Changes in corpus luteum content of prostaglandin F2 alpha and E in the adult pseudopregnant rat

Conflicting reports exist regarding the source of luteolytic PGF2 alpha in the rat ovary. To assess the quantities of different PGs, measurements of PGF2 alpha, 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 PGF2 alpha-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 PGF2 alpha in the corpus luteum during the luteolytic period, and points toward either increased luteal synthesis or luteal binding of PGF2 alpha during the luteolytic period.     

Effect of prostaglandin F2α on the hCG-stimulated progesterone production by human corpora lutea

The effect of prostaglandin PGF_2α on the hCG stimulated and basal progesterone production by human corpora lutea was examined . 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 PGF_2α (10 μg/ml) in corpora lutea of mid and late luteal phases. PGF_2α alone did not show a consistent effect on basal progesterone production. The inhibition of hCG stimulated progesterone production by PGF_2α at times corresponding to luteolysis indicates a role for that prostaglandin in the process of luteolysis in the human corpus luteum.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

Reduction by human chorionic gonadotropin of the luteolytic effect of prostaglandin F2 in ewes

The ability of human chorionic gonadotropin (HCG) to reduce the luteolytic effect of prostaglandin (PGF_2^α) was demonstrated in cycling ewes. As expected, treatment with 10 mg of PGF_2^α alone on Day 10 of the estrous cycle exerted a potent negative effect on the function and structure of corpus luteum (CL) as indicated by reduced plasma progesterone, CL progesterone, and CL weight. However, the identical PGF_2^α treatment failed to significantly reduce either luteal function or luteal weight when administered to ewes that were also treated with HCG on Days 9 and 10 of the estrous cycle. Treatment with HCG alone had a positive effect on CL as indicated by increased plasma progesterone, CL progesterone, and CL weight. Treatment with HCG did not render the CL totally insensitive to the negative effects of PGF_2^α because plasma progesterone was reduced when the dose of PGF_2^α was doubled. Whether CL regressed or continued to function after treatment with both HCG and PGF_2^α appeared to depend upon a balance between the positive and negative effects of the two hormones.     

Effect of prostaglandins on the in vitro steroidogenesis in human ovarian tissues

Although prostaglandins are luteolytic in some species, in $\text{in vitro}$ conditions they stimulate progesterone production in the corpus luteum (1). Apart from this effect prostaglandins may also stimulate other steps in the steroidogenic sequence e.g. corticosteroidogenesis in superfused rat adrenal glands (2) and aromatization of testosterone by perfused human placenta (3). With this possibility in view and also because of paucity of data on the effect of prostaglandins on steroidogenesis in human ovarian tissues we have been studying under $\text{in vitro}$ conditions the effect of prostaglandins on progesterone formation in human corpora lutea and on the utilization of C₂₁ steroids by the luteal and follicular compartments of the ovary. These studies are still in progress. However, the data obtained so far indicates that in addition to stimulating progesterone synthesis in the corpus luteum prostaglandins may also affect other steps in steroidogenesis in human ovarian tissues. We wish to report here in brief these preliminary results.     

Cyclic AMP formation of isolated human corpora lutea in response to HCG — Interference by PGF2α

Human corpora lutea of defined ages were excise at operation cut into pieces and incubated in the presence of HCG, PGF_2α and PGE₂ alone or in combination. Following incubation cAMP formation in tissue and medium was determined. HCG-stimulated tissue cAMP content was most pronounced at a corpus luteum age of 7–10 days after ovulation. This stimulation was antagonized by PGF_2α in corpora lutea older than 6 days. PGE₂ stimulated cAMP formation $\text{per se}$ and this effect was more pronounced when HCG and PGE₂ were combined. A possible role for PGF_2α as a luteolytic substance in the human is suggested.     

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-induced regression of porcine corpora lutea maintained by estrogen

Corpora lutea were marked with suture in 24 crossbred gilts on day 7 to 9 of the estrous cycle (first day of estrus = 0). All gilts were injected with 5 mg of estradiol benzoate (EB) daily from day 10 to 15 to extend the lifespan of corpora lutea, then the gilts were randomly assigned to two groups. On day 20, the 12 gilts of Group 1 were injected with 10 mg PGF_2α, and the 12 gilts of Group 2 were injected with saline. Ovaries were recovered 10 to 13 days after PGF_2α or saline injection. Ten gilts in Group 1 displayed estrus 5 ± 0.7 days after PGF_2α injection, but only two gilts in Group 2 displayed estrus during the experimental period. In gilts that displayed estrus, all marked CL had regressed. Marked CL were still present in all 12 gilts that failed to exhibit estrus during the experimental period. These results show that in the pig, PGF_2α caused regression of CL that were maintained beyond the normal luteal phase of the estrous cycle by EB treatment.     

Effect of prostaglandins on the steroidogenesis in human ovarian tissues

Although prostaglandins are luteolytic in some species, in conditions they stimulate progesterone production in the corpus luteum (1). Apart from this effect prostaglandins may also stimulate other steps in the steroidogenic sequence e.g. corticosteroidogenesis in superfused rat adrenal glands (2) and aromatization of testosterone by perfused human placenta (3). With this possibility in view and also because of paucity of data on the effect of prostaglandins on steroidogenesis in human ovarian tissues we have been studying under conditions the effect of prostaglandins on progesterone formation in human corpora lutea and on the utilization of C₂₁ steroids by the luteal and follicular compartments of the ovary. These studies are still in progress. However, the data obtained so far indicates that in addition to stimulating progesterone synthesis in the corpus luteum prostaglandins may also affect other steps in steroidogenesis in human ovarian tissues. We wish to report here in brief these preliminary results.     

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.     

Luteal function in sheep injected with prostaglandin F2α directly into the corpus luteum

Groups of ewes received either saline or prostaglandin F_2α (PGF_2α) as an injection directly into the corpus luteum. Changes in circulating progesterone levels were measured as well as subsequent histological examination of the corpora lutea. Saline, or PGF_2α given at the two lower doses (60 and 120 μg respectively), failed to suppress progesterone levels permanently, or to induce degenerative changes in the corpora lutea. Treatment with a higher dose of PGF_2α (240 μg) was followed by a marked elevation in progesterone levels. These results are discussed in relation to reported effects of PGF_2α arriving at the ovary via the arterial circulation.     

Comparison of luteolytic effectiveness of several prostaglandin analogs in heifers and relative binding affinity for bovine luteal prostaglandin binding sites

The relative binding affinities for both the prostaglandin (PG)E₁ and PGF_2α specific bovine luteal binding sites were determined for five PGE and fourteen PGF derivatives and analogs. Relative binding affinity was determined using membranes prepared from bovine corpora luteal (CL) obtained from the slaughterhouse. The parent structure of the analog was a dominant feature in determining the affinity for the respective PG binding site. Luteolysis was determined in cattle following intramuscular injection of various doses of prostaglandin once between days 6 and 14 after estrus and measuring CL regression by ovarian palpation per rectum, interval between injection and return to estrus and duration of the subsequent estrous cycle. A dose which was luteolytic was established for each of eight PGF-type compounds, and a dose which was not luteolytic was also established. There appeared to be limited association between the relative affinity for the PGF_2α specific site and the estimated luteolytic dose range of these PGF analogs when tested in cattle. Differences in luteolytic potency for the compounds tested could not be explained by differences in binding affinity. Differences in metabolism and absorption may also be important in the determination of potency.