Ischaemia-induced Arrhythmias Research Articles

Mechanisms of ischaemia-induced arrhythmias in hypertrophic cardiomyopathy: a large-scale computational study.

Lethal arrhythmias in hypertrophic cardiomyopathy (HCM) are widely attributed to myocardial ischaemia and fibrosis. How these factors modulate arrhythmic risk remains largely unknown, especially as invasive mapping protocols are not routinely used in these patients. By leveraging multiscale digital twin technologies, we aim to investigate ischaemic mechanisms of increased arrhythmic risk in HCM. Computational models of human HCM cardiomyocytes, tissue, and ventricles were used to simulate outcomes of Phase 1A acute myocardial ischaemia. Cellular response predictions were validated with patch-clamp studies of human HCM cardiomyocytes (n = 12 cells, N = 5 patients). Ventricular simulations were informed by typical distributions of subendocardial/transmural ischaemia as analysed in perfusion scans (N = 28 patients). S1-S2 pacing protocols were used to quantify arrhythmic risk for scenarios in which regions of septal obstructive hypertrophy were affected by (i) ischaemia, (ii) ischaemia and impaired repolarization, and (iii) ischaemia, impaired repolarization, and diffuse fibrosis. HCM cardiomyocytes exhibited enhanced action potential and abnormal effective refractory period shortening to ischaemic insults. Analysis of ∼75 000 re-entry induction cases revealed that the abnormal HCM cellular response enabled establishment of arrhythmia at milder ischaemia than otherwise possible in healthy myocardium, due to larger refractoriness gradients that promoted conduction block. Arrhythmias were more easily sustained in transmural than subendocardial ischaemia. Mechanisms of ischaemia-fibrosis interaction were strongly electrophysiology dependent. Fibrosis enabled asymmetric re-entry patterns and break-up into sustained ventricular tachycardia. HCM ventricles exhibited an increased risk to non-sustained and sustained re-entry, largely dominated by an impaired cellular response and deleterious interactions with the diffuse fibrotic substrate.

Prolonged ursodeoxycholic acid administration reduces acute ischaemia-induced arrhythmias in adult rat hearts

Acute myocardial ischaemia and reperfusion (I–R) are major causes of ventricular arrhythmias in patients with a history of coronary artery disease. Ursodeoxycholic acid (UDCA) has previously been shown to be antiarrhythmic in fetal hearts. This study was performed to investigate if UDCA protects against ischaemia-induced and reperfusion-induced arrhythmias in the adult myocardium, and compares the effect of acute (perfusion only) versus prolonged (2 weeks pre-treatment plus perfusion) UDCA administration. Langendorff-perfused adult Sprague–Dawley rat hearts were subjected to acute regional ischaemia by ligation of the left anterior descending artery (10 min), followed by reperfusion (2 min), and arrhythmia incidence quantified. Prolonged UDCA administration reduced the incidence of acute ischaemia-induced arrhythmias (p = 0.028), with a reduction in number of ventricular ectopic beats during the ischaemic phase compared with acute treatment (10 ± 3 vs 58 ± 15, p = 0.036). No antiarrhythmic effect was observed in the acute UDCA administration group. Neither acute nor prolonged UDCA treatment altered the incidence of reperfusion arrhythmias. The antiarrhythmic effect of UDCA may be partially mediated by an increase in cardiac wavelength, due to the attenuation of conduction velocity slowing (p = 0.03), and the preservation of Connexin43 phosphorylation during acute ischaemia (p = 0.0027). The potential antiarrhythmic effects of prolonged UDCA administration merit further investigation.

Semaphorin 3a transfection into the left stellate ganglion reduces susceptibility to ventricular arrhythmias after myocardial infarction in rats.

Myocardial infarction (MI) induces neural remodelling of the left stellate ganglion (LSG), which may contribute to ischaemia-induced arrhythmias. The neural chemorepellent Semaphorin 3a (Sema3a) has been identified as a negative regulator of sympathetic innervation in the LSG and heart. We previously reported that overexpression of Sema3a in the border zone could reduce the arrhythmogenic effects of cardiac sympathetic hyperinnervation post-MI. This study investigated whether Sema3a overexpression within the LSG confers an antiarrhythmic effect after MI through decreasing extra- and intra-cardiac neural remodelling. Sprague-Dawley rats were subjected to MI, and randomly allocated to intra-LSG microinjection of either phosphate-buffered saline (PBS), adenovirus encoding green fluorescent protein (AdGFP), or adenovirus encoding Sema3a (AdSema3a). Sham-operated rats served as controls. Two weeks after infarction, MI-induced nerve sprouting and sympathetic hyperinnervation in the LSG and myocardium were significantly attenuated by intra-LSG injection with AdSema3a, as assessed by immunohistochemistry and western blot analysis of growth-associated protein 43 and tyrosine hydroxylase. This was also confirmed by sympathetic nerve function changes assessed by cardiac norepinephrine content. Additionally, intra-LSG injection with AdSema3a alleviated MI-induced accumulation of dephosphorylated connexin 43 in the infarct border zone. Furthermore, Sema3a overexpression in the LSG reduced the incidence of inducible ventricular tachyarrhythmia by programmed electrical stimulation post-MI, and arrhythmia scores were significantly lower in the AdSema3a group than in the PBS and AdGFP groups. Semaphorin 3a overexpression in the LSG ameliorates the inducibility of ventricular arrhythmias after MI, mainly through attenuation of neural remodelling within the cardiac-neuraxis.

Activation of cardiac M3 muscarinic acetylcholine receptors has cardioprotective effects against ischaemia‐induced arrhythmias

Increasing evidence indicates the important roles of M(3) muscarinic acetylcholine receptors (M(3) mAChR) in the regulation and maintenance of cardiac function and heart disease. In the present study, we investigated whether the M(3) mAChR mediates the cardioprotection against ischaemia-induced arrhythmias and the mechanisms involved. Myocardial ischaemia was established in Wistar rats by occlusion of the left anterior descending coronary artery. Rats were treated with choline chloride (an M(3) mAChR agonist; 10mg/kg, i.v.) 10min before occlusion. In addition, 4-diphenylacetoxy-N-methylpiperidine-methiodide (4-DAMP; 0.12μg/kg, i.v.) was administered 5min before choline in the 4-DAMP-treated group. Ischaemia-induced arrhythmias were evaluated in each group for a period of 1h after occlusion. After 24h occlusion, protein and mRNA levels of L-type Ca(2+) channels and the Na(+) /Ca(2+) exchanger (NCX) were determined. Ischaemia-induced arrhythmias following coronary artery occlusion were diminished by choline and this effect was reversed in the 4-DAMP-treated group. In vitro, the effects of myocardial ischaemia were simulated by incubating isolated ventricular cardiomyocytes with Tyrode's solution (pH6.8). Intracellular Ca(2+) overload was confirmed and this was decreased by choline. Furthermore, choline reduced the L-type Ca(2+) current (I(C) (a,) (L) ) compared with cardiomyocytes incubated in Tyrode's solution (pH6.8) alone. Choline reduced the 'ischaemia'-induced upregulated expression of L-type Ca(2+) channels and NCX at both the protein and mRNA level. Based on these results, choline has an obvious protective effect against ischaemia-induced arrhythmias that is mediated via activation of cardiac M(3) mAChR, which reduces Ca(2+) overload mediated by L-type Ca(2+) channels and the NCX.

Myocardial ischemia: Some historical notes

This brief historical review alludes to some early work of clinical and experimental myocardial ischemia, illustrated with facsimiles and passages quoted from the original publications. Briefly, the first experimental ligation of a coronary artery in dog was performed some 300 yr ago by Petri Chirac (1698). The systematic experimental study on the influence of interrupting coronary circulation may be said to have begun with Erichsen (1841), followed by the work of Panum (1862), who embolized the coronary arteries of a young dog with a mixture of tallow, wax, oil and lamp black. Bezold and Boyemann (1862) were the first to produce ventricular fibrillation by clamping a rabbit's left coronary artery (“coronaria magna”). Samuelson (1881) demonstrated the first “successful reperfusion” after a 4-min ligation of a coronary artery. Conheim and von Schulthess-Rechenberg (1881) introduced the first experimental model in order to produce “ischaemia-induced arrhythmias.” Clinically, the termangina pectoris was first used by Rougnon (1801) and again by Heberden (1816). The first description of atherosclerotic changes (“ossifications”) in the coronary arteries was reported by Morgagni (1761). Jenner and Perry (1801) associated angina pectoris, the new disease described by Rougnon, with the atherosclerotic changes described by Morgagni. Jenner had already diagnosed the condition in his friend John Hunter (1794). The first coronary occlusion correctly diagnosed at bedside and confirmed by autopsy was communicated by Hammer in Vienna (1878).

CSPE Young Investigator Award Session

CSPE Young Investigator Award Session

14 Diazoxide-induced antiarrhythmic protection in the non-diabetic and diabetic rat heart does not require activation of PI3-kinase/AKT

Mitochondrial KATP channels (mitoKATP) activation interacting with phosphatidylinositol 3-kinase (PI3K)/Akt has been suggested as a key element in preconditioning protection against lethal myocardial ischaemia/reperfusion injury in healthy heart,1 while its...

Chronic vagal nerve stimulation for the treatment of human heart failure: progress in translating a vision into reality

This editorial refers to ‘Chronic vagus nerve stimulation: a new and promising therapeutic approach for chronic heart failure’, by G.M. De Ferrari et al. doi:10.1093/eurheartj/ehq391 Heart failure resulting from cardiac ischaemia and/or tachycardia is accompanied by changes in autonomic tone that typically result in increased heart rate and a reduction in heart rate variability. In this setting, autonomic dysfunction is often accompanied by evidence of neurohormonal activation (increased plasma levels of norepinephrine, angiotensin II, and endothelin-1) as well as inflammatory biomarkers and cytokines (tumor necrosis factor-α and C-reactive protein), metabolic changes, and increased systemic and cardiac oxidant stress. As illustrated in Figure 1 , these processes promote cardiovascular structural and electrical remodelling, leading to dilation of the cardiac chambers, increased interstitial fibrosis, pump failure, and an increased susceptibility to lethal ventricular arrhythmia. Figure 1 Mechanisms whereby VNS can affect the substrate for heart failure and arrhythmia. Preclinical studies have been crucial to the elucidation of the role of the autonomic nervous system in cardiac function, and the dysfunction that accompanies ischaemia, infarction, and tachycardia-mediated cardiomyopathy. In 1984, Schwartz and colleagues showed that the autonomic nervous system had a critical role in the induction of ischaemia-induced lethal ventricular arrhythmias [ventricular tachycardia (VT)/ventricular fibrillation (VF)] in dogs with a healed myocardial infarction.1 This group showed that the sympathetic nervous system had a critical role in arrhythmogenesis, and that left stellectomy (which removes sympathetic input to the heart) essentially eliminated … *Corresponding author. Tel: +1 216 444 0820, Fax: +1 216 444 9155, Email: vanwagd{at}ccf.org

Selective inhibition of persistent sodium current by F 15845 prevents ischaemia‐induced arrhythmias

Myocardial ischaemia is associated with perturbations of electrophysiological profile of cardiac myocytes. The persistent sodium current (I(Nap)) is one of the major contributors to ischaemic arrhythmias and appears as an attractive therapeutic target. We investigated the effects of F 15845, a new anti-anginal drug on I(Nap) and in integrative models of I(Nap)-induced arrhythmias. Sodium current was investigated using patch clamp technique on wild-type and DeltaKPQ-mutated hNav1.5 channels transfected in HEK293 cells. Effects of F 15845 on action potentials (APs) were studied by the glass microelectrode technique and its anti-arrhythmic activities were investigated in ischaemia- and aconitine-induced arrhythmias in the rat. We demonstrated that F 15845 is a potent blocker of I(Nap) acting from the extracellular side of the channel. Blockade of I(Nap) was voltage dependent and characterized by an almost pure tonic block. F 15845 shortened AP from rabbit Purkinje fibres, confirming its lack of pro-arrhythmic activity, and prevented AP lengthening induced by the I(Nap) activator veratridine. F 15845 did not affect APs from rabbit atria and guinea pig papillary muscle where I(Nap) is not functional, confirming its inability to affect other cardiac ionic currents. F 15845 was effective at preventing fatal ventricular fibrillation and ventricular tachycardia during coronary ligation without modifying heart rate and blood pressure, and dose dependently increased the dose threshold of aconitine required to induce ventricular arrhythmias. F 15845, a novel anti-anginal drug targeting I(Nap), demonstrates new anti-arrhythmic properties which may be of therapeutic benefit against ischaemia-induced arrhythmias.

Acute administration of cannabidiol in vivo suppresses ischaemia‐induced cardiac arrhythmias and reduces infarct size when given at reperfusion

Cannabidiol (CBD) is a phytocannabinoid, with anti-apoptotic, anti-inflammatory and antioxidant effects and has recently been shown to exert a tissue sparing effect during chronic myocardial ischaemia and reperfusion (I/R). However, it is not known whether CBD is cardioprotective in the acute phase of I/R injury and the present studies tested this hypothesis. Male Sprague-Dawley rats received either vehicle or CBD (10 or 50 microg kg(-1) i.v.) 10 min before 30 min coronary artery occlusion or CBD (50 microg kg(-1) i.v.) 10 min before reperfusion (2 h). The appearance of ventricular arrhythmias during the ischaemic and immediate post-reperfusion periods were recorded and the hearts excised for infarct size determination and assessment of mast cell degranulation. Arterial blood was withdrawn at the end of the reperfusion period to assess platelet aggregation in response to collagen. CBD reduced both the total number of ischaemia-induced arrhythmias and infarct size when administered prior to ischaemia, an effect that was dose-dependent. Infarct size was also reduced when CBD was given prior to reperfusion. CBD (50 microg kg(-1) i.v.) given prior to ischaemia, but not at reperfusion, attenuated collagen-induced platelet aggregation compared with control, but had no effect on ischaemia-induced mast cell degranulation. This study demonstrates that CBD is cardioprotective in the acute phase of I/R by both reducing ventricular arrhythmias and attenuating infarct size. The anti-arrhythmic effect, but not the tissue sparing effect, may be mediated through an inhibitory effect on platelet activation.

Activation of cardiac muscarinic M3 receptors induces delayed cardioprotection by preserving phosphorylated connexin43 and up‐regulating cyclooxygenase‐2 expression

Activation of muscarinic M(3) mucarinic acetylcholine receptors (M(3)-mAChRs) has been previously shown to confer short-term cardioprotection against ischaemic injuries. However, it is not known whether activation of these receptors can provide delayed cardioprotection. Consequently, the present study was undertaken to investigate whether stimulation of M(3)-mAChRs can induce delayed preconditioning in rats, and to characterize the potential mechanism. Rats were pretreated (24 h), respectively, with M(3)-mAChRs agonist choline, M(3)-mAChRs antagonist 4-DAMP or M(2)-mAChRs antagonist methoctramine followed by the administration of choline. This was followed by 30 min of ischaemia and then 3 h of reperfusion. Ischaemia-induced arrhythmias and ischaemia-reperfusion (I/R)-induced infarction were determined. The phosphorylation status of connexin43 (Cx43) after 30 min ischaemia, and the expression level of Hsp70, cyclooxygenase-2 (COX-2) and iNOS effected by administration of choline were also measured. Compared to the control group, pretreatment with choline significantly decreased ischaemia-induced arrhythmias, reduced the total number of ventricular premature beats, the duration of ventricular tachycardia episodes and markedly reduced I/R-induced infarct size. Furthermore, choline attenuated ischaemia-induced dephosphorylation of Cx43, and up-regulated the expression of Hsp70 and COX-2. Administration of 4-DAMP abolished these changes, while methoctramine had no effect. Our results suggest that stimulation of M(3)-mAChRs with choline elicits delayed preconditioning, which we propose is the result of up-regulation of the expression of COX-2 and inhibition of the ischaemia-induced dephosphorylation of Cx43. Therefore, M(3)-mAChRs represent a promising target for rendering cardiomyocytes tolerant to ischaemic injury.

Mitochondrial KATPopening confers protection against lethal myocardial injury and ischaemia-induced arrhythmias in the rat heart via PI3K/Akt-dependent and -independent mechanismsThis article is one of a selection of papers published in a special issue on Advances in Cardiovascular Research.

Opening of mitochondrial KATP channels (mitoKATP) has been reported to underlie protection against ischaemia-reperfusion injury induced by ischaemic preconditioning (I-PC); however, the molecular mechanisms of its antiarrhythmic effect have not been fully elucidated. We explored the involvement of phosphatidylinositol 3-kinase (PI3K)/Akt in the PC-like effect of mitoKATP opener diazoxide with particular regard to its role in protection against ischaemia-induced arrhythmias. Langendorff-perfused rat hearts were subjected to 30 min LAD occlusion with or without a prior 15 min of perfusion with diazoxide (50 micromol/L) given either alone (D-PC) or in combination with the PI3K/Akt inhibitor wortmannin (100 nmol/L). In an additional protocol, ischaemia was followed by 2 h reperfusion for infarct size (IS) determination (tetrazolium staining). The total number of premature ventricular complexes over the whole period of ischaemia, episodes of ventricular tachycardia and its duration were significantly lower in the D-PC group than in the non-preconditioned controls (158 +/- 20, 2 +/- 0.6 and 4.6 +/- 1.8 s vs. 551 +/- 61, 11 +/- 2 and 42 +/- 8 s, respectively; p < 0.05), concomitant with a 62% reduction in the size of infarction. Wortmannin modified neither arrhythmogenesis nor IS in the non-preconditioned hearts. Bracketing of diazoxide with wortmannin did not reverse the antiarrhythmic protection, whereas the IS-limiting effect was blunted. The results indicate that in contrast with the positive role of PI3K/Akt in protection against lethal myocardial injury, its activity is not involved in suppression of ischaemia-induced arrhythmias conferred by mitoKATP opening in the rat heart.

The effect of antioxidant treatment and NOS inhibition on the incidence of ischaemia-induced arrhythmias in the diabetic rat heart

Contrary to clinical trials, experimental studies revealed that diabetes mellitus (DM) may initiate, besides increased myocardial vulnerability to ischemia-reperfusion injury (I/R) and pro/antioxidant dysbalance, development of adaptation leading to an enhanced tolerance to I/R. The aims were to characterize 1) susceptibility to ischemia-induced ventricular arrhythmias in the diabetic rat heart 2) its response to antioxidant N-acetylcysteine (NAC) and a NOS inhibitor L-NAME, and 3) the effect of DM on endogenous antioxidant systems. Seven days after streptozotocin injection (65 mg/kg, i.p.), Langendorff-perfused control (C) and DM hearts were subjected to 30-min occlusion of the LAD coronary artery with or without prior 15-min treatment with L-NAME (100 microM) or NAC (4 mM). Total number of ventricular premature beats (VPB), as well the total duration of ventricular tachycardia (VT) were reduced in the DM group (from 533+/-58 and 37.9+/-10.2 s to 224.3+/-52.6 and 19+/-13.5 s; P&lt;0.05). In contrast to the antiarrhythmic effects of L-NAME and NAC in controls group (VPB 290+/-56 and 74+/-36, respectively; P&lt;0.01 vs. control hearts), application of both drugs in the diabetics did not modify arrhythmogenesis (L-NAME: VPB 345+/-136, VT 25+/-13 s; NAC: VPB 207+/-50, VT 12+/-3.9 s; P&gt;0.05 vs non-treated diabetic hearts). Diabetic state was associated with significantly elevated levels of CoQ10 and CoQ9 (19.6+/-0.8 and 217.3+/-9.5 vs. 17.4+/- 0.5 and 185.0+/-5.0 nmol/g, respectively, in controls; P&lt;0.05), as well as alpha-tocopherol (38.6+/-0.7 vs. 31.5+/-2.1 nmol/g in controls; P&lt;0.01) in the myocardial tissue. It is concluded that early period of DM is associated with enhanced resistance to ischemia-induced arrhythmias. Diabetes mellitus might induce adaptive processes in the myocardium leading to lower susceptibility to antioxidant and L-NAME treatment.

A gender-independent proarrhythmic action of 17β-estradiol in anaesthetized rabbits

Women are at increased risk of having drug-induced arrhythmias such as torsade de pointes but less susceptible to arrhythmias associated with myocardial ischaemia. We have shown previously that 17beta-estradiol had greater antiarrhythmic activity in female rats than in male rats subject to myocardial ischaemia. The aim of this work was to investigate the effects of acute administration of 17beta-estradiol in both sexes in an established in vivo model of drug-induced arrhythmias. In alpha(1)-adrenoceptor-stimulated, pentobarbital-anaesthetized rabbits, 17beta-estradiol (100, 300 or 1000 ng/kg bolus followed by 10, 30 or 100 ng/kg/min infusion) tended to increase the incidence of torsade de pointes, induced by clofilium, in both sexes: from 50% in controls to 80%, 70% and 80% in females; from 40% in controls to 60%, 70% and 80% in males with increasing doses of 17beta-estradiol (n=10 per group). The total duration of all episodes of torsade de pointes was increased significantly by the highest dose of 17beta-estradiol compared to vehicle in both female and male rabbits: from 9+/-4 s to 93+/-26 s in females; from 26+/-14 s to 96+/-20 s in males. There were no baseline differences between the sexes in heart rate, QTc interval or epicardial monophasic action potential duration. The proarrhythmic effect of acute administration of 17beta-estradiol in the alpha(1)-adrenoceptor-stimulated anaesthetized rabbit model was independent of gender. This indicates that the underlying mechanism differs from that involved in the gender-selective reduction of ischaemia-induced arrhythmias by 17beta-estradiol.

Tedisamil and lidocaine enhance each other's antiarrhythmic activity against ischaemia-induced arrhythmias in rats.

1. Combinations of the action potential-widening drug tedisamil (Class III antiarrhythmic activity), and the inactivated state sodium channel blocker lidocaine (Class Ib antiarrhythmic activity) were assessed for antiarrhythmic actions in a rat model of ischaemia-induced arrhythmias and for electrophysiological actions in normal rat myocardial tissue. 2. Both tedisamil and lidocaine dose-dependently suppressed ischaemia-induced arrhythmias. The ED(50) values were 3.0+/-1.3 and 4.9+/-0.6 micro mol kg(-1) min(-1), respectively. 3. Combinations of the two drugs acted synergistically such that the ED(50) for tedisamil was reduced to 0.8+/-0.2 micro mol kg(-1) min(-1) in the presence of 2 micro mol kg(-1) min(-1) lidocaine. Similarly, the ED(50) for lidocaine was reduced to 0.7+/-0.2 micro mol kg(-1) min(-1) in the presence of 2 micro mol kg(-1) min(-1) tedisamil (both P<0.05). 4. In a separate series of experiments in which normal ventricular tissue was electrically stimulated, 2 micro mol kg(-1) min(-1) lidocaine produced a leftward shift in the dose-response curve for tedisamil's effect on effective refractory period (P<0.05). This dose of lidocaine had no effect on its own. These data indicate that the synergistic actions of combinations of tedisamil and lidocaine were mediated, at least in part, by extension of effective refractory period in normal myocardial tissue. 5. In contrast to the strategy of developing drugs that are selective for a single electrophysiological mechanism, the results of the present study suggest that effective antiarrhythmic drugs might be developed by optimising the combination of two complimentary electrophysiological mechanisms (i.e., action potential-prolonging activity and inactivated state sodium channel blockade).

Adrenomedullin protects against ischaemia-induced arrhythmias and injury in anaesthetised rats

Adrenomedulin (AM) is a novel peptide which has pronounced effects on the cardiovascular system (Kltamura e! al., 1993). The precise role of AM during myocardlal ischaemia is, however, unknown. The aim of this study was to determine if AM has a cardioprotective effect dunng myoeardial ischaemia. Pentobarbitone-anaesthetised rats were subjected to ligation of the left main coronary artery for 30 min followed by 120 min of reperfusion. Hearts were stained with Evan's blue and triphcnyltetrazolium chloride at the end of experiments for area at risk and infarct size measurement respectively. Animals were administered an i.v. bolus dose of Ah4 (0.5 ntn~lkg and 1 nmollkg) or saline 5 min preocclusion. Both doses of AM significantly reduced the number of ventricular ectoplc beats that occurred during ischaemia from 1339*116 to 521*116 and 520574 respectively (WO.05) and the incidences of VF from 66% to 26% and 27% respectively (IQO.05). Infarct size was significantly reduced from 34*3% to l&2% of area at risk in both AM treatment groups (WO.05). Prior to occlusion, AM (1 nmol/kg) induced a significant decrease in MABP (851% decrease, P<O.OS). These results show that AM exerts cardioprotection against Ischaemia-induced arrhythmias and myocardial injury.

Anti-arrhythmic and electrophysiological effects of the endothelin receptor antagonists, BQ-123 and PD161721

The effects of the endothelin ET A, (BQ-123) and endothelin ET A/B (PD161721) receptor antagonists were investigated on ischaemia-induced arrhythmias and on the maximum following frequency. The study was carried out in Langendorff perfused rat hearts subjected to coronary artery occlusion in which the severity of arrhythmias, coronary perfusion pressure and heart rate were measured. The % incidence of ischaemia-induced irreversible ventricular fibrillation (ventricular fibrillation) was reduced significantly from 58%, in control rat hearts, to 0% (at 10 −7 and 10 −6 M PD161721 and 10 −6 M BQ-123 P<0.05). Maximum following frequency was measured in guinea-pig isolated atria. In the presence of normal extracellular [K +], BQ-123 and PD161721, at 10 −6 M, significantly decreased the maximum following frequency from 9.0±0.7 to 7.2±0.4 and from 8.3±0.4 to 6.7±0.3 Hz, respectively ( P<0.05). These effects were not potentiated by raising the extracellular [K +] with the exception of 10 −9 M PD161721. In contrast, lignocaine's ability to reduce the maximum following frequency was greater in elevated (e.g. at 1.7×10 −4 M from 8.4±0.3 to 2.5±0.6 Hz) than in normal [K +] (from 9.0±0.3 to 4.9±0.5 Hz). In conclusion, both BQ-123 and PD161721 had an anti-fibrillatory effect in isolated rat hearts that may be due, at least in part, to an ability to reduce the maximum following frequency. This latter effect is unlikely to be due to Na + channel blockade since it was not markedly potentiated by elevation of extracellular [K +].

Contribution of peroxynitrite to the beneficial effects of preconditioning on ischaemia-induced arrhythmias in rat isolated hearts

We studied the effects of urate, a peroxynitrite scavenger, on ischaemia- and peroxynitrite-induced preconditioning in rat isolated hearts. Isolated hearts perfused with Krebs–Henseleit solution were preconditioned either by 3 min of coronary artery occlusion or by peroxynitrite administration (1 μM) for 3 min, followed by 10 min of reperfusion and 30 min of coronary artery occlusion. Both ischaemia and peroxynitrite produced a marked reduction in arrhythmias. Urate (1 mM) added to the perfusate 10 min prior to ischaemic preconditioning or peroxynitrite infusion and maintained until coronary artery occlusion, markedly reversed the beneficial effects in the ischaemic and peroxynitrite-treated groups. Urate administration in the peroxynitrite-treated group increased the incidence of ventricular tachycardia from 57% ( n=11) to 100% ( n=6) and total ventricular fibrillation from 0% ( n=0) to 44% ( n=4). Similarly, urate augmented the incidence of ventricular tachycardia from 47% ( n=8) to 85% ( n=6) in the ischaemic preconditioning group. On its own, urate did not affect the severity of cardiac arrhythmias. Peroxynitrite infusion caused a marked increase in the effluent nitrate levels, from 0.05±0.1 μM ( n=5) to 0.4±0.2 μM ( n=6), and urate significantly decreased these levels to 0.08±0.03 μM ( n=9). These results suggest that peroxynitrite at low concentrations contributes to the beneficial effects of preconditioning on ischaemia-induced arrhythmias in rat isolated hearts.

RSD1019 suppresses ischaemia-induced monophasic action potential shortening and arrhythmias in anaesthetized rabbits.

The electrophysiological actions of lidocaine, tedisamil and RSD1019 were assessed on normal and ischaemic cardiac tissue using monophasic action potentials (MAPs) recorded from the epicardium of anaesthetized rabbits. Drug effects on ischaemia-induced arrhythmias were assessed simultaneously in the same rabbits. Lidocaine, infused at 2.5, 5 and 10 micromol kg(-1) min(-1) i.v., accelerated and worsened the electrophysiological derangement caused by ischaemia, had profibrillatory actions and reduced the time to the occurrence of ventricular fibrillation (VF) relative to controls. Tedisamil, infused at 0.063, 0.125 and 0.25 micromol kg(-1) min(-1) i.v., prolonged MAP duration at 90% repolarization (MAPD(90%)) before induction of ischaemia in a dose-related manner; however, this effect was not maintained 5 min after induction of ischaemia. Tedisamil had no significant antiarrhythmic actions over the dose-range tested. RSD1019, infused at 2, 4 and 8 micromol kg(-1) min(-1) i.v., produced a small increase in MAPD(90%) before induction of ischaemia and only at the highest dose tested. In contrast to tedisamil, RSD1019 suppressed ischaemia-induced MAP shortening assessed 5 min after induction of ischaemia. This effect was dose-related. RSD1019 completely prevented ischaemia-induced tachyarrhythmias at the mid and highest infusion levels tested. The results of this study illustrate a pathologically targeted approach for preventing ischaemia-induced arrhythmias. Suppression of ischaemia-induced MAP shortening, demonstrated herein for RSD1019, represents a novel antifibrillatory approach.

Ischaemia selectivity confers efficacy for suppression of ischaemia-induced arrhythmias in rats

Eight novel and three reference antiarrhythmics were investigated in anaesthetised rats for antiarrhythmic actions, as well as for effects on the electrocardiogram (ECG) under normal and “simulated ischaemic” conditions. In rats subjected to coronary artery occlusion lidocaine, (±)- trans-[2-(4-morpholinyl)-cyclohexyl]naphthyl-1-acetate, RSD1000 and (±)- trans-[2-(4-morpholinyl)-cyclohexyl]-2-(1-naphthyl)propionate, RSD1030, (Group A) produced dose-related and complete antiarrhythmic protection. Group B compounds, such as (±)- trans- N-methyl- N-[2-(1-pyrrolidinyl)-cyclohexyl]-3, 4-dichlorocinnamamide, RSD995, produced complete antiarrhythmic protection but had aberrant dose–response curves. Group C compounds, such as quinidine and flecainide, failed to give full antiarrhythmic protection and had shallow dose–response curves. The potency of Group A compounds, but not Group B or C compounds, for ECG actions indicative of Na + channel blockade (prolongation of PR and QRS intervals) were significantly increased under “simulated ischaemic” conditions ([K +] 10 mM and pH 6.4) in isolated rat hearts. Thus, compounds with ischaemia-selective actions provided superior protection against ischaemia-induced arrhythmias in rats.