High affinity interaction of endothelin-3 with recombinant ETA receptors

Pharmacological evidence has suggested that endothelin-3 (ET-3) may act via a novel form of ET receptor that is shared by ETA receptor antagonists but not by ETB receptor selective agonists. This study analyses the properties of interaction of ET-3 with recombinant bovine ETA receptor. Apparent Kd(ET-3) values as low as 50 nM were defined from 125I]ET-1 binding experiments performed at low (5 microg/ml) protein concentrations in the assays. Larger (up to 1 microM) values were artefactually obtained in experiments performed at larger protein concentrations. The three monoiodo ET-3 derivatives were synthetized. (125I]Y14)ET-3 did not recognize ETA receptors. (125I]Y6)ET-3 labelled 18% of 125I]ET-1 binding sites with a Kd value of 320 pM. (125I]Y13)ET-3 labelled 44% of 125I]ET-1 binding sites with a Kd value of 130 pM. High affinity (125I]Y6)ET-3 and (125I]Y13)ET-3 bindings were prevented by ET-1 (Kd = 5-7 pM), ET-3 (Kd = 70-250 pM), BQ-123 (Kd = 2 nM) and FR139317 (Kd = 2 nM) but not by low concentrations of 4-AlaET-1, sarafotoxin S6c or IRL1620. The three monoiodo ET-3 derivatives bound to recombinant rat ETB receptors with a pM affinity. The results suggest that ET-3, (125I]Y6)ET-3 and (125I]Y13)ET-3 should not be considered as ETB receptor specific ligands.