Role of Estradiol in Inducing an Ovulatory-Like Surge of Luteinizing Hormone in Sheep 1

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Abstract
Two experiments were performed to examine the effect of estradiol on secretion of luteinizing hormone (LH) and on the number of receptors for gonadotropin-releasing hormone (GnRH) after down-regulation of GnRH receptors in ovariectomized ewes. In the 1st experiment, ovariectomized ewes were administered 1 of 4 treatments: group 1, infusion of GnRH i.v. for 40 h; group 2, injection of 100 .mu.g estradiol i.m.; group 3, infusion of GnRH i.v. for 16 h followed immediately by an injection of 100 .mu.g estradiol i.m.; and group 4, infusion of GnRH i.v. for 40 h plus injection of 100 .mu.g estradiol i.m. after the 16th h of infusion. Ewes in groups 1, 3 and 4 responded to the infusion of GnRH with an immediate increase in serum concentrations of LH, with maximum values occurring between 2 and 4 h after the start of infusion; serum concentrations of LH then began to decline and were approaching the pretreatment baseline within 16 h. Administration of estradiol resulted in a surge of LH regardless of whether the pituitary had been desensitized by infusion of GnRH or not. In all cases the magnitude of the surge was similar to that induced by the initial infusion of GnRH. In groups 2 and 3 the surge of LH began at 12.3 .+-. 0.1 and 11.9 .+-. 0.1 h after administration of estradiol. In contrast, the ewes in group 4 had a surge of LH beginning 3.7 .+-. 0.1 h after administration of estradiol. In other ewes, treated as groups 2 and 4, measurement of GnRH receptors at 0, 8, 16 and 24 h after injection of estradiol indicated that the number of receptors was elevated (P < 0.05) at 8 h in ewes given both estradiol and GnRH, but not until 16 h in ewes given only estradiol. Since ewes receiving a constant infusion of GnRH had an ovulatory-like surge of LH earlier after treatment with estradiol than ewes not receiving an infusion of GnRH, both an increase in sensitivity of the pituitary gland to GnRH and an increased secretion of GnRH apparently are required for an ovulatoruy-like surge of LH in ewes.