Ouabain-induced Arrhythmias Research Articles

Eugenol delays the onset of ouabain-induced ventricular cardiac arrhythmias in guinea pigs.

Eugenol is an aromatic compound used in the manufacture of medicines, perfumes, cosmetics and as an anaesthetic due to the ability of the drug to block the neuronal isoform of voltage-gated Na+ channels (NaV ). Some arrhythmias are associated with gain of function in the sodium current (INa ) found in cardiomyocytes, and antiarrhythmic sodium channel blockers are commonly used in the clinical practice. This study sought to elucidate the potential mechanisms of eugenol's protection in the arrhythmic model of ouabain-induced arrhythmias in guinea pig heart. Ex vivo arrhythmias were induced using 50 μM of ouabain. The antiarrhythmic properties of eugenol were evaluated in the ex vivo heart preparation and isolated ventricular cardiomyocytes. The compound's effects on cardiac sodium current and action potential using the patch-clamp technique were evaluated. In all, eugenol decreased the ex vivo cardiac arrhythmias induced by ouabain. Furthermore, eugenol showed concentration dependent effect upon peak INa , left-shifted the stationary inactivation curve and delayed the recovery from inactivation of the INa . All these aspects are considered to be antiarrhythmic. Our findings demonstrate that eugenol has antiarrhythmic activity, which may be partially explained by the ability of eugenol to change de biophysical properties of INa of cardiomyocytes.

Sophoridine manifests as a leading compound for anti-arrhythmia with multiple ion-channel blocking effects

BackgroundSophoridine (SR) has shown the potential to be an antiarrhythmic agent. However, SR's electrophysiological properties and druggability research are relatively inadequate, which limits the development of SR as an antiarrhythmic candidate. PurposeTo facilitate the development process of SR as an antiarrhythmic candidate, we performed integrated studies on the electrophysiological properties of SR in vitro and ex vivo to gain more comprehensive insights into the multi-ion channel blocking effects of SR, which provided the foundation for the further drugability studies in antiarrhythmic and safety studies. Firstly, SR's electrophysiological properties and antiarrhythmic potentials were recorded and assessed at the cell and tissue levels by comprehensively integrating the patch clamp with the Electrical and Optical Mapping systems. Subsequently, the antiarrhythmic effects of SR were validated by aconitine and ouabain-induced arrhythmia in vivo. Finally, the safety of SR as an antiarrhythmic candidate compound was evaluated based on the guidelines of the Comprehensive in Vitro Proarrhythmia Assay (CiPA). Study designThe antiarrhythmic effect of SR was evaluated at the in vitro, ex vivo, and in vivo levels. MethodsIsolated primary cardiomyocytes and stable cell lines were prepared to explore the electrophysiologic properties of being a multiple ion-channel blocker in vitro by whole-cell patch clamp. Using electrical and optical mapping, the negative chronotropic effect of SR was determined in langendorff-perfused rat or guinea-pig hearts.The antiarrhythmic activity of SR was assessed by the ex vivo tachyarrhythmia models induced by left coronary artery ligation (LCAL) and isoproterenol (ISO). Canonical models of aconitine and ouabain-induced arrhythmia were used to verify the antiarrhythmic effects in vivo. Finally, the pro-arrhythmic risk of SR was detected in Human-Induced Pluripotent Stem Cell-Derived Cardiomyocytes (hSCCMs) using a Microelectrode array (MEA). ResultsSingle-cell patch assay validated the multiple ion-channel blockers of SR in transient outward current potassium currents (Ito), l-type calcium currents (ICa-l), and rapid activation delayed rectifier potassium currents (IKr). SR ex vivo depressed heart rates (HR) and ventricular conduction velocity (CV) and prolonged Q-T intervals in a concentration-dependent manner. Consistent with the changes in HRs, SR extended the active time of hearts and increased the action potential duration measured at 90% repolarization (APD90). SR could also significantly lengthen the onset time and curtail the duration of spontaneous ventricular tachycardia (VT) in the ex vivo arrhythmic model induced by LCAL. Meanwhile, SR could also significantly upregulate the programmed electrical stimulation (PES) frequency after the ISO challenge in forming electrical alternans and re-entrant excitation. Furthermore, SR exerted antiarrhythmic effects in the tachyarrhythmia models induced by aconitine and ouabain in vivo. Notably, the pro-arrhythmic risk of SR was shallow for a moderate inhibition of the human ether-à-go-go-related gene (hERG) channel. Moreover, SR prolonged field potential duration (FPDc) of hSCCMs in a concentration-dependent manner without early after depolarization (EAD) and arrhythmia occurrence. ConclusionOur results indicated that SR manifested as a multiple ion-channel blocker in the electrophysiological properties and exerts antiarrhythmic effects ex vivo and in vivo. Meanwhile, due to the low pro-arrhythmic risk in the hERG inhibition assay and the induction of EAD, SR has great potential as a leading candidate in the treatment of ventricular tachyarrhythmia.

The Effects of Lipopolysaccharide Derivatives in Rodent Models of Cardiac Arrhythmia.

Several previous studies have suggested that sublethal doses of Escherichia coli lipopolysaccharide (endotoxin) and monophosphoryl lipid A Re595, a nonpyrogenic derivative of Salmonella minnesota lipopolysaccharide, exhibit antiarrhythmic effects in the rat model of ischemia-reperfusion arrhythmias. In this study, the protective effect of lipopolysaccharide derivatives was also further investigated in drug (aconitine or ouabain)-induced arrhythmia models, and conclusions were drawn with particular emphasis on the molecular characteristics of different types of lipopolysaccharide. The importance of the molecular structure for the antiarrhythmic effect of monophosphoryl lipid A and E. coli lipopolysaccharide was tested in the ischemia-reperfusion arrhythmia model. In contrast to monophosphoryl lipid A from Salmonella typhimurium SL 684 which has only monophosphoryl residue in its structure, monophosphoryl lipid A Re595, obtained from S. minnesota, and E. coli lipopolysaccharide which have both mono and diphosphoryl residue reduced the duration of ventricular tachycardia (e.g., during reperfusion: vehicle: 176 ± 22.8; monophosphoryl lipid A Re595: 132.83 ± 12.1, as second, n=8-10, P < .05) and the incidence of ventricular fibrillation. The antiarrhythmic effects of E. coli lipopolysaccharide and monophosphoryl lipid A Re595 in ischemia-reperfusion arrhythmia model were absent in either aconitine- (e.g., onset time for ventricular ectopic beats: saline 25.3 5.0, E. coli lipopolysaccharide 24.3 ± 7.1; vehicle: 24.0 ± 4.5, monophosphoryl lipid A SL684 23.8 ± 4.3, as second, n=6, P > .05) or ouabain-induced arrhythmia models in mice. Therefore, we conclude that lipopolysaccharide derivatives exhibit antiarrhythmic effect only in ischemia-reperfusion arrhythmias, and lipopolysaccharide should possess diphosphoryl groups in its subcomponent composition for this antiarrhythmic effect.

Effect of Lamotrigine on Ouabain-Induced Arrhythmia in Isolated Atria of Guinea Pigs.

Background: Lamotrigine (LTG) is an antiepileptic drug used in the treatment of seizures, mood disorders, and cognitive problems. The cardiac effects of LTG, such as LTG toxicity and SUEDP, have been studied. This is an in vitro study examining the effect of LTG on isolated atria of guinea pigs. Methods: The atria of 21 male Guinea pigs were isolated and stabilized in Krebs-solution and physiologic condition. The rhythm of contraction, contractile force, and heart rate were recorded. In 7 atria, LTG at the doses of 2, 4, 8, and 16 mg/mL were added and the contractile forces and heart rates were recorded and compared together. In the next step, in 14 atria, 8 were pretreated with LTG, and 6 without pretreatment were exposed to ouabain, and the times of the onset of effect, arrhythmia, and asystole were recorded. The statistical comparisons were made by using Student’s t test and repeated measure analysis of variance followed by the Bonferroni method. Results: Lamotrigine (4, 8, and 16 mg/mL) significantly decreased the heart rate and contractile force of the isolated guinea pigs’ atria (P <.001). Pretreatment with LTG significantly increased the mean time of onset of the effect of ouabain, the onset of ouabain-induced arrhythmia, and time of ouabain-induced asystole (P <.001). Conclusion: LTG reduces the heart rate and contractile force, and also inhibit ouabain induced-arrhythmia of the isolated atria of guinea pigs.

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 (INa) and L-type calcium current (ICa,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 INa and ICa,L. These results revealed that TMCC inhibited aconitine-induced arrhythmias through effects on INa and ICa,L.

On the benefit of melatonin in protection against ouabain-induced arrhythmia through modulation of oxidative stress factors in isolated rat atria

ABSTRACTConsidering the cardioprotective and antioxidant properties of melatonin, in the present experiment, we investigated the possible involvement of oxidative stress factors in antiarrhythmic effects of melatonin in ouabain-induced arrhythmia in isolated rat atria. Male rats were divided into two groups, receiving either of melatonin (2 mg/kg) or vehicle, orally once daily for three weeks. Rats were anesthetized, and atria were isolated and incubated with ouabain in an organ bath. Time of onset of arrhythmia and asystole as well as atrial beating rate and contractile force were recorded. We also measured the activity of superoxide dismutase (SOD) and levels of thiobarbituric acid reactive substances (TBARS) in atria after injection of ouabain to animals. Pretreatment of animals with melatonin could significantly postpone the onset of arrhythmia and asystole compared with vehicle-treated group (P ≤ 0.001). Incubation of ouabain boosted the atrial beating rate in vehicle-treated group (P ≤ 0.01), while this response in melatonin-treated group was not significant (P > 0.05). Injection of ouabain decreased the activity of SOD and increased the levels of TBARS in atria (P ≤ 0.001, P ≤ 0.01, respectively), while pretreatment of animals with melatonin reversed these effects (P ≤ 0.05). It is concluded that melatonin possesses antiarrhythmic properties, and oxidative stress factors might mediate this response.

Involvement of IL-1β and IL-6 in antiarrhythmic properties of atorvastatin in ouabain-induced arrhythmia in rats

Purpose: Evidence show that statins possess wide beneficial cardioprotective and anti-inflammatory effects; therefore, in the present experiment, we investigated the antiarrhythmic properties of atorvastatin in ouabain-induced arrhythmia in isolated rat atria and the role of several inflammatory cytokines in this effect.Materials and methods: Male rats were pretreated with either of atorvastatin (10 mg/kg) or vehicle, orally once daily for 6 weeks. After induction of anesthesia, we isolated the atria and after incubation with ouabain, time of onset of arrhythmia and asystole as well as atrial beating rate and contractile force were recorded. We also measured the atrial levels of IL-1β, IL-6, and TNF-α after the injection of ouabain to animals.Results: Pretreatment with atorvastatin significantly delayed the onset of arrhythmia and asystole compared with vehicle-treated group (p < .01, p < .001, respectively). Incubation of ouabain boosted both atrial beating rate and contractile force in vehicle-treated group (p < .05), while these responses in atorvastatin-treated group were not significant (p > .05). Injection of ouabain elevated the atrial levels of IL-1β, IL-6, and TNF-α, while pretreatment of animals with atorvastatin could reverse the ouabain-induced increase in atrial IL-1β and IL-6 (p < .01 and p < .05, respectively).Conclusions: It is concluded that observed antiarrhythmic effects of atorvastatin might be attributed to modulation of some inflammatory cytokines, at least IL-1β and IL-6.

The potassium channel blocker, dalfampridine diminishes ouabain-induced arrhythmia in isolated rat atria

The aim of the present experiment was to investigate the possible antiarrhythmic effects of dalfampridine in ouabain-induced arrhythmia in rats. Twenty-four male rats including the control and dalfampridine-incubated (100 µM to 10 mM) ouabain-stimulated (40 µM) groups were used. After induction of anesthesia, the atria were isolated and the time of onset of arrhythmia and asystole were recorded. The contractile force of atria was also measured. Dalfampridine at concentration of 1 mM significantly postponed the onset of arrhythmia and asystole compared to control group (p ≤ .05). Ouabain significantly increased the atrial beating rate in control group (p ≤ .05), while pretreatment of isolated atria with dalfampridine reversed this effect. Incubation of isolated atria with ouabain did not alter the contractile force in both control- and dalfampridine-treated groups (p > .05). It is concluded that dalfampridine might possess antiarrhythmic properties in reducing the atrial arrhythmias.

On the benefit of melatonin in protection against ouabain-induced arrhythmia through modulation of oxidative stress factors in isolated rat atria

On the benefit of melatonin in protection against ouabain-induced arrhythmia through modulation of oxidative stress factors in isolated rat atria

D-Ala, D-Leu Enkephalin Delays Ouabain-Induced Arrhythmia In Isolated Rat Atria

D-Ala, D-Leu Enkephalin Delays Ouabain-Induced Arrhythmia In Isolated Rat Atria

The investigation of the cellular electrophysiological and antiarrhythmic effects of a novel selective sodium-calcium exchanger inhibitor, GYKB-6635, in canine and guinea-pig hearts.

The sodium-calcium exchanger (NCX) is considered as the major transmembrane transport mechanism that controls Ca2+ homeostasis. Its contribution to the cardiac repolarization has not yet been directly studied due to lack of specific inhibitors, so that an urgent need for more selective compounds. In this study, the electrophysiological effects of GYKB-6635, a novel NCX inhibitor, on the NCX, L-type calcium, and main repolarizing potassium currents as well as action potential (AP) parameters were investigated. Ion currents and AP recordings were investigated by applying the whole-cell patch clamp and standard microelectrode techniques in canine heart at 37 °C. Effects of GYKB-6635 were studied in ouabain-induced arrhythmias in isolated guinea-pig hearts. At a concentration of 1 μmol/L, GYKB significantly reduced both the inward and outward NCX currents (57% and 58%, respectively). Even at a high concentration (10 μmol/L), GYKB-6635 did not change the ICaL, the maximum rate of depolarization (dV/dtmax), the main repolarizing K+ currents, and the main AP parameters. GYKB-6635 pre-treatment significantly delayed the time to the development of ventricular fibrillation (by about 18%). It is concluded that GYKB-6635 is a potent and highly selective inhibitor of the cardiac NCX and, in addition, it is suggested to also contribute to the prevention of DAD-based arrhythmias.

Involvement of Inflammatory Cytokines in Antiarrhythmic Effects of Clofibrate in Ouabain-Induced Arrhythmia in Isolated Rat Atria.

Considering the cardioprotective and anti-inflammatory properties of clofibrate, the aim of the present experiment was to investigate the involvement of local and systemic inflammatory cytokines in possible antiarrhythmic effects of clofibrate in ouabain-induced arrhythmia in rats. Rats were orally treated with clofibrate (300 mg/kg), and ouabain (0.56 mg/kg) was administered to animals intraperitoneally. After induction of anesthesia, the atria were isolated and the onset of arrhythmia and asystole was recorded. The levels of inflammatory cytokines in atria were also measured. Clofibrate significantly postponed the onset of arrhythmia and asystole when compared to control group (P ≤ 0.05 and P ≤ 0.01, resp.). While ouabain significantly increased the atrial beating rate in control group (P ≤ 0.05), same treatment did not show similar effect in clofibrate-treated group (P > 0.05). Injection of ouabain significantly increased the atrial and systemic levels of all studied inflammatory cytokines (P ≤ 0.05). Pretreatment with clofibrate could attenuate the ouabain-induced elevation of IL-6 and TNF-α in atria (P ≤ 0.01 and P ≤ 0.05, resp.), as well as ouabain-induced increase in IL-6 in plasma (P ≤ 0.05). Based on our findings, clofibrate may possess antiarrhythmic properties through mitigating the local and systemic inflammatory factors including IL-6 and TNF-α.

Geraniol blocks calcium and potassium channels in the mammalian myocardium: useful effects to treat arrhythmias.

Geraniol is a monoterpene present in several essential oils, and it is known to have a plethora of pharmacological activities. In this study, we explored the contractile and electrophysiological properties of geraniol and its antiarrhythmic effects in the heart. The geraniol effects on atrial contractility, L-type Ca(2+) current, K(+) currents, action potential (AP) parameters, ECG profile and on the arrhythmia induced by ouabain were evaluated. In the atrium, geraniol reduced the contractile force (~98%, EC=1,510±160μM) and diminished the positive inotropism of CaCl2 and BAY K8644. In cardiomyocytes, the IC a,L was reduced by 50.7% (n=5) after perfusion with 300μM geraniol. Moreover, geraniol prolonged the AP duration (APD) measured at 50% (n=5) after repolarization, without changing the resting potential. The increased APD could be attributed to the blockade of the transient outward K(+) current (Ito ) (59.7%, n=4), the non-inactivation K(+) current (Iss ) (39.2%, n=4) and the inward rectifier K(+) current (IK 1 ) (33.7%, n=4). In isolated hearts, geraniol increased PRi and QTi without affecting the QRS complex (n=6), and it reduced both the left ventricular pressure (83%) and heart rate (16.5%). Geraniol delayed the time to onset of ouabain-induced arrhythmias by 128%, preventing 30% of the increase in resting tension (n=6). Geraniol exerts its negative inotropic and chronotropic responses in the heart by decreasing both L-type Ca(2+) and voltage-gated K(+) currents, ultimately acting against ouabain-induced arrhythmias.

PP103—Effects of clofibrate on ouabain-induced arrhythmia in isolated rat atria

Cardiac arrhythmia is 1 of the critical health conditions. Clofibrate is a peroxisome proliferated-activator receptor-α (PPAR-α) agonist that is widely used for reducing triglycerides in hyperlipidemic patients. Because this drug has showed several beneficial effects in relief of some cardiovascular diseases, the aim of the present study was to evaluate antiarrhythmic effects of clofibrate on ouabain-induced arrhythmia in rats.

Pleiotropic actions of amiodarone: still puzzling after half a century

Amiodarone was developed in the early 1960s based on extracts from the Khella plant and was initially used as a coronary dilator for the treatment of angina pectoris (Singh 2008). In 1970, Singh and Vaughan Williams discovered that amiodarone also directly affected myocardial tissue, prolonging the cardiac action potential and protecting against ouabain-induced arrhythmias in guinea pigs (Singh and Vaughan Williams 1970). To date, amiodarone remains one of the most commonly prescribed antiarrhythmic drugs in clinicalpracticeandisusedinthetreatmentofbothventricular

Discovery of N-(3,5-bis(1-pyrrolidylmethyl)-4-hydroxybenzyl)-4-methoxybenzenesulfamide (sulcardine) as a novel anti-arrhythmic agent

To investigate the anti-arrhythmic effects of sulfamide analogues of changrolin and to characterize the sulfate of compound 6f (sulcardine sulfate, Sul) as a novel anti-arrhythmic agent. The anti-arrhythmic effects of compounds were studied against aconitine-induced arrhythmias in rats and ouabain-induced arrhythmias in guinea pigs. The effects of Sul on transmembrane action potentials were investigated in isolated rabbit sinoatrial nodes and guinea-pig papillary muscles using intracellular recording. With a whole-cell recording technique, the effects of Sul on sodium current, calcium current, and potassium currents were examined in isolated single guinea-pig ventricular myocytes. In aconitine-induced arrhythmias of rats, sulfamide analogues of changrolin (4, 5, and 6a-6p) exhibited various anti-arrhythmic activities. The sulfate of compound 6f (Sul) increased the amount of aconitine required to induce arrhythmias in each treated animal. The ED₅₀ value of Sul in rats was 196 mg/kg. In ouabain-induced arrhythmias of guinea pigs, 25, 50, and 100 mg/kg doses of Sul increased the dose of ouabain required to induce VP, VT, and VF in a dose-dependent manner. In papillary preparations, Sul produced a concentration-dependent decrease in APA and V(max), prolonged APD(90) and ERP, whereas RP was unaffected. In the spontaneously beating sinus nodes, Sul reduced APA and V(max) in a concentration-dependent manner. The whole-cell recording studies revealed that Sul produced a reversible reduction in I(Na) (IC₅₀=26.9 μmol/L) and I(Ca,L)(IC₅₀=69.2 μmol/L), whereas the inward rectifier (I(K1)) and the delayed rectifier potassium currents (I(K)) were unaffected. As a multi-ion channel blocker, Sul may have potent efficacy in anti-atrial and ventricular arrhythmias.

Effects of Taurine–Magnesium Coordination Compound on Ionic Channels in Rat Ventricular Myocytes of Arrhythmia Induced by Ouabain

Taurine-magnesium coordination compound (TMCC) has anti-arrhythmic effects. The aim of the present study was to explore the targets of the anti-arrhythmic effect of TMCC and the electrophysiological effects of TMCC on ouabain-induced arrhythmias in rat ventricular myocytes. Sodium current (I(Na)), L-type calcium current (I(ca, L)), and transient outward potassium current (I(to)) were measured and analyzed using whole-cell patch-clamp recording technique in normal rat cardiac myocytes and rat ventricular myocytes of arrhythmia induced by ouabain. In isolated ventricular myocytes, I(Na) and I(to) were blocked by TMCC (100, 200, 400 μM) in a concentration-dependent manner, and the effects of TMCC (400 μM) were equal to that of amiodarone. However, I (ca, L) was moderately increased by TMCC (400 μM) while significantly decreased by amiodarone. Ouabain (5 μM) significantly decreased sodium, L-type calcium, and transient outward potassium currents. TMCC (100 μM) relieved abnormal sodium currents induced by ouabain through facilitation of steady-state inactivation. TMCC (200 and 400 μM) relieved abnormal L-type calcium currents induced by ouabain through facilitation of steady-state activation and retardation of steady-state inactivation. TMCC failed to further inhibit abnormal transient outward potassium currents induced by ouabain. However, amiodarone inhibited the decreasing sodium, L-type calcium, and transient outward potassium currents further. These data suggest that I(Na), I(ca, L), and I(to) may be the targets of the antiarrhythmic effect of TMCC, which can antagonize ouabain-induced changes of ionic currents in rat ventricular myocytes.

Effects of SEA0400 on Ouabain-induced Arrhythmias in Guinea Pigs

The sodium-calcium exchange (NCX) is one of the major regulators of intracellular Ca2+ concentration in cardiac myocytes. The effects of NCX blockers as antiarrhythmic agents are still controversial. We investigated antiarrhythmic effects of SEA0400 (SEA), a novel NCX inhibitor, on ouabain-induced arrhythmias in guinea pigs. In the whole animal arrhythmia model, we observed effects of SEA on the ouabain-induced arrhythmia using ECG recordings. In the isolated myocyte, we observed action potential configurations and oscillations due to calcium overload using the current clamp method. In the whole animal model, SEA at a dose range of 1-10 mg/kg (intravenous, bolus) suppressed ouabain-induced arrhythmias dose-dependently. In isolated ventricular myocytes, SEA (0.1-3 µM) suppressed ouabain-induced oscillatory activity observed between action potentials. SEA (0.1-3 µM) also suppressed ouabain-induced NCX current (INCX) that is also called transient inward current (ITI). Our results indicate that NCX is involved in arrhythmia and oscillatory activity induced by ouabain. The inhibition of these arrhythmias and oscillatory activity by SEA might result from the inhibition of NCX.Keywords: Na+-Ca2+ exchange (NCX); SEA0400; NCX inhibitors; current clamp; ECG.© 2012 JSR Publications. ISSN: 2070-0237 (Print); 2070-0245 (Online). All rights reserved.doi: http://dx.doi.org/10.3329/jsr.v4i1.7722J. Sci. Res. 4 (1), 213-225 (2012)

Calcium-Calmodulin Kinase II Mediates Digitalis-Induced Arrhythmias

Background— Digitalis-induced Na + accumulation results in an increase in Ca 2+ i via the Na + /Ca 2+ exchanger, leading to enhanced sarcoplasmic reticulum (SR) Ca 2+ load, responsible for the positive inotropic and toxic arrhythmogenic effects of glycosides. A digitalis-induced increase in Ca 2+ i could also activate calcium-calmodulin kinase II (CaMKII), which has been shown to have proarrhythmic effects. Here, we investigate whether CaMKII underlies digitalis-induced arrhythmias and the subcellular mechanisms involved. Methods and Results— In paced rat ventricular myocytes (0.5 Hz), 50 μmol/L ouabain increased contraction amplitude by 160±5%. In the absence of electric stimulation, ouabain promoted spontaneous contractile activity and Ca 2+ waves. Ouabain activated CaMKII (p-CaMKII), which phosphorylated its downstream targets, phospholamban (PLN) (Thr17) and ryanodine receptor (RyR) (Ser2814). Ouabain-induced spontaneous activity was prevented by inhibiting CaMKII with 2.5 μmol/L KN93 but not by 2.5 μmol/L of the inactive analog, KN92. Similar results were obtained using the CaMKII inhibitor, autocamtide-2 related inhibitory peptide (AIP) (1 to 2.5 μmol/L), and in myocytes from transgenic mice expressing SR-targeted AIP. Consistently, CaMKII overexpression exacerbated ouabain-induced spontaneous contractile activity. Ouabain was associated with an increase in SR Ca 2+ content and Ca 2+ spark frequency, indicative of enhanced SR Ca 2+ leak. KN93 suppressed the ouabain-induced increase in Ca 2+ spark frequency without affecting SR Ca 2+ content. Similar results were obtained with digoxin. In vivo, ouabain-induced arrhythmias were prevented by KN93 and absent in SR-AIP mice. Conclusions— These results show for the first time that CaMKII mediates ouabain-induced arrhythmic/toxic effects. We suggest that CaMKII-dependent phosphorylation of the RyR, resulting in Ca 2+ leak from the SR, is the underlying mechanism involved.

Effect of sertraline on ouabain-induced arrhythmia in isolated guinea-pig atria

The effect of sertraline a selective serotonin reuptake inhibitor antidepressant was studied on ouabain-induced toxicity (arrhythmia) in spontaneously beating isolated guinea-pig atria. The guinea-pig atrium was dissected out and suspended in modified Krebs solution under physiological conditions. Drugs were added into solutions. The changes in rate and force of contractions were measured using a physiograph. Sertraline (2-16 microg/mL) caused a dose-dependent decrease in the rate of contractions (17-46%) and in the contractile force (26-48%). Ouabain alone (1.2 microg/mL) produced arrhythmia at 7.8 min and asystole at 22 min. Pre- administration of the atria with sertraline (8 microg/mL) significantly increased the time required to produce arrhythmia by ouabain to 20.5 min, prolonged the beating of atria to more than 64.5 min and delayed the occurrence of asystolia. The pattern of contractile force induced by sertraline + ouabain was more regular than that produced by ouabain alone. These findings indicate that sertraline produces direct cardiac action, probably due to the inhibition of cardiac Na(+) and Ca(2+) channels. Our results suggest that sertraline may reduce the membrane conduction through inhibition of ionic channels which decrease ouabain-induced arrhythmia.