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

Sex Disparities in Sudden Cardiac Death

The overall incidence of sudden cardiac death is considerably lower among women than men, reflecting significant and often under-recognized sex differences. Women are older at time of sudden cardiac death, less likely to have a prior cardiac diagnosis, and less likely to have coronary artery disease identified on postmortem examination. They are more likely to experience their death at home, during sleep, and less likely witnessed. Women are also more likely to present in pulseless electrical activity or systole rather than ventricular fibrillation or ventricular tachycardia. Conversely, women are less likely to receive bystander cardiopulmonary resuscitation or receive cardiac intervention post-arrest. Underpinning sex disparities in sudden cardiac death is a paucity of women recruited to clinical trials, coupled with an overall lack of prespecified sex-disaggregated evidence. Thus, predominantly male-derived data form the basis of clinical guidelines. This review outlines the critical sex differences concerning epidemiology, cause, risk factors, prevention, and outcomes. We propose 4 broad areas of importance to consider: physiological, personal, community, and professional factors.

The protective role of estrogen and estrogen receptors in cardiovascular disease and the controversial use of estrogen therapy

Epidemiologic studies have previously suggested that premenopausal females have reduced incidence of cardiovascular disease (CVD) when compared to age-matched males, and the incidence and severity of CVD increases postmenopause. The lower incidence of cardiovascular disease in women during reproductive age is attributed at least in part to estrogen (E2). E2 binds to the traditional E2 receptors (ERs), estrogen receptor alpha (ERα), and estrogen receptor beta (ERβ), as well as the more recently identified G-protein-coupled ER (GPR30), and can exert both genomic and non-genomic actions. This review summarizes the protective role of E2 and its receptors in the cardiovascular system and discusses its underlying mechanisms with an emphasis on oxidative stress, fibrosis, angiogenesis, and vascular function. This review also presents the sexual dimorphic role of ERs in modulating E2 action in cardiovascular disease. The controversies surrounding the clinical use of exogenous E2 as a therapeutic agent for cardiovascular disease in women due to the possible risks of thrombotic events, cancers, and arrhythmia are also discussed. Endogenous local E2 biosynthesis from the conversion of testosterone to E2 via aromatase enzyme offers a novel therapeutic paradigm. Targeting specific ERs in the cardiovascular system may result in novel and possibly safer therapeutic options for cardiovascular protection.

Influence of steroid hormones on ventricular repolarization

QT interval prolongation, corrected for heart rate (QTc), either spontaneous or drug-induced, is associated with an increased risk of torsades de pointes and sudden death. Women have longer QTc than men and are at higher risk of torsades de pointes, particularly during post-partum and the follicular phase. Men with peripheral hypogonadism have longer QTc than healthy controls. The role of the main sex steroid hormones has been extensively studied with inconsistent findings. Overall, estradiol is considered to promote QTc lengthening while progesterone and testosterone shorten QTc. New findings suggest more complex regulation of QTc by sex steroid hormones involving gonadotropins (i.e. follicle-stimulating hormone), the relative concentrations of sex steroid hormones (which depends on gender, i.e., progesterone/estradiol ratio in women). Aldosterone, another structurally related steroid hormone, can also prolong ventricular repolarization in both sex. Better understanding of pathophysiological hormonal processes which may lead to increased susceptibility of women (and possibly hypogonadic men) to drug-induced arrhythmia may foster preventive treatments (e.g. progesterone in women). Exogenous hormonal intake might offer new therapeutic opportunities or, alternatively, increase the risk of torsades de pointes. Some exogenous sex steroids may also have paradoxical effects on ventricular repolarization. Lastly, variations of QTc in women linked to the menstrual cycle and sex hormone fluctuations are generally ignored in regulatory thorough QT studies. Investigators and regulatory agencies promoting inclusion of women in thorough QT studies should be aware of this source of variability especially when studying drugs over several days of administration.

Sex differences in β-adrenergic responsiveness of action potentials and intracellular calcium handling in isolated rabbit hearts

Cardioprotection in females, as observed in the setting of heart failure, has been attributed to sex differences in intracellular calcium handling and its modulation by β-adrenergic signaling. However, further studies examining sex differences in β-adrenergic responsiveness have yielded inconsistent results and have mostly been limited to studies of contractility, ion channel function, or calcium handling alone. Given the close interaction of the action potential (AP) and intracellular calcium transient (CaT) through the process of excitation-contraction coupling, the need for studies exploring the relationship between agonist-induced AP and calcium handling changes in female and male hearts is evident. Thus, the aim of this study was to use optical mapping to examine sex differences in ventricular APs and CaTs measured simultaneously from Langendorff-perfused hearts isolated from naïve adult rabbits during β-adrenergic stimulation. The non-selective β-agonist isoproterenol (Iso) decreased AP duration (APD₉₀), CaT duration (CaD₈₀), and the decay constant of the CaT (τ) in a dose-dependent manner (1–316.2 nM), with a plateau at doses ≥31.6 nM. The Iso-induced changes in APD₉₀ and τ (but not CaD₈₀) were significantly smaller in female than male hearts. These sex differences were more significant at faster (5.5 Hz) than resting rates (3 Hz). Treatment with Iso led to the development of spontaneous calcium release (SCR) with a dose threshold of 31.6 nM. While SCR occurrence was similar in female (49%) and male (53%) hearts, the associated ectopic beats had a lower frequency of occurrence (16% versus 40%) and higher threshold (100 nM versus 31.6 nM) in female than male hearts (p<0.05). In conclusion, female hearts had a decreased capacity to respond to β-adrenergic stimulation, particularly under conditions of increased demand (i.e. faster pacing rates and “maximal” levels of Iso effects), however this reduced β-adrenergic responsiveness of female hearts was associated with reduced arrhythmic activity.

Frequency-dependent acceleration of cardiac repolarization by progesterone underlying its cardiac protection against drug-induced proarrhythmic effects in female rabbits

Concurrent supplement of estradiol and progesterone has been shown to reduce the cardiac sensitivity to class III antiarrhythmic agent-induced arrhythmias in ovariectomized rabbits. To understand the underlying cardiac electrophysiological mechanisms of the hormones, present study explored the modulation of progesterone and estradiol on repolarization and its frequency dependence in papillary muscles of female rabbit right ventricles by glass microelectrode technique. Results showed that progesterone shortened action potential duration for 90% repolarization (APD(90)) whereas estradiol prolonged APD(90) and those actions on APD(90) were concentration-dependent for both hormones at 1.0-30 μM (P<0.05 or P<0.01). Further, the action of both hormones on APD(90) was found to be dependent on stimulation frequencies (0.2-3.3Hz). The shortening of APD(90) by progesterone (10 μM) was enhanced with the increase in frequencies reaching a statistic significance at frequencies ≥1.0 Hz, whereas the prolongation of APD(90) by estradiol (3 μM) was weakened with the increase in frequencies and the significant change was observed at frequencies ≤2.0 Hz (P<0.05 or P<0.01). More interestingly, the relative change of APD(90) and the incidence of early afterdepolarization induced after by dofetilide (0.1 μM), a class III antiarrhythmic agent, were significantly less or lower in the papillary muscles pretreated with progesterone than in those pretreated with estradiol (P<0.01 or P<0.05). In conclusion, progesterone has a reverse modulating affect on cardiac repolarization to that of estradiol. By acceleration of ventricular repolarization, progesterone may reduce the susceptibility of females to class III antiarrhythmic agents-induced proarrhythmic affection.

Concurrent supplement of estradiol and progesterone reduces the cardiac sensitivity to D,L-sotalol-induced arrhythmias in ovariectomized rabbits

BACKGROUND

Although the difference in the modulation of estradiol and dihydrotesterone on ventricular repolarization has been intensively studied, little information is available concerning the role of the different ovarian hormones in the modulation of repolarization in the female.

METHODS

The chronic modulation of female hormones, estradiol, and progesterone, on cardiac repolarization and the susceptibility to d,l-sotalol, a class III antiarrhythmic agent, were studied in female rabbits by ovariectomy and hormone replacement therapy (HRT) through recording and analyzing of electrocardiograms.

RESULTS

The corrected QT interval (QTc) measured 2 weeks after ovariectomy was not significantly different from that in the time-matched control rabbits. After 2 weeks of HRT, the QTc in the ovariectomized rabbits treated with estradiol alone (group E) was not significantly different from that in the control (group C); whereas in the ovariectomized rabbits treated with estradiol plus progesterone (group E + P), it was significantly shorter than those in groups E (P < .05) and C (P < .01), respectively. The corrected Tpeak-end interval (Tpec), an indicator of global dispersion of ventricular repolarization, was also significantly reduced in group E + P compared with that of group C (P < .01). In group E, d,l-sotalol-induced prolongation of QTc and the rate and the severity of arrhythmias were significantly higher, while the dose of sotalol to initiate arrhythmias was significantly lower than those in groups C or E + P, respectively (P < .05 or P < .01).

CONCLUSION

Estradiol potentiates QTc prolonging effects of d,l-sotalol and increases the susceptibility to d,l-sotalol-induced arrhythmias without significantly altering QTc itself, whereas progesterone may accelerate the process of repolarization and protect the females from drug-induced arrhythmias, thus counteracting the effect of estradiol.

Biomarkers and endogenous determinants of dofetilide-induced torsades de pointes in α(1) -adrenoceptor-stimulated, anaesthetized rabbits

BACKGROUND AND PURPOSE

Torsades de pointes (TdP) liability is a stochastic event, which indicates that unidentified factors have an important role in facilitating the initiation of TdP by increasing the probability of TdP occurrence. We sought to identify factors that facilitate drug-induced TdP.

EXPERIMENTAL APPROACH

We studied dofetilide-induced TdP in pentobarbital-anaesthetized, phenylephrine-sensitized rabbits, seeking biomarkers that discriminated between the animals that experienced TdP ('TdP+' animals) and those that did not ('TdP-' animals). As novel variables, the beat-to-beat variability and instability of ECG intervals were measured at preset times, irrespective of whether or not hearts were in stable sinus rhythm ('absolute' variability and instability). Autonomic activity was also determined.

KEY RESULTS

Dofetilide delayed repolarization and induced arrhythmias prior to TdP. The variability of the coupling interval and shape of arrhythmic beats before TdP were significantly greater in the 'TdP+' group than in the 'TdP-' group. Accordingly, the 'absolute' variability and instability of the ECG intervals were significantly elevated in the 'TdP+' group. Phenylephrine increased significantly the up-baroreflex sensitivity in the 'TdP+' group before dofetilide administration.

CONCLUSIONS AND IMPLICATIONS

'Preceding' arrhythmias have characteristics that permit prediction of TdP occurrence: the more chaotic the ventricular rhythm, the greater the probability of TdP initiation. This suggests that complexity of the arrhythmic beats may play an important mechanistic role in TdP genesis. The electrical instability quantified by the novel 'absolute' variability and instability parameters correlates with the probability of TdP occurrence. Baroreflex may contribute to TdP genesis in vivo.

The anaesthetised methoxamine-sensitised rabbit model of torsades de pointes

Current guidelines describe strategies on how the potential of non-antiarrhythmic drugs to delay ventricular repolarisation should be assessed. However, the non-clinical guidelines recommend repolarisation assays only and do not advocate experimental models that express the proarrhythmia of concern, torsades de pointes (TdP). Although the repolarisation assays may predict QT interval prolongation in man they cannot alone sufficiently predict proarrhythmia risk. Furthermore, there is also a need for more robust surrogate markers of drug-induced proarrhythmia and such validated markers are on the horizon as a result of the availability of sensitive animal models of TdP. This review will describe the methoxamine-sensitised rabbit model of TdP, one of the most frequently used proarrhythmia models, and present some of it characteristics, its pros and cons and how it historically has been used for assessing proarrhythmia liability of drugs.

Importance of vagally mediated bradycardia for the induction of torsade de pointes in an in vivo model

BACKGROUND AND PURPOSE

Bradycardia is a risk factor for the development of torsade de pointes (TdP). The aim of this work was to compare the importance of changes in heart rate and arterial blood pressure in the development of drug-induced TdP and to investigate the role of vagal influences.

EXPERIMENTAL APPROACH

Experiments were performed in open-chest, pentobarbital-anaesthetized, male rabbits which were given clofilium (20, 60 and 200 nmol kg(-1) min(-1)) with rising doses of either phenylephrine (75, 150, 225 and 300 nmol kg(-1) min(-1)), angiotensin II (0.25, 0.5, 0.75 and 1 nmol kg(-1) min(-1)) or saline. A fourth group received phenylephrine and cloflium after bilateral vagotomy. ECGs, haemodynamics and epicardial monophasic action potentials were recorded.

KEY RESULTS

TdP occurred in 57% of rabbits given phenylephrine and clofilium. Replacement of phenylephrine with saline or angiotensin II reduced the incidence of TdP to 0 and 17%, respectively. Vagotomy prevented TdP in rabbits given phenylephrine and clofilium. Increases in blood pressure induced by phenylephrine and angiotensin II were similar. Bradycardia only occurred with phenylephrine and was reduced but not abolished by vagotomy. Neither short-term variability of repolarization nor action potential triangulation could predict TdP.

CONCLUSIONS AND IMPLICATIONS

These results indicate that reflex activation of vagal nerve activity is essential for the induction of drug-induced TdP in alpha1-adrenoceptor-stimulated anaesthetized rabbits. This implies that alterations in vagal activity may also precipitate episodes of drug-induced TdP in man and that this should be considered in selecting models used in drug development.