Activation of endothelin ET(A) receptors masks the constrictor role of endothelin ET(B) receptors in rat isolated small mesenteric arteries

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-(13)-10(-7) M) produced concentration-dependent contractions (pD(2) 9.86). SRTX S6c (10(-12)-10(-7) 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(-7) M), nor incubation with BQ-788 (3 x 10(-8) M), had any significant effect 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(-6) M), responses to low concentrations of ET-1 (up to 10(-10) M) were unaffected but responses to concentrations of ET-1 above 10(-10) M were significantly inhibited. 5 SRTX S6c desensitization followed by incubation with BQ-123 (10(-6) M) or co-incubation with BQ-788 (3 x 10(-8) 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 effect was obtained by incubation with TAK-044 (10(-8) M and 3 x 10(-7) 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 effect 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.