[H-3] BQ-123, A HIGHLY SPECIFIC AND REVERSIBLE RADIOLIGAND FOR THE ENDOTHELIN ET(A) RECEPTOR SUBTYPE

The mode of binding of [H-3]BQ-123 (cyclo(-D-Trp-D-Asp-[prolyl-3,4(n)-[H-3]]Pro-D-Val-Leu-)), an endothelin receptor antagonist radioligand, was evaluated in the human neuroblastoma cell line SK-N-MC at 37 degrees C. Scatchard analysis indicated the presence of a single class of [H-3]BQ-123 binding sites with a high affinity of 3.2 nM. [H-3]BQ-123 binding achieved steady state within 7 min and dissociated with a half-time of 1.4 min, while [I-125]endothelin-1 binding barely reached a steady state even after 6 h and showed little dissociation. [H-3]BQ-123 binding was sensitive to endothelin-1 and endothelin-2 (K-i values = 0.058 and 0.10 nM, respectively) and the endothelin ET(A) receptor-selective antagonist BQ-123 (K-i = 3.3 nM), while showing low affinity for endothelin-3 (K-i = 50 nM), the endothelin ET(A) receptor-selective agonist BQ-3020 (K-i = 970 nM) and other bioactive peptides. Thus, [H-3]BQ-123 is a specific and reversible radioligand for endothelin ET(A) receptors. The rapid reversibility of [H-3]BQ-123 binding should provide a tool for estimating the equilibrium inhibition constants (K-i values) of various compounds for endothelin ET(A) receptors.