Effects of Sodium-Calcium Exchange Inhibitors, KB-R7943 and SEA0400, on Aconitine-Induced Arrhythmias in Guinea Pigs in Vivo, in Vitro, and in Computer Simulation Studies

New drug discovery of cardiac anti-arrhythmic drugs: insights in animal models

Cardiac rhythm regulated by micro-macroscopic structures of heart. Pacemaker abnormalities or disruptions in electrical conduction, lead to arrhythmic disorders may be benign, typical, threatening, ultimately fatal, occurs in clinical practice, patients on digitalis, anaesthesia or acute myocardial infarction. Both traditional and genetic animal models are: In-vitro: Isolated ventricular Myocytes, Guinea pig papillary muscles, Patch-Clamp Experiments, Porcine Atrial Myocytes, Guinea pig ventricular myocytes, Guinea pig papillary muscle: action potential and refractory period, Langendorff technique, Arrhythmia by acetylcholine or potassium. Acquired arrhythmia disorders: Transverse Aortic Constriction, Myocardial Ischemia, Complete Heart Block and AV Node Ablation, Chronic Tachypacing, Inflammation, Metabolic and Drug-Induced Arrhythmia. In-Vivo: Chemically induced arrhythmia: Aconitine antagonism, Digoxin-induced arrhythmia, Strophanthin/ouabain-induced arrhythmia, Adrenaline-induced arrhythmia, and Calcium-induced arrhythmia. Electrically induced arrhythmia: Ventricular fibrillation electrical threshold, Arrhythmia through programmed electrical stimulation, sudden coronary death in dogs, Exercise ventricular fibrillation. Genetic Arrhythmia: Channelopathies, Calcium Release Deficiency Syndrome, Long QT Syndrome, Short QT Syndrome, Brugada Syndrome. Genetic with Structural Heart Disease: Arrhythmogenic Right Ventricular Cardiomyopathy/Dysplasia, Dilated Cardiomyopathy, Hypertrophic Cardiomyopathy, Atrial Fibrillation, Sick Sinus Syndrome, Atrioventricular Block, Preexcitation Syndrome. Arrhythmia in Pluripotent Stem Cell Cardiomyocytes. Conclusion: Both traditional and genetic, experimental models of cardiac arrhythmias' characteristics and significance help in development of new antiarrhythmic drugs.

Chinese herbal medicine for the treatment of cardiovascular diseases ─ targeting cardiac ion channels

Cardiovascular disease (CVD) remains the leading cause of morbidity and mortality, imposing an increasing global health burden. Cardiac ion channels (voltage-gated Na_V, Ca_V, K_Vs, and others) synergistically shape the cardiac action potential (AP) and control the heartbeat. Dysfunction of these channels, due to genetic mutations, transcriptional or post-translational modifications, may disturb the AP and lead to arrhythmia, a major risk for CVD patients. Although there are five classes of anti-arrhythmic drugs available, they can have varying levels of efficacies and side effects on patients, possibly due to the complex pathogenesis of arrhythmias. As an alternative treatment option, Chinese herbal remedies have shown promise in regulating cardiac ion channels and providing anti-arrhythmic effects. In this review, we first discuss the role of cardiac ion channels in maintaining normal heart function and the pathogenesis of CVD, then summarize the classification of Chinese herbal compounds, and elaborate detailed mechanisms of their efficacy in regulating cardiac ion channels and in alleviating arrhythmia and CVD. We also address current limitations and opportunities for developing new anti-CVD drugs based on Chinese herbal medicines.

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

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

Sodium-Calcium Exchanger 2: A Pivotal Role in Oxaliplatin Induced Peripheral Neurotoxicity and Axonal Damage?

Oxaliplatin (OHP)-induced peripheral neurotoxicity (OIPN) is a frequent adverse event of colorectal cancer treatment. OIPN encompasses a chronic and an acute syndrome. The latter consists of transient axonal hyperexcitability, due to unbalance in Na+ voltage-operated channels (Na+VOC). This leads to sustained depolarisation which can activate the reverse mode of the Na+/Ca2+ exchanger 2 (NCX2), resulting in toxic Ca2+ accumulation and axonal damage (ADa). We explored the role of NCX2 in in vitro and in vivo settings. Embryonic rat Dorsal Root Ganglia (DRG) organotypic cultures treated with SEA0400 (SEA), a NCX inhibitor, were used to assess neuroprotection in a proof-of-concept and pilot study to exploit NCX modulation to prevent ADa. In vivo, OHP treated mice (7 mg/Kg, i.v., once a week for 8 weeks) were compared with a vehicle-treated group (n = 12 each). Neurophysiological and behavioural testing were performed to characterise acute and chronic OIPN, and morphological analyses were performed to detect ADa. Immunohistochemistry, immunofluorescence, and western blotting (WB) analyses were also performed to demonstrate changes in NCX2 immunoreactivity and protein expression. In vitro, NCX inhibition was matched by ADa mitigation. In the in vivo part, after verifyingboth acute and chronic OIPN had ensued, we confirmed via immunohistochemistry, immunofluorescence, and WB that a significant NCX2 alteration had ensued in the OHP group. Our data suggest NCX2 involvement in ADa development, paving the way to a new line of research to prevent OIPN.

Regulation of APD and Force by the Na+/Ca2+ Exchanger in Human-Induced Pluripotent Stem Cell-Derived Engineered Heart Tissue

The physiological importance of NCX in human-induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) is not well characterized but may depend on the relative strength of the current, compared to adult cardiomyocytes, and on the exact spatial arrangement of proteins involved in Ca2+ extrusion. Here, we determined NCX currents and its contribution to action potential and force in hiPSC-CMs cultured in engineered heart tissue (EHT). The results were compared with data from rat and human left ventricular tissue. The NCX currents in hiPSC-CMs were larger than in ventricular cardiomyocytes isolated from human left ventricles (1.3 ± 0.2 pA/pF and 3.2 ± 0.2 pA/pF for human ventricle and EHT, respectively, p < 0.05). SEA0400 (10 µM) markedly shortened the APD90 in EHT (by 26.6 ± 5%, p < 0.05) and, to a lesser extent, in rat ventricular tissue (by 10.7 ± 1.6%, p < 0.05). Shortening in human left ventricular preparations was small and not different from time-matched controls (TMCs; p > 0.05). Force was increased by the NCX block in rat ventricle (by 31 ± 5.4%, p < 0.05) and EHT (by 20.8 ± 3.9%, p < 0.05), but not in human left ventricular preparations. In conclusion, hiPSC-CMs possess NCX currents not smaller than human left ventricular tissue. Robust NCX block-induced APD shortening and inotropy makes EHT an attractive pharmacological model.

Two New Alkaloids from Fuzi and Their Metabolites Study

Former study suggests alkaloids from herbs of Aconitum genus plants possess excellent bioactivities, which exert great value for related deeper chemical constituent investigation. Herein, chemical isolation was performed and four alkaloids were isolated from Fuzi, of which two were new ones, and the other two were reported NMR data for the first time. Their chemical structures were identified by NMR data, high resolution MS, UV and IR analysis. Additionally, the MS fragmentation patterns were explored, formerly, that of hetisane alkaloid was rarely reported, and fragmentation mechanism of the diagnostic ion was proposed. Based on these fragment pathway, metabolites and metabolic pathways of four compounds were investigated in rat liver microsomes using UPLC-Q/TOF-MS, and dehydrogenation product was firstly found from metabolites of hetisane alkaloid.

Risk Compounds, Preclinical Toxicity Evaluation, and Potential Mechanisms of Chinese Materia Medica-Induced Cardiotoxicity

Chinese materia medica (CMM) has been applied for the prevention and treatment of diseases for thousands of years. However, arrhythmia, myocardial ischemia, heart failure, and other cardiac adverse reactions during CMM application were gradually reported. CMM-induced cardiotoxicity has aroused widespread attention. Our review aimed to summarize the risk compounds, preclinical toxicity evaluation, and potential mechanisms of CMM-induced cardiotoxicity. All relevant articles published on the PubMed, Embase, and China National Knowledge Infrastructure (CNKI) databases for the latest twenty years were searched and manually extracted. The risk substances of CMM-induced cardiotoxicity are relatively complex. A single CMM usually contains various risk compounds, and the same risk substance may exist in various CMM. The active and risk substances in CMM may be transformed into each other under different conditions, such as drug dosage, medication methods, and body status. Generally, the risk compounds of CMM-induced cardiotoxicity can be classified into alkaloids, terpenoids, steroids, heavy metals, organic acids, toxic proteins, and peptides. Traditional evaluation methods of chemical drug-induced cardiotoxicity primarily include cardiac function monitoring, endomyocardial biopsy, myocardial zymogram, and biomarker determination. In the preclinical stage, CMM-induced cardiotoxicity should be systematically evaluated at the overall, tissue, cellular, and molecular levels, including cardiac function, histopathology, cytology, myocardial zymogram, and biomarkers. Thanks to the development of systematic biology, the higher specificity and sensitivity of biomarkers, such as genes, proteins, and metabolic small molecules, are gradually applied for evaluating CMM-induced cardiotoxicity. Previous studies on the mechanisms of CMM-induced cardiotoxicity focused on a single drug, monomer or components of CMM. The interaction among ion homeostasis (sodium, potassium, and calcium ions), oxidative damage, mitochondrial injury, apoptosis and autophagy, and metabolic disturbance is involved in CMM-induced cardiotoxicity. Clarification on the risk compounds, preclinical toxicity evaluation, and potential mechanisms of CMM-induced cardiotoxicity must be beneficial to guide new CMM development and post-marketed CMM reevaluation.

Inhibition of the INa/K and the activation of peak INa contribute to the arrhythmogenic effects of aconitine and mesaconitine in guinea pigs

Aconitine (ACO), a main active ingredient of Aconitum, is well-known for its cardiotoxicity. However, the mechanisms of toxic action of ACO remain unclear. In the current study, we investigated the cardiac effects of ACO and mesaconitine (MACO), a structurally related analog of ACO identified in Aconitum with undocumented cardiotoxicity in guinea pigs. We showed that intravenous administration of ACO or MACO (25 μg/kg) to guinea pigs caused various types of arrhythmias in electrocardiogram (ECG) recording, including ventricular premature beats (VPB), atrioventricular blockade (AVB), ventricular tachycardia (VT), and ventricular fibrillation (VF). MACO displayed more potent arrhythmogenic effect than ACO. We conducted whole-cell patch-clamp recording in isolated guinea pig ventricular myocytes, and observed that treatment with ACO (0.3, 3 μM) or MACO (0.1, 0.3 μM) depolarized the resting membrane potential (RMP) and reduced the action potential amplitude (APA) and durations (APDs) in a concentration-dependent manner. The ACO- and MACO-induced AP remodeling was largely abolished by an I_Na blocker tetrodotoxin (2 μM) and partly abolished by a specific Na⁺/K⁺ pump (NKP) blocker ouabain (0.1 μM). Furthermore, we observed that treatment with ACO or MACO attenuated NKP current (I_Na/K) and increased peak I_Na by accelerating the sodium channel activation with the EC₅₀ of 8.36 ± 1.89 and 1.33 ± 0.16 μM, respectively. Incubation of ventricular myocytes with ACO or MACO concentration-dependently increased intracellular Na⁺ and Ca²⁺ concentrations. In conclusion, the current study demonstrates strong arrhythmogenic effects of ACO and MACO resulted from increasing the peak I_Na via accelerating sodium channel activation and inhibiting the I_Na/K. These results may help to improve our understanding of cardiotoxic mechanisms of ACO and MACO, and identify potential novel therapeutic targets for Aconitum poisoning.

A Proteomics- and Metabolomics-Based Study Revealed That Disorder of Palmitic Acid Metabolism by Aconitine Induces Cardiac Injury

Currently, research on cardiac injury by aconitine focuses on its effect to directly interfere with the function of cardiac ion channels. Further, abnormal lipid metabolism could cause cardiac injury via inflammatory signaling pathway. In our preliminary study, we discovered that aconitine could alter the metabolism processes of various substances, including palmitic acid. Inspired by these studies, we investigated how elevation of palmitic acid by aconitine causes cardiac injury. Aconitine induced cardiac injury in rats (0.32 mg/kg, d = 7), and the cardiac injury was confirmed by electrocardiogram and serum biochemical study. The proteomic and metabolomic results showed that the palmitic acid level increases in heart tissue, and the NOD-like receptor (NLR) signaling pathway showed a strong effect of cardiac injury. The palmitic acid results in cell viability decline and activates NLR signaling in vitro. The shRNA-mediated knockdown of NLRP3 and NOD1/2 attenuates palmitic acid-induced inhibitory effect on cells and inhibited activation of the NLR signaling pathway. Collectively, this study reveals that aconitine provoked palmitic acid elevation could aggravate cardiac injury via the NLR signaling pathway. This study suggests that drug triggered disorder of the metabolism process could evoke cardiac injury and could propose a new strategy to study drug cardiac injury.

Pseudo-ginsengenin DQ ameliorated aconitine-induced arrhythmias by influencing Ca2+ and K+ currents in ventricular myocytes

Pseudo-ginsengenin DQ (PDQ) is the product of the oxidative cyclization of protopanaxadiol. PDQ exhibits various bioactivities, including reversal of multidrug resistance in cancer, renal protective effects against acute nephrotoxicity and attenuating myocardial ischemia injury induced by isoproterenol or ligation of coronary arterials, but its effect on arrhythmias has not been clear until now. Because of the complicated effects of ginseng on the cardiovascular system, it is necessary to investigate whether PDQ affects arrhythmias, which are always concomitant with other cardiac diseases. Aconitine was used to induce arrhythmia in vivo. To understand its electrophysiological fundamental, whole-cell patch-clamp was used to record the L-type calcium current (I_Ca,L) and potassium currents (I_K and I_K1) in the ventricular myocytes in rats. Oral administration of PDQ exerted obvious antiarrhythmic effects, as indicated by the decreased incidence rate, lower number of occurrences, and shorter duration time of ventricular tachycardia and ventricular tachycardia, decreased mortality rate and increased survival time. I_Ca,L and I_K were inhibited by PDQ treatment while I_K1 was not affected. To conclude, PDQ may have an anti-arrhythmia effect through inhibiting I_Ca,L and I_K.

Pseudo-ginsengenin DQ (PDQ) is the product of the oxidative cyclization of protopanaxadiol. PDQ could ameliorate aconitine-induced arrhythmias by influencing Ca²⁺ and K⁺ currents in ventricular myocytes.

Taurine-magnesium coordination compound, a potential anti-arrhythmic complex, improves aconitine-induced arrhythmias through regulation of multiple ion channels

Taurine-magnesium coordination compound (TMCC) exhibits antiarrhythmic effects in cesium-chloride-and ouabain-induced arrhythmias; however, the mechanism underlying these effects on arrhythmia remains poorly understood. Here, we investigated the effects of TMCC on aconitine-induced arrhythmia in vivo and the electrophysiological effects of this compound in rat ventricular myocytes in vitro. Aconitine was used to induce arrhythmias in rats, and the dosages required to produce ventricular premature contraction (VPC), ventricular tachycardia (VT), ventricular fibrillation (VF), and cardiac arrest (CA) were recorded. Additionally, the sodium current (I_Na) and L-type calcium current (I_Ca,L) were analyzed in normal and aconitine-treated ventricular myocytes using whole-cell patch-clamp recording. In vivo, intravenous administration of TMCC produced marked antiarrhythmic effects, as indicated by the increased dose of aconitine required to induce VPC, VT, VF, and CA. Moreover, this effect was abolished by administration of sodium channel opener veratridine and calcium channel agonist Bay K8644. In vitro, TMCC inhibited aconitine-induced increases in I_Na and I_Ca,L. These results revealed that TMCC inhibited aconitine-induced arrhythmias through effects on I_Na and I_Ca,L.

Research progress of aconitine toxicity and forensic analysis of aconitine poisoning

Chinese herbal medicines have been extensively used in China and other countries for centuries. Aconitine, a diterpenoid alkaloid extracted from Aconitum plants, has anti-inflammatory and analgesic activities, but can also induce severe arrhythmia and neurotoxicity. Aconitine poisoning accidents caused by misuse, suicide, or homicide have been reported in recent years. In China, fatal aconitine poisoning can occasionally happen on account of accidental ingestion of some wild plants or consumption of herbal decoction made from the roots of Aconitum plants. However, it is rather difficult for forensic experts to find the specific results in present forensic autopsy of aconitine-induced death. To further clarify its potential risk following the widespread application of aconitine, toxicological characteristics and pharmacokinetics of aconitine are reviewed. Moreover, gastrointestinal, neurological, and cardiovascular symptoms were observed frequently in aconitine poisoning cases. In addition, the review also aims at providing some convincing evidences for forensic experts to identify unexplained death with postmortem examination.

L-Type Calcium Channel Inhibition Contributes to the Proarrhythmic Effects of Aconitine in Human Cardiomyocytes

Aconitine (ACO) is well-known for causing lethal ventricular tachyarrhythmias. While cardiac Na⁺ channel opening during repolarization has long been documented in animal cardiac myocytes, the cellular effects and mechanism of ACO in human remain unexplored. This study aimed to assess the proarrhythmic effects of ACO in human induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs). ACO concentration-dependently (0.3 ~ 3.0 μM) shortened the action potentials (AP) durations (APD) in ventricular-like hiPSC-CMs by > 40% and induced delayed after-depolarization. Laser-scanning confocal calcium imaging analysis showed that ACO decreased the duration and amplitude of [Ca²⁺]_i transients and increased in the beating frequencies by over 60%. Moreover, ACO was found to markedly reduce the L-type calcium channel (LTCC) currents (I_Ca,L) in hiPSC-CMs associated with a positive-shift of activation and a negative shift of inactivation. ACO failed to alter the peak and late Na⁺ currents (I_Na) in hiPSC-CMs while it drastically increased the late I_Na in Guinea-pig ventricular myocytes associated with enhanced activation/delayed inactivation of I_Na at -55 mV~ -85 mV. Further, the effects of ACO on I_Ca,L, I_Na and the rapid delayed rectifier potassium current (I_kr) were validated in heterologous expression systems by automated voltage-clamping assays and a moderate suppression of I_kr was observed in addition to concentration-dependent I_Ca,L inhibition. Lastly, increased beating frequency, decreased Ca²⁺ wave and shortened field potential duration were recorded from hiPSC-CMs by microelectrode arrays assay. In summary, our data demonstrated that LTCC inhibition could play a main role in the proarrhythmic action of ACO in human cardiomyocytes.

Dyssynchronous calcium removal in heart failure-induced atrial remodeling

We tested the hypothesis that in atrial myocytes from a rabbit left ventricular heart failure (HF) model, spatial inhomogeneity and temporal dyssynchrony of Ca removal during excitation-contraction coupling together with increased Na/Ca exchange (NCX) activity generate a substrate for proarrhythmic Ca release. Ca removal occurs via Ca reuptake into the sarcoplasmic reticulum and extrusion via NCX exclusively in the cell periphery since rabbit atrial myocytes lack transverse tubules. Ca removal kinetics were assessed by the time constant τ of decay of local peripheral subsarcolemmal (SS) and central (CT) action potential (AP)-induced Ca transients (CaTs) recorded in confocal line scan mode (using Fluo-4). Spatial and temporal dyssynchrony of Ca removal was quantified by CV TAU, defined as the standard deviation of local τ along the transverse cell axis divided by mean τ. In normal cells CT CaT decline was slower compared with the SS domain, while in HF cells decline was accelerated, became equal in SS and CT regions, and a significant increase of CV TAU indicated an increased Ca removal dyssynchrony. In HF atrial cells NCX upregulation was accompanied by an overall higher incidence of spontaneous Ca waves and a higher propensity of arrhythmogenic Ca waves, defined as waves that triggered APs due to NCX-mediated membrane depolarization. NCX inhibition normalized CV TAU in HF atrial cells and decreased the propensity of Ca waves. In summary, HF atrial myocytes show accelerated but dyssynchronous diastolic Ca removal and altered sarcoplasmic reticulum Ca-ATPase (SERCA) and NCX activity that result in increased susceptibility to arrhythmia.

Role of the dysfunctional ryanodine receptor - Na(+)-Ca(2+)exchanger axis in progression of cardiovascular diseases: What we can learn from pharmacological studies?

Abnormal Ca(2+)homeostasis is often associated with chronic cardiovascular diseases, such as hypertension, heart failure or cardiac arrhythmias, and typically contributes to the basic ethiology of the disease. Pharmacological targeting of cardiac Ca(2+)handling has great therapeutic potential offering invaluable options for the prevention, slowing down the progression or suppression of the harmful outcomes like life threatening cardiac arrhythmias. In this review we outline the existing knowledge on the involvement of malfunction of the ryanodine receptor and the Na(+)-Ca(2+)exchanger in disturbances of Ca(2+)homeostasis and discuss important proof of concept pharmacological studies targeting these mechanisms in context of hypertension, heart failure, atrial fibrillation and ventricular arrhythmias. We emphasize the promising results of preclinical studies underpinning the potential benefits of the therapeutic strategies based on ryanodine receptor or Na(+)-Ca(2+)exchanger inhibition.

Selective Na(+) /Ca(2+) exchanger inhibition prevents Ca(2+) overload-induced triggered arrhythmias

BACKGROUND AND PURPOSE

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.

EXPERIMENTAL APPROACH

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.

KEY RESULTS

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.

CONCLUSIONS AND IMPLICATIONS

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.

New antiarrhythmic targets to control intracellular calcium handling

Sudden cardiac death due to ventricular arrhythmias is a major problem. Drug therapies to prevent SCD do not provide satisfying results, leading to the demand for new antiarrhythmic strategies. New targets include Ca²⁺/Calmodulin-dependent protein kinase II (CaMKII), the Na/Ca exchanger (NCX), the Ryanodine receptor (RyR, and its associated protein FKBP12.6 (Calstabin)) and the late component of the sodium current (I_Na-Late), all related to intracellular calcium (Ca²⁺) handling. In this review, drugs interfering with these targets (SEA-0400, K201, KN-93, W7, ranolazine, sophocarpine, and GS-967) are evaluated and their future as clinical compounds is considered. These new targets prove to be interesting; however more insight into long-term drug effects is necessary before clinical applicability becomes reality.

Selective inhibition of late sodium current suppresses ventricular tachycardia and fibrillation in intact rat hearts

BACKGROUND

Enhanced late inward Na current (INa-L) modulates action potential duration (APD) and plays a key role in the genesis of early afterdepolarizations (EADs) and delayed afterdepolarizations (DADs) and triggered activity.

OBJECTIVE

The purpose of this study was to define the influence of selective block of INa-L on EAD- and DAD-mediated triggered ventricular tachycardia (VT) and ventricular fibrillation (VF) in intact hearts using (GS967), a selective and potent (IC50 = 0.13 ± 0.01 μM) blocker of INa-L.

METHODS

VT/VF were induced either by local aconitine injection (50 μg) in the left ventricular muscle of adult (3-4 months) male rats (N = 21) or by arterial perfusion of 0.1 mM hydrogen peroxide (H2O2) in aged male rats (24-26 months, N = 16). The left ventricular epicardial surface of the isolated-perfused hearts was optically mapped using fluorescent voltage-sensitive dye, and microelectrode recordings of action potentials were made adjacent to the aconitine injection site. The suppressive and preventive effects of GS967 (1 μM) against EAD/DAD-mediated VT/VF were then determined.

RESULTS

Aconitine induced VT in all 13 hearts studied. Activation map (N = 6) showed that the VT was initiated by a focal activity arising from the aconitine injection site (cycle length [CL] 84 ± 12) that degenerated to VF (CL 52 ± 8 ms) within a few seconds. VF was maintained by multifocal activity with occasional incomplete reentrant wavefronts. Administration of GS967 suppressed the VT/VF in 10 of 13 hearts (P < .001). Preexposure to GS967 for 15 minutes before aconitine injection prevented initiation of VT/VF in 5 of 8 additional hearts (P < .02). VF reoccurred within 10 minutes on washout of GS967. Microelectrode recordings (N = 7) showed that VT/VF was initiated by EAD- and DAD-mediated triggered activity at CL of 86 ± 14 ms (NS from VT CL) that preceded the VF. GS967 shortened APD, flattened the slope of the dynamic APD restitution curve, and reduced APD dispersion from 42 ± 12 ms to 8 ± 3 ms (P < .01). H2O2 perfusion in eight fibrotic aged hearts promoted EAD-mediated focal VT/VF, which was suppressed by GS967 in five hearts (P < .02).

CONCLUSION

The selective INa-L blocker GS967 effectively suppresses and prevents aconitine and oxidative stress-induced EADs, DADs, and focal VT/VF. Suppression of EADs, DADs, and reduction of APD dispersion make GS967 a potentially useful antiarrhythmic drug in conditions of enhanced INa-L.

Combined Na(+)/Ca(2+) exchanger and L-type calcium channel block as a potential strategy to suppress arrhythmias and maintain ventricular function

BACKGROUND

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.

METHODS AND RESULTS

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.

CONCLUSIONS

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.

Alternative strategies in arrhythmia therapy: evaluation of Na/Ca exchange as an anti-arrhythmic target

The search for alternative anti-arrhythmic strategies is fueled by an unmet medical need as well as by the opportunities arising from identification of novel targets and novel drugs. Na/Ca exchange is a potential target involved in several types of arrhythmias, such as those related to ischemia-reperfusion, heart failure and also some forms of genetic arrhythmias. Inhibition of Na/Ca exchange is theoretically not only anti-arrhythmic but also increases cellular Ca(2+) content. This could be an advantage in conditions of low inotropy, such as in heart failure, but may also worsen conditions such as the recovery from ischemia or relaxation abnormalities. With the available drugs such as KB-R7943 and SEA-0400 these theories have now been tested in a number of cellular and in vivo models. Experience is overall rather positive and seems less hampered by the potential drawbacks than expected. This may be because the currently available drugs are not highly selective, with additional benefit derived from concurrent effects. While this precludes a definite answer regarding the benefit of a pure NCX inhibitor, they indicate that Na/Ca exchange inhibition as part of a multi-target strategy is an avenue to be considered. Such studies will need further 'bench' work and testing in relevant preclinical models, including chronic disease.

Overexpression of M₃ muscarinic receptor is a novel strategy for preventing sudden cardiac death in transgenic mice

The present study was designed to investigate the cardiac benefits of M₃ muscarinic receptor (M₃-mAChR) overexpression and whether these effects are related to the regulation of the inward rectifying K⁺ channel by microRNA-1 (miR-1) in a conditional overexpression mouse model. A cardiac-specific M₃-mAChR transgenic mouse model was successfully established for the first time in this study using microinjection, and the overexpression was confirmed by both reverse transcriptase-polymerase chain reaction and Western blot techniques. We demonstrated that M₃-mAChR overexpression dramatically reduced the incidence of arrhythmias and decreased the mortality in a mouse model of myocardial ischemia-reperfusion (I/R). By using whole-cell patch techniques, M₃-mAChR overexpression significantly shortened the action potential duration and restored the membrane repolarization by increasing the inward rectifying K⁺ current. By using Western blot techniques, M₃-mAChR overexpression also rescued the expression of the inward rectifying K⁺ channel subunit Kir2.1 after myocardial I/R injury. This result was accompanied by suppression of upregulation miR-1. We conclude that M₃-mAChR overexpression reduced the incidence of arrhythmias and mortality after myocardial I/R by protecting the myocardium from ischemia in mice. This effect may be mediated by increasing the inward rectifying K⁺ current by downregulation of arrhythmogenic miR-1 expression, which might partially be a novel strategy for antiarrhythmias, leading to sudden cardiac death.

Triple threat: the Na+/Ca2+ exchanger in the pathophysiology of cardiac arrhythmia, ischemia and heart failure

The Na(+)/Ca(2+) exchanger (NCX) is the main Ca(2+) extrusion mechanism of the cardiac myocyte and thus is crucial for maintaining Ca(2+) homeostasis. It is involved in the regulation of several parameters of cardiac excitation contraction coupling, such as cytosolic Ca(2+) concentration, repolarization and contractility. Increased NCX activity has been identified as a mechanism promoting heart failure, cardiac ischemia and arrhythmia. Transgenic mice as well as pharmacological interventions have been used to support the idea of using NCX inhibition as a future pharmacological strategy to treat cardiovascular disease.

Methods for ECG evaluation of indicators of cardiac risk, and susceptibility to aconitine-induced arrhythmias in rats following status epilepticus

Lethal cardiac arrhythmias contribute to mortality in a number of pathological conditions. Several parameters obtained from a non-invasive, easily obtained electrocardiogram (ECG) are established, well-validated prognostic indicators of cardiac risk in patients suffering from a number of cardiomyopathies. Increased heart rate, decreased heart rate variability (HRV), and increased duration and variability of cardiac ventricular electrical activity (QT interval) are all indicative of enhanced cardiac risk. In animal models, it is valuable to compare these ECG-derived variables and susceptibility to experimentally induced arrhythmias. Intravenous infusion of the arrhythmogenic agent aconitine has been widely used to evaluate susceptibility to arrhythmias in a range of experimental conditions, including animal models of depression and hypertension, following exercise and exposure to air pollutants, as well as determination of the antiarrhythmic efficacy of pharmacological agents. It should be noted that QT dispersion in humans is a measure of QT interval variation across the full set of leads from a standard 12-lead ECG. Consequently, the measure of QT dispersion from the 2-lead ECG in the rat described in this protocol is different than that calculated from human ECG records. This represents a limitation in the translation of the data obtained from rodents to human clinical medicine. Status epilepticus (SE) is a single seizure or series of continuously recurring seizures lasting more than 30 min, and results in mortality in 20% of cases. Many individuals survive the SE, but die within 30 days. The mechanism(s) of this delayed mortality is not fully understood. It has been suggested that lethal ventricular arrhythmias contribute to many of these deaths. In addition to SE, patients experiencing spontaneously recurring seizures, i.e. epilepsy, are at risk of premature sudden and unexpected death associated with epilepsy (SUDEP). As with SE, the precise mechanisms mediating SUDEP are not known. It has been proposed that ventricular abnormalities and resulting arrhythmias make a significant contribution. To investigate the mechanisms of seizure-related cardiac death, and the efficacy of cardioprotective therapies, it is necessary to obtain both ECG-derived indicators of risk and evaluate susceptibility to cardiac arrhythmias in animal models of seizure disorders. Here we describe methods for implanting ECG electrodes in the Sprague-Dawley laboratory rat (Rattus norvegicus), following SE, collection and analysis of ECG recordings, and induction of arrhythmias during iv infusion of aconitine. These procedures can be used to directly determine the relationships between ECG-derived measures of cardiac electrical activity and susceptibility to ventricular arrhythmias in rat models of seizure disorders, or any pathology associated with increased risk of sudden cardiac death.

Aconitine blocks HERG and Kv1.5 potassium channels

ETHNOPHARMACOLOGICAL RELEVANCE

Aconitum has been widely used to treat various diseases in China for a long time. However, improper use of this drug results in severe intoxication. Aconitine (ACO), a diterpenoid alkaloid from aconitum, mainly contributes to cardio-toxic effects of aconitum and has also been commonly known to induce arrhythmias in animal models. However, its pro-arrhythmic mechanisms are not clear.

AIM OF THE STUDY

The effects of ACO on HERG and Kv1.5 channels were investigated.

MATERIALS AND METHODS

HERG and Kv1.5 channels were expressed in Xenopus laevis oocytes, and the resulting currents were recorded using a two-microelectrode voltage clamp technique.

RESULTS

In HERG channels, ACO exhibited a blockade in a voltage- and time-dependent manner. The blockade was enhanced by further activation of currents, which were consistent with an open-channel blockade. In Kv1.5 channels, ACO produced a voltage-, time-, and frequency-dependent inhibition. The blockade was enhanced by higher rates of stimulation, consistent with preferential binding of the drug to the open state. In addition, ACO blocked Kv1.5 and HERG channels in a concentration-dependent manner with an IC(50) of 0.796+/-0.123 and 1.801+/-0.332 microM, respectively.

CONCLUSIONS

ACO blocks HERG and Kv1.5 potassium channels in the open state. Blockade of potassium channels, particular the HERG channel, may be one of the important mechanisms of how ACO induces arrhythmias.

Aconite poisoning presenting as hypotension and bradycardia

The principal toxic ingredients of aconite roots include aconitine, mesaconitine and hypaconitine, which are known cardiotoxins and neurotoxins. A 58-year-old man took a decoction of 11 g each of processed 'chuanwu' (the main root of Aconitum carmichaeli) and processed 'caowu' (the root of A. kusnezoffii) as treatment for his neck pain. One hour later, he experienced numbness of tongue and the four limbs, generalized weakness, nausea, vomiting, diarrhoea and dizziness. Three hours after ingestion, he was admitted to hospital. His blood pressure was 106/53 mmHg and heart rate 65 beats/min. Six hours after ingestion, he became hypotensive (systolic blood pressure <100 mmHg) with bradycardia (heart rate <60 beats/min). As treatments for the hypotension, he was given intravenous infusions of 0.9% saline (125 mL/hour) for 15 hours (7-21 hours after ingestion) and dopamine (3 microg/kg/min) for 36 hours (10-45 hours after ingestion). He was given atropine 0.6 mg intravenously 7 and 24 hours after ingestion. He was hypotensive for 31 hours (6-36 hours after ingestion), with a systolic blood pressure of 84-106 mmHg (mean 93.5) and a diastolic blood pressure of 40-59 mmHg (mean 51.8). He had bradycardia for 36 hours (6-41 hours after ingestion), with a heart rate of 45-68 beats/min (mean 56.5). On discharge (48 hours after ingestion), his blood pressure was 117/82 mmHg and heart rate 70 beats/min. In patients with aconite poisoning, prolonged hypotension and sinus bradycardia may occur and supportive therapy with close monitoring of blood pressure and cardiac rhythm are essential.

Antiarrhythmic effect of acute oxygen-ozone administration to rats

The antiarrhythmic effects of 100; 150; and 300microg/kg i.p. oxygen/ozone mixture were tested on arrhythmias induced by i) ischemia; ii) ischemia/reperfusion; iii) aconitine (15microg/kg/i.v.); potassium chloride (1.5% i.v.) in rats. 25min of cardiac left descending coronary artery ischemia caused severe incidence of ventricular tachycardia, ventricular fibrillation and mortality. These were significantly reduced by pre-treatment of rats with oxygen/ozone mixture at doses of 150 and 300microg/kg. In separate experiments using a protocol of 5min ischemia followed by 8min reperfusion this caused arrhythmias starting within 6+/-1s. The incidence of ventricular tachycardia was 100%, while ventricular fibrillation occurred in 75% of the animals and lasted 85+/-14s. The mortality was 62.5%. These figures were significantly (P<0.01) reduced in animals treated with 150microg/kg oxygen/ozone and a substantial increase observed with 300microg/kg, whilst not affected by the lower dose of 100microg/kg. 150 and 300microg/kg oxygen/ozone prolonged the onset time for the appearance of arrhythmias induced by aconitine (300microg/kg oxygen/ozone, approximately 81% longer). Oxygen/ozone also reduced the ventricular tachycardia duration, ventricular fibrillation incidence, arrhythmia score, and increased the rat's survival rate. As for example, this latter was increased from 25% (aconitine) to 50% (aconitine+oxygen/ozone 150microg/kg). 100microg/kg oxygen/ozone was without effect. None of the oxygen/ozone doses affected the arrhythmias caused by potassium chloride 1.5% i.v. All the oxygen/ozone antiarrhythmic effects were similar to those observed with lidocaine (1.5mg/kg i.v.). In conclusion, oxygen/ozone has antiarrhythmic effects against arrhythmias caused by aconitine, myocardial ischemia and ischemia/reperfusion.

The role of the Na+/Ca2+ exchanger, I(Na) and I(CaL) in the genesis of dofetilide-induced torsades de pointes in isolated, AV-blocked rabbit hearts

BACKGROUND AND PURPOSE

The Na+/Ca2+ exchanger (NCX) may contribute to triggered activity and transmural dispersion of repolarization, which are substrates of torsades de pointes (TdP) type arrhythmias. This study examined the effects of selective inhibition of the NCX by SEA0400 on the occurrence of dofetilide-induced TdP.

EXPERIMENTAL APPROACH

Effects of SEA0400 (1 micromol x L(-1)) on dofetilide-induced TdP was studied in isolated, Langendorff-perfused, atrioventricular (AV)-blocked rabbit hearts. To verify the relevance of the model, lidocaine (30 micromol x L(-1)) and verapamil (750 nmol x L(-1)) were also tested against dofetilide-induced TdP.

KEY RESULTS

Acute AV block caused a chaotic idioventricular rhythm and strikingly increased beat-to-beat variability of the RR and QT intervals. SEA0400 exaggerated the dofetilide-induced increase in the heart rate-corrected QT interval (QTc) and did not reduce the incidence of dofetilide-induced TdP [100% in the SEA0400 + dofetilide group vs. 75% in the dofetilide (100 nmol x L(-1)) control]. In the second set of experiments, verapamil further increased the dofetilide-induced QTc prolongation and neither verapamil nor lidocaine reduced the dofetilide-induced increase in the beat-to-beat variability of the QT interval. However, lidocaine decreased and verapamil prevented the development of dofetilide-induced TdP as compared with the dofetilide control (TdP incidence: 13%, 0% and 88% respectively).

CONCLUSIONS AND IMPLICATIONS

Na+/Ca2+ exchanger does not contribute to dofetilide-induced TdP, whereas Na+ and Ca2+ channel activity is involved in TdP genesis in isolated, AV-blocked rabbit hearts. Neither QTc prolongation nor an increase in the beat-to-beat variability of the QT interval is a sufficient prerequisite of TdP genesis in rabbit hearts.

Effects of aconitine on [Ca2+] oscillation in cultured myocytes of neonatal rats

In order to investigate the effects of aconitine on [Ca2+] oscillation patterns in cultured myocytes of neonatal rats, fluorescent Ca2+ indicator Fluo-4 NW and laser scanning confocal microscope (LSCM) were used to detect the real-time changes of [Ca2+] oscillation patterns in the cultured myocytes before and after aconitine (1.0 micromol/L) incubation or antiarrhythmic peptide (AAP) and aconitine co-incubation. The results showed under control conditions, [Ca2+] oscillations were irregular but relatively stable, occasionally accompanied by small calcium sparks. After incubation of the cultures with aconitine, high frequency [Ca2+] oscillations emerged in both nuclear and cytoplasmic regions, whereas typical calcium sparks disappeared and the average [Ca2+] in the cytoplasm of the cardiomyocyte did not change significantly. In AAP-treated cultures, intracellular [Ca2+] oscillation also changed, with periodic frequency, increased amplitudes and prolonged duration of calcium sparks. These patterns were not altered significantly by subsequent aconitine incubation. The basal value of [Ca2+] in nuclear region was higher than that in the cytoplasmic region. In the presence or absence of drugs, the [Ca2+] oscillated synchronously in both the nuclear and cytoplasmic regions of the same cardiomyocyte. It was concluded that although oscillating strenuously at high frequency, the average [Ca2+] in the cytoplasm of cardiomyocyte did not change significantly after aconitine incubation, compared to the controls. The observations indicate that aconitine induces the changes in [Ca2+] oscillation frequency other than the Ca2+ overload.

Characterization of aconitine-induced block of delayed rectifier K+ current in differentiated NG108-15 neuronal cells

The effects of aconitine (ACO), a highly toxic alkaloid, on ion currents in differentiated NG108-15 neuronal cells were investigated in this study. ACO (0.3-30 microM) suppressed the amplitude of delayed rectifier K+ current (I K(DR)) in a concentration-dependent manner with an IC50 value of 3.1 microM. The presence of ACO enhanced the rate and extent of I K(DR) inactivation, although it had no effect on the initial activation phase of I K(DR). It could shift the inactivation curve of I K(DR) to a hyperpolarized potential with no change in the slope factor. Cumulative inactivation for I K(DR) was also enhanced by ACO. Orphenadrine (30 microM) or methyllycaconitine (30 microM) slightly suppressed I K(DR) without modifying current decay. ACO (10 microM) had an inhibitory effect on voltage-dependent Na+ current (I Na). Under current-clamp recordings, ACO increased the firing and widening of action potentials in these cells. With the aid of the minimal binding scheme, the ACO actions on I K(DR) was quantitatively provided with a dissociation constant of 0.6 microM. A modeled cell was designed to duplicate its inhibitory effect on spontaneous pacemaking. ACO also blocked I K(DR) in neuroblastoma SH-SY5Y cells. Taken together, the experimental data and simulations show that ACO can block delayed rectifier K+ channels of neurons in a concentration- and state-dependent manner. Changes in action potentials induced by ACO in neurons in vivo can be explained mainly by its blocking actions on I K(DR) and I Na.

Aconitine alters connexin43 phosphorylation status and [Ca2+] oscillation patterns in cultured ventricular myocytes of neonatal rats

Aconitine, a highly poisonous type of alkaloid, has a widespread effect in stimulating the membranes of cardiomyocyte. However, other effects of aconitine on cardiomyocyte are unknown. In this study, we investigated whether aconitine also affects the phosphorylation status of connexin43 (Cx43) and intracellular [Ca(2+)] oscillation patterns in cultured ventricular myocytes of neonatal rats. As determined by Western blot analysis, a decreased percentage (47.68+/-2.29%) of phosphorylated Cx43 (P-Cx43) and a concomitant increased percentage (52.32+/-2.29%) of nonphosphorylated Cx43 (NP-Cx43) were found in aconitine-treated cultures, compared to the controls (82.77+/-2.04% for P-Cx43 and 17.23+/-2.04% for NP-Cx43). Quantitative immunofluorescent microscopy revealed similar changes in phosphorylation status occurring in Cx43 containing gap junctions in the cultures under the same treatment conditions. Real-time laser scanning microscopy indicated that intracellular [Ca(2+)] oscillations were relatively stable in control cultures, with occasional calcium sparks; after being treated with aconitine, high frequency [Ca(2+)] oscillations emerged, whereas typical calcium sparks disappeared. Furthermore, Western blot analysis revealed that, after aconitine treatment, the amount of phosphorylated PKCalpha decreased significantly. These observations suggest that aconitine not only induces dephosphorylation of Cx43 and PKCalpha, but also alters intracellular [Ca(2+)] oscillation patterns in cultured cardiomyocytes.

The susceptibility of ventricular arrhythmia to aconitine in conscious Lyon hypertensive rats

AIM

The present work was designed to investigate the relationship between hemodynamic parameters and the susceptibility of ventricular arrhythmia to aconitine in conscious Lyon hypertensive rats (LH).

METHODS

Male LH and Lyon low blood pressure rats (LL) were used. After the determination of baroreflex sensitivity (BRS), ventricular arrhythmia was induced by aconitine infusion in conscious rats. Blood pressure (BP) was recorded during the period of infusion.

RESULTS

Compared with the LL rats, the LH rats possessed significantly higher BP, blood pressure variability and lower BRS. The threshold of aconitine required for ventricular fibrillation and cardiac arrest in the LH rats were significantly lower than those in the LL rats. It was found that all the hemodynamic parameters studied were not correlated with the threshold of aconitine required for arrhythmia, with the exception of BRS, which was positively related to the threshold of aconitine required for ventricular premature beat.

CONCLUSION

The LH rats possessed greater susceptibility to aconitine-induced ventricular arrhythmias when compared to the LL rats. This greater susceptibility could not be attributed to any one of the hemodynamic parameters alone studied in the LH rats. It is proposed that various hypertension-associated abnormalities, including the abnormal hemodynamics, may co-contribute to this vulnerability to ventricular arrhythmias.

Disruption of the intracellular Ca2+ homeostasis in the cardiac excitation-contraction coupling is a crucial mechanism of arrhythmic toxicity in aconitine-induced cardiomyocytes

Aconitine is an effective ingredient in Aconite tuber, an important traditional Chinese medicine. Aconitine is also known to be a highly toxic diterpenoid alkaloid with arrhythmogenic effects. In the present study, we have characterized the properties of arrhythmic cytotoxicity and explored the possible mechanisms of aconitine-induced cardiomyocytes. Results show that aconitine induces significant abnormity in the spontaneous beating rate, amplitude of spontaneous oscillations and the relative intracellular Ca(2+) concentration. Also, mRNA transcription levels and protein expressions of SR Ca(2+) release channel RyR(2) and sarcolemmal NCX were elevated in aconitine-induced cardiomyocytes. However, co-treatment with ruthenium red (RR), a RyR channel inhibitor, could reverse the aconitine-induced abnormity in intracellular Ca(2+) signals. These results demonstrate that disruption of intracellular Ca(2+) homeostasis in the cardiac excitation-contraction coupling (EC coupling) is a crucial mechanism of arrhythmic cytotoxicity in aconitine-induced cardiomyocytes. Moreover, certain inhibitors appear to play an important role in the detoxification of aconitine-induced Ca(2+)-dependent arrhythmias.

Involvement of the Na+/Ca2+ Exchanger in Ouabain-Induced Inotropy and Arrhythmogenesis in Guinea-Pig Myocardium as Revealed by SEA0400

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.

Na+/Ca2+ exchanger inhibitors modify the accumulation of tumor-diagnostic PET tracers in cancer cells

AIM

To establish the effects of Na(+)/Ca(2+) exchanger (NCX) blockers on 2-[(18)F]fluoro-2-deoxy-D-glucose ((18)FDG) and (11)C-choline accumulation in different cancer cells.

METHODS

The tumor cells were incubated with NCX inhibitors, and the uptakes of (18)FDG and (11)C-choline were measured. Flow cytometric measurements of intracellular Ca(2+) and Na(+) concentrations were carried out. The presence of the NCX antigen in the cancer cells was proved by Western blotting, flow cytometry and confocal laser scanning microscopy.

RESULTS

The NCX is expressed at a noteworthy level in the cytosol and on the cytoplasmic membrane of the examined cells. Incubation of the cells with three chemically unrelated NCX blockers (bepridil, KB-R7943 or 3',4'-dichlorobenzamil hydrochloride) resulted in an increase in the intracellular Ca(2+) concentration, with a simultaneous decrease in the intracellular Na(+) concentration. The treatment with the NCX inhibitors increased the energy consumption of the tumor cells by 50-100%. Thapsigargin abolished the NCX-induced (18)FDG accumulation in the cells. The NCX blockers applied decreased the (11)C-choline accumulation of all the investigated cancer cells by 60-80% relative to the control.

CONCLUSION

A possible masking effect of NCX medication must be taken into consideration during the diagnostic interpretation of PET scans.

SEA0400: A Novel Sodium-Calcium Exchange Inhibitor with Cardioprotective Properties1

The cardiac sodium-calcium exchanger (NCX) plays an important role in calcium homeostasis. It is the primary mechanism for removing calcium ions that enter myocytes through L-type calcium channels on a beat-to-beat basis. Its direction of transport is determined by the membrane potential and the ionic concentrations of Na+ and Ca2+, with the forward (or Ca2+-efflux) mode of transport being the dominant mode under physiological conditions. In contrast, the Ca2+-influx mode (or reverse mode) of NCX becomes important in certain pathophysiological conditions, such as myocardial ischemia and reperfusion. Recent discovery of compounds that inhibit the Ca2+-influx mode (or reverse mode) of NCX has generated intense research interest in the pharmacology of NCX. Among the newer NCX inhibitors described to date, 2-[4-[(2,5-difluorophenyl)methoxy]-phenoxy]-5-ethoxyaniline (SEA0400) appears particularly promising in attenuating cardiac, renal, and cerebral ischemia/reperfusion injuries in various experimental models. Moreover, the mixed results that have emerged from clinical trials evaluating the efficacy and safety of inhibitors of the sodium-hydrogen exchanger (an upstream target in relation to the sodium-calcium exchanger) in myocardial protection stimulated interest in evaluating NCX as an alternative therapeutic target. This article reviews the pharmacological profile of SEA0400, as presented in the published literature, and discusses the therapeutic potential of this compound in attenuating myocardial ischemia/reperfusion injury.

Role of the Na+/Ca2+ exchanger in calcium homeostasis and human sperm motility regulation

A number of cell functions, such as flagellar beating, swimming velocity, acrosome reaction, etc., are triggered by a Ca2+ influx across the cell membrane. For appropriate physiological functions, the motile human sperm maintains the intracellular free calcium concentration ([Ca2+]i) at a submicromolar level. The objective of this study was to determine the role of the Na+/Ca2+ exchanger (NCX) in the maintenance of [Ca2+]i in human spermatozoa. Spermatozoa maintained in extracellular medium containing>or=1 microM Ca2+ exhibited motility similar to that of the control. In addition to several calcium transport mechanisms described earlier, we provide evidence that the NCX plays a crucial role in the maintenance of [Ca2+]i. Three chemically unrelated inhibitors of the NCX (bepridil, DCB (3',4'-dichlorobenzamil hydrochloride), and KB-R7943) all blocked human sperm motility in a dose and incubation time dependent manner. The IC50 values for bepridil, DCB, and KB-R7943 were 16.2, 9.8, and 5.3 microM, respectively. The treatment with the above-mentioned blockers resulted in an elevated [Ca2+]i and a decreased [Na+]i. The store-operated calcium channel (SOCC) inhibitor SKF 96365 also blocked the sperm motility (IC50=2.44 microM). The presence of the NCX antigen in the human spermatozoa was proven by flow cytometry, confocal laser scanning microscopy, and immunoblotting techniques. Calcium homeostasis of human spermatozoa is maintained by several transport proteins among which the SOCC and the NCX may play a major role.

Sodium calcium exchange as a target for antiarrhythmic therapy

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 on the Na+/Ca2+ Exchanger: Involvement of Na+/Ca2+ Exchange System in Cardiac Triggered Activity

Sodium-calcium exchange (NCX) is one of the major regulators of intracellular Ca(2+) concentration in cardiac myocytes. The bi-directional and electrogenic property of NCX raises a question about whether NCX is involved in arrhythmias. We reviewed the role of NCX in cardiac triggered activity in limited experimental conditions: the digitalis-induced arrhythmia, the arrhythmia caused from sustained opening of sodium channel, and the arrhythmia caused from the inhibition of inwardly rectifying potassium current. Effects of NCX inhibitors on ventricular arrhythmias recorded on ECG or the delayed afterdepolarizations and triggered activity recorded by the current clamp method were evaluated. As an NCX inhibitor, we preferred to use SEA0400 instead of KB-R 7943. For a precise analysis, a computational reconstruction of action potential with the Luo and Rudy model was applied. The cardiac NCX system seems to play a role only in the digitalis-induced arrhythmia and may not be involved in other arrhythmias. This review highlights the relationship between triggered activity and an NCX system and also suggests the physiologic and pathologic aspect of the NCX system in cardiac arrhythmias.

Effect of Cl−channel blockers on aconitine-induced arrhythmias in rat heart

The effects of Cl- channel blockers 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB) and niflumic acid (NFA) on aconitine-induced arrhythmias were investigated. Left ventricular pressure and electrocardiogram were monitored in Langendorff-perfused rat hearts. Whole-cell patch-clamp and current-clamp techniques were used to measure sodium current (I(Na)) and action potential (AP), respectively, in single rat cardiac ventricular myocytes. Addition of the Na+ channel agonist aconitine (0.1 microM) to the perfusion solution produced polymorphic ventricular arrhythmias with a latent period of 25.5 +/- 6.3 s. NPPB could reverse aconitine-induced arrhythmias. A similar effect was observed by using NFA. NPPB and NFA reversibly depressed the upstroke of the AP in a dose-dependent manner with IC50 values of approximately 12.3 and approximately 73.1 microM, respectively, without significantly affecting the resting potential of rat ventricular myocytes. Both Cl- channel blockers inhibited I(Na) and induced a leftward shift of the steady-state inactivation of I(Na). In conclusion, the results of this study demonstrate that NPPB as well as NFA can suppress aconitine-induced arrhythmias in rat hearts mainly by inhibiting cardiac I(Na).

Forefront of Na+/Ca2+ Exchanger Studies: Molecular Pharmacology of Na+/Ca2+ Exchange Inhibitors

The Na+/Ca2+ exchanger (NCX) is an ion transporter that exchanges Na+ and Ca2+ in either Ca2+ efflux or Ca2+ influx mode, depending on membrane potential and transmembrane ion gradients. In myocytes, neurons, and nephron cells, NCX is thought to play an important role in the regulation of intracellular Ca2+ concentration. Recently, the benzyloxyphenyl derivatives KB-R7943, SEA0400, and SN-6 have been developed as selective NCX inhibitors. Currently, SEA0400 is the most potent and selective inhibitor. These inhibitors possess different isoform-selectivities, although they have similar properties, such as Ca2+ influx mode-selectivity and I1 inactivation-dependence. Recent site-directed mutagenesis has revealed that these inhibitors possess some molecular determinants (Phe-213, Val-227, Tyr-228, Gly-833, and Asn-839) for interaction with NCX1. These benzyloxyphenyl derivatives are expected to be useful tools to study the physiological roles of NCX. Moreover, such inhibitors may have therapeutic potential as a new remedy for ischemic disease, arrhythmias, heart failure, and hypertension.