Hormonal regulation of oxytocin-induced prostaglandin F(2alpha) secretion by the bovine and ovine uterus in vivo

Oxytocin-induced prostaglandin F2-alpha release is low in early bovine pregnancy but increases during the second month of pregnancy†

Circulating prostaglandin F2α metabolite (PGFM) after an oxytocin challenge was evaluated throughout the first 2 months of pregnancy in lactating Holstein cows. On day 11, 18, and 25 after artificial insemination (AI), and on days 32, 39, 46, 53, and 60 of pregnancy, cows were challenged with 50 IU oxytocin, i.m. Blood was collected before (0 min), 30, 60, 90, and 120 min after oxytocin for plasma PGFM concentrations. Ultrasound evaluations were performed for pregnancy diagnosis on day 32-60 post-AI. Nonpregnant (NP) cows on day 18 were designated by a lack of interferon-stimulated genes in peripheral blood leukocytes and Pregnant (P) based on day 32 ultrasound. On day 11, P and NP were similar with low PGFM and no effect of oxytocin on PGFM. On day 18, oxytocin increased PGFM (3-fold) in NP with little change in P cows. Comparing only P cows from day 11 to 60, basal circulating PGFM increased as pregnancy progressed, with day 11 and 18, lower than all days from day 25 to 60 of pregnancy. Oxytocin-induced PGFM in P cows on day 25 was greater than P cows on day 18 (2.9-fold). However, oxytocin-induced PGFM was lower on day 25 compared to day 53 and 60, with intermediate values on day 32, 39, and 46 of pregnancy. Thus, the corpus luteum (CL) of early pregnancy (day 11, 18) is maintained by suppression of PGF, as reflected by suppressed PGFM in this study. However, during the second month of pregnancy, uterine PGF secretion was not suppressed since basal PGFM and oxytocin-induced PGFM secretion were elevated. Apparently, mechanisms other than suppression of oxytocin receptors maintain CL after day 25 of pregnancy.

The progesterone receptor antagonist, onapristone has differential effects on the timing and control of the luteolytic mechanism depending on timing of administration in sheep

Cyclic ewes were treated with control vehicle or progesterone receptor antagonist (onapristone; 100mg i.m. twice daily) during either early (day 3-5) or late (day 12-14) luteal phase and plasma samples collected for hormone analysis and to determine endogenous and oxytocin induced PGF2α release. On day 14 and 17, ewes were euthanised and reproductive tracts collected for ovarian morphology and endometrium for oxytoxin and steroid hormone receptor analysis. Early treatment increased LH, but not progesterone or oestradiol, while late treatment elevated all three hormones. Early treatment delayed the up-regulation of endometrial oxytocin receptors and responsiveness to oxytocin challenge, delaying luteolysis. Late treatment advanced development of oxytocin receptors and responsiveness to oxytocin though not timing of luteolysis. Patterns of hormone receptor mRNA were differentially disrupted by treatments. Results provide mechanistic insight into hormonal control of the oestrous cycle and identify the ability of the luteolytic mechanism to dissociate from functional luteolysis.

Induction of PGFM pulses and luteolysis by sequential estradiol-17β treatments in heifers

The effects of sequential induction of PGFM pulses by estradiol-17β (E2) on prominence of PGFM pulses and progesterone (P4) concentration were studied in heifers. Three treatments of vehicle (n = 12) or E2 (n = 12) at doses of 0.05 or 0.1 mg were given at 12-h intervals beginning on Day 15 postovulation. Blood samples were collected every 12 h from Days 13-24 and hourly for 12 h after the first and third treatments. On Day 15, all heifers were in preluteolysis and on Day 16 were in preluteolysis in the vehicle-treated heifers (n = 11) and either preluteolysis (n = 4) or luteolysis (n = 8) in the E2-treated heifers. Peak concentration of induced PGFM pulses during preluteolysis on Day 15 was greater (P < 0.04) than for pulses during preluteolysis on Day 16. The interval from ovulation to the beginning of luteolysis was shorter (P < 0.04) in the E2-treated heifers than in the vehicle-treated heifers. An E2-induced PGFM pulse was less prominent (P < 0.008) in heifers in temporal association with a transient resurgence in P4 than in heifers with a progressive P4 decrease. The hypothesis that repeated E2 exposure stimulates increasing prominence of PGFM pulses was not supported. Instead, repeated exposure reduced the prominence of PGFM pulses, in contrast to the stimulation from the first E2 treatment. Reduced prominence of a PGF(2α) pulse during luteolysis can lead to a transient resurgence in P4 concentration.

Effect of dose of estradiol-17β on prominence of an induced 13,14-dihydro-15-keto-PGF(2α) (PGFM) pulse and relationship of prominence to progesterone, LH, and luteal blood flow in heifers

Various doses of estradiol-17β (E(2)) were used in heifers to induce a pulse of 13,14-dihydro-15-keto-prostaglandin F(2α) (PGFM). The effect of E(2) concentration on the prominence of PGFM pulses and the relationship between prominence and intrapulse concentration of progesterone (P(4)), LH, and luteal blood flow were studied. A single dose of 0 (vehicle), 0.01, 0.05, or 0.1 mg of E(2) was given (n = six/group) 14 d after ovulation. Blood samples were collected, and luteal blood flow was evaluated hourly for 10 h after the treatment. The 0.05-mg dose increased and the 0.1-mg dose further increased the prominence of the induced PGFM pulse, compared with the 0.0-mg dose and the 0.01-mg dose. The PGFM pulses were subdivided into three different prominence categories (<50, 50 to 150, and >150 pg/mL at the peak). In the 50 to 150 category, P(4) concentration increased (P < 0.05) between -2 h and 0 h (0 h = peak of PGFM pulse). In the >150 category, P(4) decreased (P < 0.05) between -1 h and 0 h, LH increased (P < 0.05) at 1 h, and luteal blood flow apparently decreased (P < 0.05) at 2 h of the PGFM pulse. The novel results supported the following hypotheses: (1) an increase in E(2) concentration increases the prominence of a PGFM pulse, and (2) greater prominence of a PGFM pulse is associated with a greater transient intrapulse depression of P(4) at the peak of the PGFM pulse. In addition, the extent of the effect of prostaglandin F(2α) on the increase in LH and changes in blood flow within the hours of a PGFM pulse was related positively to the prominence of the PGFM pulse.

Progesterone as the driving regulatory force behind serum FSH concentrations and antral follicular development in cycling ewes

Ovarian antral follicles in sheep grow in an orderly succession, producing typically three to four follicular waves per 17-day oestrous cycle. Each wave is preceded by a transient increase in circulating FSH concentrations. The mechanism controlling the number of recurrent FSH peaks and emerging follicular waves remains unknown. During the ewe’s oestrous cycle, the time between the first two FSH peaks and days of wave emergence is longer than the intervals separating the ensuing FSH peaks and follicular waves. The prolonged interpeak and interwave interval occurs early in the luteal phase when low levels of progesterone are secreted by developing, or not fully functional, corpora lutea (CL). The purpose of the present study was to determine the effect of varying progesterone (P4) levels on circulating concentrations of FSH and antral follicular development in sheep. Exogenous P4 (15 mg per ewe, i.m.) was administered twice daily to six cycling Rideau Arcott × Dorset ewes from Day 0 (ovulation) to Day 4 (the mean duration of the interwave interval); six animals served as controls. Follicular growth was monitored in all animals by daily transrectal ultrasonography (Days 0–9). Jugular blood samples were drawn twice a day from Day 0 to Day 4 and then daily until Day 9 to measure systemic concentrations of P4, FSH and 17β-oestradiol (E2). The first FSH peak after ovulation was detected on Days 1.5 ± 0.2 and 4.2 ± 0.2 in treated and control ewes, respectively (P < 0.05). The next FSH peak(s) occurred on Day 3.9 ± 0.3 in the treated group and on Day 6.4 ± 0.5 in the control group. Consequently, the treated group had, on average, three follicular waves emerging on Days 0, 3 and 6, whereas the control group had two waves emerging on Days 0 and 5. Mean serum E2 concentrations were greater (P < 0.05) in control compared with treated ewes on Days 1.3, 2.3, 3.3, 4.0 and 4.3 after ovulation. In summary, creation of mid-luteal phase levels of P4 in metoestrus shortened the time to the first post-ovulatory FSH peak in ewes, resulting in the emergence of one more follicular wave compared with control ewes during the same time frame. Therefore, P4 appears to be a key endocrine signal governing the control of periodic increases in serum FSH concentrations and the number of follicular waves in cycling sheep.

Role of follicular estradiol-17beta in timing of luteolysis in heifers

The hypothesis was tested that estradiol (E2) from the ovarian follicles controls time of luteolysis. Time of luteolysis was evaluated by multiple measures of corpus luteum (CL) structure (area, volume) and function (progesterone [P4], luteal blood flow). The hypothesis for experiment 1 was that repeated ablation of follicles would reduce circulating E2 and delay luteolysis. Heifers were randomly assigned on Day 9 (Day 0 = ovulation) to three groups. All follicles >or=4 mm were ablated on Day 9 (group FA9; n = 6); Days 9-15 (group FA15; n = 6); or Days 9-21 (group FA21; n = 7). As expected, follicular ablation delayed (P < 0.001) the rise in circulating E2 and peak E2 concentrations (FA9, Day 17.6 +/- 0.7; FA15, Day 20.3 +/- 0.3; FA21, Day 24.9 +/- 0.3). Luteolysis (based on each measure) was delayed (P < 0.005) by repeated ablation of follicles, with earlier luteolysis (based on P4 decrease) in FA9 (Day 15.2 +/- 0.8) than FA15 (Day 16.5 +/- 0.4), and a further delay in FA21 (Day 18.3 +/- 0.5). The hypothesis of experiment 2 was that exogenous treatment with E2 would stimulate prostaglandin F(2alpha) (PGF) secretion and prevent the delay in luteolysis associated with follicular ablations. Follicles >or=4 mm were ablated from Day 9 to Day 17 (n = 15). Heifers were treated on Days 13 and 15 with 1.0 mg of estradiol benzoate (FAE2; n = 7) or vehicle (FAV; n = 8). Treatment with E2 induced PGF secretion (detected by PGF metabolite) and induced earlier (P < 0.02) luteolysis in FAE2 than in FAV, whether determined by circulating P4 or by area, volume, or blood flow of CL. In summary, ablation of follicles (>or=4 mm) delayed and treatment with E2 hastened luteolysis in heifers with ablated follicles. Thus, these results are consistent with an essential role for follicle E2 in timing of luteolysis.

Conception rate in Holstein cows treated with GnRH or hCG on the fifth day post artificial insemination during summer

Lactating Holstein cows (n=158), at 213±112 days in milking and averaging 26±9kg of milk per day, were randomly assigned to one of three treatment groups: control (CG, n=52, saline), GnRH (GG, n=55, 100g gonadorelin), and hCG (HG, n=51, 2500IU) given five days after artificial insemination (AI). Rectal temperature was taken at the moment of AI and blood samples were collected five, seven, and 12 days after AI. Pregnancy was determined between 42 and 49 days after AI. Concentration of progesterone (P4) in serum (ng/ml, mean±SE) for CG, GG, and HG were, respectively, 2.7±0.4, 2.5±0.4, and 3.2±0.5 on day 5; 4.8±0.4, 4.2±0.4, and 5.7±0.5 on day 7; and 5.2±0.4, 6.9±0.4, and 8.5±0.5 on day 12 after AI. P4 concentration had proportional increase in serum between days 5 and 7 after AI (CG: 178%, GG: 168%, and HG: 178%), suggesting that the treatments did not induce a luteotropic effect on the existing corpus luteum (CL). Concentrations of P4 increased between days 7 and 12 in cows treated with GnRH and hCG (GG: 164%, and HG: 149%, P<0.01); but not in control cows (GC: 18%, P=0.31), suggesting that a new CL was formed. Treatments with GnRH or hCG increased conception rates in cows with rectal temperature below 39.7°C (CG: 10.1%, n=26, GG: 36.8%, n=27; and HG: 32.8%, n=21), but not in cows with rectal temperature above 39.7°C (CG: 15.2%, n=26; GG: 17.8%, n=28; and HG: 24.4%, n=30). These data suggest that high body temperature masked the positive impact of treatment with GnRH or hCG on day 5 after AI on conception.

Distinct phospholipase A2 enzymes regulate prostaglandin E2 and F2alpha production by bovine endometrial epithelial cells

BACKGROUND

The rate-limiting step in prostaglandin (PG) biosynthesis is catalyzed by phospholipase A2 (PLA2) enzymes which hydrolyze arachidonic acid from membrane phospholipids. Despite their importance in uterine PG production, little is known concerning the specific PLA2 enzymes that regulate arachidonic acid liberation in the uterine endometrium. The objectives of this study were to evaluate the expression and activities of calcium-independent Group VI and Group IVC PLA2 (PLA2G6 and PLA2G4C) and calcium-dependent Group IVA PLA2 (PLA2G4A) enzymes in the regulation of bovine uterine endometrial epithelial cell PG production.

METHODS

Bovine endometrial epithelial cells in culture were treated with oxytocin, interferon-tau and the PLA2G6 inhibitor bromoenol lactone, alone and in combination. Concentrations of PGF2alpha and PGE2 released into the medium were analyzed. Western blot analysis was performed on cellular protein to determine the effects of treatments on expression of PLA2G4A, PLA2G6 and PLA2G4C. Group-specific PLA2 activity assays were performed on cell lysates following treatment with oxytocin, interferon-tau or vehicle (control), alone and in combination. To further evaluate the role of specific PLA2 enzymes in uterine cell PG biosynthesis, cells were transfected with cDNAs encoding human PLA2G6 and PLA24C, treated as described above and PG assays performed.

RESULTS

Constitutive cell production of PGF2alpha was about two-fold higher than PGE2. Oxytocin stimulated production of both PGs but the increase of PGF2alpha was significantly greater. Interferon-tau diminished oxytocin stimulation of both PGs. The PLA2G6 inhibitor, bromoenol lactone, abolished oxytocin-stimulated production of PGF2alpha. Treatments had little effect on PLA2G4A protein expression. In contrast, oxytocin enhanced expression of PLA2G6 and this effect was diminished in the presence of interferon-tau. Expression of PLA2G4C was barely detectable in control and oxytocin treated cells but it was enhanced in cells treated with interferon-tau. Oxytocin stimulated PLA2 activity in assays designed to evaluate PLA2G6 activity and interferon-tau inhibited this response. In assays designed to measure PLA2G4C activity, only interferon-tau was stimulatory. Cells overexpressing PLA2G6 produced similar quantities of the two PGs and these values were significantly higher than PG production by non-transfected cells. Oxytocin stimulated production of both PGs and this response was inhibited by interferon-tau. Bromoenol lactone inhibited oxtocin stimulation of PGF2alpha production but stimulated PGE2 production, both in the absence and presence of oxytocin. Cells over-expressing PLA2G4C produced more PGE2 than PGF2alpha and interferon-tau stimulated PGE2 production.

CONCLUSION

Results from these studies indicate that oxytocin stimulation of uterine PGF2alpha production is mediated, at least in part, by up-regulation of PLA2G6 expression and activity. In addition to its known inhibitory effect on oxytocin receptor expression, interferon-tau represses oxytocin-stimulated PLA2G6 expression and activity and this contributes to diminished PGF2alpha production. Furthermore, endometrial cell PGE2 biosynthesis was associated with PLA2G4C expression and activity and interferon-tau was stimulatory to this process.

Endometrial Oxytocin Receptor Concentration and Activity in Prepubertal Ewe Lambs

We have measured endometrial oxytocin receptor concentrations during prepuberty in ewe lambs, and have investigated the effect of progesterone on the activity of these receptors. In the first study, oxytocin receptor concentrations were undetectable in 2-week prenatal lambs but had increased immediately following birth and were then maintained throughout prepubertal life. Despite the presence of oxytocin receptors animals showed no prostaglandin F(2alpha) (PGF(2alpha)) release in response to exogenous oxytocin challenge at either 3 or 5 months of age. In a second study in 4-month-old ewe lambs treatment with exogenous progesterone resulted in the appearance of PGF(2alpha) release in response to oxytocin after 10 days of treatment. Thus, during the prepubertal life, ewe lambs possess the prerequisites of a luteolytic mechanism in that they have a dormant population of oxytocin receptors in which progesterone can induce oxytocin-stimulated PGF(2alpha) release.

Effect of exogenous melatonin on in vivo and in vitro prostaglandin secretion in Rasa Aragonesa ewes

The effect of exogenous melatonin on prostaglandin secretion was measured on Rasa Aragonesa ewes. Fourteen ewes received an 18 mg melatonin implant (M+) on 10 April and were compared with 13 control animals (without implants M-). Twenty days later, intravaginal pessaries were inserted in all animals to induce a synchronized oestrus (day 0). On day 14, ewes were injected, i.v., with 0.5 IU oxytocin. Plasma 15-ketodihydro-PGF(2alpha) (PGFM) concentrations were measured to assess uterine secretory responsiveness to oxytocin. After euthanasia, pieces of endometrium were collected to determine progesterone content and PGE(2) and PGF(2alpha) secretion in vitro, in the presence or absence of either 20 microg/ml recombinant ovine interferon-tau (roIFNt) or 1 nmol/l oxytocin in the medium. Endometrial progesterone content was similar in the two treatments (M+: 50.25+/-17.34 ng/mg tissue, M-: 43.08+/-11.21 ng/mg tissue). M+ ewes that responded to oxytocin had significantly higher plasma PGFM concentrations between 10 and 80 min after oxytocin administration, a higher mean PGFM peak (P<0.001), higher plasma PGFM levels after the challenge (P<0.05) and higher plasma progesterone concentrations (P<0.01) than control ewes. In the in vitro experiment, M+ and M- control samples secreted similar amounts of PGE(2). The presence of roIFNtau and oxytocin only stimulated PGE(2) production (P<0.05) in M- tissues. Control M+ tissues secreted higher amounts of PGF(2alpha) (P=0.07) and PGF(2alpha) secretion was significantly (P<0.01) stimulated by roIFNtau. Oxytocin produced this effect only in M- samples (P<0.01). In conclusion, although previous studies have demonstrated a positive effect of melatonin on lamb production, PGF(2alpha) secretion is higher in vitro and the PGE(2):PGF(2alpha) ratio is unfavourable in response to IFNtau, which could affect embryo survival. Whether or not these mechanisms are similar in pregnant ewes remains to be elucidated.

Attenuation of PGF2alpha release in ewes infused with the oxytocin antagonist L-368,899

We have investigated the effects of systemic administration of the oxytocin antagonist (OTA) L-368,899 on luteolytic PGF(2alpha) release in ewes. In the first study, carried out in four ovariectomized ewes primed with progesterone to induce responsiveness to oxytocin, 3-h i.v. infusions of 3, 10 and 30 microg/kg/min OTA, carried out on days 12, 14, 16 and 18 in a Latin Square design, resulted in a significant attenuation of the oxytocin induced increase in PGFM concentration at all doses (OTA 139+/-8.3% of pre-oxytocin baseline; control 206.8+/-18.7%; P<0.005). In a further study, continuous infusion of cyclic ewes (n=6) with 10 microg/kg/min OTA from day 13 to day 17 of the cycle resulted in a reduction in both the frequency (OTA 1.0+/-0.4/ewe; control 2.2+/-0.2/ewe; P<0.05) and amplitude (OTA 31.8+/-11.0 pg/ml; control 68.8+/-10.4 pg/ml; P<0.05) of endogenous PGFM episodes compared to control ewes (n=5) measured during daily 8-h sampling windows on days 14-17. This reduction in PGFM concentrations was accompanied by a modest extension in the day of luteolysis (progesterone <0.5 ng/ml) to day 17.5+/-0.4 in the OTA treated group compared with day 16.4+/-0.5 in the control group (P=0.07). The results demonstrate that treatment with OTA caused a significant reduction in episodes of increased PGFM concentration during the period of luteolysis and may provide an approach by which to reduce early pregnancy failure.

Effects of progesterone on gastric emptying and intestinal transit in male rats

AIM: To study the dose-dependent of progesterone (P) effect and the interaction between the oxytocin (OT) and P on gastrointestinal motility.

METHODS: In order to monitor the gastric emptying and intestinal transit, the SD male rats were intubated via a catheter with normal saline (3 mL/kg) containing Na₂⁵¹CrO₄ (0.5 μCi/mL) and 10% charcoal. OT was dissolved into normal saline and P was dissolved into 75% alcohol.

RESULTS: Low does of P (1 mg/kg, i.p.) enhanced the gastric emptying (75% ± 3%, P < 0.05) and high dose of P (5 mg/kg, i.p.) inhibit it (42% ± 11.2%, P < 0.01). P (1 mg/kg) increased the intestinal transit (4.2 ± 0.3, P < 0.05) while the higher dose (10-20 mg/kg) had no effect. OT (0.8 mg/kg, i.p.) inhibited the gastric emptying (23.5% ± 9.8%, P < 0.01). The inhibitory effects of P (20 mg/kg) (32% ± 9.7%, P < 0.05) and OT (0.8 mg/kg) on gastric emptying enhanced each other when the two chemicals were administrated simultaneously (17% ± 9.4%, P < 0.01).

CONCLUSION: Low dose of P increased GI motility while high dose of P decreased it. During the later period of pregnancy, elevated plasma level of OT may also participate in the gastrointestinal inhibition.