Pituitary Luteinizing Hormone (LH) and Follicle-Stimulating Hormone (FSH) Responses to Gonadotropin-Releasing Hormone during the Rat Estrous Cycle: An Increased Ratio of FSH to LH is Secreted during the Secondary FSH Surge1

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Abstract
The homonal interactions required for the generation of a secondary surge of FSH on the evening of proestrus have not been clearly defined. The role of GnRH in driving a surge of FSH has been questioned by findings in previous studies. In the current study, gonadotropin secretion was measured from pituitary fragments obtained from rats at 0900 and 2400 h on each day of the estrous cycle. Pituitary fragments were perifused in basal (unstimulated) conditions or in the presence of GnRH pulses to determine whether a selective increase in basal release of FSH and/or an increase in the responsiveness to GnRH occurs during the secondary FSH surge. Each anterior pituitary was cut into eights and placed into a microchamber for perifusion. Seven pulses of GnRH (peak amplitude = 50 ng/ml; duration = .apprx. 2 min) were administered at a rate of one per hour starting at 30 min. Fractions of perfusate were collected every 5 min and frozen until RIA for LH and FSH. The mean total amount of LH or FSH secreted during the hour interval following each of the last six pulses of GnRH (or the corresponding basal hour) was calculated. Analysis of variance with repeated measures indicated that the evening secretion of LH on proestrus (2400 h) dropped significantly (p < 0.05) from a maximum on the morning of proestrus (0900 h), whereas the FSH secretion remained elevated at this time. Therefore, the ratio of FSH to LH secreted in response to GnRH pulses was highest during the secondary FSH surge and lowest on the morning of proestrus. In basal conditions, the secretion of LH decreased during the perifusion, whereas the secretion of FSH increased with increased perifusion time. We presume that these increases are due to the effects of the acute removal of gonadal negative feedback, particularly inhibin. Although there were no cycle stage differences in the total release of FSH, LH, or the ratio of FSH to LH in basal conditions, there were significant differences between groups with respect to the rate of rise of basal FSH secretion. The initial FSH secretion rate was higher on the evening of proestrus compared to the morning of proestrus. We hypothesize that both increased basal FSH release and maintained FSH responsiveness, but not LH responsiveness, to GnRH pulses contribute to the generation of the secondary FSH surge.