Pharmacological evidence for the presence of three distinct functional endothelin receptor subtypes in the rabbit lateral saphenous vein
Authors: Douglas Stephen A. , Beck George R. , Elliott John D. , Ohlstein Eliot H. 
Published: July 2012

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.
ET‐1, ET‐3, sarafotoxin S6c (STXc) and [Ala3,11]ET‐l 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 (pD2s of 12.0 ± 0.2, 12.2 ± 0.2 and 12.3 ± 0.3) indicating that this site had the characteristics of an ETB receptor. In contrast, the low affinity site had the functional characteristics of an ETC receptor since the pD2s 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 ETA receptors were not involved in mediating this effect.
SB 209670 differentially antagonized the high affinity phases of the isopeptide concentration‐response curves in a fashion dependent on the competing agonist: relative to the KB obtained against STXc (0.15 nM). SB 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 (KB = 38 nM). SB 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 SB 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 SB 209670. In addition to RES‐701 and BQ‐123, the high affinity site was insensitive to PD 142893 suggesting that it may represent an ETB2 receptor. Ro 47–0203 and SB 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 SB 209670.
ET‐1, ET‐3 and STXc produced endothelium‐dependent vasorelaxation in the precontracted saphenous vein. Antagonist IC50s were approximated as being: SB 209670, 3 nM; BQ‐788 and RES 701, 300 nM; Ro 46–2005 and PD 142893, 3μm BQ‐123, ≤ 10μm, consistent with vasorelaxation being mediated by an ETBl receptor.
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 ETB2 receptor and a distinct lower affinity site with the characteristics of an ETC receptor. In addition, an ETB1 receptor is present on the endothelium which mediates the vasodilator actions of this peptide family.