Vol. 289, Issue 3, 1385-1390, June 1999

Typical Endothelin ET_A Receptors Mediate Atypical Endothelin-1-Induced Contractions in Sheep Isolated Tracheal Smooth Muscle¹

Peter J. Henry and Sarah H. King

Department of Pharmacology, University of Western Australia, Nedlands, Australia

Contraction of vascular and nonvascular smooth muscle induced by the endothelin/sarafotoxin family of peptides frequently does not readily fit into the current classification criteria for ET_A and ET_B receptors, raising the possibility of additional atypical receptors. In the current study, isometric tension recording and radioligand binding techniques were used to characterize the ET_A receptor population in sheep isolated tracheal smooth muscle. Endothelin-1 and sarafotoxin S6b induced similar concentration-dependent contractions, although endothelin-1 was 2.6-fold more potent (P < .05, n = 15-18). The ET_A receptor-selective antagonists BQ-123 and FR139317 caused concentration-dependent inhibition of the contractions induced by endothelin-1 and sarafotoxin S6b, but both antagonists were significantly less potent in inhibiting contractions induced by endothelin-1 than sarafotoxin S6b. For example, 0.03 µM FR139317 shifted the endothelin-1 and sarafotoxin S6b concentration-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 ET_A receptors, competition binding studies using ¹²⁵I-endothelin-1 and ¹²⁵-I-sarafotoxin S6b identified only a single population of BQ-123- and sarafotoxin S6b-sensitive ET_A receptors. Additional association-, dissociation-, and saturation-binding studies revealed that ¹²⁵I-endothelin-1 binding to these ET_A receptors was pseudoirreversible, whereas ¹²⁵I-sarafotoxin S6b binding was readily reversible. Thus, marked differences in the kinetic profiles of ET_A receptor binding to endothelin-1, sarafotoxin S6b, and BQ-123, rather than the existence of another ET_A receptor subtype, may explain the stark agonist dependence of antagonist potency observed in contractile studies.