Receptor Binding and Gonadotropin-Releasing Activity of a Novel Chicken Gonadotropin-Releasing Hormone ([His5, Trp7, Tyr8]GnRH) and a D-Arg6Analog*

Publisher Website
Google Scholar
Abstract
Receptor binding and gonadotropin-releasing activity was compared for mammalian GnRH, [Gln8]GnRH (chicken I GnRH), [His5, Trp7, Tyr8]GnRH (chicken II GnRH), [Trp7, Leu8]GnRH (salmon GnRH), and [D-Arg6] chicken II GnRH. The mean ED50 values for mammalian GnRH, chicken I GnRH, chicken II GnRH, and salmon GnRH in stimulating LH release from dispersed chicken pituitary cells were 0.27 nM, 0.28 nM, 0.055 nM, and 0.11 nM, respectively. The relative potencies of the peptides compared in the same assay were 0.93, 1.0, 5.6, and 2.5. The ED50 values for chicken I GnRH, chicken II GnRH, and salmon GnRH in stimulating FSH release were 0.37 nM, 0.034 nM, and 0.18 nM, and the relative potencies were 1.0, 13.5, and 1.8. Chicken II GnRH was, therefore, more potent than chicken I GnRH and mammalian GnRH in releasing LH and appeared to have an even greater relative FSH-releasing activity than chicken I GnRH or mammalian GnRH. Introduction of D-Arg6 into chicken II GnRH enhanced the activity of this analog 4- and 2-fold relative to chicken II GnRH in LH- and FSH-releasing activity, respectively. The ED50 values of mammalian GnRH, chicken I GnRH, chicken II GnRH, and salmon GnRH in releasing LH from cultured sheep pituitary cells were 2.9 nM, 22 nM, and 104 nM, respectively. The relative potencies were 1.0, 0.016, 0.084, and 0.047. Introduction of D-Arg6 into chicken II GnRH enhanced activity 9-fold. In a rat pituitary receptor binding assay the ED50 values of mammalian GnRH, chicken I GnRH, chicken II GnRH, and salmon GnRH were 2.9 nM, 1480 nM, 19 nM, and 258 nM, respectively. [D-Arg6]Chicken II GnRH was 46 times more active than the natural chicken II GnRH peptide. The results show: 1) chicken II GnRH is more potent than chicken I GnRH, which is equipotent with mammalian GnRH in releasing LH from chicken pituitary cells. Chicken II GnRH is even more potent at releasing FSH. 2) Salmon GnRH is also more potent than chicken I GnRH and mammalian GnRH in stimulating gonadotropin release from chicken pituitary cells. It appears, therefore, that Trp in the 7 position contributes to the enhanced activity of salmon and chicken II GnRH. 3) The low activity of chicken I GnRH, chicken II GnRH, and salmon GnRH in the sheep pituitary cell bioassay and rat pituitary receptor binding assay confirms that Arg8 in mammalian GnRH is important for activity. 4) Chicken II GnRH and salmon GnRH are more active than chicken I GnRH in the mammalian assay systems, and thus, it appears that Trp7 enhances activity of these weak agonists. 5) D-Arg6 incorporation into chicken II GnRH enhances activity in both the chicken and sheep pituitary bioassays but has a substantially greater effect in the latter.