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

Antiarrhythmic effects of mirabegron on ventricular fibrillation in Langendorff-perfused rabbit ventricles

β-3 adrenoceptor (AR) counteracts the β-1 and β-2 ARs and rescues the effects of excessive catecholamines. To test the hypothesis that a β-3 AR agonist (mirabegron) can reverse the effects of isoproterenol (ISO) on ventricular fibrillation (VF), we performed optical mapping studies in six male and six female Langendorff perfused rabbit hearts at baseline and after sequential administration of ISO (100 nm), mirabegron (1000 nm), apamin (100 nm) and washout (Study I). An additional six male and six female hearts were studied with mirabegron doses ranging between 250 and 1000 nm without ISO (Study II). Patch clamp studies in human embryonic kidney 293 cells were performed to determine the effect of mirabegron on the apamin-sensitive small conductance Ca2+ activated K+ current (IKAS). Study I show that ISO increased phase singularities per VF episode (PSs/VF) in females and the dominant frequency (DF) in both sexes. Mirabegron significantly decreased PSs/VF in both sexes and significantly decreased DF in females. Study II showed no significant difference in PSs/VF between sexes at mirabegron concentrations of 250 nm and 500 nm. However, females showed significantly lower PSs/VF than males at mirabegron concentrations of 750 nm and 1000 nm. There were no differences in the DF profiles of dose-response between males and females. Mirabegron did not inhibit or activate IKAS heterologously expressed in human embryonal kidney 293 cells. Reverse transcriptase-quantitative PCR showed no differences in β-3 AR expression between sexes. We conclude that mirabegron is antiarrhythmic, and its antiarrhythmic properties are more commonly observed in females than males. KEY POINTS: Sympathetic nerve activity activates β adrenoceptors to induce cardiac arrhythmia. Among the β adrenoceptors, β-3 counteracts the effects of β-1 and β-2. Mirabegron is an US Food and Drug Administration (FDA)-approved β-3 agonist that does not by itself block cardiac ionic currents or prolong the QT interval. We showed that mirabegron significantly prevents wave breaks and reduces the dominant frequency of ventricular fibrillation. These effects are more prominent in female than in male rabbit ventricles. Because the FDA approves mirabegron for human use, its antiarrhythmic effects can be readily tested in humans.

Identifying sex similarities and differences in structure and function of the sinoatrial node in the mouse heart

Background

The sinoatrial node (SN) generates the heart rate (HR). Its spontaneous activity is regulated by a complex interplay between the modulation by the autonomic nervous system (ANS) and intrinsic factors including ion channels in SN cells. However, the systemic and intrinsic regulatory mechanisms are still poorly understood. This study aimed to elucidate the sex-specific differences in heart morphology and SN function, particularly focusing on basal HR, expression and function of hyperpolarization-activated HCN4 and HCN1 channels and mRNA abundance of ion channels and mRNA abundance of ion channels contributing to diastolic depolarization (DD) and spontaneous action potentials (APs).

Methods

Body weight, heart weight and tibia length of 2- to 3-month-old male and female mice were measured. Conscious in-vivo HR of male and female mice was recorded via electrocardiography (ECG). Unconscious ex-vivo HR, stroke volume (SV) and ejection fraction (EF) were recorded via echocardiography. Ex-vivo HR was measured via Langendorff apparatus. Volume of atria, ventricles and whole hearts were measured from the ex-vivo hearts by microcomputed tomography (micro-CT). Immunohistochemistry targeting HCN4 and HCN1 was conducted in the SN and RA tissues from both male and female hearts. The funny current (I f) of SN cells in 1 nM and following wash-on of 1 μM isoproterenol (ISO) were recorded via whole cell patch clamp. The APs of SN tissue were recorded via sharp microelectrode and optical mapping of membrane voltage. The relative abundance of mRNAs was measured in male and female mice by qPCR.

Results

Heart weight to tibia length ratio and heart volume of females were significantly smaller than males. Unconscious in-vivo HR in male mice was higher than that in females. Conscious in-vivo HR, ex-vivo HR, SV, and EF showed no notable difference between male and female mice. Immunohistochemistry revealed HCN4, HCN1, and the sum of HCN4 and HCN1, expression in the SN was notably elevated compared with the RA in both male and females, but there was no sex difference in these channels expression. There were also no significant sex differences in the V 0.5 of I f in SN cells in the presence of 1 nM ISO, however wash-on 1 μM ISO in the same cells induced a significantly increased shift of V 0.5 to more positive voltages in males than in females. The expression of mRNA coding for adrenergic receptor beta-1 (Adrb1) and cholinergic receptors muscarinic 2 (chrm2) in male mice was higher compared with that in female mice. Early diastolic depolarization (EDD) rate in APs from peripheral SN (pSN) from male mice were higher than these in female mice. Mice of both sexes showed equivalent frequency of SN APs and spatial localization of the leading site in control, and similar significant response to ISO 100 nM superfusion.

Conclusion

Males display faster in-vivo HR, but not ex-vivo HR, than females associated with increased expression of Adrb1 in male versus female. This suggests a possible difference in the β-adrenergic modulation in males and females, possibly related to the greater ISO response of I f observed in cells from males. The role of hormonal influences or differential expression of other ion channels may explain these sex-specific variations in HR dynamics. Further investigations are necessary to pinpoint the precise molecular substrates responsible for these differences.

Female cardiovascular biology and resilience in the setting of physiological and pathological stress

For years, females were thought of as smaller men with complex hormonal cycles; as a result, females have been largely excluded from preclinical and clinical research. However, in the last ten years, with the increased focus on sex as a biological variable, it has become clear that this is not the case, and in fact, male and female cardiovascular biology and cardiac stress responses differ substantially. Premenopausal women are protected from cardiovascular diseases, such as myocardial infarction and resultant heart failure, having preserved cardiac function, reduced adverse remodeling, and increased survival. Many underlying biological processes that contribute to ventricular remodeling differ between the sexes, such as cellular metabolism; immune cell responses; cardiac fibrosis and extracellular matrix remodeling; cardiomyocyte dysfunction; and endothelial biology; however, it is unclear how these changes afford protection to the female heart. Although many of these changes are dependent on protection provided by female sex hormones, several of these changes occur independent of sex hormones, suggesting that the nature of these changes is more complex than initially thought. This may be why studies focused on the cardiovascular benefits of hormone replacement therapy in post-menopausal women have provided mixed results. Some of the complexity likely stems from the fact that the cellular composition of the heart is sexually dimorphic and that in the setting of MI, different subpopulations of these cell types are apparent. Despite the documented sex-differences in cardiovascular (patho)physiology, the underlying mechanisms that contribute are largely unknown due to inconsistent findings amongst investigators and, in some cases, lack of rigor in reporting and consideration of sex-dependent variables. Therefore, this review aims to describe current understanding of the sex-dependent differences in the myocardium in response to physiological and pathological stressors, with a focus on the sex-dependent differences that contribute to post-infarction remodeling and resultant functional decline.

Interactions between Gender and Sepsis—Implications for the Future

Sex and gender dimorphisms are found in a large variety of diseases, including sepsis and septic shock which are more prevalent in men than in women. Animal models show that the host response to pathogens differs in females and males. This difference is partially explained by sex polarization of the intracellular pathways responding to pathogen–cell receptor interactions. Sex hormones seem to be responsible for this polarization, although other factors, such as chromosomal effects, have yet to be investigated. In brief, females are less susceptible to sepsis and seem to recover more effectively than males. Clinical observations produce more nuanced findings, but men consistently have a higher incidence of sepsis, and some reports also claim higher mortality rates. However, variables other than hormonal differences complicate the interaction between sex and sepsis, including comorbidities as well as social and cultural differences between men and women. Conflicting data have also been reported regarding sepsis-attributable mortality rates among pregnant women, compared with non-pregnant females. We believe that unraveling sex differences in the host response to sepsis and its treatment could be the first step in personalized, phenotype-based management of patients with sepsis and septic shock.

Sex differences in heart: from basics to clinics

Sex differences exist in the structure and function of human heart. The patterns of ventricular repolarization in normal electrocardiograms (ECG) differ in men and women: men ECG pattern displays higher T-wave amplitude and increased ST angle. Generally, women have longer QT duration because of reduced repolarization reserve, and thus, women are more susceptible for the occurrence of torsades de pointes associated with drugs prolonging ventricular repolarization. Sex differences are also observed in the prevalence, penetrance and symptom severity, and also in the prognosis of cardiovascular disease. Generally, women live longer, have less clinical symptoms of cardiac diseases, and later onset of symptoms than men. Sex hormones also play an important role in regulating ventricular repolarization, suggesting that hormones directly influence various cellular functions and adrenergic regulation. From the clinical perspective, sex-based differences in heart physiology are widely recognized, but in daily practice, cardiac diseases are often underdiagnosed and untreated in the women. The underlying mechanisms of sex differences are, however, poorly understood. Here, we summarize sex-dependent differences in normal cardiac physiology, role of sex hormones, and differences in drug responses. Furthermore, we also discuss the importance of human induced pluripotent stem cell-derived cardiomyocytes in further understanding the mechanism of differences in women and men.

Ventricular arrhythmias in acute myocardial ischaemia-Focus on the ageing and sex

Annually, approximately 17 million people die from cardiovascular diseases worldwide, half of them suddenly. The most common direct cause of sudden cardiac death is ventricular arrhythmia triggered by an acute coronary syndrome (ACS). The study summarizes the knowledge of the mechanisms of arrhythmia onset during ACS in humans and in animal models and factors that may influence the susceptibility to life-threatening arrhythmias during ACS with particular focus on the age and sex. The real impact of age and sex on the arrhythmic susceptibility within the setting of acute ischaemia is masked by the fact that ACSs result from coronary artery disease appearing with age much earlier among men than among women. However, results of researches show that in ageing process changes with potential pro-arrhythmic significance, such as increased fibrosis, cardiomyocyte hypertrophy, decrease number of gap junction channels, disturbances of the intracellular Ca²⁺ signalling or changes in electrophysiological parameters, occur independently of the development of cardiovascular diseases and are more severe in male individuals. A review of the literature also indicates a marked paucity of research in this area in female and elderly individuals. Greater awareness of sex differences in the aging process could help in the development of personalized prevention methods targeting potential pro-arrhythmic factors in patients of both sexes to reduce mortality during the acute phase of myocardial infarction. This is especially important in an era of aging populations in which women will predominate due to their longer lifespan.

Sex influences the effect of adiposity on arterial stiffness and renin‐angiotensin aldosterone system activity in young adults

Introduction

Methods

Results

Conclusion

Heart failure in diabetes

Heart failure and cardiovascular disorders represent the leading cause of death in diabetic patients. Here we present a systematic review of the main mechanisms underlying the development of diabetic cardiomyopathy. We also provide an excursus on the relative contribution of cardiomyocytes, fibroblasts, endothelial and smooth muscle cells to the pathophysiology of heart failure in diabetes. After having described the preclinical tools currently available to dissect the mechanisms of this complex disease, we conclude with a section on the most recent updates of the literature on clinical management.

Sex determination gene transformer regulates the male-female difference in Drosophila fat storage via the adipokinetic hormone pathway

Sex differences in whole-body fat storage exist in many species. For example, Drosophila females store more fat than males. Yet, the mechanisms underlying this sex difference in fat storage remain incompletely understood. Here, we identify a key role for sex determination gene transformer (tra) in regulating the male-female difference in fat storage. Normally, a functional Tra protein is present only in females, where it promotes female sexual development. We show that loss of Tra in females reduced whole-body fat storage, whereas gain of Tra in males augmented fat storage. Tra's role in promoting fat storage was largely due to its function in neurons, specifically the Adipokinetic hormone (Akh)-producing cells (APCs). Our analysis of Akh pathway regulation revealed a male bias in APC activity and Akh pathway function, where this sex-biased regulation influenced the sex difference in fat storage by limiting triglyceride accumulation in males. Importantly, Tra loss in females increased Akh pathway activity, and genetically manipulating the Akh pathway rescued Tra-dependent effects on fat storage. This identifies sex-specific regulation of Akh as one mechanism underlying the male-female difference in whole-body triglyceride levels, and provides important insight into the conserved mechanisms underlying sexual dimorphism in whole-body fat storage.

A framework for developing sex-specific engineered heart models

The convergence of tissue engineering and patient-specific stem cell biology has enabled the engineering of in vitro tissue models that allow the study of patient-tailored treatment modalities. However, sex-related disparities in health and disease, from systemic hormonal influences to cellular-level differences, are often overlooked in stem cell biology, tissue engineering and preclinical screening. The cardiovascular system, in particular, shows considerable sex-related differences, which need to be considered in cardiac tissue engineering. In this Review, we analyse sex-related properties of the heart muscle in the context of health and disease, and discuss a framework for including sex-based differences in human cardiac tissue engineering. We highlight how sex-based features can be implemented at the cellular and tissue levels, and how sex-specific cardiac models could advance the study of cardiovascular diseases. Finally, we define design criteria for sex-specific cardiac tissue engineering and provide an outlook to future research possibilities beyond the cardiovascular system.

Pharmacogenomic Effects of β-Blocker Use on Femoral Neck Bone Mineral Density

CONTEXT

Recent studies have shown that β-blocker (BB) users have a decreased risk of fracture and higher bone mineral density (BMD) compared to nonusers, likely due to the suppression of adrenergic signaling in osteoblasts, leading to increased BMD. There is also variability in the effect size of BB use on BMD in humans, which may be due to pharmacogenomic effects.

OBJECTIVE

To investigate potential single-nucleotide variations (SNVs) associated with the effect of BB use on femoral neck BMD, we performed a cross-sectional analysis using clinical data, dual-energy x-ray absorptiometry, and genetic data from the Framingham Heart Study's (FHS) Offspring Cohort. We then sought to validate our top 4 genetic findings using data from the Rotterdam Study, the BPROOF Study, the Malta Osteoporosis Fracture Study (MOFS), and the Hertfordshire Cohort Study.

METHODS

We used sex-stratified linear mixed models to determine SNVs that had a significant interaction effect with BB use on femoral neck (FN) BMD across 11 gene regions. We also evaluated the association of our top SNVs from the FHS with microRNA (miRNA) expression in blood and identified potential miRNA-mediated mechanisms by which these SNVs may affect FN BMD.

RESULTS

One variation (rs11124190 in HDAC4) was validated in females using data from the Rotterdam Study, while another (rs12414657 in ADRB1) was validated in females using data from the MOFS. We performed an exploratory meta-analysis of all 5 studies for these variations, which further validated our findings.

CONCLUSION

This analysis provides a starting point for investigating the pharmacogenomic effects of BB use on BMD measures.

Cardiac Fibroblasts Mediate a Sexually Dimorphic Fibrotic Response to β-Adrenergic Stimulation

Background

Biological sex is an important modifier of cardiovascular disease and women generally have better outcomes compared with men. However, the contribution of cardiac fibroblasts (CFs) to this sexual dimorphism is relatively unexplored.

Methods and Results

Isoproterenol (ISO) was administered to rats as a model for chronic β‐adrenergic receptor (β‐AR)‐mediated cardiovascular disease. ISO‐treated males had higher mortality than females and also developed fibrosis whereas females did not. Gonadectomy did not abrogate this sex difference. To determine the cellular contribution to this phenotype, CFs were studied. CFs from both sexes had increased proliferation in vivo in response to ISO, but CFs from female hearts proliferated more than male cells. In addition, male CFs were significantly more activated to myofibroblasts by ISO. To investigate potential regulatory mechanisms for the sexually dimorphic fibrotic response, β‐AR mRNA and PKA (protein kinase A) activity were measured. In response to ISO treatment, male CFs increased expression of β1‐ and β2‐ARs, whereas expression of both receptors decreased in female CFs. Moreover, ISO‐treated male CFs had higher PKA activity relative to vehicle controls, whereas ISO did not activate PKA in female CFs.

Conclusions

Chronic in vivo β‐AR stimulation causes fibrosis in male but not female rat hearts. Male CFs are more activated than female CFs, consistent with elevated fibrosis in male rat hearts and may be caused by higher β‐AR expression and PKA activation in male CFs. Taken together, our data suggest that CFs play a substantial role in mediating sex differences observed after cardiac injury.

Cardiovascular effects of increasing dosages of norepinephrine in healthy isoflurane-anesthetized New Zealand White rabbits

OBJECTIVE

To characterize the cardiovascular effects of increasing dosages of norepinephrine (NE) in healthy isoflurane-anesthetized rabbits.

STUDY DESIGN

Prospective experimental study.

ANIMALS

A total of nine female ovariohysterectomized New Zealand White rabbits weighing 3.4 ± 0.2 kg (mean ± standard deviation).

METHODS

Rabbits were premedicated intramuscularly with buprenorphine (0.05 mg kg^-1) and midazolam (0.5 mg kg^-1). Anesthesia was induced with intravenous propofol and maintained with a 1.1 × minimum alveolar concentration of isoflurane for this species to induce hypotension. Rabbits were administered NE infusions at three doses: low, 0.1 μg kg^-1 minute^-1; medium, 0.5 μg kg^-1 minute^-1; and high doses, 1 μg kg^-1 minute^-1 for 10 minutes each in that order. Cardiovascular variables including heart rate (HR), cardiac output (CO) by lithium dilution technique and systolic (SAP), mean (MAP) and diastolic (DAP) invasive arterial blood pressures measured in the auricular artery were recorded at baseline, 10 minutes after the start of the infusion of each NE treatment and 10 minutes after NE was discontinued. A linear mixed model and a type III anova with Tukey's post hoc comparison was performed (p < 0.05).

RESULTS

Significant increases in SAP (28% and 90%), MAP (27% and 90%) and DAP (33% and 97%) were measured with medium and high dose treatments, respectively (p < 0.001), with no changes in CO. HR decreased and stroke volume increased significantly with high dose treatment (by 17% and 15%, respectively; p < 0.05). No arrhythmias were noticed with NE treatments.

CONCLUSIONS AND CLINICAL RELEVANCE

The infusion of NE at 0.5-1.0 μg kg^-1 minute^-1 is a potentially effective treatment for hypotension in healthy isoflurane-anesthetized New Zealand White rabbits.

Antiarrhythmic mechanisms of beta blocker therapy

Sympathetic activity plays an important role in modulation of cardiac rhythm. Indeed, while exerting positive tropic effects in response to physiologic and pathologic stressors, β-adrenergic stimulation influences cardiac electrophysiology and can lead to disturbances of the heart rhythm and potentially lethal arrhythmias, particularly in pathological settings. For this reason, β-blockers are widely utilized clinically as antiarrhythmics. In this review, the molecular mechanisms of β-adrenergic action in the heart, the cellular and tissue level cardiac responses to β-adrenergic stimulation, and the clinical use of β-blockers as antiarrhythmic agents are reviewed. We emphasize the complex interaction between cardiomyocyte signaling, contraction, and electrophysiology occurring over multiple time- and spatial-scales during pathophysiological responses to β-adrenergic stimulation. An integrated understanding of this complex system is essential for optimizing therapies aimed at preventing arrhythmias.

Sex and bacterial infectious diseases

Most infectious diseases are unequally distributed between male and female subjects. This sex dimorphism is confirmed by epidemiologic studies which suggest an increased number of male septic patients, while, due to the class age of septic patients, an overrepresentation of female patients would be expected. Lifestyle, recreational activities, professional exposition and access to care are plausible reasons for this dimorphism. However, biological differences should be carefully considered, particularly the weight of X-linked variability and the role of sex hormones. Animal models clearly show that clinical response to infection is more exuberant in males than in females. This is partly explained by an attenuation of the inflammatory response by female sex hormones. However, the translation from experimental studies to the bedside remains challenging as a result of confounding factors like age, hormone changes and response to treatment.

Remote Limb Ischemic Preconditioning Attenuates Cerebrovascular Depression During Sinusoidal Galvanic Vestibular Stimulation via α1-Adrenoceptor-Protein Kinase Cε-Endothelial NO Synthase Pathway in Rats

BACKGROUND

Vasovagal syncope (VVS) is characterized by hypotension and bradycardia followed by lowering of cerebral blood flow. Remote limb ischemic preconditioning (RIPC) is well documented to provide cardio- and neuroprotection as well as to improve cerebral blood flow. We hypothesized that RIPC will provide protection against VVS-induced hypotension, bradycardia, and cerebral hypoperfusion. Second, because endothelial nitric oxide synthase has been reported as a mediator of cerebral blood flow control, we hypothesized that the mechanism by which RIPC primes the vasculature against VVS is via the α1-adrenoceptor-protein kinase Cε-endothelial nitric oxide synthase pathway.

METHODS AND RESULTS

We utilized sinusoidal galvanic vestibular stimulation in rats as a model of VVS. RIPC attenuated the lowerings of mean arterial pressure, heart rate, and cerebral blood flow caused by sinusoidal galvanic vestibular stimulation, as well as improving behavior during, and recovery after, stimulation. RIPC induced elevated serum norepinephrine, increased expression of brain α1-adrenoceptors, and reduced brain expression of norepinephrine transporter 1. Antagonizing adrenoceptors and norepinephrine transporter 1 prevented RIPC protection of cerebral perfusion during sinusoidal galvanic vestibular stimulation.

CONCLUSIONS

Taken together, this study indicates that RIPC may be a potential therapy that can prevent VVS pathophysiology, decrease syncopal episodes, and reduce the injuries associated with syncopal falls. Furthermore, the α1-adrenoceptor-protein kinase Cε-endothelial nitric oxide synthase pathway may be a therapeutic target for regulating changes in cerebral blood flow.

A multiscale computational modelling approach predicts mechanisms of female sex risk in the setting of arousal-induced arrhythmias

KEY POINTS

This study represents a first step toward predicting mechanisms of sex-based arrhythmias that may lead to important developments in risk stratification and may inform future drug design and screening. We undertook simulations to reveal the conditions (i.e. pacing, drugs, sympathetic stimulation) required for triggering and sustaining reentrant arrhythmias. Using the recently solved cryo-EM structure for the Eag-family channel as a template, we revealed potential interactions of oestrogen with the pore loop hERG mutation (G604S). Molecular models suggest that oestrogen and dofetilide blockade can concur simultaneously in the hERG channel pore.

ABSTRACT

Female sex is a risk factor for inherited and acquired long-QT associated torsade de pointes (TdP) arrhythmias, and sympathetic discharge is a major factor in triggering TdP in female long-QT syndrome patients. We used a combined experimental and computational approach to predict 'the perfect storm' of hormone concentration, I_Kr block and sympathetic stimulation that induces arrhythmia in females with inherited and acquired long-QT. More specifically, we developed mathematical models of acquired and inherited long-QT syndrome in male and female ventricular human myocytes by combining effects of a hormone and a hERG blocker, dofetilide, or hERG mutations. These 'male' and 'female' model myocytes and tissues then were used to predict how various sex-based differences underlie arrhythmia risk in the setting of acute sympathetic nervous system discharge. The model predicted increased risk for arrhythmia in females when acute sympathetic nervous system discharge was applied in the settings of both inherited and acquired long-QT syndrome. Females were predicted to have protection from arrhythmia induction when progesterone is high. Males were protected by the presence of testosterone. Structural modelling points towards two plausible and distinct mechanisms of oestrogen action enhancing torsadogenic effects: oestradiol interaction with hERG mutations in the pore loop containing G604 or with common TdP-related blockers in the intra-cavity binding site. Our study presents findings that constitute the first evidence linking structure to function mechanisms underlying female dominance of arousal-induced arrhythmias.

Sex Differences in Metabolic Cardiomyopathy

In contrast to ischemic cardiomyopathies which are more common in men, women are over-represented in diabetic cardiomyopathies. Diabetes is a risk factor for cardiovascular disease; however, there is a sexual dimorphism in this risk factor: heart disease is five times more common in diabetic women but only two-times more common in diabetic men. Heart failure with preserved ejection fraction, which is associated with metabolic syndrome, is also more prevalent in women. This review will examine potential mechanisms for the sex differences in metabolic cardiomyopathies. Sex differences in metabolism, calcium handling, nitric oxide, and structural proteins will be evaluated. Nitric oxide synthase and PPARα exhibit sex differences and have also been proposed to mediate the development of hypertrophy and heart failure. We focused on a role for these signalling pathways in regulating sex differences in metabolic cardiomyopathies.

Male and female hypertrophic rat cardiac myocyte functional responses to ischemic stress and β-adrenergic challenge are different

Background

Cardiac hypertrophy is the most potent cardiovascular risk factor after age, and relative mortality risk linked with cardiac hypertrophy is greater in women. Ischemic heart disease is the most common form of cardiovascular pathology for both men and women, yet significant differences in incidence and outcomes exist between the sexes. Cardiac hypertrophy and ischemia are frequently occurring dual pathologies. Whether the cellular (cardiomyocyte) mechanisms underlying myocardial damage differ in women and men remains to be determined. In this study, utilizing an in vitro experimental approach, our goal was to examine the proposition that responses of male/female cardiomyocytes to ischemic (and adrenergic) stress may be differentially modulated by the presence of pre-existing cardiac hypertrophy.

Methods

We used a novel normotensive custom-derived hypertrophic heart rat (HHR; vs control strain normal heart rat (NHR)). Cardiomyocyte morphologic and electromechanical functional studies were performed using microfluorimetric techniques involving simulated ischemia/reperfusion protocols.

Results

HHR females exhibited pronounced cardiac/cardiomyocyte enlargement, equivalent to males. Under basal conditions, a lower twitch amplitude in female myocytes was prominent in normal but not in hypertrophic myocytes. The cardiomyocyte Ca²⁺ responses to β-adrenergic challenge differed in hypertrophic male and female cardiomyocytes, with the accentuated response in males abrogated in females—even while contractile responses were similar. In simulated ischemia, a marked and selective elevation of end-ischemia Ca²⁺ in normal female myocytes was completely suppressed in hypertrophic female myocytes—even though all groups demonstrated similar shifts in myocyte contractile performance. After 30 min of simulated reperfusion, the Ca²⁺ desensitization characterizing the male response was distinctively absent in female cardiomyocytes.

Conclusions

Our data demonstrate that cardiac hypertrophy produces dramatically different basal and stress-induced pathophenotypes in female- and male-origin cardiomyocytes. The lower Ca²⁺ operational status characteristic of female (vs male) cardiomyocytes comprising normal hearts is not exhibited by myocytes of hypertrophic hearts. After ischemia/reperfusion, availability of activator Ca²⁺ is suppressed in female hypertrophic myocytes, whereas sensitivity to Ca²⁺ is blunted in male hypertrophic myocytes. These findings demonstrate that selective intervention strategies should be pursued to optimize post-ischemic electromechanical support for male and female hypertrophic hearts.

Electronic supplementary material

The online version of this article (doi:10.1186/s13293-016-0084-8) contains supplementary material, which is available to authorized users.

Temporal response of ectopic activity in guinea pig ventricular myocardium in response to isoproterenol and acetylcholine

Both β adrenergic and muscarinic receptor stimulation independently potentiate arrhythmogenesis. However, the effect of simultaneous stimulation on arrhythmogenesis is not well known. The purpose of this study was to determine the temporal response of arrhythmia risk to individual and combined autonomic agonists. Guinea pig hearts were excised and Langendorff-perfused. The β adrenergic receptor and muscarinic receptor agonists were isoproterenol (ISO, 0.6 μM) and acetylcholine (ACh, 10 μM), respectively. All measurements with agonists occurred over 21 min. ISO induced ectopic activity for the first 8 min. ISO also transiently shortened and then prolonged R-R interval over a similar time course. ACh added after ISO transiently induced ectopic activity for 12 min, while R-R interval invariantly prolonged. ACh alone produced few ectopic beats, while invariantly prolonging R-R interval. In contrast to ISO alone, ISO following ACh significantly increased ectopic activity and shortened R-R interval for the duration of the experiment. Animals aged 17–19 months exhibited sustained arrhythmogenesis while those aged 11–14 did not. When ACh was removed in older hearts while ISO perfused, a transient increase in ectopic activity and decreased R-R interval was observed, similar to ISO alone. These data suggest that pre-treating with and maintaining ACh perfusion can sustain ISO sensitivity, in contrast to ISO perfusion alone.

Cardiac myocyte alternans in intact heart: Influence of cell-cell coupling and β-adrenergic stimulation

BACKGROUND

Cardiac alternans are proarrhythmic and mechanistically link cardiac mechanical dysfunction and sudden cardiac death. Beat-to-beat alternans occur when beats with large Ca(2+) transients and long action potential duration (APD) alternate with the converse. APD alternans are typically driven by Ca(2+) alternans and sarcoplasmic reticulum (SR) Ca(2+) release alternans. But the effect of intercellular communication via gap junctions (GJ) on alternans in the intact heart remains unknown.

OBJECTIVE

We assessed the effects of cell-to-cell coupling on local alternans in intact Langendorff-perfused mouse hearts, measuring single myocyte [Ca(2+)] alternans synchronization among neighboring cells, and effects of β-adrenergic receptor (β-AR) activation and reduced GJ coupling.

METHODS AND RESULTS

Mouse hearts (C57BL/6) were retrogradely perfused and loaded with Fluo8-AM to record cardiac myocyte [Ca(2+)] in situ with confocal microscopy. Single cell resolution allowed analysis of alternans within the intact organ during alternans induction. Carbenoxolone (25 μM), a GJ inhibitor, significantly increased the occurrence and amplitude of alternans in single cells within the intact heart. Alternans were concordant between neighboring cells throughout the field of view, except transiently during onset. β-AR stimulation only reduced Ca(2+) alternans in tissue that had reduced GJ coupling, matching effects seen in isolated myocytes.

CONCLUSIONS

Ca(2+) alternans among neighboring myocytes is predominantly concordant, likely because of electrical coupling between cells. Consistent with this, partial GJ uncoupling increased propensity and amplitude of Ca(2+) alternans, and made them more sensitive to reversal by β-AR activation, as in isolated myocytes. Electrical coupling between myocytes may thus limit the alternans initiation, but also allow alternans to be more stable once established.