1 Contraction of the rabbit isolated saphenous vein is mediated by a heterogeneous endothelin (ET) receptor population. This study has characterized these receptor subtypes by use of several pharmacologically distinct ET receptor agonists and antagonists. 2 ET-1, ET-3, sarafotoxin S6c (STXc) and [Ala3(3,11)]ET-1 produced biphasic, concentration-dependent contractions of the saphenous vein, responses which were best fitted by a two-site model comprised of a high (pM) and a low (nM) affinity site. In contrast, IRL 1620 only recognized one of these sites. ET((16-21)) was devoid of contractile activity. ET-1, ET-3 and STXc were 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 characteristics 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 ET-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 insensitive to BQ-123, confirming that ET(A) receptors were not involved in mediating this effect. 3 SE 209670 differentially antagonized the high affinity phases of the isopeptide concentration-response curves in a fashion dependent on the competing agonist: relative to the K-B obtained against STXc (0.15 nM). SE 209670 was 10 fold less potent when ET-1 was used as the competing agonist. This differential effect was not evident at the low affinity site (K-B = 38 nM). SE 209670 produced parallel, 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 component of the concentration-response curve to STXc, whereas BQ-788 and Ro 46-2005 were approximately 3 orders of magnitude less potent than 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(B2) receptor. Ro 47-0203 and SE 209670 were equipotent at inhibiting the low affinity component of the STXc concentration-response curve. Although Ro 46-2005, BQ-788, PD 142893 and RES-701 produced significant antagonism at the low affinity site, they were at least ten fold less potent than SE 209670. 4 ET-1, ET-3 and STXc produced endothelium-dependent 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 equal to 10 mu M, consistent with vasorelaxation being mediated by an ET(B1) receptor. 5 In summary, three pharmacologically distinct ET receptor subtypes have been identified in the rabbit saphenous vein. Two contractile receptors are present on the vascular smooth muscle, a high affinity site with the characteristics of an ET(B2) receptor and a distinct 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.
