Comparative studies with the endothelin receptor antagonists BQ-123 and PD 142893 indicate at least three endothelin receptors.

Endothelin-1 (ET-1), endothelin-3 (ET-3), sarafotoxin 6b (SX6b), and sarafotoxin 6C (SX6c) were used as agonists, and BQ-123 (ETA-selective) and PD 142893 (ET receptor-nonselective) were used as antagonists to characterize the receptors mediating the effects of the ET/SX peptides on a variety of isolated smooth-muscle preparations. Contractions of the rat thoracic aorta, rat isolated perfused mesentery, and guinea pig ileum and potentiation of twitch of the rat vas deferens were mediated by ETA receptors in that they showed the order of potency ET-1 = SX6b > ET-3 >> SX6C. These effects were antagonized by BQ-123 or PD 142893. Contractions of the rabbit pulmonary artery and rat stomach strip, inhibition of twitches in the guinea pig ileum, and vasodilatations of the rat isolated perfused mesentery showed the order of potency ET-1 = SX6b = ET-3 = SX6c and were unaffected by BQ-123, suggesting involvement of ETB receptors. However, in these tissues, PD 142893 antagonized only dilatations of the rat mesentery to ET-1 but not any of the other effects of ET-1. Thus, we suggest that there are three types of endothelin receptors: ETA and two subtypes of ETB.