Typical endothelin ETA receptors mediate atypical endothelin-1-induced contractions in sheep isolated tracheal smooth muscle

Contraction of vascular and nonvascular smooth muscle induced by the endoth elin/sarafotoxin family of peptides frequently does not readily fit into th e current classification criteria for ETA and ETB receptors, raising the po ssibility of additional atypical receptors. In the current study, isometric tension recording and radioligand binding techniques were used to characte rize the ETA receptor population in sheep isolated tracheal smooth muscle. Endothelin-1 and sarafotoxin S6b induced similar concentration-dependent co ntractions, although endothelin-1 was 2.6-fold more potent (P < .05, n = 15 -18), The ETA receptor-selective antagonists BQ-123 and FR139317 caused con centration-dependent inhibition of the contractions induced by endothelin-l and sarafotoxin S6b, but both antagonists were significantly less potent i n inhibiting contractions induced by endothelin-l than sarafotoxin S6b. For example, 0.03 mu M FR139317 shifted the endothelin-1 and sarafotoxin S6b c oncentration-effect curves to the right by 1.8- and 8.3-fold, respectively (P < .01, n = 6-8). Although the observed agonist dependence of antagonist potency may indicate the presence of atypical ETA receptors, competition bi nding studies using I-125-endotheljn-1 and (125)-I-sarafotoxin S6b identifi ed only a single population of BQ-123- and sarafotoxin S6b-sensitive ETA re ceptors, Additional association-, dissociation-, and saturation-binding stu dies revealed that I-125-endothelin-1 binding to these ETA receptors was ps eudoirreversible, whereas I-125-sarafotoxin S6b binding was readily reversi ble. Thus, marked differences in the kinetic profiles of ETA receptor bindi ng to endothelin-1, sarafotoxin S6b, and BQ-123, rather than the existence of another ETA receptor subtype, may explain the stark agonist dependence o f antagonist potency observed in contractile studies.