Differential effects of progesterone treatment on the oxytocin-induced prostaglandin F2 alpha response and the levels of endometrial oxytocin receptors in ovariectomized ewes.

The oxytocin-induced uterine prostaglandin (PG) F2 alpha response and the levels of endometrial oxytocin receptors were measured in ovariectomized ewes after they had been given steroid pretreatment (SP) with progesterone and estrogen to induce estrus (day of expected estrus = Day 0) and had subsequently been treated with progesterone over Days 1-12 and/or PGF2 alpha over Days 10-12 postestrus. The uterine PGF2 alpha response was measured after an i.v. injection of 10 IU oxytocin on Days 13 and 14, using the PGF2 alpha metabolite, 13,14-dihydro-15-keto-PGF2 alpha (PGFM), as an indicator for PGF2 alpha release. The levels of oxytocin receptors in the endometrium were measured on Day 14. During the treatment with progesterone, the peripheral progesterone concentrations were elevated and remained above 1.8 ng/ml until the morning of Day 14. The PGFM responses to oxytocin in untreated controls and SP controls were low on both Days 13 and 14 whereas the levels of endometrial oxytocin receptors in the same ewes were high. Treatment with progesterone either alone or in combination with PGF2 alpha significantly (p less than 0.04) increased the PGFM response on Day 14 and reduced the levels of endometrial oxytocin receptors; treatment with PGF2 alpha alone had no effect. It is concluded that progesterone promotes the PGFM response to oxytocin while simultaneously suppressing the levels of endometrial oxytocin receptors. PGF2 alpha treatment had no effect on either the uterine secretory response to oxytocin or the levels of oxytocin receptors in the endometrium.(ABSTRACT TRUNCATED AT 250 WORDS)     

Receptors for prostaglandins F2 alpha and E2 in ovine corpora lutea during maternal recognition of pregnancy.

Plasma membrane receptors for prostaglandins (PG) F2 alpha and E2 were quantified in ovine corpora lutea obtained from nonpregnant and pregnant ewes on Days 10, 13, and 15 post-estrus, and from additional ewes on Days 25 and 40 of pregnancy. Regardless of reproductive status or day post-estrus, concentrations of luteal receptors for PGF2 alpha were 7- to 10-fold greater than those for PGE2. In pregnant ewes the concentration of receptors for PGF2 alpha was highest on Day 10 (35.4 +/- 2.8 fmol/mg) and lowest on Day 25 (22.3 +/- 2.5 fmol/mg). A difference in the concentration of luteal receptors for PGF2 alpha between pregnant and nonpregnant ewes was apparent only on Day 15 post-estrus, at which time the concentration of receptors for PGF2 alpha was higher in pregnant ewes than in nonpregnant ewes (27.1 +/- 2.7 vs. 17.7 +/- 2.7 fmol/mg). Concentrations of receptors for PGE2 in pregnant ewes were similar (p > 0.05; 2.8 +/- 0.3 to 3.7 +/- 0.2 fmol/mg) between Days 13 and 40 but were higher (p < 0.05) than in corpora lutea obtained from nonpregnant ewes on Days 10 (5.0 +/- 0.4 vs. 4.1 +/- 0.2 fmol/mg) and 15 (3.7 +/- 0.2 vs. 2.0 +/- 0.4 fmol/mg) post-estrus. Although concentrations of receptors for both PGF2 alpha and PGE2 were lowest in corpora lutea obtained from nonpregnant ewes on Day 15, this was not due to luteal regression since the weights and concentrations of progesterone in corpora lutea on Day 15 were not lower than those for corpora lutea obtained on Days 10 and 13.(ABSTRACT TRUNCATED AT 250 WORDS)     

Changes in uterine secretion of prostaglandin F2 alpha and luteal secretion of progesterone in response to oxytocin during the porcine estrous cycle.

The purpose of this experiment was to determine whether the ability of oxytocin to stimulate uterine secretion of prostaglandin F2 alpha (PGF2 alpha) and luteal secretion of progesterone changes during the porcine estrous cycle. Nineteen multiparous sows were observed for estrus. After one estrous cycle of normal length, sows were assigned randomly to receive an injection of oxytocin (30 IU, i.v.) in the EARLY (Days 4-6; n = 6), MID (Days 9-11; n = 7), or LATE (Day 15; n = 6) stage of the estrous cycle. Concentrations of 13, 14-dihydro-15-keto-PGF2 alpha (PGFM) and progesterone were determined in jugular venous serum samples collected at -60, -45, -30, -15, 0, 2, 5, 10, 15, 30, 45, 60, 90, and 120 min after injection of oxytocin. The magnitudes of the PGFM and progesterone responses and the area under the respective response curves (AUC) were calculated for each sow. Concentrations of PGFM did not change in response to oxytocin administered during the EARLY or MID portions of the estrous cycle. Concentrations increased rapidly in 4 of 6 sows that received oxytocin LATE in the estrous cycle. Both magnitude and AUC were greater LATE in the estrous cycle than at either EARLY or MID cycle (p less than 0.05). Thus, uterine secretory responsiveness to oxytocin develops between Days 11 and 15 postestrus in the sow. For progesterone, a transient increase was observed immediately following injection of oxytocin at MID cycle (p less than 0.05), but not at the other times examined. Therefore, oxytocin appears to be capable of stimulating secretion of progesterone from the functionally mature corpus luteum.     

Ovine trophoblast protein-one inhibits development of endometrial responsiveness to oxytocin in ewes

In experiment (Exp) 1, 12 cyclic ewes had catheters placed into each uterine horn on Day 7 (estrus = Day 0). On Days 11-15, 6 ewes received twice-daily intrauterine infusions of 1.5 mg serum protein (SP) into each uterine horn and 6 ewes received infusions of 1.08 mg SP + 0.42 mg ovine conceptus secretory proteins (oCSP) containing 25 micrograms ovine trophoblast protein-one (oTP-1) as determined by radioimmunoassay (25-35% bioactive by antiviral assay). SP-infused and oCSP-infused ewes had similar plasma 13,14-dihydro-15-keto prostaglandin F2 alpha (PGF2 alpha) profiles in response to oxytocin on Day 11, but SP ewes became more responsive (p less than 0.01) to oxytocin on Days 13 and 15 than oCSP ewes. SP ewes also had greater incorporation of [3H]inositol into inositol trisphosphate (IP3) (+3449%, p less than 0.01) and total inositol phosphate (IP) (+760%, p less than 0.08), in response to oxytocin, than did oCSP ewes (+553 and +168% for IP3 and total IP, respectively) in endometrium collected at ovariectomy/hysterectomy on Day 16. Mean CL weights on Day 16 and mean concentrations of progesterone in plasma collected at 12-h intervals on Days 6-16 were not different for SP and oCSP ewes, but concentrations of progesterone were lower (p less than 0.05) in SP ewes on Days 15-16 than for oCSP ewes. These results indicate that oTP-1 may prevent luteolysis by inhibiting development of endometrial responsiveness to oxytocin and, therefore, reduce oxytocin-induced synthesis of IP3 and PGF2 alpha.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of progesterone and estradiol-17 beta on uterine secretion of prostaglandin F2 alpha in response to oxytocin in ovariectomized ewes

Twenty ovariectomized ewes were used in an experiment designed to examine the interaction of progesterone, estradiol, and oxytocin in the regulation of uterine secretion of prostaglandin F2 alpha (PGF2 alpha). All ewes underwent a steroid pretreatment that mimicked the changes in progesterone and estradiol which occur during the six days immediately prior to estrus. After pretreatment, ewes were randomly assigned to 1 of 4 treatment groups: 1) control (n = 4); 2) estradiol-17 beta (n = 6); 3) progesterone (n = 4); and 4) progesterone and estradiol-17 beta (n = 6). Progesterone was injected twice daily for 15 days. The dose of progesterone varied with day postestrus in a manner designed to simulate endogenous luteal secretion of progesterone. Estradiol-17 beta was administered in s.c. Silastic implants. The implants maintained circulating concentrations of estradiol at 3 pg/ml. On Days 5, 10, and 15 of treatment, ewes were injected with oxytocin (10 IU in 1.0 ml saline, i.v.). Jugular venous blood samples were collected beginning one-half hour prior to and continuing for 2 hours post-oxytocin injection for quantification of 13,14-dihydro-15-keto-prostaglandin F2 alpha (PGFM). No changes in concentration of PGFM following injection of oxytocin were observed on Day 5 or 10 in any treatment group. Concentrations of PGFM increased following injection of oxytocin on Day 15 only in groups receiving progesterone. Both the area under the PGFM response curve (p = 0.08) and peak response (p = 0.06) were greater in ewes treated with progesterone and estradiol-17 beta than in those receiving progesterone alone.(ABSTRACT TRUNCATED AT 250 WORDS)     

Control of endometrial oxytocin receptor and uterine response to oxytocin by progesterone and oestradiol in the ewe

The effects of administration of progesterone and oestradiol on ovine endometrial oxytocin receptor concentrations and plasma concentrations of 13,14-dihydro-15-keto prostaglandin F-2 alpha (PGFM) after oxytocin treatment were determined in ovariectomized ewes. Ewes received progestagen pre-treatment, progesterone and/or oestradiol in 11 different treatment schedules. Progestagen pre-treatment decreased oxytocin receptor concentrations in endometrium from ewes treated subsequently with either progesterone for 5 days or progesterone for 5 days plus oestradiol on Days 4 and 5 of progesterone treatment. Oestradiol increased endometrial oxytocin receptor concentrations when administered on Days 4 and 5 of 5 days progesterone treatment. Progestagen pre-treatment followed by progesterone treatment for 12 days caused a large increase in oxytocin receptors and no further increase occurred when ewes were given oestradiol on Days 11 and 12, or when progesterone was withdrawn on Days 11 and 12, or these two treatments were combined. Oxytocin administration caused an increase in plasma PGFM concentrations in ewes which did not receive progestagen pre-treatment, and subsequently received progesterone treatment for 5 days and oestradiol treatment on Days 4 and 5 of progesterone treatment. Similarly treated ewes which received progestagen pre-treatment did not respond to oxytocin. Oxytocin administration also increased plasma PGFM concentrations in ewes which received progestagen pre-treatment followed by progesterone treatment for 12 days, progesterone treatment for 12 days plus oestradiol on Day 11 and 12 of progesterone treatment, progesterone withdrawal on Day 11 and 12, or progesterone withdrawal and oestradiol treatment combined. The results indicate that (1) progesterone pre-treatment affects oxytocin receptor concentrations in the endometrium and uterine responsiveness to oxytocin and (2) progesterone treatment alone for 12 days after a treatment which mimics a previous luteal phase and oestrus is sufficient to induce oxytocin receptors and increase oxytocin-induced PGF release. These results emphasize the importance of progesterone and provide information which can be used to form an hypothesis for control of luteolysis and oestrous cycle length in the ewe.     

Effect of ovine conceptus secretory proteins and purified ovine trophoblast protein-1 on interoestrous interval and plasma concentrations of prostaglandins F-2 alpha and E and of 13,14-dihydro-15-keto prostaglandin F-2 alpha in cyclic ewes

Conceptus secretory proteins (oCSP) were obtained from medium in which sheep conceptuses, collected on Day 16 of pregnancy, were cultured for 30 h. A portion of the culture medium (500 ml) was prepared for intrauterine infusion by concentrating the proteins by Amicon ultrafiltration (Mr 500 cutoff). A second portion (500 ml medium) was used to purify sheep trophoblast protein one (oTP-1). Proteins remaining after oTP-1 purification were concentrated and then passed through an anti-oTP-1 sepharose CL-4B affinity column to remove any remaining oTP-1 (oCSP-oTP-1). Serum proteins (oSP) were collected from a Day-16 pregnant ewe and diluted for infusion. Catheters were placed in the uterus of cyclic (Day 10) ewes. The following combinations of proteins were infused: 0.75 mg oCSP + 0.75 mg oSP (5 ewes), 0.75 mg oCSP - oTP-1 + 0.75 mg oSP (4 ewes), 0.05 mg oTP-1 + 1.45 mg oSP (5 ewes) and 1.5 mg oSP only (5 ewes). Infusions were twice daily on Days 12 and 13 (08:00 and 17:00 h) and once on Day 14 (08:00 h). On Day 14, ewes were injected intravenously at 08:00 h with 0.5 mg oestradiol-17 beta. Blood sampling began 30 min before oestradiol injection and continued every 30 min for 10 h. On Day 15 ewes received 10 i.u. oxytocin intravenously (08:00 h). Blood samples were collected 10 min before oxytocin and every 10 min for 1 h after oxytocin injection. Concentrations of prostaglandin (PG) F, PGE-2/PGE-1 (PGE) and 13,14-dihydro-15-keto-PGF-2 alpha (PGFM) were measured by specific radioimmunoassay. Ewes treated with oTP-1 and oCSP had longer (P less than 0.05) interoestrous intervals (27 and 25 days, respectively) compared to ewes treated with oSP and oCSP--oTP-1 (19 and 19 days, respectively) (s.e.m. = 1.56 days). These results indicate that oTP-1 alone is as potent as total conceptus secretory proteins in extending luteal maintenance. Ewes treated with oTP-1 and oCSP had no increase in PGF after oestradiol injection while production of PGF did increase 6-10 h after oestradiol in ewes treated with oSP and oCSP--oTP-1. PGFM was correlated with PGF concentrations (r = 0.57, P less than 0.01) although presence or absence of increases in production of PGFM for the treatment groups were not the same as those for PGF. No effects of treatment on PGE were detected.(ABSTRACT TRUNCATED AT 400 WORDS)     

Increases in the oxytocin-induced prostaglandin F2 alpha response and reduction in the concentrations of endometrial oxytocin receptors in ewes in response to progesterone.

Ovariectomized ewes were given progesterone and oestrogen priming as steroid pretreatment and subsequently treated with progesterone, prostaglandin F2 alpha (PGF2 alpha), or both. In Expt 1, plasma concentrations of the metabolite 13,14-dihydro-15-keto-PGF2 alpha (PGFM) were measured after an i.v. injection of oxytocin. There was little PGFM response in the untreated control ewes or in the pretreated ewes. Treatment with PGF2 alpha alone had no effect (P greater than 0.05), whereas treatment with progesterone either alone or with PGF2 alpha significantly (P less than 0.05) increased the uterine PGFM response to oxytocin. In Expt 2, chronically ovariectomized ewes had high concentrations of endometrial oxytocin receptors. Treatment with PGF2 alpha alone did not alter the concentrations of the receptors. Treatment with progesterone either alone or with PGF2 alpha significantly (P less than 0.05) reduced the concentrations of the receptors. It is concluded that progesterone promotes the PGFM response to oxytocin, but simultaneously suppresses the concentrations of endometrial oxytocin receptors.     

Effect of pregnancy on oxytocin-induced release of prostaglandin F2 alpha in heifers

The effect of pregnancy on the release of prostaglandin F2 alpha (PGF2 alpha) in response to oxytocin (OT) has been examined. Fourteen cyclic heifers received one intravenous injection of 1 IU OT (n = 6) or 100 IU OT (n = 8) 17, 18, or 19 days (Day 17-19) after the onset of estrus (Day 0). Five of these animals also received 100 IU OT at Days 6 and 13 to determine the effect of OT at different times of the cycle. Frequent blood samples were taken for 60 min before and for 90 min after OT injection for the measurement of 15-keto-13,14-dihydro-PGF2 alpha (PGFM) by radioimmunoassay. The experiment was then repeated using the same animals at Day 17-19 of pregnancy (confirmed by the recovery of an embryo the day after OT injection). Following the injection of 1 IU OT, plasma PGFM reached its peak within 30 min with the increase significantly lower (P less than 0.05) in pregnant (1.13 +/- 0.10-fold) than in nonpregnant animals (2.07 +/- 0.27-fold). However, because only 3 of the 6 cyclic animals showed a response to 1 IU OT, the dose was increased to 100 IU in subsequent experiments. The animals that received 100 IU at Days 6 and 13 had no significant increase in PGFM concentrations (1.18 +/- 0.05-fold and 1.01 +/- 0.04-fold, respectively). At Day 17-19 the increase in plasma PGFM reached its peak 5-15 min after 100 IU OT and the increase was significantly greater in nonpregnant (3.23 +/- 0.17-fold) than in pregnant (1.21 +/- 0.02-fold; P = 0.003) heifers. Six of 11 animals injected at Day 17-19 of the cycle showed a decrease in progesterone (P4) the day after OT administration. These data show that the release of PGF2 alpha in response to OT is suppressed in pregnant animals in vivo, suggesting an antiluteolytic role for the embryo in luteostasis.     

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.     

Concentrations of prostaglandins E2, F2 alpha and 6-keto-prostaglandin F1 alpha in the utero-ovarian venous plasma of nonpregnant and early pregnant ewes

The effect of pregnancy on concentrations of prostaglandins E2, F2 alpha and 6-keto-prostaglandin F1 alpha (PGE2, PGF2 alpha and 6-keto-PGF1 alpha) in utero-ovarian venous plasma was examined in ewes on Days 10 through 14 after estrus, an interval which includes the critical period for maternal recognition of pregnancy. The utero-ovarian vein ipsilateral to a corpus luteum was catheterized on Day 9 or 10 in 6 pregnant and 8 nonpregnant ewes. Five blood samples were collected at 30-min intervals for 2 h beginning at 0500 and 1700 h daily. Sampling began at 0500 h on the day after catheterization. The mean and variance within each 2-h collection period were calculated for each ewe. The natural logarithm of the variance in each collection period (ln variance) was used as an estimate of the fluctuations in secretory activity by the endometrial-conceptus complex. Patterns of the mean concentrations of PGE2 were different between pregnant and nonpregnant ewes (P less than 0.01); PGE2 being higher in the pregnant ewes beginning on Day 13. There was a trend for the patterns of ln variance in PGE2 to differ (P less than 0.1) with pregnancy status over the entire period; ln variance was greater in pregnant ewes beginning on Day 13. The patterns of the mean concentrations and ln variances for PGF2 alpha and 6-keto-PGF1 alpha did not differ between pregnant and nonpregnant ewes. There were significant increases in both of these prostaglandins over time, independent of pregnancy status (P less than 0.01). The association of higher concentrations of PGE2 in utero-ovarian venous plasma with early pregnancy is consistent with the hypothesis that PGE2, originating from the uterus and/or conceptus, is one factor involved in maintenance of the corpus luteum of pregnancy.     

Effect of oxytocin on plasma concentrations of 13,14-dihydro-15-keto prostaglandin F and the oxytocin-associated neurophysin during the estrous cycle and early pregnancy in the ewe

This study was undertaken to determine the effect of exogenous oxytocin on plasma concentrations of the prostaglandin (PG) F metabolite 13,14-dihydro-15-keto-PGF (PGFM) and the oxytocin-associated neurophysin (OT-N) during the estrous cycle and early pregnancy in the ewe. Ewes were given oxytocin (250 mU, i.v.) on Days 3 (n = 4), 8 (n = 5), 13 (n = 4) or 14 (n = 5) of the estrous cycle, and a further 6 ewes were injected on Days 13 (n = 2) and 14 (n = 4) of pregnancy. No significant rises in plasma concentrations of PGFM were observed on Days 3 and 8 of the estrous cycle and on Days 13 and 14 of pregnancy. A marked increase in plasma PGFM concentrations occurred on Day 14 of the estrous cycle with the PGFM levels rising from a mean basal value of 120 pg/ml to a mean maximum value of 415 pg/ml within 2-10 min of administering oxytocin (P less than 0.001). No increases in plasma OT-N concentrations were found in early pregnancy and only 1 of 4 ewes at Day 14 of the cycle showed any significant increase in OT-N concentrations. It is concluded that there is an increase in the responsiveness of the uterine-PGF secretory system to oxytocin during the latter stages of the estrous cycle. During early pregnancy this response was blocked by the presence of the embryo.(ABSTRACT TRUNCATED AT 250 WORDS)     

Myometrial activity and plasma progesterone and oxytocin concentrations in cycling and early-pregnant ewes

Myometrial activity and plasma progesterone (P) and oxytocin (OT) were measured in early pregnant (n = 5) and cycling (n = 5) ewes. Electromyography (EMG) leads and jugular and inferior vena cava (IVC) catheters were surgically placed in ewes about 1 wk before data collection. When ewes returned to estrus, they were bred to either an intact or vasectomized ram. Continuous EMG data were collected, and blood samples were collected twice daily from day of estrus (Day 0) until Day 18. Ewes bred with an intact ram were checked surgically for pregnancy on Day 20. Computerized, quantitative analysis of EMG events showed no difference in signal from the right to left uterine horns, and no differences between pregnant and cycling ewes (p less than 0.05) until Days 14-18 when nonpregnant ewes returned to estrus and had increased EMG activity. The mean number of EMG events 180-900 s in length decreased in pregnant ewes, but this difference was not significant (p less than 0.05). Jugular plasma progesterone (P) levels confirmed corpus luteum (CL) formation in all ewes, and no differences in P between pregnant and nonpregnant ewes were measured until Days 14-18, when cycling ewes underwent luteolysis and pregnant ewes maintained CL. IVC plasma oxytocin concentrations were increased in pregnant ewes compared to concentrations in nonpregnant ewes on Days 5-13 (p less than 0.05), and the difference was largest at Day 6 (means +/- SEM pg/ml: pregnant = 68.7 +/- 13.9, nonpregnant = 30.9 +/- 19.9).(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of oxytocin on concentration of PGF2 alpha in the uterine lumen and subsequent endometrial responsiveness to oxytocin in pigs

The aim of this study was to determine the effect of oxytocin on PGF2 alpha secretion into the uterine lumen of pigs and subsequent endometrial responsiveness to oxytocin in vitro. Cyclic, pregnant and oestradiol-induced pseudopregnant gilts were injected i.v. with vehicle or 20 iu oxytocin 10 min before hysterectomy on day 16 after oestrus. Concentrations of PGF2 alpha and 13,14-dihydro-15-keto PGF2 alpha (PGFM) were significantly increased in uterine flushings collected at hysterectomy (P < 0.05) in pregnant oxytocin-injected gilts. Concentrations of PGF2 alpha and PGFM were greater (P < 0.001) in pregnant than in pseudopregnant and cyclic gilts, and greater (P < 0.01) in pseudopregnant than in cyclic gilts. The ratio of PGFM:PGF2 alpha tended to be greater in cyclic (P < 0.06) and pseudopregnant gilts (P < 0.1) than in pregnant gilts. At 85 +/- 5 min after oxytocin injection, endometrium from each gilt was incubated for 3 h for determination of phosphoinositide hydrolysis and PGF2 alpha secretion in response to treatment with 0 or 100 nmol oxytocin l-1. Endometrial phosphoinositide hydrolysis in response to 100 nmol oxytocin l-1 in vitro was greater (P < 0.05) in cyclic oxytocin-injected gilts than in cyclic vehicle-injected gilts. Treatment with oxytocin in vitro did not stimulate phosphoinositide hydrolysis significantly in vehicle- or oxytocin-injected pregnant gilts or pseudopregnant gilts. Endometrial PGF2 alpha secretion increased after treatment with 100 nmol oxytocin l-1 in vitro in cyclic vehicle-injected (P < 0.01), cyclic oxytocin-injected (P < 0.01), pregnant vehicle-injected (P = 0.06), pseudopregnant vehicle-injected (P < 0.05) and pseudopregnant oxytocin-injected (P < 0.05) gilts, but not in pregnant oxytocin-injected gilts. The increase in PGF2 alpha in pseudopregnant oxytocin-injected gilts was less (P < 0.05) than that in cyclic oxytocin-injected gilts. These results indicate that oxytocin increases the concentration of PGF2 alpha and PGFM in the uterine lumen during pregnancy and may upregulate endometrial responsiveness to oxytocin during late dioestrus in pigs, but does not have the latter effect during early pregnancy or oestradiol-induced pseudopregnancy.     

Proteins secreted by the sheep conceptus suppress induction of uterine prostaglandin F-2 alpha release by oestradiol and oxytocin

In Exp. I, 0.5 mg oestradiol or vehicle (0.5 ml absolute ethanol + 0.5 ml 0.9% NaCl) was injected i.v. at 08:00 h on Day 14 (onset of oestrus = Day 0). Blood samples were obtained via a jugular catheter at 30 and 1 min before oestradiol and every 30 min for 10 h afterwards. Plasma was obtained and assayed for 15-keto-13,14-dihydro-PGF-2 alpha (PGFM) by radioimmunoassay. Before oestradiol, PGFM basal values were higher (P less than 0.01) in pregnant (N = 10) than nonpregnant (N = 6) ewes (193 +/- 30 vs 67 +/- 8 pg/ml). However, at 4-10 h after oestradiol, pregnant ewes (N = 5) had less variable (P less than 0.01) PGFM values than did nonpregnant ewes (N = 5). In Exp II, conceptus secretory proteins (CSP) were obtained by pooling medium from cultures of Day-16 sheep conceptuses (N = 40). Ewes received 750 micrograms CSP + 750 micrograms plasma protein (N = 6) or 1500 micrograms plasma protein (N = 6) per uterine horn at 08:00 h and 18:00 h on Days 12-14. All ewes received 0.5 mg oestradiol at 08:00 h on Day 14 and blood samples were collected as in Exp. I and assayed for PGFM. On Day 15, 3 ewes in each group received 10 i.u. oxytocin and 3 received saline i.v. at 08:00 h and blood samples were taken continuously from 10 min before to 60 min after treatment. Mean PGFM response to oestradiol was suppressed (P = 0.05) in CSP- vs plasma protein-treated ewes (371 +/- 129 vs 1188 +/- 139 pg/ml).(ABSTRACT TRUNCATED AT 250 WORDS)     

Evidence that platelet-activating factor suppresses uterine oxytocin-induced 13,14-dihydro-15-keto-prostaglandin F2 alpha release and phosphatidylinositol hydrolysis in the ewe.

This study was conducted to determine whether platelet-activating factor (PAF) (1) attenuated oxytocin-induced secretion of the prostaglandin (PG) F2 alpha metabolite, PGFM, by the ovine uterus in situ and (2) inhibited the generation of the inositol phosphate secondary messengers by endometrial tissue in response to oxytocin challenge in vitro. Ovariectomized ewes received steroid replacement to mimic the luteal phase. Six ewes received intrauterine injections of 200 micrograms PAF/uterine horn/day on Days 11-15, and 6 ewes were treated with vehicle. All ewes received 1 microgram oxytocin i.v. on Days 13-16. Pretreatment of ewes with PAF significantly suppressed PGFM release in response to oxytocin on Days 14 and 15 (p less than 0.005) compared to vehicle-treated ewes. PAF was not administered on Day 16, and the PGFM response to oxytocin was not different between groups. In a second experiment, ewes were given intrauterine injections of 200 micrograms PAF/uterine horn/day (n = 8) or vehicle (n = 7) on Days 11-15, and all ewes received 1 microgram oxytocin i.v. on Days 13 and 14. On Day 15 the uterus was removed, and the incorporation of 3H-inositol into inositol phosphates was determined in caruncular endometrium. Treatment of ewes with PAF in vivo reduced inositol monophosphate (IP1) generated by oxytocin (10(-6) M) by 56.4%, compared to that in endometrium from vehicle-treated controls, and also inhibited the incorporation of 3H-inositol into glycerophosphoinositol (GPI). If PAF was added to the endometrium during the incubation in vitro, the attenuation of inositol phosphate generation did not occur.(ABSTRACT TRUNCATED AT 250 WORDS)     

Plasma 13,14-dihydro-15-keto-PGF alpha (PGFM) in pregnant and nonpregnant heifers prior to and during surgery and following intrauterine injection of PGF2 alpha

On day 17 postestrus or postmating, heifers were given intrauterine injections of saline (2 pregnant, 2 non-pregnant) or 200 micrograms PGF2 alpha (7 pregnant, 6 nonpregnant) through cannulae installed surgically into the uterine horn ipsilateral to the corpus luteum bearing ovary. Jugular blood samples were collected prior to the laparotomy at which the cannulae were installed during surgery, and for 90 min following the intrauterine injection. Plasma was assayed for progesterone and 13,14-dihydro-15-keto-PGF2 alpha (PGFM). Laparotomies were reopened to confirm proper cannula placement and to determine if blastocysts were present in mated heifers. Concentrations of PGFM were higher in pregnant compared to nonpregnant heifers during the presurgery (68 +/- 26 vs 24 +/- 26 pg/ml; P less than .025) and surgery (186 +/- 47 vs 65 +/- 17 pg/ml; P less than .05) periods. Pregnancy status did not alter the mean concentrations of PGFM (pregnant, 554 +/- 70 pg/ml; nonpregnant, 422 +/- 81 pg/ml) or the half-life of its decline in concentration (18 min) following intrauterine injection of PGF2 alpha. Pregnancy at 17 days in cattle does not appear to influence PGF2 alpha transport from the uterine lumen or its metabolism in the uterus or elsewhere in response to an acute intrauterine injection.     

Modification of prostaglandin F-2 alpha synthesis and release in the ewe during the initial establishment of pregnancy

Pregnant (N = 10) and non-pregnant (N = 10) ewes were bled every 2 h from Days 12 to 17 after oestrus (oestrus = Day 0). Plasma concentrations of progesterone, 15-keto-13,14-dihydro-PGF-2 alpha and 11-ketotetranor-PGF metabolites were determined in all samples. The number of PGF-2 alpha pulses in non-pregnant ewes was 8.2 +/- 0.4 (mean +/- s.e.m.) with an interpulse interval of 10.7 +/- 0.7 h. Two or 3 pulses of low frequency (interpulse interval = 13.4 +/- 1.6 h) occurred in most non-pregnant ewes before the onset of luteolysis; the interpulse interval then decreased to 7.9 +/- 0.4 h for the 6.0 +/- 0.3 pulses temporally associated with luteolysis. In contrast, the number of PGF-2 alpha pulses in pregnant ewes was lower (2.5 +/- 0.7, 0-8) and the interpulse intervals longer (18.9 +/- 6.1 h). Most pulses occurred on Days 14 and 15 in the pregnant and non-pregnant ewes. The mean concentrations of both PGF-2 alpha metabolites in non-pregnant ewes were highest on Day 15 while basal levels of both metabolites remained constant at all times. In pregnant ewes, the mean concentrations of both metabolites were highest on Day 14; basal concentrations of both metabolites were also highest on Day 14. The mean concentrations of 15-keto-13,14-dihydro-PGF-2 alpha were higher in pregnant than in non-pregnant ewes on Days 13 and 14 (P less than 0.05) and higher in non-pregnant than pregnant ewes on Day 15 (P less than 0.05). The basal concentrations of the 15-keto metabolite were higher in pregnant than non-pregnant ewes at Days 13, 14, 15, 16 and 17 (P less than 0.05). Both the mean and the basal concentrations of 11-ketotetranor-PGF metabolites were higher in pregnant than in non-pregnant ewes on Day 14 (P less than 0.05). It is concluded that uterine production of PGF-2 alpha peaks at Days 14-15 after oestrus in pregnant and non-pregnant ewes. Patterns of release differ, however, in that non-pregnant ewes have a pulsatile PGF-2 alpha pattern superimposed on a constant baseline, while pregnant ewes have an increasing basal secretory pattern which is more nearly continuous, i.e. not pulsatile in form. Modification of pulsatile PGF-2 alpha synthesis and release is therefore a key aspect of prolongation of luteal function at the beginning of pregnancy in the ewe.     

Effect of systemic or intrauterine injections of indomethacin on plasma oxytocin-neurophysin concentrations in ewes over the time of normal corpus luteum regression

This study was undertaken to investigate the effect of systemic or intrauterine injections of indomethacin, a known prostaglandin (PG) synthetase inhibitor, on peripheral plasma oxytocin-associated neurophysin (OT-N) concentrations in ewes over the time of expected luteolysis. In the first experiment, 9 ewes were given i.m. injections of indomethacin (4 mg/kg live weight, n = 4) or vehicle (n = 5) 3 times/day over Days 13-15 of the estrous cycle. Blood samples were collected at hourly intervals from 0700 h on Day 13 to 1800 h on Day 15 post-estrus. In the second experiment, indomethacin (20 mg, n = 5) or the injection vehicle (n = 4) was given twice daily into the uterine horn over Days 12-14 post-estrus. Blood samples were collected at hourly intervals from Day 12 to 14. In the third experiment, 4 additional ewes were bled at 5-min intervals from 1200 to 1600 h on Day 13 of the estrous cycle. Plasma samples were analyzed for OT-N and 13,14-dihydro-15-keto-prostaglandin F2 alpha (PGFM) to provide an indirect index for ovarian oxytocin and uterine prostaglandin F2 alpha release, respectively. Results from the first experiment indicated that surges in plasma OT-N concentrations occurred in the vehicle-treated ewes but were suppressed in ewes given systemic injections of indomethacin. Intrauterine indomethacin injections did not cause a significant reduction in the maximum peak height or number of peaks when compared with the control ewes. In the third experiment, there was a marked increase in plasma OT-N concentrations, but no significant rise in plasma PGFM concentrations in one ewe.(ABSTRACT TRUNCATED AT 250 WORDS)     

Secretion of PGF2alpha and oxytocin during hyperthermia in cyclic and pregnant heifers

The effects of acute heat stress (HS) and oxytocin (OT) injection on plasma concentrations of PGF2alpha and OT were examined in cyclic (C; n = 15) and pregnant (P; n = 11) dairy heifers. On Day 17 of synchronized estrous cycles, animals were randomly assigned to either thermoneutral (TN; 20 degrees C, 20% RH) or HS (42 degrees C, 60% RH) chambers. The jugular vein of each heifer was cannulated and blood samples collected hourly for 4 h, then every 15 min for an additional 3 h. Oxytocin (100 IU) was injected (IV) 5 h after the start of blood collection. Plasma samples were assayed subsequently for concentrations of 13,14-dihydro-15-keto PGF2alpha (PGFM) and OT. During the 7-h experiment, body temperature of HS heifers reached 41.2 degrees C as compared to 38.5 degrees C in control heifers. Plasma concentrations of PGFM increased (P<0.05) and peaked 30 min after OT injection in C (890 pg/ml) and P (540 pg/ml) heifers. In C heifers, heat stress failed to alter PGFM concentrations either before or after OT injection. In the P group, PGFM concentrations following OT injection tended to be higher in HS heifers were further TN heifers (peak values of 690 vs. 410 pg/ml). Pregnant TN and HS heifers were further classified as responders or non-responders to OT challenge according to a cutoff value for PGFM of 193 pg/ml (overall mean of C heifers minus 1 SD). Five of six HS and one of five TN pregnant heifers were classified as responders (P<0.06). Oxytocin concentrations in plasma prior to injection of exogenous OT were not affected by HS or pregnancy status. It is concluded that in C heifers, acute HS in vivo does not cause any further rise in PGF2alpha secretion. However, in P heifers, HS appears to antagonize suppressive effects of the embryo on uterine secretion of PGF2alpha, as indicated by the larger proportion of P heifers responding to OT challenge.