Follicular fluid treatment during the follicular versus luteal phase of the menstrual cycle: effects on corpus luteum function

Administration of charcoal-extracted porcine follicular fluid (pFF) to rhesus monkeys at the time of menses impairs the subsequent function of the corpus luteum of the menstrual cycle. The following studies were performed: 1) to characterize the luteal phase defect induced by pFF treatment at menses, and 2) to determine whether pFF treatment in the luteal phase alters corpus luteum function. Adult, female rhesus monkeys were injected sc for 3 days with pFF (10, 5, and 5 ml) beginning on day 1 (n = 5) or day 18 (n = 4) of the menstrual cycle. Femoral venous blood was collected daily throughout the treatment cycle and during the posttreatment cycle of day 18 to 20-treated monkeys. Serum LH, FSH, 17 beta-estradiol (E2), and progesterone (P) were measured by RIA. After pFF treatment on days 1-3, FSH and E2 levels in the early follicular phase were less (P less than 0.05) than those of control cycles (n = 7). Serum LH was not suppressed by pFF treatment. Moreover, the preovulatory rise in circulating E2 and the amplitude of the LH/FSH surge were similar in control and pFF-treated monkeys. Although timely midcycle gonadotropin surges occurred in four of five pFF-treated monkeys, serum P was markedly reduced (P less than 0.05) during the first half of the luteal phase. Circulating P increased to control levels during the late luteal phase before normal onset of menses 16.3 +/- 1.0 (SE) days after the LH surge. Treatment with pFF on days 18-20 of the cycle reduced the levels of circulating FSH, but serum LH, E2, P, and the length of the luteal phase remained comparable to control cycles. Moreover, the hormonal patterns and the length of the follicular and luteal phases in the posttreatment cycle indicated normal ovarian function. Thus, pFF treatment at menses results in an aberrant ovarian cycle characterized by an insufficient, rather than short, luteal phase. Whereas pFF treatment in the early follicular phase vitiates development of the dominant follicle and the related corpus luteum, similar treatment at midluteal phase does not suppress concurrent luteal function or subsequent folliculogenesis.     

Induction of luteal phase defects in rhesus monkeys by follicular fluid administration at the onset of the menstrual cycle

Administration of charcoal-extracted porcine follicular fluid (pFF) to rhesus monkeys on days 1-3 of the menstrual cycle suppressed serum FSH, but not LH, during the early follicular phase. Although timely midcycle gonadotropin surges occurred in 5 of 6 pFF-treated monkeys, the preovulatory rise in serum estradiol was markedly diminished and serum progesterone (P) levels were subnormal at midluteal phase. The wet weight of the corpus luteum excised from pFF-treated monkeys at midluteal phase was less than that from controls. Moreover, both basal and gonadotropin (hCG)-sensitive P production by short-term suspensions of luteal cells from pFF-treated monkeys was significantly less than that by control cells. These findings provide direct support for the concept that FSH-dependent events during the early follicular phase are important determinants of the subsequent function of the corpus luteum of the menstrual cycle. Since pFF-induced luteal dysfunction was strikingly similar to spontaneous luteal phase defects found in monkeys and women, this primate model permits study of the mechanism(s) whereby FSH deficiency during recruitment and selection of the dominant follicle portends defective luteal function and infertility in women.     

Steroidogenic responsiveness of the monkey corpus luteum to exogenous chorionic gonadotropin

Groups of female rhesus monkeys were given a 5-day regimen of im injections of hCG (30, 60, 90, 180, and 360 IU, respectively) beginning 2, 6, 10, or 14 days after the midcycle LH surge (day 0). Serum progesterone concentrations in the day 2 treatment group did not differ markedly from values observed in control monkeys throughout normal menstrual cycles. In contrast, monkeys receiving hCG beginning on days 6, 10 or 14 had immediate significant increases in serum progesterone in response to the first injection of hCG. Dramatic responses were seen in the day 6 and 10 groups (2.5- and 5-fold elevations in serum progesterone, respectively); progesterone values plateaued at about 13 ng/ml during the hCG treatment interval. Monkeys receiving hCG on day 14 had 4-fold elevations in serum progesterone, but concentrations did not exceed 6 ng/ml. Serum estradiol increased significantly after hCG to concentrations between 200-300 pg/ml in all treatment groups; peak values were seen at the time of or in the days immediately after the last hCG injection. Serum testosterone concentrations were not significantly altered by hCG administration at any stage of the luteal phase. hCG did not sustain serum steroid hormone concentrations in monkeys with the corpus luteum removed on day 10 of the luteal phase. A 10-day regimen of increasing hCG doses beginning on day 10 of the luteal phase mimicked the steroid hormone secretion patterns observed in control monkeys during early pregnancy. The data show that the qualitative and quantitative steroidogenic capacities of monkey corpora lutea after a gonadotropin challenge are profoundly affected by luteal age.     

Regulation of folliculogenesis in the cycling rhesus monkey: selection of the dominant follicle.

To identify factors regulating the initiation of follicle growth in adult primates, the ovarian cycle of sexually mature rhesus monkeys was interrupted by surgical ablation of the preovulatory follicle or functioning corpus luteum (CL). In 10 of 10 animals, cautery of the largest visible follicle on Day 8-12 of the cycle blocked ovulation, and in all but one abolished the expected midcycle surges of gonadotropin secretion. In 8 monkeys of this group, surges of LH and FSH release occurred 12.4 +/- 0.9 days (d) (mean +/- SE) after cautery, coincident with elevations in serum estrogens, and succeeded by typical luteal phase patterns of circulating progesterone (P). No gonadotropin or estrogen surges were observed during the next 32 days of sampling in the remaining pair, despite visible new vesicular follicles. Removal of the CL in 5 of 5 monkeys 4-6 days after the midcycle LH surge was followed by a reduction in serum P to less than 0.25 ng/ml within 24 h and by the onset of menses within 3-4 days. After luteectomy in 4 of the 5 animals, preoperative levels of LH and FSH were maintained until 12.8 +/- 0.9 days, when typical surges of gonadotropin secretion occurred, followed by a normal luteal phase pattern of P. The fifth luteectomized monkey menstruated again 25 days after ablation without intervening surges of estrogen or gonadotropin release and did not ovulate. Sham follicle cautery did not block ipsilaternal ovulation or impair progesterone secretion by the CL in 2 of 2 monkeys. These observations indicate that, by the middle of the follicular phase, the follicle destined to ovulate had been selected, and that no other follicles were soon competent to mature. That the interval from ablation, at either phase of the cycle, until the next ovulation was the same indicates: a) that the prevailing ovarian steroidal milieu at ablation had no discernible differential effect on the time-course of resumed ovarian activity, and b) that midcycle surges of estrogen or gonadotropin secretion were not required either to initiate or synchronize subsequent follicle growth.     

The effect of naloxone on endogenous opioid regulation of pituitary gonadotropins and prolactin during the menstrual cycle

To determine the influence of ovarian sex steroid hormones on endogenous opioid regulation of pituitary FSH, LH, and PRL secretion, six women were studied during the follicular phase (days 8-9) and luteal phase (days 21-23) of their menstrual cycles. An iv bolus dose of 10 mg of the opiate antagonist naloxone was given, and plasma FSH, LH, and PRL were measured at -30, -15, 0, 15, 30, 45, 60, 90, 120, and 180 min. During the follicular phase, baseline plasma FSH and LH levels were 10.7 +/- 0.9 and 16.7 n+/- 2.0 mIU/ml (mean +/- SEM), respectively; the plasma PRL level was 11.7 +/- 1.2 ng/ml. Naloxone did not significantly alter plasma FSH, LH, or PRL during the follicular phase. Basal levels of LH were significantly lower during the luteal phase than during the follicular phase (P less than 0.01). During the luteal phase, plasma LH increased significantly from a basal level of 10.0 +/- 1.0 to 20.8 +/- 3.0 mIU at 30 min (P less than 0.001) and remained significantly elevated at 90 min. Similarly, plasma PRL increased significantly from a basal level of 11.0 +/- 0.7 to 16.2 +/- 2.7 ng/ml at 30 min (P less than 0.025), but decreased by 90 min to 12.5 +/- 1.5 ng/ml. Plasma FSH did not change after naloxone treatment. Our results suggest that endogenous opiates have a prominent inhibitory effect on pituitary gonadotropin and PRL secretion only during the luteal phase of the menstrual cycle.     

Role of endogenous opioid peptides in the initiation of the midcycle luteinizing hormone surge in normal cycling women

While compelling evidence indicates a pivotal role for endogenous opioids in the regulation of GnRH-LH pulsatile activity during the late follicular and luteal phases of the menstrual cycle, the participation, if any, of the opioidergic mechanism in the initiation of the midcycle surge has not been examined. Accordingly, we measured serum LH, FSH, estradiol (E2) and progesterone (P4) levels daily during 2 consecutive cycles in 12 normal cycling women. After a control cycle, each woman was infused with naloxone (30 micrograms/kg.h) for 24 h starting 3 days before the anticipated spontaneous midcycle surge. Blood samples were obtained at 15-min intervals for 8 h before, during, and 16 h after the naloxone infusion. Serum LH and FSH concentrations were measured in all samples, and serum E2 and P4 concentrations at 2-h intervals. Pulsatile LH secretion was analyzed using the cluster program. The opioidergic blockade elicited a robust increase in LH pulsatile activity and a 3-fold rise in serum FSH levels in 6 of the 12 women. This increased gonadotropin secretion lasted more than 24 h and was characterized by a progressive increase in LH pulse amplitude, which was 9-fold greater during the last 8 h of naloxone infusion [mean LH pulse amplitude, 36.5 +/- 4.5 (+/- SE) vs. 4.1 +/- 0.4 IU/L; P less than 0.001]. This increase was accompanied by a corresponding increase in transverse mean serum LH levels (83.3 +/- 13 vs. 20.7 +/- 3.2 IU/L; P less than 0.001), but no alteration of the interpulse interval (93 +/- 11 vs. 85 +/- 4 min). The peak serum LH concentrations exceeded 100 IU/L in all 6 of these women. This naloxone-advanced gonadotropin surge, resembling closely the spontaneous midcycle surge, resulted in a significantly shortened (P less than 0.001) follicular phase and a more than 2-fold elevation of serum P4, followed by assumed ovulation and normal luteal function. These 6 women had serum E2 levels immediately before naloxone infusion that were comparable to those during the preovulatory peak during the control cycle. In the 6 women who did not have a naloxone-induced increase in gonadotropin secretion the preinfusion serum E2 levels were substantially lower (P less than 0.001) than the values during the control cycle. These findings suggest that a transient decrease in opioidergic activity may contribute to the initiation of the midcycle gonadotropin surge in women.     

Monitoring the menstrual cycle of humans and lowland gorillas based on urinary profiles of bioactive follicle-stimulating hormone and steroid metabolites

A sensitive and specific in vitro granulosa cell aromatase bioassay was adapted to measure bioactive FSH (bio-FSH) levels in urine samples. Urinary levels of bio-FSH, immunoreactive LH, estrone conjugates, and pregnanediol-3-glucuronide (PdG) were measured in first morning urine samples during the menstrual cycle in six cycling women and four lowland gorillas. The cycle length of women was relatively constant [28 +/- 1 (+/- SD) days], but varied from 28-38 days for lowland gorillas; the length of the luteal phases was relatively constant for both. All subjects had a midcycle LH peak and a luteal phase elevation in PdG. In addition, urinary estrogen excretion displayed a midcycle elevation that preceded the LH peak and a luteal phase increase similar to that of PdG. The bio-FSH levels in urine of cycling women, although at almost 100-fold higher concentrations, exhibited a pattern that closely resembled that of serum bio-FSH levels reported earlier, with an early follicular phase rise and a midcycle peak. Statistical analysis indicated a highly significant correlation (r = 0.90) between serum and urinary bio-FSH levels during the human menstrual cycle and in women in several hypo- and hypergonadotropic states, including oral contraceptive pill users, hypothalamic amenorrhea, premature ovarian failure, and postmenopause. Although a midcycle bio-FSH surge was also detected in lowland gorillas, two peaks of bio-FSH levels were consistently found during the follicular phase. The late follicular phase increase in bio-FSH levels was presumably involved in follicle selection and preceded the midcycle FSH peak by about 6 days, whereas the timing of the early follicular phase peak was variable, suggesting the involvement of complex regulatory mechanisms. These findings suggest that measurement of urinary bio-FSH levels in humans reflects serum bio-FSH in subjects in several physiological and pathological states. Studies of urinary bio-FSH levels in humans and nonhuman primates are useful in monitoring menstrual cycles, and the gorillas may be a model for understanding human reproductive cycles. The urinary granulosa cell aromatase bioassay should be useful for future assessment of bio-FSH levels in situations where serum measurements are impractical or in animal species for which specific FSH RIAs are not available.     

Administration of insulin-like growth factor I to rhesus monkeys does not augment gonadotropin-stimulated ovarian steroidogenesis

Although it is well established that IGF-I is able to amplify the actions of FSH and LH on ovarian cells in vitro, little information is available regarding the effects of IGF-I on ovarian function in vivo. To address this question, rhesus monkeys whose spontaneous gonadotropin secretion was interrupted with a GnRH antagonist received continuous iv infusions of saline, IGF-I (240 microg/kg.d), or IGF-I (240 microg/kg.d) plus human GH (hGH) (200 microg/kg.d) 7 d before and continuing throughout a 15-d iv infusion of hFSH and hLH during which serum LH concentrations were maintained at 7-10 mIU/ml and FSH concentrations were incrementally increased every 3 d from 7.5 to 17.5 mIU/ml. Serum estradiol concentrations in saline-treated control animals did not differ (P > 0.05) from animals treated with IGF-I + hGH. In contrast, serum estradiol levels in IGF-I-treated animals were significantly less (P < 0.05) than those of control or IGF-I + hGH-treated animals. Serum androstenedione levels did not differ among the three treatment groups. Analysis of follicular fluids on the final day of gonadotropin infusion indicated that intrafollicular IGF-I concentrations paralleled serum IGF-I concentrations in all treatment groups. Measurement of the ratio of IGF-I to IGF-binding protein-3 in follicular fluids indicated that there was not a disproportionate increase in I-binding protein-3 in animals infused with either IGF-I alone or IGF-I + hGH. Concentrations of GH in follicular fluids of IGF-I treated animals were less than control animals suggesting that the diminished responsiveness of ovaries to FSH in the IGF-I treatment group may have been due to reduced GH.     

Progesterone does not inhibit gonadotropin-induced follicular maturation in the female rhesus monkey (Macaca mulatta)

Female rhesus monkeys were treated with exogenous human menopausal gonadotropin (Pergonal) in the presence of exogenous systemic progesterone provided by Silastic implants and during the luteal phase of the menstrual cycle to determine if progesterone influences gonadotropin-induced follicular maturation. Peripheral serum and ovarian vein plasma 17 beta-estradiol concentrations were used to assess the response to gonadotropin treatment. When systemic levels of progesterone were elevated to 6--10 ng/ml in eight animals during the midfollicular phase of the menstrual cycle, each animal responded to exogenous gonadotropin with a sustained increase in serum 17 beta-estradiol concentration, which reached levels of approximately 800 pg/ml during treatment. To determine if the corpus luteum itself influences the ovarian response to exogenous gonadotropin, animals were treated with exogenous gonadotropin for 7 days, commencing during either the follicular phase of the cycle or the midluteal phase of the cycle. Regardless of the time of initiation of gonadotropin treatment, peripheral serum 17 beta-estradiol levels rose to approximately 1 ng/ml in all animals studied. 17 beta-Estradiol levels were elevated in ovarian venous blood from the ovary, which contains the active corpus luteum and the contralateral ovary. These observations demonstrate that exogenous gonadotropins can stimulate follicular maturation in the presence of progesterone.     

Immunoreactive LH-like substances in serum of hypophysectomized and prepubertal monkeys: inactive in an in vitro LH bioassay.

Serum LH levels in rhesus monkeys are commonly measured by a radioimmunoassay (RIA) utilizing ovine LH as the radioligand and a novel antiserum to ovine LH. Although meeting most criteria of validity, this RIA cross-reacts with serum from demonstrably hypophysectomized monkeys as well as serum from pre-pubertal monkeys. We have utilized the recently developed rat interstitial cell-testosterone assay (RICT) for measurement of serum LH in rhesus monkeys during various endocrine states and have compared the results with those obtained by RIA. The RICT in vitro bioassay was as sensitive, precise, and accurate as the RIA. Partially purified pituitary monkey LH preparations, varying by as much as 100-fold in purity, had similar potency estimates by both assays. The expected biphasic pattern of serum LH levels during the menstrual cycle as well as the usual increase in serum LH levels after ovariectomy was observed with both assays. In each case, however, basal serum LH levels were overestimated by the RIA and peak serum LH levels were underestimated by the RIA as compared to results obtained by the RICT bioassay. The most striking differences between the two assays were observed with serum from hypophysectomized and prepubertal monkeys. Whereas LH-like activity in such samples was high when measured by the RIA, it was undetectable by the RICT bioassay. The ratios of serum LH (RIA/RICT) varied from greater than 7.2 to greater than 17.4 in such sera. These results demonstrate the serious limitations of the ovine LH RIA for measurement of serum LH in rhesus monkeys. It appears that the RICT bioassay may be a suitable alternative method, especially in circumstances other than the menstrual cycle and after ovariectomy.     

Effects of progesterone on estrogen-induced gonadotropin release in male rhesus macaques

The effects of progesterone (P) on estrogen (E)-induced gonadotropin release were studied in 10 adult male rhesus macaques castrated more than 2 yr earlier. The intent was to determine whether physiological levels of P (approximately 400 pg/ml) found in the systemic circulation of intact males would block E-induced gonadotropin release and whether the responses of castrated males were similar to those of castrated females with and without pretreatment with 17 beta-estradiol (E2). Different doses of P were administered in Silastic capsules (0.3, 4.0, and 5.0 cm) implanted sc. A 0.3-cm implant maintained serum P levels at about 400 pg/ml (equivalent to physiological levels in intact males); 5.0-cm implants produced serum levels of about 4.0 ng/ml (similar to luteal phase levels in females). In male monkeys treated for approximately 3 weeks with E2, only the highest dose (approximately 4.0 ng/ml) of P blocked FSH induced by estradiol benzoate (E2B). LH was blocked in one third of the animals thus treated. The same P dose was ineffective in blocking E2-induced LH release in spayed females pretreated with E2, but did block FSH release. Gonadectomized males and females not treated beforehand with E2 released LH in response to an E2B challenge, but FSH was not elevated in the peripheral circulation under these experimental conditions. These results suggest that luteal phase levels of P block E2-induced FSH release in gonadectomized males and females. With the same treatment regimens, P blocks E2 action in some males, but all females responded to E2B by releasing LH. These data also suggest that estrogen priming is necessary for FSH, but not LH, release in adult rhesus macaques of both sexes. The prerequisite of E treatment for the induction of positive feedback appears to be associated with the level of gonadotropin suppression before E2B treatment.     

Serum bioactive follicle-stimulating hormone during the human menstrual cycle and in hyper- and hypogonadotropic states: application of a sensitive granulosa cell aromatase bioassay

A sensitive in vitro assay based on the stimulation of estrogen production by cultured rat granulosa cells was recently developed for the measurement of biologically active FSH. This bioassay system is specific for FSH, highly sensitive, and capable of measuring basal FSH levels in rat serum. The granulosa cell aromatase bioassay was improved by the use of additives known to enhance FSH activity and by pretreatment of serum with 12% polyethylene glycol to remove inhibitory substances. We applied this method to the measurement of bioactive FSH levels in serum samples from human subjects. As determined in daily blood samples during ovulatory menstrual cycles in seven women, bioactive FSH levels exhibited a pattern closely resembling that of immunoreactive FSH. The mean bioactive serum FSH levels were 29.9, 20.5, 39.2, and 14.8 mIU/ml for the early follicular phase, late follicular phase, preovulatory surge, and luteal phase, respectively. The bio- to immunoratio (B:I) throughout the menstrual cycle ranged from 1.4-3.4, with a mean of 2.5. The ratios for early follicular phase, late follicular phase, preovulatory surge, and luteal phase were 2.7, 2.3, 1.4, and 2.6, respectively. The correlation coefficient (r) of the serum FSH values obtained by bioassay and RIA was 0.91. FSH bioactivity was also measured in patients in each of the following categories with the following mean values: oral contraceptive pill users (undetectable), hypothalamic amenorrhea (18.7 mIU/ml; B:I, 2.6), premature ovarian failure (163 mIU/ml; B:I, 1.7), and postmenopausal women (191 mIU/ml; B:I, 1.6). These findings suggest that measurement of immunoreactive FSH levels correctly reflects the biological activity of FSH in serum of cycling women and patients in certain hyper- and hypogonadotropic states. The granulosa cell aromatase bioassay represents a new tool for future assessments of biologically active FSH in physiological and pathophysiological conditions.     

The effect of intraluteal infusion of deglycosylated human chorionic gonadotropin on the corpus luteum in rhesus monkeys

Removal of the carbohydrates from hCG results in an antagonist (degly-hCG) that competitively inhibits hCG/LH-stimulated adenylate cyclase in macaque luteal tissue in vitro, but its effect in vivo is controversial. To examine the effect of degly-hCG on the lifespan and steroidogenic activity of the primate corpus luteum, the antagonist was administered to female rhesus monkeys (n = 7) beginning at the midluteal phase of the menstrual cycle. In a control cycle the saline vehicle was infused via an osmotic minipump directly into the corpus luteum. In a subsequent cycle, one of three dose rates of degly-hCG (0.001, 0.009, and 0.09 nmol/h) was infused into the corpus luteum. Pump implantation and infusion began 5-9 days after the midcycle LH surge and continued for 7 days. Peripheral venous blood was collected daily from day 8 of the cycle until menses, and serum progesterone levels were determined by RIA. Progesterone levels and patterns were similar in animals that received either the saline vehicle or degly-hCG, and the length of the luteal phase in monkeys receiving any dose of degly-hCG (16.4 +/- 0.5 days) was not different from that in animals receiving a control infusion (16.1 +/- 0.9 days). In a corollary study, an intraluteal infusion of degly-hCG (0.009 nmol/h) in the midluteal phase did not prevent stimulation of progesterone levels after im injection of hCG (15 IU/day for 5 days). We conclude that whereas degly-hCG is a useful tool to examine gonadotropin action in vitro, it is not a potent gonadotropin antagonist in vivo.     

Effect of reduced luteinizing hormone concentrations on corpus luteum function during the menstrual cycle of rhesus monkeys

To further define the relationship between plasma LH concentrations and progesterone secretion by the primate corpus luteum, we examined luteal function in rhesus monkeys in response to reduced LH concentrations during the luteal phase of the menstrual cycle. Five anovulatory rhesus monkeys received a pulsatile infusion of synthetic GnRH (6 micrograms/pulse; one pulse per h, iv) to restore menstrual cyclicity. During the early luteal phase (4-5 days after ovulation), the amount of GnRH administered per pulse was reduced to 1/250th or 1/750th of the standard GnRH infusion regimen. Plasma LH concentrations, determined by bioassay, were reduced by approximately 50% during cycles maintained by reduced GnRH concentrations compared with the standard GnRH dosage. Serum progesterone concentrations were maintained for 5-6 days after GnRH reduction and declined thereafter, and premature menstruations were observed in four of seven cycles maintained by the 1/250th GnRH reduction and four of six cycles maintained with the 1/750th GnRH reduction. These results are consistent with the hypothesis that luteal regression during the nonfertile menstrual cycles of primates is due primarily to an alteration in luteal cell responsiveness to LH, rather than a reduction in the gonadotropic drive to the corpus luteum per se. When plasma LH concentrations were reduced during the early luteal phase to values below those found during the onset of luteal regression in control cycles, luteal function was maintained for 5-6 days. However, as the luteal phase progressed, the reduced LH concentrations were unable to sustain progesterone secretion, and premature menses occurred in some, but not all, animals.     

Circulating immunoreactive inhibin levels during the normal human menstrual cycle

Serum inhibin concentrations were measured daily by RIA in six normal women throughout one menstrual cycle. The RIA was specific for inhibin, and inhibin subunits and related proteins cross-reacted minimally in it. In the early to midfollicular phase, inhibin levels changed little, while in the late follicular phase, inhibin levels rose, in parallel with estradiol (r = 0.43; P less than 0.05; n = 22), to a peak level of 714 (407-1267) U/L (geometric mean +/- 67% confidence limits) coincident with the midcycle LH and FSH surges. An inverse relationship was found between serum inhibin and FSH during the mid- to late follicular phase (r = 0.42; P less than 0.01; n = 45). Inhibin levels rose further during the luteal phase to a peak level of 1490 (1086-2028) U/L 7-8 days after the LH surge, and they correlated positively with serum progesterone (r = 0.76; P less than 0.001; n = 49) and inversely with serum FSH (r = 0.43; P less than 0.01; n = 49) throughout the luteal phase. We conclude that 1) circulating inhibin is detectable throughout the normal menstrual cycle; 2) in the late follicular phase, inhibin levels rise in parallel with estradiol, consistent with the concept that both are products of the maturing follicle; 3) in the luteal phase, the profile of inhibin suggests that it is a secretory product of the corpus luteum; and 4) the inverse relationship between inhibin and FSH in the follicular phase is consistent with the inhibin hypothesis, while at midcycle there is loss of the inhibitory effect of inhibin on FSH secretion. The inverse relationship between FSH and inhibin during the luteal phase suggests a hitherto unsuspected role for inhibin in the feedback regulation of FSH secretion.     

beta-Endorphin in hypophyseal portal blood: variations throughout the menstrual cycle

Concentrations of beta-endorphin were measured in the venous effluent of the hypothalamus (hypophyseal portal blood) at various phases of the menstrual cycle and after ovariectomy in rhesus and pigtailed monkeys. In the rhesus, beta-endorphin concentrations were high during the mid- to late follicular phase [737 +/- 256 pg/ml (mean +/- SE)] and the luteal phase (1675 +/- 1108) of the menstrual cycle, but were undetectable (less than 133) at menstruation. Concentrations were also high in pigtailed monkeys during stages of the menstrual cycle other than at menstruation (4870 +/- 1090 pg/ml), but undetectable (less than 133) 4--12 months after ovariectomy. These results indicate that beta-endorphin concentrations in hypophyseal portal blood are related to menstrual cycle events, probably changes in ovarian steroids; this in turn suggests that beta-endorphin may participate in the ovarian feedback regulation of gonadotropin secretion.     

Effects of porcine follicular fluid on gonadotropin concentrations in rhesus monkeys

The administration of steroid-free charcoal-treated porcine follicular fluid to long term castrated female rhesus monkeys lowered basal serum concentrations of FSH and had almost no effect on serum LH. Treatment with porcine follicular fluid before the administration of exogenous LRH inhibited the release of FSH, but also affected the release of LH. This inhibition was especially striking on the suppression of the peak of release of both FSH or LH at 20 min. These findings suggest that an inhibin-like material present in follicular fluid could play an important role in the secretion of FSH and LH in rhesus monkeys.     

Inhibition of folliculogenesis in the monkey following early follicular phase administration of an LRH agonist

This study was undertaken to determine if early follicular phase administration of a synthetic luteinizing hormone releasing hormone (LRH) agonist would produce luteal phase defects in the monkey. [D-His(im-Bzl)6,Pro9]LRH N-ethylamide was administered to groups of rhesus monkeys on days 1-3 of the menstrual cycle. Two responses were observed: a) anovulatory menstrual cycles of less than 14 days duration, and b) ovulatory menstrual cycles characterized by unusually long follicular phases. All 4 monkeys with shortened menstrual cycles had prominent increases in serum gonadotrophin and oestradiol concentrations during treatment with the LRH agonist; early menses in these animals was attributed to uterine bleeding upon oestrogen withdrawal. Serum FSH concentrations declined, serum LH concentrations were unaltered, and only 2 of 8 monkeys had elevations in serum oestradiol during ovulatory menstrual cycles. The mean interval from cessation of treatment with the LRH agonist to the next preovulatory gonadotrophin surge was 21.5 +/- 3.2 days in ovulatory menstrual cycles. Corpus luteum function was normal following treatment with the LRH agonist in ovulatory cycles. The results indicate that both the long and short menstrual cycles observed following early follicular phase administration of the LRH agonist to monkeys can be attributed to a profound inhibition in follicle recruitment. [D-His(im-Bzl)6,Pro9]LRH N-ethylamide did not alter corpus luteum function in the monkeys.     

Ovarian responses in macaques to pulsatile infusion of follicle-stimulating hormone (FSH) and luteinizing hormone: increased sensitivity of the maturing follicle to FSH

This study investigated the relationship between plasma gonadotropin concentrations and the initiation and maintenance of preovulatory follicular growth in macaques. Eight adult cynomolgus monkeys were treated with a GnRH antagonist [AcD2Nal1-4ClDPhe2, DTrp3, DArg6, DAla10]GnRH X HOAc to block endogenous gonadotropin secretion. In four animals, a pulsatile infusion of human FSH and human LH (hLH) (one 3-min pulse/h) was initiated, and the amount of hFSH delivered per pulse was increased every 3-4 days until serum estradiol concentrations rose. Thereafter, the amount of FSH delivered per pulse was reduced by 12.5%/day for 5 days, whereas the amount of LH delivered per pulse was not altered. Results indicated that plasma FSH concentrations in the range of 15-20 mIU/ml were associated with the initiation of estrogen production; in addition, a progressive reduction in plasma FSH concentration to 8-10 mI/ml over the subsequent 5 days was accompanied by continued rises in estradiol concentrations and preovulatory follicular growth. In contrast, in four control animals, maintenance of plasma FSH concentrations at 8-10 mIU/ml for 13 days did not result in elevation in serum estradiol concentrations or antral follicular growth. These observations demonstrate that after stimulation by elevated FSH concentration, follicles can continue to mature in the presence of FSH concentrations which are unable to support the growth of less mature follicles. Thus, this may account for the mechanisms by which the maturing follicle continues to develop during the mid-through late follicular phase of the menstrual cycle, whereas other less mature follicles undergo atresia.     

Blockade of the spontaneous midcycle gonadotropin surge in monkeys by RU 486: a progesterone antagonist or agonist?

Preovulatory ovarian secretion of progesterone (P4), several hours before the onset of the typical midcycle gonadotropin surge, occurs in humans and monkeys. We investigated the potentially obligatory role of preovulatory P4 secretion in stimulating the midcycle LH surge by administering a potent P4 antagonist, RU 486(17 beta-hydroxy-11 beta-[4-dimethylaminophenyl-1]17 alpha-[prop-1-ynyl]estra-4,9-dien-3-one), to sexually mature, normally ovulatory cynomolgus monkeys on days 10-12 of the menstrual cycle (n = 18). Monkeys were randomized to receive RU 486 alone (5 mg/day, im; group I); RU 486 plus dexamethasone (1 mg/day, im; group II); dexamethasone alone (group III); or vehicle (ethanol; 0.5 ml; group IV). Before drug treatment, the follicular phases were quite similar among groups. The administration of RU 486 blocked (delayed) the expected gonadotropin surge, despite rising estrogen concentrations (greater than 250 pg/ml). The expected LH surge was delayed by RU 486 (n = 5) or RU 486 with dexamethasone (n = 3) until 36 +/- 7 (+/- SEM) and 27 +/- 8 days in groups I and II, respectively. In contrast, groups III (n = 3) and IV (n = 5) had timely midcycle surges after the administration of dexamethasone or vehicle alone (4 +/- 2 and 6 +/- 2 days, respectively). The intermenstrual interval was lengthened by RU 486 administration in both group I and II animals (61 +/- 6 and 54 +/- 6 days) compared to controls (30 +/- 2; P less than 0.0001). In summary, RU 486 effectively blocked imminent midcycle gonadotropin surges, delayed subsequent folliculogenesis, and significantly extended the menstrual cycle length. If RU 486 acted as a pure P4 antagonist, then P4 is necessary for timely midcycle gonadotropin surges to occur. However, recent evidence showing agonistic properties of RU 486 (in the virtual absence of P4) at both endometrial and pituitary levels may favor a P4-like (agonistic) blockade of the estrogen-induced FSH/LH surges by RU 486.