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 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.     

Luteolysis, estrus and ovulation, and blood prostaglandin F after intramuscular administration of 15, 30 or 60 mg prostaglandin F2α

During diestrus in three consecutive estrous cycles, each of six heifers was given (im) 30 mg, 15 mg (twice at 6-hr intervals) and 60 mg prostaglandin F_2α (PGF_2α) tham salt. Neither the decline in blood progesterone, the increase in blood estradiol, the duration or the peak of the LH surge, the interval to onset of estrus, nor the interval to ovulation was affected significantly by dose of PGF_2α. Thus, relative to that after 30 mg PGF_2α im, two injections of 15 mg at 6-hr intervals or 60 mg PGF_2α did not hasten luteolysis. Thirty mg was an ample im dose of PGF_2α to cause luteolysis. Regardless of im dose of PGF_2α, blood PGF peaked at about 6.0 ng/ml within 10 minutes and returned to basal values (<1.0 ng/ml) within 90 minutes. In another trial, after a single iv injection of 5 mg PGF_2α, blood PGF peaked (25 ng/ml) within 5 minutes and returned to basal values within 15 minutes. During a 30-minute infusion (0.5 mg/minute) of PGF_2α, blood PGF plateaued at 29.5 ng/ml with a metabolic clearance rate of 17.0 liters per minute.     

Influence of perioestrous suprabasal progesterone levels on cycle length,oestrous behaviour and ovulation in heifers

In order to induce suprabasal plasma concentrations of progesterone after luteolysis and to determine their effect on oestrous behaviour and ovulation, heifers subcutaneously received silicone implants containing 2.5 (n = 4), 5 (n = 4), 6 (n = 3), 7.5 (n = 3) or 10 (n = 4) g of progesterone, or an empty implant (controls, n = 5) between days 8 and 25 of the cycle (ovulation designated Day 0). Growth of dominant follicles and time of ovulation were determined by ultrasound, and signs of oestrus were recorded and scored. Blood was collected at 2–4 h intervals from Days 15 to 27 and assayed for progesterone concentration. In all heifers, plasma concentrations of progesterone sharply decreased during Days 16–18. Control heifers had their lowest progesterone levels on Days 20.5 and 21, standing oestrus on Day 19.5 ± 0.4 (mean ± SEM), and ovulated on Day 20.7 ± 0.4. A similar pattern was observed in heifers treated with 2.5 and 5 g progesterone. Heifers treated with 6, 7.5 and 10 g of progesterone showed an extended (P < 0.05) interovulatory interval. Onset of prooestrus and time of maximum expression of signs of oestrus were not significantly different from those in controls. However, there was an absence of standing oestrus in most of the cases, signs of oestrus lasted longer (P < 0.05) and were weaker in intensity when doses increased. In these groups, the lowest progesterone concentrations were attained shortly after implant removal. Some heifers treated with 6 and 7.5 g of progesterone had standing oestrus and post oestrous bleeding as seen in the controls but ovulation occurred from Days 24.5 to 27. When plasma progesterone concentrations were over 1 nmol 1⁻¹, disturbed oestrus and delayed ovulation occurred. The extended period of prooestrus and oestrus and delayed ovulation were similar to that described in cases of repeat breeding. It is suggested that suprabasal plasma concentrations of progesterone, after luteolysis, may lead to asynchrony between onset of oestrus and ovulation and consequently be a cause of repeat breeding in cattle.     

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.     

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.     

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.     

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.     

Direct effect of PGF2α pulses on PRL pulses, based on inhibition of PRL or PGF2α secretion in heifers

The relationships between PRL and PGF_2α and their effect on luteolysis were studied. Heifers were treated with a dopamine-receptor agonist (bromocriptine; Bc) and a Cox-1 and -2 inhibitor (flunixin meglumine [FM]) to inhibit PRL and PGF_2α, respectively. The Bc was given (Hour 0) when ongoing luteolysis was indicated by a 12.5% reduction in CL area (cm²) from the area on Day 14 postovulation, and FM was given at Hours 0, 4, and 8. Blood samples were collected every 8-h beginning on Day 14 until Hour 48 and hourly for Hours 0 to 12. Three groups of heifers in ongoing luteolysis were used: control (n = 7), Bc (n = 7), and FM (n = 4). Treatment with Bc decreased (P < 0.003) the PRL concentrations averaged over Hours 1 to 12. During the greatest decrease in PRL (Hours 2-6), LH concentrations were increased. Progesterone concentrations averaged over hours were greater (P < 0.05) in the Bc group than in the controls. In the FM group, no PGFM pulses were detected, and PRL concentrations were reduced. Concentrations of PGFM were not reduced in the Bc group, despite the reduction in PRL. Results supported the hypothesis that a decrease (12.5%) in CL area (cm²) is more efficient in targeting ongoing luteolysis (63%) than using any day from Days 14 to ≥19 (efficiency/day, 10-24%). The hypothesis that PRL has a role in luteolysis was supported but was confounded by the known positive effect of LH on progesterone. The hypothesis was supported that the synchrony of PGFM and PRL pulses represents a positive effect of PGF_2α on PRL, rather than an effect of PRL on PGF_2α.     

Evaluation of models to induce low progesterone during the early luteal phase in cattle

Two experiments were designed to evaluate models for generation of low circulating progesterone concentrations during early pregnancy in cattle. In Experiment 1, 17 crossbred heifers (Bos taurus) were assigned to either prostaglandin F_2α (PGF_2α) administration on Days 3, 3.5, and 4 (PG3; n = 9) or to control (n = 8). Blood samples were collected from heifers from Days 1 to 9 for progesterone assay. Progesterone concentrations were decreased (P < 0.03) between 18 and 48 h after first PGF_2α treatment in heifers assigned to PG3 compared with that of controls. In Experiment 2, 39 crossbred heifers detected in estrus were inseminated (Day 0) and assigned to either (1) PGF_2α administration on Days 3, 3.5, and 4 (PG3; n = 10), (2) PGF_2α administration on Days 3, 3.5, 4, and 4.5 (PG4; n = 10), (3) Progesterone Releasing Intravaginal Device (PRID) insertion on Day 4.5 with PGF_2α administration on Days 5 and 6 (PRID + PGF_2α; n = 10), or (4) control (n = 9). Blood samples were collected daily until Day 15, and conceptus survival rate was determined at slaughter on Day 16. Progesterone concentrations during the sampling period in the PG3 and PG4 groups did not differ but were less than that of controls (P < 0.01). After an initial peak, progesterone concentrations in the PRID + PGF_2α group were similar to that of controls. More heifers in the PG4 group (6 of 10) had complete luteal regression than did those in the PG3 group (3 of 10). Conceptus survival rate on Day 16 did not differ between groups. There was a significant correlation between progesterone concentration on Days 5 and 6 and conceptus size on Day 16. In summary, treatment with PGF_2α on Days 3, 3.5, and 4 postestrus appeared to provide the best model to induce reduced circulating progesterone concentrations during the early luteal phase in cattle.     

Dose and time dependent luteotropic and luteolytic action of prostaglandin F2α in the luteinized rabbit ovary

The mechanism of stimulatory and inhibitory action of PGF_2α on ovarian steroidogenesis both under and conditions has been studied in the pseudo-pregnant rabbits. Short term incubation of the ovaries with PGF_2α (2.82 × 10⁻⁵M) resulted in an increased synthesis of progesterone and 20α-OH P. The addition of PGF_2α in the medium and further incubation of the ovaries obtained from rabbits that had been constantly infused with PGF_2α (0.5 μg/min.) for two hours resulted in increased synthesis of these progestins. The ratio of progesterone to 20α -OH P was also enhanced under these conditions and thus supported the luteotropic action of small doses of PGF_2α under short term incubations. However, as the amount of PGF_2α infused was increased to 5 μg/min., the addition of PGF_2α under conditions strikingly decreased the production of these progestins. The ratio of progesterone to 20α -OH P was also decreased and thus was indicative of luteolytic action of higher doses of PGF_2α. High doses of PGF_2α (5.64 × 10⁻⁴M) failed to I cause any significant change in the progestin synthesis under short term incubation. These results thus suggest that the luteotropic and luteolytic action of PGF_2α in the luteinized rabbit ovary is dose and time dependent.     

Failure of exogenous LH to prevent PGF2α-induced luteolysis in beef cows

Henderson and McNatty (Prostaglandins 9:779, 1975) proposed that LH from the preovulatory LH surge attached to receptors on luteal cells and that this attachment might protect the early corpus luteum from PGF_2α induced luteolysis. To test this hypothesis, experiments were performed on heifers at day 10–12 of the cycle. Both jugular veins were catheterized and infusions of either saline (0.64 ml/min) or LH-NIH-B9 (10 μg/min; 0.64 ml/min) were given. Saline infusions were from 0–12 h; LH infusions were for 10 h and were preceded by a 2 h saline infusion. All animals were given 25 mg PGF_2α im at 6 h (6 h into the saline infusion and 4 h into the LH infusion). Blood samples were taken at 0.5 h, 1 h and 4 h intervals from 0–12, 13–18 h and 22–24 h respectively. Serum was assayed for LH and progesterone by radioimmunoassay methods. Two animals received saline and two received LH in each experiment. Eact treatment was replicated 6 times. LH infusion resulted in a mean serum LH of 57 ng/ml compared to 0.90 ng/ml in saline infused animals. This elevation of LH did not alter PGF_2α induced luteolysis as indicated by decline in serum progesterone. This experiment does not support the hypothesis that the newly formed corpus luteum is resistant to PGF_2α because of protection afforded by the protestrus LH surge.     

Effect of in vivo prostaglandin treatment on H-PGF2α uptake in hamster corpora lutea

Pregnant hamsters were administered (SC) prostaglandin or vehicle on the morning of the 4th day of pregnancy. Serum progesterone was significantly depressed (p<.01) at 0.5, 2, and 6 hours after treatment with 100 μg PGF_2α. Serum progesterone levels were unchanged 2 hours and 6 hours after treatment with 100 μg PGF_2β and 2 hours after treatment with 1 mg PGF_2β. Progesterone levels were depressed to less than 1 ng/ml 6 hours after treatment with 1 mg PGF_2β. The specific uptake of ³H-PGF_2α in whole hamster corpora lutea was significantly depressed 2 hours and 6 hours following 100 μg PGF_2α treatment. A 15% depression in specific uptake occurred 0.5 hour post-treatment. Treatment with 100 μg PGF_2β resulted in no change. Administration of 1 mg PGF_2β resulted in depressed ³H-PGF_2α uptake at both 2 and 6 hours post-treatment.Prostacyclin (PGI₂) treatment resulted in no change in either ³H-PGF_2α specific uptake or serum progesterone 2 hours after 100 μg treatment SC. These parameters were both reduced approximately 30% 6 hours post-treatment. Treatment with 6-keto-PGF_1α resulted in a complete lack of measurable ³H-PGF_2α uptake and serum progesterone levels less than 1 ng/ml at both 2 and 6 hours after treatment with 1 mg SC.     

Prostaglandin biosynthesis by the rat uterus during the oestrus cycle, temporal correlation with plasma oestradiol and progesterone concentrations, identification of 6 keto PGF1α as the major metabolite

Prostaglandin biosynthesis was studied in the rat uterus during the oestrous cycle. Uterine homogenates were incubated for 20 minutes in the presence of exogenous substrate (2.10⁻⁵M). PGF_2α and PGE₂ were measured by R.I.A.. A sharp peak PGF_2α and a smaller peak of PGE₂ were observed at prooestrus, 20 h. Another small PGE₂ peak occurred at dioestrus II, 15 h. The lowest values of both PGs were found on dioestrus, 15 h. Plasma oestradiol concentration were highest at proestrus, 15 h and 20 h. A sharp progesterone peak occurred at prooestrus, 20 h. The PGF_2α peak is next to the oestradiol peak and is superimposable or lags slightly beyond the progesterone peak.Incubation with ¹⁴C arachidonic acid and subsequent analysis of extracts by TLC and scanning showed that the major metabolite is PGI₂, identified as 6 keto PGF_1α. The conversion rate of arachidonic acid into 6 keto PGF_1α is 5 times higher than into PGF_2α. 6 keto PGF_1α was further identified by GC/MS. No significant difference was observed between 6 keto PGF_1α production during oestrus and dioestrus.     

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.     

Effect of estradiol-17β on peripheral plasma concentration of 15-keto,14-dihydro PGF2α and luteolysis in cyclic cattle

Friesian heifers (n = 10) were assigned randomly to receive an intravenous injection of estradiol-17^β (E₂; 3 mg) or saline: ethanol vehicle solution (6 ml; 1:1) on day 13 of the estrous cycle. Blood was collected collected from the jugular vein by venipuncture into heparinized vacutainer tubes at 30 minute intervals for 2 hours (h) preinjection, 10.5 h postinjection and then at 3 h intervals until estrus. Repeated hormone measurements of 15-keto-13,14-dihydro-PGF_2α (PGFM) and progesterone (P₄) were evaluated by split-plot analysis of variance. Mean concentration of PGFM for the 12.5 h acute sampling phase was 164.1 ± .14 pg/ml. A treatment by time interaction was detected (P < .01). After treatment with E₂, PGFM concentrations began to increase at approximately 3.5 h, reached a mean peak of 330.4 ± 44.5 pg/ml (n = 5) at 5.5 ± .3 h, and returned to basal concentration by 9.0 ± .6 h. Vehicle treatment did not alter concentrations of PGFM. Injections of E₂ on day 13 of the estrous cycle caused luteolysis (P₄ concentration < 1 ng/ml) to occur earlier following injection (96.9 ± 10.6 h < 153.6 ±17.7 h; P, 0.05) than did the vehicle control treatment. During the chronic sampling phase of 3 h intervals, 39 of 606 samples (6.4%) were classified as PGFM spikes (323.0 ± 50.0 pg/ml); 21 (53%) of the spikes occurred at a mean interval of 18.9 ± 3.86 h before the time of completed luteolysis. Exogenous E₂ induced an acute increase in PGFM that may be indicative of uterine PGF_2α production. Peaks of PGFM in plasma were temporally associated with luteolysis on a within cow basis.     

Prostaglandin release from the guinea-pig uterus superfused in vitro. Effect of stage of estrous cycle, progesterone, estradiol, oxytocin and A23187

Prostaglandin (PG) E₂ was the major PG released from the superfused guinea-pig uterus on Day 7, followed by in descending order 6-oxo-PGF_1α, thromboxane (TX) B₂ and PGF_2α. However, the outputs of all four substances were low and were very similar. By Day 15, PGF_2α output from the superfused uterus had increased 21.9-fold, whereas the outputs of PGE₂, 6-oxo-PGF_1α and TXB₂ had increased only 1.8-, 2.9- and 1.2-fold, respectively. A mechanism is apparently “switched on” between Days 7 and 15 which causes a fairly specific increase in the release of PGF_2α from the uterus.Progesterone and/or estradiol had no effect on PG or TX release when superfused over the uterus on Day 7, nor did they have any effect on PG and TX release from the Day 15 uterus when administered separately. When administered together, however, they significantly inhibited PGF_2α, PGE₂ and 6-oxo-PGF_1α, but not TXB₂, release from the Day 15 uterus. Oxytocin had no effect on PG release from the Day 7 or Day 15 uterus, while A23187 stimulated PGF_2α, 6-oxo-PGF_1α and, to a lesser extent, PGE₂ release from the uterus on both Days 7 and 15 Oxytocin is apparently not important for stimulating PGF_2α release from the guinea-pig uterus in relation to luteolysis, whereas increasing intracellular free Ca⁺⁺ levels may be part of the mechanism for “switching on” uterine PG synthesis. Furthermore, changes in intracellular free Ca⁺⁺ levels in the endometrium may be responsible for the pulsatile nature of PGF_2α release from the uterus.     

Effect of induced suprabasal progesterone levels around estrus on plasma concentrations of progesterone, estradiol-17beta and LH in heifers

A controlled study was carried out to investigate the effects of suprabasal plasma progesterone concentrations on blood plasma patterns of progesterone, LH and estradiol-17beta around estrus. Heifers were assigned to receive subcutaneous silicone implants containing 2.5 g (n=4), 5 g (n=4), 6 g (n=3), 7.5 g (n=3) or 10 g (n=4) of progesterone, or implants without hormone (controls, n=5). The implants were inserted on Day 8 of the cycle (Day 0=ovulation) and left in place for 17 d. The time of ovulation was determined by ultrasound scanning. Blood was collected daily from Days 0 to 14 and at 2 to 4-h intervals from Days 15 to 27. Control heifers had the lowest progesterone concentrations on Days 20.5 to 21 (0.5 +/- 0.1 nmol L(-1)); a similar pattern was observed in heifers treated with 2.5 and 5 g of progesterone. In the same period, mean progesterone concentrations in the heifers treated with 6, 7.5 and 10 g were larger (P < 0.05) than in the controls, remaining between 1 and 2.4 nmol L(-1) until implant removal. A preovulatory estradiol increase started on Days 16.4 to 18.4 in all the animals. In the controls and in heifers treated with 2.5 and 5 g of progesterone, estradiol peaked and was followed by the onset of an LH surge. In the remaining treatments, estradiol release was prolonged and increased (P < 0.05), while the LH peak was delayed (P < 0.05) until the end of the increase in estradiol concentration. The estrous cycle was consequently extended (P < 0.05). In all heifers, onset of the LH surge occurred when progesterone reached 0.4 to 1.2 nmol L(-1). The induction of suprabasal levels of progesterone after spontaneous luteolysis caused endocrine asynchronies similar to those observed in cases of repeat breeding. It is suggested that suprabasal concentrations of progesterone around estrus may be a cause of disturbances oestrus/ovulation.     

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.