RESUMEN
BACKGROUND AND PURPOSE Endothelin-1 (ET-1) causes long-lasting vasoconstrictions. These can be prevented by ET(A) receptor antagonists but are only poorly reversed by these drugs. We tested the hypothesis that endothelin ET(A) receptors are susceptible to allosteric modulation by endogenous agonists and exogenous ligands. EXPERIMENTAL APPROACH Rat 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 RESULTS Endothelins caused contractions with equal maximum but differing potency (ET-1 = ET-2 > ET-3). ET-1(1-15) neither mimicked nor antagonized these effects in the absence and presence of ET(16-21). 4(Ala) ET-1 (ET(B) agonist) and BQ788 (ET(B) antagonist) were without effects. BQ123 (peptide ET(A) antagonist) reduced the sensitivity and relaxed the contractile responses to endothelins. Both effects depended on the agonist (pK(B): ET-3 = ET-1 > ET-2; % relaxation: ET-3 = ET-2 > ET-1). Also, with PD156707 (non-peptide ET(A) 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 IMPLICATIONS These findings indicate allosteric modulation of arterial smooth muscle ET(A) receptor function by endogenous agonists and by exogenous endothelin receptor antagonists. This may have consequences for the diagnosis and pharmacotherapy of diseases involving endothelins.
Asunto(s)
Arterias Mesentéricas/fisiología , Músculo Liso Vascular/fisiología , Receptor de Endotelina A/fisiología , Animales , Unión Competitiva , Carbocianinas/farmacología , Dioxoles/farmacología , Antagonistas de los Receptores de la Endotelina A , Endotelinas/farmacología , Fluoresceína-5-Isotiocianato/farmacología , Técnicas In Vitro , Arterias Mesentéricas/efectos de los fármacos , Contracción Muscular/efectos de los fármacos , Músculo Liso Vascular/efectos de los fármacos , Péptidos Cíclicos/farmacología , Ratas , Receptor de Endotelina A/agonistasRESUMEN
BACKGROUND AND PURPOSE: Calcitonin gene-related peptide (CGRP) has been proposed to relax vascular smooth muscle cells (VSMC) via cAMP and can promote dissociation of endothelin-1 (ET-1) from ET(A) receptors. The latter is not mimicked by other stimuli of adenylate cyclases. Therefore, we evaluated the involvement of G-protein ßγ subunits (Gßγ) in the arterial effects of CGRP receptor stimulation. EXPERIMENTAL APPROACH: To test the hypothesis that instead of α subunits of G-proteins (Gαs), Gßγ mediates the effects of CGRP receptor activation, we used (i) rat isolated mesenteric resistance arteries (MRA), (ii) pharmacological modulators of cyclic nucleotides; and (iii) low molecular weight inhibitors of the functions of Gßγ, gallein and M119. To validate these tools with respect to CGRP receptor function, we performed organ bath studies with rat isolated MRA, radioligand binding on membranes from CHO cells expressing human CGRP receptors and cAMP production assays in rat cultured VSMC. KEY RESULTS: In isolated arteries contracted with K(+) or ET-1, IBMX (PDE inhibitor) increased sodium nitroprusside (SNP)- and isoprenaline (ISO)- but not CGRP-induced relaxations. While fluorescein (negative control) was without effects, gallein increased binding of [(125) I]-CGRP in the absence and presence of GTPγS. Gallein also increased CGRP-induced cAMP production in VSMC. Despite these stimulating effects, gallein and M119 selectively inhibited the relaxing and anti-endothelinergic effects of CGRP in isolated arteries while not altering contractile responses to K(+) or ET-1 or relaxing responses to ISO or SNP. CONCLUSION AND IMPLICATIONS: Activated CGRP receptors induce cyclic nucleotide-independent relaxation of VSMC and terminate arterial effects of ET-1 via Gßγ.