Effect of prostaglandin E2 alpha on the hCG-stimulated progesterone production by human corpora lutea

The effect of prostaglandin PGF2 alpha on the hCG stimulated and basal progesterone production by human corpora lutea was examined in vitro. hCG (40 i.u./ml) stimulated progesterone formation in corpora lutea of early (days 16-19 of a normal 28 day cycle), mid (days 20-22) and late (days 23-27) luteal phases. This stimulation was inhibited by PGF2 alpha (10 micrograms/ml) in corpora lutea of mid and late luteal phases. PGF2 alpha alone did not show a consistent effect on basal progesterone production. The inhibition of hCG stimulated progesterone production by PGF2 alpha at times corresponding to luteolysis indicates a role for that prostaglandin in the process of luteolysis in the human corpus luteum.     

Induction of 20α-hydroxysteroid dehydrogenase in rat corpora lutea of pregnancy by prostaglandin F2α

20α-OH-SDH is a marker of luteolysis in rat corpora lutea and appearance of this enzyme is inhibited by prolactin but stimulated by LH or hCG. PGF₂α induced 20 α-OH-SDH activity in corpora lutea of pregnant rats and a significant fall in peripheral plasma progesterone concentrations when administered i.m. for two consecutive days. Rats treated with PGF₂ α on days 8 and 9 of pregnancy were resorbing implants by day 10. Exogenous progesterone, but not estrogen, prevented implant resorption, yet 20 α-OH-SDH appeared in the corpora marking luteolysis. HCG, LH and prolactin, but not FSH, prevented pregnancy termination and inhibited induction of 20 α-OH-SDH in rats treated with PGF₂ α in early pregnancy. PGF₂α also induced 20α-OH-SDH in luteal tissue of intact and hypophysectomized rats treated on days 14 and 15 of pregnancy, but neither exogenous steroids or gonadotrophins blocked the induction of the enzyme in rats treated at this time. The increase in lutein 20α-OH-SDH activity during the peripartal period was partially blocked by administration of the prostaglandin biosynthesis inhibitor, indomethacin, suggesting a role for endogenous prostaglandins in the induction of 20α-OH-SDH at term. It appears that PGF₂α acts directly on the ovary to induce 20α-OH-SDH activity by preventing the luteotrophic action of prolactin. Other luteal NADPH-dependent dehydrogenase activities are not markedly stimulated following PGF₂α administration.     

PGF2α-induced loss of corpus luteum gonadotrophin receptors

In experiments and we have previously shown that PGF_2α directly antagonized the action of gonadotrophins on the corpus luteum. To determine if this action of PGF_2α may occur as a consequence of an induced loss of gonadotrophin receptors, binding of hCG to rat luteal tissue was measured following PGF_2α treatment . In immature rats which were treated with exogenous gonadotrophin to luteinize the gonads, PGF_2α produced a marked and highly significant decrease in circulating progesterone when administered 24 hours before sacrifice. Although the affinity constant (Ka; 1.2-2 × 10¹⁰ L/M) of the luteal receptor to hCG was not affected, PGF_2α treatment produced a marked fall in the binding capacity of the luteal tissue to hCG. This response was absent, however, when PGF_2α was incubated directly with luteal receptor or administered during early pseudopregnancy when corpora lutea are more resistant to luteolysis. Experiments are in progress to determine if the decrease in capacity of luteal receptors to bind hCG is the mechanism or a consequence of luteolysis produced by PGF_2α.     

Effect of prostaglandin F2α on endocrine-ovarian function in the domestic cat

The effect of prostaglandin F_2α(PGF_2α) on endocrine and ovarian function during the early luteal phase of the domestic cat was investigated. Queens were induced to ovulate and then injected subcutaneously with 0.5–5.0 mg PGF_2α/kg body weight. The greatest dose was found to approach toxicity. Concentrations of progesterone were similar in cats following treatment with PGF_2α compared to values of controls. Development and regression of corpora lutea as determined by serial laparoscopy were similar in all groups. These data indicate that PGF_2α at the tested dosages, given during the early luteal phase is not luteolytic in this species and suggest that these regimens would be ineffective for the premature termination of pseudopregnancy.     

Luteal phase variations in endogenous concentrations of prosta-glandins PGE and PGF and in the capacity for their in vitro formation in the human corpus luteum

One evidence for a luteolytic role for prostaglandin F₂α in the human is the increase in luteal PGF at times corresponding to luteolysis as reported earlier by us and other groups. There have been other contradictory reports on this point. In the present experiments we have measured the concentrations of PGE and PGF in 16 more human corpora lutea and have determined the capacity of those tissues to form PGE and PGF in vitro. PGF concentrations were highest in the mid luteal phase but were accompanied by high PGE concentrations. On the other hand, in the late luteal phase PGF concentrations, lower than in mid luteal but generally higher than in early luteal phase, were significantly higher than PGE concentrations. This pattern in PGE and PGF concentrations was also evident in the capacity of these tissues to form these compounds in vitro. In view of the known capacity of PGE₂ to counteract the luteolytic effect of PGF₂α, these variations in the relative concentrations of PGE and PGF during the luteal phase may be of significance in the process of luteolysis in the human.     

Prostaglandin E1 and F2α specific binding in bovine corpora lutea: Comparison with luteolytic effects

Preliminary characterization indicated the presence of separate prostaglandin (PG)E₁ and (PG)F_2α binding sites in membrane fractions prepared from bovine corpora lutea. These differ in the rate and temperature dependence of the specific binding. Equilibrium binding data indicate the apparent dissociation constants as 1.32 × 10⁻⁹M and 2.1 × 10⁻⁸M for PGE₁ and PGF_2α, respectively. Competition of several natural prostaglandins for the PGE₁ and PGF_2α bovine luteal specific binding sites indicates specificity for the 9-keto or 9α-hydroxyl moiety, respectively. Differences in relative ability to inhibit ³H-PG binding were found due to sensitivity to the absence or presence of the 5,6-cis-double bond as well.Bovine luteal function was affected following treatment of heifers with 25 mg PGF_2α as measured by reduced estrous cycle length, decreased corpus luteum size and significantly decreased plasma progesterone levels. In contrast, treatment with 25 mg PGE₁ resulted in cycle lengths comparable to those of non-treated herdmates with no apparent modification in corpus luteum size. However, plasma progesterone levels were increased significantly following PGE₁ treatment compared to pretreatment values. In so far as data obtained on PGF_2α relative binding affinity to the bovine CL can be compared to data obtained independently on PGF_2α induced luteolysis in the bovine, PGF_2α relative binding to the CL and luteolysis appeared to be associated. By similar reasoning, there was no apparent relationship between PGE₁ relative binding affinity in the luteal fractions and luteolysis in estrous cyclic cattle.     

Prostaglandin F2a inhibition of epinephrine stimulated cyclic AMP and progesterone production by rat corpora lutea of various ages

Epinephrine can mimic the stimulatory effects of LH in vitro on cyclic AMP (cAMP) and progesterone production by isolated rat corpora lutea. The aim of the present study was to test whether the effects of epinephrine in vitro on the rat corpus luteum, as with LH, can be inhibited by prostaglandin F_2a (PGF_2a. The stimulatory effect of epinephrine on tissue levels of cAMP in 1-day-old corpora lutea was not inhibited by PGF₂. A dose-dependent inhibition by PGF_2a (0.5–50 μM) was seen for 3-day-old corpora lutea and this inhibition could not be overcome by higher concentrations of epinephrine (0.165–165 μM). The stimulation by epinephrine on progesterone production was inhibited by PGF_2a (5 μM) in 3- and 5-day-old, but not in 1-day-old corpora lutea. Thus, PGF_2a can inhibit the stimulatory effect of epinephrine in 3- and 5-day-old corpora lutea, but not in the newly formed corpora lutea (1-day-old) and PGF_2a shows in this respect the same agedependent inhibitory pattern as in relation to LH stimulation.     

Prostaglandin E1 and F2α specific binding in bovine corpora lutea: Comparison with luteolytic effects

Preliminary characterization indicated the presence of separate prostaglandin (PG)E₁ and (PG)F_2α binding sites in membrane fractions prepared from bovine corpora lutea. These differ in the rate and temperature dependence of the specific binding. Equilibrium binding data indicate the apparent dissociation constants as 1.32 × 10⁻⁹M and 2.1 × 10⁻⁸M for PGE₁ and PGF_2α, respectively. Competition of several natural prostaglandins for the PGE₁ and PGF_2α bovine luteal specific binding sites indicates specificity for the 9-keto or 9α-hydroxyl moiety, respectively. Differences in relative ability to inhibit ³H-PG binding were found due to sensitivity to the absence or presence of the 5,6-cis-double bond as well.Bovine luteal function was affected following treatment of heifers with 25 mg PGF_2α as measured by reduced estrous cycle length, decreased corpus luteum size and significantly decreased plasma progesterone levels. In contrast, treatment with 25 mg PGE₁ resulted in cycle lengths comparable to those of non-treated herdmates with no apparent modification in corpus luteum size. However, plasma progesterone levels were increased significantly following PGE₁ treatment compared to pretreatment values. In so far as data obtained in vitro on PGF_2α relative binding affinity to the bovine CL can be compared to data obtained independently in vitro on PGF_2α induced luteolysis in the bovine, PGF_2α relative binding to the CL and luteolysis appeared to be associated. By similar reasoning, there was no apparent relationship between PGE₁ relative binding affinity in the luteal fractions and luteolysis in estrous cyclic cattle.     

Luteolysis in the rhesus monkey: Ovarian venous estrogen, progesterone, and prostaglandin F2α-metabolite

The role of prostaglandin F₂α (PGF₂α) in luteolysis in the non-human primate is poorly understood. We have recently reported that chronic PGF₂α infusion to the corpus luteum via Alzet pump, induced premature, functional luteolysis in the rhesus monkey. In the present study we sought to determine the ovarian events leading to spontaneous luteolysis in the monkey. Rhesus monkeys underwent laparotomy during the early luteal (4–5 days after the preovulatory estradiol surge, PES), mid-luteal (7–9 days PES), and late luteal (10–14 days PES) phases or at the first day of menses (M). Concentrations of progesterone, estradiol, estrone, and 13, 14-dihydro-15-keto-PGF₂α (PGFM) were measured in the ovarian venous effluents ipsilateral and contralateral to the ovary bearing the corpus luteum. Steroid levels in the ovarian vein on the corpus luteum side were significantly higher than the non-corpus luteum side throughout the cycle. PGFM levels were similar on both sides until the late luteal phase, when the effluent of the ovary bearing the corpus luteum contained significantly more PGFM (206±3) vs. 123±9 pg/ml, mean±sem); this disparity increased further at the time of menses (241±38 vs. 111±22 pg/ml). These data are the first to show an asymmetric secretion of PGFM in the ovarian venous effluent in the primate and suggest that PGF₂α of ovarian and possibly of corpus luteum origin may be directly involved in luteal demise.     

Effect of prostaglandin F2α on release of progesterone and leukotriene B-4 by cells from corpora lutes or mares

Corpora lutca were recovered from mares either 4 to 5 days or 12 to 13 days after ovulation. Mixed populations of luteal cells were prepared by collagenase digestion and were incubated for 24 h in the presence or absence of prostaglandin (PG) F-2 α (250 ng/ml). PGF-2 α significantly (P = 0.03) reduced progesterone secretion by cells from late diestrous corpora lutea and tended (P = 0.06) to reduce secretion by early diestrous cells. PGF-2 α had no significant effect on leukotriene B-4 (LTB-4) production by cells from early diestrous corpora lutea, but significantly (P = 0.03) increased LTB-4 production by late diestrous luteal cells. It seems possible that LTB-4 could play a role as an intermediary in the action of PGF-2 α in luteolysis in the mare.     

Corpus luteum susceptibility to prostaglandin F2α (PGF2α) luteolysis in hysterectomized prepuberal and mature gilts

The susceptibility of induced corpora lutea (CL) of prepuberal gilts and spontaneously formed CL of mature gilts to prostaglandin F_2α (PGF_2α) luteolysis was studied. Prepuberal gilts (120 to 130 days of age) were induced to ovulate with Pregnant Mare Serum Gonadotropin and Human Chorionic Gonadotropin (HCG). The day following HCG was designated as Day 0. Mature gilts which had displayed two or more estrous cycles of 18 to 22 days were used (onset of estrus = Day 0). Gilts were laparotomized on Day 6 to 9, their CL marked with sterile charcoal and totally hysterectomized. On Day 20, gilts were injected IM with either distilled water (DW), 2.5 mg PGF_2α or 5.0 mg PGF_2α. An additional group of prepuberal gilts was injected with 1.25 mg PGF_2α, a dose of PGF_2α equivalent, on a per kilogram body weight basis, to the 2.5 mg PGF_2α dose given to the mature gilts. The percentages of luteal regression on Day 27 to 30 for mature and prepuberal gilts given DW, 2.5 mg PGF_2α and 5.0 mg PGF_2α were 0.0 vs 4.4, 43.5 vs 96.8 and 47.7 vs 91.6, respectively; the percentage of luteal regression for the prepuberal gilts given 1.25 mg PGF_2α was 75.1. These results indicate that induced CL of the prepuberal gilt were more susceptible to PGF_2α luteolysis than spontaneously formed CL of the mature gilt and that pregnancy failure in the prepuberal gilt could be due to increased susceptibility of induced CL to the natural luteolysin.     

A biochemical hypothesis to explain the mechanism of luteal regression

It seems likely that luteal regression may involve a direct biochemical action of prostaglandin F₂α (PGF₂α) on the luteal cell since there are now several reports that PGF₂α can directly inhibit steroidogenesis . However, the mechanism of such an action of PGF₂α remains obscure.This article initially reviews the central role of adenosine 3,^I5^I-mono-phosphate (c-AMP) in initiating and maintaining the structural and functional changes occurring on luteinisation. A mechanism is suggested, supported by results obtained using granulosa cells in tissue culture, in which PGF₂α initiates functional luteolysis by inhibiting further synthesis of c-AMP. This mechanism is then used in conjunction with further observations to provide a possible explanation for the inability of PGF₂α to regress newly formed corpora lutea. Finally, the possible mechanisms of structural regression are discussed.     

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 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.     

An intra-corpus luteum site for the luteolytic action of prostaglandin F2α in the rhesus monkey

It has not been possible to demonstrate prostaglandin F₂α (PGF₂α) participation in primate luteolysis under conditions of systemic administration or of acute intraluteal injection. These study designs were hampered by the short biological half-life in the first instance and brevity of administration in the latter. In this study, luteolysis has resulted from chronic, intraluteal delivery of PGF₂ α. Using the Alzet osmotic pump-cannula system, normally cycling rhesus monkeys were continuously infused, until menses occurred, with PGF₂ α (10 ng/1/hr) directly into the corpus luteum (CL, n=6), into the stroma of the ovary bot bearing the corpus luteum (NCL, n=3), or subcutaneously (SC, n=5). An additional 5 monkeys received vehicle (V) into the corpus luteum. All experiments commenced 5–7 days after the preovulatory estradiol surge. Luteal function was assessed by the daily measurements of plasma progesterone, estradiol, and LH. Intraluteal PGF₂α caused premature functional luteolysis in all monkeys, as reflected by a highly significant decline in circulating progesterone and estradiol and the early onset of menstruation, when compared to the other groups. V, NCL, and SC infusions had no effect on either circulating steroid levels or luteal phase lengths. None of the experimental groups showed any change in plasma LH concentrations. These are the first data to indicate that PGF₂α can induce functional luteolysis in the primate, and the site of action appears to be the corpus luteum.     

Time related changes in luteal prostaglandin synthesis and steroidogenic capacity during pregnancy, normal and antiprogestin induced luteolysis in the bitch

In nonpregnant and pregnant dogs the corpora lutea (CL) are the only source of progesterone (P4) which shows an almost identical secretion pattern until the rapid decrease of P4 prior to parturition. For the nonpregnant dog clear evidence has been obtained that physiological luteal regression is devoid of a functional role of the PGF2α-system and seems to depend on the provision of StAR. Yet in pregnant dogs the rapid prepartal luteal regression, coinciding with an increase of PGF2α, may be indicative for different regulatory mechanisms. To assess this situation and by applying semi-quantitative Real Time (Taq Man) RT-PCR, expression patterns were determined for the following factors in CL of pregnant and prepartal dogs and of mid-pregnant dogs treated with the antiprogestin Aglepristone: cyclooxygenase 2 (Cox2), prostaglandin E2 synthase (PGES), prostaglandin F2α synthase (PGFS), its receptors (EP2, EP4 an FP), the steroidogenic acute regulatory protein (StAR), 3β-hydroxysteroid-dehydrogenase (3βHSD) and the progesterone receptor (PR). Peripheral plasma P4 concentrations were determined by RIA. CL were collected via ovariohysterectomy from pregnant bitches (n = 3–5) on days 8–12 (Group 1, pre-implantation period), days 18–25 (Group 2, post-implantation period), days 35–40 (Group 3, mid-gestation period) and during the prepartal progesterone decline (Group 4). Additionally, CL were obtained from groups of 5 mid-pregnant dogs (days 40–45) 24 h, respectively 72 h after the second treatment with Aglepristone. Expression of Cox2 and PGES was highest during the pre-implantation period, that of PGFS and FP during the post-implantation period. EP4 and EP2 revealed a constant expression pattern throughout pregnancy with a prepartal upregulation of EP2. 3βHSD and StAR decreased significantly from the pre-implatation period to prepartal luteolysis, it was matched by the course of P4 concentrations. Expression of the PR was higher during mid-gestation and prepartal luteolysis than in the two preceding periods. After application of Aglepristone the overall mRNA-expression resembled the situation during prepartal luteolysis except for EP2, which remained unchanged.These data suggest that – as in the nonpregnant bitch – also in the pregnant bitch luteal production of prostaglandins is associated with luteal support rather than luteolysis. On the other hand induction of luteolysis by the PR blocker Aglepristone points to a role of luteal P4 as an autocrine factor in a positive loop feedback system controlling the availability of P4, StAR and 3βHSD.     

Changes in corpus luteum content of prostaglandin F2α and E in the adult pseudopregnant rat

Conflicting reports exist regarding the source of luteolytic PGF_2α in the rat ovary. To assess the quantities of different PGs, measurements of PGF_2α, PGE and PGB were performed by radioimmunoassay in the adult pseudopregnant rat ovary throughout the luteal lifespan. Ovaries of 84 rats were separated by dissection into two compartments, corpora lutea of pseudopregnancy and remainder of ovary. Tissue samples were homogenized and prostaglandins extracted and determined by radioimmunoassay. During the mid-luteal and late-luteal phases, levels of PGs were significantly higher in the corpora lutea of pseudopregnancy than in the remainder of ovary. An increase of PGF_2α-content in the corpus luteum was registered with peak-levels of 53.9 ± 8.5 (mean ± SEM, N=18) ng/g tissue wet weight at day 13 of pseudopregnancy. PGE-levels reached peak-values at day 11 of pseudopregnancy (271.6 ± 28.4 ng/g w w, mean ± SEM, N=12). PGB-levels were below detection limits in all compartments for all ages studied. The present study demonstrates increased availability of PGF_2α in the corpus luteum during the luteolytic period, and points toward either increased luteal synthesis or luteal binding of PGF_2α during the luteolytic period.     

Effect of PGF2α on progesterone secretion and adenylate cyclase activity in ovine luteal tissue

Ovine luteal slices were used to study the effects of prostaglandins (PG) F₂α on luteinizing hormone (LH)-stimulated secretion of progesterone and adenylate cyclase activity. The accumulation of progesterone in incubation medium and adenylate cyclase activity was similar after incubation of luteal slices with Medium 199 alone or Medium 199 containing PGF₂α (250 ng/ml) for 3 hr. Addition of luteinizing hormone (LH; 100 ng/ml) resulted in a 2–3 fold increase in both the rate of progesterone accumulation and adenylate eyclase activity by 3 hr. When luteal slices were incubated in the presence of both LH and PGF₂α the rates of progesterone accumulation and adenylate cyclase activity were identical to those in flasks containing LH alone after 1 hr; however, after 3 hr both LH stimulated progesterone accumulation and adenylate cyclase activity were inhibited to levels similar to those observed in control slices.In a second experiment, after 60–120 min of exposure to PGF₂α the rate of progesterone accumulation in the medium was not different from that in untreated control slices. In addition, after this experiment the luteal slices were homogenized and the basal, sodium fluoride, LH, isoproterenol (ISO) and PGE₂ sensitive adenylate cyclase activities were determined to evaluate the hormonal specificity of the negative effect of the pretreatment with PGF₂α. Both LH and ISO stimulated adenylate cyclase activities were reduced after PGF₂α pretreatment. However, fluoride ion stimulated adenylate cyclase activity was not significantly effected by PGF₂α pretreatment and PGE₂ sensitive adenylate cyclase was effected only slightly.     

Influence of prostaglandin F autoantibodies on the estrous cycle of the guinea pig

Adult female guinea pigs were actively immunized with prostaglandin F_2α conjugated to bovine serum albumin (BSA). Control animals, immunized against BSA continued to cycle normally, while the animals immunized against prostaglandin F_2α stopped cycling after one to three normal cycles. Laparotomy at 30 days after the last estrus revealed no recently formed corpora lutea. During the remaining 70 days of observation the antibody titer increased to 1:700, accompanied by increasing total serum estrogens (136 pg/ml at day 100) and a slow decline in circulating progesterone levels (0.6 ng/ml at day 100). The ovaries at day 100 contained degenerated corpora lutea and luteinized follicles. The suppression of the estrous cycle in the present experiments was interpreted as resulting from prolongation of luteal function as well as from inhibition of ovulation.     

Effects of systemic administration of indomethacin to cyclic ewes on endometrial concentrations of prostaglandins effects on estrous cycle lenght and on progesterone, luteinizing hormone and prolactin patterns

Experiments were designed to evaluate in cyclic sheep the effects of systemic administration of a prostaglandin synthetase inhibitor (indomethacin). Indomethacin (100 mg, 3 times daily, S.C.) was administered from day 7 of the estrous cycle for 16 days to five ewes in which the cycle was synchronized as well as the cycles of five control ewes. All control ewes had cycles of approximately 17 days duration, but three of five Indomethacin treated ewes showed no estrous behavior before their slaughter at 23 days after induced ovulation. Autopsy revealed normal corpora lutea which had not undergone luteolysis, as confirmed by progesterone determination in blood. The two remaining Indomethacin treated ewes showed an estrous behavior on day 19 and 20 respectively together with a “prevulatory surge” of luteinizing hormone and prolactin which was not followed by follicular rupture. These results show that inhibition of PGF₂α synthesis by systemic administration of Indomethacin to the ewe is able to prevent luteolysis.When luteolysis did occur however, it was not followed by an ovulation despite a normal gonadotropin surge, indicating that inhibition of prostaglandin synthesis by systematic administration of Indomethacin interferes with the luteolysis and follicle rupture processes.