Specificity of gonadotrophin-releasing hormone binding sites of the human corpus luteum: comparison with receptors of rat pituitary gland

The effects of a number of analogues of gonadotrophin-releasing hormone (GnRH) on the binding of a radiolabelled GnRH agonist (GnRH-A; [D-Ser(But)6, des Gly10]GnRH-ethylamide) to homogenates of human corpus luteum (CL) and rat pituitary tissue were compared. Specific binding was inhibited by GnRH and GnRH-like peptides only. Both the C-terminal amide and N-terminal region of the GnRH molecule were required for binding in both tissues. However, amino acid substitutions at position 6 markedly enhanced, and at position 8 markedly reduced, binding potencies in rat pituitary tissue compared with human CL binding sites. These results indicate that GnRH-binding sites of rat pituitary and human luteal tissue have a similar degree of specificity for GnRH-like peptides, and a similar requirement for both N- and C-terminal regions of the peptide, but that differences in specificity related to the mid-chain region of GnRH exist between human luteal and rat pituitary binding sites.