Sinus Tachycardia: a Multidisciplinary Expert Focused Review.

Sinus tachycardia (ST) is ubiquitous, but its presence outside of normal physiological triggers in otherwise healthy individuals remains a commonly encountered phenomenon in medical practice. In many cases, ST can be readily explained by a current medical condition that precipitates an increase in the sinus rate, but ST at rest without physiological triggers may also represent a spectrum of normal. In other cases, ST may not have an easily explainable cause but may represent serious underlying pathology and can be associated with intolerable symptoms. The classification of ST, consideration of possible etiologies, as well as the decisions of when and how to intervene can be difficult. ST can be classified as secondary to a specific, usually treatable, medical condition (eg, pulmonary embolism, anemia, infection, or hyperthyroidism) or be related to several incompletely defined conditions (eg, inappropriate ST, postural tachycardia syndrome, mast cell disorder, or post-COVID syndrome). While cardiologists and cardiac electrophysiologists often evaluate patients with symptoms associated with persistent or paroxysmal ST, an optimal approach remains uncertain. Due to the many possible conditions associated with ST, and an overlap in medical specialists who see these patients, the inclusion of experts in different fields is essential for a more comprehensive understanding. This article is unique in that it was composed by international experts in Neurology, Psychology, Autonomic Medicine, Allergy and Immunology, Exercise Physiology, Pulmonology and Critical Care Medicine, Endocrinology, Cardiology, and Cardiac Electrophysiology in the hope that it will facilitate a more complete understanding and thereby result in the better care of patients with ST.

Gonadotrophin-releasing hormone receptor autoantibodies induce polycystic ovary syndrome-like features in a rat model.

NEW FINDINGS: What is the central question of this study? Is there a causal relationship between gonadotrophin-releasing hormone (GnRH) receptor-activating autoantibodies and polycystic ovary syndrome (PCOS)? What is the main finding and its importance? Induction of GnRH receptor-activating autoantibodies in rats resulted in increased luteinizing hormone pulsatility and testosterone concentrations, disrupted oestrous cycles, increased atretic follicles, and activation of insulin signalling in the pituitary and ovary. These changes replicate those seen in humans with PCOS, suggesting that GnRH receptor-activating autoantibodies might be involved in the pathogenesis of PCOS. ABSTRACT: Gonadotrophin-releasing hormone receptor-activating autoantibodies (GnRHR-AAb) are associated with polycystic ovary syndrome (PCOS). In the present study, we examined the impact of GnRHR-AAb on reproductive function in GnRHR-immunized female rats. All immunized rats produced high titres of GnRHR-AAb targeting a dominant epitope located in the central region of the second extracellular loop of the GnRHR. Increased pulsatile luteinizing hormone secretion and testosterone concentrations were found in immunized rats. These rats exhibited disrupted oestrous cycles, increased ovarian follicular atresia, and activation of insulin signalling in the pituitary and ovary, as indicated by increased mRNA expressions of insulin receptor substrate, phosphatidylinositol 3-kinase and glucose transporter 1. No significant changes in inflammatory cytokines were detected in the ovarian tissue. These features mimic those observed in humans with PCOS. Our findings support the concept that chronic stimulation of the GnRHR by GnRHR-AAb, with an associated increase in pituitary luteinizing hormone secretion and ovarian androgen overproduction, might represent a new aetiological mechanism for PCOS.

Autoimmune activation of the GnRH receptor induces insulin resistance independent of obesity in a female rat model.

Polycystic ovary syndrome (PCOS), a metabolic and reproductive disease, is frequently associated with type 2 diabetes. We have demonstrated activating autoantibodies (AAb) directed toward the second extracellular loop (ECL2) of the gonadotropin-releasing hormone receptor (GnRHR) are present in a significant subgroup of PCOS patients. It is unclear whether GnRHR-AAb can induce peripheral tissue insulin resistance (IR) in animal models. Sixteen rats were divided equally into a GnRHR ECL2 peptide-immunized group (IMM group) and a control group (CON group). Sera GnRHR-AAb titer, luteinizing hormone (LH), and testosterone (T) were higher in IMM rats compared with CON rats. No significant difference in fasting blood glucose was observed between the two groups. However, the plasma glucose level at other time points of the IMM group was higher than that of the CON group during an intraperitoneal glucose tolerance test (IPGTT) and an insulin tolerance test (ITT) (p < 0.01). These data support the likelihood of the GnRHR-AAb induction of glucose intolerance and IR. Compared with the CON group, the IMM group showed a significant increase in insulin-stimulated phosphorylation of IRS-1 (p-IRS-1 S636/639) and a decrease in insulin-stimulated phosphorylation of Akt (p-AKT S473). Expression of the glucose transport genes including GLUT-2 in liver and GLUT-4 in white adipose tissue and skeletal muscle was significantly decreased in IMM rats compared with the CON rats. Serum levels of proinflammatory cytokines (TNF-α, IL-1α, and IL-18) were increased, while anti-inflammatory cytokines (IL-4 and IL-10) were decreased in the IMM group. Taken together, elevated GnRHR-AAb enhanced LH, hyperandrogenism, and inflammation. These changes are likely related to the observed peripheral tissue IR through the downregulation of the insulin-stimulated IRS/PI3K/Akt/Glut signaling pathway.

The Role of GnRH Receptor Autoantibodies in Polycystic Ovary Syndrome.

OBJECTIVE: Is polycystic ovary syndrome (PCOS) associated with activating autoantibodies (AAb) to the second extracellular loop (ECL2) of gonadotropin-releasing hormone receptor (GnRHR)? DESIGN AND METHODS: We retrospectively screened sera from 40 patients with PCOS and 14 normal controls (NCs) with regular menses using enzyme-linked immunosorbent assay (ELISA) for the presence of GnRHR-ECL2-AAb. We obtained similar data from 40 non-PCOS ovulatory but infertile patients as a control group (OIC) of interest. We analyzed GnRHR-ECL2-AAb activity in purified immunoglobulin (Ig)G using a cell-based GnRHR bioassay. RESULTS: The mean ELISA value in the PCOS group was markedly higher than the NC (P = .000036) and the OIC (P = .0028) groups. IgG from a sample of 5 PCOS subjects, in contrast to a sample of 5 OIC subjects, demonstrated a dose-dependent increase in GnRHR-stimulating activity qualitatively similar to the acute action of the natural ligand GnRH and the synthetic agonist leuprolide. The GnRHR antagonist cetrorelix significantly suppressed (P < .01) the elevated GnRHR activity induced by IgG from 7 PCOS patients while the IgG activity level from 7 OIC subjects was unchanged. Five other OIC subjects had relatively high ELISA values at or above the 95% confidence limits. On further study, 3 had normal or low activity while 2 had elevated IgG-induced GnRHR activity. One suppressed with cetrorelix while the other did not. The copresence of PCOS IgG increased the responsiveness to GnRH and shifted the dosage response curve to the left (P < .01). CONCLUSIONS: GnRHR-ECL2-AAb are significantly elevated in patients with PCOS compared with NCs. Their presence raises important etiological, diagnostic, and therapeutic implications.

Elevated activity levels of activating autoantibodies to the GnRH receptor in patients with polycystic ovary syndrome.

OBJECTIVES: 1) To confirm the correlation of GnRH receptor (GnRHR) activating autoantibody (AAb) activity with polycystic ovary syndrome (PCOS) diagnosis in large well defined cohorts; and 2) to evaluate suppression of AAb activity with GnRH antagonist medication in transfected GnRHR cells exposed to serum of PCOS patients. DESIGN: Cross-sectional matched case-control study. SETTING: University-based research facility. PATIENTS: Sera from 200 patients with PCOS from the Pregnancy in Polycystic Ovary Syndrome II (PPCOS II) trial and from 200 race, parity-, age-, and body mass index (BMI)-matched ovulatory unexplained infertile control patients from the Assessment of Multiple Intrauterine Gestations from Ovarian Stimulation (AMIGOS) trial were obtained and used for this study. INTERVENTIONS: GnRHR AAb activity was determined with the use of the GeneBlazer cell-based fluorescence resonance energy transfer assay with and without cetrorelix, a GnRH antagonist. MAIN OUTCOME MEASURES: 1) GnRHR AAb activity in PCOS patients compared with control subjects; and 2) effectiveness of GnRH antagonist in suppressing GnRHR AAb activity. RESULTS: GnRHR AAb activity levels in the PCOS group were significantly higher than in the control group. With cetrorelix, GnRHR AAb activity was largely suppressed in the PCOS group but not in the control group. These differences remained significant after adjusting for within-pair differences in age, BMI, and antimüllerian hormone (AMH) levels. CONCLUSIONS: We confirmed higher GnRHR AAb activity levels in the sera of a large cohort of PCOS patients compared with unexplained infertile control subjects. Addition of cetrorelix resulted in significant suppression of AAb activity levels in PCOS patients as a group whereas control subjects were unaffected. GnRHR AAb, along with patient age and AMH level, may provide a promising future diagnostic test for PCOS.

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 ß1-adrenergic receptor (ß1AR). 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 ß-adrenoceptor agonist isoproterenol was significantly enhanced in immunized animals, suggesting a positive allosteric effect of ß1AR 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 ß1AR autoantibodies would lead to a hyperadrenergic state and overstimulation of cardiac ß1AR. These data support evidence for an autoimmune basis for postural tachycardia syndrome.

A functional cell-based bioassay for assessing adrenergic autoantibody activity in postural tachycardia syndrome.

BACKGROUND: Activating autoantibodies (AAb) to adrenergic receptors (AR) have previously been reported in patients with postural tachycardia syndrome (POTS). These AAb may contribute to a final common pathway for overlapping disease processes, reflecting a possible autoimmune contribution to POTS pathophysiology. In prior studies, measurement of AAb activity was inferred from costly, low-throughput, and laborious physiological assays. In the present study, we developed and validated an alternative cell-based bioassay for measuring AAb activity in serum by means of pre-treatment with monoamine oxidase (MAO). METHODS: A total of 37 POTS patients and 61 sex-matched healthy control participants were included. Serum was pre-treated with MAO to remove endogenous catecholamines that could falsely inflate AR activation by AAb. A receptor-transfected cell-based bioassay was used to detect presence of α1AR-AAb and ß1AR-AAb in serum. RESULTS: MAO effectively degraded catecholamines as demonstrated by suppression of norepinephrine-induced α1AR activation in POTS (6.4 â€‹± â€‹0.7 vs. 5.5 â€‹± â€‹0.9; P â€‹= â€‹0.044) and in controls (4.1 â€‹± â€‹0.5 vs. 3.9 â€‹± â€‹0.6; P â€‹= â€‹0.001). Mean activity values were greater in the POTS vs. Controls for α1AR-AAb (6.2 â€‹± â€‹1.2 vs. 5.3 â€‹± â€‹1.0; P â€‹< â€‹0.001) and ß1AR-AAb (5.7 â€‹± â€‹1.8 vs. 4.1 â€‹± â€‹0.9; P â€‹< â€‹0.001). Compared to controls, more POTS patients were positive for α1AR-AAb activity (22% vs 4%; P â€‹= â€‹0.007) and ß1AR-AAb activity (52% vs. 2%; P â€‹< â€‹0.001). CONCLUSIONS: The co-presence of norepinephrine in serum samples can artifactually elevate α1AR and ß1AR activity, which can be avoided by serum pre-treatment with MAO. Using this novel bioassay, we show that POTS patients have increased α1AR-AAb and ß1AR-AAb activity compared to healthy controls in the largest POTS cohort reported to-date.

Angiotensin II Type 1 Receptor Autoantibodies in Postural Tachycardia Syndrome.

BACKGROUND: Both the adrenergic and renin-angiotensin systems contribute to orthostatic circulatory homeostasis, which is impaired in postural orthostatic tachycardia syndrome (POTS). Activating autoantibodies to the α1-adrenergic and ß1/2-adrenergic receptors have previously been found in sera from patients with POTS. We hypothesized that patients with POTS might also harbor activating autoantibodies to the angiotensin II type 1 receptor (AT1R) independently of antiadrenergic autoimmunity. This study examines a possible pathophysiological role for AT1R autoantibodies in POTS. METHODS AND RESULTS: Serum immunoglobulin G from 17 patients with POTS, 6 patients with recurrent vasovagal syncope, and 10 normal controls was analyzed for the ability to activate AT1R and alter AT1R ligand responsiveness in transfected cells in vitro. Of 17 subjects with POTS, 12 demonstrated significant AT1R antibody activity in immunoglobulin G purified from their serum. No significant AT1R antibody activity was found in the subjects with vasovagal syncope or healthy subjects. AT1R activation by POTS immunoglobulin G was specifically blocked by the AT1R blocker losartan. Moreover, POTS immunoglobulin G significantly shifted the angiotensin II dosage response curve to the right, consistent with an inhibitory effect. All subjects with POTS were positive for one or both autoantibodies to the AT1R and α1-adrenergic receptor. CONCLUSIONS: Most patients with POTS harbor AT1R antibody activity. This supports the concept that AT1R autoantibodies and antiadrenergic autoantibodies, acting separately or together, may exert a significant impact on the cardiovascular pathophysiological characteristics in POTS.

Antiadrenergic autoimmunity in postural tachycardia syndrome.

AIMS: Postural tachycardia syndrome (POTS), a common and debilitating cardiovascular disorder, is characterized by an exaggerated heart rate increase during orthostasis and a wide spectrum of adrenergic-related symptoms. To determine the aetiology of POTS, we examined a possible pathophysiological role for autoantibodies against α1-adrenergic (α1AR) and ß1/2-adrenergic receptors (ß1/2AR). METHODS AND RESULTS: Immunoglobulin G (IgG) derived from 17 POTS patients, 7 with recurrent vasovagal syncope (VVS), and 11 normal controls was analysed for its ability to modulate activity and ligand responsiveness of α1AR and ß1/2AR in transfected cells and to alter contractility of isolated rat cremaster arterioles in vitro. Immunoglobulin G activation of α1AR and ß1/2AR was significantly higher in POTS compared with VVS and controls in cell-based assays. Eight, 11, and 12 of the 17 POTS patients possessed autoantibodies that activated α1AR, ß1AR and ß2AR, respectively. Pharmacological blockade suppressed IgG-induced activation of α1AR and ß1/2AR. Eight of 17 POTS IgG decreased the α1AR responsiveness to phenylephrine and 13 of 17 POTS IgG increased the ß1AR responsiveness to isoproterenol irrespective of their ability to directly activate their receptors. Postural tachycardia syndrome IgG contracted rat cremaster arterioles, which was reversed by α1AR blockade. The upright heart rate correlated with IgG-mediated ß1AR and α1AR activity but not with ß2AR activity. CONCLUSION: These data confirm a strong relationship between adrenergic autoantibodies and POTS. They support the concept that allosteric-mediated shifts in the α1AR and ß1AR responsiveness are important in the pathophysiology of postural tachycardia.

Cardiac myosin-Th17 responses promote heart failure in human myocarditis.

In human myocarditis and its sequela dilated cardiomyopathy (DCM), the mechanisms and immune phenotype governing disease and subsequent heart failure are not known. Here, we identified a Th17 cell immunophenotype of human myocarditis/DCM with elevated CD4+IL17+ T cells and Th17-promoting cytokines IL-6, TGF-ß, and IL-23 as well as GM-CSF-secreting CD4+ T cells. The Th17 phenotype was linked with the effects of cardiac myosin on CD14+ monocytes, TLR2, and heart failure. Persistent heart failure was associated with high percentages of IL-17-producing T cells and IL-17-promoting cytokines, and the myocarditis/DCM phenotype included significantly low percentages of FOXP3+ Tregs, which may contribute to disease severity. We demonstrate a potentially novel mechanism in human myocarditis/DCM in which TLR2 peptide ligands from human cardiac myosin stimulated exaggerated Th17-related cytokines including TGF-ß, IL-6, and IL-23 from myocarditic CD14+ monocytes in vitro, and an anti-TLR2 antibody abrogated the cytokine response. Our translational study explains how an immune phenotype may be initiated by cardiac myosin TLR ligand stimulation of monocytes to generate Th17-promoting cytokines and development of pathogenic Th17 cells in human myocarditis and heart failure, and provides a rationale for targeting IL-17A as a therapeutic option.

ß1-Adrenergic and M2 Muscarinic Autoantibodies and Thyroid Hormone Facilitate Induction of Atrial Fibrillation in Male Rabbits.

Activating autoantibodies to the ß1-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 ß1-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 ß1-adrenergic autoantibody-mediated cardiac arrhythmias in the rabbit.

PURPOSE: Previous studies demonstrated that burst pacing and subthreshold infusion of acetylcholine in ß1-adrenergic receptor (ß1AR)-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 ß1AR antibodies in the rabbit. METHODS: Six New Zealand white rabbits were immunized with a ß1AR second extracellular loop peptide to produce sympathomimetic ß1AR 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. ß1AR antibody levels were negatively correlated with the atrial effective refractory period. Postimmune sera-induced ß1AR activation in transfected cells in vitro was also blocked by the RI peptide. CONCLUSIONS: ß1AR-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.

AT2R autoantibodies block angiotensin II and AT1R autoantibody-induced vasoconstriction.

Activating autoantibodies to the angiotensin type 1 receptor (AT1R) are associated with hypertensive disorders. The angiotensin type 2 receptor (AT2R) is known to counter-regulate the actions of AT1R. We investigated whether AT2R autoantibodies produced in immunized rabbits will activate AT2R and suppress the vasopressor responses to angiotensin II and AT1R-activating autoantibodies. Five rabbits immunized with a peptide corresponding to the second extracellular loop of AT2R developed high AT2R antibody titers. Rabbit anti-AT2R sera failed to directly dilate isolated rat cremaster arterioles; however, when co-perfused with angiotensin II or AT1R-activating autoantibodies, the anti-AT2R sera significantly inhibited their contractile effects. Rabbit anti-AT2R sera recognized a predominant sequence near the N-terminus of the AT2R second extracellular loop. A decoy peptide based on this sequence effectively reversed the opposing effect of the anti-AT2R sera on angiotensin II-induced contraction of rat cremaster arterioles. A similar blockade of the anti-AT2R sera effect was observed with the AT2R antagonist PD 123319 and the guanylyl cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one. Rabbit anti-AT2R sera reacted specifically with AT2R. No cross-reactivity with AT1R was observed. Blood pressure did not change in immunized animals. However, the pressor responses to incremental angiotensin II infusions were blunted in immunized animals. Thirteen subjects with primary aldosteronism demonstrated increased AT2R autoantibody levels compared with normal controls. In conclusion, AT2R autoantibodies produced in immunized rabbits have the ability to activate AT2R and counteract the AT1R-mediated vasoconstriction. These autoantibodies provide useful and selective tools for the study of their roles in blood pressure regulation and possible therapeutic intervention.

Novel retro-inverso peptide inhibitor reverses angiotensin receptor autoantibody-induced hypertension in the rabbit.

Activating autoantibodies to the angiotensin II type 1 receptor (AT1R) have been implicated in hypertensive disorders. We investigated whether AT1R antibodies produced in immunized rabbits will activate AT1R and contribute to hypertension by a direct contractile effect on the vasculature and whether they can be blocked by a novel decoy peptide. A multiple antigenic peptide containing the AT1R epitope AFHYESQ, which is the receptor-binding epitope of AT1R-activating autoantibodies, was used to immunize 6 rabbits. AT1R antibody activity was analyzed in AT1R-transfected cells, and their contractile effects were assayed using isolated perfused rat cremaster resistance arterioles. A retro-inverso D-amino acid epitope-mimetic peptide was tested for AT1R antibody inhibition in vitro and in vivo. All immunized animals produced high AT1R antibody titers and developed elevated blood pressure. No changes in measured blood chemistry values were observed after immunization. Rabbit anti-AT1R sera induced significant AT1R activation in transfected cells and vasoconstriction in the arteriole assay, both of which were blocked by losartan and the retro-inverso D-amino acid peptide. A single intravenous bolus injection of the retro-inverso d-amino acid peptide (1 mg/kg) into immunized rabbits dropped the mean arterial pressure from 122±11 to 82±6 mm Hg. Rabbit anti-AT1R sera partially suppressed angiotensin II-induced contraction of isolated rat cremaster arterioles, and the pressor response to angiotensin II infusion was attenuated in immunized animals. In conclusion, AT1R-activating autoantibodies and the retro-inverso d-amino acid peptide, respectively, have important etiologic and therapeutic implications in hypertensive subjects who harbor these autoantibodies.

Activating autoantibodies to the ß1/2-adrenergic and M2 muscarinic receptors associate with atrial tachyarrhythmias in patients with hyperthyroidism.

We have previously demonstrated that activating autoantibodies to ß1-adrenergic receptor (ß1AR) 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 ß1AR, ß2AR, 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 ß1AR, M2R, and ß2AR autoantibodies, respectively. In patients with GD and sinus rhythm, 17, 39, and 75 % were ELISA positive for ß1AR, M2R, and ß2AR autoantibodies, respectively. ß1AR 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 ß1AR and M2R autoantibody activity was elevated in both GD groups but higher in those with AF than those with sinus rhythm. ß2AR autoantibody activity was also increased in both groups. In conclusion, ß1AR, ß2AR, and M2R autoantibodies were elevated in GD. ß1AR and M2R autoantibodies appear to be related to concurrent AF, while ß2AR autoantibodies were equally prevalent in those with a sinus tachycardia and those with AF.

Prevalence of angiotensin II type 1 receptor (AT1R)-activating autoantibodies in primary aldosteronism.

Autoantibodies to the angiotensin II type 1 receptor (AT1R) have been reported in patients with primary aldosteronism, including aldosterone producing adenoma (APA) and idiopathic adrenal hyperplasia (IAH). Sera from 25 primary aldosteronism subjects (12 with IAH and 13 with APA) and 15 normotensive control subjects were assayed for AT1R autoantibodies by enzyme-linked immunosorbent assay and an AT1R-transfected cell-based bioassay. Nine of 12 IAH subjects (75%) and six of 13 APA subjects (46%) were positive for AT1R autoantibodies in the bioactivity assay. The mean AT1R autoantibody activity for the IAH and APA subjects was significantly greater than controls (P < .001 and P < .01, respectively), and this in vitro activity was suppressed by the AT1R blocker losartan. None of the controls had significant AT1R autoantibody activity. Enzyme-linked immunosorbent assay values were less sensitive but were positive in some subjects with IAH and APA. The mean arterial pressure of these primary aldosteronism subjects correlated modestly with AT1R autoantibody activity. These data confirm the presence of active AT1R autoantibodies in a high percentage of subjects with primary aldosteronism irrespective of their underlying etiology. These observations have both pathophysiological and clinical implications.

Atrial tachyarrhythmias induced by the combined effects of ß1/2-adrenergic autoantibodies and thyroid hormone in the rabbit.

Activating autoantibodies (AAb) to ß-adrenergic receptors (ßAR) are associated with atrial fibrillation in patients with Graves' disease. In the present study, we examined the interaction of thyroid hormone with ß1/2AR-AAb in inducing atrial tachyarrhythmias in the rabbit. Immunization of rabbits with a ß1AR or ß2AR second extracellular loop peptide produced high titers of ß1AR-AAb or ß2AR-AAb. Thyroid hormone in combination with ß1AR-AAb or ß2AR-AAb induced a significant number of sustained sinus tachycardia and atrial tachycardia, respectively. Both combinations resulted in significantly increased inductions of sustained arrhythmias compared to AAb alone. Thyroid hormone alone induced sustained sinus and junctional tachycardia. Sera from immunized rabbits specifically bound to and activated ß1AR or ß2AR in transfected cells in vitro. This study demonstrates thyroid hormone qualitatively accentuates the specific arrhythmogenic action of these AAb and quantitatively enhances their rate. Our data support a dual role of AAb and thyroid hormone in Graves'-associated tachyarrhythmias.

The Propensity for Inducing Atrial Fibrillation: A Comparative Study on Old versus Young Rabbits.

It is well established that atrial fibrillation (AF) is far more common in elderly humans. Autonomic activation is thought to be an operative mechanism for AF propensity. The aim of the study was to investigate the impact of age on atrial tachyarrhythmia induction in a rabbit model. Six old (aged 4-6 years) and 9 young (aged 3-4 months) New Zealand white rabbits were subjected to a catheter-based electrophysiological study. Atrial tachyarrhythmia susceptibility was tested by burst pacing before and after infusion of increasing concentrations of acetylcholine. Both young and old rabbits were in normal sinus rhythm at the beginning of the infusion/burst pacing protocol. The old rabbits had faster heart rates and a marked increase in atrial tachyarrhythmias compared to the young rabbits. Nonsustained and sustained AF events were more frequent in the old rabbits. No significant fibrosis was observed in the atria of either young or old rabbits. In conclusion, the old rabbits have a greater propensity for induction of AF. The significantly faster heart rates in the old rabbits suggest that dominant sympathetic activity may play an important role in the propensity for AF in this group.

Autoimmune basis for postural tachycardia syndrome.

BACKGROUND: Patients with postural tachycardia syndrome (POTS) have exaggerated orthostatic tachycardia often following a viral illness, suggesting autoimmunity may play a pathophysiological role in POTS. We tested the hypothesis that they harbor functional autoantibodies to adrenergic receptors (AR). METHODS AND RESULTS: Fourteen POTS patients (7 each from 2 institutions) and 10 healthy subjects were examined for α1AR autoantibody-mediated contractility using a perfused rat cremaster arteriole assay. A receptor-transfected cell-based assay was used to detect the presence of ß1AR and ß2AR autoantibodies. Data were normalized and expressed as a percentage of baseline. The sera of all 14 POTS patients demonstrated significant arteriolar contractile activity (69±3% compared to 91±1% of baseline for healthy controls, P<0.001) when coexisting ß2AR dilative activity was blocked; and this was suppressed by α1AR blockade with prazosin. POTS sera acted as a partial α1AR antagonist significantly shifting phenylephrine contractility curves to the right. All POTS sera increased ß1AR activation (130±3% of baseline, P<0.01) and a subset had increased ß2AR activity versus healthy subjects. POTS sera shifted isoproterenol cAMP response curves to the left, consistent with enhanced ß1AR and ß2AR agonist activity. Autoantibody-positive POTS sera demonstrated specific binding to ß1AR, ß2AR, and α1AR in transfected cells. CONCLUSIONS: POTS patients have elevated α1AR autoantibodies exerting a partial peripheral antagonist effect resulting in a compensatory sympathoneural activation of α1AR for vasoconstriction and concurrent ßAR-mediated tachycardia. Coexisting ß1AR and ß2AR agonistic autoantibodies facilitate this tachycardia. These findings may explain the increased standing plasma norepinephrine and excessive tachycardia observed in many POTS patients.

Autoimmune mechanisms activating the angiotensin AT1 receptor in 'primary' aldosteronism.

CONTEXT: The mechanisms causing excessive aldosterone production and hypertension in primary aldosteronism (PA) are complex and often incompletely recognized. Autoantibodies to the angiotensin AT1 receptor (AT1R) have been reported in some PA patients with an aldosterone-producing adenoma but not with idiopathic adrenal hyperplasia. OBJECTIVE: We investigated whether these autoantibodies will activate AT1R and thereby potentially contribute to the pathophysiology of PA. DESIGN: AT1R autoantibody activity in sera and/or IgG purified from 13 biochemically confirmed PA patients was measured using AT1R-transfected cells, and their contractile effects were assayed using perfused rat cremaster arterioles. Aldosterone stimulation was measured in vitro using isolated human adrenal carcinoma (HAC15) adrenal cells. These data were compared with sera obtained from a group of normotensive control subjects who were expected to have negligible AT1R autoantibodies. RESULTS: Sera from each of the 13 PA patients significantly increased AT1R activation in AT1R-transfected cells compared with 20 control subjects, and this activity was inhibited by the selective AT1R blocker losartan. Sera and IgG purified from AT1R autoantibody-positive sera demonstrated significant vasoconstrictive effects in isolated rat cremaster arterioles and were blocked by losartan. Moreover, the AT1R autoantibody-positive IgG directly stimulated aldosterone production in the cultured adrenal cells and enhanced angiotensin-induced aldosterone production in these cells, and these effects were blocked by candesartan. CONCLUSIONS: These data support a probable pathophysiological role for AT1R autoantibodies in PA and thereby raise important etiological and therapeutic implications.