Gonadotrophin Surge-inhibiting Factor Research Articles

Ovarian gonadotrophin surge-attenuating factor (GnSAF): where are we after 20 years of research?

When gonadotrophin-stimulated IVF methods were being developed in the 1970s and 1980s, understanding of the physiology of FSH improved. In addition to its classic actions of stimulating aromatase activity and oestradiol secretion by ovarian granulosa cells, FSH was found to stimulate the ovarian production of an uncharacterized hormone known by its specific effect of reducing pituitary responsiveness to GnRH. This hormone has been called gonadotrophin surge-attenuating factor (GnSAF), gonadotropin surge-inhibiting factor (GnSIF), various abbreviations (GnSAF/IF, GnSIF/AF) and also attenuin. Although first described in the 1980s, GnSAF has still not been convincingly characterized and no published candidate amino acid sequences conclusively relate to GnSAF bioactivity. On the basis of superovulation studies and in vitro experimentation into the roles of steroids in regulating LH, GnRH and GnRH self-priming, the concept that GnSAF has a role in the regulation of LH secretion, the timing of the LH surge and the prevention of premature luteinization developed. For at least a decade, understanding of the specific GnSAF effects of reducing pituitary sensitivity to GnRH, especially GnRH self-priming and antagonizing the stimulatory effects of oestradiol on GnRH-induced LH secretion, supported this concept. However, improved knowledge of the changes in GnSAF bioactivity in follicular fluid and serum in women requires revision of this concept. The present authors propose that the main role of GnSAF is probably the negative regulation of pulsatile LH secretion, mainly during the first half of the follicular phase, indicating a critical role in the regulation of folliculogenesis and oestradiol secretion.

Ovarian gonadotrophin surge-attenuating factor (GnSAF): where are we after 20 years of research?

When gonadotrophin-stimulated IVF methods were being developed in the 1970s and 1980s, understanding of the physiology of FSH improved. In addition to its classic actions of stimulating aromatase activity and oestradiol secretion by ovarian granulosa cells, FSH was found to stimulate the ovarian production of an uncharacterized hormone known by its specific effect of reducing pituitary responsiveness to GnRH. This hormone has been called gonadotrophin surge-attenuating factor (GnSAF), gonadotropin surge-inhibiting factor (GnSIF), various abbreviations (GnSAF/IF, GnSIF/AF) and also attenuin. Although first described in the 1980s, GnSAF has still not been convincingly characterized and no published candidate amino acid sequences conclusively relate to GnSAF bioactivity. On the basis of superovulation studies and in vitro experimentation into the roles of steroids in regulating LH, GnRH and GnRH self-priming, the concept that GnSAF has a role in the regulation of LH secretion, the timing of the LH surge and the prevention of premature luteinization developed. For at least a decade, understanding of the specific GnSAF effects of reducing pituitary sensitivity to GnRH, especially GnRH self-priming and antagonizing the stimulatory effects of oestradiol on GnRH-induced LH secretion, supported this concept. However, improved knowledge of the changes in GnSAF bioactivity in follicular fluid and serum in women requires revision of this concept. The present authors propose that the main role of GnSAF is probably the negative regulation of pulsatile LH secretion, mainly during the first half of the follicular phase, indicating a critical role in the regulation of folliculogenesis and oestradiol secretion.

Effects of progesterone (P) and antiprogestin RU486 on LH and FSH release by incubated pituitaries from rats treated with the SERM LY11701 8-HCl and/or recombinant human FSH.

The aim of the present study was to investigate the role of the estrogen (ES) background on the effects of P or its antagonist RU486 on basal and LHRH-stimulated LH and FSH secretion. To do this, pituitaries collected from: intact rats in proestrus; rats injected with the ES antagonist LY11701 8-HCl; rats injected with recombinant-human FSH (r-hFSH) to stimulate ovarian hormonogenesis; and rats injected with both LY11701 8-HCl and r-hFSH were incubated with or without LHRH (10 nM) in the presence of P (100 nM) or RU486 (10 nM). RU486 decreased basal and LHRH-stimulated release of LH and FSH and LHRH self-priming in pituitaries from control rats, while P increased both pituitary responsiveness and LHRH self-priming. These effects were absent in pituitaries from rats treated either with the ES antagonist or r-hFSH, which, in the absence of P or RU486 in the incubation medium, reduced gonadotropin release. Because r-hFSH did not increase E2 serum concentration significantly, the putative FSH-dependent ovarian non-steroidal gonadotropin surge inhibiting factor (GnSIF) might be the hormonal cause of the reduced secretion of LH and FSH. Combined treatment with LY117018-HCl and r-hFSH had additive inhibitory effects on gonadotropin release. These results indicate that ES-inducible P receptor (PR) in the pituitary can be activated in a ligand-independent manner by intracellular messengers giving rise to enhanced basal and LHRH-stimulated gonadotropin secretion. The results also suggested that the r-hFSH-stimulated ovarian bioactive entity GnSIF and RU486 may share a similar mechanism of action involving pituitary PR.

Immunoneutralization of follicle stimulating hormone does not affect gonadotrophin surge-inhibiting factor/attenuating factor bioactivity during the rat ovarian cycle.

The physiological role of follicle stimulating hormone (FSH) in the regulation of the release of the putative ovarian factor gonadotrophin surge-inhibiting or -attenuating factor (GnSIF/AF) was investigated. Blood FSH concentrations were immunoneutralized in female rats during different days of the ovarian cycle. FSH antiserum was injected in different protocols to neutralize the FSH surge(s) and/or basal FSH concentrations. All animals were killed on the morning of the day of pro-oestrus of the control rats. Hemi-pituitaries were incubated and gonadotrophin-releasing hormone (GnRH) stimulated and basal luteinizing hormone (LH) and FSH releases and pituitary contents were measured. The biological effectiveness of the FSH antiserum was established by monitoring the histology of vaginal smears and inhibin concentrations in trunk-blood. The predominance of small leukocytes in the smears (low oestradiol bioactivity) and the decreased (bioactive) inhibin secretion imply that anti-FSH neutralizes FSH bioactivity. Simultaneously, the results showed that all anti-FSH injections did not affect the biphasic LH response to GnRH in vitro, which shows an unaltered presence of GnSIF/AF bioactivity in the blood circulation. The results suggest that under physiological conditions GnSIF/AF bioactivity, which keeps the LH responsiveness of the pituitary gland suppressed to the action of GnRH, is not supported by inhibin and not controlled by FSH.

P-231. Functional antagonism between GnRH and gonadotropin surge-inhibiting factor in vivo

P-231. Functional antagonism between GnRH and gonadotropin surge-inhibiting factor in vivo

O-138. Gonadotrophin surge-inhibiting factor inhibits GnRH-stimulated mitogen-activated protein kinase activation

O-138. Gonadotrophin surge-inhibiting factor inhibits GnRH-stimulated mitogen-activated protein kinase activation

The Nature and Function of Putative Gonadotropin Surge-Attenuating/Inhibiting Factor (GnSAF/IF)

I. Introduction REGULATION of the ovarian cycle is by a complex series of positive and negative feedback mechanisms between ovary, hypothalamus, and pituitary. The role of steroidal feedback between ovary, pituitary, and hypothalamus has been known for many years, and our knowledge of the mechanisms involved continues to expand. Nonsteroidal ovarian secretagogues also have endocrine regulatory function, some acting centrally. The best known of these is inhibin, although its central function is now uncertain. Since the late 1970s it has become evident that there are ovarian nonsteroidal products with specific inhibitory effects on pituitary responsiveness to GnRH. As early as 1979 (1) it was suggested that follicular fluid contained a factor other than inhibin that suppressed GnRH-induced LH secretion in vitro. Recent reviews of a novel ovarian factor, which has been called gonadotropin surge-attenuating factor (GnSAF) or gonadotropin surge-inhibiting factor (GnSIF), have supported the evidence for its exi...

Identification of gonadotrophin surge-inhibiting factor (GnSIF)/attenuin in human follicular fluid.

Evidence from several laboratories suggests that the ovaries of many species produce a non-steroidal factor called gonadotrophin surge-inhibiting or attenuating factor (GnSIF) which may regulate the response of the pituitary to gonadotrophin-releasing hormone (GnRH) and as such, may modulate the timing and/or amplitude of the luteinizing hormone (LH) surge. We have recently isolated a candidate GnSIF from porcine follicular fluid (PFF). Porcine GnSiF is a 69 kDa protein which has undetectable inhibin and follistatin immunological and biological activity. The present study was designed to purify and identify GnSIF from human follicular fluid. GnSIF activity was measured as suppression of GnRH-stimulated LH secretion from rat pituitary cells in primary culture. Human follicular fluid (approximately 500 ml) was recovered from patients undergoing in-vitro fertilization (IVF). GnSIF was purified by heparin-sepharose, Q-sepharose, S-sepharose, and hydrophobic interaction chromatography followed by isoelectric focusing. Gel electrophoresis and Western blot were used to identify human GnSIF and compare it with porcine GnSIF. Using these steps, we obtained a highly-purified preparation of GnSIF that manifests in-vitro bioactivity and chromatography characteristics similar to those observed for porcine GnSIF and that hybridizes with a porcine GnSIF antibody. Following treatment with human chorionic gonadotrophin/human menopausal gonadotrophin (HMG/HCG), human follicular fluid contained roughly 25% of the GnSIF (per mg protein) present in porcine follicular fluid. We conclude that GnSIF is present in human follicular fluid and may participate in the regulation of gonadotrophin secretion in this species.

Gonadotrophin surge-inhibiting/attenuating factor governs luteinizing hormone secretion during the ovarian cycle: physiology and pathology.

The stabilization of low luteinizing hormone (LH) concentrations during the period of the ovarian cycle preceding the mid-cycle LH surge seems to be of importance for embryo viability and survival. Several studies have stressed the importance of the timely excess of threshold levels of LH for optimal oocyte quality: LH already initiates the resumption of meiosis when a relatively low threshold level is reached, whereas a further outburst of LH release is necessary to reach the threshold level to induce ovulation. Hence, the mechanism of LH release should have the ability, on the one hand, to limit the LH secretion rate, and on the other, to allow an increased secretion rate, as during the LH surge. The functional antagonistic coupling of gonadotrophin-releasing hormone (GnRH) and the ovarian protein gonadotrophin surge-inhibiting factor or -attenuating factor (GnSIF/AF) provides such a mode of action for the control of LH concentration during the ovarian cycle. One of the important regulatory steps in this process is the de-novo synthesis of the so far unidentified pituitary proteins induced by GnRH. The induction of these proteins is a prerequisite for the increase in the rate of LH release. Because their synthesis is a time-consuming process, the effect becomes visible in the typical biphasic pituitary LH responses to the pulsatile or continuous administration of GnRH: initially low, with an increase after some time. This phenomenon is also known as the GnRH self-priming action. It is assumed that the synthesis of these self-priming-associated proteins is necessary to eliminate the inhibitory effect of GnSIF/AF. GnSIF/AF eliminates the effect of self-priming by neutralizing the biological activity of the pituitary proteins, which are responsible for the increased rate of LH release. Thus, the pituitary gland is kept in a GnRH-hyporesponsive state. The major advantage of such a slow protein synthesis-dependent regulatory mechanism is that it prevents sudden increases in the LH secretion rate in response to GnRH. Thus it stabilizes low LH concentrations to prevent the premature reinitiation of meiosis. However, the enhanced secretion of GnRH and/or the suppressed release or action of GnSIF/AF may finally lead to the restoration of the intrinsic LH responsiveness of the gonadotrophs at the start of the mid-cycle LH surge. The antagonistic interaction between GnRH and GnSIF/AF, and its implication in the control of LH release under physiological and pathological conditions, are discussed.

In vitro bioactivity of gonadotrophin surge attenuating factor is not affected by an antibody to human inhibin

The effect of anti-inhibin antibodies on gonadotrophin surge attenuating/inhibiting factor (GnSAF/GnSIF) and its effect on gonadotrophin secretion in a pituitary cell bioassay were determined by culturing rat pituitary cells in a serum-free medium to which inhibin and a partially purified preparation of gonadotrophin surge inhibiting factor (GnSIF) were added. The samples were treated with an anti-inhibin antiserum and basal and GnRH-stimulated FSH and LH secretion were measured by radioimmunoassay of the culture supernatant. Inhibin had a dose-dependent inhibitory effect on both basal and GnRH-stimulated FSH secretion and also on GnRH-stimulated LH secretion. Anti-inhibin antibody blocked the inhibition of FSH secretion, except at the highest two doses of human inhibin (1.25 ng ml-1, P = 0.04; 2.5 ng ml-1, P = 0.0004). The GnSIF preparation inhibited basal and GnRH-stimulated FSH and GnRH-stimulated LH secretion and was not affected by the anti-inhibin antibody. The novel hormone, GnSIF, is different from inhibin.

Purification of a candidate gonadotropin surge inhibiting factor from porcine follicular fluid.

Several lines of evidence suggest that the ovaries of many species produce a nonsteroidal substance, termed gonadotropin surge inhibiting factor (GnSIF), which inhibits the midcycle gonadotropin surge and attenuates the pituitary response to endogenous or exogenous GnRH. We have previously reported the partial purification of GnSIF from porcine follicular fluid (pFF) and its differentiation from inhibin. We present now the purification of GnSIF to homogeneity and determination of the partial NH2-terminal amino acid sequence. The bioassay for GnSIF used rat pituitary cells in short-term culture that were incubated with test fractions for 48 h, washed, and then incubated with 10 nM GnRH plus test fractions for 4 h. GnSIF activity is defined as the suppression of GnRH-stimulated LH secretion. GnSIF was purified from 500 ml of pFF using sequential heparin-Sepharose, anion-exchange, and cation-exchange liquid chromatography followed by gel permeation, hydrophobic interaction, and mono-Q HPLC steps. Using these six purification steps, we have obtained an apparently homogenous preparation that stains as a single band on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. GnSIF has an apparent mol wt of 69K. Limited NH2-terminal sequence analysis reveals that GnSIF has no sequence homology with other reproductive hormones including the inhibins, activins, and follistatins. Over the dose range tested, GnSIF had no effect on basal LH or FSH secretion by pituitary cells in culture and only slightly inhibited GnRH-stimulated FSH secretion at the highest dose tested. In addition, there was no inhibin or follistatin immunoactivity in the GnSIF preparation. As such, GnSIF appears to be a novel protein in pFF that inhibits GnRH-stimulated LH secretion, and which may participate along with other ovarian proteins and steroids in the regulation of pituitary gonadotropin secretion.

Identification of gonadotropin surge inhibiting factor (GNSIF) activtty in human follicular fluid

Identification of gonadotropin surge inhibiting factor (GNSIF) activtty in human follicular fluid

Endocrinology: On the dynamics between gonadotrophin surge-inhibiting factor and gonadotrophin releasing hormone (GnRH): role of self-priming and desensitization in the luteinizing hormone response to GnRH after follicle stimulating hormone treatment

The effects were studied of follicle stimulating hormone (FSH)-induced production of gonadotrophin surge-inhibiting factor (GnSIF) on three phases of the pituitary responsiveness to gonadotrophin releasing hormone (GnRH): the unprimed, primed and desensitized phases. Rats were injected with FSH on two occasions during the oestrous cycle. Spontaneous luteinizing hormone (LH) surges were measured as well as GnRH-induced LH surges on the day of pro-oestrus during infusions with 100-4000 pmol GnRH/rat/10 h, in phenobarbital blocked rats. The spontaneous LH surges were attenuated or completely inhibited by the FSH treatment. FSH suppresses and prolongs the unprimed LH response and delays GnRH self-priming, especially during infusions with low concentrations of GnRH. This treatment does not affect the total LH response (area under curve) to the highest concentrations of GnRH and after ovariectomy. On the other hand, this response is suppressed during infusions with the lower concentrations of GnRH. Hence, FSH, via GnSIF, delays maximal priming of the LH response to GnRH, whereas the suppression of LH release is a consequence of the GnRH-induced progressed state of desensitization. The inconsistent effects of FSH on the mid-cycle LH surges are explained as a result of the interaction between the relative strengths of GnRH and GnSIF.

Purification of Gonadotropin Surge-Inhibiting Factor from Sertoli Cell-Enriched Culture Medium

Gonadotropin surge-inhibiting factor (GnSIF) is a novel biological factor in follicular fluid that inhibits the pre-ovulatory LH and FSH surges. Recent studies suggest that the biological activity of GnSIF can be attributed to inhibin. Preliminary studies showed that the Sertoli cell-conditioned medium contains GnSIF-like biological activity. Using 32 liters of primary Sertoli cell-enriched culture medium and an in vitro pituitary bioassay, GnSIF was isolated. GnSIF is a single monomeric polypeptide (Mr of 37,000) with a partial N-terminal amino acid sequence of NH 2-SDXXPQL which is distinct from any existing protein sequences. The identity of the inhibin that was separated from GnSIF was confirmed by direct protein sequencing. The purified GnSIF inhibited GnRH-stimulated LH- and basal FSH-release from pituitary cells cultured in vitro dose-dependently with a half maximal effective dose of about 1.4 ng/ml and 3.5 ng/ml, respectively. The half maximal effective dose of inhibin to suppress the pituitary GnRH-stimulated LH- and basal FSH-release was 2.5 ng/ml and 0.8 ng/ml, respectively. This study demonstrates that GnSIF is a unique protein that shares some biological activities with inhibin.

The identification of gonadotrophin surge inhibiting factor and its role in the regulation of pituitary gonadotrophin secretion

Evidence from several laboratories suggests that the ovaries of rats, pigs, monkeys and women produce a non-steroidal factor which inhibits or attenuates the mid-cycle gonadotrophin surge. This substance is present in follicular fluid and we have called it gonadotrophin surge inhibiting factor (GnSIF). Utilizing a rat pituitary cell bioassay, we have monitored GnSIF activity during purification from porcine follicular fluid (PFF). Rat pituitary cells in short-term culture were incubated with GnSIF for 48 h, washed and then exposed to gonadotrophin releasing hormone (GnRH) plus GnSIF for 4 h. GnSIF activity was defined as suppression of GnRH-stimulated LH secretion over 4 h, and inhibin activity was expressed as suppression of basal follicle stimulating hormone (FSH) secretion over 48 h. Approximately 5 l of PFF was fractionated through heparin-Sepharose, Q-Sepharose, Mono-S, hydroxylapatite, and gel permeation chromatography steps. This fractionation removes all detectable inhibin and follistatin immunoactivity and bioactivity. The purified GnSIF inhibits GnRH-stimulated LH secretion with little or no effect on basal FSH release. In summary, we have obtained a purified preparation of GnSIF which contains undetectable inhibin and follistatin. As such, GnSIF appears to be distinct from other known gonadal proteins regulating reproductive function, and may participate along with the inhibins, activins and follistatins in the gonadal regulation of pituitary gonadotrophin secretion.

The effect of gonadotrophin surge-inhibiting factor on the self-priming action of gonadotrophin-releasing hormone in female rats in vitro.

The present study was designed to explore further the functional antagonism between gonadotrophin-releasing hormone (GnRH) and the ovarian factor, gonadotrophin surge-inhibiting factor (GnSIF). In all experiments, pituitary tissue was exposed to various amounts of GnSIF, after which the self-priming action of GnRH was studied. GnSIF was increased in vivo by FSH treatment and increased in vitro by adding various amounts of follicular fluid (FF) to cultured pituitary cells. Treatment with 3 or 10 IU FSH suppressed the initial LH response and delayed the maximally primed LH response to GnRH. Treatment with FSH was only effective in intact rats on days 1 and 2 of dioestrus. There was no difference in the rate of maximal LH release irrespective of treatment with either FSH or saline. Since FSH treatment was ineffective in long-term ovariectomized rats, it was concluded that the initial suppressive effect of FSH on LH release was mediated by GnSIF. Cycloheximide prevented the self-priming action of GnRH by inhibiting GnRH-induced protein synthesis. The initial protein synthesis-independent GnRH-stimulated LH release, which was already suppressed by FSH treatment, remained suppressed in the presence of cycloheximide. Pretreatment with GnRH in vivo increased the protein synthesis-independent GnRH-induced LH release during subsequent incubation of the glands. This increase did not occur after FSH treatment. Pituitary cells, cultured for 20 h in medium only, failed to elicit the self-priming effect of GnRH. Preincubation with FF maintained the self-priming effect. This was independent of the concomitant presence of various amounts of oestradiol. Preincubation with bovine FF suppressed the initial GnRH-stimulated LH release dose-dependently. Porcine FF, human FF and testicular extract suppressed the release of LH in a similar way. It was concluded that GnSIF suppresses the initial LH response to continuous GnRH stimulation. Increased levels of GnSIF caused by FSH treatment also delayed the primed LH release. The mechanism of functional antagonism between GnSIF and GnRH could give rise to the occurrence of the phenomenon of GnRH self-priming.

In vivo evidence that inhibin is a gonadotropin surge-inhibiting/attenuating factor.

Hyperstimulation of ovarian function through exogenous gonadotropin administration suppresses the preovulatory surges of both LH and FSH. This phenomenon has been demonstrated to be the result of a non-steroidal factor released from the ovary that has been designated as either gonadotropin surge-inhibiting factor or attenuating factor (GnSIF/AF). To examine the possibility that inhibin might possess the activity ascribed to this factor, endogenous inhibin was immunoneutralized in female rats under conditions known to stimulate GnSIF/AF activity. Inhibin-like immunoreactivity was found to be significantly elevated in FSH-treated rats prior to the time of the gonadotropin surges. Spontaneous preovulatory surges of both LH and FSH were observed in saline-treated (control) rats that were subsequently treated with either anti-inhibin serum (AS) or normal sheep serum (NS). FSH-treatment completely prevented the occurrence of gonadotropin surges in rats subsequently treated with NS. In FSH-injected rats subsequently treated with AS, however, normal preovulatory surges of both LH and FSH were observed. These results indicate that inhibin may be the factor responsible for the suppression of the preovulatory gonadotropin surges in FSH-treated rats and, at least in this species, may be a GnSIF/AF.

Identification of Gonadotropin Surge-Inhibiting Factor (GnSIF) in Follicular Fluid and its Differentiation from Inhibin1

Exposure of rat pituitary cell cultures to charcoal-extracted porcine follicular fluid (pFF) inhibits gonadotropin-releasing hormone (GnRH) stimulation of luteinizing hormone (LH) and follicle-stimulating hormone (FSH) release in a dose-dependent manner. The inhibition of GnRH-stimulated gonadotropin secretion (gonadotropin surge-inhibiting factor [GnSIF] activity) by pituitary cells in vitro requires up to 24-48 h preexposure to pFF. One microliter of pFF inhibits approximately 50% of the GnRH-stimulated LH release and is defined as 1 unit of GnSIF activity. Basal LH secretion is unaltered under these conditions. GnSIF activity is distinct from that of inhibin, which selectively suppresses basal release of FSH but not LH. A partially purified preparation of inhibin contains less than 1% as much GnSIF activity as inhibin activity. GnSIF activity is resistant to moderate heat treatment (60 degrees C for 60 min) and is fully recovered after acetone precipitation. Chromatography of pFF on heparin/Sepharose affinity matrix effectively separates inhibin from GnSIF. Whereas inhibin has a high affinity for heparin, GnSIF activity does not associate with this affinity matrix and is recovered in the column void volume. In summary, we have developed an in vitro bioassay for the detection of GnSIF activity in pFF. Moreover GnSIF activity seemingly derives from a molecular entity distinct from inhibin, having different physicochemical characteristics and differential effects on pituitary gonadotropin secretion.

Human menopausal gonadotropin stimulation in monkeys: Blockade of the luteinizing hormone surge by a highly transient ovarian factor

Women and monkeys treated with human menopausal gonadotropin (hMG) often fail to demonstrate timely luteinizing hormone (LH) or follicle-stimulating hormone (FSH) surges despite serum estradiol (E 2 ) levels sufficient to elicit positive feedback of LH and FSH secretion. Here we explored the mechanism of this blockade of estrogen-mediated positive feedback by hMG and the duration of this effect in acutely ovariectomized monkeys. Cycling cynomolgus monkeys ( n =14) were administered hMG (22.5IU, intramuscularly) daily beginning on cycle day 3. Three monkeys received an E 2 -benzoate challenge on day 10, resulting in peak E 2 levels of 892±313pg/ml, without subsequent LH or FSH surges. Four monkeys underwent bilateral ovariectomy on day 12, followed within 60 to 90minutes by gonadotropin-releasing hormone (GnRH) administration (15 μ g, intravenously). Five intact monkeys underwent a similar GnRH challenge. LH response to GnRH in the intact monkeys was significantly suppressed in comparison with the ovariectomized group. None of the animals manifested an FSH response to GnRH administration. Two additional monkeys did have a spontaneous LH surge during hMG administration. We conclude that hMG stimulates the production of an ovarian factor(s) which blocks the pituitary LH response to GnRH. This blockade of GnRH action on the pituitary may be the mechanism by which hMG stimulation prevents estrogen-mediated positive feedback of LH secretion. The putative ovarian factor(s) has a relatively short circulatory half-life and/or binding time, resulting in loss of its blocking activity within 90minutes after bilateral ovariectomy. Evidence is presented to suggest that this factor(s), designated gonadotropin surge-inhibiting factor, belongs to an amorphous group of nonsteroidal ovarian hormones that remain to be further characterized.

Human menopausal gonadotropin stimulation in monkeys: blockade of the luteinizing hormone surge by a highly transient ovarian factor

Women and monkeys treated with human menopausal gonadotropin (hMG) often fail to demonstrate timely luteinizing hormone (LH) or follicle-stimulating hormone (FSH) surges despite serum estradiol (E2) levels sufficient to elicit positive feedback of LH and FSH secretion. Here we explored the mechanism of this blockade of estrogen-mediated positive feedback by hMG and the duration of this effect in acutely ovariectomized monkeys. Cycling cynomolgus monkeys (n = 14) were administered hMG (22.5 IU, intramuscularly) daily beginning on cycle day 3. Three monkeys received an E2-benzoate challenge on day 10, resulting in peak E2 levels of 892 +/- 313 pg/ml, without subsequent LH or FSH surges. Four monkeys underwent bilateral ovariectomy on day 12, followed within 60 to 90 minutes by gonadotropin-releasing hormone (GnRH) administration (15 micrograms, intravenously). Five intact monkeys underwent a similar GnRH challenge. LH response to GnRH in the intact monkeys was significantly suppressed in comparison with the ovariectomized group. None of the animals manifested an FSH response to GnRH administration. Two additional monkeys did have a spontaneous LH surge during hMG administration. We conclude that hMG stimulates the production of an ovarian factor(s) which blocks the pituitary LH response to GnRH. This blockade of GnRH action on the pituitary may be the mechanism by which hMG stimulation prevents estrogen-mediated positive feedback of LH secretion. The putative ovarian factor(s) has a relatively short circulatory half-life and/or binding time, resulting in loss of its blocking activity within 90 minutes after bilateral ovariectomy. Evidence is presented to suggest that this factor(s), designated gonadotropin surge-inhibiting factor, belongs to an amorphous group of nonsteroidal ovarian hormones that remain to be further characterized.