Sequential Study of Luteinizing Hormone (LH) and “Progestin” Secretion on the Afternoon of Proestrus in the Rat

Abstract

Sequential measurements were made of LH concentrations in systemic blood and of progesterone and 20α-OH-pregn-4-en-3- one secretion rates into ovarian venous blood of female rats on the afternoon of proestrus. On the days of estrus, diestrus I and diestrus II, evaluation was limited to a single time period between 3:30 and 5:00 PM. LH was quantitated by double antibody radioimmunoassay. Progesterone and 20α-OH-pregn-4-en-3-one were measured by gasliquid chromatography following purification through thin-layer chromatography. Elevations in LH level were observed as early as 2:00 PM and as late as 8:00 PM on the day of proestrus (uterine ballooning). On the days of estrus, diestrus I and diestrus II, LH remained at basal levels. No change in progesterone secretion occurred between diestrus II and 4 : 3 0 PM of proestrus. Secretion rates (μg/hr/ovary) of progesterone rose sharply after 4:30 PM and exhibited peak levels (40-fold change) at 8:00 PM. 20a-OHpregn- 4-en-3-one secretion rates (μg/hr/ovary) were highly variable. Highest peak values were obtained between 2:30 and 3:00 PM on the day of proestrus and declined rapidly at the time of progesterone increase. Analysis of individual animals revealed no change in progesterone secretion prior to elevations in plasma LH concentrations. The lag period between LH and progesterone secretion appeared to shorten if LH was released late in the afternoon. These results are interpreted to indicate that progesterone secretion is not increased prior to the LH surge and probably does not play a role in initiating LH release. 20α-OH-pregn-4-en-3-one secretion is increased prior to, or simultaneously with, the release of LH, and may have a physiological role in inducing and/or facilitating LH release. (Endocrinology89: 39, 1971)