Agonist‐dependent modulation of arterial endothelin_A receptor function

Abstract

BACKGROUND AND PURPOSEEndothelin-1 (ET-1) causes long-lasting vasoconstrictions. These can be prevented by ETA receptor antagonists but are only poorly reversed by these drugs. We tested the hypothesis that endothelin ETA receptors are susceptible to allosteric modulation by endogenous agonists and exogenous ligands. EXPERIMENTAL APPROACHRat isolated mesenteric resistance arteries were pretreated with capsaicin and studied in wire myographs, in the presence of L-NAME and indomethacin to concentrate on arterial smooth muscle responses. KEY RESULTSEndothelins caused contractions with equal maximum but differing potency (ET-1 = ET-2 > ET-3). ET-11-15 neither mimicked nor antagonized these effects in the absence and presence of ET16-21. 4 Ala ET-1 (ETB agonist) and BQ788 (ETB antagonist) were without effects. BQ123 (peptide ETA antagonist) reduced the sensitivity and relaxed the contractile responses to endothelins. Both effects depended on the agonist (pKB: ET-3 = ET-1 > ET-2; % relaxation: ET-3 = ET-2 > ET-1). Also, with PD156707 (non-peptide ETA antagonist) agonist-dependence and a discrepancy between preventive and inhibitory effects were observed. The latter was even more marked with bulky analogues of BQ123 and PD156707. CONCLUSIONS AND IMPLICATIONSThese findings indicate allosteric modulation of arterial smooth muscle ETA receptor function by endogenous agonists and by exogenous endothelin receptor antagonists. This may have consequences for the diagnosis and pharmacotherapy of diseases involving endothelins. Abbreviations 4 Ala

EndothelinA–EndothelinB Receptor Cross Talk in Endothelin-1–Induced Contraction of Smooth Muscle

“…23 On the other hand, Meens et al 23 suggested that in the rat mesenteric arteriole, the partial relaxation of the plateau contraction to ET-1 by ET A receptor antagonist was due to the quasi-irreversible binding of ET-1 to the ET A receptor. 49 Consistent with this suggestion is the greater inhibitory efficacy of ET A receptor antagonist when added before ET-1 as compared with during the contraction in rat mesenteric arteriole rings 23,50 and in rat basilar artery in situ. 34 Furthermore, this differential inhibitory effect does not seem to extend to ET B receptor antagonists because in rabbit pulmonary artery, 10 mM BQ788 challenge before and during the 0.1 nM ET-1-induced contraction caused similar magnitudes of inhibition 20 (;95% and ;85% inhibition, respectively).…”

“…34 Furthermore, this differential inhibitory effect does not seem to extend to ET B receptor antagonists because in rabbit pulmonary artery, 10 mM BQ788 challenge before and during the 0.1 nM ET-1-induced contraction caused similar magnitudes of inhibition 20 (;95% and ;85% inhibition, respectively). Moreover, the greater inhibitory effect associated with selective ET A receptor antagonist challenge before ET-1 as compared with during the plateau contraction to ET-1 23,34,50 was unrelated to possible limited intracellular access of ET A receptor peptide antagonists such as BQ123 51 because the nonpeptide ET A receptor antagonists, PD156707 and SB234551 (selective ET A receptor antagonists; Table 1), also inhibited contraction to a greater magnitude when added before ET-1 in mesenteric arterioles. 23,50 Then again, the essentially complete relaxation of several ET-1-contracted vessels by ET A receptor antagonist [43][44][45][46][47] would not have been predicted based on the proposed limitation of the relaxation by the quasi-irreversible binding of ET-1 to the ET A receptor.…”

“…Moreover, the greater inhibitory effect associated with selective ET A receptor antagonist challenge before ET-1 as compared with during the plateau contraction to ET-1 23,34,50 was unrelated to possible limited intracellular access of ET A receptor peptide antagonists such as BQ123 51 because the nonpeptide ET A receptor antagonists, PD156707 and SB234551 (selective ET A receptor antagonists; Table 1), also inhibited contraction to a greater magnitude when added before ET-1 in mesenteric arterioles. 23,50 Then again, the essentially complete relaxation of several ET-1-contracted vessels by ET A receptor antagonist [43][44][45][46][47] would not have been predicted based on the proposed limitation of the relaxation by the quasi-irreversible binding of ET-1 to the ET A receptor. 23 Indeed, the observations that ET A receptor antagonist nearly completely relaxed the contraction to ET-1, as observed in dog basilar artery and rat aorta and spermatic artery rings, 43-47 represents the second category described above in which cross talk was absent.…”

Orthosteric Binding of ρ-Da1a, a Natural Peptide of Snake Venom Interacting Selectively with the α1A-Adrenoceptor

“…In the converse situation where a toxin (for example MT7) binds to a receptor at a site distinct from the orthosteric pocket, receptors are able to bind simultaneously both the toxin and an agonist – illustrating “permissive” antagonism characteristic of allosteric modulators [48]. As different agonists adopt distinct poses in the orthosteric binding site, they have the capacity to differentially affect the affinity of an allosteric modulator for the receptor, thus pK B values of the modulator are altered depending on the agonist used [48], [49]. Our finding that the pK B of ρ-Da1a is the same for four agonists belonging to two distinct structural classes, and known to display signaling bias at the α 1A -AR [20], corroborates our other data showing that ρ-Da1a has no effect on the dissociation rate of either 3 H-prazosin or 125 I-HEAT, and that ρ-Da1a affinity for the α 1A -AR is reduced by mutation of residues within the orthosteric pocket.…”

Endothelin‐1 and endothelin‐2 initiate and maintain contractile responses by different mechanisms in rat mesenteric and cerebral arteries

“…After release from the endothelium, ET‐1 causes contraction of vascular smooth muscle cells via endothelin ET A receptors (Rizzoni et al ., ; receptor nomenclature follows Alexander et al ., ). In most ex vivo preparations of freshly isolated arteries, there is no observable effect on vasomotor tone attributable to ET B receptors in endothelium or smooth muscle (Meens et al ., ; Compeer et al ., 2012a). Activated ET A receptors can stimulate several signal‐transduction pathways including NADPH‐oxidases, phospholipases, Rho‐kinase (RhoK) and cellular influx of calcium ions (Badr et al ., ; Neylon, ; Masaki, ).…”