Endothelin-induced nociception in mice: mediation by ETA and ETB receptors.

Endothelins (ET-1, ET-2 or ET-3) or endothelin precursors (big-ET-1[1-38], big-ET-2[1-37] or big-ET-3[1-41]) injected i.p. in mice have previously been shown to elicit a characteristic nociceptive behavioral response. In this study, we investigated the endothelin receptor type (ETA or ETB) that mediates this behavioral response. Mice were injected i.p. with ET-1, ET-2, ET-3, big-ET-1[1-38], big-ET-2[1-37], big-ET-3[1-41], sarafotoxin S6a, sarafotoxin S6b, sarafotoxin S6c, ET-1 with Ala substitutions for Cys3 and Cys11 or His-Leu-Asp-Ile-Ile-Trp, and quantal dose-response curves were obtained for each of the compounds (except the latter). Co-administration of enzyme inhibitors with the big-endothelins was used to establish the requisite conversion to endothelins and big-ET-1[22-38], big-ET-2[22-37] and ET-3[22-41] amide, and the ETA-selective antagonist cyclo[-D-Asp-Pro-D-Val-Leu-D-Trp-] was used to determine receptor specificity. The ED50 values were 2.9, 3.3 and 23.9 micrograms/kg i.p. for ET-1, ET-2 and ET-3, respectively, 0.6, 0.6 and 13.1 micrograms/kg i.p. for sarafotoxin S6a, sarafotoxin S6b and sarafotoxin S6c, respectively, and 5.3 micrograms/kg i.p. for ET-1 with Ala substitutions for Cys3 and Cys11. Big-ET-1[22-38], big-ET-2[22-37], big-ET-3[22-41] amide and ET-C produced less than 25% effect up to 2000 micrograms/kg. The big-ET-1-induced effects were blocked by the enzyme inhibitors phosphoramidon and thiorphan (ID50 = 0.9 mg/kg) but not by ubenimex (bestatin), captopril or perindopril. Cyclo[-D-Asp-Pro-D-Val-Leu-D-Trp-] blocked ET-1- and ET-2-induced effects but not ET-3-, ACh- or phenyl-p-quinone-induced effects. These results suggest that endothelin-induced nociceptive behavioral response in mice can be mediated via both ET receptor types, ETA and ETB. Further, the ET-1 carboxy-terminal hexapeptide is insufficient to produce the effect, and the Cys3-Cys11 disulfide bridge of ET-1 is not required.