Effect of PGF2 alpha administration on the release of endogenous PGF2 alpha and oxytocin in indomethacin-treated goats

Repeated intramuscular injection of 1 mg prostaglandin F2 alpha (PGF2 alpha) during the luteal phase of the oestrous cycle of the goat hastened luteolysis and resulted in rapid increases in jugular concentrations of 13,14-dihydro-15-keto PGF2 alpha (PGFM), the primary metabolite of PGF2 alpha, and of oxytocin; similar injections of PGF2 alpha in indomethacin-treated goats had a reduced effect on PGFM and oxytocin concentrations, and failed to induce luteolysis. The same injections of PGF2 alpha were without effect on PGFM and oxytocin concentrations in ovariectomised goats. The results suggest that exogenous PGF2 alpha, or endogenous PGF2 alpha released at luteolysis, may induce the release of ovarian oxytocin in the goat.     

Hormone control of prostaglandin F(2 alpha) production and oxytocin receptor concentrations in bovine endometrium in explant culture.

The effect of progesterone and estradiol on basal and oxytocin-stimulated prostaglandin F(2 alpha) production and on oxytocin receptor concentrations in endometrium from long term ovariectomized cows was investigated using an explant culture system. Uteri were obtained from cows at slaughter and endometrial explants were cultured in triplicate for up to 96 h in either control media, or media containing progesterone or estradiol. Basal prostaglandin F(2 alpha) production was unaffected by progesterone treatment but was stimulated by estradiol treatment in a dose dependent manner. Oxytocin receptor concentrations remained unchanged in control culture and were unaffected by treatment with estradiol while treatment with progesterone caused a dose-dependent inhibition. Responsiveness to oxytocin, in terms of increased prostaglandin F(2 alpha) production, developed "spontaneously" over the first 24 h of culture and was unaffected by treatment with progesterone or estradiol. In summary the results reveal a dose-dependent inhibition of oxytocin receptor concentration by progesterone and a dose-dependent stimulation of basal PGF(2 alpha) release by estradiol. The reason for the "spontaneous" development of responsiveness to oxytocin remains unknown but may result from the removal of tissue from the influence of an, as yet unidentified, inhibitory factor.     

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

In long-term ovariectomized ewes and cows, endometrial oxytocin receptors rest at relatively high levels but oxytocin is unable to induce prostaglandin F(2alpha) release. A series of studies were carried out to investigate the roles of physiological levels of progesterone and estradiol in "activating" these receptors in terms of permitting oxytocin-induced prostaglandin F(2alpha) release. In long-term ovariectomized cows, treatment with progesterone, but not estradiol, resulted in the induction of responsiveness to oxytocin. This responsiveness appeared within 2 d of progesterone treatment, reached a maximum by 6 d and was maintained to Day 18. In ovariectomized ewes, while estradiol treatment did induce temporary responsiveness to oxytocin after 3 d of treatment, treatment with progesterone was required to induce sustained responsiveness that appeared by Day 9 of treatment and was maintained to Day 12. Measurement of endometrial receptors for oxytocin revealed a significant decline in oxytocin receptors by Day 6 of progesterone treatment when responsiveness to oxytocin was maximal, demonstrating that receptor concentrations were not a limiting factor. The most likely mechanism by which progesterone treatment induces responsiveness to oxytocin may be through the up regulation of post receptor signaling pathways and/or enzymes involved in prostaglandin synthesis.     

Delayed luteolysis and suppression of the pulsatile release of oxytocin after indomethacin treatment in the goat

Luteolysis in the goat was characterised by the pulsatile appearance of both oxytocin and 13,14-dihydro-15-keto-prostaglandin F2 alpha (PGFM), the primary pulmonary metabolite of prostaglandin (PG) F2 alpha, in the peripheral circulation. The episodic, surge release of oxytocin was superimposed on declining levels which paralleled the fall in progesterone concentrations. Daily subcutaneous administration of the prostaglandin synthetase inhibitor indomethacin (10 mg kg-1) between days 11 and 16, delayed luteolysis and suppressed both the decline in oxytocin concentrations and the pulsatile appearance of both oxytocin and PGFM in the peripheral circulation. Although there was little evidence of the release of the two hormones being synchronised, the results suggest that PGF2 alpha may stimulate pulsatile oxytocin release at luteolysis.     

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.     

Effect of constant infusion of oxytocin on luteal lifespan and oxytocin-induced release of prostaglandin F2α in heifers

Two experiments were conducted to determine whether constant infusion of oxytocin would prolong the luteal phase and inhibit uterine prostaglandin F2 alpha (PGF2 alpha) secretion in heifers. In Experiment 1, twelve heifers, treated with saline (SAL) or oxytocin (OXY) via jugular cannulae infusions (INF) or osmotic minipumps (OMP), were allotted at estrus into four treatment groups (n = 3). Treatments were: SAL-INF, SAL-OMP, OXY-INF and OXY-OMP. Physiological saline or oxytocin was given from Days 10 to 23 (Day 0 = estrus) of the estrous cycle. Method of treatment (jugular cannula infusion or osmotic minipump) had no effect (P greater than 0.05) on estrous cycle length or pattern of secretion of progesterone; therefore, data were pooled. Estrous cycle lengths were extended (P less than 0.01) for heifers which received oxytocin (25.3 +/- 0.4 d) compared to saline (20.5 +/- 0.4 d). Luteolysis did not occur in oxytocin-treated heifers until after treatment ceased. Experiment 2 was designed and conducted identically to Experiment 1 with the addition of a "challenge" injection of oxytocin (100 IU oxytocin, i.v.) given on Day 16 of the estrous cycle. Treatment of heifers with oxytocin extended (P less than 0.05) estrous cycle length by an average of 3 d compared to heifers treated with saline. The "challenge" injection induced (P less than 0.05) secretion of PGF2 alpha (as measured by the stable PGF2 alpha metabolite, 15-keto-13,14-dihydro-PGF2 alpha) in saline-treated but not oxytocin-treated heifers. In both Experiment 1 and 2, serum concentrations of FSH were elevated (P less than 0.05) in oxytocin-treated heifers. No increase was observed for LH or prolactin. The rise in estradiol-17 beta at luteolysis was not affected (P greater than 0.10) by treatment. In summary, constant infusion of oxytocin extended luteal lifespan, prolonged secretion of progesterone, and inhibited oxytocin-induced secretion of PGF2 alpha. Constant infusion of oxytocin did not affect serum concentrations of estradiol-17 beta, LH or prolactin; however, serum concentrations of FSH were elevated during the oxytocin treatment period.     

Relationship between uterine secretion of prostaglandin F2 alpha induced by oxytocin and endogenous concentrations of estradiol and progesterone at three stages of the bovine estrous cycle

The purpose of this experiment was to determine whether differences among cows in the ability of oxytocin to stimulate uterine secretion of prostaglandin F2 alpha (PGF2 alpha) were related to the endogenous ovarian steroid environment. Sexually mature heifers were treated with oxytocin (.33 IU/kg BW) at three stages of the estrous cycle: early (d 3 to 5; n = 5), middle (d 10 to 11; n = 5) or late (d 16 to 17; n = 5). To assess uterine responsiveness to oxytocin, concentrations of 13,14-dihydro-15-keto-PGF2 alpha (PGFM) were quantified in jugular venous plasma samples collected at 1/2-h intervals for 8 h postinjection. The ovarian steroid environment at the time of injection was estimated by measuring the concentrations of progesterone and estradiol in jugular venous plasma samples collected at 4-h intervals for 12 h immediately prior to injection. Concentrations of PGFM increased immediately following injection of oxytocin either early or late in the estrous cycle. The response was much less during the middle of the estrous cycle. The magnitudes of response, early and late in the estrous cycle, were similar and greater than that observed during the middle of the estrous cycle (P less than .05). There was a positive relationship (R2 greater than .8; P less than .05) between magnitude of the response to oxytocin and ratio of estradiol to progesterone both early and late in the estrous cycle. Thus, individual differences in uterine secretion of PGF2 alpha in response to oxytocin were related to stage of the cycle and to differences in the endogenous ovarian steroid environment within each stage of the estrous cycle.     

Differences in Reproductive Performance, Embryo Development, Interferon-tau Secretion by the Conceptus and Luteal Function in Ewe Lambs Synchronized in Oestrus before or after the Spontaneous Onset of Luteal Activity Preceding Puberty

In mid-September, 1 month before the insertion of intravaginal pessaries to induce sexual activity, blood samples were collected every 4 days from 16 ewe lambs aged 7 months, in order to determine the incidence of ovulations by measurement of plasma progesterone concentrations. It has been studied whether the response to a progestagen treatment of ewe lambs apparently close to puberty could be modified by the onset of the ovarian events preceding puberty. The effect of the presence or absence of ovulations prior to progestagen treatment on the potential reproductive performance (fertility, litter size and fecundity), embryo development [embryo quality and interferon-tau (IFNτ) secretion], luteal function (progesterone secretion in vitro ) and endometrial progesterone content was studied in seven ovulating (Ov+) and nine nonovulating ewe lambs (Ov−) on day 14 after mating. The best potential reproductive results were obtained with Ov+ animals, although these differences could not be initially attributed to either different progesterone secretion in vitro or concentration of endometrial progesterone. Irrespective of the experimental groups, secretion of progesterone by luteal tissue from ewe lambs with normal embryos was significantly greater (p<0.05) than that of animals with abnormal embryos or with no embryos. Normal embryos secreted a higher amount of IFN-τ than those embryos classified as abnormal (p<0.07). In conclusion, ewe lambs which exhibit luteal activity before puberty have the highest levels of reproductive performances after a progestagen treatment. Corpora lutea from ewe lambs with normal embryos had higher rates of progesterone secretion in vitro and their embryos had a higher IFN-τ production by the embryos, indicating greater capacity for subsequent development.     

Fish meal supplementation alters uterine prostaglandin F2alpha synthesis in beef heifers with low luteal-phase progesterone

The objective of the current study was to evaluate the effect of omega-3 fatty acids in fish meal on mitigating uterine PGF2alpha synthesis in heifers with low luteal-phase concentrations of progesterone. Animals were individually fed a corn silage-based diet supplemented with fish meal (5% of DMI; n = 12) or corn gluten meal (6% of DMI; n = 13). Estrous cycles were synchronized using PGF2alpha beginning on d 25 of supplementation. Random heifers from each supplement group (n = 6 fish meal, and n = 7 corn gluten meal) were given three additional i.m. injections of PGF2alpha (25 mg) at 12-h intervals beginning at 0600 on d 3 after estrus to induce formation of corpora lutea that secrete lower concentrations of progesterone. Jugular blood samples were collected daily commencing on d 1 and continuing through d 16 of the estrous cycle to determine serum progesterone concentrations. Oxytocin was administered i.v. (100 IU) to heifers on d 16 after estrus to stimulate uterine PGF2alpha synthesis. Before statistical analyses, heifers were sorted to either normal or low luteal-phase progesterone as determined from serum progesterone on d 9 of the estrous cycle. After sorting, treatment groups consisted of 1) normal luteal progesterone + fish meal (n = 6); 2) low luteal progesterone + fish meal (n = 6); 3) normal luteal progesterone + corn gluten meal (n = 6); and 4) low luteal progesterone + corn gluten meal (n = 7). Serum concentrations of the PGF2alpha metabolite following oxytocin stimulation tended (P = 0.09) to be greater in heifers with low luteal-phase progesterone compared with heifers with normal luteal-phase progesterone. Fish meal supplementation mitigated this response in heifers with low luteal-phase progesterone (P < 0.05), but had no effect on heifers with normal luteal-phase progesterone. In conclusion, the omega-3 fatty acids in fish meal seem to decrease uterine PGF2alpha synthesis in heifers with low luteal-phase serum concentrations of progesterone.     

The effect of progesterone on oxytocin-stimulated intracellular Ca2+ mobilisation and prostaglandin secretion in porcine endometrium

We have studied in the porcine endometrium the expression of oxytocin receptor (OTR) mRNA and the effect of progesterone (P4) on oxytocin/oxytocin receptor (OT/OTR) function concerning intracellular Ca2+ mobilisation ([Ca2+]i), prostaglandin F2alpha (PGF2alpha) and E2 (PGE2; PG) secretion. Tissue was taken from cyclic and early pregnant pigs (days 14-16). A higher expression of OTR mRNA (P < 0.05) was observed in the endometrium of cyclic than pregnant pigs. The stimulatory (P < 0.05) effect of OT (10(-7) M) on [Ca2+]i mobilisation was noticed within 15-60 s and 30-60 s in endometrial stromal cells of cyclic and pregnant pigs, respectively. In the presence of P4 (10(-5) M) basal and OT-stimulated [Ca2+]i concentrations decreased in stromal cells during luteolysis and pregnancy. In stromal cells P4 delayed mobilisation of [Ca2+]i in response to OT by 15 s during luteolysis and had no effect during pregnancy. In cyclic and pregnant epithelial cells OT stimulated mobilisation of [Ca2+]i in 45 s and 60 s, respectively. Oxytocin increased (P < 0.05) PGF2alpha secretion during luteolysis and pregnancy and PGE2 during luteolysis from endometrial slices. Progesterone did not inhibit this stimulatory effect. During luteolysis OT increased (P < 0.05) PGF2alpha in epithelial and stromal cells and PGE2 secretion in epithelial cells. In the presence of P4 this effect of OT was reduced only in stromal cyclic cells (6 h culture). The presence of P4 decreased the effect of OT on [Ca2+]i mobilisation only in stromal cells. We found that, in most conditions, P4 did not inhibit the OT-stimulated secretion of PG in the porcine endometrium.     

Steroids as local regulators of ovarian activity in domestic animals

The presented overview gives clear evidence for steroids as local regulators of follicular and luteal activity. In the follicle, estrogen receptor-alpha (ERalpha) and ERbeta expression are demonstrated in cow, ewe and pig. Besides species specific effects in general, there is evidence that estradiol-17beta (E(2)) exerts a dose-dependent inhibition on the secretion of progesterone (P(4)) by both theca interna cells (TI) and granulosa cells (GC). GC enhance the ability of the TI to produce androstendione by supplying them with progestin precursor. Androgen produced by TI enhances the ability of the GC to make E(2), and high concentrations of E(2) in the preovulatory follicle inhibit 3beta-HSD in both TI and GC and thus, may promote the use of the pathway Delta(5) for TI androgen production. The authors suggest that E(2) acts within the follicle to exert positive feedback on androgen and E(2) production, and exerts mitotic and anti-atretic or anti-apoptotic effects on follicular cells. Parts of the E(2)-mediated local action are regulated by stimulating effects on hormone receptors (LH, FSH, oxytocin). Gap junctions permit transfer of nutrients and cytokines to and from the avascular GC and oocyte, and formation is stimulated by estrogens. In bovine corpus luteum (CL) there is evidence that P(4) may directly regulate the production of P(4), oxytocin and prostaglandins (PGs) in a cycle dependent fashion. In most of domestic animal species, there is clear evidence for CL production of E(2) with clear stimulatory and luteotropic effects on P(4), and an intraluteal circuit that involves paracrine effects of E(2), oxytocin and PGF(2alpha) (especially in pigs). In contrast, there are species (ruminants, mares) in which the evidence for important local effects of E(2) is less clear, although expression of ERalpha, ERbeta and progesterone receptor (PR) is documented. Progesterone is very important for the regulation of CL lifetime by effects on the endometrium and release of the luteolytic signal PGF(2alpha). In conclusion, steroids as local regulators of ovarian activity are now documented and may stimulate further research in this field.     

Acquisition of luteolytic capacity involves differential regulation by prostaglandin F2alpha of genes involved in progesterone biosynthesis in the porcine corpus luteum

Luteolytic capacity is defined as the ability of corpora lutea (CL) to undergo luteolysis after prostaglandin (PG) F2alpha treatment. The mechanisms causing acquisition of luteolytic capacity are not yet identified but CL without luteolytic capacity have PGF2alpha receptors and respond to PGF2alpha with some changes in gene expression. Inhibition of progesterone biosynthesis is a key feature of luteolysis and therefore we postulated that genes involved in progesterone biosynthesis would be regulated by PGF2alpha differently in CL with or without luteolytic capacity. Gilts on day 9 after estrus (lack luteolytic capacity) or day 17 of pseudopregnancy (with luteolytic capacity) were treated with saline or a PGF2alpha analog (cloprostenol) and CL were collected 0.5 (Experiment I) or 10 h (Experiment II) later. In Experiment III, large luteal cells from CL on day 9 or 17 were cultured for 1, 12 and 24h with or without PGF2alpha. PGF2alpha decreased LDL receptor mRNA (27%), steroidogenic acute regulatory protein (StAR) mRNA (41%), StAR protein (75%), LH receptor mRNA (55%), and LH receptor protein (45%) at 10 h after treatment in day 17 but not day 9 CL. PGF2alpha increased DAX-1 mRNA at 0.5 h (43%) and 10 h (46%) after PGF2alpha in day 17 but not day 9 CL but decreased 3betaHSD mRNA ( approximately 20% at 10 h) in both days 9 and 17 CL. In vitro, PGF2alpha decreased StAR mRNA at 12 h only in day 17 luteal cells; however, continuous treatment with PGF2alpha for 24 h decreased StAR mRNA in both days 9 and 17 luteal cells. Thus, luteolytic capacity involves a critical change in responsiveness of DAX-1, StAR, and LH receptor to PGF2alpha that results in inhibition of luteal progesterone biosynthesis.     

Oxytocin stimulates secretion of prostaglandin F(2alpha) from endometrial cells of swine in the presence of progesterone

Oxytocin (OT) stimulates endometrial secretion of prostaglandin (PG) F(2 alpha) during corpus luteum regression in swine but there is differential responsiveness to OT among endometrial cell types. To determine if progesterone influenced responsiveness of luminal epithelial, glandular epithelial, and stromal cells to 100 nM OT during luteolysis in swine, cells were isolated from endometrium of 15 gilts by differential enzymatic digestion and sieve filtration on day 16 postestrus and cultured continuously in the presence of 0, 10 or 100 nM progesterone. For phospholipase C (PLC) activity and PGF(2 alpha) secretion, stromal cells were most responsive to OT (P<0.01) in the absence of progesterone, whereas luminal epithelial cells were unresponsive and glandular epithelial cells displayed an intermediate response to OT (P<0.09). Progesterone enhanced PLC activity linearly in glandular epithelial cells (P<0.05) and influenced it quadratically in stromal cells (P=0.05). The effect of OT and progesterone on PLC activity in luminal epithelial cells was not significant, and progesterone did not increase PLC activity in response to OT in any cell type. Culture in the presence of progesterone, enhanced PGF(2 alpha) secretion in response to OT in luminal epithelial cells (P<0.05) but not in glandular epithelial or stromal cells. Progesterone also increased overall PGF(2 alpha) release from glandular epithelial (P<0.05) and stromal cells (P<0.06) across both levels of OT treatment. These results indicate that progesterone enhanced PGF(2 alpha) secretion from luminal epithelial cells in response to OT and increased basal PGF(2 alpha) release from glandular epithelial and stromal cells.     

Oxytocin modulates the pulsatile secretion of prostaglandin F2alpha in initiated luteolysis in cattle

Subluteolytic doses of prostaglandin F2alpha analogue (oestrophan) given i.m. and oxytocin (OT) antagonist (CAP) and noradrenaline (NA) infused into the abdominal aorta were used to test the importance of luteal OT in pulsatile secretion of prostaglandin F2alpha (PGF) during luteolysis in heifers (n = 17). In experiment 1, heifers were pre-infused for 30 minutes with saline on either day 17 of the oestrous cycle (group 1; n = 4) or on day 18 of the oestrous cycle (group 2; n = 3), and with CAP (8 mg per animal) on day 17 of the oestrous cycle (group 3; n = 4). Next, heifers were injected with oestrophan (30 microg per animal). Injection of oestrophan in Group 3 increased OT concentrations (P < 0.001) to values similar to those observed during spontaneous luteolysis (50 to 70 pg ml(-1)). PGFM concentrations in this group also increased (P < 0.001), but were lower (P < 0.05) than the values in groups 1 and 2, CAP given prior to oestrophan decreased both PGFM elevation (P < 0.06) and its area under the curve (P < 0.01), compared to the saline pretreated heifers. In experiment 2 NA (4 mg) was infused twice for 30 minutes at five hour intervals to release OT on day 17 of the oestrous cycle (n = 6). However, during hormone analysis it appeared that three of six heifers had elevated PGFM concentrations (group 1) and three others did not (group 2). NA caused the correlated increase of progesterone and OT secretion (r = 0.68; P < 0.05) in both groups but it only influenced PGF secretion in group 1 only (P < 0.05). We postulate that OT can amplify and modulate the course of induced luteolysis as a regulator of the amplitude of pulsatile PGF secretion. PGF analogue stimulates secretion of endogenous PGF from the uterus in cattle and this may be an important component of the luteolytic response to exogenous PGF.     

Reproductive response of yearling beef heifers to a melengestrol acetate-prostaglandin F2 alpha estrus synchronization system.

A study was designed to evaluate estrus response and fertility after treatment with melengestrol acetate (MGA) and prostaglandin F2 alpha (PGF2 alpha) in yearling beef heifers. Three hundred four heifers at three locations were allotted to one of two treatments: Treatment 1 served as a nonsynchronized control (CON); and heifers in Treatment 2 received .5 mg of MGA.animal-1.d-1 for 14 d and 25 mg of prostaglandin F2 alpha (PGF2 alpha) 17 d after MGA (MGA-PGF). Heifers in CON and MGA-PGF groups were artificially inseminated 12 h after observed estrus for 21 and 6 d after PGF2 alpha, respectively. Blood samples were collected from each heifer 10 d before and on the day MGA feeding began and 10 d before and on the day PGF2 alpha was administered. Heifers with concentrations of serum progesterone greater than 1 ng/mL on either date before administration of MGA or PGF2 alpha were considered pubertal. More (P = .02) prepubertal heifers that received MGA attained puberty by initiation of breeding than did CON heifers (72 vs 45%, respectively). The proportion of heifers that displayed estrus within 6 d after PGF2 alpha was greater (P less than .001) for MGA-PGF than for CON heifers (77 vs 25%, respectively) but was also influenced by location (P = .03). Conception rate at first service for MGA-PGF heifers that attained puberty during MGA feeding and before PGF2 alpha was not different (P = .50) from that of CON that attained puberty during the same period.(ABSTRACT TRUNCATED AT 250 WORDS)     

Efficacy of an intravaginal progesterone insert and an injection of PGF2alpha for synchronizing estrus and shortening the interval to pregnancy in postpartum beef cows, peripubertal beef heifers, and dairy heifers

The objective was to test the efficacy of an intravaginal progesterone insert and injection of PGF2alpha for synchronizing estrus and shortening the interval to pregnancy in cattle. Cattle were assigned to one of three treatments before a 31-d breeding period that employed artificial insemination. Control cattle were not treated, and treated cattle were administered PGF2alpha or an intravaginal progesterone-releasing insert (CIDR) for 7 d and treated with PGF2alpha on d 6. The treatments were applied in one of three experiments that involved postpartum beef cows (Exp. 1; n = 851; 56+/-0.6 d postpartum), beef heifers (Exp. 2; n = 724; 442.5+/-2.8 d of age), and dairy heifers (Exp. 3; n = 260; 443.2+/-4.5 d of age). Luteal activity before treatment was determined for individual cattle based on blood progesterone concentrations. In Exp. 1, there was a greater incidence of estrus during the first 3 d of the breeding period in CIDR+PGF2alpha-treated cows compared with PGF2alpha-treated or control cows (15, 33, and 59% for control, PGF2alpha, and CIDR+PGF2alpha, respectively; P < 0.001). The improved estrous response led to an increase in pregnancy rate during the 3-d period (7, 22, and 36% for control, PGF2alpha, and CIDR+PGF2alpha, respectively; P < 0.001) and tended to improve pregnancy rate for the 31-d breeding period for cows treated with CIDR+PGF2alpha, (50, 55, and 58% for control, PGF2alpha, and CIDR+PGF2alpha, respectively, P = 0.10). Improvements in rates of estrus and pregnancy after CIDR+PGF2alpha, were also observed in beef heifers. Presence of luteal activity before the treatment period affected synchronization and pregnancy rates because anestrous cows (Exp. 1) or prepubertal heifers (Exp. 2) had lesser synchronization rates and pregnancy rates during the first 3 d of the breeding period as well as during the entire 31-d breeding period. The PGF2alpha, and CIDR+PGF2alpha but not the control treatments were evaluated in dairy heifers (Exp. 3). The CIDR+PGF2alpha-treated heifers had a greater incidence of estrus (84%) during the first 3 d of the breeding period compared with the PGF2alpha-treated heifers (57%), but pregnancy rates during the first 3 d or during the 31-d breeding period were not improved for CIDR+PGF2alpha compared with PGF2alpha-treated heifers. In summary, the concurrent treatment of CIDR and PGF2alpha improved synchronization rates relative to PGF2alpha alone or control. Improved estrus synchrony led to greater pregnancy rates for beef cows and beef heifers but failed to improve pregnancy rates for dairy heifers.     

Inhibitory effects of CIDR-based ovulation-synchronization protocols on uterine PGF2alpha secretion at the following luteal phase in early postpartum non-cycling beef cows

We investigated whether CIDR-based ovulation-synchronization protocols inhibit secretion of prostaglandin (PG) F2alpha from the uterus in the following luteal phase in non-cycling beef cows. Ten early (a month) postpartum non-cycling Japanese Black beef cows were treated with (1) Ovsynch (GnRH analogue on Day 0, PGF2alpha analogue on Day 7, and GnRH analogue on Day 9; n=3), (2) Ovsynch+CIDR (Ovsynch protocol plus a CIDR for 7 days from Day 0; n=4), or (3) estradiol benzoate (EB) Ovsynch+CIDR (EB on Day 0 in lieu of the first GnRH treatment followed by the Ovsynch+CIDR protocol; n=3). An oxytocin challenge was administered on Day 24 to examine uterine PGF2alpha secretion. Plasma concentrations of 13,14-dihydro-15-keto- PGF2alpha were lower at 30-120 min after oxytocin administration in the Ovsynch+CIDR group and 75 min after administration in the EB Ovsynch+CIDR group than in the Ovsynch group (P<0.05). Plasma progesterone concentrations were higher from Days 1 to 7 in the Ovsynch+CIDR group and from Days 1 to 5 in the EB Ovsynch+CIDR group than in the Ovsynch group (P<0.05). The progesterone concentrations were higher on Days 27 and 29 in both CIDR-treated groups than in the Ovsynch group (P<0.05). In conclusion, in non-cycling beef cows, CIDR-based ovulation-synchronization protocols inhibit uterine PGF2alpha secretion in the following luteal phase and prevent premature luteolysis as is seen with the Ovsynch protocol.     

Effect of an oxytocin antagonist on prostaglandin F2 alpha secretion and the course of luteolysis in sows.

The role of oxytocin (OT) in the regulation of prostaglandin F2 alpha (PGF2 alpha) secretion during luteolysis in gilts was studied using a highly specific OT antagonist (CAP-581). In Experiment 1 gilts on Days 14 to 19 of the oestrous cycle in Latin square design were used, to determine the dose and time of application of OT and CAP. In Group I (n = 6) gilts were treated intravenously with saline or with 10, 20 and 30 IU of OT. Concentrations of the main PGF2 alpha metabolite i.e. 13,14-dihydro-15-keto-prostaglandin F2 alpha (PGFM) were measured in blood samples as uterine response to the treatment. Twenty IU of OT was the most effective to stimulate PGFM release and this dose was used after CAP treatment in gilts of Groups II, III and IV. Gilts of Group II (n = 3) were injected into the uterine horns (UH) with saline (5 ml/horn) or CAP (2 mg, 3 mg and 4 mg; half dose/horn) and OT was injected (i.v.) 30 min thereafter. Any of the CAP doses given into the UH affected PGFM plasma concentrations stimulated by OT. In Group III (n = 4) gilts were infused (i.v.) for 30 min with CAP (9 mg, 14 mg and 18 mg/gilt) followed by 20 IU of OT. All doses of CAP effectively inhibited OT-stimulated PGF2 alpha release, therefore 9 mg was selected for the further studies. Gilts of Group IV (n = 4) received OT 4, 6 and 8 h after CAP to define how long CAP blocks the OT receptors. Concentrations of PGFM increased after any of this period of time. Thus, we concluded that 9 mg of CAP infused every 4 h will effectively block OT receptors. In Experiment 2, gilts (n = 4) received CAP as a 30-min infusion every 4 h on Days 12-20 of the oestrous cycle. Control gilts (n = 3) were infused with saline. CAP infusions diminished the height of PGFM peaks (P < 0.05). Frequency of the PGFM (P < 0.057) and OT (P < 0.082) peaks only tended to be lower in the CAP-treated gilts. Peripheral plasma concentrations of progesterone (P4) and oestradiol-17 beta (E2) and the time of luteolysis initiation as measured by the decrease of P4 concentration were the same in CAP- and saline-treated gilts. The macroscopic studies of the ovaries in gilts revealed lack of differences between groups. We conclude that OT is involved in the secretion of luteolytic PGF2 alpha peaks but its role is limited to controlling their height and frequency. Blocking of OT receptors did not prevent luteolysis in sows.     

Effects of plane of nutrition, growth hormone and unsaturated fat on growth hormone, insulin and prolactin receptors in prepubertal lambs

The influence of plane of nutrition, growth hormone (GH) treatment and dietary polyunsaturated fat on serum concentrations of GH and insulin (INS) and binding capacities of GH, INS and prolactin (PRL) in liver, mammary parenchyma and adipose tissue was assessed in prepubertal ewe lambs. Ten lambs were assigned to each of four treatment groups. Treatments included: (A) lambs with ad libitum access to a high-energy ration; (G) lambs fed as group A and treated with bovine GH (.08 mg/kg/d); (R) lambs with feed intake restricted to limit ADG to about 120 g; and (S) lambs with ad libitum access to a ration including formaldehyde-protected sunflower seed. Diets, all approximately isonitrogenous and isocaloric, were fed from about 7 to 22 wk of age. Weekly blood samples were collected during wk 6 to 14 of the trial. Averaged across sampling dates, mean serum concentrations of GH were elevated in G lambs (P less than .05) and INS concentrations differed in the order G greater than A greater than R = S (P less than .05). Crude membranes for binding assays were prepared from liver, mammary parenchyma and adipose tissue. Mean concentrations of GH receptors in liver and PRL receptors in mammary parenchyma were elevated in group S lambs (P less than .01). Dietary polyunsaturated fat increased the number of GH receptors in liver and PRL receptors in mammary parenchyma. Increased availability of receptors may mediate the stimulation of mammary growth observed in lambs fed polyunsaturated fat.     

Effect of feed treatment and exogenous estrogen and progestogen on puberty and lambing rates in ewe lambs

Four hundred seventy-four ewe lambs (5 to 6 mo of age) were assigned within breed to two postweaning feed treatments; 1 = alfalfa pellets ad libitum and 2 = alfalfa pellets plus 20% barley or wheat ad libitum. Ten days prior to the start of breeding, approximately one-half of the ewe lambs within each feed treatment were treated with a single injection of 2.5 mg of estradiol valerate and 1.5 mg of norgestomet and implanted with 3 mg of norgestomet for 8 d. At the start of breeding, fertile Suffolk rams were fitted with marking harnesses and penned with ewe lambs; evidence of estrus and breeding was determined on a weekly basis by visual examination for breeding marks. In the second year of the study, 7 to 13 d after recording estrous activity, all marked ewe lambs were bled; and blood was assayed for progesterone. In 1983, 60% of the ewe lambs showed estrus and 11% lambed compared with 48% exhibiting estrus and 30% lambing in 1984 (P less than .01). More (P less than .01) ewe lambs on feed treatment 1 displayed estrus, but more (P less than .05) lambed in the feed treatment 2 group. Steroid treatment resulted in more (P less than .01) ewe lambs showing estrus but fewer lambing (P less than .01). Examination of progesterone concentrations for evidence of ovulation and corpus luteum development indicated that treatment with steroids caused a large percentage of ewe lambs to exhibit estrus. However, many of these failed to develop a corpus luteum and become pregnant.(ABSTRACT TRUNCATED AT 250 WORDS)