The possible mode of action of prostaglandins: X. Antagonism between prostaglandin F2a and prolactin or human chorionic gonadotropin : A comparative study

A comparative assessment of the efficacy of prolactin or HCG in reversing the luteolytic property of a single dose (2.0 mg/kg) of PGF₂a was conducted in pregnant (day 10) rats. It was found that prolactin maintained pregnancy in 90% of the test animals. HCG though found to maintain pregnancy in 70% of the PGF₂a-treated pregnants, the fetal survival rate was, however, recorded to be 62% against the control value of 98%. Conversely, prolactin replacement maintained the fetal survival rate as high as 93.4%. Moreover, the growth of the fetuses, placentae and corpus luteum in the pregnants having prolactin in conjunction with PGF₂a was also found to be greater compared to the animals which h ad a combined regimen of PGF₂a and HCG, but identical to controls. On the other hand, similar combined regimen when applied to hysterectomized pregnant rats, it was observed that though the vaginal diestrus was maintained by the prolactin or HCG in the presence of PGF₂a, the prolactin regimen was found to be superb compared to HCG in the maintenance of luteal weight and functional activity. It was concluded that the antifertility effect of PGF₂a in the rat is primarily the consequence of luteolysis and prolactin seems to be a much more appropriate hormonal replacement compared to HCG, a long-acting LH, in antagonizing the luteolytic property of PGF₂a.     

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.     

Prostaglandin F in the peripheral plasma of the rhesus monkey in normal pregnancy and after the administration of dexamethasone and PGF2α

The concentration of prostaglandin F (PGF) has been measured in the peripheral plasma of normal rhesus monkeys ( ) during the final third of gestation, and in monkeys treated with dexamethasone or PGF₂α after day 145 of pregnancy. Daily administration of PGF₂α (10–15 mg/day im) reliably induced abortion within 3–6 days. However, dexamethasone (8 mg/day im from day 145) had no effect on the length of gestation.The concentration of PGF in the femoral venous plasma of untreated or dexamethasone-treated monkeys was highly variable, both in serial samples taken from the same animal and in samples taken from different animals at the same time of gestation. There was no indication of an effect of dexamethasone treatment on the plasma PGF levels, nor did the concentration of PGF increase during late pregnancy before spontaneous parturition. These results show that the myometrium of the pregnant rhesus monkey is highly sensitive to exogenous PGF₂α during late gestation. However, a significant increase in the peripheral plasma concentration of PGF prior to the onset of labor was not observed.     

Clearance of prostaglandin F2ALPHA during circulatory shock.

The rate of disappearance of total circulating radioactivity following an intravenous bolus injection of 3HPGF_2α was determined during splanchnic artery occlusion (SAO) shock in the dog. In addition, the pattern of PGF_2α metabolite formation was assessed in both shocked and nonshocked animals. Although the clearance of circulating prostaglandin metabolites is significantly impaired during SAO shock as a result of decreased renal function, neither the pattern nor the time course of PGF_2α metabolite formation appears to be altered. Thus, increases in circulating prostaglandin concentrations during SAO shock reflect an increase in the rate of $\text{de novo}$ synthesis and release of these materials, and are not the result of decreased prostaglandin degradation.     

Pregnancy and fetal development in hamsters treated with prostaglandin F2α

The influence of PGF_2α on pregnancy and fetal outcome was investigated in the hamster. Following subcutaneous treatment with 50, 100, 200, 400, and 800 μg PGF_2α prenatal loss was significantly increased only at the highest dose level. The offspring of the treated animals were all alive and normal. Fetal weight was not affected. However, following intravenous injection of 100 and 200 μg PGF_2α there was a significant reduction in fetal weight, and at the 400 μg dose level an increase in fetal resorptions. All fetuses recovered were alive and normal. These results indicate that PGF_2α is not teratogenic in hamsters despite the apparent greater sensitivity of the hamster embryo to prostaglandin.     

Suppression of fertility in female hamsters with polyvinylpyrrolidone — Silicone rubber (PSR) implants containing prostaglandin F2α

Fertility has been suppressed in female hamsters by premating implantation of polyvinylpyrrolidone-silicone rubber (PSR) capsules containing PGF_2α. When tubular PSR capsules (0.5 cm diameter, 1 cm length) with 4 mg PGF_2α (tham salt) were implanted subcutaneously 3 days before mating, no pregnancies occurred among the treated animals. Removal of the capsules was associated with a recovery of fertility. Administration of PGF_2α by subcutaneous bi-daily injections, 2 mg/day on days 3 and 2 prior to mating, and by implantation of PSR capsules, whose exposed surfaces were sealed, produced no obvious effect on either the rate of pregnancy or upon the number of fetuses/pregnancy. From the present results, PSR capsules provide an effective means of implanting prostaglandin and appear to be suitable for their long term administration.     

Corpus luteum function in the guinea pig: effect of pgf2 alpha and inhibitors of prostaglandin and progesterone biosynthesis.

Exogenous PGF_2α failed to consistently alter estrous cycle length of the guinea pig. A wide range of dose levels were administered with varying frequency, at different stages of the estrous cycle, in different vehicles and by various routes. Massive doses of PGF_2α (5.0–10.0 mg) produced a significant (p<.05), although transient, lowering of plasma progesterone levels. Smaller doses were ineffectual. An i.p. injection of 25 mg of PGF_2α was toxic in four of five treated animals. It would appear that the intact guinea pig is extremely resistant to the luteolytic effects of parenterally administered PGF_2α.Estradiol-17β, administered s.c. on days 3 to 10 of the estrous cycle, significantly (p<.05) reduced corpus luteum diameter and plasma progesterone levels. Estrous cycle length was unaffected. Clomiphene, in the same experiment, caused premature vaginal opening in some treated animals, but corpus luteum size and plasma progesterone levels were unaffected and no ovulations occurred.The prolactin secretion inhibitor, CB-154, administered early in the estrous cycle, did not have any effect on estrous cycle length of the guinea pig alone or in combination with PGF_2α. The prostaglandin precursor, arachidonic acid, also failed to influence estrous cycle length when administered on days 8 and 9 of the cycle. Plasma progesterone levels remained unaltered.Oral administration of a prostaglandin synthetase inhibitor, (MK-715), caused a small, but non-significant (p>.05) prolongation of the estrous cycle. The progesterone biosynthesis inhibitors, aminoglutethimide and 6β-hydroxy-3α, 5α-cyclo-androstane-17-one did not effect estrous cycle length or plasma progesterone levels of the guinea pig.     

Plasma hormone levels in beef heifers during prostaglandin-induced parturition

Plasma hormone levels were determined at frequent intervals in 12 two-year-old Hereford heifers after treatment with prostaglandin F₂α on day 267 of gestation to induce parturition. Of the 12 animals, six also received a daily injection of estradiol-17β until calving occurred. An additional six heifers received no treatment and therefore served as a control group. The treatment with PGF₂α was effective since the treated heifers calved a mean of 12 days sooner than the untreated heifers (P < .01). A metabolite of PGF₂α, 13, 14-dihydro-15-keto PGF₂α, rose to a peak level in the plasma within one hour after injection (im) of PGF₂α then declined to a baseline level within four hours. The plasma progesterone level had fallen significantly within two hours after injection, but no change in plasma estradiol-17β or estrone was observed. In the eight heifers that responded, plasma estrogen levels were greater at the time of treatment than in the four heifers that did not respond.     

Termination of pregnancy in cases of fetal death in utero by intravenous prostaglandin F2α

Delivery was induced by an intravenous infusion of prostaglandin F_2α (PGF_2α) in gradually increasing doses in 30 consecutive cases of fetal death in utero after the 28th week of gestation. Twenty patients delivered during the first day of prostaglandin administration, 9 on the second day, and 1 patient not until the third day of infusion. It is concluded, that intravenous PGF_2α appears to be superior to oxytocin in termination of pregnancy under these conditions.     

Action of prostaglandin F2α on pregnancy in hamsters: Luteolytic and extra-ovarian effects

Pregnancies in hamsters may be terminated with 10 μg PGF_2α administered b.i.d. on days 4, 5 and 6 of gestation. Small (250 μg and above) daily injections of progesterone on the same days will reverse this PG effect; in contradistinction, 10 mg of progesterone per day failed to maintain normal pregnancies in hamsters spayed on day 5. Daily administration of 3 mg of progesterone and 1 μg of estrone essentially normalized the gestation; administration of PGF_2α at 10 mg on days 5, 6 and 7 of pregnancy in steroid-maintained rats, resulted in pregnancy termination in all animals, while 1 mg was partly effective. These data demonstrate an extra-ovarian site of action of prostaglandin F_2α on pregnancy in hamsters.     

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.     

Possible mode of action of prostaglandins: V - differential effects of prostaglandin F2α before and after the establishment of placental physiology in pregnant rats

Prostaglandin F₂α (PGF₂α) at a dose level of 2.00 mg/kg body weight could cause complete resorption of the implanted blastocysts when injected either on day 10 or day 11 of pregnancy in rats. The same injection apparently failed to induce abortion or resorption in rats having functional placentae on day 12 or day 13 of pregnancy. It was moreover observed that a concomitant exogenous administration of either prolactin or progesterone alongwith PGF₂α could successfully reverse the abortifacient property of PGF₂α and keep the status of the ovaries, embryos, placentae etc. identical to that obtained in the control. It was suggested from the experimental evidences that the abortifacient effects of PGF₂α in the rat might possibly be mediated through the pituitary or hypothalamo-pituitary complex.     

Seminal plasma affects prostaglandin synthesis in the porcine oviduct

Seminal fluids introduced to the female reproductive tract at mating can affect subsequent events, such as ovulation, fertilization, conception, and pregnancy. Bioactive molecules present in seminal plasma can modify the cellular composition, structure, and function of local tissues and of tissues distal to the tract. The oviduct plays a decisive role in reproduction providing a beneficial milieu for gamete maturation, fertilization, and early embryonic development. Therefore we have investigated whether intrauterine infusion of seminal plasma can modulate prostaglandin (PG) synthesis in the porcine oviduct through regulation of gene and protein expression of enzymes of prostaglandin synthesis pathway. Among several enzymes involved in the prostaglandin synthesis pathway tested in the present study PGF_2α synthase (PTGFS) and prostaglandin 9-ketoreductase (CBR1), which convert PGE₂ to PGF_2α, expression were significantly down-regulated in the oviducts on Day 1 after seminal plasma infusion into the uterine horns. The effects of the treatment were transient and by Day 5 levels of PTGFS and CBR1 were comparable in seminal plasma-treated and control animals. Additionally, increased PGE₂ to PGF_2α and PGFM to PGF_2α ratios in the oviductal tissues were indicated. Our results clearly demonstrate that seminal plasma affects prostaglandin synthesis in the porcine oviduct. Altered PTGFS and CBR1 expression in consequence changed PGE₂ to PGF_2α and PGFM to PGF_2α ratios in the porcine oviduct.     

Induction of parturition in the mare with prostaglandin F2α

Thirty-one mares of Quarter Horse and Thoroughbred breeding were utilized in two experiments to evaluate the efficacy of prostaglandin F₂α (PGF₂α)_for induction of equine parturition and to monitor the effects of this treatment on viability of the resulting foals.Three of five mares given 5 mg PGF₂α (im) on day 338 of gestation foaled 19.6 ± 8.2 hr postinjection. In the second experiment immediately following 3 daily injections of 10 mg estradiol cypionate (ECP) given on days 326, 327 and 328 of gestation, seven mares were infused (iv) with PGF₂α at the rate of 1.3 mg/hr for 24 hr or until parturition occurred. Four of the seven mares foaled in 8.8 ± 1.8 hr after the start of infusion. Side effects including sweating, hypothermia, increased respiration rate and diarrhea were evident in both injected and infused mares, but effects were transient. Neither the injection, nor infusion route of administration of PGF₁α adversely affected the viability of foals. However, some mares induced to foal 12 days prior to expected parturition had foals with slightly weaker pasterns than those of control mares.     

Comparison of three subcutaneous modes of prostaglandin F2α administration for pregnancy termination in the hamster

Dose response relationships for pregnancy termination in hamsters following administration of prostaglandin F_2α (PGF_2α by three subcutaneous methods were determined in 526 hamsters. The median effective dose (ED₅₀) for PGF_2α given as a single subcutaneous injection in 500 μl of saline was 22.2 μg. Administration of the prostaglandin with an Alzet® osmotic minipump (subcutaneous insertion for 24 hours) required 1.35 times more PGF_2α (ED₅₀ = 30.0 μg). The least effective method of prenancy termination in the hamster involved administration of PGF_2α by a single subcutaneous injection in 20.4 μl of saline (the same volume delivered by the minipump in 24 hours); the ED₅₀ for this method of administration was 41.3 μg of PGF_2α.     

Oxytocin,prostaglandins E and F,estradiol, progesterone,sodium, and potassium in preovulatory bovine follicles either developed normally or stimulated by follicle stimulating hormone

To determine the differences between stimulation by follicle stimulating hormone (FSH) and normal development of follicles in heifers and the endocrinologic events associated with ovulatory development of follicles in heifers, follicular fluid was aspirated from follicles (n = 627) at 24, 48, and 72 h after estrus synchronization (via prostaglandin F_2α (PGF_2α)) from control animals (n = 10/time period) and a treatment group (n = 10/time period) that received FSH. No endocrinologic or ionic differences were noted between follicles harvested from control or FSH-stimulated animals within follicular size or time (24, 48, or 72 h). No interaction of treatment (FSH stimulation vs controls) by time was detected; thus the effect and timing of the LH surge was similar between treatments.At 48 h after PGF_2α administration, sodium from follicular fluid decreased and potassium increased, signifying a considerable physiologic change in the follicle. Follicular prostaglandins E and F increased ten-fold by 72 h, and these changes were primarily found in the estrogen-inactive follicles at 72 h. Though progesterone concentrations within follicular fluid tended to increase throughout the three periods and increased with enlargement of follicular size, a major increase was detected 72 h after PGF_2α injection. Estradiol concentrations tended to increase with enlargement of follicular size within a time period, but estradiol concentrations in the follicles greater than 10 mm in diameter decreased with time (24, 48, and 72 h) after PGF_2α. Concentrations of oxytocin increased with increases in follicular diameter, and the time trends were similar to changes in follicular progesterone. By 72 h after PGF_2α administration, follicular estradiol) was decreasing in concentration and progesterone and oxytocin were increasing, thus signifying a change in the secretory role of the granulosa cells.     

Interaction between uterine PGE and PGF2α production and the nitridergic system during embryonic implantation in the rat

Embryonic implantation is a complex process in which both maternal andembryonic signals are involved. In the present study, we evaluated changes in uterine prostaglandins production and nitric oxide synthase (NOS) activity during the course of early pregnancy and their interaction during implantation in rats. Uterine phospholipase A₂ (PLA₂) activity is increased on days 5 (day of ovoimplantation) and 6, compared to preimplantation days (3 and 4). This enhanced activity might be responsible for the observed increase in uterine PGE and PGF_2α production observed on day 5 of pregnancy, which induces endometrial vascular permeability and decidualization. When embryo access to the uterus is impaired, the increase of PG production is suppressed. During postimplantation, PGE levels return to preimplantation values, while PGF_2α decreased with respect to preimplantation values. Uterine NOS activity is also increased on day 4 and reaches a maximum on day 5, with a profile similar to PGE and PGF_2α Dexamethasone administered in vivo decreased uterine NOS activity on day 4 of pregnancy but not on day 5, suggesting the presence of at least two types of NOS enzymes in the early days of pregnancy. A competitive inhibitor of NOS, L-NAME (600 and 1000 μM) induced a decrease in PGE and PGF_2α production in uterine tissue on day 5 of pregnancy. These results suggest the existence of a physiologically relevant nitridergic system which modulates prostaglandin production in the rat uterus during embryonic implantation.     

Inhibition of uterine prostaglandin-F2α production by bovine conceptus secretory proteins

The effect of bovine conceptus secretory proteins (CSP) on uterine prostaglandin (PG)-F_2α production was evaluated in dairy cattle following injection of estradiol-17β. Intrauterine injections of dialyzed serum proteins (Control, n=5) or CSP (n=5) were administered from days 15 through 18 post-estrus. Following intrauterine treatments on day 18, all cows were injected with E₂ (3 mg) to stimulate uterine PGF_2α production. Plasma concentrations of progesterone (P₄) and 15-keto-13,14-dihydro-PGF_2α (PGFM) were determined by RIA. The PGFM responses following E₂ challenge were decreased (p<0.01) for cows receiving CSP versus serum proteins into the uterine lumen. Individual PGFM, P₄ and cycle length responses are discussed. Data suggest that proteins secreted by the bovine conceptus suppress uterine PGF_2α production during pregnancy recognition in the cow.     

Corpus luteum function in the ewe: Effect of PGF2α and prostaglandin synthetase inhibitors

A series of experiments were conducted to evaluate the effects of mode and frequency of administration and estrous cycle stage on the response of the cycling ewe to PGF_2α. The effects of dexamethasone, arachadonic acid and prostaglandin synthetase inhibitors on estrous cycle length and plasma progesterone levels were also determined.Intramuscular administration of 5 or 10 mg of PGF_2α, on days 8 and 9 after estrus (5 ewes/group), significantly (p<.01) shortened the mean length of the estrous cycle and the interval from the end of treatment to estrus. Mean plasma progesterone levels, 24 hours after initial injection, were significantly (p<.01) lowered. When administered on day 8 only, these doses were considerably less effective in shortening estrous cycle length or lowering plasma progesterone levels. Intravaginal administration of PGF_2α, by polyurethane tampon, was also largely ineffective.Treatment of ewes with 10 mg of PGF_2α i.m., on days 3 and 4 of the estrous cycle, resulted in a return to estrus in 2 days in 25% of the treated animals. Plasma progesterone levels of PGF_2α-treated ewes were significantly lower than controls on the second, third and fourth days after the start of dosing. It would appear that PGF_2α exerts a retarding effect on developing CL functionality.The prostaglandin synthetase inhibitors, aspirin, flufenamic acid and 1-p-chlorobenzylidene-2-methyl-5-methoxy-3-indenylacetic acid, were administered orally or parenterally for 16 days beginning on day 8 of the estrous cycle. These compounds failed to prolong estrous cycle length. Parenteral administration of dexamethasone did not result in PGF_2α release in the cycling ewe, at least not in quantities sufficient to induce luteolysis. The prostaglandin precursor, arachadonic acid, also was not luteolytic when given parenterally to cycling ewes.     

Changes in the metabolism of PGE1 and PGF2α in rat placenta during pregnancy

The metabolism of PGE₁ and PGF_2α were studied in an in vitro system using placentae from 11-day pregnant rats. PGE₁ was metabolized faster than PGF_2α. The same system was employed to study the quantitative metabolism of these prostaglandins at various stages of pregnancy in the rat. Results of these investigations showed that metabolism became maximal between days 9–12 and between days 15–22 of gestation. On days 12–15 of pregnancy, metabolism decreased, and was at its lowest point on day 14. Maximum prostaglandin metabolism during the sensitive period of days 9–12 of gestation may act as a protective device against the detrimental effects of prostaglandin. Possible correlation of prostaglandin regulation with hormonal balance is discussed.