Effect of high dose exogenous oestrogen on midcycle luteinizing hormone surge in human spontaneous cycles

Luteinizing hormone-independent rise of progesterone as the physiological trigger of the ovulatory gonadotropins surge in the human

The current ovarian cycle paradigm postulates that ovulation is triggered by a critically sustained elevation of estradiol. However, an in-depth look into the published data reveals considerable uncertainty about the relative roles of progesterone and estradiol in the ovulation process.This review provides compelling evidences that the role of estradiol in ovulation has been misinterpreted and that the true physiological trigger of ovulation is a luteinizing hormone-independent preovulatory progesterone surge in the circulation to approximately 0.5 ng/mL. Furthermore, the current work reconciles the ability of progesterone to trigger ovulation, with its well-established ability to block ovulation during pregnancy, or when administered in the form of a synthetic progestin in birth control formulations and with experimental data that estradiol benzoate triggers ovulation in the complete absence of progesterone.

Attenuating activity of the ovary on LH response to GnRH during the follicular phase of the cycle

OBJECTIVE

Oestradiol sensitizes the pituitary to GnRH, while gonadotrophin surge attenuating factor (GnSAF) may oppose this action. Using the LH response to GnRH during treatment with FSH as an in vivo bioassay for GnSAF, we tested the hypothesis that the augmented LH response to GnRH in the late follicular phase is related to reduced production of GnSAF from the ovulatory follicle.

DESIGN

Prospective intervention study.

PATIENTS

Ten healthy, normally cycling women.

MEASUREMENTS

The LH response to 10 μg GnRH i.v. (ΔLH) was investigated on days 2 and 3 and on days v (follicle size 16-17 mm) and v + 1 of cycle 1 (control) and cycle 2. On days 2 and v, a single s.c. injection of either normal saline (cycle 1) or 450 IU recombinant FSH (cycle 2) was given after the end of the GnRH experiment.

RESULTS

FSH injection increased both serum oestradiol and inhibin B. In cycle 1, ΔLH remained unchanged from days 2 to 3 but increased significantly from days v to v + 1. In contrast, in cycle 2, ΔLH decreased significantly from days 2 to 3 (P < 0·05) and showed a nonsignificant increase from day v to day v + 1. The percentage difference in ΔLH between cycle 1 and cycle 2 was similar on days 3 (-66·9 ± 17·5%) and v + 1 (-65·2 ± 3·6%).

CONCLUSIONS

These results suggest that during the follicular phase of the menstrual cycle, GnSAF is produced by small antral follicles, while the contribution of the ovulatory follicle is minimal.

Progesterone and ovulation

The role of progesterone (P) in the mechanism of ovulation is controversial at best. The contraceptive application of P was established in rodents in 1936 and with orally absorbed progestogenes was put to human use. There were hints on the proovulatory actions of P administered before the time of ovulation in rats by 1948. Similarly, in 1954 the observation of high P level in the preovulatory follicle pointed to a role in ovulation. Neither of these two observations was further investigated and the positive feedback effect of P exerted on gonadotropins was described in 1968. Still the positive feedback between P and gonadotropins were not recognized as a physiologic mechanism, much less utilized pharmacologically. The apparent contradiction between these two different actions of P continues upto now. The paper sets out to expose this controversy and tries to resolve it using extensive literary data and the author's experiences with synthetic progestogenes in contraception, in the treatment of infertility and with the antigestagen mifepristone in blocking ovulation. The precise mechanisms lying behind these applications are explored and discussed in detail. The putative role of oestradiol (E2) in the mechanism of eliciting the gonadotropin surge is extensively discussed but refuted as the ovulatory signal. The time sequence between the rise of P and gonadotropins contradicts the common wisdom of LH causing luteinization. The positive feedback effect of P on the E2 sensitized ovulatory axis on the hypothalamic and pituitary level is discussed and its local role in the mechanism of follicular rupture is also taken into account. The final proof seems to be the antiovulatory effect of mifepristone, which blocked both GnRH pulsatility, pituitary sensitivity to GnRH and follicular rupture in several experiments. Thus, the dogma of LH peak causing follicular rupture and subsequent luteinization seems questionable, the putative role of E2 to initiate the ovulatory cascade has to be discarded and P's role as a trigger of the physiological mechanisms leading to ovulation should be firmly recognized.

Ovarian regulators of gonadotropin secretion

Great progress in the ovarian mechanisms that control gonadotrophin secretion in women has recently been achieved. Estradiol (E2) is the main component of the ovarian negative effect on basal gonadotropin secretion during the normal menstrual cycle. However, nonsteroidal substances such as inhibins and activins that can affect follicle-stimulating hormone (FSH) secretion in vitro may also participate in the control of FSH secretion in vivo. Recent evidence has shown that the ovaries also produce another nonsteroidal substance, named gonadotropin surge-attenuating factor (GnSAF), that specifically attenuates GnRH-induced LH secretion and the endogenous LH surge in superovulated women. It is possible that during the normal menstrual cycle GnSAF controls the amplitude of the midcycle LH surge by antagonizing the stimulating effect of E2 on the pituitary.

Attenuation of gonadotrophin release and reserve in superovulated women by gonadotrophin surge attenuating factor (GnSAF)

In-vivo and in-vitro studies have provided evidence that a non-steroidal ovarian factor, called gonadotrophin surge attenuating factor (GnSAF), attenuates the endogenous LH surge in superovulated women. To study the mechanism of action of GnSAF, the LH response to two i.v. pulses of GnRH (10 micrograms each, 2 h apart) was investigated in eight normally ovulating women during the late follicular phase of a spontaneous and an FSH superovulated cycle. The maximal LH increase in response to the first pulse (initial release) was considered as representing the acutely releasable pool and the delta LH area under the whole curve (integrated response) the reserve pool of LH. Both the initial release and the integrated response to GnRH were markedly attenuated in the FSH as compared to the spontaneous cycles. The response to the second pulse was significantly greater than the response to the first pulse (self-priming effect of GnRH) in both the spontaneous and the FSH cycles. However, in the FSH cycles the self-priming effect of GnRH was markedly reduced as compared to the spontaneous cycles. We conclude that during superovulation induction in women the two pools of pituitary LH are markedly attenuated. It is suggested that GnSAF attenuates both the GnRH-induced initial release of LH and the self-priming effect of GnRH on the pituitary.

Superovulation induction in women suppresses luteinizing hormone secretion at the pituitary level

In superovulated women the pituitary response to GnRH is markedly attenuated by an unspecified ovarian factor(s). To examine the site of attenuation, the response of the pituitary to GnRH was investigated in five normally ovulating women during the late follicular phase of 3 cycles, i.e. a spontaneous (control) cycle, a cycle treated with 'pure' FSH, and a cycle treated with a combination of 'pure' FSH and pulsatile GnRH, via a pump (15 micrograms/pulse). The oestradiol levels (mean +/- SEM) at the time of the GnRH challenge were respectively 646 +/- 35, 1692 +/- 282 and 5976 +/- 1129 pmol/l. The size of the leading follicle was similar in all groups. Serum LH levels during treatment with FSH decreased significantly, while during treatment with FSH plus GnRH they increased initially and then decreased progressively. The response of pituitary LH to GnRH was significantly attenuated during treatment with FSH and FSH plus GnRH, as compared to the spontaneous cycles, but was not abolished. The attenuation was significantly greater in the FSH plus GnRH cycles (94%) than in the FSH cycles (59%). We conclude that in superovulated cycles, the attenuation of the pituitary response to GnRH increases with the degree of ovarian hyperstimulation. It is suggested that the responsible unspecified ovarian factor(s) exerts its effects at least at the pituitary level.