1 Endothelin-1 (ET-1) produces constriction of the rat mesenteric vascular bed in vivo via ET A and ET B receptor subtypes. The aim of this study was to investigate the relative roles of these receptor subtypes in rat isolated, endothelium-denuded, small mesenteric arteries, under pressure, by use of ET-1; the ET A receptor antagonist, BQ-123; the ET B receptor selective agonist, sarafotoxin S6c (SRTX S6c); the ET B receptor selective antagonist, BQ-788; and the ET A /ET B antagonist, TAK-044. 2 In 3rd generation mesenteric arteries, ET-1 (10 713 ± 10 77 M) produced concentration-dependent contractions (pD 2 9.86). SRTX S6c (10 712 ± 10 77 M) also induced concentration-dependent contractions in 53% of arteries studied, although the E max was much less than that obtained with ET-1 (10.7+2.9% vs 101.9+2.6% of the 60 mM KCl-induced contraction). 3 Neither ET B receptor desensitization, by a supra-maximal concentration of SRTX S6c (10 77 M), nor incubation with BQ-788 (3610 78 M), had any signi®cant e ect on the ET-1 concentration-response curve, although both treatments tended to enhance rather than inhibit responses to ET-1. 4 In the presence of BQ-123 (10 76 M), responses to low concentrations of ET-1 (up to 10 710 M) were una ected but responses to concentrations of ET-1 above 10 710 M were signi®cantly inhibited. 5 SRTX S6c desensitization followed by incubation with BQ-123 (10 76 M) or co-incubation with BQ-788 (3610 78 M) and BQ-123 caused inhibition of responses to all concentrations of ET-1, resulting in a rightward shift of the ET-1 concentration-response curve. The same e ect was obtained by incubation with TAK-044 (10 78 M and 3610 77 M). 6 Thus, responses of rat small mesenteric arteries to ET-1 are mediated by both ET A and ET B receptors. The relative role of ET B receptors is greater than that predicted by the small responses to SRTX S6c or by resistance of ET-1-induced contraction to ET B receptor desensitization or BQ-788. The e ect of ET B receptor desensitization or blockade is only revealed in the presence of ET A receptor blockade, suggesting the presence of a`crosstalk' mechanism between the receptors. These results support the concept that dual receptor antagonists, like TAK-044, may be required to inhibit completely constrictor responses to ET-1.

No abstract

1 To characterize the receptors mediating the effects of the endothelin/sarafotoxin family of peptides on the responses to electrical stimulation of the rat vas deferens (RVD) and guinea-pig ileum (GPI) we have used endothelin-I (ET-1), ET-3, sarafotoxin 6b (SX6b) and SX6c as agonists and the endothelinreceptor antagonists BQ-123 (ETA receptor selective) and PD 142893 (non-selective). 2 In the RVD, ET-1 and SX6b increased the twitches induced by field stimulation starting at a threshold concentration of 10-`' M while the threshold concentration for ET-3 was 3 x 10-9 M. SX6c (up to 3 x 10-8M) did not potentiate the twitches. SX6b produced significantly (P<0.05) greater potentiations than ET-1 at concentrations of 3 x 10-9 M and higher, and I0-7 M ET-3 also produced a significantly greater effect than ET-1 at the same concentration. Thus, at threshold the rank order of peptides was ET-1 = SX6b>ET-3>>>SX6c, and at concentrations of 3 x 108 M and higher, SX6b> ET-3> ET-1 >>> SX6c.3 In the presence of BQ-123 or PD 142893 (10-' M) the threshold concentrations for ET-1 to augment the twitches were increased 30 fold. In the same conditions neither SX6b nor ET-3 potentiated the responses. The relative activities of the endothelin/sarafotoxin peptides and the effectiveness of the endothelin receptor antagonists are consistent with postjunctional ETA receptors mediating these effects. 4 In the transmurally stimulated GPI the endothelin/sarafotoxin peptides produced two effects; an increase in the basal tension of the tissues and an inhibition of the twitch responses. To increase the basal tension the peptides had the order of potency ET-1 > SX6b >> ET-3 = SX6c. These direct effects of ET-1 or SX6b were strongly antagonized (100 fold) by either BQ-123 (10-'M) or PD 142893 (10-' M). Thus, ETA receptors mediate contractions of the GPI induced by these peptides.5 The endothelin/sarafotoxin peptides were approximately equipotent at depressing twitches of the GPI in response to transmural stimulation (EC,0s, 4 x 10 -to 1.5 x 10-10 M). The depressions induced by ET-1 were unaffected by either BQ-123 (10-' M) or PD 142893 (10-5 M). BQ-123 produced a small (three fold) antagonism of the inhibitory effects of ET-3 or SX6c. These results indicate that a receptor of the ETB type mediates the inhibitory effects of the endothelin/sarafotoxin peptides on neurotransmission in the GPI.6 Thus, both ETA receptors and ETB receptors mediate the effects of the endothelin/sarafotoxin peptides on neurotransmission.