Hormone changes occurring during second trimester abortion induced with 15 (S)-15-methyl prostaglandin F2alpha.

Changes in progesterone, human placental lactogen (HPL), cortisol and estradiol-17B were measured during second trimester abortion induced by I.M. 15-methyl PGF2alpha. A rapid decline in progesterone and HPL was found, indicating perhaps an initial effect on the placenta. A rapid rise in cortisol was found, but it is not clear if this is due to stress or part of the termination mechanism. The changes of estradiol were not as distinct and may reflect opposite effects of the prostaglandin on the placenta and adrenals. Similar hormonal changes were observed regardless of the duration of gestation.     

Effect of prostaglandins on in vitro synthesis of progesterone and oestradiol-17β in placenta from early human pregnancy

In vitro synthesis of progesterone and estradiol-17β from endogenous precursors was studied in the placenta from women in early stage of gestation (< 7 weeks). Radioimmunoassay techniques were used to measure progesterone and estradiol-17β.It was shown that placental tissue from as early as six weeks of gestation can synthesize both progesterone and estradiol-17β in vitro. Prostaglandins F_2α and E₂ in concentration of 100 μg/ml of the incubation media did not have any significant effect on the in vitro synthesis of progesterone and estradiol-17β in the placental tissue.It seems unlikely that the abortifacient effect of natural prostaglandins PGE₂ and PGF_2α is due to their direct action on the synthesis of progesterone and estradiol-17β in the placenta.     

Partial purification of human placental 15-hydroxyprostaglandin dehydrogenase: Kinetic properties

The enzyme system, 15-hydroxyprostaglandin dehydrogenase, which catalyzes the first inactivation step in the catabolism of the prostaglandins has been isolated and purified 107-fold from human placenta. Kinetic studies reveal different Michaelis-Menten constants for most of the naturally occurring prostaglandins. The K_m for PGE₂ was found to be 10 μM, for PGE₁, 27 μM; for PGA₂, 32 μM; for PGA₁, 33 μM; and for PGF_2α 59 μM. The enzyme has a sharp pH-optimum between 7.5 and 8.8. Prostaglandin dehydrogenase appears to be isoenzymic as judged by separation on polyacrylamide disc gel electrophoresis. Inhibition studies with the partially purified enzyme indicate that progesterone and estrogen may influence the conversion of biologically active prostaglandins into the biologically inactive 15-ketoprostaglandins. These findings offer evidence for the control of prostaglandin metabolism in the human placenta.     

Treatment with a single vaginal suppository containing 15-methyl PGF2α methyl ester at expected time of menstruation

Termination of early pregnancy, by vaginal administration of prostaglandin analogues, one to three weeks after the first missed menstrual period, has advantages and disadvantages in comparison with vacuum aspiration. Some of these may be reduced if the patient is treated earlier. In the present study the effect and safety of one vaginal administration of 2.5 to 3 mg 15-methyl-PGF_2α methyl ester around the expected time of menstruation was evaluated in 16 women exposed to the risk of pregnancy.The overall number of treatment cycles was 35 and pregnancy was confirmed by plasma β-HCG in eight. The treatment resulted in bleeding in all the pregnant cycles while in the nonpregnant ones it only provoked spotting and bleeding did not begin until the expected time of menstruation. Treatment with 2.5 mg 15-methyl-PGF_2α methyl ester resulted in complete abortion in one of three women. If the dose was increased to 3 mg all five treated women aborted. In nonpregnant patients no changes in the levels of estradiol-17β or progesterone at any time during the 24-hour observation period were found. Serum cortisol and prolactin but not TSH levels started to increase two hours after the start of treatment and reached a maximum after five hours. The increase coincided with the onset of uterine pain.Ovulatory cycles as judged from basal body temperature occurred in the first menstrual cycle following treatment in all nonpregnant patients. Although possible to use as a “once a month treatment” it seems preferable since the dose is the same, to postpone treatment until menstruation is delayed for a week or more.     

Luteolytic action of 15-keto prostaglandin F2α in the rhesus monkey

The naturally-occurring metabolite of prostaglandin F_2α, 15-keto prostaglandin F_2α (15-keto PGF_2α), elicited rapid and sustained declines in serum progesterone concentrations when administered to rhesus monkeys beginning on day 22 of normal menstrual cycles. Evidence for luteolysis of a more convincing nature was obtained in studies where a single dose of 15-keto PGF_2α was given on day 20 of ovulatory menstrual cycles in which intramuscular injections of hCG were also given on days 18–20; serum progesterone concentrations fell precipitously in monkeys within 24 hours following intramuscular administration of 15-keto PGF_2α. However, corpus luteum function was impaired in only 4 of 11 early pregnant monkeys when 15-keto PGF_2α was administered on days 30 and 31 from the last menses, a time when the ovary is essential for the maintenance of pregnancy. Gestation failed in 2 additional monkeys 32 and 60 days after treatment with 15-keto PGF_2α, but progressed in an apparently normal manner in the remaining 5 animals. Two pregnant monkeys treated with 15-keto PGF_2α on day 42 from the last menstrual period, a time when the ovary is no longer required for gestation, continued their pregnancies uneventfully. Corpus luteum function was not impaired in 9 control monkeys which received injections of vehicle or hCG at appropriate times during the menstrual cycle or pregnancy.     

Effects of prostaglandins, dibutyryl camp, LHRH, estrogens, progesterone, and potassium on output of prostaglandin F2α, 13, 14-dihydro-15-keto-prostaglandin F2α, HCG, estradiol, and progesterone by placental minces

In order to compare the endocrine response of placental minces to luteinizing hormone releasing hormone (LHRH) and dibutyryl cAMP (dbcAMP) and to screen for effects of potential stimulatory and inhibitory substances, the simultaneous outputs of PGF_2α, 13,14-dihydro-15-keto-prostaglandin F_2α (PGFM), progesterone, 17β-estradiol, and hCG were evaluated during a 4 hour incubation in 5 placentas. The output of hCG was highest for 12-week placentas, intermediate for a 16 week placenta, and lowest for term placentas. The output of 17β-estradiol by 12 and 16 week placentas in the presence of 30 μM dehydroepiandrosterone sulfate (DHEAS) was greater than that by term placentas. Progesterone output was apparently independent of gestational age although some variation between 12-week placentas was demonstrated. Output of PGF_2α was lower in 12 and 16-week placentas than in term placentas and that of PGFM was lower in 12-week placentas than in term placentas. LHRH (100 nM) produced stimulation of PGF_2α output (P<.005) and a trend toward inhibition of progesterone output (which failed to achieve statistical significance) but no stimulation of hCG under these conditions. Stimulation of the outputs of hCG (P<.005) and PGF_2α (P<.001) and inhibition of that of progesterone (P<.005) was produced by 20 mM dbcAMP. DHEAS inhibited output of progesterone (P<.01) and PGF_2α (P<.01). There were no effects of potassium, estrogens, progesterone, or prostaglandins on output of any measured substance.     

Effects of PGF2α and PGF2α, 1–15 lactone on the corpus luteum and on early pregnancy in the rhesus monkey

The effects of prostaglandin (PG)F_2α and PGF_2α, 1–15 lactone were compared in luteal phase, non-pregnant and in early pregnant rhesus monkeys. Animals treated with either PG after pretreatment with human chorionic gonadotropin (hCG) had peripheral plasma progesterone concentrations that were not statistically different from those in animals treated with hCG and vehicle. However, menstrual cycle lengths in monkeys treated with PGF_2α, 1–15 lactone were significantly (P <0.02) shorter than those in vehicle treated animals. In the absence of hCG pretreatment, plasma progesterone concentrations were significantly (P <0.008) lower by the second day after the initial treatment with either PGF_2α or PGF_2α, 1–15 lactone than in vehicle treated monkeys. Menstrual cycle lengths in monkeys treated with either PG were significantly (P <0.04) shorter than those in animals treated with vehicle. There were no changes in plasma progesterone concentrations in early pregnant monkeys treated with PGF_2α, and pregnancy was not interrupted. In contrast, plasma progesterone declined and pregnancy was terminated in 5 of 6 early pregnant monkeys treated with PGF_2α, 1–15 lactone. These data indicate that PGF_2α, 1–15 lactone decreases menstrual cycle lengths in non-pregnant rhesus monkeys. More importantly, PGF_2α, 1–15 lactone terminates early pregnancy in the monkey at a dose which is less than an ineffective dose of PGF_2α.     

Effects of (15S)-15-methyl prostaglandin F2 methyl ester and estrogens upon the corpus luteum and conceptus of the rhesus monkey

The corpus luteum inhibiting properties of eighteen 15-methyl prostaglandin analogs were determined in the rhesus monkey during concomitant stimulation of the corpus luteum with chorionic gonadotropin. The methyl ester of (15S)-15-methyl PGF_2^α (15M-PGF_2^α, 12.5 mg/monkey) lowered serum progesterone to 12% of pretreatment values within 24 hours, however progesterone returned to normal limits within 48 hours. Elongation of the top side-chain by two carbons (2a,2b-dihomo-15M-PGF_2^α methyl ester, 13 mg/monkey), substitution of a hydroxymethyl group at carbon 1 (2-decarboxy-2-hydroxymethyl-15M-PGF_2^α, 12 mg/monkey), or the formation of the carbon 1 amide (15M-PGF_2^α amide, 12.5 mg/monkey) improved the inhibitory activity of 15M-PGF_2^α; serum progesterone for these 3 analogs was depressed to 15–30% of pretreatment levels within 24 hours, and did not return to control values. Luteal function was not inhibited (12 or more mg/monkey) when the 15-methyl group was placed in the R configuration, the top side chain was shortened by two carbons, an amino group was substituted for carbon 1, the 5-oxa modification was added, or the 1,9-lactone was formed. Some other modifications of 15M-PGF_2^α were also inactive, although not all were tested at equivalent doses: 2,2-difluoro; 4,5-cis-didehydro; 9,11-dideoxy-9α,11α-dichloro; 11-deoxy; 17-phenyl; 1,15-lactone; and the p-benzamidophenyl ester of 2a,2b-dihomo-15M-PGF_2^α. (15S)-15-Methyl PGE₂ methyl ester (1 mg/monkey) depressed serum progesterone concentrations to 42% of pretreatment values within 24 hours; 2a,2b-dihomo-11-deoxy-(15S)-15-methyl PGE₂ methyl ester was inactive (5 mg/monkey). A corpus luteum inhibiting action of certain 15-methyl prostaglandins can be demonstrated in the rhesus monkey.     

Effect of 15(S)15-methyl-PGF2α-methyl ester vaginal suppositories on circulating hormone levels in early pregnancy

Intravaginal administration of 15-methyl-PGF_2α-methyl ester in the form of suppositories terminated pregnancy in 70 percent of the cases whose last menstrual periods ranged from 35 to 56 days. The use of these suppositories in 49 patients, between 57 to 80 days of gestation, dilated the cervix by 10 mm or more, in one hundred percent of the cases. A decrease in circulating levels of estradiol-17β and progesterone was observed following 15-methyl-PGF_2α administration. The mean estradiol-17β levels declined by about 55.9 percent at 9 hours whereas, the corresponding fall in progesterone was 32.7 percent. This was indicative of a direct action of 15-methyl-PGF_2α on the corpus luteum. The vaginal use of 15-methyl-PGF_2α-methyl ester suppositories thus appears to be a promising method for the termination of early pregnancy and for pre-operative cervical dilatation. The termination of early pregnancy appears to be partly due to the luteolytic effect of 15-methyl-PGF_2α besides stimulating uterine contractions.     

Concentrations of circulating hormones normalized to pulses of a prostaglandin F2α metabolite during spontaneous luteolysis in mares

The temporal relationships between a pulse of 13,14-dihydro-15-keto-PGF_2α (PGFM) and the concentrations of circulating hormones during the luteolytic period were studied for 11 pulses in 11 mares (Equus caballus) using samples collected hourly. Mean PGFM pulses encompassed 4 h before to 4 h after the peak, and hormone data were normalized to the PGFM peak (Hour 0). Concentration of progesterone decreased (P < 0.05) between Hours –4 and –3 and continued to decrease linearly throughout the PGFM pulse. The concentrations of cortisol and prolactin increased (P < 0.004) during Hours –4 to 0 and decreased (P < 0.002) during Hours 0 to 4. Estradiol concentration increased (P < 0.02) during Hours –4 to 0 but did not change significantly after Hour 0. Concentrations of follicle-stimulating hormone and luteinizing hormone did not change significantly during the PGFM pulse, and the oxytocin results were equivocal. Percentage of corpus luteum area with color-Doppler signals of blood flow did not change significantly between Hours –4 and 0 and first began to decrease (P < 0.004) between Hours 0 and 2. Results demonstrated that concentrations of progesterone decreased linearly during a PGFM pulse, and cortisol, prolactin, and estradiol increased during the ascending portion of the pulse. The progesterone and gonadotropin results supported the hypothesis that the initial progesterone and gonadotropin increases that have been reported to occur in response to a single bolus luteolytic treatment with prostaglandin F_2α do not occur in response to the natural secretion of prostaglandin F_2α.     

Release of prostaglandin F, regression of corpora lutea and induction of premature parturition in goats treated with estradiol-17β

Premature parturition was induced in five pregnant goats infused intravenously with 4.65–8.4 mg estradiol-17β but not in one treated with 5.85 mg estradiol-17α. A single intramuscular injection of 12 mg estradiol benzoate (8.8 mg estradiol-17β equivalents) was also effective. These doses were estimated to provide plasma concentrations of estradiol-17β in the physiological range for animals at spontaneous parturition. Circulating plasma concentrations of progesterone decreased and lactogenesis occurred before all instances of induced parturition but no such changes resulted from infusion of estradiol-17α. Placental delivery was normal in all animals but neonates of more than 10 days prematurity were non-viable.In three of the five goats infused with estradiol-17β, evidence was obtained for release of F-group prostaglandins from the uterus at the time of onset of the decline in progesterone.     

Effect of intravenous infusion of 15-methyl-PGF2α on plasma hormone levels during early pregnancy

Ten pregnant women (7th–8th week of pregnancy) obtained an intravenous infusion of 15-methyl-prostaglandin-F_2α (2.5 μg/min) until clinical signs of abortion occurred or up to 7 hours. Surgical removal of the products of conception was performed 4–7 hours after the start of the infusion. Blood samples were taken prior to and during the infusion and then during the post-abortion period. The plasma levels of both progesterone and estradiol exhibited a significant decrease (p<0.001 and p<0.05, respectively) one hour after the beginning of infusion and remained reduced by approximately 35 and 45 per cent, respectively, during the entire infusion period. A drop in the levels of both steroids was seen after surgical interruption. This was followed by a gradual decrease to non-pregnancy levels. The levels of cortisol increased significantly (p<0.01) by approximately 60 per cent, starting with the second hour of infusion. Following surgical interruption, the levels dropped to pre-infusion values. 17-Hydroxyprogesterone levels increased (p<0.05) above the pretreatment levels by approximately 25 per cent, starting with the third hour of infusion. These levels were not correlated with those of cortisol during the infusion period. Following surgical interruption the plasma levels of 17-hydroxyprogesterone returned to non-pregnancy levels.     

Estrogen interaction with the anterior pituitary of female rats differential cytosol binding, nuclear translocation and stimulation of RNA synthesis by 17β-estradiol and tamoxifen

The interaction of tamoxifen (trans-1-(p-β-dimethylaminoethoxyphenyl)-1,2-diphenylbut-1-ene) with the cytosol estrogen receptor of the anterior pituitary of female rats was studied. No differences were recorded between incubations of cytosol samples with 17β-[³H]estradiol performed in the presence or absence of unlabeled 17β-estradiol and tamoxifen, respectively, thus suggesting that these interactions were at common receptor sites and excluding possible cooperative interactions. Competition experiments and Scatchard plot analysis of saturation experiments add further evidence for common receptor sites. A dissociation constant for tamoxifen of K_d = 2 nM was recorded. Tamoxifen was found to be bound to a moiety sedimenting in the 4–5 S region, on a 6–24% linear sucrose density gradient at low salt concentrations, whereas 17β-estradiol sedimented in the 8–9 S area. These data suggest possible conformational changes of the receptor in the presence of tamoxifen. Furthermore, nuclear estrogen receptor levels remained elevated for at least 80 h after the application of tamoxifen alone or in a combination with 17β-estradiol, and a concomitant inhibition of cytosol receptor replenishment was noted. Tamoxifen and 17β-estradiol, respectively, were found to stimulate progesterone receptor levels when applied through 5 days. Tamoxifen plus 17β-estradiol administration elevated progesterone receptor contents above those found for each of the two compounds alone. On the other hand, tamoxifen enhanced the 17β-estradiol-induced prolactin serum levels, but did not stimulate prolactin serum levels by itself. These data combine to suggest that tamoxifen interacts with common estrogen receptor sites at the rat anterior pituitary.     

Blood LH after PGF2α in diestrous and ovariectomized cattle

Two experiments were conducted to determine whether the increased serum LH which occurs within 12 hr after a luteolytic dose of PGF_2α is dependent upon changes in progesterone or estradiol secretion. In the first experiment, exogenous progesterone abolished the increase in serum LH caused by a subcutaneous injection of 25 mg PGF_2α in diestrous heifers, but not in ovariectomized heifers. In the second experiment, progesterone pessaries were removed at 6 hr after a subcutaneous injection of 25 mg PGF_2α. LH remained at pre-PGF_2α values while the pessaries were in place, but began to increase within 1 hr after they were removed. Blood estradiol also remained at pre-PGF_2α values until the pessaries were removed, and began to increase at 2 hr after pessary removal. We conclude that the increase in serum LH within 12 hr after PGF_2α treatment in diestrous cattle is dependent upon withdrawal of progesterone; it is not due to increased serum estradiol.     

Endocrine Activity of Extraembryonic Membranes Extends beyond Placental Amniotes

Background

During development, all amniotes (mammals, reptiles, and birds) form extraembryonic membranes, which regulate gas and water exchange, remove metabolic wastes, provide shock absorption, and transfer maternally derived nutrients. In viviparous (live-bearing) amniotes, both extraembryonic membranes and maternal uterine tissues contribute to the placenta, an endocrine organ that synthesizes, transports, and metabolizes hormones essential for development. Historically, endocrine properties of the placenta have been viewed as an innovation of placental amniotes. However, an endocrine role of extraembryonic membranes has not been investigated in oviparous (egg-laying) amniotes despite similarities in their basic structure, function, and shared evolutionary ancestry. In this study, we ask whether the oviparous chorioallantoic membrane (CAM) of chicken (Gallus gallus) has the capability to synthesize and receive signaling of progesterone, a major placental steroid hormone.

Methodology/Principal Findings

We quantified mRNA expression of key steroidogenic enzymes involved in progesterone synthesis and found that 3β-hydroxysteroid dehydrogenase, which converts pregnenolone to progesterone exhibited a 464 fold increase in the CAM from day 8 to day 18 of embryonic development (F_{5, 68} = 89.282, p<0.0001). To further investigate progesterone synthesis, we performed explant culture and found that the CAM synthesizes progesterone in vitro in the presence of a steroid precursor. Finally, we quantified mRNA expression and performed protein immunolocalization of the progesterone receptor in the CAM.

Conclusions/Significance

Collectively, our data indicate that the chick CAM is steroidogenic and has the capability to both synthesize progesterone and receive progesterone signaling. These findings represent a paradigm shift in evolutionary reproductive biology by suggesting that endocrine activity of extraembryonic membranes is not a novel characteristic of placental amniotes. Rather, we hypothesize that these membranes may share an additional unifying characteristic, steroidogenesis, across amniotes at large.     

Effects of dibuturyl cAMP, LHRH, and aromatase inhibitor on simultaneous outputs of prostaglandin F2α, and 13,14-dihydro-15-keto-prostaglandin F2α by term placental explants

Explants from term human placentas were maintained in culture with daily changes of medium. Daily output of PGF_2α and PGFM¹ decreased during the course of the incubation. Addition of 4 μg/ml DHEAS or 67 μg/ml LDL cholesterol had no effect on output of PGF_2α or PGFM. Addition of 1.6, 3.2, or 6.4 μg/ml of LHRH to the culture plates had no effect on output of PGFM or PGF_2α, but LHRH increased hCG output. Dibutyryl cAMP (1mM, 2mM, and 4mM) increased output of PGF_2α, PGFM, and hCG. Aromatase inhibitor decreased hCG output, but it was without effect on output of PGF_2α, or PGFM. Significant correlations were demonstrated between progesterone, PGFM, PGF_2α, and hCG, suggesting that PGF_2α originates in the syncytiotrophoblast cell. The ability of LHRH to stimulate output of hCG but not PGF_2α while dbcAMP stimulated both suggests that either PGF_2α and hCG arise in different cells or that LHRH does not act through cAMP.     

Dose-related action of estradiol on placental prostanoid prediction

Prostanoids play an important role throughout all of pregnancy and during the initiation and progress of labor. The human placenta at term produces large quantities of prostanoids, yet little is known of the factors that regulate their biosynthesis. Herein, we report the effect of estradiol or estradiol and progesterone on the basal release of placental prostanoids from fresh human term placental explants using a perifusion system.The basal release of prostaglandin E₂ (PGE₂, prostaglandin F_2α (PGF_2α), thromboxane (TxB₂) and 6-keto-prostaglandin F_1α (6-keto-PGF_1α) increased about 50% from the fifth to the ninth hour in culture, while the release of 13, 14-dihydro-15-keto-PGF (PGFM) remained constant and hCG release decreased. The dose-related effect of estradiol (20–2,000 ng/ml) in the perifusing medium starting at the fifth hour of perifusiOn (i.e., the zero treatment time) effected no change in the release of TxB₂, PGF_2α, PGFM or hCG. A biphasic action on the release of 6-keto-PGF,. was observed, i.e. it was significantly decreased when incubated with 20 ng/ml of estradiol, but effected an increase after exposure to 200 ng/ml. The concomitant addition of progesterone (2,000 ng/ml) with estradiol (200 ng/ml) significantly inhibited the stimulatory action of estradiol at this dose. The release of PGE₂ was inhibited in a dose-related fashion with increasing dose of estradiol. The addition of progesterone with estradiol (2,000 and 200 ng/ml, respectively) reversed the inhibition of PGE₂ by estradiol alone.These data demonstrate that physiologic levels of estradiol affect 6-keto-PGF_α and PGE₂ release from the human term placenta, but do not significantly alter production of TxB₂, PGFM or hCG under these conditions.     

Cortisol metabolism by human liver in vitro—IV. Metabolism of 9α-fluorocortisol by human liver microsomes and cytosol

The oxidative and reductive biotransformations of 9α-fluorocortisol (fluorocortisol) by human liver microsomes and cytosol have been characterized. 9α-Fluorination greatly simplified cortisol metabolism in microsomes: dehydrogenation of the 11β-hydroxyl group and A-ring (4-ene-5β and 3α-keto) reduction, the principle pathways, were completely blocked. Fluorocortisol was essentially metabolized by the remaining pathways, 20β-reduction and 6β-hydroxylation. In cytosol, 20β-reduction replaced the A-ring reduction of cortisol as the sole biotransformation. The major structure-metabolism relationships of fluorocortisol in man, i.e. complete and extensive inhibition of 11β-dehydrogenation and 4-ene-5β-reduction, respectively, were attributed to hepatic enzyme systems. Their mechanistic basis is discussed with reference to the electronic and conformational changes induced by 9α-fluorination.     

Influence of low plasma progesterone concentrations on the release of prostaglandin F2α during luteolysis and oestrus in heifers

A study was conducted to determine the effect of suprabasal plasma concentrations of progesterone on the release of prostaglandin F_2α (PGF_2α) at luteolysis and oestrus. Heifers received silicone implants containing 2.5 (n = 4), 5 (n = 4), 6 (n = 3), 7.5 (n = 3), 10 (n = 4), or 15 (n = 3) g of progesterone, or an empty implant (controls, n = 4) between Days 8 and 25 post ovulation. Blood was collected frequently between Days 14 and 28 and assayed for progesterone and 15-ketodihydroprostaglandin F_2α. Basal progesterone concentrations in control heifers did not differ from those in heifers with 2.5- or 5-g implants and remained around 0.4−0.5 nmol l⁻¹ until ovulation in all three groups. In the heifers treated with 6–15 g of progesterone, basal concentrations were maintained at higher (P < 0.05) levels compared with those in the controls, ranging from 0.8 to 1.6 nmol 1⁻¹. The effect of these elevated progesterone levels was to delay ovulation by prolonging the growth of the ovulatory follicle, which continued growing until the implant was removed. In all experimental groups, the first significant increase of the PGF_2α metabolite occurred between Days 15.3 and 16.3 (P > 0.05) and was associated with the onset of a decrease in progesterone concentrations, which had reached levels below 3 nmol 1⁻¹ by Days 17.4−19.1. PGF_2α metabolite peaks associated with luteolysis were frequent until Day 20. In the period from Day 20 until implant removal, sporadic peaks were observed, ranging in number from 1.0 ± 1.2 (mean ± SEM) in the control group to 3.0 ± 1.4 peaks in the heifers treated with 7.5 g of progesterone (P > 0.05). The number of PGF_2α metabolite peaks during that period was higher (P < 0.05) in heifers treated with 10 and 15 g than in controls. A positive correlation was found between the basal concentration of progesterone and the number of PGF_2α peaks after luteolysis (r = 0.54; P < 0.01). Plasma progesterone concentrations above approximately 1.4 nmol l⁻¹ were able to maintain the release of PGF_2α until the progesterone implants were removed and plasma levels decreased to basal values. These heifers had a preovulatory PGF_2α release pattern resembling that found in repeat breeder heifers.