Control of the Preovulatory Endocrine Events in the Ewe: Interrelationship of Estradiol, Progesterone, and Luteinizing Hormone*

Abstract

Two experiments were conducted to examine the possibility that, in the ewe, regression of the corpus luteum initiates the following series of preovulatory events: 1) a sustained increase in tonic LH secretion resulting from progesterone withdrawal; 2) a sustained increase in estradiol secretion stimulated by the rising titer of LH; and 3) the estradiol-induced preovulatory LH surge. In the first experiment, the corpus luteum was removed surgically during the midluteal phase of the estrous cycle. This procedure induced a premature withdrawal of progesterone accompanied by premature sustained increases in circulating LH and estradiol and a premature preovulatory LH surge. These changes closely resembled those which follow spontaneous luteolysis. The premature sustained increases in circulating LH and estradiol and the LH surge were prevented when luteal phase levels of serum progesterone were maintained by insertion of progesterone implants at the time of corpus luteum removal, and they were reinstated when the progesterone implants were removed. In the second experiment, hCG was injected into ewes during seasonal anestrus to determine whether an exogenous LH-like stimulus could initiate a preovulatory increase in estradiol secretion in the absence of progesterone withdrawal. This treatment evoked a rise in circulating estradiol of sufficient magnitude and duration to induce an LH surge, followed by ovulation. The results of these two experiments lend strong support to the hypothesis that during the estrous cycle of the ewe, the withdrawal of progesterone at luteolysis initiates the subsequent preovulatory events by permitting a sustained increase in tonic LH secretion which, in turn, stimulates the increase in estradiol secretion necessary to trigger the LH surge.