Ablation of the androgen receptor gene modulates atrial electrophysiology and arrhythmogenesis with calcium protein dysregulation

Electrical and Structural Insights into Right Ventricular Outflow Tract Arrhythmogenesis

The right ventricular outflow tract (RVOT) is the major origin of ventricular arrhythmias, including premature ventricular contractions, idiopathic ventricular arrhythmias, Brugada syndrome, torsade de pointes, long QT syndrome, and arrhythmogenic right ventricular cardiomyopathy. The RVOT has distinct developmental origins and cellular characteristics and a complex myocardial architecture with high shear wall stress, which may lead to its high vulnerability to arrhythmogenesis. RVOT myocytes are vulnerable to intracellular sodium and calcium overload due to calcium handling protein modulation, enhanced CaMKII activity, ryanodine receptor phosphorylation, and a higher cAMP level activated by predisposing factors or pathological conditions. A reduction in Cx43 and Scn5a expression may lead to electrical uncoupling in RVOT. The purpose of this review is to update the current understanding of the cellular and molecular mechanisms of RVOT arrhythmogenesis.

The Impact of Testosterone on the QT Interval: A Systematic Review

Humans and mammals have sex-specific differences in cardiac electrophysiology, linked to the action of sex hormones in the cardiac muscle. These hormones can upregulate or downregulate the expression of ionic channels modulating the cardiac cycle through genomic and non-genomic interactions. Systematic search in PubMed, Medline and EMBASE including keywords pertaining to testosterone and QT interval. Included experimental studies and observation studies and case reports presenting the results of testosterone administration, excess or deficiency in humans and animals. Testosterone has been shown to shorten the action potential duration, by enhancing the expression of K⁺ channels and downregulating I_CaL increasing the repolarization reserve of the cardiac muscle. This effect has been observed in both genders and animals. Testosterone deficient states can promote arrhythmogenesis. The evidence in this paper may be used to guide clinical considerations, such as increased clinical surveillance of patients in testosterone deficient states using ECG.

New aspects of endocrine control of atrial fibrillation and possibilities for clinical translation

Hormones are potent endo-, para-, and autocrine endogenous regulators of the function of multiple organs, including the heart. Endocrine dysfunction promotes a number of cardiovascular diseases, including atrial fibrillation (AF). While the heart is a target for endocrine regulation, it is also an active endocrine organ itself, secreting a number of important bioactive hormones that convey significant endocrine effects, but also through para-/autocrine actions, actively participate in cardiac self-regulation. The hormones regulating heart-function work in concert to support myocardial performance. AF is a serious clinical problem associated with increased morbidity and mortality, mainly due to stroke and heart failure. Current therapies for AF remain inadequate. AF is characterized by altered atrial function and structure, including electrical and profibrotic remodelling in the atria and ventricles, which facilitates AF progression and hampers its treatment. Although features of this remodelling are well-established and its mechanisms are partly understood, important pathways pertinent to AF arrhythmogenesis are still unidentified. The discovery of these missing pathways has the potential to lead to therapeutic breakthroughs. Endocrine dysfunction is well-recognized to lead to AF. In this review, we discuss endocrine and cardiocrine signalling systems that directly, or as a consequence of an underlying cardiac pathology, contribute to AF pathogenesis. More specifically, we consider the roles of products from the hypothalamic-pituitary axis, the adrenal glands, adipose tissue, the renin–angiotensin system, atrial cardiomyocytes, and the thyroid gland in controlling atrial electrical and structural properties. The influence of endocrine/paracrine dysfunction on AF risk and mechanisms is evaluated and discussed. We focus on the most recent findings and reflect on the potential of translating them into clinical application.

Macrophage migration inhibitory factor increases atrial arrhythmogenesis through CD74 signaling

Macrophage migration inhibitory factor (MIF), a pleiotropic inflammatory cytokine, is highly expressed in patients with atrial fibrillation (AF). CD74 (major histocompatibility complex, class II invariant chain) is the main receptor for MIF. However, the role of the MIF/CD74 axis in atrial arrhythmogenesis is unclear. In this study, we investigated the effects of MIF/CD74 signaling on atrial electrophysiological characteristics and determined its underlying mechanisms. Confocal fluorescence microscopy, patch clamp, and western blot analysis were used to study calcium homeostasis, ionic currents, and calcium-related signaling in MIF-treated HL-1 atrial cardiomyocytes with or without anti-CD74 neutralized antibodies treatment. Furthermore, electrocardiographic telemetry recording and echocardiography were obtained from mice treated with MIF. Compared with controls, MIF-treated HL-1 myocytes had increased calcium transients, sarcoplasmic reticulum (SR) calcium content, Na⁺/Ca²⁺ exchanger (NCX) efflux rate, calcium leak, transient outward potassium current, and ultra-rapid delayed rectifier potassium current. Furthermore, MIF could induce expression of SR Ca²⁺ATPase, NCX, phosphorylation of ryanodine receptor 2 (RyR2), and activation of calcium/calmodulin kinase II (CaMKII) when compared with control cells. MIF-mediated electrical dysregulation and CaMKII-RyR2 signaling activation were attenuated through blocking of CD74. Moreover, MIF-injected mice had lesser left atrium fractional shortening, greater atrial fibrosis, and atrial ectopic beats than control (nonspecific immunoglobulin treated) or MIF combined with anti-CD74 neutralized antibody-treated mice. Consequently, our study on MIF/CD74 signaling has pointed out a new potential therapeutic intervention of AF patients with MIF elevation.

Cardiac arrhythmias secondary to hormone therapy in trans women

Introduction: With greater social acceptance and the evolution of transgender medicine as a specialty, more trans women are seeking hormone therapy (HT). Several studies have identified an increase in cardiovascular disease in trans women, however no studies have investigated the incidence of arrhythmias. Using two cases from the authors' clinic as examples, we propose that hormone therapy in trans women may increase the risk of cardiac arrhythmias. Areas covered: A literature search of sex hormones and cardiac arrhythmias was conducted. Using sex hormone studies completed in cis individuals and animal models we identified several similarities to trans women on HT. In cis men, low levels of testosterone are associated with increased rates of atrial fibrillation and right ventricular outflow tract arrhythmias. The role of estradiol remains less clear but there is evidence to suggest that the administration of exogenous estrogen may increase the rates of cardiac arrhythmias in cis women. Expert opinion: Research in the field of transgender medicine is expanding. As more trans women initiate HT, we will have a larger database from which to collect information regarding the benefits and risks of treatment, including the potential side effect of arrhythmias.

Age as a Critical Determinant of Atrial Fibrillation: A Two-sided Relationship

The incidence of atrial fibrillation (AF), the most common sustained arrhythmia and a major public health burden, increases exponentially with age. However, mechanisms underlying this long-recognized association remain incompletely understood. Experimental and human studies have demonstrated the involvement of aging in several arrhythmogenic processes, including atrial electrical and structural remodelling, disturbed calcium homeostasis, and enhanced atrial ectopic activity/increased vulnerability to re-entry induction. Given this wide range of putative mechanisms, the task of delineating the specific effects of aging responsible for AF promotion is not simple, as aging is itself associated with increasing prevalence of a host of AF-predisposing conditions, including heart failure, coronary artery disease, and hypertension. Although we usually think of old age promoting AF, there is also evidence that young age may actually have a protective effect against AF occurrence. For example, the low AF incidence among populations of young patients with significant structural congenital heart disease and substantial atrial enlargement/remodelling suggests that younger age might protect against fibrillation in the diseased atrium; efforts at understating how younger age may prevent AF might be helpful in elucidating missing mechanistic links between AF and age. The goal of this paper is to review the epidemiologic and pathophysiologic evidence regarding mechanisms underlying age-related AF. Although the therapeutic options for AF have recently improved, major gaps still remain and a better understanding of the special relationship between age and AF may be important for the identification of new targets for therapeutic innovation.

Fibroblast growth factor 23 dysregulates late sodium current and calcium homeostasis with enhanced arrhythmogenesis in pulmonary vein cardiomyocytes

Fibroblast growth factor 23 (FGF23), elevated in chronic renal failure, increases atrial arrhythmogenesis and dysregulates calcium homeostasis. Late sodium currents (I_Na-Late) critically induces ectopic activity of pulmoanry vein (the most important atrial fibrillation trigger). This study was to investigate whether FGF23 activates the I_Na-Late leading to calcium dysregulation and increases PV arrhythmogenesis. Patch clamp, western blot, and confocal microscopy were used to evaluate the electrical activities, calcium homeostasis, and mitochondrial reactive oxygen species (ROS) in PV cardiomyocytes with or without FGF23 (0.1 or 1 ng/mL) incubation for 4~6 h. Compared to the control, FGF23 (1 ng/mL, but not 0.1 ng/mL)-treated PV cardiomyocytes had a faster beating rate. FGF23 (1 ng/mL)-treated PV cardiomyocytes had larger I_Na-Late, calcium transients, and mitochondrial ROS than controls. However, ranolazine (an inhibitor of I_Na-Late) attenuated FGF23 (1 ng/mL)-increased beating rates, calcium transients and mitochondrial ROS. FGF23 (1 ng/mL)-treated PV cardiomyocytes exhibited larger phosphorylation of calcium/calmodulin-dependent protein kinase II (CaMKII). Chelerythrine chloride (an inhibitor of protein kinase C) decreased I_Na-Late in FGF23 (1 ng/mL)-treated PV cardiomyocytes. However, KN93 (a selective CaMKII blocker) decreased I_Na-Late in control and FGF23 (1 ng/mL)-treated PV cardiomyocytes to a similar extent. In conclusion, FGF23 increased PV arrhythmogenesis through sodium and calcium dysregulation by acting protein kinase C signaling.

Androgen receptor is expressed in mouse cardiomyocytes at prenatal and early postnatal developmental stages

Background

Previous studies show that androgens are involved in hypertrophy and excitability of cardiomyocytes and that their effects are mediated through their receptor. The aim of this study was to evaluate the presence of androgen receptor (AR) in mouse heart during prenatal and early postnatal stages.

Results

The expression of AR and related genes, alpha myosin heavy chain -Myh6-, beta myosin heavy chain -Myh7- and atrial natriuretic factor –Nppa- was simultaneously evaluated by semiquantitative RT-PCR. AR was also detected by immunohistochemistry. Androgen receptor mRNA was detected in hearts from 10.5 days post coitum to 16 postnatal days. A higher expression of AR mRNA in atria compared to ventricles was observed in neonatal mouse. A positive correlation between mRNA levels of AR and Nppa was observed in mouse heart at early postnatal development. Androgen receptor expression is similar in males and females during cardiac development. Finally, androgen receptor protein was observed by immunohistochemistry in myocardial cells of atria and ventricles from 12.5 days onwards and restricted after 16.5 days post-coitum to nuclei of cardiomyocytes.

Conclusion

Present results provide evidence that androgen receptor is expressed from prenatal stages in mouse heart, supporting the proposition that androgens could be involved in mammalian heart development.

Increment of late sodium currents in the left atrial myocytes and its potential contribution to increased susceptibility of atrial fibrillation in castrated male mice

BACKGROUND

The incidence of atrial fibrillation (AF) is correlated with decreased levels of testosterone in elderly men. Late sodium current may exert a role in AF pathogenesis.

OBJECTIVE

The purpose of this study was to explore the effect of testosterone deficiency on AF susceptibility and the therapeutic effect of late sodium current inhibitors in mice.

METHODS

Male ICR mice (5 weeks old) were castrated to establish a testosterone deficiency model. One month after castration, dihydrotestosterone 5 mg/kg was administered subcutaneously for 2 months. Serum total testosterone level was assessed by enzyme-linked immunosorbent assay. High-frequency electrical stimulation was used to induce atrial arrhythmias. Whole-cell patch-clamp technique was used to for single-cell electrophysiologic study.

RESULTS

Serum dihydrotestosterone levels of castration mice declined significantly but recovered with administration of exogenous dihydrotestosterone. In comparison with sham mice, the number of AF episodes significantly increased by 13.5-fold, AF rate increased by 3.75-fold, and AF duration prolonged in castrated mice. Dihydrotestosterone administration alleviated the occurrence of AF. Action potential duration at both 50% and 90% repolarization were markedly increased in castrated mice compared to sham controls. The late sodium current was enhanced in castrated male mice. These alterations were alleviated by treatment with dihydrotestosterone. Systemic application of the I_Na-L inhibitors ranolazine, eleclazine, and GS967 inhibited the occurrence of AF in castrated mice.

CONCLUSION

Testosterone deficiency contributed to the increased late sodium current, prolonged action potential repolarization, and increased susceptibility to AF. Blocking of late sodium current is beneficial against the occurrence of AF in castrated mice.

Renal failure induces atrial arrhythmogenesis from discrepant electrophysiological remodeling and calcium regulation in pulmonary veins, sinoatrial node, and atria

BACKGROUND

Renal failure (RF) increases the risk of atrial fibrillation (AF), but arrhythmogenic mechanism is unclear. The present study investigated the electrophysiological effects of RF on AF trigger (pulmonary veins, PVs) and substrate (atria) and evaluated potential underlying mechanisms.

METHODS

Electrocardiographic, echocardiographic, and biochemical studies were conducted in rabbits with and without antibiotic-induced mild (creatinine=1.5-6.0 mg/dl) and advanced (creatinine>6.0 mg/dl) RF. Conventional microelectrode techniques, western blotting, and histological examinations were performed using the isolated rabbit PV, left atrium (LA), right atrium (RA) and sinoatrial node (SAN).

RESULTS

Advanced RF rabbits (n=18) had a higher incidence (33.3% vs. 11.1% and 0%, p<0.05) of atrial arrhythmia than mild RF (n=18) and control (n=18) rabbits. Advanced RF rabbits exhibited faster PV spontaneous activities, longer action potential duration (APD) in the LA, higher fibrosis in the LA, and slower SAN beating rates than control rabbits, but had a similar APD and fibrosis in the RA. Caffeine (1 mM) increased advanced RF PV arrhythmogenesis, which is blocked by flecainide (10 μM), or KB-R7943 (10 μM). Moreover, advanced RF rabbits had a higher expression of the Na+/Ca2+ exchanger, protein kinase A, phosphorylated ryanodine receptor (Serine 2808), and phosphorylated phospholamban (Serine 16) in PVs, and a higher expression of Cav 1.2 in the LA, and a lower expression of hyperpolarization-activated cyclic nucleotide-gated potassium channel 4 in the SAN.

CONCLUSIONS

Advanced RF increases atrial arrhythmia by modulating the distinctive electrophysiological characteristics of the PV, LA, and SAN.

Testosterone replacement increases aged pulmonary vein and left atrium arrhythmogenesis with enhanced adrenergic activity

BACKGROUND

Aging and testosterone deficiency contribute to the pathogenesis of atrial fibrillation (AF). We determine the effects of testosterone replacement on the electrophysiology and arrhythmogenesis of pulmonary vein (PV) and left atrium (LA) in aged rabbits.

METHODS

Electrocardiography, heart rate variability, echocardiography, Western blot and conventional microelectrodes were used in aged rabbits (age, >2 years) with and without (control) testosterone treatment (10mg/kg, 12 weeks).

RESULTS

Testosterone-treated aged rabbits had longer corrected QT interval, higher low frequency/high frequency, greater left ventricle (LV) mass but lower LA total emptying fraction and LV ejection fraction than control rabbits. In tissue preparations, the spontaneous rate was faster for testosterone-treated PVs than for control PVs. Angiotensin II concentration-dependently increased the amplitude of delayed afterdepolarizations (DADs) in testosterone-treated PVs but only did so at the highest angiotensin II concentration (100 nM) in control PVs. Isoproterenol increased the incidence of early afterdepolarizations (EADs) and DADs in testosterone-treated PVs but not in control PVs. Testosterone-treated PVs had more H2O2-induced burst firing and EADs than control PVs. Testosterone-treated LAs had more isoproterenol-induced DADs and spontaneous activity than did control LAs. However, acetylcholine infusion and rapid atrial pacing (10-20 Hz) induced AF in control LAs but not in testosterone-treated LAs. In addition, as compared with control LAs, testosterone-treated LAs expressed more androgen receptor, β1-adrenergic receptor, and Cav 1.2 and less G protein-coupled receptor kinase-2 and Kv 4.2.

CONCLUSIONS

Testosterone replacement increased arrhythmogenesis in PV and LA by enhancing adrenergic activity in aged rabbits.