Similarities between ischemic preconditioning and 17beta-estradiol mediated cardiomyocyte KATP channel activation leading to cardioprotective and antiarrhythmic effects during ischemia/reperfusion in the intact rabbit heart

Sex differences in mortality and hospitalization in heart failure with preserved and mid-range ejection fraction: a systematic review and meta-analysis of cohort studies

Introduction

The influence of sex on the prognosis of heart failure with preserved or intermediate ejection fraction (HFpEF and HFmrEF) remains uncertain. This study aimed to investigate whether sex differences impact the prognosis of patients diagnosed with HFpEF and HFmrEF.

Methods

A comprehensive search across three databases (PubMed, the Cochrane Library, and Embase) was conducted to identify sex-related prognostic cohort studies focusing on HFpEF and HFmrEF. Risk estimates were synthesized using the random effects model. The analysis included 14 cohorts comprising 41,508 HFpEF patients (44.65% males) and 10,692 HFmrEF patients (61.79% males).

Results

Among HFpEF patients, men exhibited significantly higher rates of all-cause mortality (13 studies; hazard ratio (HR): 1.24, 95% confidence interval (CI): 1.15 to 1.33)) and cardiovascular disease mortality (5 studies; HR: 1.22, 95% CI: 1.14 to 1.31) compared to women. However, no significant difference was observed in HF admissions. For HFmrEF patients, men displayed notably higher all-cause mortality (HR: 1.21, 95% CI: 1.12 to 1.31) but no significant differences in cardiovascular mortality or HF admissions.

Discussion

These findings suggest that male patients diagnosed with HFpEF and HFmrEF may face a more unfavorable prognosis in terms of all-cause mortality. Variations were noted in cardiovascular mortality and HF admissions, indicating potential complexities in sex-related prognostic factors within these heart failure categories. In summary, male patients with HFpEF and HFmrEF may have a more unfavorable prognosis.

Estrogen-Mediated Gaseous Signaling Molecules in Cardiovascular Disease

Gender difference is well recognized as a key risk factor for cardiovascular disease (CVD). Estrogen, the primary female sex hormone, improves cardiovascular functions through receptor (ERα, ERβ, or G protein-coupled estrogen receptor)-initiated genomic or non-genomic mechanisms. Gaseous signaling molecules, including nitric oxide (NO), hydrogen sulfide (H₂S), and carbon monoxide (CO), are important regulators of cardiovascular function. Recent studies have demonstrated that estrogen regulates the production of these signaling molecules in cardiovascular cells to exert its cardiovascular protective effects. We discuss current understanding of gaseous signaling molecules in cardiovascular disease (CVD), the underlying mechanisms through which estrogen exerts cardiovascular protective effects by regulating these molecules, and how these findings can be translated to improve the health of postmenopausal women.

Synthesis and Biological Activity of a Bis-steroid-Methanocyclobutanaphthalene- dione Derivative against Ischemia/Reperfusion Injury via Calcium Channel Activation

BACKGROUND

There is some experimental data on the effect exerted by some steroid derivatives against ischemia/reperfusion injury; however, the molecular mechanism is very confusing, perhaps this phenomenon could be due to the protocols used and/or differences in the chemical structure of each one of the steroid derivatives.

OBJECTIVES

The aim of this study was to synthesize a new bis-steroid-methanocyclobutanaphthalene- dione derivative using some tools chemical.

METHODOLOGY

The biological activity exerted by the bis-steroid-methanocyclobutanaphthalene- dione derivative against ischemia/reperfusion injury was evaluated in an isolated heart model using noradrenaline, milrinone, dobutamine, levosimendan, and Bay-K- 8644 as controls. In addition, other alternative experiments were carried out to evaluate the biological activity induced by the bis-steroid-methanocyclobuta-naphthalene-dione derivative against left ventricular pressure in the absence or presence of nifedipine.

RESULTS

The results showed that 1) the bis-steroid-methanocyclobuta-naphthalene-dione derivative significantly decreases the ischemia-reperfusion injury translated as a decrease in the the infarct area in a similar manner to levosimendan drug; 2) both bis-steroidmethanocyclobuta- naphthalene-dione and Bay-K-8644 increase the left ventricular pressure and 3) the biological activity exerted by bis-steroid-methanocyclobuta-naphthalenedione derivative against left ventricular pressure is inhibited by nifedipine.

CONCLUSION

In conclusion, the bis-steroid-methanocyclobuta-naphthalene-dione derivative decreases the area of infarction and increases left ventricle pressure via calcium channels activation; this phenomenon could constitute a new therapy for ischemia/reperfusion injury.

shRNA interference of NLRP3 inflammasome alleviate ischemia reperfusion-induced myocardial damage through autophagy activation

Myocardial ischemia-reperfusion (I/R) injury always occur during the recovery of myocardial blood supply with high morbidity and mortality. Although, various therapeutic schedules were applied in clinic, there are real problems that have to be resolved on curative effect. Nod-like receptor protein 3 (NLRP3) inflammasome has moderation effects on cellular damage and inflammatory reaction after I/R injury. Our research aims to investigate a more effective approach to restrain the activation of NLRP3 inflammasome in treating myocardial I/R injury. Results indicated that cell viability, Bax/Bcl-2 expression were affected hardly by sh-NLRP3 transfection in normal cells. However, the decreased cell viability and increased Bax/Bcl-2 expression level caused by I/R were remarkably suppressed through sh-NLRP3 transfection. Besides that, the reduced levels of pro-autophagy proteins (Beclin1, Agt7, LC3II/LC3I) while enhanced level of anti-autophagy protein (p62) and apoptosis-related proteins (Bax/Bcl-2) were significantly repressed via sh-NLRP3 transfection. Nevertheless, the autophagy inhibitor 3 MA could reverse the results. Moreover, in vivo experiment suggested that NLRP3 was up-regulated in wild type (WT) rats with I/R injury. The expansion of infarct size induced by ischemia was tremendously constricted in NLRP3 knockout (KO) rats. NLRP3 silence had nearly no impact on myocardial enzymes (AST, LDH and CK) expressions, inflammatory factors (TNF-α and IL-1β) expressions and cell apoptosis in rats without I/R injury. Nonetheless, the elevated levels of myocardial enzymes, inflammatory factors and cell apoptosis caused by I/R injury were vastly inhibited in NLRP3 KO rats. Furthermore, NLRP3 KO itself would lead to higher level of pro-autophagy proteins (Beclin1, Agt7, LC3II/LC3I) while lower level of anti-autophagy protein (p62) in vivo. The decreased expressions of pro-autophagy proteins while increased expressions of anti-autophagy protein induced by I/R injury were remarkably suppressed by NLRP3 KO. Taken together, our study indicated that shRNA interference of NLRP3 inflammasome attenuated myocardial I/R injury via autophagy activation. These findings demonstrated that NLRP3 KO may a promising therapy in myocardial I/R injury.

Mitochondrial ATP-Sensitive Potassium Channels Play a Role in Reducing Both Myocardial Infarction and Reperfusion Arrhythmia in Remote Ischemic Preconditioned Hearts

Background

Mitochondrial ATP-sensitive potassium (mKATP) channels play a role in reperfusion arrhythmias (RAs) in ischemia-reperfusion (I/R) injury. Evidence suggests that remote ischemic preconditioning (RIPC) reduces RAs, however not much is known on the mechanistic role of mKATP in RIPC. We evaluated whether mKATP channels are associated with reducing arrhythmia and infarct size in RIPC.

Methods

Isolated rat hearts received 30 minutes of regional ischemia followed by 2 hours of reperfusion through the Langendorff perfusion system. RIPC was induced by 3 cycles of 5 minutes occlusion and 5 minutes release of the bilateral femoral artery. The animals were randomly divided into 4 groups as follows: 1) CON, I/R injury but not RIPC, 2) RIPC, 3) HD+RIPC, pretreatment of the selective mKATP channel blocker, 5-hydroxydecanoate (5-HD), in RIPC, and 4) HD, pretreatment of 5-HD in CON. Cardiodynamics and infarct size were determined. The severity of arrhythmia was quantitated via the Curtis and Walker scoring system as well as the Lepran scoring system.

Results

RIPC significantly reduced the infarct size over AR (25.7 ± 2.6%) compared to CON (37.0 ± 2.6%, P < 0.05). The selective mKATP channel blocker 5-HD significantly inhibited the infarct-reducing effect of RIPC (39.3 ± 3.0%, P < 0.05 vs. RIPC). Additionally, RIPC significantly reduced the arrhythmia score compared to CON (14.6 ± 1.9 to 8.7 ± 0.4, P = 0.023, by Curtis and Walker’s system, 16.1 ± 2.1 to 9.1 ± 0.5, P = 0.006, by Lepran’s system). The anti-arrhythmic effect of RIPC was blocked by 5-HD (15.5 ± 1.6 and 16.0 ± 1.2, by Curtis and Walker’s and Lepran’s system, respectively).

Conclusions

The selective mKATP channel blocker, 5-HD, inhibited the infarct-limitation and anti-arrhythmic effect of RIPC. The mKATP channels play a role in the reduction of both infarct size and RAs in RIPC.

Effects of estradiol on measurements of conduit artery endothelial function after ischemia and reperfusion in premenopausal women

In premenopausal women, ovarian steroids are felt to play a role in the prevention of cardiovascular disease. We aimed to assess whether menstrual cycle variations in estrogen can modify the response to ischemia-reperfusion (IR) injury in humans. In an investigator-blinded crossover study, 10 healthy premenopausal women with regular menstrual cycles were studied. They had flow-mediated dilatation (FMD) measured by ultrasound in the radial artery before and after IR (15 min of brachial artery ischemia, 15 min of reperfusion) during both the early and late follicular phases of the menstrual cycle. The order of these visits was not randomized. IR significantly blunted FMD in the early follicular phase (pre-IR: 7.1% ± 1.0%; post-IR: 3.6% ± 1.0%, P = 0.01) when estradiol levels were low (148.4 ± 19.8 pmol/L). Conversely, FMD was preserved after IR during the late follicular phase (pre-IR: 7.2% ± 0.9%; post-IR: 7.0% ± 0.8%, P = NS, P = 0.03 compared with early follicular) when estradiol levels were high (825.7 ± 85.8 pmol/L, P < 0.001 compared with early follicular). There was a significant inverse relationship between estradiol concentration and IR-induced endothelial dysfunction (i.e., change in FMD after IR) (r = 0.59, r² = 0.36, P < 0.01). These findings demonstrate, for the first time in humans, a clear relationship between the cyclical changes in serum concentrations of estradiol and the endothelium's response to IR.

Rapid estrogen actions on ion channels: A survey in search for mechanisms

A survey of nearly two hundred reports shows that rapid estrogenic actions can be detected across a range of kinds of estrogens, a range of doses, on a wide range of tissue, cell and ion channel types. Striking is the fact that preparations of estrogenic agents that do not permeate the cell membrane almost always mimic the actions of the estrogenic agents that do permeate the membrane. All kinds of estrogens, ranging from natural ones, through receptor modulators, endocrine disruptors, phytoestrogens, agonists, and antagonists to novel G-1 and STX, have been reported to be effective. For actions on specific types of ion channels, the possibility of opposing actions, in different cases, is the rule, not the exception. With this variety there is no single, specific action mechanism for estrogens per se, although in some cases estrogens can act directly or via some signaling pathways to affect ion channels. We infer that estrogens can bind a large number of substrates/receptors at the membrane surface. As against the variety of subsequent routes of action, this initial step of the estrogen's binding action is the key.

Characterization of the sex-dependent myocardial S-nitrosothiol proteome

Premenopausal women exhibit endogenous cardioprotective signaling mechanisms that are thought to result from the beneficial effects of estrogen, which we have shown to increase protein S-nitrosylation in the heart. S-nitrosylation is a labile protein modification that increases with a number of different forms of cardioprotection, including ischemic preconditioning. Herein, we sought to identify a potential role for protein S-nitrosylation in sex-dependent cardioprotection. We utilized a Langendorff-perfused mouse heart model of ischemia-reperfusion injury with male and female hearts, and S-nitrosylation-resin-assisted capture with liquid chromatography tandem mass spectrometry to identify S-nitrosylated proteins and modification sites. Consistent with previous studies, female hearts exhibited resilience to injury with a significant increase in functional recovery compared with male hearts. In a separate set of hearts, we identified a total of 177 S-nitrosylated proteins in female hearts at baseline compared with 109 S-nitrosylated proteins in male hearts. Unique S-nitrosylated proteins in the female group included the F1FO-ATPase and cyclophilin D. We also utilized label-free peptide analysis to quantify levels of common S-nitrosylated identifications and noted that the S-nitrosylation of sarcoplasmic/endoplasmic reticulum Ca(2+)-ATPase 2a was nearly 70% lower in male hearts compared with female, with no difference in expression. Furthermore, we found a significant increase in endothelial nitric oxide synthase expression, phosphorylation, and total nitric oxide production in female hearts compared with males, likely accounting for the enhanced S-nitrosylation protein levels in female hearts. In conclusion, we identified a number of novel S-nitrosylated proteins in female hearts that are likely to contribute to sex-dependent cardioprotection.

The effects of zileuton and montelukast in reperfusion-induced arrhythmias in anesthetized rats

Background

5-Lipoxygenase is an enzyme involved in the synthesis of leukotriene eicosanoids from arachidonic acid. The therapeutic potential of zileuton, an inhibitor of 5-lipoxygenase, and montelukast, a cysteinyl leukotriene receptor antagonist, for the treatment of ischemia/reperfusion (I/R) injury of the heart has been proposed in a few studies. However, the effects of zileuton and montelukast on I/R-induced arrhythmias have not been determined.

Objective

We assessed the possible protective effects of zileuton and montelukast against I/R-induced arrhythmias.

Methods

Forty-five male Wistar albino rats were divided into 5 groups, each containing 9 rats. Group 1: control, Groups 2 and 3: rats treated with montelukast (10 and 30 mg/kg IP); and Groups 4 and 5: rats treated with zileuton (1 and 3 mg/kg IV) 15 minutes before the induction of ischemia. Ischemia and reperfusion were induced by occluding the left main coronary artery of anesthetized rats for 6 minutes followed by reopening the artery for 6 minutes.

Results

Both doses of zileuton decreased the mean [SE] arrhythmia score (zileuton 1 mg/kg: 1.4 [0.8]; zileuton 3 mg/kg: 1.3 [0.5] vs control: 2.9 [0.3]; P < 0.05), the duration of ventricular tachycardia, and the total length of arrhythmias, but montelukast was not effective to decrease the ventricular arrhythmias during the 6 minutes of reperfusion.

Conclusions

The results indicate for the first time that zileuton exerts an antiarrhythmic effect at different doses and that montelukast is not effective against I/R-induced arrhythmias. These results indicate that zileuton may be a candidate for drug treatment of I/R-induced arrhythmias.

The effect of uraemia on the duration of arrhythmias in the context of cardioprotective ischaemic conditioning strategies

BACKGROUND

Sudden cardiac death is a leading cause of death in patients with chronic kidney disease and end stage renal disease. Ischaemic conditioning strategies confer profound myocardial protection and, in the absence of uraemia, have been reported to reduce myocardial dysrhythmias. Recent data confirms that ischaemic conditioning can protect the uraemic heart. However, the effect of uraemia on myocardial arrhythmogenesis in the context of ischaemia-reperfusion injury and whether ischaemic conditioning can modulate this is unknown.

OBJECTIVE

We investigated the effect of underling chronic uraemia on the duration of arrhythmias in the context of cardioprotective ischaemic conditioning strategies.

METHODS

We examined the effect of chronic uraemia on arrhythmias occurring in the context of myocardial ischaemia-reperfusion injury and the ability of ischaemic preconditioning (IPC), remote ischaemic preconditioning (RIPC) and ischaemic postconditioning (iPOST) to suppress arrhythmogenesis in uraemic and non-uraemic animals.

RESULTS

IPC led to a reduction in the frequency and duration of arrhythmias occurring during ischaemia and reperfusion. Neither RIPC nor iPOST affected the duration or frequency of ischaemic or reperfusion arrhythmias. Underlying uraemia did not alter the frequency or duration of ischaemic arrhythmias in any of the experiments however it was associated with a reduction in reperfusion arrhythmia duration in the IPC and iPOST experiments.

CONCLUSIONS

These studies demonstrate that underlying chronic uraemia does not reduce the threshold for arrhythmia timing or duration resulting from myocardial ischaemia-reperfusion and underlying uraemia did not alter the effects of these cardioprotective ischaemic conditioning strategies in the context of arrhythmia duration.

SUMMARY

This novel work in a rodent model of chronic uraemia establishes that underlying uraemia does not increase the susceptibility to myocardial ischaemia-reperfusion induced arrhythmias. When compared with the non-uraemic heart, the uraemic heart has a similar response to the effects of ischaemic conditioning strategies in terms of their effect on arrhythmia timing and duration.

GPER: a novel target for non-genomic estrogen action in the cardiovascular system

A key to harnessing the enormous therapeutic potential of estrogens is understanding the diversity of estrogen receptors and their signaling mechanisms. In addition to the classic nuclear estrogen receptors (i.e., ERα and ERβ), over the past decade a novel G-protein-coupled estrogen receptor (GPER) has been discovered in cancer and other cell types. More recently, this non-genomic signaling mechanism has been found in blood vessels, and mediates vasodilatory responses to estrogen and estrogen-like agents; however, downstream signaling events involved acute estrogen action remain unclear. The purpose of this review is to discuss the latest knowledge concerning GPER modulation of cardiovascular function, with a particular emphasis upon how activation of this receptor could mediate acute estrogen effects in the heart and blood vessels (i.e., vascular tone, cell growth and differentiation, apoptosis, endothelial function, myocardial protection). Understanding the role of GPER in estrogen signaling may help resolve some of the controversies associated with estrogen and cardiovascular function. Moreover, a more thorough understanding of GPER function could also open significant opportunities for the development of new pharmacological strategies that would provide the cardiovascular benefits of estrogen while limiting the potentially dangerous side effects.

Neuroprotective effects of 17β-estradiol associate with KATP in rat brain

Previous studies have indicated that estrogen protects the brain from ischemic damage and regulates K(ATP) channel activity; the present study was designed to address the involvement of K(ATP) channels in the neuroprotective effects of estrogen in focal cerebral ischemia: in experiment 1, K(ATP) mRNA and protein in the cortices of rats were compared among groups of ovariectomized rats (Ovx-1), Sham-operated rats (Sham-1), and ovariectomized rats administered 17β-estradiol (Estr-1). In experiment 2, neurobehavioral scores and infarct volume of rats were evaluated after middle cerebral artery occlusion in ovariectomized rats (Ovx-2), Sham-operated rats (Sham-2), ovariectomized female rats administered 17β-estradiol (Estr-2), and ovariectomized rats administered both 17β-estradiol and stereotactic injections of glibenclamide (Estr+G). Our results showed that the Kir6.2 and SUR1 mRNA and protein levels in the brain cortices of female ovariectomized rats were lower than those in Sham rats. However, the expression levels of Kir6.2 and SUR1 in brain cortices of ovariectomized rats recovered after supplementation with 17β-estradiol. The protective effects of 17β-estradiol were abolished by glibenclamide, a K(ATP) channel blocker. This indicates that estradiol significantly upregulates the expression of K(ATP) channel subunits and channel activity in the brain cortices of ovariectomized rats. This regulation is associated with the neuroprotective effects of estradiol.

Interventricular heterogeneity as a substrate for arrhythmogenesis of decoupled mitochondria during ischemia in the whole heart

Myocardial ischemia results in metabolic changes, which collapse the mitochondrial network, that increase the vulnerability of the heart to ventricular fibrillation (VF). It has been demonstrated at the single cell level that uncoupling the mitochondria using carbonyl cyanide p-(tri-fluoromethoxy)phenyl-hydrazone (FCCP) at low concentrations can be cardioprotective. The aim of our study was to investigate the effect of FCCP on arrhythmogenesis during ischemia in the whole rabbit heart. We performed optical mapping of isolated rabbit hearts ( n = 33) during control and 20 min of global ischemia and reperfusion, both with and without pretreatment with the mitochondrial uncoupler FCCP at 100, 50, or 30 nM. No hearts went into VF during ischemia under the control condition, with or without the electromechanical uncoupler blebbistatin. We found that pretreatment with 100 ( n = 4) and 50 ( n = 6) nM FCCP, with or without blebbistatin, leads to VF during ischemia in all hearts, whereas pretreatment with 30 nM of FCCP led to three out of eight hearts going into VF during ischemia. We demonstrated that 30 nM of FCCP significantly increased interventricular (but not intraventricular) action potential duration and conduction velocity heterogeneity in the heart during ischemia, thus providing the substrate for VF. We showed that wavebreaks during VF occurred between the right and left ventricular junction. We also demonstrated that no VF occurred in the heart pretreated with 10 μM glibenclamide, which is known to abolish interventricular heterogeneity. Our results indicate that low concentrations of FCCP, although cardioprotective at the single cell level, are arrhythmogenic at the whole heart level. This is due to the fact that FCCP induces different electrophysiological changes to the right and left ventricle, thus increasing interventricular heterogeneity and providing the substrate for VF.

Testosterone enhances estradiol's cardioprotection in ovariectomized rats

After menopause, the development of cardiovascular disease (CVD) is due not only to estrogen decline but also to androgen decline. This study examined the effects of either estradiol (E(2)) or testosterone replacement alone or E(2)-testosterone combination on isolated myocytes in ovariectomized (Ovx) rats subjected to ischemia/reperfusion (I/R). Furthermore, we determined whether the effects are associated with β(2)-adrenoceptor (β(2)-AR). Five groups of adult female Sprague-Dawley rats were used: Sham operation (Sham) rats, bilateral Ovx rats, Ovx rats with E(2) 40 μg/kg per day (Ovx+E), Ovx rats with testosterone 150 μg/kg per day (Ovx+T), and Ovx rats with E(2) 40 μg/kg per day+testosterone 150 μg/kg per day (Ovx+E/T). We determined the lactate dehydrogenase (LDH) release, percentage of rod-shaped cells and apoptosis of ventricular myocytes from rats of all groups subjected to I/R. Then, we determined the above indices and contractile function with or without a selective β(2)-AR antagonist ICI 118 551. We also determined the expression of β(2)-AR. Our data show that either E(2) or testosterone replacement alone or E(2) and testosterone in combination decreased the LDH release, increased the percentage of rod-shaped cells, reduced apoptotic cells (%), and combination treatment appeared to be more effective than either E(2) or testosterone replacement alone. ICI 118 551 abolished the effects of the three. Combination supplementation also enhanced the expression of β(2)-AR. We concluded that in Ovx rats, testosterone enhances E(2)'s cardioprotection, while E(2) and testosterone in combination was more effective and the protective effects may be associated with β(2)-AR. The study highlights the potential therapeutic application for CVD in postmenopausal women.

Mechanisms of the beneficial actions of ischemic preconditioning on subcellular remodeling in ischemic-reperfused heart

Cardiac function is compromised by oxidative stress which occurs upon exposing the heart to ischemia reperfusion (I/R) for a prolonged period. The reactive oxygen species (ROS) that are generated during I/R incur extensive damage to the myocardium and result in subcellular organelle remodeling. The cardiac nucleus, glycocalyx, myofilaments, sarcoplasmic reticulum, sarcolemma, and mitochondria are affected by ROS during I/R injury. On the other hand, brief periods of ischemia followed by reperfusion, or ischemic preconditioning (IPC), have been shown to be cardioprotective against oxidative stress by attenuating the cellular damage and alterations of subcellular organelles caused by subsequent I/R injury. Endogenous defense mechanisms, such as antioxidant enzymes and heat shock proteins, are activated by IPC and thus prevent damage caused by oxidative stress. Although these cardioprotective effects of IPC against I/R injury are considered to be a consequence of changes in the redox state of cardiomyocytes, IPC is considered to promote the production of NO which may protect subcellular organelles from the deleterious actions of oxidative stress. The article is intended to focus on the I/R-induced oxidative damage to subcellular organelles and to highlight the cardioprotective effects of IPC. In addition, the actions of various endogenous cardioprotective interventions are discussed to illustrate that changes in the redox state due to IPC are cardioprotective against I/R injury to the heart.

Estrogen signaling and cardiovascular disease

Estrogen has pleiotropic effects on the cardiovascular system. The mechanisms by which estrogen confers these pleiotropic effects are undergoing active investigation. Until a decade ago, all estrogen signaling was thought to occur by estrogen binding to nuclear estrogen receptors (estrogen receptor-α and estrogen receptor-β), which bind to DNA and function as ligand-activated transcription factors. Estrogen binding to the receptor alters gene expression, thereby altering cell function. Estrogen also binds to nuclear estrogen receptors that are tethered to the plasma membrane, resulting in acute activation of signaling kinases such as PI3K. An orphan G-protein-coupled receptor, G-protein-coupled receptor 30, can also bind estrogen and activate acute signaling pathways. Thus, estrogen can alter cell function by binding to different estrogen receptors. This article reviews the different estrogen receptors and their signaling mechanisms, discusses mechanisms that regulate estrogen receptor levels and locations, and considers the cardiovascular effects of estrogen signaling.

The G protein-coupled estrogen receptor GPER/GPR30 as a regulator of cardiovascular function

Endogenous estrogens are important regulators of cardiovascular homeostasis in premenopausal women and delay the development of hypertension and coronary artery disease. These hormones act via three different estrogen receptors affecting both gene transcription and rapid signaling pathways in a complex interplay. In addition to the classical estrogen receptors ERα and ERβ, which are known mediators of estrogen-dependent vascular effects, a G protein-coupled estrogen receptor termed GPER that is expressed in the cardiovascular system has recently been identified. Endogenous human 17β-estradiol, selective estrogen receptor modulators (SERMs) including tamoxifen and raloxifene, and selective estrogen receptor downregulators (SERDs) such as ICI 182,780 are all agonists of GPER, which has been implicated in the regulation of vasomotor tone and protection from myocardial ischemia/reperfusion injury. As a result, understanding the individual role of ERα, ERβ, and GPER in cardiovascular function has become increasingly complex. With accumulating evidence that GPER is responsible for a variety of beneficial cardiovascular effects of estrogens, this receptor may represent a novel target to develop effective strategies for the treatment of cardiovascular diseases by tissue-specific, selective activation of estrogen-dependent molecular pathways devoid of side effects seen with conventional hormone therapy.

Mitochondria are sources of metabolic sink and arrhythmias

Mitochondria have long been recognized for their central role in energy transduction and apoptosis. More recently, extensive work in multiple laboratories around the world has significantly extended the role of cardiac mitochondria from relatively static arbitrators of cell death and survival pathways to highly dynamic organelles that form interactive functional networks across cardiomyocytes. These coupled networks were shown to strongly affect cardiomyocyte responses to oxidative stress by modulating cell signaling pathways that strongly impact physiological properties. Of particular importance is the role of mitochondria in modulating key electrophysiological and calcium cycling properties in cardiomyocytes, either directly through activation of a myriad of mitochondrial ion channels or indirectly by affecting cell signaling cascades, ATP levels, and the over-all redox state of the cardiomyocyte. This important recognition has ushered a renewed interest in understanding, at a more fundamental level, the exact role that cardiac metabolism, in general and mitochondria, in particular, play in both health and disease. In this article, we provide an overview of recent advances in our growing understanding of the fundamental role that cardiac mitochondria play in the genesis of lethal arrhythmias.

Cardiac mitochondria and arrhythmias

Despite a high prevalence of sudden cardiac death throughout the world, the mechanisms that lead to ventricular arrhythmias are not fully understood. Over the last 20 years, a growing body of evidence indicates that cardiac mitochondria are involved in the genesis of arrhythmia. In this review, we have attempted to describe the role that mitochondria play in altering the heart's electrical function by introducing heterogeneity into the cardiac action potential. Specifically, we have focused on how the energetic status of the mitochondrial network can alter sarcolemmal potassium fluxes through ATP-sensitive potassium channels, creating a 'metabolic sink' for depolarizing wave-fronts and introducing conditions that favour catastrophic arrhythmia. Mechanisms by which mitochondria depolarize under conditions of oxidative stress are characterized, and the contributions of several mitochondrial ion channels to mitochondrial depolarization are presented. The inner membrane anion channel in particular opens upstream of other inner membrane channels during metabolic stress, and may be an effective target to prevent the metabolic oscillations that create action potential lability. Finally, we discuss therapeutic strategies that prevent arrhythmias by preserving mitochondrial membrane potential in the face of oxidative stress, supporting the notion that treatments aimed at cardiac mitochondria have significant potential in attenuating electrical dysfunction in the heart.

Sex-related resistance to myocardial ischemia-reperfusion injury is associated with high constitutive ARC expression

The female sex has been associated with improved myocardial salvage after ischemia and reperfusion (I/R). Estrogen, specifically 17beta-estradiol, has been demonstrated to mediate this phenomenon by limiting cardiomyocyte apoptosis. We sought to quantitatively assess the effect of sex, ovarian hormone loss, and I/R on myocardial Bax, Bcl-2, and apoptosis repressor with caspase recruitment domain (ARC) expression. Male (n = 48), female (n = 26), and oophorectomized female (n = 20) rabbits underwent 30 min of regional ischemia and 3 h of reperfusion. The myocardial area at risk and infarct size were determined using a double-staining technique and planimetry. In situ oligo ligation was used to assess apoptotic cell death. Western blot analysis was used to determine proapoptotic (Bax) and antiapoptotic (Bcl-2 and ARC) protein levels in all three ischemic groups and, additionally, in three nonischemic groups. Infarct size (43.7 +/- 3.2%) and apoptotic cell death (0.51 +/- 0.10%) were significantly attenuated in females compared with males (56.4 +/- 1.6%, P < 0.01, and 4.29 +/- 0.95%, P < 0.01) and oophorectomized females (55.7 +/- 3.4%, P < 0.05, and 4.36 +/- 0.51%, P < 0.01). Females expressed significantly higher baseline ARC levels (3.62 +/- 0.29) compared with males (1.78 +/- 0.18, P < 0.01) and oophorectomized females (1.08 +/- 0.26, P < 0.01). Males expressed a significantly higher baseline Bax-to-Bcl-2 ratio (4.32 +/- 0.99) compared with females (0.65 +/- 0.13, P < 0.01) and oophorectomized females (0.42 +/- 0.10, P < 0.01). I/R significantly reduced Bax-to-Bcl-2 ratios in males. In all other groups, ARC levels and Bax-to-Bcl-2 ratios did not significantly change. These results support the conclusion that in females, endogenous estrogen limits I/R-induced cardiomyocyte apoptosis by producing a baseline antiapoptotic profile, which is associated with estrogen-dependent high constitutive myocardial ARC expression.

Estrogen-mediated protection in myocardial ischemia-reperfusion injury

Before menopause, a woman has a relatively low risk for developing cardiovascular disease. After menopause, however, the risk increases nearly twofold and cardiovascular disease remains the number one cause of death among women. Observational trials and studies in animal models of cardiovascular disease suggested that females have reduced injury after myocardial ischemia and reperfusion injury. However, two large clinical trials, the women's health initiative (WHI) and the heart estrogen and progestin replacement study (HERS), found an increase in cardiovascular incidences in women taking hormone replacement therapy. The discrepancy between these data highlights the need for further research on the mechanism of estrogen in the cardiovascular system. Animal studies have demonstrated protective effects by endogenous estrogen (gender differences) and also by the administration of exogenous estrogen. In vivo studies suggest a possible anti-inflammatory mechanism of estrogen. Exogenous estrogen has been shown to have anti-oxidant activities. Pre-treatment with estrogen prior to myocardial ischemia and reperfusion causes a decrease in neutrophil infiltration into the irreversibly injured myocardium, decrease in C-reactive protein expression, and deposition of the membrane attack complex. This review will summarize the protection afforded by estrogen as well as discuss several possible mechanisms of protection for exogenous estrogen administration.

Estradiol abolishes reduction in cell death by the opioid agonist Met5-enkephalin after oxygen glucose deprivation in isolated cardiomyocytes from both sexes

There is evidence for differences in the response to the treatment of cardiovascular disease in men and women. In addition, there are conflicting results regarding the effectiveness of pharmacologically induced protection or ischemic preconditioning in females. We investigated whether the ability of Met5-enkephalin (ME) to reduce cell death after oxygen-glucose deprivation (OGD) is influenced by the presence of 17β-estradiol (E2) in a nitric oxide (NO)- and estrogen receptor-dependent manner. On postnatal day 7 to 8, murine cardiomyocytes from wild-type or inducible NO synthase (iNOS) knockout mice were separated by sex, isolated by collagenase digestion, cultured for 24 h, and subjected to 90 min OGD and 180 min reoxygenation at 37°C ( n = 4 to 5 replicates). Cell cultures were incubated in E2 for 15 min or 24 h before OGD. ME was used to increase cell survival. Cell death was assessed by propidium iodide. More than 300 cells were examined for each treatment. Data are presented as means ± SE. As a result, in both sexes, ME-induced cell survival was lost in the presence of E2, and the ability of ME to improve cell survival was restored after treatment with the estrogen receptor antagonist ICI-182780. Furthermore, iNOS was necessary for ME to increase cell survival following OGD in vitro. We conclude that ME-induced reduction in cell death is abolished by E2 in a sex-independent manner via activation of estrogen receptors, and this interaction is dependent on iNOS.

Cardioprotection in females: a role for nitric oxide and altered gene expression

A number of epidemiological and animal studies have suggested a cardioprotective role for estrogen. This review will focus on the cardioprotective role of estrogen in ischemia-reperfusion injury. Estrogen binding to receptors can lead to altered gene expression and estrogen has been shown to induce expression of a number of genes that have been suggested to be important in cardioprotection. Estrogen is reported to increase expression of the plasma membrane glucose transporter GLUT4 and to increase carbohydrate metabolism. Estrogen has also been reported to increase mitochondrial biogenesis and to alter mitochondrial generation of reactive oxygen species. Estrogen results in upregulation of cardiac eNOS and nNOS, which have been shown previously to be important mediators of cardioprotection. Nitric oxide has been shown to result in S-nitrosylation and inhibition of the L-type calcium channel, thereby reducing calcium loading during ischemia. Nitric oxide has also been reported to inhibit complex I and inhibition of complex I has been reported to reduce activation of the mitochondrial permeability transition pore. Nitric oxide has been shown to result in activation of the mitochondrial K(ATP) channel, which has been shown to be involved in cardioprotection. Estrogen can also activate rapid non-genomic pathways that activate cardioprotective-signaling pathways such as the phosphatidylinositol-3-kinase (PI-3 kinase) pathway which has also been shown to initiate protection. Taken together, estrogen by genomic and non-genomic pathways can result in the initiation of a number of signaling pathways that enhance cardioprotection.

Treatment with an estrogen receptor-beta-selective agonist is cardioprotective

This study was designed to investigate whether treatment with an estrogen receptor-beta (ER-beta)-selective agonist (2,3-bis(4-hydroxyphenyl)-propionitrile, DPN) can provide cardioprotection in female mice lacking endogenous estrogen. To study the effect of ER-beta stimulation in ischemia-reperfusion injury, we treated ovariectomized (ovx) female mice with 0.1 mg/kg/day of 17beta-estradiol, 0.8 mg/kg/day of DPN, or vehicle for 2 weeks. Isolated hearts were Langendorff perfused for 25 min prior to a 1-min treatment with isoproterenol, followed by 20 min of normothermic global ischemia and 40 min of reperfusion. Left ventricular developed pressure (LVDP) and heart rate were measured. Recovery of function at the end of 40 min of reperfusion was expressed as a percentage of pre-ischemic rate pressure product (RPP=LVDP x heart rate). Hearts from ovx female mice had a significantly lower recovery of LVDP than the hearts from intact female mice (12.4+/-1.6% vs. 19.6+/-1.6%, p<0.05, respectively). Furthermore, hearts from ovx female mice treated with DPN exhibited significantly better functional recovery than hearts from either vehicle-treated ovx female mice (20.1+/-2.2% vs. 12.4+/-1.6%, p<0.05, respectively) or wild type male mice (20.1+/-2.2% vs. 6.4+/-0.6%, p<0.05, respectively). DPN did not increase uterine weight in ovx females compared to vehicle treatment. Gene profiling showed that treatment with DPN resulted in upregulation of a number of protective genes such as heat shock protein 70, the antiapoptotic protein, growth arrest and DNA damage 45 beta, and cyclooxygenase 2.

Actions of estrogen and estrogen receptors in nonclassical target tissues

Hormonal effects on classical endocrine target organs such as the female reproductive tract, mammary gland, ovary, and neuroendocrine system have been thoroughly studied, with significant advancements in our understanding of estrogen actions and disease conditions from both cell culture as well as new experimental animal models. Knowledge of the highly appreciated effects of estrogen in nonclassical endocrine organ systems, arising from epidemiological and clinical findings in the cardiovascular, immune, GI tract, and liver, is only now becoming clarified from the development and use of knock-out or transgenic animal models for the study of both estrogen and ER activities. There are considerable epidemiological data showing that premenopausal females (Barrett-Connor 1997; Crabbe et al. 2003) have reduced risk for cardiovascular disease. However, a recent large clinical trial failed to show cardioprotection for postmenopausal females on estrogen-progestin replacement (Rossouw et al. 2002). In fact, the Women's Health Initiative Study showed increased cardiovascular risk for females taking an estrogen-progestin combination. These studies suggest that we need a better understanding of the mechanisms responsible for cardioprotection in females.