Binding characteristics of [125i]TTA 386, ETA-selective antagonist

Abstract The data presented in this manuscript describe the binding characteristics of the ET A -selective antagonist, [ 125 I]TTA 386 (hexamethyleneimino carbonyl-Leu-Tri-Ala-β-Ala-Tyr-Phe). This radioligand bound with high affinity and specificity to cloned human ET A receptors and rat mesenteric artery ET A receptors. The apparent dissociation constants (K d s) and maximum binding capacities were 1.0 nM and 8.5 pmol/mg for cloned human ET A receptors and 0.8 nM and 170 fmol/mg for rat mesenteric artery membranes respectively. Binding of [ 125 I]TTA 386 was fast reaching equilibrium by 45 min and 15 min for human ET A and rat mesenteric artery membrane, respectively. Addition of excess unlabeled ligand resulted in the dissociation of bound radioligand from both preparations. Competition of [ 125 I]TTA 386 binding by unlabeled ET-1, ET-3, TTA 386 and BQ123 revealed appropriate ET A pharmacology. This radioligand did not display any binding to cloned human ET B receptors.