Measurement of luteal and placental gonadotrophin-releasing hormone (GnRH) binding sites: role of inactivation of GnRH tracer

The ability of radiolabelled gonadotrophin-releasing hormone (GnRH) and GnRH analogues to bind to homogenates and membranes of rat, rabbit and sheep pituitary and to luteal homogenates and membranes from a number of species was measured. In addition, inactivation of tracer during binding incubation was estimated by measurement of the ability of the unbound tracer fraction to bind to fresh GnRH-receptor or anti-GnRH antibody. Following incubation of rat, sheep and rabbit pituitary gland with a radiolabelled GnRH agonist tracer, appreciable amounts of specific binding of GnRH agonist to fresh rat pituitary or human placental GnRH receptors could still be demonstrated. In contrast, no specific binding of [I-125]-labelled GnRH analogues were measured following incubation of the tracer with homogenates or membranes of bovine, sheep and porcine luteal tissue, nor with rabbit and rat placental homogenates. However, during incubation of GnRH tracers with these tissues, almost complete inactivation of GnRH tracers occurred. There was an inverse relationship between binding and inactivation of [I-125]-labelled GnRH for a number of human corpora lutea. We conclude that degradation of GnRH tracers may prevent the measurement of specific GnRH binding sites in some tissues.