PHARMACOLOGICAL EVIDENCE FOR THE PRESENCE OF 3 DISTINCT FUNCTIONAL ENDOTHELIN RECEPTOR SUBTYPES IN THE RABBIT LATERAL SAPHENOUS-VEIN

1 Contraction of the rabbit isolated saphenous vein is mediated by a h eterogeneous endothelin (ET) receptor population. This study has chara cterized these receptor subtypes by use of several pharmacologically d istinct ET receptor agonists and antagonists. 2 ET-1, ET-3, sarafotoxi n S6c (STXc) and [Ala3(3,11)]ET-1 produced biphasic, concentration-dep endent contractions of the saphenous vein, responses which were best f itted by a two-site model comprised of a high (pM) and a low (nM) affi nity site. In contrast, IRL 1620 only recognized one of these sites. E T((16-21)) was devoid of contractile activity. ET-1, ET-3 and STXc wer e equipotent at the high affinity site (pD(2)s of 12.0 +/- 0.2, 12.2 /- 0.2 and 12.3 +/- 0.3) indicating that this site had the characteris tics of an ET(B) receptor. In contrast, the low affinity site had the functional characteristics of an ET(C) receptor since the pD(2)s for E T-3 (10.2 +/- 0.3) and STXc (10.6 +/- 0.3) were significantly greater than that for ET-1 (9.1 +/- 0.1). These contractile responses were ins ensitive to BQ-123, confirming that ET(A) receptors were not involved in mediating this effect. 3 SE 209670 differentially antagonized the h igh affinity phases of the isopeptide concentration-response curves in a fashion dependent on the competing agonist: relative to the K-B obt ained against STXc (0.15 nM). SE 209670 was 10 fold less potent when E T-1 was used as the competing agonist. This differential effect was no t evident at the low affinity site (K-B = 38 nM). SE 209670 produced p arallel, concentration-dependent rightward shifts in the concentration -response curve to STXc Ro 47-0203 was approximately 1 to 2 orders of magnitude less potent than SE 209670 at inhibiting the high affinity c omponent of the concentration-response curve to STXc, whereas BQ-788 a nd Ro 46-2005 were approximately 3 orders of magnitude less potent tha n SE 209670. In addition to RES-701 and BQ-123, the high affinity site was insensitive to PD 142893 suggesting that it may represent an ET(B 2) receptor. Ro 47-0203 and SE 209670 were equipotent at inhibiting th e low affinity component of the STXc concentration-response curve. Alt hough Ro 46-2005, BQ-788, PD 142893 and RES-701 produced significant a ntagonism at the low affinity site, they were at least ten fold less p otent than SE 209670. 4 ET-1, ET-3 and STXc produced endothelium-depen dent vasorelaxation in the precontracted saphenous vein. Antagonist IC (50)s were approximated as being: SE 209670, 3 nM; BQ-788 and RES 701, 300 nM; Ro 46-2005 and PD 142893, 3 mu M; BQ-123, greater than or equ al to 10 mu M, consistent with vasorelaxation being mediated by an ET( B1) receptor. 5 In summary, three pharmacologically distinct ET recept or subtypes have been identified in the rabbit saphenous vein. Two con tractile receptors are present on the vascular smooth muscle, a high a ffinity site with the characteristics of an ET(B2) receptor and a dist inct lower affinity site with the characteristics of an ET(C) receptor . In addition, an ET(B1) receptor is present on the endothelium which mediates the vasodilator actions of this peptide family.