Agonistic Autoantibodies as Vasodilators in Orthostatic Hypotension

Abstract

Agonistic autoantibodies to the β-adrenergic and muscarinic receptors are a novel investigative and therapeutic target for certain orthostatic disorders. We have identified the presence of autoantibodies to β2-adrenergic and/or M3 muscarinic receptors by enzyme-linked immunosorbent assay in 75% (15 of 20) of patients with significant orthostatic hypotension. Purified serum IgG from all 20 patients and 10 healthy control subjects were examined in a receptor-transfected cell-based cAMP assay for β2 receptor activation and β-arrestin assay for M3 receptor activation. There was a significant increase in IgG-induced activation of β2 and M3 receptors in the patient group compared to controls. A dose response was observed for both IgG activation of β2 and M3 receptors and inhibition of their activation with the non-selective β blocker propranolol and muscarinic blocker atropine. The antibody effects on β2 and/or M3 (via production of nitric oxide) receptor-mediated vasodilation were studied in a rat cremaster resistance arteriole assay. Infusion of IgG from patients with documented β2 and/or M3 receptor agonistic activity produced a dose-dependent vasodilation. Sequential addition of the β blocker propranolol and the nitric oxide synthase inhibitor NG-nitro-L-arginine methyl ester partially inhibited IgG-induced vasodilation (% 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 antibody activation of vascular β2 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.

Implications of a Vasodilatory Human Monoclonal Autoantibody in Postural Hypotension

“…The patient was not on any immunomodulatory medications at the time. His serum demonstrated significantly elevated titer and functional activity of anti-␤ 2 AR autoantibodies (22).…”

“…cAMP Assay-C5F2-mediated ␤ 2 AR activation of cAMP production in CHO cells was measured using the cAMP Hunter eXpress GPCR assay kit (DiscoveRx) as described (22). Briefly, 30,000 cAMP Hunter eXpress ␤ 2 AR-expressing CHO cells were dispensed into each well of a 96-well culture plate and incubated overnight.…”

“…Isolated Arteriole Assay-The vasodilatory effect of C5F2 via activation of ␤ 2 AR on resistance vessels was examined using an isolated rat cremaster arteriole assay as described (22,24). Briefly, cremaster arterioles were removed from anesthetized Sprague-Dawley rats (180 -250 g).…”

“…We recently reported the association of vasodilatory polyclonal autoantibodies to the ␤ 2 -adrenergic receptor (␤ 2 AR) and/or M3 muscarinic receptor in postural hypotension patients (21,22). These autoantibodies possess sufficient bioactivity to alter the postural vascular response, thus contributing to the pathophysiology of postural hypotension, especially the "idiopathic" type, i.e.…”

State of the art paper The role of G protein coupled receptor kinases in neurocardiovascular pathophysiology

“…Several features of idiopathic orthostatic hypotension have been attributed to hypoactive or hyperactive states of adrenergic receptors of the sympathetic nervous system. Recent data indicate that autoimmune lesions of the β-AR signaling pathway could be one of the contributing factors to orthostatic intolerance [47,48], but still there are no available data about the contribution of GRK to this syndrome.…”