Dissociation between pituitary GnRH binding sites and LH response to GnRH in vitro

Experiments were performed to study gonadotroph responsiveness to gonadotrophin releasing hormone (GnRH) in vitro in dispersed pituitary cells from ovariectomised rats and mice when GnRH binding sites were increased or reduced, respectively. Maximal/basal LH release after GnRH treatment of intact female rat pituitary cells was 4.7 to 9.7-fold (range n = 3 expts.) compared to 3.4 to 5.0-fold for cells from ovariectomised rat donors. Both basal and maximal GnRH-stimulated LH release from ovariectomised (OVX) rat pituitary cells were 1.5 to 3-fold greater than from intact rat cells, which corresponded to increased LH content of the cells. There was no significant change in the GnRH ED50 concentration (intact = 2.3 +/- 0.03 X 10(-10) M; OVX = 3.3 +/- 0.08 X 10(-10) M (mean +/- SEM, n = 3 expts.)), despite a 57-88% increase in GnRH binding sites in ovariectomised rat pituitary cells. Basal and maximal LH release from ovariectomised mouse pituitary cells was 1.5 to 3-fold greater than that from intact mouse pituitary cells. There was no change in the GnRH ED50 concentration (intact = 4.3 +/- 2.3 X 10(-9) M; OVX = 3.4 +/- 0.9 X 10(-9) M (mean +/- SEM, n = 3 expts.)), even though GnRH binding sites were reduced by 40-73% in the cells from ovariectomised mice. These data indicate that changes in GnRH binding sites of the magnitude observed after ovariectomy play no part in the regulation of gonadotroph responsiveness to GnRH, which is determined by changes in post-receptor events, one of which is an increase in cellular LH.