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

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