Cardiac; Calcium; Cell Survival; Cells, Cultured; Dogs; Guanidines; Heart; In Vitro Techniques; Male; Myocardial Reperfusion Injury; Myocardium; Myocytes, Cardiac; Phenyl Ethers; Rats; Rats, Sprague-Dawley; Rats, Wistar; Sarcolemma; Sodium-Calcium Exchanger; Sulfones; Thiourea