Luteal oxytocin: characteristics and control of synchronous episodes of oxytocin and PGF2 alpha secretion at luteolysis in ruminants

Estrous cycle stage-dependent manner of type I interferon-stimulated genes induction in the bovine endometrium

Interferon tau (IFN-τ) is a ruminant-specific type I IFN secreted by a conceptus before its attachment to the uterus. IFN-τ induces the expression of IFN-stimulated genes (ISGs) via the type I IFN receptor (IFNAR), which is composed of IFNAR1 and IFNAR2 subunits in the endometrium. However, expression patterns of IFNARs during the estrous cycle have not been reported. We hypothesized that the response to a type I IFN changes along with IFNARs and the IFN-regulatory factors (IRFs) driving transcription of IFN signal-related genes and modulating a type I IFN signal during the estrous cycle. We investigated the estrous cycle stage-dependent type I IFN induction of ISGs and expression patterns of IFN signal-related genes in bovine endometrial tissues. Endometrial tissue pieces collected from bovine uteri at each estrous stage (early, mid, and late) were cultured with or without recombinant bovine IFN-α or concentrated pregnant uterine flushing (PUF) on day 18 after confirming the presence of a conceptus. IFN-α and PUF each significantly increased the expression of ISGs in endometrial tissues. The induction levels of the typical ISGs (MX1-a and ISG15) were significantly higher at the mid stage and correlated with high expression of IRFs at the mid stage. The immunostaining of IFNARs showed strong fluorescence intensities in luminal and glandular epithelia at the early and mid stages. Collectively, these results suggest that the endometrium exhibits estrous cycle stage-dependent responsiveness to type I IFN that may be associated with the expression of IFNARs and IRFs for pregnancy recognition.

A mathematical model of the bovine oestrous cycle: simulating outcomes of dietary and pharmacological interventions

A mathematical model was constructed to simulate the bovine oestrous cycle by using nonlinear differential equations to describe the biological mechanisms which regulate the cycle. The model predicts circulating concentrations of gonadotrophin-releasing hormone, follicle-stimulating hormone, luteinizing hormone, oestradiol, inhibin and progesterone. These hormones collectively provide control and feedback mechanisms between the hypothalamus, pituitary gland and ovaries, which regulate ovarian follicular dynamics, corpus luteum function and ovulation. When follicular growth parameters are altered, the model predicts that cows will exhibit either two or three follicular waves per cycle, as seen in practice. Changes in other parameters allow the model to simulate: effects of nutrition on follicle recruitment and size of the ovulatory follicle; effects of negative energy balance on postpartum anoestrus; and effects of pharmacological intervention on hormone profiles and timing of ovulation. It is concluded that this model provides a sound basis for exploring factors that influence the bovine oestrous cycle in order to test hypotheses about nutritional and hormonal influences which, with further validation, should help to design dietary or pharmacological strategies for improving reproductive performance in cattle.

Effect of phthalate esters on the secretion of prostaglandins (F2alpha and E2) and oxytocin in cultured bovine ovarian and endometrial cells

The influence of phthalate esters di-2-ethylhexyl phthalate (DEHP) and mono-2-ethylhexyl phthalate (MEHP) on uterine prostaglandin (PGF2alpha and PGE2) and ovarian oxytocin secretion was investigated. Endometrial, granulosa, and luteal cells from cows on days 8-12 of the estrous cycle were treated with DEHP or MEHP (0.1, 1, or 10 ng/mL). We found that DEHP and MEHP stimulated (P < 0.05) secretion of PGF2alpha and inhibited (P < 0.001) secretion of PGE2 from endometrial cells. The ratio of PGF2alpha to PGE2 was markedly altered. The endocrine disrupting chemicals also enhanced secretion of oxytocin (P < 0.05) from ovarian cells. Our results indicated that DEHP and its metabolite MEHP could affect the process of the estrous cycle by impairing secretion of prostaglandin from the uterus and oxytocin from the ovary.

Oxytocinase/insulin-regulated aminopeptidase is distributed throughout the sheep, female reproductive tract and is regulated by oestrogen in the uterus

Using [(125)I]Angiotensin IV (Ang IV) for the autoradiographic localisation of oxytocinase/insulin-regulated aminopeptidase (IRAP), we demonstrate for the first time that IRAP is distributed throughout the female reproductive tract. The highest concentration of IRAP was detected in the outer myometrial layer of the uterus with lower levels in the inner myometrial layer and in the luminal epithelium. High levels of the enzyme was also detected in the inner mucosal lining of the ampulla segment of the fallopian tubes with lower levels in the interstitial and isthmus. In the ovary, a high level of IRAP was found in the corpus albicans with lower levels throughout the ovarian cortex and the surrounding connective tissue. In the uterine body of ovariectomised (OVX) ewes, oestrogen treatment resulted in a significant decrease (P<0.05) in the level of IRAP in the outer myometrium. These findings indicate an important role for IRAP in reproductive physiology in regulating the action of peptide hormones.

Influence of different isoforms of recombinant trophoblastic interferons on prostaglandin production in cultured bovine endometrial cells

In ruminants, interferon produced by the trophectoderm (IFN-tau) is recognized as the embryonic signal responsible for maternal recognition of pregnancy. IFN-tau is believed to act by down-regulating estrogen receptors, thus preventing appearance of oxytocin receptors responsible for the release of prostaglandin F(2alpha) (PGF(2alpha)) by the endometrium. The present study was undertaken to determine in vitro the biological activities of different IFN-tau isoforms and document putative alternate luteotrophic mechanisms. Endometrial cells in primary cultures were treated with five different rIFN-tau isoforms: two ovine isoforms (ro-4 and ro-11) and three bovine isoforms (rb-1a, rb-2b and rb-3b). Their effect was quantified by measurement of PGE(2) and PGF(2alpha) production by ELISA and induction of cyclooxygenase (COX-2) by Western and Northern analysis and correlated with antiviral activity previously reported. The overall pattern of response to the IFNs tested suggests that low concentrations (<1 microg/ml) reduced the production of both PGs and higher concentrations (>1 microg/ml) stimulated preferentially PGE(2); however, exceptions were noted. Isoform rb-2b with high antiviral activity inhibited PG production in both cell types at all concentrations tested. IFNs rb-1a and ro-11 had similar antiviral activities, inhibiting PG at low concentrations and stimulating them at high concentrations. Isoform rb-3b stands out relative to the other IFNs tested because it induced a variable non-dose-dependent effect on PG production and low antiviral activity. An increase in COX-2 protein expression and messenger was correlated with increased PG production. The results showing two distinct responses to IFN-tau depending on its concentration and/or isoform and the absence of correlation with antiviral activity suggest that complex transduction mechanisms are involved.

Gene regulation in the magnocellular hypothalamo-neurohypophysial system

The hypothalamo-neurohypophysial system (HNS) is the major peptidergic neurosecretory system through which the brain controls peripheral physiology. The hormones vasopressin and oxytocin released from the HNS at the neurohypophysis serve homeostatic functions of water balance and reproduction. From a physiological viewpoint, the core question on the HNS has always been, "How is the rate of hormone production controlled?" Despite a clear description of the physiology, anatomy, cell biology, and biochemistry of the HNS gained over the last 100 years, this question has remained largely unanswered. However, recently, significant progress has been made through studies of gene identity and gene expression in the magnocellular neurons (MCNs) that constitute the HNS. These are keys to mechanisms and events that exist in the HNS. This review is an inventory of what we know about genes expressed in the HNS, about the regulation of their expression in response to physiological stimuli, and about their function. Genes relevant to the central question include receptors and signal transduction components that receive and process the message that the organism is in demand of a neurohypophysial hormone. The key players in gene regulatory events, the transcription factors, deserve special attention. They do not only control rates of hormone production at the level of the gene, but also determine the molecular make-up of the cell essential for appropriate development and physiological functioning. Finally, the HNS neurons are equipped with a machinery to produce and secrete hormones in a regulated manner. With the availability of several gene transfer approaches applicable to the HNS, it is anticipated that new insights will be obtained on how the HNS is able to respond to the physiological demands for its hormones.

Phosphorylation of myristoylated alanine-rich C kinase substrate (MARCKS) protein is associated with bovine luteal oxytocin exocytosis

The ruminant corpus luteum, in addition to producing progesterone, synthesizes and secretes oxytocin (OT) during the estrous cycle. Secretion of oxytocin occurs by exocytosis of membrane-encapsulated granules of this hormone. Exocytosis of oxytocin involves transport of granules through a cytoskeletal matrix including an actin cortex closely associated with the plasma membrane (PM). Actin filaments crosslinked by various proteins give rise to the structural integrity of the cortex. Myristoylated alanine-rich C kinase substrate (MARCKS), a protein specifically phosphorylated by protein kinase C (PKC), crosslinks actin filaments and anchors the actin network to the inner leaflet of the PM. There is evidence that the intact actin cortex may serve as a barrier, precluding fusion of transport vesicles with the PM. In some secretory cells, phosphorylation of MARCKS has resulted in its translocation from the PM to the cytoplasm with an associated disassembly of the actin cortex. Prostaglandin F(2alpha) (PGF(2alpha)) stimulation of the bovine corpus luteum during the midluteal phase of the estrous cycle activates PKC, which is associated with an increase in OT secretion in vivo and in vitro. Data are presented demonstrating that stimulation of bovine luteal cells with PGF(2alpha) on Day 8 of the cycle promotes rapid phosphorylation of MARCKS protein and causes its translocation from the PM to the cytoplasm and concomitant, enhanced exocytosis of OT. These data are consistent with the premise that MARCKS plays a role in the exocytotic process.

Is tumor necrosis factor alpha a trigger for the initiation of endometrial prostaglandin F(2alpha) release at luteolysis in cattle?

To determine the physiological significance of tumor necrosis factor alpha (TNFalpha) in the regulation of luteolytic prostaglandin (PG) F(2alpha) release by the bovine endometrium, the effect of TNF-alpha on PGF(2alpha) output by the endometrial tissues in vitro was investigated and compared with the effect of oxytocin (OT). Furthermore, the presence of specific receptors for TNFalpha in the bovine endometrium during the estrous cycle was determined. Endometrial slices (20-30 mg) taken from six stages of the estrous cycle (estrus: Day 0; early I: Days 2-3; early II: Days 5-6; mid-: Days 8-12; late: Days 15-17; and follicular: Days 19-21), as determined by macroscopic examination of the ovaries and uterus, were exposed to TNFalpha (0.06-6 nM) and/or OT (100 nM). OT stimulated PGF(2alpha) output at the follicular stage and at estrus (P < 0.001), but not at the late luteal stage. On the other hand, the stimulatory effects of TNFalpha on PGF(2alpha) output were observed not only at the follicular stage but also at the late luteal stage (P < 0.001). When the endometrial tissues at late luteal stage were simultaneously exposed to TNFalpha (0.6 nM) and OT (100 nM), the stimulatory effect on PGF(2alpha) output was higher than the effect of TNFalpha or OT alone (P < 0.05). Specific binding of TNFalpha to the bovine endometrial membranes was observed throughout the estrous cycle. The concentration of TNF-alpha receptor at the early I luteal stage was less than the concentrations at other luteal stages (P < 0.01). The dissociation constant (K(d)) values of the endometrial membranes were constant during the estrous cycle. The overall results lead us to hypothesize that TNFalpha may be a trigger for the output of PGF(2alpha) by the endometrium at the initiation of luteolysis in cattle.

Intracellular regulation of endometrial PGF(2a) and PGE(2) production in dairy cows during early pregnancy and following treatment with recombinant interferon-tau

Objectives were to examine how the conceptus and recombinant bovine interferon-tau (rbIFN-tau) regulate intracellular components of the PGF(2a) synthetic pathway and to determine if arachidonic acid (AA) is limiting in endometrial tissue of pregnant cows. In Experiment 1, uteri were collected from either cyclic or pregnant dairy cows on Day 17 post-estrus. Intercaruncular explants were dissected and incubated for 60 min to quantify PGF(2a) production in response to oxytocin (10(-6) M), A23187 (10(-5) M), melittin (10(-5) M), and phorbol 12, 13 dibutyrate (PDBu, 10(-6) M). Additional explants from the same cows were incubated for 24 h with and without AA. Oxytocin and A23187 did not stimulate PGF(2a) in explants from either cyclic or pregnant cows. Both PDBu, melittin, and A23187 + melittin stimulated PGF(2a) production in explants of cyclic cows, but not in explants of pregnant cows. The addition of AA to explant cultures for 24 hr did not increase PGF(2a) production during a subsequent 60-min incubation. In Experiment 2, explants were collected from cows that received intrauterine infusions of either BSA (1.9 mg/1.2 ml) or rbIFN-tau (0.2 mg rbIFN-tau + 1.7 mg BSA/1.2 ml) twice a day from Days 14 to 17 of the estrous cycle. Treatments of rbIFN-tau attenuated PGF(2a) secretion induced by in vitro PDBu and A23187 treatments. However, rbIFN-tau treatment in vivo had no effect on the in vitro induction of PGF(2a) secretion by melittin. IFN-tau may regulate the PGF(2a) synthetic pathway by reducing activity of PKC or PKC mediated events.

The role of luteal oxytocin in episodic secretion of prostaglandin F2alpha at luteolysis in the ewe

The role of luteal oxytocin in the generation of luteolytic episodes of prostaglandin F2alpha at luteolysis was investigated. On day 10 of the cycle Dorset ewes underwent either surgical removal of the corpora lutea (lutectomy; n = 4) or sham operation (sham; n = 4). Lutectomised ewes were then administered progesterone by twice daily i.m. injection in corn oil (20 mg/day) until day 14 when treatment was ceased to simulate luteolysis. The concentration of 13, 14 dihydro-15-keto prostaglandin F2alpha (PGFM) was measured in peripheral blood samples collected at 20-min intervals for 8 h on days 12-16 of the cycle. Progesterone and oestradiol concentrations were similar in the two groups over the whole experimental cycle while oxytocin fell dramatically following lutectomy. No prostaglandin F2alpha release episodes were seen on day 12 or 13, while from days 14-16 both groups exhibited a similar episode frequency (lutectomy 0.9/ewe/8 h; sham 0.8/ewe/8 h). Analysis of episode characteristics revealed lower episode amplitude (p<0.05) but longer episode duration (p<0.05) in the lutectomy group. The results demonstrate that a normal frequency of prostaglandin F2alpha release episodes occurs independently of luteal oxytocin secretion. However, luteal oxytocin is involved in regulating the pattern of release, perhaps causing the release of episodes of the magnitude required for the successful completion of luteolysis.

Effects of administration of oxytocin on embryonic survival in progestogen supplemented cattle

Embryonic survival after administration of oxytocin (OT) was examined in 42 beef cows. All cows were bred (Day 0) and randomly assigned to receive either 25 mL saline (CON; n = 10), 100 IU OT + 20 mL saline (OT; n = 12), 100 IU OT + 1 g flunixin meglumine (OT + FM; inhibitor of prostaglandin endoperoxide synthase; n = 10), or 100 IU OT + lutectomy (OT + LUT; n = 10) administered (i.m.) at 8-h intervals on Days 5-8 after mating. Lutectomies were performed by transrectal digital pressure prior to initiation of treatments (0600, Day 5). All cows were fed 4 mg/head/day of melengesterol acetate (an orally administered exogenous progestogen) through Days 3-30 and were bled by jugular venipuncture at 0600 and 0700 h on Day 5 for determination of 13,14-dihydro-15-keto-PGF2a (PGFM). Pregnancy rates, as determined by transrectal ultrasonography at Day 30, were reduced in OT (33.3%) and OT + LUT (30%) groups compared to CON and OT + FM (80%; p < or = 0.03). Number of short cycles were increased in OT (n = 6/12) group compared to CON (n = 0/10; p < or = 0.009) and OT + FM (n = 1/10; p < or = 0.045). Mean change in PGFM from the 0600 to 0700 h bleed was different (p < or = 0.01) between the OT + LUT (31.6 +/- 11.0 pg/mL) group versus CON (-11.2 +/- 10.6 pg/mL) and OT + FM (-13.8 +/- 10.6 pg/mL) groups. Administration of oxytocin appears to decrease embryonic survival by stimulating uterine PGF2a. Thus, previous reports indicating that removal of the corpus luteum during progestogen supplementation and prior to PGF2a administration increases embryonic survival can be explained through interruption of the luteal oxytocin-uterine PGF2a feedback loop.

Luteolysis: a neuroendocrine-mediated event

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

Cellular mechanisms by which oxytocin mediates uterine prostaglandin F2 alpha synthesis in bovine endometrium: role of calcium

The objective of these experiments was to determine the role of Ca2+ during oxytocin-stimulated prostaglandin (PG) F2 alpha release from bovine endometrial tissue in vitro. Uteri were collected from dairy cows on the day after spontaneous luteal regression. Caruncular endometrial explants were dissected and incubated in vitro to determine phospholipase C activity or PGF2 alpha release. A23,187 (a calcium ionophore) and maitotoxin (an activator of voltage-gated L-type calcium channels) stimulated release of PGF 2 alpha in a concentration-dependent manner (P < 0.05). Thapsigargin (induces accumulation of Ca2+ in the cytoplasm by inhibiting endoplasmic reticulum Ca2+/ATPase pumps) stimulated release of PGF2 alpha in a concentration-dependent manner as well (P < 0.13). Oxytocin (10(-6) M), AIF4- (a nonspecific activator of G-proteins; 10(-5) M), A23,187 (10(-5) M), and melittin (a stimulator of phospholipase A2; 10(-4) M) stimulated PGF2 alpha release when explants were incubated in Ca(2+)-free medium (P < 0.10); however, oxytocin, A23,187, or melittin were unable to stimulate PGF2 alpha release when explants were incubated in Ca(2+)-free medium containing the calcium chelator EGTA (P < 0.10). This treatment did not prevent oxytocin or AIF4- from stimulating phospholipase C activity (P < 0.08). CoCl2 (a nonspecific Ca2+ channel blocker) and methoxyverapamil (a specific voltage-gated L-type Ca2+ channel blocker) prevented oxytocin from stimulating PGF2 alpha release (P < 0.05). Our results suggest that both extracellular and intracellular Ca2+ may be required for oxytocin to stimulate PGF2 alpha secretion in bovine endometrial tissue.

Oestrogenic effects of ICI 182,780, a putative anti-oestrogen, on the secretion of oxytocin and prostaglandin F2 alpha during oestrous cycle in the intact ewe

The effect of ICI 182,780, oestrogen antagonist, on the concentration of oxytocin and uterine PGF2 alpha was investigated in intact Border Leicester Merino cross ewes during the late oestrous cycle. Twelve cyclic ewes (n = 6 per group) were randomly assigned to receive, at 6 h intervals, intra-muscular injection of either peanut oil or ICI 182,780 (1.5 mg kg-1 day-1) in oil for 2 days, starting at 1900 h on day 13 until 1300 h on day 15 post-oestrus. Hourly blood samples were collected via a jugular catheter from 0800 h on day 14 for 37 h and then daily over days 16, 17 and 18 post-oestrus. Peripheral plasma concentrations of oxytocin, the metabolite of prostaglandin F2 alpha, 15-keto-13,14-dihydro-prostaglandin F2 alpha, (PGFM) and progesterone were measured by radioimmunoassay. All ewes treated with ICI 182,780 exhibited functional luteal regression as indicated by a marked reduction in plasma progesterone concentrations to less than 1000 pg/ml over the period of 18-36 h during sampling period on days 14 and 15 of the oestrous cycle. In five of six vehicle-treated ewes, progesterone concentrations declined between day 16 and day 18 post-oestrus. In the remaining control ewe, progesterone concentrations reach less than 1000 pg/ml within 36 h of the commencement of the sampling period. During the frequent sampling period, the number of oxytocin pulses in the ICI 182,780 treated ewes was significantly higher compared to control ewes (2.7 +/- 0.3 vs. 0.8 +/- 0.3). The mean amplitude of oxytocin pulses observed was also greater (70.4 +/- 19.5 pg/ml) in ewes treated with ICI 182,780, but was not significantly different from the control ewes (33.5 +/- 12.9 pg/ml). Oxytocin pulses may however have occurred following the initial two ICI 182,780 injections but before commencing blood sampling. The oxytocin pulses were detected at a mean of 3.2 +/- 0.2 h following each injection with ICI 182,780 during blood sampling. In the ICI 182,780-treated ewes, the pulsatile pattern of plasma PGFM in jugular blood samples over the 37 h sampling period on days 14 and 15 post-oestrus had a higher amplitude (512.9 +/- 158.9 vs 121.7 +/- 78.7 pg/ml) and pulse area (618.1 +/- 183.3 vs 151.5 +/- 102.9 (ph/ml)tau) compared to the vehicle-treated ewes (P < 0.05) respectively.. The average number of PGFM pulses observed per ewe was 3.0 +/- 0.7 in the ICI 182,780-treated group and was significantly (P < 0.02) higher than the number of pulses (0.5 +/- 0.3) observed in ewes treated with vehicle alone. The PGFM pulses were detected at 4.2 +/- 0.6 h following each injection with ICI 182,780 during blood sampling. The percentage of PGFM pulses that occurred coincidently with significant elevation of oxytocin concentrations was 44.4% in ICI 182,780-treated compared to 66.6% in control ewes. We conclude that administration of oestrogen antagonist ICI 182,780 accelerated development of the luteolytic mechanism by enhancing pulsatile secretion of oxytocin and PGFM which suggests that ICI 182,780 acts as an agonist for oxytocin and prostaglandin f2 alpha release in intact ewes when administered at 1.5 mg/kg/day over day 13 to 15 post-oestrus.

Cellular mechanisms involved during oxytocin-induced prostaglandin F2alpha production in endometrial epithelial cells in vitro: role of cyclooxygenase-2

PGs are important regulators of reproductive processes. At the time ofluteolysis in vivo, PGF2alpha is produced by endometrial cells, in response to oxytocin (OT). The mechanism by which OT induces the release of PGF2alpha remains to be defined. We have used 13 different cultures of bovine epithelial endometrial cells to study the effect of OT on the regulation of PGF2alpha and to identify the possible involvement of cyclooxygenases (COXs). OT induced a dose-dependent increase of both inositol phosphates (IPs) and [Ca2+]i concentration in epithelial cells labeled with [3H]-myoinositol or loaded with fura-2 (using a fluorescent microscope imaging system), respectively. OT induced a dose-dependent increase of both PGF2alpha production and COX-2 gene expression (as demonstrated by RT-PCR and Northern blots). PGF2alpha production was increased from 13.3 +/- 2.0 to 166.8 +/- 22.5 ng/ml (P < 0.0001). On the other hand, COX-2/beta-actin mRNA gene expression (as determined by densitometric analysis) was increased 5.1 +/- 0.7-fold (P < 0.001) with OT (10[-7] M) treatment, compared with control. Addition of indomethacin (1 microM) and a specific COX-2 inhibitor (NS-398, 1 microM) blocked the OT-induced PGF2alpha production. COX-1 and phospholipase A2 mRNA were expressed at steady-state levels, but no effect of OT was detected on their regulation. Combined to OT, 10 microq/ml of recombinant ovine interferon-tau (roIFN-tau) was able to decrease significantly (P < 0.0001) the dose-dependent increase of PGF2alpha production. Furthermore, partial bovine COX-1 (777 pb) and COX-2 (449 bp) cDNAs were cloned and sequenced. An homology of 83% and 97% was found in relation with rat and sheep, for COX-1, respectively. COX-2 was found to bear 84%, 86%, and 87% of homology in relation to rat, guinea pig, and human, respectively. Collectively, these results demonstrate, for the first time, that COX-2 is involved in the mechanism by which OT regulates PGF2alpha production in the endometrium.

IFN-tau increases PGE2 production and COX-2 gene expression in the bovine endometrium in vitro

Prostaglandins (PGs) are well known for their role in reproductive processes. At the time of pregnancy recognition, PGF2alpha is luteolytic and PGE2 may be antiluteolytic and luteotropic. During the preimplantation period, interferon-tau (IFN-tau) is produced by the conceptus and plays a crucial role in maternal recognition of pregnancy in domestic ruminants. We have demonstrated previously that recombinant bovine and ovine interferon-tau (rbIFN-tau and roIFN-tau) stimulate PGE2 production in epithelial cells, changing the primary PG produced by these cells from F2alpha to E2. In stromal cells, where PGE2 is the major PG produced, roIFN-tau induced an increase of both types of PGs. The aim of this paper is to identify the possible involvement of cyclooxygenases (COXs) in the modulation of PG production by trophoblastic interferons. Epithelial and stromal cells cultured in vitro were isolated from bovine endometrium and stimulated with increasing doses (1, 10 and 20 microg/ml) of roIFN-tau. PG levels in the culture media were measured by enzyme immunoassays (EIA) and total RNA was extracted from the cells. Northern blot analysis was performed to quantify cyclooxygenase COX-1 (constitutive), COX-2 (inducible) and phospholipase A2 (PLA2) messenger RNA (mRNA) production in response to treatment. The results indicate that roIFN-tau treatment did not affect COX-1 and PLA2 mRNA production in either cell type, whereas COX-2 expression was upregulated in both. The up-regulation of COX-2 transcript was greater in stromal than in epithelial cells. The increase in COX-2 mRNA levels was concurrent with increased production of PGE2 and PGF2alpha in stromal cells and principally PGE2 in epithelial cells. Furthermore, addition of indomethacin (1 microM) and a specific COX-2 inhibitor (NS-398, 1 microM) blocked the roIFN-tau-stimulation of PG production in both cell types. The mechanism whereby elevated COX-2 expression results in a selective increase of PGE2 in epithelial cells remains to be elucidated. In stromal cells, an increase in COX-2 mRNA levels may explain increased PG production. The overall effect of roIFN-tau in the two cell types is a net increase in PGE2 output.

Cellular mechanisms by which oxytocin stimulates uterine PGF2 alpha synthesis in bovine endometrium: roles of phospholipases C and A2

The objective of these experiments was to identify the cellular mechanisms by which oxytocin stimulates prostaglandin (PG) F2 alpha synthesis in bovine endometrial tissue. Uteri were collected on the day after spontaneous luteal regression. Caruncular endometrial explants were dissected and incubated in vitro to assess PGF2 alpha release or phospholipase (PL) C activity. Oxytocin (10(-6) M) stimulated PGF2 alpha release and PLC activity within 30 min of incubation (P < 0.01). The highest stimulation was observed at 100 min (P < 0.01). Oxytocin stimulated PLC activity at 10(-9) M and higher doses, whereas an increase in PGF2 alpha release was not detected until 10(-8) M (P < 0.09). Melittin, a stimulator of PLA2 activity, stimulated PGF2 alpha release at 10(-6) M and higher doses (P < 0.01). Aristolochic acid, an inhibitor of PLA2 activity, blocked the ability of oxytocin to stimulate PGF2 alpha release at 10(-5) M and higher doses (P < 0.01). Aristolochic acid (10(-4) M) reduced the stimulation of PGF2 alpha release induced by A1F4-, a nonspecific stimulator of G protein (10(-5) M) and melittin (10(-4) M; P < 0.05). Aristolochic acid had no effect on the ability of oxytocin or A1F4- to stimulate PLC activity (P > 0.10). By comparing the time course of stimulation and dose-response relationships between PGF2 alpha and PLC activity, it appears that oxytocin may stimulate PGF2 alpha secretion by activating PLC. The effects of melittin and aristolochic acid indicate that PLA2 may play a role in mediating the stimulatory effect of oxytocin on PGF2 alpha secretion, as well.

Effect of an oxytocin receptor antagonist on ovarian and uterine synthesis and release of prostaglandin F2 alpha in pseudopregnant rats

There is substantial experimental evidence suggesting that oxytocin has a role in luteolysis in ruminates. Endogenous pulses of uterine prostaglandin (PG) F2 alpha occur synchronously with pulses of oxytocin during luteolysis; leading us to propose a possible feedback loop between uterine PGF2 alpha and luteal oxytocin. In rates, the mechanism whereby oxytocin acts has not been well elucidated. In the present report, the effects of an oxytocin receptor antagonist in pseudopregnant rats were investigated. Pseudopregnancy was induced in immature female rats by gonadotrophin treatment; this resulted in the formation of corpus luteum that remained functional for 9 +/- 1 days. The pseudopregnant rats were assigned to one of the following four groups. In the first group the relationship between the release of ovarian and uterine PGF2 alpha was tested. We also studied the serum progesterone during the pseudopregnancy. We found that PGF2 alpha released into the incubation medium from ovaries of pseudopregnant rats increased (p < 0.05) and was maximal on day 9 of pseudopregnancy. This concentration remained high until day 10 of pseudopregnancy and then decreased. The PGF2 alpha released from the uterus to the incubation medium rose (p < 0.05) on day 8 of pseudopregnancy and reached the peak value on day 10. the serum progesterone was increased (p < 0.001) on day 2 pseudopregnancy and was greater on day 5 (p < 0.001). The second and third group received a specific oxytocin receptor antagonist (1-deamino-2-O-methyltyrosine) in two different concentrations (0.05 or 0.2 mumol/l before the peak of PG release. Both doses employed decreased (p < 0.001) the release into the incubating medium of PGF2 alpha from ovaries and uterus. Indeed, after the treatment, the progesterone levels were higher (p < 0.001) than control on day 10 of pseudopregnancy. In the fourth group, a potent inhibitor of cyclooxygenase activity was administered on day 8 of pseudopregnancy into the ovarian bursa. The serum progesterone levels increased (p < 0.01) compared to control suggesting a possible role of ovarian PG in the luteolytic phase of the corpus luteum regression. Thus, our findings show that oxytocin is luteolytic in pseudopregnant rats and this action is mediated by oxytocin receptors, as it was blocked by a specific oxytocin receptor antagonist.

Preliminary findings of altered follicular activity in Holstein cows with coagulation factor XI deficiency

Factor XI (F XI) deficiency is an autosomal recessive coagulopathy found in Holstein cattle. Affected animals have a 50% greater prevalence of repeat breeding. Therefore, several parameters describing ovarian function were studied. Daily blood sampling revealed that progesterone concentrations were slower to decline from a peak at day 16 (p < 0.01) to values less than 3 nmol/L in F XI-deficient cows (5.14 +/- 0.69 days (mean +/- SD) versus 4.05 +/- 0.63 days in control animals), resulting in an oestrous cycle length of 24.7 +/- 2.1 days compared to 22.9 +/- 3.0 days, respectively. This was not due to an alteration in the availability of prostaglandin F2 alpha (PGF2 alpha) or oxytocin (OT) involved in luteolysis. No significant differences (p > 0.05) were seen between normal (n = 7) and F XI-deficient (n = 7) cows in the peak values or the area under the curve for the pulse in 13,14-dihydro-15-keto PGF2 alpha in response to OT challenge or in the parameters describing the pulse of ovarian OT secretion after PGF2 alpha injection (n = 7 for each) between days 12 and 14. Ovulatory follicular development was assessed by ultrasound monitoring and plasma 17 beta-oestradiol values at 8-h intervals after a luteolytic injection of cloprostenol (n = 6 for each). Follicular diameter was smaller (p < 0.05) and accompanied by lower peak oestradiol values near the time of ovulation in F XI-deficient cows. The results suggest that the oestrous cycle in F XI-deficient cows is characterized by a slower process of luteolysis that may be associated with smaller follicular development.

Oxytocin in mares: lack of evidence for oxytocin production by or action on preovulatory follicles

It is not known whether the equine preovulatory follicle produces oxytocin or is a target tissue for oxytocin, as has been reported for other species, especially ruminants. Bovine granulosa cells secrete oxytocin, and oxytocin modulates the production of progesterone by granulosa cells in vitro. We examined whether oxytocin plays a comparable role in the equine preovulatory follicle. To test the hypothesis that the equine preovulatory follicle produces oxytocin during estrus and that its production increases in late estrus, preovulatory follicles were isolated during early (Days 1 to 2; n = 4) and late (Days 4 to 5; n = 4) estrus. Granulosa cells, pieces of theca interna and pieces of follicle wall (theca with attached granulosa cells) were cultured for 3 d with or without equine gonadotropins. Culture media were collected, replaced at 3, 6, 12, 24, 48, and 72 hr of culture, and assayed for oxytocin. Granulosa cells from preovulatory follicles secreted negligible amounts of oxytocin during 3 d of culture, irrespective of gonadotropin treatment or stage of estrus. Likewise, negligible amounts of oxytocin were measured in theca and follicle wall cultures at both developmental stages, in the presence or absence of gonadotropins. Furthermore, follicular fluid from early or late estrous follicles contained only negligible amounts of oxytocin. To determine if oxytocin affects steroidogenesis by equine granulosa cells, granulosa cells from follicles obtained on Day 2 of estrus were cultured with graded doses of oxytocin (0, 1, 10, 100, and 1,000 ng/ml) in defined medium supplemented with testosterone (0.5 microM) and culture media were assayed for estradiol-17 beta and progesterone. Estradiol was secreted throughout the culture period, and its production was not significantly affected by oxytocin treatment (P > 0.05). Progesterone secretion was relatively low during the first 24 hr of culture, increased dramatically on the second day of culture, and remained high through the third day. No dose of oxytocin had a significant effect on progesterone secretion (P > 0.05). In conclusion, the results indicate that equine preovulatory follicles, isolated during early or late estrus, are neither a source of oxytocin nor a target for oxytocin action on steroidogenesis. Although ovarian oxytocin appears to play a role in regulating follicular function in some other mammalian species, our data provide no support for such a role for oxytocin in mares.

Role of protein kinase C in the inhibitory action of trophoblast interferons on expression of the oxytocin receptor in sheep endometrium

PhosphoIipid/Ca(2+) -dependent protein kinase C (PKC) and oxytocin receptor were measured in sheep endometrial explants after culture for up to 96 h. Oxytocin receptor binding and PKC activity were reduced by up to 90% in explants exposed to recombinant ovine trophoblast interferon (rolFN-τ), recombinant bovine IFN-α(1) or ovine conceptus secretory proteins (a source of IFN-τ). Inhibition occurred in both caruncular and intercaruncular endometrium taken between days 7 and 10 of the oestrous cycle and in intercaruncular (but not caruncular) endometrium on day 6. Down-regulation of PKC by continued exposure of expiants to 4β-phorbol myristate acetate, or treatment with PKC inhibitors reduced both oxytocin receptor binding and PKC activity by up to 70%. Tyrosine kinase inhibitors were ineffective. Addition of oxytocin or progesterone, which reduce oxytocin receptor bindingin vivo, also lowered oxytocin receptor bindingin vitro in the absence of any effect on PKC. The data indicate that IFN-τ inhibits oxytocin receptor synthesis by a mechanism involving PKC inhibition, but that a non-PKC pathway also operates to control oxytocin receptor binding in non-pregnant animals. These conclusions were supported by measuring PKC activity and oxytocin receptor binding in endometrium without culture. Prolonged exposure of the endometrium to IFN-τin vivo may lead to PKC down regulation by a mechanism analogous to that involved in the action of continuous activation by agonist, and this may represent one function of the prolonged secretion of IFN-τ over a 10-day period in early pregnancy.

Intramammary pressure and luteal oxytocin after PGF2 alpha administration in cycling and early pregnant ewes

The aim of this work was to investigate whether luteal oxytocin released after non-luteolytic prostaglandin F2 alpha (PGF2 alpha) challenge could explain the intramammary pressure (IMP) rises previously described in pregnant ewes after the normal life span of a corpus luteum. Blood oxytocin levels and IMP after challenge were measured in cycling and pregnant lactating ewes until the response ceased (approximately 55 d post oestrus). Oxytocin release was not significantly different for cycling and pregnant ewes, and fell to its lowest levels on days 16-18 post oestrus in both groups, i.e. with or without luteolysis. However, although IMP response fell with luteolysis in cycling ewes, the response persisted until day 55 in pregnant ewes. This lack of correlation between oxytocin release and IMP response raised interesting questions including: is there an unknown oxytocic compound that can be released after PGF2 alpha stimulation of the corpora lutea?

The role of the endometrial oxytocin receptor in determining the length of the sterile oestrous cycle and ensuring maintenance of luteal function in early pregnancy in ruminants

The oxytocin receptor, a seven transmembrane domain, G protein-linked receptor molecule, plays a central role in determining the endocrine function of the ruminant uterine endometrium. During nonpregnant cycles the control of this molecule by circulating steroid hormones leads to regression of the corpora lutea. The kinetics of the mechanisms involved determine the time at which luteolysis occurs, and therefore the length of the oestrous cycle. In pregnancy, secretions of the trophoblast block endometrial oxytocin receptor gene expression and lead to luteal maintenance. An understanding of the molecular mechanisms involved in the steroidal control of oxytocin receptor gene expression will provide an explanation for the relative constancy of oestrous cycle lengths in non-pregnant animals. Unravelling the way in which trophoblast products block expression of the oxytocin receptor gene will lead to a better understanding of the reasons for the high rate of embryonic loss in domestic ruminants.

Auto/paracrine role of prostaglandins in corpus luteum function

The homeostasis and ephemerality of the corpus luteum (CL) involves an intriguing interplay amongst pituitary, placental and intraovarian regulators. Recent findings have indicated a local pathway of synthesis for the cyclooxygenase-derived prostaglandins (PGs) in luteal cells of all mammalian species investigated. Thus, an autocrine or paracrine role of intraluteal PGs in modulation of luteal steroidogenesis is implicated. The presence of immune cells in the ovary indicates a constitutive role of these cells and their secretory products, in particular the cytokines, some of which have been demonstrated to greatly influence luteal PG and progesterone production. Despite the plentitude of investigations, a precise role for PGs other than PGF2 alpha in regulation of CL function is still obscure, mainly lacking evidence of cell-specific expression of various classes of PG receptors and their intracellular signaling mechanisms.

Oxytocin gene expression and action in bovine preovulatory follicles

We have studied oxytocin (OT) gene expression, secretion, and action in bovine preovulatory follicles during the follicular phase of the estrous cycle. OT is secreted in vitro by follicular granulosa cells, but not by theca cells. Both OT content of granulosa cells and their ability to secrete OT in culture increased dramatically when follicles were obtained after the gonadotropin surge (LH surge) that triggers ovulation. These changes were correlated with increased levels of messenger RNA (mRNA) for OT in granulosa cells obtained after vs. before the LH surge. When granulosa cells were obtained before the surge, both OT secretion and OT mRNA levels increased with time in culture, and the increases were greatly enhanced in the presence of LH. Estradiol, at concentrations found in follicular fluid of preovulatory follicles before the LH surge, inhibited OT secretion in vitro, whereas concentrations found in follicular fluid after the LH surge were not inhibitory. Progesterone, at physiological concentrations, stimulated OT secretion in vitro. We have shown previously that OT increases progesterone secretion by granulosa cells obtained before the LH surge. Taken together these results show that, during the follicular phase in cattle, OT secretion and gene expression are coordinately regulated and suggest that they are regulated by both gonadotropins and intrafollicular steroids. Increases in OT after the LH surge may play a role in the follicular/luteal phase shift in steroidogenesis from estradiol/androgen to progesterone.

Prostaglandin F2 alpha activates protein kinase C in human ovarian cells

Recent studies in several non-primate species have suggested that prostaglandin F2 alpha (PGF2 alpha) inhibits luteal cell progesterone production by activating the calcium and phospholipid-dependent protein kinase, protein kinase C (PKC). This study investigated the presence of PKC in human ovarian cells and assessed the ability of PGF2 alpha and its structural analogue, cloprostenol, to generate inositol polyphosphates and activate PKC. PKC was detected in cultured human granulosa-lutein cells and human luteal cells (from mid-late luteal phase). The major proportion of PKC detected was cytosol-associated in both cell types. Cloprostenol increased the generation of inositol polyphosphates in cultured human granulosa-lutein cells in a dose- and time-dependent manner. In addition both cloprostenol and PGF2 alpha activated PKC (as assessed by redistribution of enzyme activity from a principally cytosol-associated form to a membrane-associated form) in both granulosa-lutein and luteal cells. Short-term exposure of both cell types to phorbol myristate acetate (4 beta-PMA) activated PKC, whilst prolonged exposure of human granulosa-lutein cells to 4 beta-PMA led to a > 85% loss of total PKC activity. The inactive phorbol ester, 4 alpha-PMA, had no effect on PKC activity when exposed to cells for up to 20 h. These results demonstrate the presence of PKC in human ovarian cells and the ability of PGF2 alpha to induce translocation/activation of this kinase.(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacologic manipulation of fertility

The professional application of agents to the manipulation of fertility of cows requires basic and applied knowledge of the physiologic mechanisms that are affected and of the pharmacologic agents that are used. In all areas of the pharmacologic manipulation of fertility, the achievement is less than the ideal, and further research is required to improve the efficiency of treatments. The induction of estrus in acyclic animals can involve a reduction in the depth of anestrus, pretreatment with progestagen to ensure estrous behavior and the formation of a normal corpus luteum, and then treatment with exogenous gonadotropin. Responsiveness to treatment can be variable and reflects the depth of anestrus of the animals. Improved treatment regimens require a knowledge of the basic mechanisms involved with the depth of anestrus, a means of assessing the depth of anestrus, and an understanding of the hormonal requirements of ovarian follicles for development and maturation in animals at different depths of anestrus. The optimal precision in the synchronization of estrus (and ovulation) in cyclic animals requires the synchronization of both follicular waves and the end of progestational phase. The end of progestational phase can be synchronized effectively using prostaglandin F2a (or analogs), or by treatment with progestagens with or without luteolytic agents. Procedures to synchronize follicular waves need to be established. The induction of superovulation can be achieved readily using gonadotropins prior to estrus synchronization using prostaglandin F2a. The responses to standard treatments in terms of ovulation rates and yield of transferable embryos are highly variable. The development of procedures to reduce this variability requires an understanding of the intra-ovarian mechanisms involved in recruitment of follicles for a wave of follicular growth, in the selection of dominant follicles for further development, and in the mechanisms controlling follicular atresia. Cystic ovarian disease can be treated effectively using HCG or GnRH (follicular cysts) or prostaglandin F2a (luteal cysts). The basic mechanisms resulting in failure of estrogen positive feedback on LH secretion (that results in cystic follicles) remain to be determined. Small but significant increases in pregnancy rates can be achieved treating cows with prostaglandin during the post-partum period, with prostaglandin to induce estrus for insemination, with GnRH or HCG at estrus, and with GnRH or progestagen treatment during diestrus. Beneficial effects of treatment have been shown in some trials but not in others.(ABSTRACT TRUNCATED AT 400 WORDS)

In vivo desensitization of a high affinity PGF2 alpha receptor in the ovine corpus luteum

The corpus luteum (CL) of the sheep exhibits a differential sensitivity to PGF2 alpha in vivo in terms of an increase in oxytocin (OT) secretion and a decrease in progesterone secretion, pointing to the presence in vivo of both high and low affinity receptors for PGF2 alpha. The presence of the high affinity PGF2 alpha receptor was assessed by monitoring the secretion rate of OT from the ovine CL in response to subluteolytic infusions of PGF2 alpha. Rapid desensitization to PGF2 alpha occurred after only one hour of infusion, while a minimum rest period of six hours was required to restore sensitivity. The possibility that these findings could be explained by the depletion and resynthesis of OT was excluded by demonstrating an increase in OT secretion rate with supra-physiological levels of PGF2 alpha two hours after desensitization. Collectively, these results indicate the presence of a high affinity receptor for PGF2 alpha in the ovine CL which exhibits desensitization and recovery in vivo. The temporal nature of the desensitization and recovery of the high affinity PGF2 alpha receptor controlling luteal OT secretion may contribute to the pulsatile nature of PGF2 alpha release from the ovine uterus.

Prostaglandin-induced secretion of oxytocin and prolactin in red (Cervus elaphus) and Pere David's (Elaphurus davidianus) deer hinds: evidence for oxytocin of luteal origin

Peripheral plasma concentrations of oxytocin in female red deer during the luteal phase of the oestrous cycle (9.3 +/- 2.1 fmol/ml) exceeded those in the follicular phase (3.1 +/- 1.4) or during seasonal anoestrus (3.2 +/- 1.3). In both red and Père David's deer hinds during the mid-luteal phase of the cycle, systemic administration of a luteolytic dose of the prostaglandin F2 alpha analogue, cloprostenol, caused the concentration of oxytocin in the peripheral circulation to rise. Mean (+/- SEM) concentrations increased from 8.1 +/- 0.7 to 97 +/- 8 fmol/ml in red and from 6.2 +/- 0.7 to 153 +/- 30 fmol/ml in Père David's hinds within 5 min of treatment. During seasonal anoestrus oxytocin secretion in response to cloprostenol was reduced to less than 10% of that during the breeding season, in both species. Cloprostenol treatment raised circulating concentrations of prolactin in both species during the breeding season, and during anoestrus in red deer only. The concentration of oxytocin in a single corpus luteum removed at laparotomy from one red deer hind at the mid-luteal phase of the cycle was 66 nmol/g wet wt; identification was authenticated by HPLC. These results suggest that the corpus luteum secretes oxytocin in the Cervidae, as established previously in the Bovidae, and that luteal oxytocin secretion is stimulated by prostaglandin.

Type I conceptus interferons: maternal recognition of pregnancy signals and potential therapeutic agents

In summary, Type I conceptus interferons of sheep, cows, and goats (perhaps all ruminants), which signal maternal recognition of pregnancy, may be useful for enhancing fertility in animal agriculture and may have a role in human and veterinary medicine as well. The Type I conceptus interferons share antiviral, anticellular, and immunosuppressive properties with Type I leukocyte interferons, but lack their cytotoxic effects, which may make them especially useful as therapeutic agents in human and veterinary medicine. The unique N-terminal structure of oTP-1 may account for its distinctive biological properties, including its antiluteolytic activity.