Prostaglandin F2alpha specific binding in equine corpora lutea

A century of research on the uteroovarian pathway for uterine-induced luteolysis in mammals

Year 2023 is the 100-year anniversary of the discovery in guinea pigs that the lifespan of the corpus luteum (CL) is controlled by the uterus. The CL is the gatekeeper between two fundamental reproductive events - the estrous cycle and pregnancy. Uteroluteal research for the initial 33 years was productive but limited to laboratory species until the inclusion of farm animals in 1956. In the early 1960s, it was found that uterine luteolysin in sows travels unilaterally from a uterine horn to the adjacent CL which likely accounted for the heyday of uteroluteal research in the 1960s-70s. The luteolytic properties of PGF2α were demonstrated in rats in 1969. In 1971, (1) surgical separation of the lengthwise adherence between the uteroovarian vein and ovarian artery interfered with luteolysis in ewes, (2) species with primarily unilateral vs systemic uterine-induced luteolysis have a strong vs an absent or weak unilateral venoarterial transfer pathway, and (3) vascular infusions identified PGF2α as a uterine luteolysin. Vascular and PGF2α studies were beginning to merge. In 1973, a venoarterial pathway was firmly demonstrated in ewes and later in heifers by surgical anastomosis of a uterine vein or ovarian artery from a uterine-intact side to the corresponding vessel on the unilaterally hysterectomized side. More recent studies described how prostaglandins likely transfer through the walls of uterine and ovarian vessels using concentration gradients in sows and a prostaglandins transporter system in cows.

Endometrial and luteal responses to a prostaglandin F2alpha pulse: A comparison between heifers and mares

In heifers and mares, multiple pulses of prostaglandin F2alpha (PGF) are generally associated with complete luteal regression. Although PGF pulses occur before and during luteolysis, little is known about the role of minor PGF pulses during preluteolysis on subsequent luteal and endometrial PGF production that may initiate luteolysis. Heifers (n = 7/group) and mares (n = 6/group) were treated with a single minor dose of PGF (3.0 and 0.5 mg, respectively) during mid-luteal phase (12 and 10 days postovulation in heifers and mares, respectively). After treatment, a transient decrease in progesterone (P4) concentrations occurred in heifers between Hours 0–2 but at Hour 4 P4 was not different from pre-treatment. In mares, P4 was unaltered between Hours 0 and 4. Concentrations of P4 decreased in both species by Hour 24 and complete luteolysis occurred in mares by Hour 48. Luteal and endometrial gene expression were evaluated 4 hours post-treatment. In heifers, luteal mRNA abundance of PGF receptor and PGF dehydrogenase were decreased while PTGS2, PGF transporter, and oxytocin receptor were increased. In the heifer endometrium, receptors for oxytocin, P4, and estradiol were upregulated. In mares, luteal expression of PGF receptor was decreased while PGF transporter and oxytocin receptor were increased. The decrease in P4 between Hours 4 and 24 and changes in gene expression were consistent with upregulation of endogenous synthesis of PGF. The hypotheses were supported that a single minor PGF treatment upregulates endogenous machinery for PGF synthesis in heifers and mares stimulating endogenous PGF synthesis through distinct regulatory mechanisms in heifers and mares.

Early Pregnancy in Jennies in the Caribbean: Corpus Luteum Development and Progesterone Production, Uterine and Embryo Dynamics, Conceptus Growth and Maturation

Simple Summary

An understanding of the basic mechanisms of reproduction in donkeys is essential, for several reasons. Some donkey breeds are threatened or endangered, and efforts to save these species depend on improved knowledge of reproductive processes. In some parts of the world, donkeys continue to be valued for purposes of work, recreation, or even meat or milk production, as well as the breeding of mules, and reproduction is essential to maintain suitable populations. In others, donkey populations have become feral and represent a nuisance or even a danger to human populations, and improved contraceptive methods are required. Whether for enhancing or inhibiting reproduction, species-specific information is valuable. While the mare has been extensively studied, few studies have explored early pregnancy in jennies. Therefore, this study characterized early embryo development and differences in progesterone profile and changes in the corpus luteum between pregnant and non-pregnant jennies.

Abstract

We aimed to characterize early embryo development and changes in corpus luteum (CL) development and progesterone profile in pregnant vs. non-pregnant jennies. Eight jennies were enrolled in the study. In the first two cycles, the jennies were monitored by transrectal ultrasonography and had blood harvested for hormone profile assay. In the third cycle, jennies were bred by a jack of proven fertility. Jennies were then monitored and sampled for up to 30 days of pregnancy. Data were evaluated by random-effects multiple linear regression, and correlations were expressed as Pearson’s correlation coefficient. Progesterone concentration rose rapidly from ovulation (D0) until D7, plateaued until D12–14, then precipitously declined between D14 and 15, remaining low until the next ovulation in non-pregnant cycles. In the pregnant jennies, the progesterone concentration rose to maximal concentrations on D7–11, being higher at this stage than in non-pregnant cycles, then declined gradually up to D30. In all cycles, the volume of the CL increased steadily until D6, when it plateaued in pregnant jennies. For non-pregnant jennies, CL volume decreased slowly from D6 to D11 and then had a faster drop. Uterine tone increased following ovulation, becoming turgid around the day of embryo fixation (D15.0 ± 0.9). An embryonic vesicle (EV) was first detected on D9.3 ± 0.5 (2.4 ± 0.5 mm). The EV remained spherical until D18.6 ± 1.4. The embryo proper was first detected ventrally in the vesicle on D20.8 ± 1.1 and the embryonic heartbeat by D22.0 ± 0.9. The allantoic sac was identified at D24.0 ± 0.9, and at D30, the allantoic sac filled the ventral half of the EV. This study provides evidence that higher cumulative concentrations of progesterone are correlated to size of the EV, and there were changes in the luteal dynamics and progesterone profiles in pregnant vs. non-pregnant jennies.

Endogenous and exogenous effects of PGF2α during luteolysis in mares

An inhibitor of PGF2α biosynthesis (flunixin meglumine, FM) was used to study the role of endogenous PGF2α on the luteolytic effect of exogenous PGF2α in mares. A 2-h infusion of PGF2α at a constant rate (total dose, 0.1 mg) on Day 10 (ovulation = Day 0) was used to mimic the maximal concentrations of a spontaneous pulse of a PGF2α metabolite (PGFM). Treatment with FM (1.7 mg/kg) was done 1 h before and 5 h after the start of PGF2α infusion. In hourly blood samples beginning 1 h before the start of PGF2α infusion, progesterone decreased (P < 0.05) similarly by 5 h in each of the PGF2α and PGF2α+FM groups but not in the controls (n = 5). In a study of spontaneous luteolysis, the same FM dose was given every 6 h from Day 13 until Day 17 or earlier if CL regression was indicated by an 80% decrease in luteal blood-flow signals. Blood was sampled for progesterone assay each day and 8 h of hourly blood sampling was done each day to characterize PGFM concentrations and pulses. Progesterone (P4) was lower (P < 0.05) in controls than in an FM group (n = 7) by Day 15. Luteolysis (P4 < 1 ng/mL) ended on Days 14-19 in individual controls. In contrast, luteolysis did not end until after Day 20 in 4 of 7 FM-treated mares. In the three mares with completion of luteolysis before Day 20 in the FM group, the interval from beginning to end of luteolysis was longer (P < 0.02) (4.5 ± 0.6 days) than in the controls (3.0 ± 0.4 days). During 8-h sessions of hourly blood sampling on Day 14, concentration of PGFM was significantly lower in the FM group for the minimal, mean, and maximal per session. Pulses of PGFM were identified by a CV methodology on each day in 7 of 7 and 3 of 7 mares in the controls and FM group, respectively. The four FM-treated mares without a CV-identified pulse were the four mares in which luteolysis did not occur before Day 20. In mares with detected pulses, PGFM was lower at each nadir and at the peak (86% lower) in the FM group than in controls, but the interval between nadirs or base of a pulse was not different between groups. Hypothesis 1 that endogenous PGF plays a role in the luteolytic effect of exogenous PGF2α was not supported. Hypothesis 2 that an inhibitor of PGF2α biosynthesis prevented or minimized the prominence of PGFM pulses and increased the frequency of persistent CL was supported.

Luteolysis and the Auto-, Paracrine Role of Cytokines From Tumor Necrosis Factor α and Transforming Growth Factor β Superfamilies

Successful pregnancy establishment demands optimal luteal function in mammals. Nonetheless, regression of the corpus luteum (CL) is absolutely necessary for normal female cyclicity. This dichotomy relies on intricate molecular signals and rapidly activated biological responses, such as angiogenesis, extracellular matrix (ECM) remodeling, or programmed cell death. The CL establishment and growth after ovulation depend not only on the luteinizing hormone-mediated endocrine signal but also on a number of auto-, paracrine interactions promoted by cytokines and growth factors like fibroblast growth factor 2, vascular endothelial growth factor A, and tumor necrosis factor α (TNF), which coordinate vascularigenesis and ECM reorganization as well as steroidogenesis. With the organ fully developed, the release of the uterine prostaglandin F2α activates luteolysis, an intricate process supported by intraluteal interactions that ensure the loss of steroidogenic function (functional luteolysis) and the involution of the organ (structural luteolysis). This chapter provides an overview of the local action of cytokines during luteal function, with particular emphasis on the role of TNF and transforming growth factor β superfamilies during luteolysis.

Evidence for a PGF2α auto-amplification system in the endometrium in mares

In mares, prostaglandin F2α (PGF2α) secreted from the endometrium is a major luteolysin. Some domestic animals have an auto-amplification system in which PGF2α can stimulate its own production. Here, we investigated whether this is also the case in mares. In an in vivo study, mares at the mid-luteal phase (days 6-8 of estrous cycle) were injected i.m. with cloprostenol (250 µg) and blood samples were collected at fixed intervals until 72 h after treatment. Progesterone (P4) concentrations started decreasing 45 min after the injection and continued to decrease up to 24 h (P < 0.05). In turn, 13,14-dihydro-15-keto-PGF2α (PGFM) metabolite started to increase 4h after an injection and continued to increase up to 72 h (P < 0.05). PGF receptor (PTGFR) mRNA expression in the endometrium was significantly higher in the late luteal phase than in the early and regressed luteal phases (P < 0.05). In vitro, PGF2α significantly stimulated (P < 0.05) PGF2α production by endometrial tissues and endometrial epithelial and stromal cells and significantly increased (P < 0.05) the mRNA expression of prostaglandin-endoperoxide synthase-2 (PTGS2), an enzyme involved in PGF2α synthesis in endometrial cell. These findings strongly suggest the existence of an endometrial PGF2α auto-amplification system in mares.

Pitfalls in animal reproduction research: how the animal guards nature's secrets

The estrous cycles of heifers and mares are used for illustrating pitfalls at the animal level in research in reproductive biology. Infrequent monitoring for characterizing the change in hormone concentrations or for detecting a reproductive event can be a pitfall when the interval for obtaining data exceeds the interval between events. For example, hourly collection of blood samples has shown that the luteolytic period (decreasing progesterone) encompasses 24 hours in heifers and mares. Collection of samples every 6-24 hours results in the illusion that luteolysis requires 2-3 days, owing to the occurrence of luteolysis on different days in individuals. A single treatment with PGF2α that causes complete regression of the corpus luteum is an example of an overdose pitfall. A nonphysiological progesterone increase occurs and will be misleading if used for making interpretations on the nature of luteolysis. A pitfall can also occur if a chosen reference point or end point is a poor representation of a physiological event. For example, if on a selected day after ovulation the animals in treatment A are closer on average to luteolysis than animals in treatment B, treatment A will appear to have had an earlier luteolytic effect. Among the techniques that are used directly in the animal, ultrasonography appears to be most prone to research pitfalls. Research during a given month can be confounded by seasonal effects, even in species that ovulate throughout the year. The presence of unknown factors or complex interactions among factors and the sensitivity of the animal to a research procedure separate from the direct effect of a treatment are also research challenges. A hidden factor should be considered nature's challenge to open-minded biologists but a pitfall for the close-minded.

Plasma clearance and half-life of prostaglandin F2alpha: a comparison between mares and heifers

Horses are about five times more sensitive to the luteolytic effect of prostaglandin F2alpha (PGF) than cattle, as indicated by a recommended clinical dose of 5 mg in horses and 25 mg in cattle. Novel evaluations of the PGF plasma disappearance curves were made in mares and in heifers, and the two species were compared. Mares and heifers (n = 5) of similar body weight were injected (Min 0) intravenously with PGF (5 mg per animal). Blood was sampled every 10 sec until Min 3, every 30 sec until Min 5, every 10 min until Min 60, and every 30 min until Min 240. The mean PGF concentration was greater (P < 0.05) in mares than in heifers at Min 1 through Min 60 and at Mins 180 and 240. The mean time to maximum PGF concentration was not different between mares (42.0 ± 8.6 sec) and heifers (35.0 ± 2.9 sec). The apparent plasma clearance, distribution half-life, elimination half-life, and maximum plasma PGF concentration were 3.3 ± 0.5 L h(-1) kg(-1), 94.2 ± 15.9 sec, 25.9 ± 5.0 min, and 249.1 ± 36.8 ng/ml, respectively, in mares and 15.4 ± 2.3 L h(-1) kg(-1), 29.2 ± 3.9 sec, 9.0 ± 0.9 min, and 51.4 ± 22.6 ng/ml, respectively, in heifers. Plasma clearance was about five times less (P < 0.0005), maximum plasma PGF concentration was five times greater (P < 0.002), and the distribution half-life and elimination half-life were about three times longer (P < 0.005) in mares than in heifers. The fivefold greater plasma clearance of PGF in heifers than in mares corresponds to the recommended fivefold greater clinical dose of PGF in cattle and supported the hypothesis that the metabolic clearance of PGF is slower in mares than heifers.

Dynamics of circulating progesterone concentrations before and during luteolysis: a comparison between cattle and horses

The profile of circulating progesterone concentration is more dynamic in cattle than in horses. Greater prominence of progesterone fluctuations in cattle than in horses reflect periodic interplay in cattle between pulses of a luteotropin (luteinizing hormone; LH) and pulses of a luteolysin (prostaglandin F2alpha; PGF2alpha). A dose of PGF2alpha that induces complete regression of a mature corpus luteum with a single treatment in cattle or horses is an overdose. The overdose effects on the progesterone profile in cattle are an immediate nonphysiological increase taking place over about 30 min, a decrease to below the original concentration, a dose-dependent rebound 2 h after treatment, and a progressive decrease until the end of luteolysis. An overdose of PGF2alpha in horses results in a similar nonphysiological increase in progesterone followed by complete luteolysis; a rebound does not occur. An overdose of PGF2alpha and apparent lack of awareness of the rebound phenomenon has led to faulty interpretations on the nature of spontaneous luteolysis. A transient progesterone suppression and a transient rebound occur within the hours of a natural PGF2alpha pulse in cattle but not in horses. Progesterone rebounds are from the combined effects of an LH pulse and the descending portion of a PGF2alpha pulse. A complete transitional progesterone rebound occurs at the end of preluteolysis and the beginning of luteolysis and returns progesterone to its original concentration. It is proposed that luteolysis does not begin in cattle until after the transitional rebound. During luteolysis, rebounds are incomplete and gradually wane. A partial rebound during luteolysis in cattle is associated with a concomitant increase in luteal blood flow. A similar increase in luteal blood flow does not occur in mares.

Regression and resurgence of the CL following PGF2α treatment 3 days after ovulation in mares

The present study was designed to characterize and compare the physiology and ultrasonographic morphology of the corpus luteum (CL) during regression and resurgence following a single dose of native prostaglandin F2alpha (PGF) given 3 days after ovulation, with a more conventional treatment given 10 days after ovulation. On the day of pre-treatment ovulation (Day 0), horse mares were randomly assigned to receive PGF (Lutalyse; 10 mg/mare, i.m.) on Day 3 (17 mares) or Day 10 (17 mares). Beginning on either Days 3 or 10, follicle and CL data and blood samples were collected daily until post-treatment ovulation. Functional and structural regression of the CL in response to PGF treatment were similar in both the Day 3 and 10 groups, as indicated by an abrupt decrease in circulating concentrations of progesterone, decrease in luteal gland diameter and increase in luteal tissue echogenicity. As a result, the mean +/- S.E.M. interovulatory interval was shorter (P < 0.0001) in the Day 3 group (13.2 +/- 0.9 days) than in the Day 10 group (19.2 +/- 0.7 days). Within the Day 3 group, functional resurgence of the CL was detected in 75% of the mares (12 of 16) beginning 3 days after PGF treatment, as indicated by transient major (6 mares) and minor (6 mares) increases (P < 0.05 and < 0.1, respectively) in progesterone. Correspondingly, mean length of the interovulatory interval was longer (P < 0.03) in mares with major resurgence (15.8 +/- 1.6 days) than in mares with minor (11.2 +/- 1.2 days) and no resurgences (13.5 +/- 0.3 days) in progesterone. Structural resurgence of the CL in the Day 3 group and functional and structural resurgence in the Day 10 group were not detected. In conclusion, PGF treatment 3 days after ovulation resulted in structural and functional regression of the CL and hastened the interval to the next ovulation, despite post-treatment resurgences in progesterone.

Induction of luteolysis in mares by ultrasound-guided intraluteal treatment with PGF2alpha

To evaluate the technique of ultrasound-guided luteal injection in mares, PGF2alpha was administered under ultrasound guidance to horse mares (n = 7 to 9 per group) on Day 9 postovulation via either a systemic (i.m.; zero, 0.01, 0.1, or 5 mg/dose) route or a local intraluteal (i.l.; zero, 0.01 or 0.1 mg/dose) route. The luteolytic efficacy of each treatment was determined based on post-treatment decreases in progesterone concentration, interval to uterine edema (IE) and interovulatory interval (IOI). Local administration of PGF2alpha directly into the CL consistently induced luteolysis, at doses up to 50-fold lower than the lowest effective systemic dose. Significant decreases in IOI and IE occurred in mares treated with 5 mg PGF2alpha i.m. or 0.1 mg PGF2alpha i.l., but did not occur in mares treated with 0.1 or 0.01 mg PGF2alpha i.m., 0.01 mg PGF i.l., vehicle i.l. or vehicle i.m.. Progesterone concentrations were reduced to less than 10% of pretreatment values by two days post treatment in mares treated with 5 mg PGF2alpha i.m. or 0.1 mg PGF2alpha i.l.. PGF2alpha doses of 0.1 mg i.m. and 0.01 mg i.l. were associated with smaller but significant progesterone decreases (to 66% and 46% of pre-treatment values, respectively) by two days post treatment. Progesterone values after administration of i.l. vehicle did not differ from pre-treatment values by two days post treatment, but were significantly lower (53% of pre-treatment values) by four days post treatment. Intramuscular treatment with vehicle or 0.01 mg of PGF2alpha did not significantly reduce progesterone concentrations below pretreatment values. Overall, the minimum effective luteolytic dose of PGF2alpha given intraluteally was between 0.01 and 0.1 mg. Based on the results of this study, ultrasound-guided i.l. injection appears to be a repeatable method for studying the direct effect of other chemicals on luteal function. However, the current procedure carries some risk, since three i.l. injections were associated with ovarian abscesses.

Effect of PGF2alpha and 13,14-dihydro-15-keto-PGF2alpha (PGFM) on corpora luteal function in nonpregnant mares

The objective of this study was to determine if the primary circulating metabolite of PGF2alpha, 13,14-dihydro-15-keto-PGF2alpha (PGFM), is biologically active and would induce luteolysis in nonpregnant mares. On Day 9 after ovulation, mares (n = 7/group) were randomly assigned to receive: 1) saline control, 2) 10 mg PGF2alpha or 3) 10 mg PGFM in 5 mL 0.9% sterile saline i.m. On Days 0 through 16, blood was collected for progesterone analysis. In addition, blood was collected immediately prior to treatment, hourly for 6 h, and then at 12 and 24 h after treatment for progesterone and PGFM analysis; PGFM was measured to verify that equivalent amounts of hormone were administered to PGF2alpha- and PGFM-treated mares. Mares were considered to have undergone luteolysis if progesterone decreased to < or = 1.0 ng/mL within 24 h following treatment. Luteolysis was induced in 0/7 control, 7/7 PGF2alpha-treated, and 0/7 PGFM-treated mares. There was no difference (P>0.1) in the occurrence of luteolysis in control and PGFM-treated mares. More (P<0.001) PGF2alpha-treated mares underwent luteolysis than control or PGFM-treated mares. There was no difference (P>0.1) in progesterone concentrations between control and PGFM-treated mares on Days 10 through 16. Progesterone concentrations were lower (P<0.01) on Days 10 through 14 in PGF2alpha-treated compared with control and PGFM-treated mares. There was no difference (P>0.05) in PGFM concentrations between PGF2alpha- and PGFM-treated mares; PGFM concentrations in both groups were higher (P<0.001) than in control mares. These results do not support the hypothesis that PGFM is biologically active in the mare, since there was no difference in corpora luteal function between PGFM-treated and control mares.

Luteolysis: a neuroendocrine-mediated event

In many nonprimate mammalian species, cyclical regression of the corpus luteum (luteolysis) is caused by the episodic pulsatile secretion of uterine PGF2alpha, which acts either locally on the corpus luteum by a countercurrent mechanism or, in some species, via the systemic circulation. Hysterectomy in these nonprimate species causes maintenance of the corpora lutea, whereas in primates, removal of the uterus does not influence the cyclical regression of the corpus luteum. In several nonprimate species, the episodic pattern of uterine PGF2alpha secretion appears to be controlled indirectly by the ovarian steroid hormones estradiol-17beta and progesterone. It is proposed that, toward the end of the luteal phase, loss of progesterone action occurs both centrally in the hypothalamus and in the uterus due to the catalytic reduction (downregulation) of progesterone receptors by progesterone. Loss of progesterone action may permit the return of estrogen action, both centrally in the hypothalamus and peripherally in the uterus. Return of central estrogen action appears to cause the hypothalamic oxytocin pulse generator to alter its frequency and produce a series of intermittent episodes of oxytocin secretion. In the uterus, returning estrogen action concomitantly upregulates endometrial oxytocin receptors. The interaction of neurohypophysial oxytocin with oxytocin receptors in the endometrium evokes the secretion of luteolytic pulses of uterine PGF2alpha. Thus the uterus can be regarded as a transducer that converts intermittent neural signals from the hypothalamus, in the form of episodic oxytocin secretion, into luteolytic pulses of uterine PGF2alpha. In ruminants, portions of a finite store of luteal oxytocin are released synchronously by uterine PGF2alpha pulses. Luteal oxytocin in ruminants may thus serve to amplify neural oxytocin signals that are transduced by the uterus into pulses of PGF2alpha. Whether such amplification of episodic PGF2alpha pulses by luteal oxytocin is a necessary requirement for luteolysis in ruminants remains to be determined. Recently, oxytocin has been reported to be produced by the endometrium and myometrium of the sow, mare, and rat. It is possible that uterine production of oxytocin may act as a supplemental source of oxytocin during luteolysis in these species. In primates, oxytocin and its receptor and PGF2alpha and its receptor have been identified in the corpus luteum and/or ovary. Therefore, it is possible that oxytocin signals of ovarian and/or neural origin may be transduced locally at the ovarian level, thus explaining why luteolysis and ovarian cyclicity can proceed in the absence of the uterus in primates. However, it remains to be established whether the intraovarian process of luteolysis is mediated by arachidonic acid and/or its metabolite PGF2alpha and whether the central oxytocin pulse generator identified in nonprimate species plays a mediatory role during luteolysis in primates. Regardless of the mechanism, intraovarian luteolysis in primates (progesterone withdrawal) appears to be the primary stimulus for the subsequent production of endometrial prostaglandins associated with menstruation. In contrast, luteolysis in nonprimate species appears to depend on the prior production of endometrial prostaglandins. In primates, uterine prostaglandin production may reflect a vestigial mechanism that has been retained during evolution from an earlier dependence on uterine prostaglandin production for luteolysis.

Identification of a single (FP) receptor associated with prostanoid-induced Ca2+ signals in Swiss 3T3 cells

Thus far, the prostanoid FP-receptor has been characterized only on the basis of agonist studies. It is currently classified as a receptor having particular sensitivity to prostaglandin F2 alpha (PGF2 alpha) but with the ability to recognize prostaglandins D2 and E2 (PGD2 and PGE2). We have re-examined this concept by studying second messenger Ca2+ signals to PGF2 alpha, PGD2 and PGE2, and performing radioligand binding studies in Swiss 3T3 cells. The same rank order of potency was obtained for both the Ca2+ transient signal and competition for PGF2 alpha binding sites. The potency rank order, PGF2 alpha > PGD2 > PGE2, was identical to that obtained from functional studies in isolated tissues, such as the cat iris. Additional support for the concept that PGF2 alpha, PGD2, and PGE2 interact with a single receptor to elicit a Ca2+ signal was provided by successive addition studies. Thus, cells pretreated with a supramaximal concentration of PGF2 alpha exhibited little or no response to subsequent administration of PGD2 or PGE2. Likewise, cells pretreated with a large concentration of PGD2 or PGE2 exhibited minimal responsiveness to successive addition of the corresponding alternative prostaglandins. Pretreatment with a maximally effective concentration of PGF2 alpha, PGD2, or PGE2 rendered the cells refractory to the FP-receptor selective agonist fluprostenol, which further supports the hypothesis that Ca2+ transient signals in response to prostanoids in Swiss 3T3 cells are mediated by the FP-receptor. The Ca2+ transient responses to PGF2 alpha, PGD2, and PGE2 also exhibited a similar modest reduction when extracellular Ca2+ was removed. Finally, the DP-receptor antagonist BW A868C did not block the Ca2+ transient response to PGD2, indicating an absence of DP-receptor involvement. Moreover, Ca2+ responses to the thromboxane A2 mimetic U-46619 were unaffected by the TP-antagonist BM 13505, which indicates no involvement of the TP-receptor. These results support the contention that the FP-receptor has particular sensitivity to PGF2 alpha but will also recognize PGD2 and PGE2.

Corpus luteal function in nonpregnant mares following intrauterine administration of prostaglandin E2 or estradiol-17β

The objective of this study was to test the hypothesis that intrauterine administration of prostaglandin E(2) (PGE(2)) or estradiol-17beta (E-17beta) would prolong CL function in nonpregnant mares. Nonpregnant mares were continuously infused with 240 mug/d of PGE(2), 6 mug/d of E-17beta, or vehicle (sham-treated) on Days 10 to 16 post ovulation (ovulation = Day 0), using osmotic minipumps surgically placed into the uterine lumen on Day 10 (n = 11 per group). Nonpregnant and pregnant mares served as negative and positive controls, respectively (n = 11 per group). Mares were defined as having prolonged CL function if plasma progesterone remained > 2.5 ng/ml and if ovulation did not occur on Days 9 to 30. Corpus luteal function was prolonged until Day 30 in 1 11 nonpregnant mares, 4 11 sham-treated mares, 6 11 E-17beta-treated mares, 8 11 PGE(2)-treated mares, and 11 11 pregnant mares. The incidence of prolonged CL function was similar (P=0.16) in the sham-treated and nonpregnant mares. The hypothesis that PGE(2) would prolong CL function in nonpregnant mares was supported, since the incidence of prolonged CL function was higher (P=0.003) in PGE(2)-treated versus nonpregnant mares, tended to be higher (P=0.09) in PGE(2)-versus sham-treated mares, and was not lower (P=0.11) in PGE(2)-treated versus pregnant mares. The hypothesis that E-17beta would prolong CL function in nonpregnant mares was not supported, since the incidence of prolonged CL function was not higher (P=0.34) in E-17beta-versus sham-treated mares, and was lower (P=0.02) in E-17beta-treated versus pregnant mares. These results demonstrate that intrauterine administration of a pharmacologic dose of PGE(2) initiated prolonged CL function in nonpregnant mares. Further experiments are needed to confirm the role of conceptus secretion of PGE(2) in CL maintenance, and to determine the mechanism of action of PGE(2) within the equine reproductive tract.

Effect of prostaglandin F2 alpha on release of progesterone and leukotriene B-4 by cells from corpora lutea of mares

Corpora lutea 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 alpha (250 ng/ml). PGF-2 alpha 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 alpha 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 alpha in luteolysis in the mare.

Binding of a novel prostaglandin F 2 alpha-derivative (ZK 71 677) to pseudopregnant rat ovaries

Prostaglandin (PG)F 2 alpha is believed to regulate the life span of the corpus luteum and this physiologically induced change could be related to the binding properties of PGF 2 alpha in the corpus luteum. Corpora lutea formation was therefore stimulated in juvenile rats with PMSG and HCG. In membrane particles obtained after differential centrifugation, radioligand binding studies were performed with PGF 2 alpha and a novel PGF 2 alpha-derivative (ZK 71 677) on day 7 after HCG administration. Evaluation of the binding parameters revealed a competitive interaction between PGF 2 alpha and ZK 71 677 for the PGF 2 alpha-receptor molecule. When other prostaglandin analogues were used to establish biological potency, the data obtained from receptor binding analysis compared well with the abortifacient potency of these compounds in pregnant rats. The results provide further evidence for the nature and specificity of the PGF 2 alpha-receptor molecule.

Effect of PGF-2 alpha on progesterone production in swine luteal cells at different stages of the luteal phase

Suspensions of luteal cells were prepared by enzymatic dispersion of pig corpora lutea obtained at specific times during the estrous cycle. Luteal cells from early corpora lutea produced more progesterone (4.73 +/- 0.84 nmol/10(6) cells, day 3) than those from late diestrus (0.73 +/- 0.04 nmol/10(6) cells, day 15); (P less than 0.05). Bovine LH enhanced progesterone production in a dose dependent manner particularly in cells from 9 to 15 day corpora lutea. Also PGF-2 alpha enhanced progesterone output in cells from mid-late corpora lutea. PGF-2 alpha did not exert any antigonadotropic effect since it further increased the progesterone production induced by LH. Luteal cells produced PGF-2 alpha with levels ranging between 1.6 and 2.7 pmol/10(6) cells throughout the whole luteal phase. The cellular content of cAMP was markedly increased by LH (556 +/- 60%) while it was not affected by PGF-2 alpha. Plasma membrane receptors for PGF-2 alpha were not detected in the analyzed tissue.

Binding of prostaglandin F2 alpha and 20 alpha-hydroxysteroid-dehydrogenase activity of immature rat ovaries throughout pseudopregnancy

PGF2 alpha is involved in the initiation of progesterone decline in the late luteal phase in the pseudopregnant rat. 20 alpha-Hydroxysteroid- dehydrogenase (20 alpha-OH-SDH), an important enzyme in progesterone metabolism, may participate in luteal regression. It was therefore of interest to investigate whether prostaglandin F2 alpha (PGF2 alpha) receptor binding in membrane particles and 20 alpha-OH-SDH activity in a cytosolic fraction in superovulated ovaries of immature rats are related. Scatchard analysis of the radioligand binding data revealed two classes of PGF2 alpha receptors (KD 10(-10) mol/l and 10(-8) mol/l). The number of binding sites varied from day 1 through day 21 of pseudopregnancy. Maximal binding/mg protein was obtained on day 7 with a gradual decrease until day 21 after HCG administration. 20 alpha-OH-SDH activity was low in 1 to 9 days old corpora lutea and increased markedly during days 11 to 13. Maximal enzyme activity was monitored on days 15 to 21 after administration of HCG. The time dependent increase in 20 alpha-OH-SDH activity was partially suppressed by the treatment of the rats with indomethacin, a potent inhibitor of prostaglandin biosynthesis. Since PGF2 alpha-receptor content does not coincide with the increase in enzyme activity (maximal receptor content preceded maximal enzyme activity by about 6 days), PGF2 alpha may not be the only direct factor responsible for the induction of ovarian 20 alpha-OH-SDH, to terminate luteal function.

Inhibitory characteristics of prostaglandin F2 alpha in the rat luteal cell

Enzymatically dispersed and enriched preparations of rat luteal cells were used to characterize the antigonadotropic effects of prostaglandin (PG) F2 alpha. The half-maximal dose (ED50) of LH for stimulation of cAMP accumulation and progesterone secretion was 100 and 25 ng/ml, respectively. Methylisobutylxanthine (MIX) had no effect on the ED50 of LH on cAMP accumulation but reduced the ED50 of LH on progesterone secretion from 25 to 10 ng/ml. PGF2 alpha inhibited the tropic responses to LH by 55-70% within minutes at concentrations of PGF2 alpha within the physiological range. For example, 2-4 nM PGF2 alpha inhibited LH-stimulated cAMP accumulation by 50% (IC50). PGF2 alpha reduced the maximum cAMP response to LH but had no effect on the ED50 of LH for cAMP accumulation whereas PGF2 alpha increased the ED50 of LH on progesterone secretion by 5-7-fold. Inhibition by PGF2 alpha appeared to be unrelated to an effect on cAMP phosphodiesterase activity or to changes in parameters of LH receptor binding activity. No inhibition by PGF2 alpha was evident on LH-stimulated cAMP accumulation in isolated membranes. PGF2 alpha had little effect on cAMP accumulation in response to cholera toxin or forskolin but produced significant inhibition of progesterone secretion in response to cholera toxin or dibutyryl cAMP [Bu)2cAMP). It is concluded that the antigonadotropic effect of PGF2 alpha in the luteal cell is due to two interrelated actions: inhibition of activation of cAMP accumulation by LH and inhibition of the luteal cell response to cAMP. Since PGF2 alpha had no effect in the broken cells, it is suggested that the action of PGF2 alpha may be mediated by a second messenger in the intact cell.

Characteristics of receptors for prostaglandin F-2 alpha in bovine and equine corpora lutea

Prostaglandin F-2 alpha (PGF-2 alpha) receptors of bovine and equine corpora lutea (C.L.) were studied. From both the equilibrium binding data and the dissociation kinetics behaviour, two affinity classes of receptors are evident in the mare, with apparent dissociation constants (Kd) of 1,5 x 10(-9) M and 3.5 x 10(-8) M. Bovine PGF-2 alpha receptors present a homogeneous population of binding sites with Kd = 1 x 10(-8) M. Both bovine and equine C.L. receptors bind PGF-2 alpha in a specific manner; only 13, 14-dihydro-PGF-2 alpha considerably cross-reacts with these receptors. Since in the mare uterine PGF-2 alpha reaches the ovary systemically with consequent extensive degradation, it is suggested that this metabolite could act as a luteolysin in this species. The dissociation kinetics indicate that the hormone-receptor (H-R) reaction evolves from a state of loose binding to one of tight binding as the time of incubation proceeds. This could be explained by internalization of the H-R complex.

Luteolytic prostaglandins. Synthesis and biological activity

Analogues of PGF2 alpha with enhanced luteolytic activity were synthesized using the Corey synthesis. The luteolytic activity of the new prostaglandins was tested in the hamster. In addition the smooth muscle activity of the new compounds was compared with that of PGA2 on the longitudinal strip of rat stomach fundus. Structure-activity relationships in the new series of 17,18,19,20-tetranor-16-thienyl-oxy-PGF2 alpha are discussed.

Specific binding of prostaglandin F2 alpha to membranes of rat corpora lutea

Prostaglandin F2 alpha (PGF2 alpha) binds specifically to a partially purified membrane preparation from rat corpora lutea. The high affinity, low capacity binding component asa a Kd = 4.7 nM and has a capacity of 0.38 pmol/mg protein. Binding kinetics were temperature-dependent with an association rate constant of 2.5 x 10(5) 1/mol-sec and a dissociation rate constant of 4.3 x 10(-4) sec-1 at 22 degrees C. Little competition for binding was shown by other prostaglandins and prostaglandin metabolites; the PGF2 alpha analogue ICI 81008 (16-m-trifluoromethylphenyl-prostaglandin F2 alpha) showed a binding affinity similar to that of PGF2 alpha. The specific binding of PGF2 alpha to luteal cell membranes was confirmed by electron microscopy using a ferritin--PGF2 alpha conjugate. Ferritin--PGF2 alpha was found predominantly on luteal cell surfaces; little binding occurred on other types of cells present. These data demonstrate specific binding of PGF2 alha to rat luteal membranes. It is suggested that the luteolytic action of PGF2 alpha in the rat may be receptor-mediated.