Gonadotropin-Releasing Hormone Receptors in Human Pituitary: Ligand Structural Requirements, Molecular Size, and Cationic Effects*

Abstract

A specific, high affinity receptor for GnRH in human pituitaries obtained post mortem is described. The human pituitary GnRH receptor bound GnRH, a GnRH agonist [(D-Ala⁶,N^α-MeLeu⁷,Pro⁹NEt)-GnRH], and a GnRH antagonist [Ac-D-Nal(2)¹,D-α-Me-4-ClPhe²_iD-3-Pal³,D-Arg6,D-Ala¹⁰)- GnRH] with similar affinities (K_ds of 4.81 nM, 0.32 nM, and 0.32 nM) to those of the rat pituitary (K_dS of 4.71 nM, 0.31 nM, and 0.40 nM). A second GnRH antagonist [(D-pGlu¹,D-Phe²,DTrp^3,6)- GnRH], however, was bound with a much lower affinity by the human receptor (K_d of 4.21 nM) than that of the rat (K^d of 0.09 nM). Monovalent and divalent cations affected [¹²⁵I]GnRH agonist binding to rat and human pituitary receptors differently. In the presence of Mg²⁺ or Ca²⁺, binding to the human receptor was significantly lower than in the rat. At near physiological concentrations, Na⁺ and K⁺ (100 mM and 10 mM, respectively) inhibited [^125I]GnRH agonist binding to the receptors to a similar extent in both species. At unphysiological concentrations (10 mM Na⁺ and 100 mM K⁺) these ions decreased binding to the human pituitary receptor to a greater extent than to the rat receptor. Using a ligand-immunoblotting technique, the human receptor or binding component of the receptor complex was found to be of greater molecular size (64,000 daltons) than that of the rat (60,000 daltons). The results show that the human and rat pituitary GnRH receptors have similar binding affinities for GnRH and certain GnRH analogs but differ in their binding of an antagonist, their sensitivity to cationic effects on GnRH agonist binding, and in the molecular size of the receptor GnRH-binding protein.