Antiarrhythmic actions of the ATP-regulated K+ current activated by pinacidil

Distinct Electrogram Features and Ventricular Arrhythmia Induction Modes Between Repolarization and Conduction Heterogeneities

BACKGROUND

Recent clinical studies have indicated the presence of localized electrical abnormalities in idiopathic ventricular fibrillation and J-wave syndrome patients.

OBJECTIVES

This study aims to characterize the specific electrical signatures of localized repolarization and conduction heterogeneities and their respective role in vulnerability to arrhythmias.

METHODS

Optical mapping was performed in porcine right ventricles with local: 1) repolarization shortening; 2) conduction slowing; or 3) structural heterogeneity induced by locally perfusing: 1) pinacidil (20 μmol/L, n = 13); or 2) flecainide (2 μmol/L, n = 13) via an epicardial catheter; or 3) by local epicardial tissue destruction (9 radiofrequency lesions n = 12). Electrograms were recorded (n = 5 in each group) and spontaneous and induced arrhythmias were quantified and optically mapped.

RESULTS

Electrograms were normal in (1) but showed local fragmentation in 40% of preparations in (2) with greater effects observed at high pacing frequencies dependent on the wavefront direction. In (3), the structural substrate alone increased the width and number of peaks in the electrograms, and addition of flecainide induced pronounced fragmentation (≥3 peaks and ≥70 ms) in all cases. Occurrence of spontaneous arrhythmias was significantly increased in (1) and (2) (P < 0.0001 and 0.05, respectively, vs baseline) and were triggered by ectopies. Vulnerability to arrhythmias at high pacing frequencies (≥2 Hz) was the lowest in (1) and greatest in (2).

CONCLUSIONS

Microstructural substrates have the most pronounced impact on electrograms, especially when combined with sodium channel blockers, whereas local action potential duration shortening does not lead to electrogram fragmentation even though it is associated with the highest prevalence of spontaneous arrhythmias.

Hypoxia Promotes Atrial Tachyarrhythmias via Opening of ATP-Sensitive Potassium Channels

BACKGROUND

Hypoxia-ischemia predisposes to atrial arrhythmia. Atrial ATP-sensitive potassium channel (KATP) modulation during hypoxia has not been explored. We investigated the effects of hypoxia on atrial electrophysiology in mice with global deletion of KATP pore-forming subunits.

METHODS

Whole heart KATP RNA expression was probed. Whole-cell KATP current and action potentials were recorded in isolated wild-type (WT), Kir6.1 global knockout (6.1-gKO), and Kir6.2 global knockout (6.2-gKO) murine atrial myocytes. Langendorff-perfused hearts were assessed for atrial effective refractory period (ERP), conduction velocity, wavefront path length (WFPL), and arrhymogenicity under normoxia/hypoxia using a microelectrode array and programmed electrical stimulation. Heart histology was assessed.

RESULTS

Expression patterns were essentially identical for all KATP subunit RNA across human heart, whereas in mouse, Kir6.1 and SUR2 (sulphonylurea receptor subunit) were higher in ventricle than atrium, and Kir6.2 and SUR1 were higher in atrium. Compared with WT, 6.2-gKO atrial myocytes had reduced tolbutamide-sensitive current and action potentials were more depolarized with slower upstroke and reduced peak amplitude. Action potential duration was prolonged in 6.1-gKO atrial myocytes, absent of changes in other ion channel gene expression or atrial myocyte hypertrophy. In Langendorff-perfused hearts, baseline atrial ERP was prolonged and conduction velocity reduced in both KATP knockout mice compared with WT, without histological fibrosis. Compared with baseline, hypoxia led to conduction velocity slowing, stable ERP, and WFPL shortening in WT and 6.1-gKO hearts, whereas WFPL was stable in 6.2-gKO hearts due to ERP prolongation with conduction velocity slowing. Tolbutamide reversed hypoxia-induced WFPL shortening in WT and 6.1-gKO hearts through ERP prolongation. Atrial tachyarrhythmias inducible with programmed electrical stimulation during hypoxia in WT and 6.1-gKO mice correlated with WFPL shortening. Spontaneous arrhythmia was not seen.

CONCLUSIONS

KATP block/absence leads to cellular and tissue level atrial electrophysiological modification. Kir6.2 global knockout prevents hypoxia-induced atrial WFPL shortening and atrial arrhythmogenicity to programmed electrical stimulation. This mechanism could be explored translationally to treat ischemically driven atrial arrhythmia.

Cardiac transmembrane ion channels and action potentials: cellular physiology and arrhythmogenic behavior

Cardiac arrhythmias are among the leading causes of mortality. They often arise from alterations in the electrophysiological properties of cardiac cells and their underlying ionic mechanisms. It is therefore critical to further unravel the pathophysiology of the ionic basis of human cardiac electrophysiology in health and disease. In the first part of this review, current knowledge on the differences in ion channel expression and properties of the ionic processes that determine the morphology and properties of cardiac action potentials and calcium dynamics from cardiomyocytes in different regions of the heart are described. Then the cellular mechanisms promoting arrhythmias in congenital or acquired conditions of ion channel function (electrical remodeling) are discussed. The focus is on human-relevant findings obtained with clinical, experimental, and computational studies, given that interspecies differences make the extrapolation from animal experiments to human clinical settings difficult. Deepening the understanding of the diverse pathophysiology of human cellular electrophysiology will help in developing novel and effective antiarrhythmic strategies for specific subpopulations and disease conditions.

Ischemic Postconditioning Reduces Reperfusion Arrhythmias by Adenosine Receptors and Protein Kinase C Activation but Is Independent of KATP Channels or Connexin 43

Ischemic postconditioning (IPoC) reduces reperfusion arrhythmias but the antiarrhythmic mechanisms remain unknown. The aim of this study was to analyze IPoC electrophysiological effects and the role played by adenosine A₁, A_2A and A₃ receptors, protein kinase C, ATP-dependent potassium (K_ATP) channels, and connexin 43. IPoC reduced reperfusion arrhythmias (mainly sustained ventricular fibrillation) in isolated rat hearts, an effect associated with a transient delay in epicardial electrical activation, and with action potential shortening. Electrical impedance measurements and Lucifer-Yellow diffusion assays agreed with such activation delay. However, this delay persisted during IPoC in isolated mouse hearts in which connexin 43 was replaced by connexin 32 and in mice with conditional deletion of connexin 43. Adenosine A₁, A_2A and A₃ receptor blockade antagonized the antiarrhythmic effect of IPoC and the associated action potential shortening, whereas exogenous adenosine reduced reperfusion arrhythmias and shortened action potential duration. Protein kinase C inhibition by chelerythrine abolished the protective effect of IPoC but did not modify the effects on action potential duration. On the other hand, glibenclamide, a K_ATP inhibitor, antagonized the action potential shortening but did not interfere with the antiarrhythmic effect. The antiarrhythmic mechanisms of IPoC involve adenosine receptor activation and are associated with action potential shortening. However, this action potential shortening is not essential for protection, as it persisted during protein kinase C inhibition, a maneuver that abolished IPoC protection. Furthermore, glibenclamide induced the opposite effects. In addition, IPoC delays electrical activation and electrical impedance recovery during reperfusion, but these effects are independent of connexin 43.

Cardiac Na+/Ca2+ exchange stimulators among cardioprotective drugs

We previously reviewed our study of the pharmacological properties of cardiac Na+/Ca2+ exchange (NCX1) inhibitors among cardioprotective drugs, such as amiodarone, bepridil, dronedarone, cibenzoline, azimilide, aprindine, and benzyl-oxyphenyl derivatives (Watanabe et al. in J Pharmacol Sci 102:7-16, 2006). Since then we have continued our studies further and found that some cardioprotective drugs are NCX1 stimulators. Cardiac Na+/Ca2+ exchange current (INCX1) was stimulated by nicorandil (a hybrid ATP-sensitive K+ channel opener), pinacidil (a non-selective ATP-sensitive K+ channel opener), flecainide (an antiarrhythmic drug), and sodium nitroprusside (SNP) (an NO donor). Sildenafil (a phosphodiesterase-5 inhibitor) further increased the pinacidil-induced augmentation of INCX1. In paper, here I review the NCX stimulants that enhance NCX function among the cardioprotective agents we examined such as nicorandil, pinacidil, SNP, sildenafil and flecainide, in addition to atrial natriuretic (ANP) and dofetilide, which were reported by other investigators.

Pinacidil, a KATP channel opener, stimulates cardiac Na+/Ca2+ exchanger function through the NO/cGMP/PKG signaling pathway in guinea pig cardiac ventricular myocytes

Pinacidil, a nonselective ATP-sensitive K⁺ (KATP) channel opener, has cardioprotective effects for hypertension, ischemia/reperfusion injury, and arrhythmia. This agent abolishes early afterdepolarizations, delayed afterdepolarizations (DADs), and abnormal automaticity in canine cardiac ventricular myocytes. DADs are well known to be caused by the Na⁺/Ca²⁺ exchange current (I_NCX). In this study, we used the whole-cell patch-clamp technique and Fura-2/AM (Ca²⁺-indicator) method to investigate the effect of pinacidil on I_NCX in isolated guinea pig cardiac ventricular myocytes. In the patch-clamp study, pinacidil enhanced I_NCX in a concentration-dependent manner. The half-maximal effective concentration values were 23.5 and 23.0 μM for the Ca²⁺ entry (outward) and Ca²⁺ exit (inward) components of I_NCX, respectively. The pinacidil-induced I_NCX increase was blocked by L-NAME, a nitric oxide (NO) synthase inhibitor, by ODQ, a soluble guanylate cyclase inhibitor, and by KT5823, a cyclic guanosine monophosphate (cGMP)-dependent protein kinase (PKG) inhibitor, but not by N-2-mercaptopropyonyl glycine (MPG), a reactive oxygen species (ROS) scavenger. Glibenclamide, a nonselective KATP channel inhibitor, blocked the pinacidil-induced I_NCX increase, while 5-HD, a selective mitochondria KATP channel inhibitor, did not. In the Fura-2/AM study pinacidil also enhanced intracellular Ca²⁺ concentration, which was inhibited by L-NAME, ODQ, KT5823, and glibenclamide, but not by MPG and 5-HD. Sildenafil, a phosphodiesterase 5 inhibitor, increased further the pinacidil-induced I_NCX increase. Sodium nitroprusside, a NO donor, also increased I_NCX. In conclusion, pinacidil may stimulate cardiac Na⁺/Ca²⁺ exchanger (NCX1) by opening plasma membrane KATP channels and activating the NO/cGMP/PKG signaling pathway.

Rat Models of Ventricular Fibrillation Following Acute Myocardial Infarction

A number of animal models have been designed in order to unravel the underlying mechanisms of acute ischemia-induced arrhythmias and to test compounds and interventions for antiarrhythmic therapy. This is important as acute myocardial infarction (AMI) continues to be the major cause of sudden cardiac death, and we are yet to discover safe and effective treatments of the lethal arrhythmias occurring in the acute setting. Animal models therefore continue to be relevant for our understanding and treatment of acute ischemic arrhythmias. This review discusses the applicability of the rat as a model for ventricular arrhythmias occurring during the acute phase of AMI. It provides a description of models developed, advantages and disadvantages of rats, as well as an overview of the most important interventions investigated and the relevance for human pathophysiology.

KATP Channels in the Cardiovascular System

KATP channels are integral to the functions of many cells and tissues. The use of electrophysiological methods has allowed for a detailed characterization of KATP channels in terms of their biophysical properties, nucleotide sensitivities, and modification by pharmacological compounds. However, even though they were first described almost 25 years ago (Noma 1983, Trube and Hescheler 1984), the physiological and pathophysiological roles of these channels, and their regulation by complex biological systems, are only now emerging for many tissues. Even in tissues where their roles have been best defined, there are still many unanswered questions. This review aims to summarize the properties, molecular composition, and pharmacology of KATP channels in various cardiovascular components (atria, specialized conduction system, ventricles, smooth muscle, endothelium, and mitochondria). We will summarize the lessons learned from available genetic mouse models and address the known roles of KATP channels in cardiovascular pathologies and how genetic variation in KATP channel genes contribute to human disease.

Pro- and antiarrhythmic effects of ATP-sensitive potassium current activation on reentry during early afterdepolarization-mediated arrhythmias

BACKGROUND

Under conditions promoting early afterdepolarizations (EADs), ventricular tissue can become bi-excitable, that is, capable of wave propagation mediated by either the Na current (INa) or the L-type calcium current (ICa,L), raising the possibility that ICa,L-mediated reentry may contribute to polymorphic ventricular tachycardia (PVT) and torsades de pointes. ATP-sensitive K current (IKATP) activation suppresses EADs, but the effects on ICa,L-mediated reentry are unknown.

OBJECTIVE

To investigate the effects of IKATP activation on ICa,L-mediated reentry.

METHODS

We performed optical voltage mapping in cultured neonatal rat ventricular myocyte monolayers exposed to BayK8644 and isoproterenol. The effects of pharmacologically activating IKATP with pinacidil were analyzed.

RESULTS

In 13 monolayers with anatomic ICa,L-mediated reentry around a central obstacle, pinacidil (50 μM) converted ICa,L-mediated reentry to INa-mediated reentry. In 33 monolayers with functional ICa,L-mediated reentry (spiral waves), pinacidil terminated reentry in 17, converted reentry into more complex INa-mediated reentry resembling fibrillation in 12, and had no effect in 4. In simulated 2-dimensional bi-excitable tissue in which ICa,L- and INa-mediated wave fronts coexisted, slow IKATP activation (over minutes) reliably terminated rotors but rapid IKATP activation (over seconds) often converted ICa,L-mediated reentry to INa-mediated reentry resembling fibrillation.

CONCLUSIONS

IKATP activation can have proarrhythmic effects on EAD-mediated arrhythmias if ICa,L-mediated reentry is present.

Thiazolidinedione drugs promote onset, alter characteristics, and increase mortality of ischemic ventricular fibrillation in pigs

PURPOSE

Despite favorable metabolic and vascular effects, thiazolidinedione (TZD) drugs have not convincingly reduced cardiovascular mortality in clinical trials, raising the possibility of countervailing, off-target effects. We previously showed that TZDs block cardiac ATP-sensitive potassium (K(ATP)) channels in pigs. In this study, we investigated whether TZDs affect onset, spectral characteristics, and mortality of ischemic ventricular fibrillation (VF) and whether such effects are recapitulated by a non-selective K(ATP) blocker (glyburide) or a mitochondrial K(ATP) blocker (5-hydroxydecanoate).

METHODS

A total of 121 anesthetized pigs were pre-treated with TZD (pioglitazone or rosiglitazone, 1 mg/kg IV, resulting in clinically relevant plasma concentrations), glyburide (1 mg/kg IV), 5-hydroxydecanoate (5 mg/kg IV) or inert vehicle. Ischemia was produced by occlusion of the left anterior descending coronary artery. In a subset of pigs treated with rosiglitazone or vehicle, ischemic preconditioning was performed.

RESULTS

VF developed in all but 6 pigs. In non-preconditioned pigs, onset of VF occurred sooner with pioglitazone (11±3 min, p<0.05) or rosiglitazone (14±3 min, p=0.06) than with vehicle (20±2 min). Defibrillation of VF was successful in 44% of pigs treated with vehicle, compared with 0% with pioglitazone (p=0.057) and 33% with rosiglitazone (NS). After ischemic preconditioning, defibrillation was successful in 62% of pigs treated with vehicle, compared with 26% treated with rosiglitazone (p=0.03). TZDs attenuated slowing of conduction due to ischemia and shifted ECG power spectra during VF toward higher frequencies. All effects of TZDs were recapitulated by glyburide, but not by 5-hydroxydecanoate, supporting an interaction of TZDs with the sarcolemmal K(ATP) channel.

CONCLUSION

In a porcine model, TZDs promote onset and increase mortality of ischemic VF, associated with alterations of conduction and VF spectral characteristics. Similar effects in a clinical setting might adversely impact cardiovascular mortality.

Effect of the ATP-sensitive K⁺ channel opener nicorandil in a canine model of proarrhythmia

Increased action potential duration (APD) induces early afterdepolarization (EAD) in vitro and torsade de pointes in vivo, and ATP-sensitive K(+) channel openers decrease APD in cardiac tissue. We tested whether the ATP-sensitive K(+) channel opener nicorandil has antiarrhythmic effects on class III antiarrhythmic drug-induced ventricular arrhythmia. In 10 anesthetized dogs with chronic atrioventricular block, we recorded monophasic action potentials (MAPs) from the left and right ventricular (LV and RV) endocardium. The class III antiarrhythmic drug nifekalant (1 mg/kg, IV) was administered at 5 minute intervals (total doses; 2-6 mg/kg) until the appearance of EADs, premature ventricular contractions (PVCs), or polymorphic ventricular tachycardias (PVTs). Five minutes after the end of nifekalant administration, nicorandil (1.0 mg/kg) was administered over 5 minutes. Nifekalant decreased the ventricular escape rate from 75 ± 5 beats/minute to 45 ± 10 beats/minute and increased RV-MAP duration (MAPD) from 217 ± 32 msec to 308 ± 2 msec (P < 0.01) and LV-MAPD from 232 ± 32 msec to 353 ± 82 msec (P < 0.01). EADs were recorded in 9 dogs, frequent premature ventricular contractions (PVCs) developed in 10 dogs, incessant PVTs developed in 3 dogs, and monomorphic ventricular tachycardia developed in 3 dogs after nifekalant administration. Nicorandil decreased RV-MAPD to 267 ± 57 msec and LV-MAPD to 279 ± 44 msec. It suppressed EADs, decreased the incidence of PVCs, and abolished PVT. Nicorandil may be clinically useful for treatment of PVCs and PVTs accompanying acquired long QT syndrome.

Animal models of ventricular arrhythmias

Limitations in understanding of arrhythmias stem from lack of animal models which serve as surrogates for man. The purpose of this review is to discuss iatrogenic and naturally occurring animal models that are useful in our understanding of the mechanisms of ventricular arrhythmia and of antiarrhythmic and proarrhythmic agents. It is not surprising however that some information obtained from studies on infrahuman mammals may not be extrapolated to man. Need for anesthesia affects profoundly the electrophysiology of the heart, including autonomic affects. Most of the animal are modification of the Harris' 2-stage model. A model proposed by Schwartz, Billman and Stone has evolved as one that produces arguably the most information on the pathophysiology of arrhythmia production, including the role of the autonomic nervous system and the interaction with pharmacological agents. Intoxication with digitalis and escalating doses of epinephrine are commonly used models for production of ventricular arrhythmias. No matter what model of ventricular arrhythmias is used, programmed electrical stimulation can be useful to uncover increased tendency for arrhythmia, even if no arrhythmia occurs spontaneously. Models of spontaneous ventricular arrhythmia occur in German shepherd puppies, Boxer dogs, Doberman pinchers with dilated cardiomyopathy, and in large dogs with gastric dilatation or splenic torsion. Models are necessary because they allow for controlled studies and methods of exploration impossible, for legal and ethical reasons, in humans. Nonetheless, ethical considerations in using animal models are still important, and there is a continual search for non-animal models to explore ventricular arrhythmias.

Evidence that activation following failed defibrillation is not caused by triggered activity

BACKGROUND

Earliest postshock activation following failed defibrillation shocks slightly lower than the defibrillation threshold (DFT) in large animals appears to arise from a focus. We tested the hypothesis that these foci are caused by early or delayed afterdepolarizations (EADs or DADs) by performing epicardial electrical mapping and giving the EAD inhibitor pinacidil or the DAD inhibitor flunarizine to see if the foci were extinguished or altered in timing or location.

METHODS AND RESULTS

A sock containing 504 electrodes was placed over the entire ventricular epicardium of 12 open-chested pigs. After the DFT was determined and additional shocks given, pinacidil was administered to 6 pigs and flunarizine to 6 pigs. Then, the DFT was again determined and additional shocks were given. Pinacidil significantly shortened the effective refractory period (ERP) (162 +/- 16 vs 130 +/- 28 msec) and action potential duration (APD(90)) (179 +/- 6 vs 149 +/- 19 msec) and significantly increased the peak frequency of the power spectrum of a left ventricle (LV) electrode during ventricular fibrillation (VF) (9.3 +/- 0.6 vs 10.5 +/- 1.0 Hz), while flunarizine did not significantly alter the ERP (162 +/- 8 vs 167 +/- 18 msec) or APD(90) (187 +/- 12 vs 191 +/- 20) but significantly reduced the peak frequency (9.2 +/- 0.5 vs 7.5 +/- 1.0 Hz). These findings suggest the drugs had their expected electrophysiological effects. However, the DFT was not significantly changed by either drug. Following the same strength shock 10% below the predrug DFT, earliest postshock activation arose in a focal epicardial pattern from the anterior-apical LV both before and after the drugs. The time from the shock until the appearance of this activation was not significantly different before and after either drug.

CONCLUSION

The lack of change in DFT as well as the lack of change in the incidence, location, and timing of the postshock focus with sub-DFT strength shocks before and after pinacidil and flunarizine provide evidence that these foci are not caused by triggered activity.

Ischemic tolerance of rat hearts in acute and chronic phases of experimental diabetes

Different from clinical studies of diabetes mellitus (DM), experimental data reveal both, higher and lower vulnerability of the heart to ischemic injury. We have previously demonstrated an enhanced resistance to ischemia-induced arrhythmias in isolated rat hearts in the acute phase of DM. Our objectives were thus to extend our knowledge to the effects of DM of different duration on myocardial infarction, in conjunction with susceptibility to arrhythmias, in the in vivo model. DM was induced by streptozotocin (45 mg/kg, i.v.) and following 1 week (acute phase) and 8 weeks (chronic phase), anesthetized open-chest diabetic and age-matched control rats were subjected to 30-min regional ischemia (occlusion of LAD coronary artery) followed by 4-h reperfusion for the evaluation of the infarct size (tetrazolium staining). In the control rats, ventricular tachycardia (VT) represented 45.4% of total arrhythmias and occurred in 90% of the animals. In the acute phase of DM, arrhythmia profile was similar to that in the control animals, and the incidence and severity of arrhythmias were not enhanced. On the other hand, the size of infarct area normalized to the size of area at risk was significantly smaller in the diabetics than in the controls (47.2 +/- 2.8 vs. 70.2 +/- 2.1%, respectively; p < 0.05). In the chronic phase, only 17.7% of arrhythmias occurred as VT in 44% of the diabetics (p < 0.05 vs. controls). Severity of arrhythmias was also lower (arrhythmia score: 2.1 +/- 0.3 vs. 2.9 +/- 0.3 in the controls, respectively; p < 0.05). This effect was not due to a smaller infarct size, since the latter did not differ from that in the controls.

IN CONCLUSION

diabetic rat hearts exhibit rather lower, than higher sensitivity to ischemia. In acute phase of DM, diabetic hearts are more resistant to irreversible cell damage, whereas in the chronic phase they exhibit reduced susceptibility to arrhythmias; these discrepancies might reflect different pathogenesis of arrhythmias and myocardial infarction.

Progress toward the development of a safe and effective agent for treating reentrant cardiac arrhythmias: synthesis and evaluation of ibutilide analogues with enhanced metabolic stability and diminished proarrhythmic potential

A series of ibutilide analogues with fluorine substituents on the heptyl side chain was prepared and evaluated for class III antiarrhythmic activity, metabolic stability, and proarrhythmic potential. It was found that fluorine substituents stabilized the side chain to metabolic oxidation. Many of the compounds also retained the ability to increase the refractoriness of cardiac tissue at both slow and fast pacing rates. The potential for producing polymorphic ventricular tachycardia in the rabbit model was dependent on the chirality of the benzylic carbon. The S-enantiomers generally had less proarrhythmic activity than the corresponding racemates. One compound from this series (45E, trecetilide fumarate) had excellent antiarrhythmic activity and metabolic stability and was devoid of proarrhythmic activity in the rabbit model. It was chosen for further development.

Ventricular arrhythmias with left bundle branch block pattern and inferior axis: assessment of their mechanisms on the basis of response to ATP, nicorandil and verapamil

The present study investigated the mechanism of ventricular arrhythmias showing left bundle branch block (LBBB) pattern with an inferior axis. The effects of 3 drugs, adenosine triphosphate (ATP), nicorandil and verapamil, were evaluated in 17 patients. ATP suppressed the arrhythmias in 14 patients and nicorandil suppressed them in 8 of those 14. Verapamil suppressed 5 of the 6 ATP-nicorandil-sensitive arrhythmias. Four patients with ATP- or nicorandil-sensitive arrhythmias were not sensitive to verapamil. On the other hand, 3 of the ATP-insensitive arrhythmias were sensitive to neither nicorandil nor verapamil. The QT intervals and QTc were shortened by nicorandil in 5 of the 6 patients who were sensitive to all 3 drugs. One mechanism of suppression by nicorandil could be related to less Ca++ entering the myocardium, which would decrease the duration of the action potential as indicated by the shortened QT intervals. The results suggest that the mechanism of some ventricular arrhythmias is related to triggered activity. Arrhythmias that are sensitive to ATP or nicorandil, but not to verapamil, may be caused by abnormal automaticity. On the other hand, arrhythmias that are insensitive to all 3 drugs might be related to reentry. The features of ventricular arrhythmias with LBBB pattern and inferior axis differ and therefore the causative mechanisms are not the same.

Acute diabetes modulates response to ischemia in isolated rat heart

Diabetic hearts are suggested to exhibit either increased or lower sensitivity to ischemia. Detrimental effects of prolonged ischemia can be attenuated by preconditioning, however, relatively little is known about its effects in the diseased myocardium. This study was designed to test the susceptibility to ischemia-induced arrhythmias and the effect of preconditioning in the diabetic heart. Rats were made diabetic with streptozotocin (45 mg/kg, i.v.). After 1 week, isolated Langendorff-perfused hearts were subjected to 30 min occlusion of LAD coronary artery without or with preceding preconditioning induced by one cycle of 5 min ischemia and 10 min reperfusion. Glycogen and lactate contents were estimated in the preconditioned and non-preconditioned hearts before and after ischemia. Diabetic hearts were more resistant to ischemia-induced arrhythmias: incidence of ventricular tachycardia (VT) decreased to 42% and only transient ventricular fibrillation (VF) occurred in 17% of the hearts as compared to the non-diabetic controls (VT 100% and VF 70% including sustained VF 36%; p < 0.05). Preconditioning effectively suppressed the incidence and severity of arrhythmias (VT 33%, VF 0%) in the normal hearts. However, this intervention did not confer any additional protection in the diabetic hearts. Despite higher glycogen content in the diabetic myocardium and greater glycogenolysis during ischemia, production of lactate in these hearts was significantly lower than in the controls. Preconditioning caused a substantial decrease in the accumulation of lactate in the normal hearts, whereby in the diabetic hearts, this intervention did not cause any further reduction in the level of lactate. In conclusion, diabetic rat hearts exhibit lower susceptibility to ischemic injury and show no additional response to preconditioning. Reduced production of glycolytic metabolites during ischemia can account for the enhanced resistance of diabetic hearts to ischemia as well as for the lack of further protection by preconditioning.

The antihypertensive and cardioprotective effects of (-)-MJ-451, an ATP-sensitive K(+) channel opener

ATP-sensitive K(+) (K(ATP)) channel openers have been shown to be a potential class of therapeutic agents for the control of cardiovascular diseases, including angina, arrhythmias, and hypertension. In this study, the pharmacological activity of 6-cyano-3S,4R-dihydro-2, 2-dimethyl-2H-3-hydroxy-4-[5S-(1-hydroxymethyl)-2-oxo-1-pyrrolidinyl] -1-benzopyran ((-)-MJ-451), a synthetic K(ATP) opener, was evaluated in anesthetized rat models and in isolated rat thoracic rings. Results demonstrated that intravascular injection of (-)-MJ-451 (0. 02, 0.05 and 0.1 mg/kg) produced an immediate, dose-related reduction in mean arterial blood pressure in anesthetized spontaneously hypertensive rats (SHR), which persisted for more than 3 h and was not accompanied by reflex tachycardia. The hemodynamic changes were completely abolished by pretreatment with glibenclamide (4 mg/kg, i.v. bolus), a selective K(ATP) channel blocker. In isolated thoracic aorta, (-)-MJ-451 (10 nM-3 microM) produced a concentration-dependent vasodilator effect on the phenylephrine (0.3 microM)-induced vasoconstriction. Moreover, (-)-MJ-451 relaxed the thoracic aorta contracted by low (5, 20 and 30 mM), but not high (40 and 60 mM) concentrations of extracellular potassium. In addition, (-)-MJ-451 showed cardioprotective effects in the rat model of 45-min left coronary artery occlusion followed by 1-h reperfusion. In myocardial ischemia, pretreatment with (-)-MJ-451 (2, 5 and 10 microg/kg, i.v. bolus) significantly reduced the incidence of ventricular fibrillation and the mortality, also reducing the total number of ventricular premature contractions, total duration of ventricular tachycardia and ventricular fibrillation. A significant reduction in infarct size was noted in three (-)-MJ-451 (2, 5 and 10 microg/kg)-treated groups. Also, the cardioprotective effects of (-)-MJ-451 were virtually abolished by pretreating the rats with glibenclamide (4 mg/kg, i.v. bolus). In conclusion, (-)-MJ-451, through opening the K(ATP) channel, exerted antihypertensive and cardioprotective effects. Therefore, it is suggested that (-)-MJ-451 has potential in the treatment of hypertension or acute myocardial infarction.

Ventricular arrhythmias in normal hearts

Idiopathic ventricular tachycardia (VT) is characterized by two predominant forms. The most common form originates from the right ventricular outflow tract and presents as repetitive monomorphic VT or exercise-induced VT. The tachycardia is adenosine sensitive and is thought to be because of cAMP-mediated triggered activity. The other major form of idiopathic VT is owing to verapamil-sensitive intrafascicular re-entrant tachycardia, which most often originates in the region of the left posterior fascicle. Both forms of idiopathic VT can be readily treated with radiofrequency catheter ablation.

KATP channels and 'border zone' arrhythmias: role of the repolarization dispersion between normal and ischaemic ventricular regions

1. In order to investigate the role of KATP channel activation and repolarization dispersion on the 'border zone' arrhythmias induced by ischaemia-reperfusion, the effects of glibenclamide and bimakalim, agents modifying action potential (AP) duration, were studied in an in vitro model of myocardial 'border zone'. 2. The electrophysiological effects of 10 microM glibenclamide and 1 microM bimakalim (n=8 each), respectively KATP channel blocker and activator, were investigated on guinea-pig ventricular strips submitted partly to normal conditions (normal zone, NZ) and partly to simulated ischaemic then reperfused conditions (altered zone, AZ). 3. By preventing the ischaemia-induced AP shortening (P<0.0001), glibenclamide reduced the dispersion of AP duration 90% (APD90) between NZ and AZ (P<0.0001), and concomitantly inhibited the 'border zone' arrhythmias induced by an extrastimulus (ES), their absence being significantly related to the lessened APD90 dispersion (chi2=8.28, P<0.01). 4. Bimakalim, which also reduced the APD90 dispersion (P<0.005) due to differential AP shortening in normal and ischaemic tissues, decreased the incidence of myocardial conduction blocks (25% of preparations versus 83% in control, n=12, P<0.05) and favoured 'border zone' spontaneous arrhythmias (75% of preparations versus 25% in control, P<0.05). 5. During reperfusion, unlike bimakalim, glibenclamide inhibited the ES-induced arrhythmias and reduced the incidence of the spontaneous ones (12% of preparations versus 92% in control, P<0.05), this latter effect being significantly related (chi2=6.13, P<0.02) to the lessened ischaemia-induced AP shortening in the presence of glibenclamide (P<0.0001). 6. These results suggest that KATP blockade may protect the ischaemic-reperfused myocardium from 'border zone' arrhythmias concomitantly with a reduction of APD90 dispersion between normal and ischaemic regions. Conversely, KATP channel activation may modify the incidence of conduction blocks and exacerbate the ischaemia-induced 'border zone' arrhythmias.

Effects of K+ channel modulators on the relationship between action potential duration and Ca2+ transients in single ventricular myocytes of the guinea pig

Effects of K+ channel modulators, cromakalim and E4031 [1-[2-(6-methyl-2-pyridyl)-ethyl]-4-(4-methylsulfonylaminobenzoyl) piperidine], on the relationship between the action potential duration (APD) and Ca2+ transients were examined in single myocytes isolated from guinea pig cardiac left ventricle. Application of cromakalim decreased APD at 90% repolarization (APD90) and Ca2+ transient elicited at 0.5 Hz (IC50s=0.6 and 3 microM, respectively). Application of 0.3 microM E4031 increased these parameters by 110% and 45%, respectively. Under voltage-clamp, the relation between the duration of depolarization to 0 mV and Ca2+ transients could be described by the sum of two exponential components; the time constants were approximately 5 and 280 msec, respectively. The first component was abolished by 10 microM ryanodine, suggesting the involvement of Ca2+-induced Ca2+ release (CICR). Neither cromakalim nor E4031 directly affected Ca2+ current and Ca2+ transients under voltage clamp. When APD was changed by K+ channel modulators, the relation between APD90 and Ca2+-transients was almost similar to that obtained by changing the depolarization duration under voltage-clamp. CICR was changed significantly only when APD90 was markedly shortened by cromakalim. The extensively prolonged AP and Ca2+ transient in the presence of E4031 were reduced by an addition of cromakalim. It is concluded that these two K+ channel modulators can significantly alter the AP-induced Ca2+ transient mainly by changing APD, which regulates both Ca2+ influx and extrusion.

Propafenone blocks ATP-sensitive K+ channels in rabbit atrial and ventricular cardiomyocytes

Propafenone, a class I antiarrhythmic agent, inhibits several membrane currents (I(Na), I(Ca), I(K), Ito), however, its effects on ATP-sensitive potassium current (I(K)ATP) of cardiac cells have not been tested. We evaluated the blocking effects of 0.1 to 100 microM propafenone applications at 35 degrees C on the whole-cell I(K)ATP as triggered by dinitrophenol (75 microM) in adult rabbit dissociated atrial and ventricular cardiomyocytes in comparison. The block of I(K)ATP by propafenone was dose-dependent, fully reversible and voltage-independent. The dose-response relation, as evaluated at 0 mV for atrial myocytes (ED50 = 1.26+/-0.17 microM, Hill number = 1.25+/-0.22) was significantly shifted to the left vs. that in ventricular myocytes (ED50 = 4.94+/-0.59 microM, Hill number = 1.22+/-0.14). It is concluded that propafenone blocks cardiac I(K)ATP at a single site with 4 times higher affinity for the drug in atrial myocytes. This block of cardiac I(K)ATP might play a role in the beneficial and adverse effects of the drug.

Slow ventricular tachycardia located in the epicardium of the left ventricular base and characterized by effects of adenosine triphosphate, nicorandil and verapamil

A 31-year-old male with slow ventricular tachycardia (VT) developed a nonsustained VT with prolongation of the JT intervals after injection of contrast medium and saline into the marginal vein of the coronary sinus. The earliest activation site of the VT existed in the epicardium of the left ventricular base. Adenosine triphosphate prevented induction of VT and prolongation of JT intervals. Ventricular premature contractions showing the same morphology as the VT were also inhibited by nicorandil and verapamil. The mechanism of the VT was suggested to be abnormal automaticity due to an increase in the Ca++ current into cells after prolongation of the action potential duration induced by hypothermia.

Suppression of reperfusion arrhythmias by preconditioning is inhibited by an ATP-sensitive potassium channel blocker, 5-hydroxydecanoate, but not by protein kinase C blockers in the rat

It has been suggested that preconditioning (PC) against infarction in the rat heart is mediated by opening of ATP-sensitive potassium (KATP) channels, which may be induced by receptor-triggered activation of protein kinase C (PKC). However, the mechanism of suppression of reperfusion arrhythmias by PC remains unclear. This study first examined whether suppression of reperfusion arrhythmias by PC requires the activation of KATP channels, PKC, and opioid receptors. In anesthetized rats, reperfusion arrhythmias were induced by occluding the left main coronary artery for 5 min and subsequent reperfusion. In untreated control rats, the incidence of reperfusion ventricular tachycardia (VT) was 100%, and 80% of the VT deteriorated to ventricular fibrillation (VF). PC with 2-min ischemia/5-min reperfusion reduced the incidences of VT and VF to 30% and 0, respectively. Although a KATP channel blocker, 5-hydroxydecanoate (5-HD), alone caused no significant effect on the incidence of reperfusion VT, this agent blocked the suppression of reperfusion VT by PC (VT incidence, 91%). The incidence of reperfusion VF in the 5-HD-treated rats tended to be lower than that in the untreated controls and was not different between preconditioned and nonpreconditioned groups (30 vs. 27%). PKC inhibitors staurosporine and calphostin C modified neither reperfusion arrhythmias nor the antiarrhythmic effect of PC on reperfusion VT and VF. Naloxone (6 mg/kg, i.v.) did not alter the incidence or duration of VT in the nonpreconditioned heart. However, suppression of reperfusion VT by PC was prevented by naloxone (VT incidence, 70%). The incidence of reperfusion VF was similarly low in the naloxone-treated rats in both nonpreconditioned and preconditioned groups (20% vs. 0). In the second series of experiments, the effect of 5-HD on repetitive PC was assessed. Repetitive PC was performed with three cycles of 2-min ischemia/5-min reperfusion, which totally abolished reperfusion VT and VF. This antiarrhythmic effect of repetitive PC was not inhibited by 5-HD. These results suggest that KATP channels and opioid receptors may be partly involved in suppression of reperfusion arrhythmias, although their roles may be compensated for by other antiarrhythmic mechanisms in repetitive PC. In contrast with PC against infarction, PKC is unlikely to play a major role in PC against reperfusion arrhythmias in the rat.

Effects of nicorandil, a potassium channel opener, on idiopathic ventricular tachycardia

OBJECTIVES

We assessed the effects of the adenosine triphosphate (ATP)-sensitive potassium channel opener, nicorandil, on ATP- and verapamil-responsive ventricular tachycardias (VTs).

BACKGROUND

Adenosine- or ATP-sensitive VTs are thought to be due to a nonreentrant mechanism, presumably delayed afterdepolarization. We suggest that this potassium channel opener may suppress ATP- and verapamil-sensitive VTs.

METHOD

The subjects included 13 patients with idiopathic VTs, 7 of whom had sustained VT and 6 of whom had nonsustained VT. We evaluated the effects of ATP, nicorandil and verapamil on VTs.

RESULTS

Sustained VT: Verapamil had preventive effects on seven VTs. Four VTs were terminated by ATP, and of these, nicorandil terminated two and prevented exercise-induced VT in the two others. Three ATP-insensitive VTs, which were determined to be due to a reentry by an electrophysiologic study, were not terminated by nicorandil. Nonsustained VT: All six VTs were inhibited by ATP, and five of these were suppressed by nicorandil. Verapamil inhibited four of the five VTs. QT intervals and the corrected QT intervals were significantly shortened by nicorandil.

CONCLUSIONS

Nicorandil suppresses ATP- and verapamil-responsive VTs. One of the mechanisms of suppression by nicorandil might be related to a reduction of calcium in the myocardium, because it reduces the action potential duration.

Antiarrhythmic drugs and cardiac ion channels: mechanisms of action

In this review a description and an analysis are given of the interaction of antiarrhythmic drugs with their molecular target, i.e. ion channels and receptors. Our approach is based on the concept of vulnerable parameter, i.e. the electrophysiological property which plays a crucial role in the genesis of arrhythmias. To prevent or stop the arrhythmia a drug should modify the vulnerable parameter by its action on channel or receptor targets. In the first part, general aspects of the interaction between drugs channel molecules are considered. Drug binding depends on the state of the channel: rested, activated pre-open, activated open, or inactivated state. The change in channel behaviour with state is presented in the framework of the modulated-receptor hypothesis. Not only inhibition but also stimulation can be the result of drug binding. In the second part a detailed and systematic description and an analysis are given of the interaction of drugs with specific channels (Na+, Ca2+, K+, "pacemaker") and non-channel receptors. Emphasis is given to the type of state-dependent block involved (rested, activated and inactivated state block) and the change in channel kinetics. These properties vary and determine the voltage- and frequency-dependence of the change in ionic current. Finally, the question is asked as to whether the available drugs by their action on channels and receptors modify the vulnerable parameter in the desired way to stop or prevent arrhythmias.

The sulfonylurea controversy: more questions from the heart

Myocardial ischemia and infarction are associated with substantially increased morbidity and mortality among patients with diabetes mellitus. Although many factors contribute to the increased morbidity and mortality, in patients with non-insulin-dependent (type II) diabetes mellitus, one contributor may be the use of sulfonylurea drugs, the most widely used oral hypoglycemic agents. Such a possibility, which first arose over a 25 years ago when it was observed that patients taking sulfonylurea drugs had increased cardiovascular mortality, has recently resurfaced after the discovery that sulfonylureas act by inhibiting adenosine triphosphate (ATP)-sensitive potassium channels. In the pancreas, inhibition of ATP-sensitive potassium channels induces release of insulin; but in the heart, inhibition of these channels prevents ischemic preconditioning, an endogenous cardioprotective mechanism that protects the heart from lethal injury. This review outlines the current understanding of the molecular and cellular pharmacodynamics of sulfonylurea drugs and discusses the potential clinical consequences of inhibition of ATP-sensitive potassium channels in the heart of diabetic patients with cardiac disease in whom the use of sulfonylureas may be harmful.

Effects of the ATP-sensitive K channel opener nicorandil on the QT interval and the effective refractory period in patients with congenital long QT syndrome. Investigator Group for K-Channel Openers and Arrhythmias

Congenital or idiopathic long QT syndrome is characterized by a frequently lethal ventricular arrhythmia called torsades de pointes (TdP) as well as a prolonged QT interval. The long QT interval related to an abnormal gene of the Na+ channel has been shown to be shortened by mexiletine. However, the action of K+ channel openers on the QT interval associated with abnormal genes of the K channel has yet to be studied. Seven patients of five families with long QT syndrome were included in this study, of whom six had syncope and six had documented TdP. Either long QT interval or sudden cardiac death had been observed in family members of all seven patients. At 1 to 3 weeks after admission, when TdP or frequent ventricular arrhythmia had subsided, nicorandil, an ATP-sensitive K channel opener, was administered orally at a dose of 15 mg/day in five patients and at 30 mg/day in the remaining two patients, and the effects were assessed on the third day after drug administration. In four patients, the effective refractory period was measured in the right ventricle before and after administration of K channel opener administration. The QT interval (QTc) prior to administration of the K channel opener was 0.60 +/- 0.09 ms (mean +/- SD) (0.61 +/- 0.10 second(1/2)), which was shortened to 0.54 +/- 0.05 ms (0.55 +/- 0.06 second(1/2)) on the third day of drug administration (P < .05 for both): 10.4 +/- 8.0% (8.6 +/- 5.5%). The QT interval at varying preceding R-R intervals on Holter electrocardiograms showed a shift toward the right as a result of the drug administration. The effective refractory period showed a significant prolongation, 256 +/- 26 ms versus 280 +/- 22 ms before and after drug administration, respectively (P <.05). Intravenous administration of nicorandil resulted in no significant change in heart rate or blood pressure, while QTc showed a tendency to shorten, but nonsignificantly (P = .08). However, a hump on the monophasic action potential was abolished, especially at the long preceding R-R interval induced by premature stimulation of the ventricle. It is concluded that nicorandil shortens the QT interval slightly when administered orally, whereas the effective refractory period shows a slight prolongation. The physiologic and clinical significance of these effects needs to be studied further.

ATP-sensitive K+ channels in cardiac muscle from cold-acclimated goldfish: characterization and altered response to ATP

ATP-sensitive potassium channels (K(ATP)) play an important, if incompletely defined, role in myocardial function in mammals. With the discovery that K(ATP) channels are also present at high densities in the hearts of vertebrate ectotherms, speculation arises as to their function during periods of cold-acclimation and depressed ATP synthesis. We used single-channel and intracellular recording techniques to examine the possibility that channel activity would be altered in cardiac muscle from goldfish (Carassius auratus) acclimated at 7+/-1 degrees C relative to control (21+/-1 degrees C). As previously observed in mammals, K(ATP) channels in isolated ventricular myocytes were inwardly rectified with slope conductances of 63 pS. However, channel mean open-time and overall open-state probability (Po) were significantly increased in cells from the cold-acclimated animals. In addition, K(ATP) channels in cells from fish acclimated at 7 degrees were nearly insensitive to the inhibitory effects of 2 mM ATP, whether studied at 7 or at 21 degrees C. Transmembrane action potential duration (APD) in hearts of cold-acclimated fish studied at 21 degrees was significantly shorter than that observed in hearts of warm-acclimated fish at the same temperature; this difference was eliminated by the K(ATP) channel antagonist glibenclamide (5 microM). These data suggest that K(ATP) channels in the hearts of cold-acclimated animals are more active and less sensitive to ATP-inhibition than those in warm-acclimated fish, possibly reflecting a functional adaptation to promote tolerance of low temperatures in this species.

Electrophysiological mechanisms for ventricular arrhythmias in patients with myocardial ischemia: anesthesiologic considerations, Pt II

This is the second half of a two-part review article that discusses ventricular tachyarrhythmias, either induced by acute ischemia or consequent to chronic myocardial ischemia, and their anesthestic implications. The first half of the article was published in the June 1997 Issue of The Journal.

Antiarrhythmic and electrophysiological effects of the novel KATP channel opener, rilmakalim, in rabbit cardiac cells

1. The effects of rilmakalim, a potassium channel opener, were studied on rabbit cardiac Purkinje, ventricular muscle and atrial fibers, with the use of conventional microelectrode techniques. 2. Rilmakalim (0.24-7.2 microM) shortened, in a concentration-dependent manner, the action potential duration (APD) in Purkinje and ventricular muscle without affecting other parameters of the action potential. Pinacidil (30-300 microM) also decreased the APD of Purkinje fibers. 3. Rilmakalim (2.4 microM) and cromakalim (100 microM) hyperpolarized and abolished abnormal automaticity of cardiac Purkinje fibers pretreated with barium (0.2-0.3 mM). Glibenclamide (5 microM) blocked the hyperpolarizing effect. 4. Stable early afterdepolarizations induced in Purkinje fibers by berberine (100 microM) were reversibly blocked by rilmakalim (2.4 microM), which also suppressed late afterdepolarizations induced in Purkinje fibers treated with ouabain (0.3-0.5 microM). 5. The rate of spontaneous discharges of the rabbit sinoatrial node was not affected by rilmakalim (7.2 microM) or by pinacidil (100 microM). Both agents were also unable to affect the APD of atrial muscle fibers. 6. In cardiac Purkinje fibers, tetraethylammonium (TEA; 20 mM) significantly reduced the effects of rilmakalim (2. 4 microM) on the APD. However, neither TEA nor glibenclamide (100 microM) reduced the shortening of the APD induced by dinitrophenol (30 microM) or by salicylate (1 mM).

Comparison of proarrhythmogenic effects of two potassium channel openers, levcromakalim (BRL 38227) and nicorandil (RP 46417): a high-resolution mapping study on rabbit heart

This study was designed (a) to test and (b) to compare proarrhythmic effects of levcromakalim and nicorandil; and (c) determine the mechanism of arrhythmia initiation by using high-resolution ventricular epicardial mapping on 44 Langendorff-perfused rabbit hearts. Eighteen hearts were kept intact and received incremental doses (1-500 microM) of levcromakalim, nicorandil, and isosorbide dinitrate. In 26 hearts, a thin layer of epicardium was obtained after endocardial cryotechnique (frozen hearts). In intact hearts, isosorbide dinitrate did not produce any arrhythmia. In contrast, levcromakalim induced spontaneous ventricular fibrillation (VF) in all hearts at 50 microM, whereas only one VF occurred at 500 microM nicorandil. These three drugs produced a dose-dependent bradycardia in intact hearts. In frozen hearts, arrhythmias were induced by 5 microM levcromakalim and 50 microM nicorandil. Isosorbide dinitrate had no proarrhythmogenic effect. Epicardial mapping showed that most of induced ventricular tachycardias were based on reentry around an arc of functional conduction block. Ventricular conduction velocities did not change, but levcromakalim and nicorandil shortened ventricular effective refractory period. We conclude that (a) levcromakalim and nicorandil, used in toxic concentrations, have direct proarrhythmic effects; (b) nicorandil proarrhythmogenic effects are 10 times less marked than those of levcromakalim (arrhythmia is solely the result of the potassium channel opener property of nicorandil); and (c) most of ventricular tachycardias induced are based on reentry.

Early after-depolarisations induced by noradrenaline may be initiated by calcium released from sarcoplasmic reticulum

We investigated the effect of 10(-8) M noradrenaline (NA) on [Ca2+]i and electrical activity of single myocytes of guinea-pig ventricular myocardium loaded with Indo 1-AM. Membrane potential was recorded by means of the patch electrode and patch amplifier set to the current clamp mode. Cells were stimulated at a rate of 30/min by 3 ms pulses of the current injected through the recording electrode. Superfusion of NA resulted in slight shortening of action potentials (APs), increase in rate of rise and amplitude of the respective Ca2+ transients, and appearance of secondary Ca2+ transients of two kinds: 1. appearing before repolarisation of AP and decay of the preceding Ca2+ transient were completed and 2. appearing between the APs. We named them early after-transients (EAT) and delayed after-transients (DAT), respectively. Without any additional intervention EATs caused some prolongation of APs duration and DATs resulted in subthreshold delayed after-depolarisations (DADs). When sarcolemmal K+ conductance was decreased by tetraethylammonium (TEA) in the patch electrode or 20 microM BaCl2 in the Tyrode solution, EATs initiated early after depolarizations (EADs) and DATs initiated suprathreshold DADs triggering full-sized APs. Superfusion of 30.0 mM Na+ (replaced with LiCl) resulted in reduction of AP duration by 70% and appearance of DATs. Also, the frequent multiple oscillations of Ca2+ concentration were often observed. Neither DATs nor the oscillations had any affect on electrical activity of the cells. Their electrogenicity could not be increased by TEA or 20.0 microM Ba2+. EATs and DATs and their respective EADs and DADs could not be initiated by NA or low Na+ superfusion in the cells pretreated with 2 x 10(-7) M thapsigargin, a selective blocker of Ca(2+)-ATPase of sarcoplasmic reticulum (SR). We conclude that in contrast to the current hypothesis, EADs can be initiated by Ca2+ released early in the cardiac cycle from the overloaded SR, and that electrogenicity of both types of Ca2+ oscillations critically depends on the sarcolemmal K+ conductance.

Cardiovascular effects of sulphonylurea derivatives

The classical sulphonylurea derivatives like glibenclamide and tolbutamide are widely prescribed in non-insulin dependent diabetes mellitus in order to stimulate insulin secretion. The insulinotropic effect of these agents is based on the closure of adenosine-5'-triphosphate (ATP)-sensitive potassium channels (KATP-channels) in the beta-cells of the pancreas. Interestingly, the cardiovascular system also shares these KATP-channels. The open state probability of these channels is regulated by the intracellular concentration of ATP. During ischaemia, the KATP-channels are thought to open by a fall in the cytosolic ATP concentration. The increase in the extracellular adenosine concentration, and the release of endothelium-derived hyperpolarizing factor (EDHF) during ischaemia may further contribute to the opening of cardiovascular KATP-channels. Sulphonylurea derivatives like glibenclamide and tolbutamide have been reported to block the opening of KATP-channels in several types of tissues including myocardial and vascular smooth muscle cells. Since the opening of KATP-channels is regarded as an endogenous cardioprotective mechanism, the blocking effect of sulphonylurea derivatives in the cardiovascular system may have deleterious effects. Human studies on this issue have just been initiated, and preliminary results point towards a significant interaction between glibenclamide and cardiovascular KATP-channels at clinically relevant concentrations. In this regard, the introduction of more pancreas specific sulphonylurea derivatives like glimepiride, which do not interact with cardiovascular KATP-channels, is a promising development.

Prevention of reoxygenation-induced arrhythmias in guinea pig papillary muscles

Effects of various agents on reoxygenation-induced arrhythmias, action potentials, and tension of guinea pig papillary muscles were recorded to investigate the site of action. Triggered activities due to delayed afterdepolarizations (DADs) and aftercontractions were elicited on reoxygenation after 60-min substrate-free hypoxia. Low extracellular Ca2+ (0.1 mM) abolished arrhythmias, and high Ca2+ (4.9 mM) increased the amplitudes of DADs and aftercontractions. D-600 at the high concentration (20 microM) decreased the incidence of arrhythmias (p < 0.05 vs. no drug) and decreased the recovery of developed tension after reoxygenation (p < 0.001). Ryanodine (1 microM) abolished aftercontractions and arrhythmias but did not affect the recovery of developed tension. Tetrodotoxin (TTX 3 microM) and nicorandil (100 microM) decreased the incidence of arrhythmias (p < 0.05), but did not affect the recovery of developed tension or the amplitudes of aftercontractions. TTX caused only a slight decrease in Ca2+ transients in a fluo-3-loaded guinea pig ventricular myocyte. The Ca2+ entry through the Ca2+ channels apparently synchronized Ca2+ release from the sarcoplasmic reticulum, and D-600 at the high concentration apparently decreased the incidence of arrhythmias. TTX and nicorandil decreased arrhythmias, probably by decreasing the Na+ current or by increasing the ATP-sensitive K+ current, respectively.

Ionic mechanisms of ischemia-related ventricular arrhythmias

The aim of this review is the utmost simplification of the cellular electrophysiologic background of ischemia-related arrhythmias. In the acute and subacute phase of myocardial infarction, arrhythmias can be caused by an abnormal impulse generation, abnormal automaticity or triggered activity caused by early or delayed afterdepolarizations (EAD and DAD), or by abnormalities of impulse conduction (i.e., reentry). This paper addresses therapeutic intervention aimed at preventing the depolarization of "pathologic" slow fibers, counteracting the inward calcium (Ca) influx that takes place through the L-type channels (Ca antagonists), or hyperpolarizing the diastolic membrane action potential, increasing potassium (K) efflux (K-channel openers) in arrhythmias generated by an abnormal automaticity (ectopic tachycardias or accelerated idioventricular rhythms). If the cause enhanced impulse generation is related to triggered activity, and since both EAD and DAD are dependent on calcium currents that can appear during a delayed repolarization, the therapeutic options are to shorten the repolarization phase through K-channel openers or Ca antagonists, or to suppress the inward currents directly responsible for the afterdepolarization with Ca blockers. Magnesium seems to represent a reasonable choice, as it is able to shorten the action potential duration and to function as a Ca antagonist. Abnormalities of impulse conduction (re-entry) account for the remainder of arrhythmias that occur in the acute and subacute phase of ischemia and for most dysrhythmias that develop during the chronic phase. Reentrant circuits due to ischemia are usually Na channel-dependent. Drug choice will depend on the length of the excitable gap: in case of a short gap (ventricular fibrillation, polymorphic ventricular tachycardia, etc.), the refractory period has been identified as the most vulnerable parameter, and therefore a correct therapeutic approach will be based on drugs able to prolong the effective refractory period (K-channel blockers, such as class III antiarrhythmic drugs); on the other hand, for those arrhythmias characterized by a long excitable gap (most of the monomorphic ventricular tachycardias), the most appropriate therapeutic intervention consists of depressing ventricular excit-ability and conduction by use of sodium-channel blockers such as mexiletine and lidocaine. Compared with other class I antiarrhythmic agents, these drugs minimally affect refractoriness and exhibit a use-dependent effect and a voltage dependent action (i.e., more pronounced on the ischemic tissue because of its partial depolarization).

Which cardiac potassium channel subtype is the preferable target for suppression of ventricular arrhythmias?

Prolongation of the cardiac action potential duration is the hallmark of Class III antiarrhythmic activity. Action potential duration prolongation may be achieved by several means: enhancement of inward current and, more commonly, blockade of one or more of the many outward currents that are carried by K+. However, it is far from clear whether blockade of one particular K+ channel is more efficacious than blockade of another. The objective of this review is to consider this question with particular reference to ischaemic heart disease, a condition for which effective prevention of ventricular arrhythmias continues to be sought.

Effects of pinacidil, verapamil, and heart rate on afterdepolarizations in the guinea-pig heart in vivo

Recently, ionic current simulation in the Luo-Rudy model has elucidated putative mechanisms of afterdepolarizations under various experimental conditions. The present study was aimed at gaining insight into the differential mechanism of different types of afterdepolarizations in the guinea-pig heart in vivo. The effects of pharmacological and heart rate perturbations on early (EADs) and delayed (DADs) afterdepolarizations, induced by either digoxin, CsCl, or BayK 8644 were studied, using mid-myocardial left ventricular monophasic action potential (MAP) recordings. Digoxin insignificantly shortened sinus cycle length (SCL) and CsCl and BayK 8644 differentially prolonged SCL and MAP duration. Digoxin induced phase 3-EADs and DADs and CsCl or BayK 8644 induced phase 2- and phase 3-EADs. Pinacidil shortened MAP duration, suppressed almost all the phase 2-EADs and some of the phase 3-EADs, but not the DADs. In a few cases, DADs were manifested following the abolishment of phase 2-EADs by pinacidil, but this phenomenon did not occur in the presence of hexamethonium. Verapamil prolonged SCL, did not significantly affect phase 2-EADs, but suppressed almost all of the DADs, including those which appeared after pinacidil, and all but one of the phase 3-EADs. The effects of pinacidil and verapamil were independent of the mode of afterdepolarization induction. A pacing-induced heart rate increase, which shortened MAP duration, and vagal stimulation, which prolonged MAP duration, attenuated and enhanced phase 2-EADs, respectively. The amplitude of phase 3-EADs was inversely related to the heart rate. These data, taken together, are consistent with those obtained previously by others in a computer model and recent observations on CsCl-induced EADs in the guinea-pig Purkinje fibers in vitro which have indicated that the mechanism of phase 2-EADs is different from that of DADs and that late phase 3-EADs generated under conditions of Ca2+ overload and DADs share similar properties.

Proarrhythmia with class III antiarrhythmic drugs: definition, electrophysiologic mechanisms, incidence, predisposing factors, and clinical implications

Antiarrhythmic drugs can and do induce unexpected and sometimes fatal reactions by either producing new symptomatic arrhythmias or by aggravating existing arrhythmias. The definition of proarrhythmia has changed since controlled clinical studies showed a dichotomy between arrhythmia suppression and mortality. The nature of proarrhythmic reactions is linked to the electrophysiologic effects of various antiarrhythmic drugs. Whereas Class I agents without accompanying effects on repolarization generally produce ventricular tachycardia (often incessant) or fibrillation, Class III agents typically produce torsades de pointes that may deteriorate into ventricular fibrillation. The precise mechanism of torsades de pointes is not fully elucidated, although early after-depolarization and increases in spatial or temporal dispersion of repolarization are likely possibilities. Proarrhythmic risk is lowest for amiodarone and is probably related to the drug's complex electrophysiologic profile. The incidence of torsades with sotalol increases with dose and the baseline values of the QT interval; the incidence with d-sotalol and other pure Class III agents remains unclear. Prospective, randomized, placebo-controlled studies to evaluate this are under way. The fact that d-sotalol increases mortality in postinfarction patients suggests that it may possibly be a common property of most, if not all, pure Class III compounds. The ongoing clinical trials with various Class III agents are likely to provide the critical information on this important therapeutic issue.

Suppression of reperfusion arrhythmia by ischemic preconditioning in the rat: is it mediated by the adenosine receptor, prostaglandin, or bradykinin receptor?

The mechanism for the suppression of reperfusion arrhythmia by preconditioning (PC) remains unknown. This study aimed to examine the roles of the adenosine receptor, prostaglandin (PG), and bradykinin (BK) receptor in PC. Under pentobarbital anesthesia, the coronary artery of the rat was occluded for 5 min and then reperfused. In untreated controls, this protocol induced ventricular tachycardia (VT) in 100% of the rats and ventricular fibrillation (VF) in 60%. PC with 2 min ischemia/5 min reperfusion prior to the 5 min coronary occlusion significantly reduced the incidence of reperfusion VT and VF to 30% and 0%, respectively. This antiarrhythmic effect of the PC was not blocked when rats were pretreated with 8-phenyltheophylline (8-PT, 10 mg/kg), aspirin-DL-lysin (18 mg/kg), or a specific BK receptor antagonist, Hoe140 (20 nmol/kg). None of these agents alone significantly modified the incidence of reperfusion VT or VF. These results suggest that neither the adenosine receptor, endogenous PG, nor BK receptor play a major role in the mechanism of suppression of perfusion arrhythmias by PC in the rat heart.

Electrophysiologic mechanisms of the long QT interval syndromes and torsade de pointes

PURPOSE

To review the current understanding of the mechanisms and treatment of the long QT interval syndromes and torsade de pointes.

DATA SOURCES

Personal databases of the authors and a search of the MEDLINE database from 1966 to 1994.

STUDY SELECTION

Experimental and clinical studies and topical reviews on the electrophysiologic mechanisms and treatment of torsade de pointes were analyzed.

RESULTS

The long QT interval syndromes have been classified into acquired and hereditary forms, both of which are associated with a characteristic type of life-threatening polymorphic ventricular tachycardia called torsade de pointes. The acquired form is caused by various agents and conditions that reduce the magnitude of outward repolarizing K+ currents, enhance inward depolarizing Na+ or Ca2+ currents, or both, thereby triggering the development of early afterdepolarizations that initiate the tachyarrhythmia. The hereditary form appears to result from an abnormal response to adrenergic or sympathetic nervous system stimulation. At least some cases of the hereditary long QT interval syndromes may result from a single gene defect that alters the intracellular regulatory proteins responsible for the modulation of K+ channel function. Treatment of the acquired form is primarily directed at identifying and withdrawing the offending agent, although emergent therapy using maneuvers and agents that favorably modulate transmembrane ion currents can be lifesaving. In torsade de pointes associated with the hereditary long QT interval syndromes, early diagnosis leading to treatments designed to both shorten the QT interval and block the beta-adrenergic-induced instability of the QT interval is essential.

CONCLUSIONS

The long QT interval syndromes and torsade de pointes are potentially life-threatening conditions caused by various agents, conditions, and genetic defects. The mechanisms responsible for these conditions and available treatment options for them are reviewed.

Electrophysiologic effects of potassium channel openers

Potassium-channel openers or activators have been introduced as a new class of antihypertensive and antianginal agents that act by increasing membrane conductance to potassium, mainly through augmentation of the ATP-sensitive potassium current. Recent in vitro studies have shown that K(+)-channel openers exert concentration-dependent effects on cardiac electrophysiology. A shortening of the cardiac action potential by acceleration of repolarization has been reported in multicellular preparations as well as in isolated myocytes. However, drug concentrations that affect the action potential duration of myocardial cells are considerably higher (10- to 100-fold) than those needed for effects on vascular smooth muscle cells. Studies in which mostly high concentrations of K(+)-channel openers were used have demonstrated that these drugs may accelerate automaticity and may promote reentrant activity. Particular interest has focused on the question whether opening of potassium channels may be potentially arrhythmogenic in the setting of acute myocardial ischemia. On the other hand, recent studies have shown that K(+)-channel openers are effective in suppressing polymorphic ventricular tachyarrhythmias induced by early afterdepolarizations and triggered activity in vivo. The clinical relevance of these experimental studies to the clinical situation is still unclear. Some K(+)-channel openers have been shown to produce electrocardiographic T-wave changes in patients in whom their effectiveness as antihypertensives was tested. However, this effect was not associated with adverse effects and has not been demonstrated for all compounds.(ABSTRACT TRUNCATED AT 250 WORDS)

Cardiovascular effects of sulphonylurea derivatives. Implications for the treatment of NIDDM?

Sulphonylurea derivatives are widely used in the treatment of non-insulin-dependent diabetes mellitus. The mechanism of action of the insulino-tropic effect of these agents is based on the closure of adenosine-5'-triphosphate (ATP)-sensitive potassium channels (KATP-channels) in the beta cells of the pancreas. In the last decade, these KATP-channels have been demonstrated in myocardial cells as well as in vascular smooth muscle cells. During myocardial ischaemia, the KATP-channels are thought to open by a fall in the cytosolic ATP concentration. The increase in the extracellular adenosine concentration, and the release of endothelium-derived hyperpolarizing factor (EDHF) during ischaemia may further contribute to the opening of cardiovascular KATP-channels. Independently from the mechanism of opening, sulphonylurea derivatives have been reported to block the opening of cardiovascular KATP-channels. Related to the role of KATP-channel-opening in the (patho)physiology of ischaemia, the use of sulphonylurea derivatives significantly modifies the outcome of experimental myocardial infarction. Sulphonylurea derivatives impair the recovery of the contractile function and increase the ultimate infarct size in animal models. In contrast, sulphonylurea derivatives have a beneficial effect on the incidence of ventricular fibrillation as occurs after ischaemic incidents of the myocardium. Based on these experimental observations, human studies are indicated to investigate whether the use of these drugs modifies the clinical outcome of cardiovascular events in patients with non-insulin dependent diabetes mellitus.

An experimental model of the production of early after depolarizations by injury current from an ischemic region

An ischemic myocardial region contains cells with a depolarized resting membrane potential. This depolarization leads to an intercellular current flow between the ischemic region and the surrounding normal myocardial cells which has been termed an "injury current". We have devised an experimental model system in which an isolated guinea pig ventricular cell is electrically coupled to a model depolarized cell in order to evaluate the effects of this injury current on the electrical properties of a normal ventricular cell exposed to drugs which increase calcium current or decrease potassium current. Using low doses of isoproterenol, forskolin, or Bay K 8644 (or 8-bromo-cyclic adenosine monophosphate in the pipette) we found that the action potential duration of the isolated cell was lengthened, but that early afterdepolarizations (EADs) were not produced unless the cell was also coupled to a depolarized cell model representing an adjacent ischemic region. A similar prolongation of the action potential was produced by low doses of quinidine, but EADs were not produced unless coupling to a depolarized cell model was added. EADs could not be produced in any cells in the absence of the drugs even though the coupling to the depolarized cell model was increased up to the level at which the action potential was indefinitely prolonged. At higher isoproterenol concentrations, EADs or spontaneous activity were produced without coupling to the depolarized cell model. Under these conditions, coupling of the cell to a cell model with normal resting membrane potential stopped the spontaneous activity and prevented the occurrence of EADs even with high levels of resistive coupling.(ABSTRACT TRUNCATED AT 250 WORDS)

Pinacidil attenuates positive inotropic but not chronotropic responses to norepinephrine in isolated dog atrial and ventricular preparations

We investigated whether pinacidil, a K+ATP channel opener like acetylcholine and adenosine, attenuated the positive chronotropic and inotropic responses to norepinephrine in isolated, blood-perfused dog atrial and ventricular preparations. Pinacidil (0.01-0.3 mumol) decreased atrial and ventricular contractile force to a much greater extent than sinus rate in a dose-related manner. Pinacidil dose-dependently attenuated increases in atrial and ventricular forces induced by norepinephrine but not increases in sinus rate. Pinacidil similarly attenuated the positive atrial and ventricular inotropic responses to Bay k 8644 and CaCl2. The pinacidil doses producing a fifty percent decrease (ED50) of the atrial and ventricular contractile force were not significantly different from the respective pinacidil doses producing a fifty percent inhibition (ID50) of the positive inotropic responses to norepinephrine, Bay k 8644 and CaCl2. Ouabain (5 and 15 nmol) did not affect the decreases in atrial and ventricular contractile force in response to pinacidil. These results suggest that the K+ATP-channel activator pinacidil, unlike acetylcholine or adenosine, functionally attenuates increases in ventricular and atrial contractile force in the responses to norepinephrine and other cardiotonics due to shortening of the action potential duration induced by K+ATP-channel activation in the dog heart.

Mechanism-specific antiarrhythmic effects of the potassium channel activator levcromakalim against repolarization-dependent tachycardias

INTRODUCTION

The hypothesis that levcromakalim, a potassium channel (IK-ATP) activator with antihypertensive properties, has a mechanism-specific antiarrhythmic action against repolarization-dependent ventricular tachycardias (VTs) was tested in dogs.

METHODS AND RESULTS

A low dose of levcromakalim (0.01 mg/kg) was selected, which decreased blood pressure by 25% but had almost no electrophysiologic effect on AV nodal or ventricular conduction or effective refractory period. In dogs with chronic AV block, the antiarrhythmic action of this dose of levcromakalim was evaluated in three models of abnormal impulse formation: (1) d-sotalol (2 mg/kg) induced torsades de pointes VT, initiated by early afterdepolarizations (EADs), (2) sustained ouabain-induced VTs, which are dependent on delayed afterdepolarizations (DADs), and (3) VT occurring 24 hours after left anterior descending coronary artery occlusion, which are likely based on abnormal automaticity. Levcromakalim abolished d-sotalol induced U waves, ventricular ectopic beats, and self-terminating bouts of torsades de pointes. Induction of torsades de pointes by pacing was also completely prevented. The cycle length of the idioventricular rhythm, which was lengthened after d-sotalol from 1490 +/- 515 to 1700 +/- 610 msec (P < 0.05), remained similar after levcromakalim (1655 +/- 580 msec). The QT(U) duration, which was increased after d-sotalol from 410 +/- 55 to 550 +/- 40 msec (P < 0.05), normalized to 405 +/- 70 msec (P < 0.05). Levcromakalim did not suppress but rather enhanced ouabain-induced VT by decreasing the cycle length slightly from 315 +/- 35 to 290 +/- 35 msec (P < 0.05). Pretreatment with a beta blocker prevented this acceleration in rate. Finally, levcromakalim had no effect on VT 24 hours after infarction.

CONCLUSION

A low dose of levcromakalim has specific antiarrhythmic properties against repolarization-dependent arrhythmias, but it does not affect VTs based on other mechanisms of abnormal impulse formation.

Reversibility of electrophysiologic abnormalities of subendocardial Purkinje fibers induced by ischemia

INTRODUCTION

During the subacute phase of infarction in the canine heart, the subendocardial Purkinje fibers subtended by the infarct show depolarization greater than can be accounted for by the decrease in [K+]i, and generate abnormal action potentials and spontaneous rhythms due to abnormal automaticity. We have used pinacidil to hyperpolarize these fibers and evaluate the extent to which an increase in resting potential can normalize action potential generation.

METHODS AND RESULTS

Twenty-four hours after two-stage ligation of the canine left anterior descending coronary artery, preparations of subendocardial Purkinje fibers were studied in vitro by recording transmembrane potentials through standard microelectrodes and exposing the preparation to pinacidil and increases in [K+]o. Pinacidil increased resting potential to the estimated value of EK, abolished the abnormal automaticity, and restored action potentials of normal amplitude with normal values of Vmax. This effect often persisted after washout of pinacidil. Elevation of [K+]o from 4.0 to 20.0 mM slightly increased maximum diastolic potential, suggesting that the excess (over the change in EK) depolarization was caused by a decrease in gK1.

CONCLUSION

The ventricular arrhythmias seen during the subacute stage of infarction probably are caused by abnormal automaticity. Our findings support the conclusion that this abnormal automaticity arises in partially depolarized subendocardial Purkinje fibers. This loss of resting potential is due in large part to a decrease in gK1. Restoration of resting potential to the value of EK permits the Purkinje fibers to develop essentially normal action potentials. An agent capable of reversing the partial block of IK,1 thus might be an effective drug for some types of arrhythmias.