Inadequate ischaemia-selectivity limits the antiarrhythmic efficacy of mibefradil during regional ischaemia and reperfusion in the rat isolated perfused heart

OCT2013, an ischaemia-activated antiarrhythmic prodrug, devoid of the systemic side effects of lidocaine

BACKGROUND AND PURPOSE

Sudden cardiac death (SCD) caused by acute myocardial ischaemia and ventricular fibrillation (VF) is an unmet therapeutic need. Lidocaine suppresses ischaemia-induced VF, but its utility is limited by side effects and a narrow therapeutic index. Here, we characterise OCT2013, a putative ischaemia-activated prodrug of lidocaine.

EXPERIMENTAL APPROACH

The rat Langendorff-perfused isolated heart, anaesthetised rat and rat ventricular myocyte preparations were utilised in a series of blinded and randomised studies to investigate the antiarrhythmic effectiveness, adverse effects and mechanism of action of OCT2013, compared with lidocaine.

KEY RESULTS

In isolated hearts, OCT2013 and lidocaine prevented ischaemia-induced VF equi-effectively, but OCT2013 did not share lidocaine's adverse effects (PR widening, bradycardia and negative inotropy). In anaesthetised rats, i.v. OCT2013 and lidocaine suppressed VF and increased survival equi-effectively; OCT2013 had no effect on cardiac output even at 64 mg·kg^-1 i.v., whereas lidocaine reduced it even at 1 mg·kg^-1 . In adult rat ventricular myocytes, OCT2013 had no effect on Ca²⁺ handling, whereas lidocaine impaired it. In paced isolated hearts, lidocaine caused rate-dependent conduction slowing and block, whereas OCT2013 was inactive. However, during regional ischaemia, OCT2013 and lidocaine equi-effectively hastened conduction block. Chromatography and MS analysis revealed that OCT2013, detectable in normoxic OCT2013-perfused hearts, became undetectable during global ischaemia, with lidocaine becoming detectable.

CONCLUSIONS AND IMPLICATIONS

OCT2013 is inactive but is bio-reduced locally in ischaemic myocardium to lidocaine, acting as an ischaemia-activated and ischaemia-selective antiarrhythmic prodrug with a large therapeutic index, mimicking lidocaine's benefit without adversity.

Characterisation of mexiletine's translational therapeutic index for suppression of ischaemia-induced ventricular fibrillation in the rat isolated heart

The 'translational therapeutic index' (TTI) is a drug's ratio of nonclinical threshold dose (or concentration) for significant benefit versus threshold for adversity. In early nonclinical research, discovery and safety studies are normally undertaken separately. Our aim was to evaluate a novel integrated approach for generating a TTI for drugs intended for prevention of ischaemia-induced ventricular fibrillation (VF). We templated the current best available class 1b antiarrhythmic, mexiletine, using the rat Langendorff preparation. Mexiletine's beneficial effects on the incidence of VF caused by 120 min regional ischaemia were contrasted with its concurrent adverse effects (on several variables) in the same hearts, to generate a TTI. Mexiletine 0.1 and 0.5 µM had no adverse effects, but did not reduce VF incidence. Mexiletine 1 µM reduced VF incidence to 0% but had adverse effects on atrioventricular conduction and ventricular repolarization. Separate studies undertaken using an intraventricular balloon revealed no detrimental effects of mexiletine (1 and 5 µM) on mechanical function, or any benefit against reperfusion-related dysfunction. Mexiletine's TTI was found to be less than two, which accords with its clinical therapeutic index. Although non-cardiac adversity, identifiable from additional in vivo studies, may reduce the TTI further, it cannot increase it. Our experimental approach represents a useful early-stage integrated risk/benefit method that, when TTI is found to be low, would eliminate unsuitable class 1b drugs prior to next stage in vivo work, with mexiletine's TTI defining the gold standard that would need to be bettered.

A pharmacological characterization of electrocardiogram PR and QRS intervals in conscious telemetered rats

INTRODUCTION

The conscious telemetered rat is widely used as an early in vivo screening model for assessing the cardiovascular safety of novel pharmacological agents. The current study aimed to identify its utility in assessing electrocardiogram (ECG) PR and QRS interval changes.

METHOD

Male Han-Wistar rats (~250 g) were implanted with radio-telemetry devices for the recording of ECG and haemodynamic parameters. Animals (n = 4-8) were treated with single doses of calcium (nifedipine, diltiazem or verapamil; CCBs) or sodium channel blockers (quinidine or flecainide; SCBs) or their corresponding vehicles in an ascending dose design. Data was recorded continuously up to 24 h post-dose. Pharmacokinetic analysis of blood samples was performed to allow comparison of effects to published data in other species.

RESULTS

Of the CCBs, only diltiazem (300 mg/kg) prolonged the PR interval (49 ± 2 versus vehicle: 43 ± 1 ms), although this was not statistically significant (p = .11). QA interval decreased with nifedipine (30 ± 1 versus 24 ± 0 ms) and diltiazem (34 ± 1 versus 27 ± 1 ms) but increased with verapamil (30 ± 0 versus 37 ± 1 ms) demonstrating pharmacological activity of each agent. Both SCBs, caused statistically significant (p < .05) increases in both intervals - quinidine (100 mg/kg; PR: 50 ± 2 versus 43 ± 1 ms; QRS: 22 ± 2 versus 18 ± 1 ms) and flecainide (9 mg/kg; PR: 56 ± 1 versus 46 ± 1 ms; QRS: 27 ± 1 versus 21 ± 1 ms). Drug plasma exposure was confirmed in all animals.

DISCUSSION

At similar plasma concentrations to other species, the conscious telemetered rat demonstrates limited utility in assessing PR interval prolongation by CCBs, despite significant contractility effects being observed. However, results with SCBs demonstrate a potential application for evaluating drug-induced QRS prolongation.

Contractile function assessment by intraventricular balloon alters the ability of regional ischaemia to evoke ventricular fibrillation

BACKGROUND AND PURPOSE

In drug research using the rat Langendorff heart preparation, it is possible to study left ventricular (LV) contractility using an intraventricular balloon (IVB), and arrhythmogenesis during coronary ligation-induced regional ischaemia. Assessing both concurrently would halve animal requirements. We aimed to test the validity of this approach.

EXPERIMENTAL APPROACH

The electrocardiogram (ECG) and LV function (IVB) were recorded during regional ischaemia of different extents in a randomized and blinded study.

KEY RESULTS

IVB-induced proarrhythmia was anticipated, but in hearts with an ischaemic zone (IZ) made deliberately small, an inflated IVB reduced ischaemia-induced ventricular fibrillation (VF) incidence as a trend. Repeating studies in hearts with large IZs revealed the effect to be significant. There were no changes in QT interval or other variables that might explain the effect. Insertion of an IVB that was minimally inflated had no effect on any variable compared with 'no IVB' controls. The antiarrhythmic effect of verapamil (a positive control drug) was unaffected by IVB inflation. Removal of an inflated (but not a non-inflated) IVB caused a release of lactate commensurate with reperfusion of an endocardial/subendocardial layer of IVB-induced ischaemia. This was confirmed by intracellular (31) phosphorus ((31) P) nuclear magnetic resonance (NMR) spectroscopy.

CONCLUSIONS AND IMPLICATIONS

IVB inflation does not inhibit VF suppression by a standard drug, but it has profound antiarrhythmic effects of its own, likely to be due to inflation-induced localized ischaemia. This means rhythm and contractility cannot be assessed concurrently by this approach, with implications for drug discovery and safety assessment.

Ventricular cycle length irregularity affects the correlation between ventricular rate and coronary flow in isolated, Langendorff perfused guinea pig hearts

INTRODUCTION

Heart rate affects coronary flow, but the mechanism is complex. The relationship between rhythm and flow is unclear, especially in experimental settings used for determining drug actions. The present study examined whether ventricular irregularity influences coronary flow independently of heart rate.

METHODS

Guinea pig hearts were perfused (Langendorff mode) at constant pressure. Hypokalemic Krebs solution facilitated spontaneous development of arrhythmias. The ECG, left ventricular and perfusion pressures were recorded, and the coronary flow was measured. Beat-to-beat ventricular cycle length variability was quantified. Hearts were retrospectively allocated to arbitrary 'Low' or 'High' RR variability groups.

RESULTS

A positive linear correlation was found between mean ventricular rate and coronary flow. The slope of the regression line was significantly greater in the 'High' versus 'Low' RR variability group, with greater coronary flow values in the 'High' RR variability group in the physiological heart rate range. During regular rhythm, left ventricular pressure exceeded perfusion pressure and prevented coronary perfusion at peak systole. However, ventricular irregularity significantly increased the number of beats in which left ventricular pressure remained below perfusion pressure, facilitating coronary perfusion.

DISCUSSION

In isolated hearts, cycle length irregularity increases the slope of the positive linear correlation between mean ventricular rate and coronary flow via producing beats in which left ventricular pressure remains below perfusion pressure. This means that changes in rhythm have the capacity to influence coronary flow independently of heart rate in isolated hearts perfused at constant pressure, which should be noted in drug studies on arrhythmias performed in Langendorff hearts.

Absolute beat-to-beat variability and instability parameters of ECG intervals: biomarkers for predicting ischaemia-induced ventricular fibrillation

BACKGROUND AND PURPOSE

Predicting lethal arrhythmia liability from beat-to-beat variability and instability (BVI) of the ECG intervals is a useful technique in drug assessment. Most investigators use only arrhythmia-free ECGs for this. Recently, it was shown that drug-induced torsades de pointes (TdP) liability can be predicted more accurately from BVI measured irrespective of rhythm, even during arrhythmias (absolute BVI). The present study tested the broader applicability of this assessment by examining whether absolute BVI parameters predict another potential lethal arrhythmia, ischaemia-induced ventricular fibrillation (VF).

EXPERIMENTAL APPROACH

Langendorff-perfused rat hearts were subjected to regional ischaemia for 15 min. Absolute BVI parameters were derived from ECG intervals measured in 40 consecutive ventricular complexes (irrespective of rhythm) immediately preceding VF onset and compared with time-matched values in hearts not expressing VF.

KEY RESULTS

Increased frequency of non-sinus beats and 'R on T' arrhythmic beats, shortened mean RR and electrical diastolic intervals, and increased BVI of cycle length and repolarization predicted VF occurrence. Absolute BVI parameters that quantify variability of repolarization (e.g. 'short-term variability' of QT interval) had the best predictive power with high sensitivity and specificity. In contrast, VF was not predicted by any BVI parameter derived from the last arrhythmia-free interlude before VF.

CONCLUSIONS AND IMPLICATIONS

The novel absolute BVI parameters that predicted TdP in rabbit also predict ischaemia-induced VF in rat, indicating a diagnostic and mechanistic congruence. Repolarization inhomogeneity represents a pivotal biomarker of ischaemia-induced VF. The newly validated biomarkers could serve as surrogates for VF in pre-clinical drug investigations.

Feasibility of targeting ischaemia-related ventricular arrhythmias by mimicry of endogenous protection by endocannabinoids

BACKGROUND AND PURPOSE

The hypothesis that endocannabinoids protect hearts against ventricular fibrillation (VF) induced by myocardial ischaemia and reperfusion was examined, and the concept that cannabinoids may represent a new class of anti-VF drug was tested.

EXPERIMENTAL APPROACH

In rat isolated hearts (Langendorff perfusion), VF evoked by reperfusion after 60 min regional ischaemia is known to be exacerbated by inhibitors of endogenous protectants such as nitric oxide. This preparation was used to assay the effects of cannabinoid agonists and antagonists, and the protocols were varied to examine mechanisms.

KEY RESULTS

Reperfusion-induced VF was not facilitated by relatively selective CB1 (1 μM AM251) or CB2 (1 μM AM630) antagonists. VF evoked during early (30 min) acute ischaemia was also unaffected. However, AM251 significantly increased the incidence of VF and the duration of VF episodes occurring during the later stage of acute ischaemia (30-60 min). AM630 had no such effects. In a separate study, cannabinoid perfusion (anandamide or 2-arachidonoylglycerol, both 0.01-1 μM) failed to reduce VF incidence concentration-dependently during 30 min ischaemia. In all these studies, changes in ancillary variables (QT, PR, heart rate) were unrelated to changes in VF.

CONCLUSIONS AND IMPLICATIONS

Endocannabinoids are not endogenous anti-VF mediators during reperfusion, but may have a weak protective effect during the late stages of ischaemia, mediated via CB1 agonism. This does not suggest endocannabinoids are important endogenous protectants in these settings, or that CB1 (or CB2) receptors are useful novel targets for developing drugs for VF.

Stabilizing Microtubules Decreases Myocardial Ischaemia-Reperfusion Injury

Microtubules are important components of the cytoskeleton that forms the backbone of myocardial architecture, sustaining its form and size. This study investigated whether stabilizing microtubules with paclitaxel (0.1 or 1 μM) could decrease myocardial ischaemia-reperfusion injury, and reduce myocardial infarct size and the incidence of ischaemic ventricular arrhythmia. Isolated rat hearts were used, with arrhythmia induced by regional ischaemia and myocardial infarcts induced by ischaemia-reperfusion. In these ex vivo rat models, paclitaxel decreased myocardial ischaemia-reperfusion injury, significantly reducing the incidence and severity of ischaemic ventricular arrhythmia and significantly decreasing infarct size.

Taxol, a microtubule stabilizer, prevents ischemic ventricular arrhythmias in rats

Microtubule integrity is important in cardio-protection, and microtubule disruption has been implicated in the response to ischemia in cardiac myocytes. However, the effects of Taxol, a common microtubule stabilizer, are still unknown in ischemic ventricular arrhythmias. The arrhythmia model was established in isolated rat hearts by regional ischemia, and myocardial infarction model by ischemia/reperfusion. Microtubule structure was immunohistochemically measured. The potential mechanisms were studied by measuring reactive oxygen species (ROS), activities of oxidative enzymes, intracellular calcium concentration ([Ca(2+) ](i) ) and Ca(2+) transients by using fluorometric determination, spectrophotometric assays and Fura-2-AM and Fluo-3-AM, respectively. The expression and activity of sarcoplasmic reticulum Ca(2+)-ATPase (SERCA2a) was also examined using real-time polymerase chain reaction, Western blot and pyruvate/Nicotinamide adenine dinucleotide-coupled reaction. Our data showed that Taxol (0.1, 0.3 and 1 μM) effectively reduced the number of ventricular premature beats and the incidence and duration of ventricular tachycardia. The infarct size was also significantly reduced by Taxol (1 μM). At the same time, Taxol preserved the microtubule structure, increased the activity of mitochondrial electron transport chain complexes I and III, reduced ROS levels, decreased the rise in [Ca(2+)](i) and preserved the amplitude and decay times of Ca(2+) transients during ischemia. In addition, SERCA2a activity was preserved by Taxol during ischemia. In summary, Taxol prevents ischemic ventricular arrhythmias likely through ameliorating abnormal calcium homeostasis and decreasing the level of ROS. This study presents evidence that Taxol may be a potential novel therapy for ischemic ventricular arrhythmias.

Minocycline attenuates ischemia-induced ventricular arrhythmias in rats

Minocycline has been shown to protect against myocardial ischemia-reperfusion injury. This study investigated the effects of minocycline on ischemia-induced ventricular arrhythmias in rats. Anesthetized male rats were once treated with minocycline (45mg/kg, i.p.) 1h before ischemia in the absence and/or presence of 2-(4-morpholinyl)-8-phenyl-1(4H)-benzopyran-4-one hydrochloride (LY294002, 0.3mg/kg, i.v., a PI3K inhibitor) and 5-hydroxydecanoic acid [5-HD, 10mg/kg, i.v., a specific inhibitor of mitochondrial ATP-sensitive potassium (K(ATP)) channels] which were once injected 10min before ischemia and then subjected to ischemia for 30min. Ventricular arrhythmias were assessed. L-type Ca(2+) current was measured by the patch-clamp technique. During the 30-minute ischemia, minocycline significantly reduced the incidence of ventricular fibrillation (VF) (P<0.05). The duration of VT+VF, the number of VT+VF episodes and the severity of arrhythmias were all significantly reduced by minocycline compared to those in myocardial ischemia group (P<0.05 for all). Administration of LY294002 or 5-HD abolished the protective effects of minocycline on VF incidence, the duration of VT+VF, the number of VT+VF episodes and the severity of arrhythmias (P<0.05 for all). In addition, minocycline inhibited L-type Ca(2+) currents of normal myocardial cell membrane in a dose-dependent manner. This study suggested that minocycline could attenuate ischemia-induced ventricular arrhythmias in rats in which PI3K/Akt signaling pathway, mitochondrial K(ATP) channels and L-type Ca(2+) channels may be involved.

Druggable targets for sudden cardiac death prevention: lessons from the past and strategies for the future

Sudden cardiac death (SCD) is most commonly caused by ventricular fibrillation (VF). The single largest cohort of victims is the population with little or no prior overt heart disease. Effective prevention will require long-term prophylaxis by drugs in large numbers of people identified by risk factors. This means that safe as well as effective drugs are required. Drugs with overt effects on cardiac electrophysiology have failed in the clinic owing to poor effectiveness and/or adverse effects. This article examines possible new drug targets. We have focused on acute myocardial ischaemia as it is the most strikingly proarrhythmic pathology, and the most common cause of coronary artery disease-related VF and SCD according to inferences from epidemiology, drug trials and decades of animal research. To set the scene we have briefly explored drugs that have failed in the clinic in order to identify possible targets that have been overlooked or underexploited. We conclude that the best strategy is identification of pathology-specific targets that render drugs active only where and when their action is required.

Prevalent role of Akt and ERK activation in cardioprotective effect of Ca(2+) channel- and beta-adrenergic receptor blockers

We studied cardioprotective as well as Akt and extracellular signal-activated kinase (ERK) activating effect of a Ca(2+) antagonist and a beta-adrenergic receptor blocker during ischemia-reperfusion, and compared these properties of the substances with that of a poly(ADP-ribose) polymerase (PARP) inhibitor used as a positive control throughout the experiments. Langendorff-perfused isolated rat hearts were subjected to 25 min global ischemia followed by 45 min reperfusion, and recovery of energy metabolism as well as functional cardiac parameters were monitored. Although to varying extents, all substances improved recovery of creatine phosphate, ATP, intracellular pH, and reutilization of inorganic phosphate. These favorable changes were accompanied by improved recovery of heart function parameters and reduced infarct size. In addition and again to varying extents, all studied substances decreased oxidative damage (lipid peroxidation and protein oxidation), and activated Akt, glycogen synthase kinase (GSK)-3beta, and ERK1/2. Correlation between cardioprotective and kinase activating effectivity of the compounds proved to be statistically significant. Physiological significance of these kinase activations was established by demonstrating that inhibition of Akt by LY294002 and ERK1/2 by PD98059 compromised the cardioprotective effect of all the substances studied. In conclusion, we demonstrated for the first time that activation of phosphatidylinositol-3-kinase (PI-3K)-Akt and ERK2 pathways significantly contributed to cardioprotective effects of a Ca(2+) antagonist and a beta-adrenergic receptor blocker. Furthermore, we found a strong correlation between cardioprotective and kinase-activating potencies of the substances studied (Verapamil, Metoprolol and two PARP inhibitors), which indicated the potentiality of these kinases as drug-targets in the therapy of ischemic heart disease.

Na(+)/Ca(2+) exchanger inhibition exerts a positive inotropic effect in the rat heart, but fails to influence the contractility of the rabbit heart

BACKGROUND AND PURPOSE

The Na(+)/Ca(2+) exchanger (NCX) may play a key role in myocardial contractility. The operation of the NCX is affected by the action potential (AP) configuration and the intracellular Na(+) concentration. This study examined the effect of selective NCX inhibition by 0.1, 0.3 and 1.0 microM SEA0400 on the myocardial contractility in the setting of different AP configurations and different intracellular Na(+) concentrations in rabbit and rat hearts.

EXPERIMENTAL APPROACH

The concentration-dependent effects of SEA0400 on I(Na/Ca) were studied in rat and rabbit ventricular cardiomyocytes using a patch clamp technique. Starling curves were constructed for isolated, Langendorff-perfused rat and rabbit hearts. The cardiac sarcolemmal NCX protein densities of both species were compared by immunohistochemistry.

KEY RESULTS

SEA0400 inhibited I(Na/Ca) with similar efficacy in the two species; there was no difference between the inhibitions of the forward or reverse mode of the NCX in either species. SEA0400 increased the systolic and the developed pressure in the rat heart in a concentration-dependent manner, for example, 1.0 microM SEA0400 increased the maximum systolic pressures by 12% relative to the control, whereas it failed to alter the contractility in the rabbit heart. No interspecies difference was found in the cardiac sarcolemmal NCX protein densities.

CONCLUSIONS AND IMPLICATIONS

NCX inhibition exerted a positive inotropic effect in the rat heart, but it did not influence the contractility of the rabbit heart. This implies that the AP configuration and the intracellular Na(+) concentration may play an important role in the contractility response to NCX inhibition.

Nupafant, a PAF-antagonist prototype for suppression of ventricular fibrillation without liability for QT prolongation?

BACKGROUND AND PURPOSE

PAF antagonists inhibit ischaemia-induced ventricular fibrillation (VF) in animals. However, unfavourable ancillary actions (on QT interval and coronary flow) have been reported with the PAF antagonist, BN-50739. If these are class actions, they would preclude development of PAF antagonists as novel anti-VF drugs. Our purpose was to examine this proposition using the hitherto untested PAF antagonist, nupafant.

EXPERIMENTAL APPROACH

Two rat heart preparations (Langendorff and 'dual coronary' perfusion) were used to assay nupafant's effects on ischaemia-induced VF, coronary flow and QT interval, and to test for the site-selectivity necessary if any effects on VF are caused by PAF antagonism.

KEY RESULTS

Global (whole-heart) delivery of 10 microM nupafant, reduced the incidence of ischaemia-induced VF and widened QT interval without affecting coronary flow. Importantly, lower concentrations (0.1 and 1 microM) had no effect on VF, yet widened QT almost identically to 10 microM nupafant. When nupafant was delivered selectively to (and entrapped within) the involved region it partially protected against VF (P<0.05). This occurred without change in QT interval. Selective nupafant delivery to the uninvolved region was without effect.

CONCLUSIONS AND IMPLICATIONS

Nupafant protects against ischaemia-induced VF primarily by site-selective actions in the ischaemic region but, unlike BN-50739, the effect is unrelated to its QT widening action, and is not compromised by any effect on coronary flow. This establishes proof of concept that VF suppression by PAF antagonism need not invariably be associated with QT prolongation or vasodilatation, justifying further development of this drug class.

Spectral and correlation analyses of the verapamil-induced conversion of ventricular fibrillation to tachycardia in isolated rat hearts

Ventricular tachycardia (VT) is considered to be the most common precursor of ventricular fibrillation (VF). However, the mechanisms underlying the transition from VT to VF remain unclear. Here, we investigated whether and how perfusion of the heart with verapamil, a blocker of L-type calcium channels, changed the macro-dynamics of the heart between VT and VF. The experiments were performed with Langendorff perfused isolated rat hearts, in which left ventricular pressure and left ventricular cardiomyogram were measured. Sustained VT or VF was induced by burst pacing of the left ventricular muscles. During sustained VF, verapamil perfusion resulted in the conversion of VF to VT. A cross-correlation analysis between left ventricular cardiomyogram and left ventricular pressure revealed that the correlation coefficient was small during VF, but became larger during VT. This study showed that inactivation of L-type Ca(2+) channels occurred during verapamil-induced conversion of pacing-induced sustained VF to VT, and characterized the changes in macro-dynamics of the heart associated with the transition.

Mibefradil is more effective than verapamil for restoring post-ischemic function of isolated hearts of guinea pigs with acute renal failure

The deleterious intracellular Ca(2+) overload in the ischemic-reperfusion injury of the heart can be even more expressed in subjects with acute renal failure in whom maintenance of intracellular Ca(2+) has already been disturbed in normoxia. To study the influence of acute renal failure in ischemic-reperfusion injury on the heart, we used isolated Langendorff's hearts of guinea pigs with gentamicin-induced acute renal failure. We examined arrhythmias, heart contractility and myocardial cell damage during reperfusion. Two specific Ca(2+) channel antagonists, mibefradil (0.1 and 1 microM) and verapamil (0.1 microM), were used to test the possible involvement of T-type and L-type Ca(2+) channels in these processes. We exposed hearts to 50 min of zero-flow global ischemia and 60 min of reperfusion. During reperfusion, unrecoverable ventricular fibrillation appeared more often in hearts of animals with acute renal failure than in control hearts (80% vs. 0%, respectively). Mibefradil, but not verapamil, applied either pre- or post-ischemically, terminated ventricular fibrillation in all hearts of animals with acute renal failure. Mibefradil (0.1 microM only) improved contractility in hearts of animals with acute renal failure during reperfusion by 30%. During reperfusion, lactate dehydrogenase (LDH) release rate increased less in hearts of guinea pigs with acute renal failure than in control hearts and only verapamil decreased it additionally. Thus, our results suggest a more important role of T- than of L-type Ca(2+) channels in ischemic-reperfusion injury in isolated guinea pig hearts with acute renal failure.

Ruthenium red-induced transition from ventricular fibrillation to tachycardia in isolated rat hearts:

INTRODUCTION

Ventricular tachycardia (VT) is considered to be the most common precursor of ventricular fibrillation (VF) and sudden cardiac death. However, the mechanisms underlying the transition from VT to VF remain unclear despite more than a century of study. Here, we investigated whether perfusion of the heart with blockers of mitochondrial Ca(2+) uniporter changed the macrodynamics of the heart between VT and VF.

METHODS

The experiments were performed using Langendorff perfused isolated rat hearts in which left ventricular pressure (LVP) and left ventricular cardiomyogram (LVCMG) were measured. Sustained VT or VF was induced by burst pacing of the left ventricular muscles.

RESULTS

During pacing-induced sustained VF, perfusion of the heart with ruthenium red (RR) or Ru 360, blockers of mitochondrial Ca(2+) uniporter, resulted in the reversible conversion of VF to VT. In contrast, during pacing-induced sustained VT, perfusion of the heart with spermine, an activator of mitochondrial Ca(2+) uptake, resulted in the reversible conversion of VT to VF, and the effect was antagonized by cotreatment with RR. In addition, RR-induced conversion of VF to VT was antagonized by cotreatment with S(-)-Bay K8644 (Bay K), an activator of L-type Ca(2+) channels, suggesting that the inactivation of L-type Ca(2+) channels was responsible for the RR-induced effect on the macrodynamics of hearts. In fact, perfusion with verapamil, an antagonist of L-type Ca(2+) channels, during pacing-induced sustained VF, resulted in the conversion of VF to VT.

CONCLUSION

This study demonstrated that perfusion of isolated rat hearts with blockers of Ca(2+) uptake by mitochondria resulted in the reversible conversion of pacing-induced sustained VF to VT, suggesting that changes in mitochondrial Ca(2+) uptake were possibly involved in the transition between VT and VF.

Does QT Widening in the Langendorff-perfused Rat Heart Represent the Effect of Repolarization Delay or Conduction Slowing?

It has been suggested that class I antiarrhythmic drugs and ischemia can widen the QT interval in the Langendorff-perfused rat heart preparation as a consequence of slowed ventricular conduction. If this were so, it would undermine the clinical relevance of the preparation and its effectiveness as an antiarrhythmic bioassay. To test this, the authors determined whether three different class I drugs could prolong QT in the preparation and whether this effect was augmented by ischemia and elevation of the potassium (K+) content of the perfusion solution. Baseline drug-free QT intervals correlated inversely with the K+ content (3 microM vs. 5 mM). QT intervals widened during the first 3-5 minutes of ischemia (P < 0.05), then returned gradually to baseline. Lidocaine (3.88 microM and 12.93 microM) had no effect on the QT interval before or during ischemia, whereas quinidine (7.90 microM but not 0.79 microM) and flecainide (1.48 microM but not 0.74 microM) caused QT widening before and during ischemia (P < 0.05). Elevating perfusion solution K+ content from 3 microM to 5 mM reduced the QT-widening effects of quinidine and flecainide (P < 0.05). Because lidocaine, a relatively selective sodium (Na+) channel blocker, failed to widen QT interval whereas quinidine and flecainide (combined Na+ and K+ channel blockers) did so, and because K+ elevation reduced rather than potentiated the drug-induced QT widening, it is unlikely that Na+ channel blockade and conduction slowing play any role in ischemia- or class I drug-induced QT widening in this model.