Strength-duration characteristics of estrogen effects on gonadotropin response to gonadotropin-releasing hormone in women. I. Effects of varying duration of estradiol administration.

This study was designed to investigate the effects of increased serum concentrations of estradiol of varying durations upon the gonadotropin responses to synthetic gonadotropin-releasing hormone (GnRH or LRF). Beginning at 8:00 PM on the first day of the menstrual cycle, subjects received im injections of estradiol benzoate (E2B), 5 mug/kg initially, followed by 2.5 mug/kg every 12 h for a total of 3, 5, 7, 9, or 11 injections. Twelve h after the last E2B injection, or 36, 60, 84, 108, or 132 h after the first injection of E2B (2 subjects at each time interval), each subject received 100 mug GnRH, iv. In addition, each subject received 100 mug GnRH iv during one of the seven days of the antecedent (control) menstrual cycle during which no exogenous estradiol was administered. When GnRH was infused 36 h after the initiation of E2B pretreatment, there was no significant LH or FSH increase. In contrast, LH and FSH responses were augmented and prolonged when compared with control cycles when GnRH was administered at 84, 108, or 132 h. At 60 h, responses of LH were augmented, although not to as great a degree. FSH responses were not augmented at 60 h. Expressed as maximal increase from baseline, gonadotropin responses following E2B were 1 1/2 to 9 times those achieved during control cycles (without E2B). Since mean serum estradiol concentrations at 36 h (185.9 +/- 20.0), when gonadotropin responses were absent, were similar to those at 60 (157.7 +/- 31.6), 84 (186.2 +/- 38.1), 108 (181.3 +/- 46.7), and 132 h (128.0 +/- 43.0 pg/ml), when responses were augmented, these results support the concept that the modulating effect of estradiol on pituitary response is dependent upon the duration of exposure of the hypothalamic-pituitary system to increased concentrations of estradiol. It is probable that the duration of the late follicular phase rise in serum estradiol is responsible, at least in part, for the augmented gonadotropin response seen at midcycle.