BackgroundPatients 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 ResultsFourteen 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 β1AR and β2AR 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 β2AR 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 β1AR activation (130±3% of baseline, P<0.01) and a subset had increased β2AR activity versus healthy subjects. POTS sera shifted isoproterenol cAMP response curves to the left, consistent with enhanced β1AR and β2AR agonist activity. Autoantibody‐positive POTS sera demonstrated specific binding to β1AR, β2AR, and α1AR in transfected cells.ConclusionsPOTS patients have elevated α1AR autoantibodies exerting a partial peripheral antagonist effect resulting in a compensatory sympathoneural activation of α1AR for vasoconstriction and concurrent βAR‐mediated tachycardia. Coexisting β1AR and β2AR agonistic autoantibodies facilitate this tachycardia. These findings may explain the increased standing plasma norepinephrine and excessive tachycardia observed in many POTS patients.

The epithelial Na ؉ channel, ENaC, is exposed to a wide range of proton concentrations in the kidney, lung, and sweat duct. We, therefore, tested whether pH alters ENaC activity. In Xenopus oocytes expressing human ␣-, ␤-, and ␥ENaC, amiloridesensitive current was altered by protons in the physiologically relevant range (pH 8.5-6.0). Compared with pH 7.4, acidic pH increased ENaC current, whereas alkaline pH decreased current (pH 50 ‫؍‬ 7.2). Acidic pH also increased ENaC current in H441 epithelia and in human primary airway epithelia. In contrast to human ENaC, pH did not alter rat ENaC current, indicating that there are species differences in ENaC regulation by protons. This resulted predominantly from species differences in ␥ENaC. Maneuvers that lock ENaC in a high open-probability state ("DEG" mutation, proteolytic cleavage) abolished the effect of pH on human ENaC, indicating that protons alter ENaC current by modulating channel gating. Previous work showed that ENaC gating is regulated in part by extracellular Na ؉ ("Na ؉ self-inhibition"). Based on several observations, we conclude that protons regulate ENaC by altering Na ؉ self-inhibition. First, protons reduced Na ؉ self-inhibition in a dose-dependent manner. Second, ENaC regulation by pH was abolished by removing Na ؉ from the extracellular bathing solution. Third, mutations that alter Na ؉ self-inhibition produced corresponding changes in ENaC regulation by pH. Together, the data support a model in which protons modulate ENaC gating by relieving Na ؉ self-inhibition. We speculate that this may be an important mechanism to facilitate epithelial Na ؉ transport under conditions of acidosis.

Acid-sensing ion channel-1a (ASIC1a) is a potential therapeutic target for multiple neurological diseases. We studied here ASIC1a glycosylation and trafficking, two poorly understood processes that are pivotal in determining the functional outcome of an ion channel. We found that most ASIC1a in the mouse brain was fully glycosylated. Inhibiting glycosylation with Tunicamycin reduced ASIC1a surface trafficking, dendritic targeting and acid-activated current density. N-glycosylation of the two glycosylation sites, Asn393 and Asn366, has differential effects on ASIC1a biogenesis. Maturation of Asn393 increased ASIC1a surface and dendritic trafficking, pH sensitivity and current density. In contrast, glycosylation of Asn366 was dispensable for ASIC1a function and may be a rate-limiting step in ASIC1a biogenesis. Lastly, we revealed that acidosis reduced the density and length of dendritic spines in a time- and ASIC1a-dependent manner. ASIC1a N366Q, which showed increased glycosylation and dendritic targeting, potentiated acidosis-induced spine loss. Conversely, ASIC1a N393Q, which had diminished dendritic targeting and inhibited ASIC1a current dominant-negatively, had the opposite effect. These data tie N-glycosylation of ASIC1a with its trafficking. More importantly, revealing a site-specific effect of acidosis on dendritic spines suggests an important role of these processes in regulating synaptic plasticity and long-term consequences in diseases that generate acidosis.

The extracellular domain of the epithelial sodium channel ENaC is exposed to a wide range of The epithelial Na ϩ channel ENaC 2 is a heterotrimer of homologous ␣, ␤, and ␥ subunits (1, 2). ENaC functions as a pathway for Na ϩ absorption across epithelial cells in the kidney collecting duct, lung, distal colon, and sweat duct (reviewed in Refs. 3 and 4). Na ϩ transport is critical for the maintenance of Na ϩ homeostasis and for the control of the composition and quantity of the fluid on the apical membrane of these epithelia. ENaC mutations and defects in its regulation cause inherited forms of hypertension and hypotension (5) and may contribute to the pathogenesis of lung disease in cystic fibrosis (6).ENaC is a member of the DEG/ENaC family of ion channels. A common structural feature of these channels is a large extracellular domain that plays a critical role in channel gating. For example, in ASICs, the extracellular domain functions as a receptor for protons, which transiently activate the channel by titrating residues that form an acidic pocket (7). FaNaCh is a ligand-gated family member in Helix aspersa, activated by the peptide FMRFamide (8). In Caenorhabditis elegans MEC family members, the extracellular domain is thought to respond to mechanical signals (9).ENaC differs from other family members because it is constitutively active in the absence of a ligand/stimulus. However, a convergence of data indicate that ENaC gating is modulated by a variety of molecules that bind to or modify its extracellular domains, including proteases (10 -12), Na ϩ (13-15), protons (16), and the divalent cations Zn 2ϩ and Ni 2ϩ (17,18). These findings suggest that the ENaC extracellular domain might regulate epithelial Na ϩ transport by sensing and integrating diverse signals in the extracellular environment.In the current study, we tested the hypothesis that ENaC activity is regulated by changes in the extracellular Cl Ϫ concentration. Several observations suggested that Cl Ϫ might be a strong candidate to regulate the channel. First, transport of Na ϩ and Cl Ϫ are often coupled to maintain electroneutrality. Second, ENaC is exposed to large changes in extracellular Cl Ϫ concentration. For example, in the kidney collecting duct, the urine Cl Ϫ concentration varies widely (19). As the predominant anion, its concentration parallels that of Na ϩ in most clinical states. However, under conditions of metabolic alkalosis and metabolic acidosis, the Na ϩ and Cl Ϫ concentrations can become dissociated as a result of increased urinary bicarbonate (alkalosis) or ammonium (acidosis) (19). Thus, ENaC is well positioned to respond to changes in Cl Ϫ concentration. Third, crystallization of ASIC1a revealed a binding site for a Cl Ϫ ion at the base of the thumb domain (7). The Cl Ϫ is coordinated by Arg-310 and Glu-314 from one subunit and Lys-212 from an adjacent subunit. Although the functional role of Cl Ϫ binding to ASIC1a is unknown, it supports the hypothesis that extracellular Cl Ϫ might regulate the activity of DEG/ENaC ion chann...

Background:The epithelial Na ϩ channel ENaC functions as a pathway for Na ϩ absorption across epithelia.