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.

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.

Implications of a vasodilatory human monoclonal autoantibody in postural hypotension.

Functional autoantibodies to the autonomic receptors are increasingly recognized in the pathophysiology of cardiovascular diseases. To date, no human activating monoclonal autoantibodies to these receptors have been available. In this study, we describe for the first time a ß2-adrenergic receptor (ß2AR)-activating monoclonal autoantibody (C5F2) produced from the lymphocytes of a patient with idiopathic postural hypotension. C5F2, an IgG3 isotype, recognizes an epitope in the N terminus of the second extracellular loop (ECL2) of ß2AR. Surface plasmon resonance analysis revealed high binding affinity for the ß2AR ECL2 peptide. Immunoblotting and immunofluorescence demonstrated specific binding to ß2AR in H9c2 cardiomyocytes, CHO cells expressing human ß2AR, and rat aorta. C5F2 stimulated cyclic AMP production in ß2AR-transfected CHO cells and induced potent dilation of isolated rat cremaster arterioles, both of which were specifically blocked by the ß2AR-selective antagonist ICI-118551 and by the ß2AR ECL2 peptide. This monoclonal antibody demonstrated sufficient activity to produce postural hypotension in its host. Its availability provides a unique opportunity to identify previously unrecognized causes and new pharmacological management of postural hypotension and other cardiovascular diseases.

Atrial tachycardia provoked in the presence of activating autoantibodies to ß2-adrenergic receptor in the rabbit.

BACKGROUND: A recent clinical study of patients with inappropriate sinus tachycardia reported that autoantibodies to ß-adrenergic receptors (ß2ARs) could act as agonists to induce atrial arrhythmias. OBJECTIVE: To test the hypothesis that activating autoantibodies to the ß2AR in the rabbit atrium are arrhythmogenic. METHODS: Five New Zealand white rabbits were immunized with a ß2AR second extracellular loop peptide to raise ß2AR antibody titers. A catheter-based electrophysiologic study was performed on anesthetized rabbits before and after immunization. Arrhythmia occurrence was determined in response to burst pacing before and after the infusion of acetylcholine in incremental concentrations of 10 µM, 100 µM, and 1 mM at 1 mL/min. RESULTS: In the preimmune studies when ß2AR antibody titers were undetectable, of a total of 20 events, only 3 episodes of nonsustained (<10 seconds) atrial arrhythmias were induced. In the postimmune studies when ß2AR antibody titers ranged from 1:160,000 to 1:1.28 million, burst pacing induced 10 episodes of nonsustained or sustained (≥10 seconds) arrhythmias in 20 events (P = .04 vs preimmune; χ(2) and Fisher exact test). Taking into account only the sustained arrhythmias, there were 6 episodes in 20 events in the postimmune studies compared with 0 episodes in 20 events in the preimmune studies (P = .02). Immunized rabbits demonstrated immunoglobulin G deposition in the atria, and their sera induced significant activation of ß2AR in transfected cells in vitro compared to the preimmune sera. CONCLUSIONS: Enhanced autoantibody activation of ß2AR in the rabbit atrium leads to atrial arrhythmias mainly in the form of sustained atrial tachycardia.

Agonistic autoantibodies as vasodilators in orthostatic hypotension: a new mechanism.

Agonistic autoantibodies to the ß-adrenergic and muscarinic receptors are a novel investigative and therapeutic target for certain orthostatic disorders. We have identified the presence of autoantibodies to ß2-adrenergic and/or M3 muscarinic receptors by ELISA in 75% (15 of 20) of patients with significant orthostatic hypotension. Purified serum IgG from all 20 of the patients and 10 healthy control subjects were examined in a receptor-transfected cell-based cAMP assay for ß2 receptor activation and ß-arrestin assay for M3 receptor activation. There was a significant increase in IgG-induced activation of ß2 and M3 receptors in the patient group compared with controls. A dose response was observed for both IgG activation of ß2 and M3 receptors and inhibition of their activation with the nonselective ß blocker propranolol and muscarinic blocker atropine. The antibody effects on ß2 and/or M3 (via production of NO) receptor-mediated vasodilation were studied in a rat cremaster resistance arteriole assay. Infusion of IgG from patients with documented ß2 and/or M3 receptor agonistic activity produced a dose-dependent vasodilation. Sequential addition of the ß-blocker propranolol and the NO synthase inhibitor N(G)-nitro-L-arginine methyl ester partially inhibited IgG-induced vasodilation (percentage of maximal dilatory response: from 57.7±10.4 to 35.3±4.6 and 24.3±5.8, respectively; P<0.01; n=3), indicating that antibody activation of vascular ß2 and/or M3 receptors may contribute to systemic vasodilation. These data support the concept that circulating agonistic autoantibodies serve as vasodilators and may cause or exacerbate orthostatic hypotension.