Evidence for gender differences in electrophysiological properties of canine Purkinje fibres

Beneficial Effect of Calcium Treatment for Hyperkalemia Is Not Due to "Membrane Stabilization"

OBJECTIVES

Hyperkalemia is a common life-threatening condition causing severe electrophysiologic derangements and arrhythmias. The beneficial effects of calcium (Ca 2+ ) treatment for hyperkalemia have been attributed to "membrane stabilization," by restoration of resting membrane potential (RMP). However, the underlying mechanisms remain poorly understood. Our objective was to investigate the mechanisms underlying adverse electrophysiologic effects of hyperkalemia and the therapeutic effects of Ca 2+ treatment.

DESIGN

Controlled experimental trial.

SETTING

Laboratory investigation.

SUBJECTS

Canine myocytes and tissue preparations.

INTERVENTIONS AND MEASUREMENTS

Optical action potentials and volume averaged electrocardiograms were recorded from the transmural wall of ventricular wedge preparations ( n = 7) at baseline (4 mM potassium), hyperkalemia (8-12 mM), and hyperkalemia + Ca 2+ (3.6 mM). Isolated myocytes were studied during hyperkalemia (8 mM) and after Ca 2+ treatment (6 mM) to determine cellular RMP.

MAIN RESULTS

Hyperkalemia markedly slowed conduction velocity (CV, by 67% ± 7%; p < 0.001) and homogeneously shortened action potential duration (APD, by 20% ± 10%; p < 0.002). In all preparations, this resulted in QRS widening and the "sine wave" pattern observed in severe hyperkalemia. Ca 2+ treatment restored CV (increase by 44% ± 18%; p < 0.02), resulting in narrowing of the QRS and normalization of the electrocardiogram, but did not restore APD. RMP was significantly elevated by hyperkalemia; however, it was not restored with Ca 2+ treatment suggesting a mechanism unrelated to "membrane stabilization." In addition, the effect of Ca 2+ was attenuated during L-type Ca 2+ channel blockade, suggesting a mechanism related to Ca 2+ -dependent (rather than normally sodium-dependent) conduction.

CONCLUSIONS

These data suggest that Ca 2+ treatment for hyperkalemia restores conduction through Ca 2+ -dependent propagation, rather than restoration of membrane potential or "membrane stabilization." Our findings provide a mechanistic rationale for Ca 2+ treatment when hyperkalemia produces abnormalities of conduction (i.e., QRS prolongation).

Sex and racial disparities in catheter ablation

Sex and racial disparities in the presentation, diagnosis, and management of cardiac arrhythmias are recognized. Sex-specific differences in electrophysiological parameters are well known and are predominantly related to differences in ion channel expression and the influence of sex hormones. However, the relationship between hormonal or racial influence and arrhythmia mechanisms, presentation, and management needs to be better defined. Women and racial and ethnic groups are less likely to undergo catheter ablation procedures for treatment of cardiac arrhythmias. Underrepresentation of women and racial/ethnic groups in clinical trials has resulted in significant knowledge gaps. Whether sex and racial disparities in arrhythmia management reflect barriers in access to care, physician bias, patient values, and preferences or other factors requires further study.

Comparative Efficacy of Dofetilide Versus Amiodarone in Patients With Atrial Fibrillation

OBJECTIVES

The study's goal was to compare the efficacy and safety of dofetilide (DOF) versus amiodarone (AMIO) in patients with atrial fibrillation (AF).

BACKGROUND

Comparative efficacy of DOF versus AMIO in patients with AF has not been well established. In addition, proarrhythmia has been a concern with DOF therapy.

METHODS

Rhythm control was attempted by using DOF in 657 consecutive patients (mean age 72 ± 9 years; 35% women) with AF (n = 528) or atrial flutter and AF (n = 129) between January 2014 and December 2018.

RESULTS

DOF was successfully initiated in 573 (87%) of 657 patients, including 510 (89%) with persistent AF and 63 (11%) with paroxysmal AF. During a mean follow-up of 19 ± 7 months, sinus rhythm was maintained in 361 (63%) of the 573 DOF-treated patients. At 12 months, patients on DOF had a similar likelihood of experiencing recurrent atrial arrhythmias compared with the 2,476 consecutive patients treated with AMIO for rhythm control during the study period (37% vs. 39%; p = 0.56). The efficacy of DOF and AMIO was also similar in specific subgroups of patients, including patients >75 years of age, with a low left ventricular ejection fraction, obesity, renal insufficiency, and prior catheter ablation for AF. Among patients with atypical atrial flutter, likelihood of recurrent atrial flutter was similar between the DOF (43 of 108 [40%]) and AMIO (211 of 555 [38%]; p = 0.69) groups.

CONCLUSIONS

When properly initiated and monitored, DOF has efficacy comparable to that of amiodarone for rhythm control in patients with AF.

Sex-Related Differences in Drug-Induced QT Prolongation and Torsades de Pointes: A New Model System with Human iPSC-CMs

Numerous drugs have the potential to prolong the QT interval and may cause accidental cardiac arrest (torsades de pointes [TdP]). Women are at a higher risk than men for experiencing drug-induced TdP. Due to the lack of appropriate tools, few studies have investigated whether genetic differences between men and women have any effects on drug-induced proarrhythmia. Sex hormones are believed to play a predominant role in the induction of TdP. Recently, progress in induced pluripotent stem cell (iPSC) technologies has made it possible to utilize human iPSC-derived cardiomyocytes (hiPSC-CMs) to investigate the influence of both genetics and sex hormones on cardiac ion channel gene expression and cardiomyocyte function. In this study, we investigated genetic and hormonal effects on sex differences of drug-induced QT prolongation and TdP with hiPSC-CMs from healthy male and female donors. We found that despite batch variations in beating rates and field potential durations (FPD), female-derived hiPSC-CMs showed steeper slopes of FPD to interspike interval ratios and were more sensitive to IKr blocker-induced FPD prolongation. 17β-estradiol increased FPD and 5α-dihydrotestosterone shortened FPD, but the addition of sex hormones had limited effect on the responses of hiPSC-CMs to IKr blockades. The differential expression of KCNE1 gene and reduced repolarization reserve in female-derived hiPSC-CMs compared with male-derived hiPSC-CMs may partially explain why females are more susceptible to proarrhythmias. Human iPSC-CMs can be a useful new model to study mechanisms of sex differences in cardiomyocyte repolarization processes and aid in the prediction of drug-induced proarrhythmias in both men and women.

Molecular Pathophysiology of Congenital Long QT Syndrome

Ion channels represent the molecular entities that give rise to the cardiac action potential, the fundamental cellular electrical event in the heart. The concerted function of these channels leads to normal cyclical excitation and resultant contraction of cardiac muscle. Research into cardiac ion channel regulation and mutations that underlie disease pathogenesis has greatly enhanced our knowledge of the causes and clinical management of cardiac arrhythmia. Here we review the molecular determinants, pathogenesis, and pharmacology of congenital Long QT Syndrome. We examine mechanisms of dysfunction associated with three critical cardiac currents that comprise the majority of congenital Long QT Syndrome cases: 1) IKs, the slow delayed rectifier current; 2) IKr, the rapid delayed rectifier current; and 3) INa, the voltage-dependent sodium current. Less common subtypes of congenital Long QT Syndrome affect other cardiac ionic currents that contribute to the dynamic nature of cardiac electrophysiology. Through the study of mutations that cause congenital Long QT Syndrome, the scientific community has advanced understanding of ion channel structure-function relationships, physiology, and pharmacological response to clinically employed and experimental pharmacological agents. Our understanding of congenital Long QT Syndrome continues to evolve rapidly and with great benefits: genotype-driven clinical management of the disease has improved patient care as precision medicine becomes even more a reality.

Sex differences in the mechanisms underlying long QT syndrome

Sexual dimorphism is a well-established phenomenon, but its degree varies tremendously among species. Since the early days of Einthoven's development of the three-lead galvanometer ECG, we have known there are marked differences in QT intervals of men and women. It required over a century to appreciate the profound implications of sex-based electrophysiological differences in QT interval on the panoply of sex differences with respect to arrhythmia risk, drug sensitivity, and treatment modalities. Little is known about the fundamental mechanism responsible for sex differences in electrical substrate of the human heart, in large part due to the lack of tissue availability. Animal models are an important research tool, but species differences in the sexual dimorphism of the QT interval, the ionic currents underlying the cardiac repolarization, and effects of sex steroids make it difficult to interpolate animal to human sex differences. In addition, in some species, different strains of the same animal model yield conflicting data. Each model has its strengths, such as ease of genetic manipulation in mice or size in dogs. However, many animals do not reproduce the sexual dimorphism of QT seen in humans. To match sex linked prolongation of QT interval and arrhythmogenic phenotype, the current data suggest that the rabbit may be best suited to provide insight into sex differences in humans. In the future, emerging technologies such as induced pluripotent stem cell derived cardiac myocyte systems may offer the opportunity to study sex differences in a controlled hormonal situation in the context of a sex specific human model system.

Structure, function and clinical relevance of the cardiac conduction system, including the atrioventricular ring and outflow tract tissues

It is now over 100years since the discovery of the cardiac conduction system, consisting of three main parts, the sinus node, the atrioventricular node and the His-Purkinje system. The system is vital for the initiation and coordination of the heartbeat. Over the last decade, immense strides have been made in our understanding of the cardiac conduction system and these recent developments are reviewed here. It has been shown that the system has a unique embryological origin, distinct from that of the working myocardium, and is more extensive than originally thought with additional structures: atrioventricular rings, a third node (so called retroaortic node) and pulmonary and aortic sleeves. It has been shown that the expression of ion channels, intracellular Ca(2+)-handling proteins and gap junction channels in the system is specialised (different from that in the ordinary working myocardium), but appropriate to explain the functioning of the system, although there is continued debate concerning the ionic basis of pacemaking. We are beginning to understand the mechanisms (fibrosis and remodelling of ion channels and related proteins) responsible for dysfunction of the system (bradycardia, heart block and bundle branch block) associated with atrial fibrillation and heart failure and even athletic training. Equally, we are beginning to appreciate how naturally occurring mutations in ion channels cause congenital cardiac conduction system dysfunction. Finally, current therapies, the status of a new therapeutic strategy (use of a specific heart rate lowering drug) and a potential new therapeutic strategy (biopacemaking) are reviewed.

A systematic approach for uptake of evidence on sex-specific issues in guidelines--a pilot study

RATIONALE, AIMS AND OBJECTIVES

Increasing evidence indicates that sex-specific issues may have impact on prevention, diagnosis, or treatment. These issues are not systematically considered during the development of Dutch clinical practice guidelines. The aim of this study is to identify how members of guideline development groups discuss sex-specific evidence, and whether and how the outcomes of these discussions are reflected in the guideline.

METHODS

Six guideline development committees (GDCs) were studied. Each committee was supported by a staff member from the guideline organization who was trained and received feedback to facilitate uptake of evidence on sex differences in the process of guideline development. Non-participant observation and transcription of audio recordings from 22 GDC meetings were performed. Content analysis of meeting transcripts and guidelines were studied to analyse characteristics of discussion episodes on sex-specific research data-based issues (subject matter, initiator and group approach towards the topic and themes) and whether or not conclusions on evidence were reflected in the final guideline text.

RESULTS

Of the 87 identified discussion episodes, 68 dealt with sex-specific research evidence potentially relevant to guidelines. Respectively 51%, 28% and 21% of the latter episodes were initiated by committee members, staff members and chairpersons. Group approaches towards the subject matter were generally positive. Data from 60% of those episodes were reflected in the final guideline text. Sex-specific data on reproductive issues were more often discussed and reflected in guideline texts than data on other health issues. Discussion episodes on sex-specific evidence initiated by chairpersons were most often reflected in the guidelines.

CONCLUSIONS

This pilot study indicates that GDCs regularly focused on sex-specific issues. The participation of a trained staff member contributed to this.

Dog left ventricular midmyocardial myocytes for assessment of drug-induced delayed repolarization: short-term variability and proarrhythmic potential

BACKGROUND AND PURPOSE

Evaluation of the potential for delayed ventricular repolarization and proarrhythmia by new drugs is essential. We investigated if dog left ventricular midmyocardial myocytes (LVMMs) that can be used as a preclinical model to assess drug effects on action potential duration (APD) and whether in these cells, short-term variability (STV) or triangulation could predict proarrhythmic potential.

EXPERIMENTAL APPROACH

Beagle LVMMs and Purkinje fibres (PFs) were used to record APs. Effects of six reference drugs were assessed on APD at 50% (APD(50)) and 90% (APD(90)) of repolarization, STV(APD), triangulation (ratio APD(90)/APD(50)) and incidence of early afterdepolarizations (EADs) at 1 and 0.5 Hz.

KEY RESULTS

LVMMs provided stable recordings of AP, which were not affected by four sequential additions of dimethyl sulphoxide. Effects of dofetilide, d-sotalol, cisapride, pinacidil and diltiazem, but not of terfenadine, on APD in LVMMs were found to be comparable with those recorded in PFs. LVMMs, but not PFs, exhibited a proarrhythmic response to I(Kr) blockers. Incidence of EADs was not related to differences in AP prolongation or triangulation, but corresponded to beat-to-beat variability of repolarization, here quantified as STV of APD.

CONCLUSIONS AND IMPLICATIONS

LVMMs provide a suitable preclinical model to assess the effects of new drugs on APD and also yield additional information about putative indicators of proarrhythmia that add value to an integrated QT/TdP risk assessment. Our findings support the concept that increased STV(APD) may predict drug-induced proarrhythmia.

Validation of a voltage-sensitive dye (di-4-ANEPPS)-based method for assessing drug-induced delayed repolarisation in beagle dog left ventricular midmyocardial myocytes

INTRODUCTION

Evaluation of drug candidates in in-vitro assays of action potential duration (APD) is one component of preclinical safety assessment. Current assays are limited by technically-demanding, time-consuming electrophysiological methods. This study aimed to assess whether a voltage-sensitive dye-based assay could be used instead.

METHODS

Optical APs were recorded using di-4-ANEPPS in electrically field stimulated beagle left ventricular midmyocardial myocytes (LVMMs). Pharmacological properties of di-4-ANEPPS on the main cardiac ion channels that shape the ventricular AP were investigated using IonWorks and conventional electrophysiology. Effects of 9 reference drugs (dofetilide, E4031, D-sotalol, ATXII, cisapride, terfenadine, alfuzosin, diltiazem and pinacidil) with known APD-modulating effects were assessed on optically measured APD at 1 Hz.

RESULTS

Under optimum conditions, 0.1 microM di-4-ANEPPS could be used to monitor APs paced at 1 Hz during nine, 5 s exposures without altering APD. di-4-ANEPPS had no effect on either hI(ERG), hI(Na), hI(Ks) and hI(to) currents in transfected CHO cells (up to 10 microM) or I(Ca,L) current in LVMMs (at 16 microM). di-4-ANEPPS had no effect on APs recorded with microelectrodes at 1 or 0.5 Hz over a period of 30 min di-4-ANEPPS displayed the sensitivity to record changes in optically measured APD in response to altered pacing frequencies and sequential vehicle additions did not affect the optically measured APD. APD data obtained with 9 reference drugs were as expected except (i) D-sotalol-induced increases in duration were smaller than those caused by other I(Kr) blockers and (ii) increases in APD were not detected using low concentrations of terfenadine.

DISCUSSION

Early in drug discovery, the di-4-ANEPPS-based method can reliably be used to assess drug effects on APD as part of a cardiac risk assessment strategy.

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.

Towards automation of a valuable preclinical cardiac safety pharmacology assay: Evaluation of the effects of cardiac ion channel blockers on cardiac repolarisation in vitro

INTRODUCTION

Purkinje fibre repolarisation assays are valuable tools for identifying compounds which affect cardiac ion channels. The throughput of compound testing in this assay is low therefore we designed a novel recording system to improve screening and animal tissue usage efficiencies.

METHODS

The system was used to evaluate compounds using standard sharp microelectrode techniques. Animal tissue usage efficiencies were quantified by adding up the total number of Purkinje fibres from which recordings were attempted and dividing this by the number of experimental data sets generated, to arrive at a 'fibres per data set' ratio. Test compounds were dofetilide (3 x 10(-10) to 10(-8) M), cisapride (10(-8) to 3 x 10(-7) M), terodiline (10(-6) to 3 x 10(-5) M) and verapamil (3 x 10(-7) to 10(-5) M).

RESULTS

Using the novel modified system, 21 data sets were generated from 29 fibres, compared to 24 data sets from 41 fibres using the conventional manual recording system, demonstrating a 24% improvement in the efficiency of animal tissue usage. Comparing data from the manual and modified systems revealed differences in absolute values for all parameters including APD90 (308.73 +/- 9.97 ms, n = 24, compared to 275.27 +/- 8.25 ms, n = 21, respectively; P < 0.05). Differences in the magnitude of changes in action potential parameters between the systems were also evident for all compounds including terodiline (1 x 10(-5) M) which caused a -27.1 +/- 16.5% reduction in APD50 in the manual system, compared to a - 55.2 +/- 2.2% reduction in the modified system.

DISCUSSION

Although the value of the present study is limited by the small sample sizes, it has demonstrated utility of the modified system in improving efficiency of animal tissue usage. It offers potential utility in a higher throughput screening environment for examining the electrophysiological properties of novel compounds in native cardiac tissues, particularly where functional patch clamp data are limited.

Analysis of arrhythmogenic profile in a canine model of chronic atrioventricular block by comparing in vitro effects of the class III antiarrhythmic drug nifekalant on the ventricular action potential indices between normal heart and atrioventricular block heart

The chronic atrioventricular block dog is a useful model for predicting the future onset of drug-induced long QT syndrome in clinical practice. To better understand the arrhythmogenic profile of this model, we recorded the action potentials of the isolated ventricular tissues in the presence and absence of the class III antiarrhythmic drug nifekalant. The action potential durations of the Purkinje fiber and free wall of the right ventricle were longer in the chronic atrioventricular block dogs than in the dogs with normal sinus rhythm. Nifekalant in concentrations of 1 and 10 microM prolonged the action potential durations of Purkinje fiber and the free wall in a concentration-dependent manner. The extent of prolongation was greater in the chronic atrioventricular block dogs than in the normal dogs. However, increase of temporal dispersion of ventricular repolarization including early afterdepolarization was not detected by nifekalant in either group of dogs, indicating lack of potential to trigger arrhythmias in vitro. These results suggest that the ventricular repolarization delay in the chronic atrioventricular block model by nifekalant may largely depend on the decreased myocardial repolarization reserve, whereas the trigger for lethal arrhythmia was not generated in the in vitro condition in contrast to the in vivo experiment.

Dofetilide: a new class III antiarrhythmic agent

Dofetilide is a relatively new class III antiarrhythmic agent that selectively blocks the rapid component of the cardiac ion channel delayed rectifier current. This results in an increase in the action potential duration and effective refractory period of the myocyte, thereby terminating reentrant tachyarrhythmias and preventing their re-induction. Oral dofetilide is effective in the conversion of atrial fibrillation and flutter to sinus rhythm and in the maintenance of sinus rhythm after conversion. It is generally well tolerated but like other antiarrhythmic agents in its class, torsades de pointes may be induced as a consequence of therapy. This risk is minimized by dosage adjustment according to creatinine clearance and QT(c) interval, by selecting patients without known risk factors for torsades and by initiating treatment in a monitored hospital setting for the first 3 days. Unlike other antiarrhythmic agents, oral dofetilide did not increase mortality in patients with a recent myocardial infarction or congestive heart failure, hence its importance as an alternative medication for the pharmacological conversion of atrial fibrillation and flutter, and maintenance of sinus rhythm after conversion in patients at high risk of sudden death.

A rabbit Langendorff heart proarrhythmia model: predictive value for clinical identification of Torsades de Pointes

BACKGROUND AND PURPOSE

The rabbit isolated Langendorff heart model (SCREENIT) was used to investigate the proarrhythmic potential of a range of marketed drugs or drugs intended for market. These data were used to validate the SCREENIT model against clinical outcomes.

EXPERIMENTAL APPROACH

Fifty-five drugs, 3 replicates and 2 controls were tested in a blinded manner. Proarrhythmia variables included a 10% change in MAPD(60), triangulation, instability and reverse frequency-dependence of the MAP. Early after-depolarisations, ventricular tachycardia, TdP and ventricular fibrillation were noted. Data are reported at nominal concentrations relative to EFTPC(max). Proarrhythmic scores were assigned to each drug and each drug category.

KEY RESULTS

Category 1 and 2 drugs have the highest number of proarrhythmia variables and overt proarrhythmia while Category 5 drugs have the lowest, at every margin. At 30-fold the EFTPC(max), the mean proarrhythmic scores are: Category 1, 101+/-24; Category 2, 101+/-14; Category 3, 72+/-20; Category 4, 59+/-16 and Category 5, 22+/-9 points. Only drugs in Category 5 have mean proarrhythmic scores, below 30-fold, that remain within the Safety Zone.

CONCLUSIONS AND IMPLICATIONS

A 30-fold margin between effects and EFTPC(max) is sufficiently stringent to provide confidence to proceed with a new chemical entity, without incurring the risk of eliminating potentially beneficial drugs. The model is particularly useful where compounds have small margins between the hERG IC(50) and predicted EFTPC(max). These data suggest this is a robust and reliable assay that can add value to an integrated QT/TdP risk assessment.

Assessing the proarrhythmic potential of drugs: current status of models and surrogate parameters of torsades de pointes arrhythmias

Torsades de pointes (TdP) is a potentially lethal cardiac arrhythmia that can occur as an unwanted adverse effect of various pharmacological therapies. Before a drug is approved for marketing, its effects on cardiac repolarisation are examined clinically and experimentally. This paper expresses the opinion that effects on repolarisation duration cannot directly be translated to risk of proarrhythmia. Current safety assessments of drugs only involve repolarisation assays, however the proarrhythmic profile can only be determined in the predisposed model. The availability of these proarrhythmic animal models is emphasised in the present paper. It is feasible for the pharmaceutical industry to establish one or more of these proarrhythmic animal models and large benefits are potentially available if pharmaceutical industries and patient-care authorities embraced these models. Furthermore, suggested surrogate parameters possessing predictive power of TdP arrhythmia are reviewed. As these parameters are not developed to finalisation, any meaningful study of the proarrhythmic potential of a new drug will include evaluation in an integrated model of TdP arrhythmia.

Can optical recordings of membrane potential be used to screen for drug-induced action potential prolongation in single cardiac myocytes?

INTRODUCTION

Potential-sensitive dyes have primarily been used to optically record action potentials (APs) in whole heart tissue. Using these dyes to record drug-induced changes in AP morphology of isolated cardiac myocytes could provide an opportunity to develop medium throughout assays for the pharmaceutical industry. Ideally, this requires that the dye has a consistent and rapid response to membrane potential, is insensitive to movement, and does not itself affect AP morphology.

MATERIALS AND METHODS

We recorded the AP from isolated adult guinea-pig ventricular myocytes optically using di-8-ANEPPS in a single-excitation dual-emission ratiometric system, either separately in electrically field stimulated myocytes, or simultaneously with an electrical AP recorded with a patch electrode in the whole-cell bridge mode. The ratio of di-8-ANEPPS fluorescence signal was calibrated against membrane potential using a switch-clamp to voltage clamp the myocyte.

RESULTS

Our data show that the ratio of the optical signals emitted at 560/620 nm is linearly related to voltage over the voltage range of an AP, producing a change in ratio of 7.5% per 100 mV, is unaffected by cell movement and is identical to the AP recorded simultaneously with a patch electrode. However, the APD90 recorded optically in myocytes loaded with di-8-ANEPPS was significantly longer than in unloaded myocytes recorded with a patch electrode (355.6+/-13.5 vs. 296.2+/-16.2 ms; p<0.01). Despite this effect, the apparent IC50 for cisapride, which prolongs the AP by blocking IKr, was not significantly different whether determined optically or with a patch electrode (91+/-46 vs. 81+/-20 nM).

DISCUSSION

These data show that the optical AP recorded ratiometrically using di-8-ANEPPS from a single ventricular myocyte accurately follows the action potential morphology. This technique can be used to estimate the AP prolonging effects of a compound, although di-8-ANEPPS itself prolongs APD90. Optical dyes require less technical skills and are less invasive than conventional electrophysiological techniques and, when coupled to ventricular myocytes, decreases animal usage and facilitates higher throughput assays.

Gender differences in the slow delayed (IKs) but not in inward (IK1) rectifier K+ currents of canine Purkinje fibre cardiac action potential: key roles for IKs, beta-adrenoceptor stimulation, pacing rate and gender

As the beagle dog is a commonly used preclinical species to test the effects of new drugs on cardiac repolarisation and Purkinje fibres have become an established in vitro preparation to assess the effects of these new drugs on action potential duration (APD), the main aim of this study was therefore to evaluate the relative contribution of the inward (I(K1)) and slow delayed (I(Ks)) rectifier cardiac K(+) currents to action potential repolarisation in beagle Purkinje fibres under three different experimental conditions: (i) selective block of I(K1) with BaCl(2), (ii) selective block of I(Ks) with (-) chromanol 293B under basal conditions and (iii) selective block of I(Ks) during beta-adrenoceptor stimulation. Furthermore, the dependence of this contribution on gender and pacing rate was investigated. Microelectrode techniques were employed to measure APD in Purkinje fibres from adult female and male dogs. At stimulation rates of 3.33, 1.0 and 0.2 Hz, the degree of prolongation of APD evoked by BaCl(2) (10 microM) was comparable in fibres from female and male dogs. At the same stimulation rates, 10 microM (-) chromanol 293B did not change the APD in fibres from female and male dogs. During beta-adrenoceptor stimulation with 0.1 microM isoproterenol, an APD prolonging effect of (-) chromanol 293B was detected. In the presence of isoproterenol, action potentials in fibres from male dogs get shorter when changing the stimulation rate from 1.0 to 0.2 Hz, while the opposite is seen in fibres from female dogs. This alteration was completely reversed by (-) chromanol 293B. In conclusion, our findings confirm that beta-adrenoceptor stimulation is one condition where there may be an increased role of I(Ks) in action potential repolarisation. Gender differences in the autonomic modulation of I(Ks) could be a contributing factor to the reported increased susceptibility of female hearts to arrhythmias.

Recent advances in understanding sex differences in cardiac repolarization

A number of gender differences exist in the human electrocardiogram (ECG): the P-wave and P-R intervals are slightly longer in men than in women, whilst women have higher resting heart rates than do men, but a longer rate-corrected QT (QT(C)) interval. Women with the LQT1 and LQT2 variants of congenital long-QT syndrome (LQTS) are at greater risk of adverse cardiac events. Similarly, many drugs associated with acquired LQTS have a greater risk of inducing torsades de pointes (TdP) arrhythmia in women than in men. There are also male:female differences in Brugada syndrome, early repolarisation syndrome and sudden cardiac death. The differences in the ECG between men and women, and in particular those relating to the QT interval, have been explored experimentally and provide evidence of differences in the processes underlying ventricular repolarization. The data available from rabbit, canine, rat, mouse and guinea pig models are reviewed and highlight involvement of male:female differences in Ca and K currents, although the possible involvement of rapid and persistent Na current and Na-Ca exchange currents cannot yet be excluded. The mechanisms underlying observed differences remain to be elucidated fully, but are likely to involve the influence of gonadal steroids. With respect to the QT interval and risk of TdP, a range of evidence implicates a protective role of testosterone in male hearts, possibly by both genomic and non-genomic pathways. Evidence regarding oestrogen and progesterone is less unequivocal, although the interplay between these two hormones may influence both repolarization and pro-arrhythmic risk.

QT PRODACT: In Vivo QT Assay in the Conscious Dog for Assessing the Potential for QT Interval Prolongation by Human Pharmaceuticals

The goal of the present study was to examine the utility of the conscious dog model by assessing the QT-interval-prolonging potential of ten positive compounds that have been reported to induce QT interval prolongation in clinical use and seven negative compounds considered not to have such an effect. Three doses of test compounds or vehicle were administered orally to male beagle dogs (n=4), and telemetry signals were recorded for 24 h after administration. All positive compounds (astemizole, bepridil, cisapride, E-4031, haloperidol, MK-499, pimozide, quinidine, terfenadine, and thioridazine) caused a significant increase in the corrected QT (QTc) interval, with a greater than 10% increase achieved at high doses. In contrast, administration of negative compounds (amoxicillin, captopril, ciprofloxacin, diphenhydramine, nifedipine, propranolol, and verapamil) did not produce any significant change in the QTc interval, with the exception of nifedipine that may have produced an overcorrection of the QTc interval due to increased heart rate. The estimated plasma concentrations of the positive compounds that caused a 10% increase in the QTc interval were in good agreement with the plasma/serum concentrations achieved in humans who developed prolonged QT interval or torsade de pointes (TdP). Although careful consideration should be given to the interpretation of QT data with marked heart rate change, these data suggest that an in vivo QT assay using the conscious dog is a useful model for the assessment of QT interval prolongation by human pharmaceuticals.