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(E
T-3) values as low as 50 nM were defined from [I-125]ET-1 binding experimen
ts performed at low (5 mu g/ml) protein concentrations in the assays. Large
r (up to 1 mu M) values were artefactually obtained in experiments performe
d at larger protein concentrations. The three monoiodo ET-3 derivatives wer
e synthetized, ([I-125]Y-14)ET-3 did not recognize ETA receptors, ([I-125]Y
-6)ET-3 labelled 18% of [I-125]ET-1 binding sites with a Kd value of 320 pM
, ([I-125]Y-13)ET-3 labelled 44% of [I-125]ET-1 binding sites with a Kd val
ue of 130 pM. High affinity ([I-125]Y-6)ET-3 and ([I-125]Y-13)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 re
combinant rat ETB receptors with a pM affinity. The results suggest that ET
-3, ([I-125]Y-6)ET-3 and ([I-125]Y-13)ET-3 should not be considered as ETB
receptor specific ligands. (C) 1999 Academic Press.