Atrial tachyarrhythmias induced by the combined effects of β1/2-adrenergic autoantibodies and thyroid hormone in the rabbit

Autoantibodies in Atrial Fibrillation-State of the Art

Atrial fibrillation (AF) is the most common type of cardiac arrhythmia. To date, a lot of research has been conducted to investigate the underlying mechanisms of this disease at both molecular and cellular levels. There is increasing evidence suggesting that autoimmunity is an important factor in the initiation and perpetuation of AF. Autoantibodies are thought to play a pivotal role in the regulation of heart rhythm and the conduction system and, therefore, are associated with AF development. In this review, we have summarized current knowledge concerning the role of autoantibodies in AF development as well as their prognostic and predictive value in this disease. The establishment of the autoantibody profile of separate AF patient groups may appear to be crucial in terms of developing novel treatment approaches for those patients; however, the exact role of various autoantibodies in AF is still a matter of ongoing debate.

Mechanisms and Management of Thyroid Disease and Atrial Fibrillation: Impact of Atrial Electrical Remodeling and Cardiac Fibrosis

Atrial fibrillation (AF) is the most common cardiac arrhythmia associated with increased cardiovascular morbidity and mortality. The pathophysiology of AF is characterized by electrical and structural remodeling occurring in the atrial myocardium. As a source of production of various hormones such as angiotensin-2, calcitonin, and atrial natriuretic peptide, the atria are a target for endocrine regulation. Studies have shown that disorders associated with endocrine dysregulation are potential underlying causes of AF. The thyroid gland is an endocrine organ that secretes three hormones: triiodothyronine (T3), thyroxine (T4) and calcitonin. Thyroid dysregulation affects the cardiovascular system. Although there is a well-established relationship between thyroid disease (especially hyperthyroidism) and AF, the underlying biochemical mechanisms leading to atrial fibrosis and atrial arrhythmias are poorly understood in thyrotoxicosis. Various animal models and cellular studies demonstrated that thyroid hormones are involved in promoting AF substrate. This review explores the recent clinical and experimental evidence of the association between thyroid disease and AF. We highlight the current knowledge on the potential mechanisms underlying the pathophysiological impact of thyroid hormones T3 and T4 dysregulation, in the development of the atrial arrhythmogenic substrate. Finally, we review the available therapeutic strategies to treat AF in the context of thyroid disease.

M2 muscarinic autoantibodies and thyroid hormone promote susceptibility to atrial fibrillation and sinus tachycardia in an autoimmune rabbit model

NEW FINDINGS

What is the central question of this study? Do autoantibodies to the M2 muscarinic receptor (M2R-AAbs) have the potential to facilitate specific sustained tachyarrhythmias in the presence of thyroxine (T4 ) in rabbits? What is the main finding and its importance? The M2R-AAb and T4 jointly destabilized the electrophysiological properties, thus promoting the occurrence of atrial and sinus tachyarrhythmias in rabbits. These findings provide a practical basis for understanding the pathophysiological role of M2R-AAb alone and with T4 in arrhythmia induction and might provide an innovative option for treatment of Graves' disease with rhythm disturbance.

ABSTRACT

Activating autoantibodies toward the β1/2 -adrenergic receptors (β1/2AR-AAbs) and M2 muscarinic receptor (M2R-AAbs) are present in a high proportion of patients with Graves' disease. We previously demonstrated that β1/2AR-AAbs with or without the presence of M2R-AAbs in combination with excessive thyroxine (T4 ) increased the induction of sustained tachyarrhythmias in an autoimmune rabbit model. However, the separate role of M2R-AAbs and their interaction with T4 are not clear. The aim of this study was to investigate the impact of M2R-AAbs and T4 on the induction of cardiac arrhythmias in a similar rabbit model. Ten New Zealand White rabbits were randomly divided into two groups. In group A (n = 6), the rabbits were immunized with the second extracellular loop peptide of M2R and subjected to 2 weeks of T4 treatment. In group B (n = 4), the rabbits were treated only with T4 for 2 weeks. After induction of general anaesthesia, rabbits were subjected to an electrophysiological study at 0 (pre-immune), 6 (post-immune) and 8 weeks (post-immune+T4 treatment) in group A and at 0 (baseline) and 8 weeks (T4 treatment) in group B. Each rabbit served as its own control. In group A, high levels and activity of M2R-AAbs were detected in all immunized animals. Thyroxine in combination with immunization significantly increased induction of sustained sinus tachycardia and atrial fibrillation in comparison to the pre-immune state. In group B, T4 predominantly induced sustained sinus tachycardia. This study demonstrated that M2R-AAbs and T4 jointly increased the susceptibility to both sinus and atrial tachyarrhythmias. The data supported the pathophysiological role of M2R-AAbs in hyperthyroidism-associated supraventricular tachyarrhythmias.

Elevated β1-Adrenergic Receptor Autoantibody Levels Increase Atrial Fibrillation Susceptibility by Promoting Atrial Fibrosis

Objective

Beta 1-adrenergic receptor autoantibodies (β1ARAbs) have been identified as a pathogenic factor in atrial fibrillation (AF), but the underlying pathogenetic mechanism is not well understood. We assessed the hypothesis that elevated β1ARAb levels increase AF susceptibility by promoting atrial fibrosis.

Methods

A total of 70 patients with paroxysmal AF were continuously recruited. The serum levels of β1ARAb and circulating fibrosis biomarkers were analyzed by ELISA. Linear regression was used to examine the correlations of β1ARAb levels with left atrial diameter (LAD) and circulating fibrosis biomarker levels. Furthermore, we established a rabbit β1ARAb overexpression model. We conducted electrophysiological studies and multielectrode array recordings to evaluate the atrial effective refractory period (AERP), AF inducibility and electrical conduction. AF was defined as irregular, rapid atrial beats > 500 bpm for > 1000 ms. Echocardiography, hematoxylin and eosin staining, Masson's trichrome staining, and picrosirius red staining were performed to evaluate changes in atrial structure and detect fibrosis. Western blotting and PCR were used to detect alterations in the protein and mRNA expression of TGF-β1, collagen I and collagen III.

Results

Patients with a LAD ≥ 40 mm had higher β1ARAb levels than patients with a smaller LAD (8.87 ± 3.16 vs. 6.75 ± 1.34 ng/mL, P = 0.005). β1ARAb levels were positively correlated with LAD and circulating biomarker levels (all P < 0.05). Compared with the control group, the rabbits in the immune group showed the following: (1) enhanced heart rate, shortened AERP (70.00 ± 5.49 vs. 96.46 ± 3.27 ms, P < 0.001), increased AF inducibility (55% vs. 0%, P < 0.001), decreased conduction velocity and increased conduction heterogeneity; (2) enlarged LAD and elevated systolic dysfunction; (3) significant fibrosis in the left atrium identified by Masson's trichrome staining (15.17 ± 3.46 vs. 4.92 ± 1.72%, P < 0.001) and picrosirius red staining (16.76 ± 6.40 vs. 4.85 ± 0.40%, P < 0.001); and (4) increased expression levels of TGF-β1, collagen I and collagen III.

Conclusion

Our clinical and experiential studies showed that β1ARAbs participate in the development of AF and that the potential mechanism is related to the promotion of atrial fibrosis.

Adrenergic Autoantibody-Induced Postural Tachycardia Syndrome in Rabbits

Background Previous studies have demonstrated that functional autoantibodies to adrenergic receptors may be involved in the pathogenesis of postural tachycardia syndrome. The objective of this study was to examine the impact of these autoantibodies on cardiovascular responses to postural changes and adrenergic orthosteric ligand infusions in immunized rabbits. Methods and Results Eight New Zealand white rabbits were coimmunized with peptides from the α1-adrenergic receptor and β1-adrenergic receptor (β1AR). Tilt test and separate adrenergic agonist infusion studies were performed on conscious animals before and after immunization and subsequent treatment with epitope-mimetic peptide inhibitors. At 6 weeks after immunization, there was a greater percent increase in heart rate upon tilting compared with preimmune baseline. No significant difference in blood pressure response to tilting was observed. The heart rate response to infusion of the β-adrenoceptor agonist isoproterenol was significantly enhanced in immunized animals, suggesting a positive allosteric effect of β1AR antibodies. In contrast, the blood pressure response to infusion of the α1-adrenergic receptor agonist phenylephrine was attenuated in immunized animals, indicating a negative allosteric effect of α1-adrenergic receptor antibodies. Injections of antibody-neutralizing peptides suppressed the postural tachycardia and reversed the altered heart rate and blood pressure responses to orthosteric ligand infusions in immunized animals at 6 and 30 weeks. Antibody production and suppression were confirmed with in vitro bioassays. Conclusions The differential allosteric effect of α1-adrenergic receptor and β1AR autoantibodies would lead to a hyperadrenergic state and overstimulation of cardiac β1AR. These data support evidence for an autoimmune basis for postural tachycardia syndrome.

β1-Adrenergic and M2 Muscarinic Autoantibodies and Thyroid Hormone Facilitate Induction of Atrial Fibrillation in Male Rabbits

Activating autoantibodies to the β1-adrenergic and M2 muscarinic receptors are present in a very high percentage of patients with Graves' disease and atrial fibrillation (AF). The objective of this study was to develop a reproducible animal model and thereby to examine the impact of these endocrine-like autoantibodies alone and with thyroid hormone on induction of thyroid-associated atrial tachyarrhythmias. Five New Zealand white rabbits were coimmunized with peptides from the second extracellular loops of the β1-adrenergic and M2 muscarinic receptors to produce both sympathomimetic and parasympathomimetic antibodies. A catheter-based electrophysiological study was performed on anesthetized rabbits before and after immunization and subsequent treatment with thyroid hormone. Antibody expression facilitated the induction of sustained sinus, junctional and atrial tachycardias, but not AF. Addition of excessive thyroid hormone resulted in induced sustained AF in all animals. AF induction was blocked acutely by the neutralization of these antibodies with immunogenic peptides despite continued hyperthyroidism. The measured atrial effective refractory period as one parameter of AF propensity shortened significantly after immunization and was acutely reversed by peptide neutralization. No further decrease in the effective refractory period was observed after the addition of thyroid hormone, suggesting other cardiac effects of thyroid hormone may contribute to its role in AF induction. This study demonstrates autonomic autoantibodies and thyroid hormone potentiate the vulnerability of the heart to AF, which can be reversed by decoy peptide therapy. These data help fulfill Witebsky's postulates for an increased autoimmune/endocrine basis for Graves' hyperthyroidism and AF.

A peptidomimetic inhibitor suppresses the inducibility of β1-adrenergic autoantibody-mediated cardiac arrhythmias in the rabbit

PURPOSE

Previous studies demonstrated that burst pacing and subthreshold infusion of acetylcholine in β1-adrenergic receptor (β1AR)-immunized rabbits induced sustained sinus tachycardia. The aim of this study was to examine the anti-arrhythmogenic effect of a newly designed retro-inverso (RI) peptidomimetic inhibitor that specifically targets the β1AR antibodies in the rabbit.

METHODS

Six New Zealand white rabbits were immunized with a β1AR second extracellular loop peptide to produce sympathomimetic β1AR antibodies. A catheter-based electrophysiological study was performed on anesthetized rabbits before and after immunization and subsequent treatment with the RI peptide inhibitor. Each rabbit served as its own control.

RESULTS

No sustained arrhythmias were induced at preimmune baseline. At 6 weeks after immunization, there was a marked increase in induced sustained tachyarrhythmias, predominantly sinus tachycardia, which was largely suppressed by the RI peptide. The atrial effective refractory period was shortened significantly in immunized rabbits compared to their preimmune state. The RI peptide reversed and prolonged this shortening. β1AR antibody levels were negatively correlated with the atrial effective refractory period. Postimmune sera-induced β1AR activation in transfected cells in vitro was also blocked by the RI peptide.

CONCLUSIONS

β1AR-activating autoantibodies are associated with reduction of the atrial effective refractory period and facilitate arrhythmia induction in this model. The RI peptide reversal may have important therapeutic implications in subjects who harbor these autoantibodies.

Activating autoantibodies to the β1/2-adrenergic and M2 muscarinic receptors associate with atrial tachyarrhythmias in patients with hyperthyroidism

We have previously demonstrated that activating autoantibodies to β1-adrenergic receptor (β1AR) and M2 muscarinic receptor (M2R) facilitate atrial fibrillation (AF) in patients with Graves' disease (GD). The objectives of this expanded study were to examine the prevalence of β1AR, β2AR, and M2R autoantibodies in hyperthyroidism subjects. Sera from 81 patients including 31 with GD and AF, 36 with GD and sinus rhythm, 9 with toxic multinodular goiter, 5 with subacute thyroiditis, and 10 control subjects were examined for these autoantibodies by ELISA. Sera from 20 ELISA-positive GD subjects, 10 with AF and 10 with sinus rhythm, were assayed for autoantibody bioactivity using cell-based bioassays. In patients with GD and AF, 45, 65, and 77 % were ELISA positive for β1AR, M2R, and β2AR autoantibodies, respectively. In patients with GD and sinus rhythm, 17, 39, and 75 % were ELISA positive for β1AR, M2R, and β2AR autoantibodies, respectively. β1AR and M2R autoantibodies were co-present in 39 % of patients with GD and AF compared to 14 % in GD with sinus rhythm (p = 0.026). Patients with toxic multinodular goiter or subacute thyroiditis had a low prevalence of autoantibodies. The mean β1AR and M2R autoantibody activity was elevated in both GD groups but higher in those with AF than those with sinus rhythm. β2AR autoantibody activity was also increased in both groups. In conclusion, β1AR, β2AR, and M2R autoantibodies were elevated in GD. β1AR and M2R autoantibodies appear to be related to concurrent AF, while β2AR autoantibodies were equally prevalent in those with a sinus tachycardia and those with AF.