Effect of number of receptors for gonadotropin-releasing hormone on the release of luteinizing hormone 1

The relationship between number of receptors for gonadotropin-releasing hormone (GnRH) and the ability of the anterior pituitary gland to release luteinizing hormone (LH) was examined in ovariectomized ewes. A GnRH antagonist was used to regulate the number of available receptors. The dose of GnRH antagonist required to saturate approximately 50 and 90% of GnRH receptors in ovariectomized ewes was determined. Thirty min after intracarotid infusion of GnRH antagonist, ewes were killed and the number of unsaturated (i.e., those available for binding) pituitary GnRH receptors was quantified. Infusion of 10 and 150 µg GnRH antagonist over a 5-min period reduced binding of the labeled ligand to approximately 50 and 12% of controls, respectively. The effect of reducing the number of GnRH receptors on release of LH after varying doses of the GnRH agonist, D-Ala6-GnRH-Pro9-ethylamide (D-Ala6-GnRH) was then evaluated. One of four doses of D-Ala6-GnRH (0.125, 2.5, 50 and 400 µg) was given i.v. to 48 ovariectomized ewes whose GnRH receptors had not been changed or were reduced to approximately 50 or 12% of control ewes. In ewes with a 50% reduction in GnRH receptors, total release of LH (area under response curve) was lower than that obtained for controls (P<0.01) at the 0.125-µg dose of D-Ala (6.1 ± 0.7 cm2 vs. 13.5 ± 0.7 cm2) but was not different at the 2.5-, 50- or 400-µg doses of D-Ala6-GnRH. Total release of LII was greater (P<0.01) in control ewes than in ewes with 12% available GnRH receptors at all doses of D-Ala6-GnRH (controls=3.5 ± 0.7, 45.0 ± 4.7, 94.0 ± 10.6 and 130.3 ± 22.2 cm2; 12% available GnRH receptorsı1.1 ± 0.4, 13.8 ± 5.2, 53.2 ± 7.2 and 62.9 ± 3.1 cm2). The relationship between release of LH and number of GnRH receptors occupied by D-Ala6 -GnRH was not linear throughout the LH response curve. From these data we suggest that reduction of GnRH receptor numbers may influence tonic release of LH, but that near maximal release of LH is not affected unless the number of GnRH receptors is reduced by more than 50%.