1 Human Reproduction Unit, Department of Physiology, University of Sydney, Royal North Shore Hospital of Sydney, St. Leonards, NSW 2065, Australia; and 2 Center for Animal Biotechnology, Institute of Biosciences and Technology, Texas A & M University, College Station, Texas 77843-2471
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ABSTRACT |
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Pulsatile release of uterine prostaglandin
F2
(PGF2
) induces luteolysis in
ruminants. However, the mechanism(s) that initiates and maintains
luteolysis has not been defined. The present study tested the
hypothesis that the endogenous
PGF2
pulse generator is
uterine-derived platelet-activating factor (PAF). Ovariectomized ewes
were given exogenous progesterone (P), estradiol (E), or both (P+E,
mimicking the normal luteal phase). Only ewes treated with steroids
released PAF into the uterine lumen and had increased
PAF:acetylhydrolase activity in the uterine lumen. Steroid treatment
also influenced the capacity of the uterus to release
PGF2
in response to exogenous
PAF. PAF infusion did not affect plasma
PGF2
metabolite (PGFM) levels
in control (no steroid treatment) ewes but increased plasma PGFM levels
in P+E ewes (P < 0.001) and ewes
treated with P or E alone (P < 0.05). Infusion of PAF followed by or coincident with oxytocin (OT)
acted in a synergistic manner to increase plasma PGFM levels. Repeated infusion of PAF into the uterus at 1-h intervals induced tachyphylaxis of the PGFM response to PAF; however, sensitivity of the uterus to PAF
returned spontaneously by the 6th h. Interferon-
(IFN-
) inhibits
pulsatile release of PGF2
during pregnancy to prevent luteolysis. Exogenous recombinant ovine
IFN-
(50 µg) inhibited the uterine response to PAF alone or the
combined effects of PAF and OT. These results indicate that uterine PAF
fulfills many of the criteria for an endogenous
PGF2
pulse-generator: steroid induction of PAF production and uterine responsiveness to PAF-induced release of PGF; synergistic stimulation of PAF-induced PGF release by
OT; inhibition of PAF effects by IFN-
; and PAF's ability to induce
pulses of PGF with a periodicity during a period of chronic exposure of
the uterus to PAF.
prostaglandin F2; platelet-activating factor:acetylhydrolase; endometrium; estradiol-17
; progesterone; sheep; pulsatility
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INTRODUCTION |
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SOME BIOLOGICAL MEDIATORS and hormones are released in
a pulsatile manner to enhance the response of target cells. Whereas continuous exposure of target cells to ligand may downregulate responses (tachyphylaxis), the pulsatile exposure of cells to ligand
may prevent tachyphylaxis and thus enhance or maintain cellular
responses; for example, the pulsatile release of insulin from
pancreatic islet cells, induction and maintenance of gonadotrophin release from the anterior pituitary by pulsatile release of
gonadotrophin-releasing hormone (GnRH) from the hypothalamus, and
pulsatile release of uterine prostaglandin
F2
(PGF2
) in ruminants to induce luteolysis (the structural demise and cessation of progesterone production by the ovarian corpus luteum). On the other hand, constant exposure to ligand causes downregulation of cellular responses; for
example, continuous infusion of GnRH prevents gonadotrophin release
(58). Although several inputs into the hypothalamus coordinate the
pulsatility, immortalized hypothalamic cell lines release GnRH with an
interpulse interval similar to that in vivo, which suggests a
pulse-generator endogenous to GnRH neurones (58). The nature of the
putative endogenous pulse-generator remains ill defined.
The pulsatile release of luteolytic
PGF2 in sheep is generally
coordinate with pulsatile release of oxytocin (OT) from the corpus
luteum (CL) and posterior pituitary and of
PGF2
from the uterus (24).
These two events may provide for a coupled positive feedback loop (20),
although unequivocal proof for the existence of such a loop has not
been provided. Although constant infusion of OT in ruminants prevents
pulsatile release of PGF2
and luteolysis (21),
exhaustion of 75% of ovarian OT by use of norepinephrine (29) and
inhibition of OT receptors (OTR) with an antagonist (30) did not
disrupt luteolysis. Furthermore, pulsatile release of
PGF2
from the uterus occurs
spontaneously even in ovariectomized (54) and hypothalamic-pituitary
stalk-sectioned ewes (16, 35). Although the pulses of PGF are of
smaller amplitude, their persistence in these ewes questions the role
of OT. The spontaneous pulsatile release of
PGF2
requires steroid hormone treatment of ewes and may result from an endogenous pulse generator (54).
Platelet-activating factor (PAF,
1-O-alkyl-2-acetyl-sn-glycero-3-phosphorylcholine)
is an ether phospholipid that is involved in a number of
pathophysiological pathways (for reviews see Refs. 13,
34). PAF is produced by the ovine
uterus, mimics the actions of OT in inducing
PGF2 release (11), and is present in the uterus when the pulsatile release of
PGF2
occurs (11). Therefore,
this study tested the hypothesis that uterine PAF is an endogenous
uterine pulse generator for pulsatile release of
PGF2
. This hypothesis is
supported by evidence that 1) the
production and action of PAF on the uterus is steroid hormone
dependent; 2) the actions of PAF are
potentiated by OT and inhibited by interferon-
(IFN-
), the
endogenous antiluteolytic factor for pregnancy recognition (36); and
3) the PAF-induced pulsatile release
of PGF occurred, even during chronic exposure of the uterus to PAF.
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MATERIALS AND METHODS |
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Animals.
Merino ewes were penned indoors and exposed to a 12:12-h light-dark
cycle. All ewes were anesthetized by pentobarbital sodium induction
with
halothane-O2-N2O
maintenance and ovariectomized with aseptic techniques. They were
allowed to recover for 6 wk before beginning treatment. All
experiments were performed according to the Australian Code of Practice
for the Care and Use of Animals for Scientific Purposes (43) and were
approved by the Institutional Animal Care and Ethics Committee.
Uterine flushings and endometrial samples.
On the equivalent of days 14, 15, and
16 of the luteal phase, endometrial
explants and luminal fluids were collected from ewes in each steroid
replacement group. For endometrial tissue collection, ewes were
anesthetized by pentobarbital sodium induction and euthanized with an
overdose of saturated KCl. For ewes in which uterine PAF and
PAF:acetylhydrolase were measured, each uterine horn was immediately
removed without excess mesentery, blotted free of all blood, and placed
on ice. The lumen of each was flushed with 3 ml of ice-cold sterile
Tyrode solution (in mM: 137 NaCl, 2.68 KCl, 11.9 NaHCO3, 1 MgCl2 · 6H2O,
0.41 NaH2PO4 · 2H2O,
0.5 glucose, 5 HEPES, and 0.5% Na azide, pH 7.4) just below the
uterotubal junction and was collected at the cervical end. The uterine
flushings were divided into 0.5-ml aliquots and stored at
20°C. The luminal fluid protein concentration was measured using a Bio-Rad Total Protein Assay kit (Bio-Rad Laboratories, Hercules, CA).
Actions of PAF:acetylhydrolase.
The PAF:acetylhydrolase activity was also measured in supernatants of
caruncular endometrium homogenized in Tyrode solution and centrifuged
(2,500 g, 4°C, 15 min). The
protein concentration of the supernatants was measured using a Bio-Rad
Total Protein Assay kit (Bio-Rad Laboratories). Samples were then
stored at 20°C until assayed for PAF:acetylhydrolase activity.
Treatments.
The PAF
(1-O-hexadecyl/octadecyl-2-acetyl-sn-glycero-3-phosphocholine;
Sigma Chemical) was maintained as a stock solution in chloroform (1 mg/ml). Aliquots were evaporated to dryness under N2 in siliconized glass test tubes
and brought to solution in Ca2+-
and Mg2+-free PBS supplemented
with 2.5 mg BSA/ml (PBS-BSA). PAF was sterilized by filtration through
a 0.22-µm filter (Millipore, Sydney, NSW, Australia), prepared as
aliquants in sterile 5-ml plastic tubes (Flow Labs, Sydney, NSW,
Australia), and then stored at 20°C until used. Control
infusions were PBS-BSA prepared, thawed, and stored in the same manner.
Assays.
PAF:acetylhydrolase activity in luminal fluids and endometrial
homogenates was assayed at three protein concentrations as previously
described (39, 46). Briefly, 5 µmol/l
[3H]PAF (5.25 GBq/mmol) was incubated with each sample for 15 min at 37°C in a
shaking water bath. The reaction was stopped by adding 170 µg BSA and
then ice-cold TCA (9.2% wt/vol) to facilitate protein precipitation.
Aliquots of the supernatant were collected, and the free
3H acetate was counted on a
Packard Tricarb model 1500 scintillation counter (Canberra Packard,
Sydney, Australia). The limit of detection for the assay was 0.1 pmol
acetate
recovered · min1 · mg
protein
1.
Data analysis. All statistical analyses were performed using the statistical package SPSS for Windows (Release 6.10). For PAF in caruncular endometrial culture medium and uterine flushings, and for PAF:acetylhydrolase activity in endometrial homogenates and uterine flushings, homogeneity of variance was assessed by Bartlett's test. Where the data were heterogeneous, medians were compared using Mann-Whitney tests and Kruskal-Wallis one-way ANOVA. Repeated-measures ANOVA was used for analysis of PGFM responses.
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RESULTS |
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The effect of steroid hormones on PAF release by the endometrial
explants in vitro and the concentration of PAF in uterine flushings
from ovariectomized ewes given E, P, or P+E are shown in Fig.
1. To assess the potential turnover of PAF
released from this source, PAF:acetylhydrolase activity values in
uterine flushings and endometrial homogenates (Fig.
2) on days
14-16 were compared with values for untreated
ewes.
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The amount of PAF released in vitro and detected in uterine flushings was variable; thus both the medians and means of the results are shown (Fig. 1). Ewes treated with E alone released more PAF on day 14 than on day 15 or 16, when the amount was greater for untreated ewes (P < 0.05). Endometrium from ewes given P only released more PAF (P < 0.05) than untreated ewes only on day 16, although there was a clear trend for high levels on day 15. Release of PAF from ewes treated with P+E was higher on day 16 than on days 14 and 15 and also higher than from untreated ewes (P < 0.05). As a further control, some animals were treated with oil as a vehicle instead of being left untreated. PAF released was assessed on day 16. The results showed that there was no difference between vehicle-treated and untreated animals (P > 0.05).
All hormone treatments increased PAF concentrations in the uterine flushings (P < 0.05) compared with those of untreated ewes on days 15 and 16. PAF was not detected in uterine flushings from control ewes (no steroid hormone replacement) but was detected in all other groups except for P-only-treated ewes on day 14 (Fig. 1B).
The PAF:acetylhydrolase specific activity in uterine flushings (Fig. 2A) for all hormone-treated ewes on each day was higher (P < 0.05) than for untreated ewes. Ewes receiving E alone had significantly lower specific activity (P < 0.05) than those receiving either P alone or P+E. In both E and P treatment groups, the PAF:acetylhydrolase specific activity decreased significantly (P < 0.05) after day 14, whereas activity in P+E-treated ewes was maintained at a consistently high level throughout the study period.
The PAF:acetylhydrolase activity in endometrial explant tissues (Fig. 2B) was lower than that in uterine flushings. For both P- and E-treated ewes, there was no change in activity (P > 0.05) between days 14 and 16; however, there was an increase (P < 0.05) in PAF:acetylhydrolase activity between days 14 and 16 for P+E-treated ewes.
PAF released by some cell types binds to domain II of albumin, and in
this configuration its degradation by PAF:acetylhydrolase is prevented
(7). To determine whether the PAF measured in uterine flushings and the
PAF released by endometrial explants were in this protected
conformation, endometrial culture medium and uterine flushings were
exposed to PAF:acetylhydrolase (Table 1).
Endometrium-derived and uterine flushing PAF was hydrolyzed by
PAF:acetylhydrolase and was not affected by acid-treated serum, because
PAF:acetylhydrolase is acid labile, confirming the specificity of the
reaction.
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Control ewes having oil injections instead of steroids showed no change
in plasma PGFM levels after intrauterine infusion of PAF (200 µg/horn) or PBS-BSA (P > 0.05).
Ewes treated with P, E, or P+E also showed no change in PGFM after
PBS-BSA infusion (P > 0.05). Because
there were no differences (P > 0.05)
in response among control groups, results were pooled to form the one
control group shown in Fig. 3 and used in
analysis. The infusion of PAF into the uterus on days
15 and 16 of ewes
receiving either P or E alone induced a small but significant PGFM
response (P < 0.05) compared with
control ewes (Fig. 3, A and
B, respectively), whereas a much
larger increase (P < 0.001) in PGFM
occurred in P+E-treated ewes. In P+E ewes the response was greater than
for control ewes and ewes treated with E or P. The duration of the PGFM
response to PAF on day 15 was ~15
min (Fig. 3A), whereas on
day 16 the response lasted for up to 1 h (Fig. 3B).
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PAF and OT were administered on days
15 and 16 to
P+E-treated ewes. Both PAF and OT alone increased
(P < 0.05) plasma PGFM within 10 min
of administration (Fig.
4A).
When PAF was administered before OT, the increase in plasma PGFM in
response to OT was six to seven times
(P < 0.05) greater than that induced
by PAF or OT alone. To determine whether this synergistic effect
resulted from a priming action of PAF on the uterus, PAF and OT were
also administered simultaneously (Fig.
4B). This treatment resulted in a
PGFM response equivalent to the sequential treatment, but peak values
occurred 10 min earlier, probably due to the earlier time of
administration of OT (Fig. 4B).
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Ovine conceptus trophectoderm secretes IFN-, which inhibits
luteolytic pulses of PGF2
during early pregnancy (36). The P+E-treated ewes given roIFN-
24 h
before PAF challenge on days 15 and
16 did not respond with an increase in
plasma PGFM (Fig.
5A).
Similarly, pretreatment with roIFN-
on day
14, followed by PAF and OT on days
15 and 16, resulted in
a reduced PGFM response compared with ewes not treated with roIFN-
(P < 0.05, Fig.
5B). These results indicate that the
effect of roIFN-
administration on day
14 continued to inhibit the effects of PAF and OT
through day 16.
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It is currently not known whether PAF is produced or released from the
endometrium in a pulsatile manner. We therefore asked the question,
does prolonged exposure to PAF cause chronic downregulation of the
uterine response? Thus exposure to PAF over an 8-h period was performed
by hourly infusion into P+E-treated ewes on days 14 and 15 (Figs.
6, A and
B). On both days, a strong response to the initial PAF challenge was followed by complete tachyphylaxis by
the third challenge. Uterine sensitivity to PAF was again detected at
the sixth challenge, with responsiveness being 64 ± 4 and 57 ± 7%
of the first response on days 14 and
15, respectively. Further challenges
induced another refractory period. On days
14 and 15, the 6-h
response to PAF was greater (P < 0.01) than the 5-h and 7-h challenges
(P < 0.05).
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CONCLUSIONS |
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Results of the present study are the first to demonstrate that PAF may
be responsible for the pulsatile release of
PGF2 from the uterus. PAF
release by the uterus was dependent on sex steroids, and roIFN-
prevented PAF-induced PGF2
release from the uterus. The uteri from ovariectomized ewes treated
with E+P hormone replacement similar to the luteal phase became
spontaneously responsive to PAF at ~6-h intervals, which is similar
to the frequency of PGF pulses for intact ewes during the luteolytic
period. After PGF2
release, the uterus was
desensitized to PAF for ~6 h.
The synergistic interaction between PAF and OT in the generation of PGF
pulses supports substantial evidence for a role of OT in luteolysis in
sheep (19, 37). However, the persistence of PGF pulses in both
ovariectomized (54) and hypothalamic-pituitary stalk-sectioned (16, 35)
ewes has raised doubts about an essential role for OT in the luteolytic
process. The observation that PGF pulsatility occurred in
steroid-replaced ovariectomized ewes is tempered by the observation
that the amplitude of the PGF pulses was markedly reduced compared with
intact ewes (54). Studies with intact ewes indicated that uterine
PGF2 levels often increase
before the release of ovarian OT (40) and that inhibitors of PG
synthesis (41, 48) or action (22, 44) can block OT release. Results of
the present study indicate that PAF-induced low-amplitude PGF pulses
synergistically enhanced subsequent effects of OT to stimulate PGF
secretion. The synergism between PAF and OT was blocked by IFN-
,
regardless of whether PAF and OT were administered simultaneously or sequentially.
The presence of PAF in the uterus has been reported for rat (59), human (4), rabbit (9), and sheep (11), but this is the first report of its presence in uterine flushings. The high degree of variability in PAF release from endometrial explants and in uterine flushings was unexpected and may reflect the combined effects of episodic release of PAF and its relatively short half-life due to the presence of PAF:acetylhydrolase in luminal fluids and endometrial tissue. It was shown (6) that good agreement in quantitative measurement of PAF was achieved when bioassays and radioimmunoassay were compared. Thus the variability seen is unlikely to be due to assay artifacts. In many cell types, most PAF produced is retained by the cell and may act as a juxtracrine hormone (60); that is, it is presented to its receptor on the outer leaf of the plasma membrane of adjacent cells. In human endometrial cells, high concentrations of PAF are associated with stromal cells (4).
The uterine endometrium possesses the enzymes necessary for PAF synthesis (31), and PAF accumulates in the endometrial stroma rather than in the epithelium (4). Synthesis of PAF in human endometrium is dependent primarily on progesterone (4). If stromal cells accumulate PAF in ewes, intraluminal administration of PAF may not best mimic its normal sites of action. This may explain why relatively high concentrations of PAF were required to be administered into the lumen to elicit responses. Further studies of PAF's site of release, action in the uterus, and the assessment of PAF levels in the endometrial tissue are required.
Angle et al. (9) showed that concentrations of PAF in the rabbit
endometrium increased ~20-fold during the first half of the luteal
phase. In pregnant rabbits, PAF content declined to postovulatory
levels around the time for luteolysis, whereas levels of PAF in
endometrium were high throughout pseudopregnancy. Although regulation
of luteolysis in the rabbit is not well defined,
PGF2 is considered to be the
uterine-derived luteolysin (14). Thus high levels of PAF in the uterus
of pseudopregnant females may be required for the generation of
luteolytic PGF2
.
PAF biosynthesis may occur by two alternate pathways, "the re-modelling," where phospholipase A2 converts 1-O-alkyl-2-aracodonyl-glycerophosphocholine into with the lyso-PAF with the release of arachidonic acid (55), and the alternative or "de-novo" pathway, which does not use phospholipase A2 (33). The precursor and final converting enzymes of the de novo and remodeling pathways have been reported in the rabbit uterus (32). Although the remodeling pathway has not been demonstrated in the ovine uterus, the spontaneous production of prostaglandins in the uterus in the ovariectomized ewe model infers that phospholipase A2 activity is present. It may be possible, therefore, that PAF present in the uterus is a by-product of PG production. The possibility for coordinate regulation of the synthesis of these lipid mediators requires further study.
In mice (46), PAF:acetylhydrolase activity in the endometrium and uterine lumen is high in the mid-luteal phase but falls rapidly around the time of luteolysis because of the effects of estrogen. In the present study, E-treated ewes had less PAF:acetylhydrolase activity than ewes treated with P or P+E treatment. This study does not demonstrate whether the change in activity is due to changes in the concentration of protein. Relatively low levels of PAF:acetylhydrolase activity would be advantageous, because PAF in the uterine flushings and released by the ovine endometrium is susceptible to hydrolysis by PAF:acetylhydrolase, whereas the PAF released by the embryo (8) or endothelial cells (7) is highly resistant to the actions of PAF:acetylhydrolase. PAF present on the surface of cells can act as a juxtacrine hormone, but it is also susceptible to PAF:acetylhydrolase (32). A method of demonstrating the actions of PAF on the endometrium independent of the effects of its metabolism by PAF:acetylhydrolase may be the use of a nonhydrolyzable analog, such as methyl-carbamyl PAF. Investigation of such a method is a priority for the future.
The effect of PAF on PGF secretion was greatest when the hormone replacement regimen mimicked the normal luteal phase, and it increased as the cycle progressed toward the time for luteolysis. The ovine PAF receptor has not been defined, but a heptahelical G protein-linked PAF receptor has been identified in several species (53). The mRNA for the human PAF receptor was localized in the uterus by in situ hybridization (10). It was not detected in the proliferative phase but was abundant in the late luteal phase in both stromal and epithelial cells. Infusion of the PAF receptor antagonist WEB 2086 in the uterus caused some release of PGFM (11). In this regard, its actions were more like a partial agonist than a true antagonist. Such partial agonism has not been reported for the actions of these agents on G protein-linked receptors. The apparent actions of WEB 2086 as a partial agonist have also been seen in mouse embryo implantation studies (45) and in a perfused rat heart model (25). On the basis of this pharmacological evidence, it is thought that another class of PAF receptor may also exist in the uterus (28).
Although the ovine PAF receptor has not been characterized, a functional ovine PAF receptor in endometrial explants is presumed to be responsible for PAF-induced activation of phospholipase C and mobilization of polyinositol phosphates (12). Similar responses to PAF have been reported for human endometrial explants (3), also including activation of phospholipase D (1, 2).
PAF synthesis and PAF receptor expression are both influenced by the presence of steroids in some tissues. The human PAF receptor gene has two steroid-responsive elements and one estradiol-responsive region in the promoter region (52). Estradiol treatment enhanced PAF production and PAF-receptor mRNA expression in cultures of human endometrial cells (50). After ovariectomy, the PAF concentration in the uterus of rats significantly declined, and this could be restored by treatment with estradiol (42). Treatment of human endometrial cell cultures with estradiol enhanced PAF-induced PGE2 production (5) and PAF-induced phospholipase D activity (2). Therefore, the presence of steroids in the uterus may stimulate both PAF production and PAF-receptor formation to sensitize the uterus to PAF.
The results presented in Fig. 6 show that the PAF ligand may play an important role in pulsatile release of PGF by the uterus. Many G protein-linked heptahelical receptors undergo a cycle of agonist-induced desensitization, recycling, and resensitization (26, 27). Desensitization of the receptor involves functional uncoupling of the receptor by gonadotropin-releasing hormone-associated proteins, including the regulators of G protein signaling (RGS) proteins (17) and ligand-induced receptor sequestration into the intracellular compartment after endocytosis of clathrin-coated vesicles (26). PAF-induced sequestration of its receptor was ligand dependent, and recycling of the active form did not require the synthesis of a new receptor (26). In B cells, PAF-induced expression of RGS 1 resulted in downregulation of signal transduction (18), which may determine the duration of tachyphylaxis. This may provide a mechanism for periodic response to ligand of cells that are independent of frequency of release of the ligand.
Results of the present study suggest a new hypothesis for the
generation of luteolytic pulses of
PGF2 by the ovine uterus; this
is presented in Fig. 7, with the primary points of
control indicated by the letters A-H. Luteal phase steroids (A) induce production of PAF, stimulate PAF-receptor mRNA production (51, 52) by
the endometrium, and cause the uterus to become sensitive to PAF (B).
The responsiveness of the uterus to PAF (at the receptor or
postreceptor level) requires both E and P for low-amplitude PGF2
pulses (C). The
low-amplitude PGF pulses may act on the CL to induce OT release (D)
(40), which then binds to uterine OTR (E) to induce high-amplitude
pulses of PGF2
(F) because of
the sequential action of PAF and OT on the uterus. After a high-amplitude pulse, the uterus becomes relatively desensitized to PAF
for ~6 h, followed by spontaneous resensitization and subsequent high-amplitude pulses of PGF that culminate in complete luteolysis (G).
Thus, even if relatively constant levels of PAF production occur in the
uterus, pulsatile release of PGF may occur because of the periodicity
of responsiveness of the uterus to PAF.
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IFN- completely blocks the release of PGFM in response to PAF (H)
and inhibits the synergism between PAF and OT. IFN-
also inhibits
transcription of the genes for the estrogen receptor and OTR
(38). McCracken et al. (37) showed that the production of
subluteolytic pulses of PGF by the uterine endometrium is similar between cyclic and pregnant ewes between days
12 and 14 and that they were unaffected by the presence of IFN-
. Also, OT release has
been shown (49) to be similar between cyclic and pregnant ewes during
this period. Perhaps pregnancy has the effect of uncoupling the
PAF-induced luteolytic PGF pulses and OT release and action on the
uterus. The ability of IFN-
to block the synergism of these agents
is consistent with this possibility. The hypothesis presented here is
suited for testing in intact animals during natural luteolysis and pregnancy.
In conclusion, PAF fulfills many of the criteria of an endogenous
initiator of PGF2 release from
the ovine uterus. The synergistic interaction between PAF and OT,
compared with the modest PGF pulses produced by PAF alone, may account
for the high amplitude of PGF responsible for luteolysis. The results
also provide an explanation for the apparent coupling of uterine
PGF2
and ovarian OT pulses.
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ACKNOWLEDGEMENTS |
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We thank Drs. K. Battye and A. Ammit for assistance with some experiments; Ray Kearns and staff for supply, care, and maintenance of animals and assistance with animal surgery; Dr. J. Chen for advice with biostatistics; and K. O'Neill for manuscript preparation.
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FOOTNOTES |
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Present address of T. Ott: Department of Animal and Veterinary Sciences, 216 Agricultural Sciences Building, University of Idaho, Moscow, ID 83844-2330.
The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.
Address for reprint requests and other correspondence: C. O'Neill, Human Reproduction Unit, Department of Physiology, Univ. of Sydney, Royal North Shore Hospital of Sydney, St. Leonards, NSW 2065, Australia (E-mail: chriso{at}med.usyd.edu.au).
Received 8 October 1998; accepted in final form 6 January 1999.
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REFERENCES |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
1.
Ahmed, A.,
R. A. Ferriani,
R. Plevin,
and
S. K. Smith.
Platelet-activating factor mediates phosphatidylcholine hydrolysis by phospholipase D in human endometrium.
Biol. Reprod.
47:
59-65,
1992[Abstract].
2.
Ahmed, A.,
M. A. Shoaibi,
R. Plevin,
and
S. K. Smith.
Oestradiol-17 modulates PAF-evoked phospholipase D activity but not inositide-lipid hydrolysis in human endometrial cell line, HEC-1B.
Cell Signal.
7:
403-409,
1995[Medline].
3.
Ahmed, A.,
and
S. K. Smith.
Platelet-activating factor stimulates phospholipase C activity in human endometrium.
J. Cell. Physiol.
152:
207-214,
1992[Medline].
4.
Alecozay, A. A.,
B. G. Casslen,
R. M. Riehl,
F. D. Deleon,
M. J. K. Harper,
M. Silvia,
T. A. Nouchi,
and
D. J. Hanahan.
Platelet-activating factor in human luteal phase endometrium.
Biol. Reprod.
41:
578-586,
1989[Abstract].
5.
Alecozay, A. A.,
M. J. Harper,
R. S. Schenken,
and
D. J. Hanahan.
Paracrine interactions between platelet-activating factor and prostaglandins in hormonally treated human luteal phase endometrium in vitro.
J. Reprod. Fertil.
91:
301-312,
1991[Abstract].
6.
Ammit, A. J.,
and
C. O'Neill.
Comparison of a radioimmunoassay and bioassay for embryo-derived platelet-activating factor.
Human Reprod.
6:
872-878,
1991[Abstract].
7.
Ammit, A. J.,
and
C. O'Neill.
Studies of the nature of the binding by albumin of platelet-activating factor released from cells.
J. Biol. Chem.
272:
18772-18778,
1997
8.
Ammit, A. J.,
and
C. O'Neill.
The role of albumin in the release of platelet-activating factor by mouse preimplantation embryos in vitro.
J. Reprod. Fertil.
109:
309-318,
1997[Abstract].
9.
Angle, M. J.,
F. Paltauf,
and
J. M. Johnston.
Selective hydrolysis of ether-containing glycerophospholipids by phospholipase A2 in rabbit lung.
Biochim. Biophys. Acta
962:
234-240,
1988[Medline].
10.
Baldi, E.,
L. Bonaccorsi,
G. Finetti,
M. Luconi,
M. Muratori,
T. Susini,
G. Forti,
M. Serio,
and
M. Maggi.
Platelet-activating factor in human endometrium.
J. Steroid Biochem. Molec. Biol.
49:
359-363,
1994[Medline].
11.
Battye, K. M.,
G. Evans,
and
C. O'Neill.
Ovine endometrium synthesizes and releases platelet-activating factor, which can cause the release of prostaglandin F2 by the uterus in situ.
Biol. Reprod.
54:
355-363,
1996[Abstract].
12.
Battye, K. M.,
C. O'Neill,
and
G. Evans.
Evidence that platelet activating factor suppresses uterine oxytocin-induced 13,14-dihydro-15-keto-prostaglandin F2 release and phosphatidylinositol hydrolysis in the ewe.
Biol. Reprod.
47:
213-219,
1992[Abstract].
13.
Braquet, P.,
and
M. Rola-Pleszczynski.
The role of PAF in immunological responses: a review.
Prostaglandins
34:
142-147,
1987[Medline].
14.
Carlson, J. C.,
P. Hahn,
K. So,
and
A. Chan.
Studies on prostaglandin and luteolysis in the pseudopregnant rabbit.
Endocr. Res.
16:
193-204,
1990[Medline].
15.
Collier, M.,
C. O'Neill,
A. J. Ammit,
and
D. M. Saunders.
Measurement of human embryo-derived platelet-activating factor (PAF) using a quantitative bioassay of platelet aggregation.
Human Reprod.
5:
323-328,
1990[Abstract].
16.
Denamur, R.,
J. Martinet,
and
R. V. Short.
Secretion de la progesterone par les corps jaunes de la brebis, apres hypophysectomie de la tige pituitarie et hysterectomie.
Acta Endocrinol. (Copenhagen)
52:
72-90,
1996.
17.
Dohlman, H. G.,
and
J. Thorner.
RGS proteins and signalling by heterotrimeric G proteins.
J. Biol. Chem.
272:
3871-3874,
1997
18.
Druey, K. M.,
K. J. Blumer,
V. H. Kang,
and
J. H. Kehrl.
Inhibition of G-protein-mediated MAP kinase activation by a new mammalian gene family.
Nature
379:
742-746,
1996[Medline].
19.
Fairclough, R. J.,
L. C. Moore,
A. J. Peterson,
and
W. B. Watkins.
Effect of oxytocin on plasma concentrations of 13,14-dihydro-15-keto prostaglandin F and oxytocin neurophysin during the oestrous cycle and early pregnancy in the ewe.
Biol. Reprod.
31:
36-43,
1980[Abstract].
20.
Flint, A. P. F.,
W. M. F. Leat,
E. L. Sheldrick,
and
H. J. Stewart.
Stimulation of phosphoinositide hydrolysis by oxytocin and the mechanism by which oxytocin controls prostaglandin synthesis in the ovine endometrium.
Biochem. J.
237:
797-805,
1986[Medline].
21.
Flint, A. P. F.,
and
E. L. Sheldrick.
Continuous infusion of oxytocin prevents induction of uterine oxytocin receptor and blocks luteal regression in cyclic ewes.
J. Reprod. Fertil.
75:
623-631,
1985[Abstract].
22.
Grazzini, E.,
G. Guillon,
B. Mouilac,
and
H. H. Zingg.
Inhibition on oxytocin receptor function by direct binding of progesterone.
Nature
392:
509-512,
1998[Medline].
23.
Homanics, G. E.,
and
W. J. Silvia.
Effects of progesterone and estradiol-17 on uterine secretion of prostaglandin F2a in response to oxytocin in ovariectomized ewes.
Biol. Reprod.
38:
804-811,
1988[Abstract].
24.
Hooper, S. B.,
W. B. Watkins,
and
G. D. Thorburn.
Oxytocin, oxytocin-associated neurophysin, and prostaglandin F2a concentrations in the utero-ovarian vein of pregnant and non-pregnant sheep.
Endocrinology
119:
2590-2597,
1986[Abstract].
25.
Hu, W.,
I. K. McNicholl,
P. C. Choy,
and
R. Y. K. Man.
Partial agonist effect of the platelet-activating factor receptor antagonists, WEB 2086 and WEB 2170, in the rat perfused heart.
Pharmacology
110:
645-650,
1993.
26.
Ishii, I.,
E. Saito,
T. Izumi,
M. Ui,
and
T. Shimizu.
Agonist-induced sequestration, recycling, and resensitization of platelet-activating factor receptor. Role of sytoplasmic tail phosphorylation in each process.
J. Biol. Chem.
273:
9878-9885,
1998
27.
Izumi, T.,
and
T. Shimizu.
Platelet-activating factor receptor: gene expression and signal transduction.
Biochim. Biophys. Acta
1259:
317-333,
1995[Medline].
28.
Kato, K.,
G. D. Clark,
N. G. Bazan,
and
C. F. Zorumski.
Platelet-activating factor as a potential retrograde messenger in CA1 hippocampal long-term potentiation.
Nature
367:
175-179,
1994[Medline].
29.
Kotwica, J.,
and
D. Skarzynski.
Influence of oxytocin removal from the corpus luteum on secretory function and duration of the oestrous cycle.
J. Reprod. Fertil.
97:
411-417,
1993[Abstract].
30.
Kotwica, J.,
D. Skarzynski,
M. Bogacki,
P. Melin,
and
B. Starostka.
The use of an oxytocin antagonist to study the function of ovarian oxytocin during luteolysis in cattle.
Theriogenology
48:
1287-1299,
1997.
31.
Kudolo, G. B.,
and
M. J. Harper.
Lyso-PAF:acetyl-CoA acetyltransferase and CDP-choline cholinephosphotransferase activities in the rabbit endometrium.
J. Lipid Mediators Cell Signal.
11:
145-158,
1995[Medline].
32.
Kudolo, G. B.,
Y. Q. Yang,
D. B. Chen,
M. A. Jones,
and
M. J. Harper.
Differential metabolism of exogenous platelet-activating factor by glandular epithelial and stromal cells of rabbit endometrium.
J. Reprod. Fertil.
105:
315-324,
1995[Abstract].
33.
Lee, T.,
B. Malone,
and
F. Snyder.
A new de novo pathway for the formation of 1-alkyl-2-acetyl-sn-glycerols, precursors of platelet activating factor. Biochemical characterization of 1-alkyl-2-lyso-sn-glycero-3-P:acetyl-CoA acetyltransferase in rat spleen.
J. Biol. Chem.
261:
5373-5377,
1986
34.
Lee, T.,
and
F. Snyder.
Overview of PAF biosynthesis and catabolism.
In: Biosynthesis and Catabolism, 1992, p. 1-22.
35.
Mallory, D. S.,
C. M. Gusr,
and
R. A. Dailey.
Effects of pituitary stalk transection and types of barrier on pituitary and luteal function during the oestrus cycle of the sheep.
Domest. Anim. Endocrinol.
3:
253-259,
1986.
36.
Martal, J.,
M. C. Lacroix,
C. Loudis,
M. Saunier,
and
S. Winterberger-Torres.
Trophoblastin an antiluteolytic protein present in early pregnancy in sheep.
J. Reprod. Fertil.
56:
63-73,
1979[Abstract].
37.
McCracken, J. A.,
E. E. Custer,
J. A. Eldering,
and
A. G. Robinson.
The central oxytocin pulse generator: a pacemaker for the ovarian cycle.
Acta Neurobiol. Exp.
56:
819-832,
1996[Medline].
38.
Mirando, M. A.,
J. P. Harney,
Y. Zhou,
T. F. Ogle,
T. L. Ott,
R. J. Moffatt,
and
F. W. Bazer.
Changes in progesterone and oestrogen receptor mRNA and protein and oxytocin receptors in endometrium of ewes after intrauterine injection of ovine trophoblast interferon.
J. Mol. Endocrinol.
10:
185-192,
1993[Abstract].
39.
Miwa, M.,
T. Miyake,
T. Yamanaka,
J. Sugatani,
Y. Suzuki,
S. Sakata,
Y. Araki,
and
M. Matsumoto.
Characterisation of serum platelet-activating factor (PAF) acetylhydrolase. Correlation between deficiency of serum PAF acetylhydrolase and respiratory symptoms in asthmatic children.
J. Clin. Invest.
82:
1983-1991,
1988[Medline].
40.
Moore, L. G.,
V. J. Choy,
R. L. Elliot,
and
W. B. Watkins.
Evidence for pulsatile release of PGF-2 inducing the release of ovarian oxytocin during luteolysis in the ewe.
J. Reprod. Fertil.
76:
159-166,
1986[Abstract].
41.
Morgan, G. L.,
R. D. Geisert,
J. P. McCann,
F. W. Bazer,
T. L. Ott,
M. A. Mirando,
and
M. Stewart.
Failure of luteolysis and extension of the interoestrous interval in sheep treated with the progesterone antagonist mifepristone (RU 486).
J. Reprod. Fertil.
98:
451-457,
1993[Abstract].
42.
Nakayama, R.,
K. Yasuda,
T. Okumura,
and
K. Saito.
Effect of 17beta-estradiol on PAF and prostaglandin levels in oophorectomized rat uterus.
Biochim. Biophys. Acta
1085:
235-240,
1991[Medline].
43.
National Health and Medical Research Council Commonwealth
Scientific and Industrial Research Organisation and Australian
Agricultural Council. Australian Code of Practice for the Care
and Use of Animals for Scientific Purposes. Canberra: Australian
Government Publishing Service, 1995.
44.
Okuda, K.,
Y. Uenoyama,
A. Miyamoto,
A. Okano,
F. J. Schweigert,
and
D. Schams.
Effects of prostaglandins and oestradion-17 on oxytocin binding in cultured bovine luteal cells.
Reprod. Fertil. Dev.
7:
1045-1051,
1995[Medline].
45.
O'Neill, C.
Platelet-activating factor-antagonists reduce implantation in mice at low doses only.
Reprod. Fertil. Dev.
7:
51-57,
1995[Medline].
46.
O'Neill, C.
Activity of platelet-activating factor acetylhydrolase in the mouse uterus during the oestrus cycle, throughout the preimplantation phase of pregnancy, and throughout the luteal phase of pseudopregnancy.
Biol. Reprod.
52:
965-971,
1995[Abstract].
47.
Ott, T. L.,
G. Van Heeke,
H. W. Johnson,
and
F. W. Bazer.
Cloning and expression in Saccharomyces cerevisiae of a synthetic gene for the type-1 trophoblast interferon ovine trophoblast protein-1: purification and antiviral activity.
J. Interferon Res.
11:
357-364,
1991[Medline].
48.
Poyser, N. L.
The refractoriness of uterine prostaglandin F2a productiona possible explanation (Abstract).
J. Reprod. Fertil.
13:
19,
1990.
49.
Rhodes, L.,
and
P. W. Nathanielsz.
Myometrial activity and plasma progesterone and oxytocin concentrations in cyclic and early pregnant ewes.
Biol. Reprod.
42:
834-841,
1990[Abstract].
50.
Sato, S.,
K. Kume,
T. Takan,
H. Mutoh,
Y. Taketani,
and
T. Shimizu.
Up-regulation of intracellular Ca2+ signaling and mRNA expression of platelet-activating factor receptor by oestradiol in human uterine endometrial cells.
Adv. Exp. Med. Biol.
416:
95-100,
1996[Medline].
51.
Schams, D.,
E. Schallenberger,
H. D. D. Meyer,
B. Bullermann,
H. J. Breitinger,
R. Koll,
G. Enzehofer,
T. A. M. Kruip,
D. L. Walters,
and
H. Kark.
Ovarian oxytocin during oestrus cycle in cattle.
In: Oxytocin Clinical and Laboratory Studies. Amsterdam: Elsevier Biomedical, 1985, p. 317-334.
52.
Shimizu, T.,
H. Mutoh,
and
S. Kato.
Platelet-activating receptor. Gene structure and tissue specific regulation.
Adv. Exp. Med. Biol.
416:
79-84,
1996[Medline].
53.
Shulka, S. D.,
J. A. W. Thuston,
C. Y. Zhu,
and
A. Dhar.
Platelet-activating factor receptorsignal mechanisms and molecular biology.
In: Platelet-Activating Factor
Signal Mechanisms and Molecular Biology, edited by S. D. Shulka. Boca Raton, FL: CRC, 1993, p. 41-59.
54.
Silvia, W. J.,
and
R. E. Raw.
Regulation of pulsatile secretion of prostaglandin F2 from the ovine uterus by ovarian steroids.
J. Reprod. Fertil.
98:
341-347,
1993[Abstract].
55.
Snyder, F.,
T. Lee,
M. L. Blank,
B. Malone,
D. Woodard,
and
M. Robinson.
Platelet-activating factor: alternate pathways of biosynthesis, mechanism of inactivation, and reacylation of lyso-PAF with arachidonate.
Adv. Prostaglandin Thromboxane Leukotriene Res.
15:
693-696,
1985[Medline].
56.
Van Heeke, G.,
T. L. Ott,
A. Strauss,
D. Ammaturo,
and
F. W. Bazer.
High-yield expression and secretion of the pregnancy recognition hormone ovine interferon-tau by Pichia pastoris.
J. Interferon Cytokine Res.
16:
119-126,
1996[Medline].
57.
Wardlow, M. L.,
C. P. Cox,
K. E. Meng,
D. E. Greene,
and
R. S. Farr.
Substrate specificity and partial characterisation of the PAF-acetylhydrolase in serum that rapidly inactivates platelet-activating factor.
J. Immunol.
136:
3441-3446,
1986
58.
Wetsel, W. C.,
M. M. Valenca,
I. Merchenthaler,
Z. Liposits,
F. J. Lopez,
R. I. Weiner,
P. L. Mellon,
and
A. Negro-Vilar.
Intrinsic pulsatile secretory activity of immortalized luteinizing hormone-releasing hormone-secreting neurons.
Proc. Natl. Acad. Sci. USA
89:
4149-4153,
1992[Abstract].
59.
Yasuda, K.,
K. Satouchi,
and
K. Saito.
Platelet-activating factor in normal rat uterus.
Biochem. Biophys. Res. Commun.
138:
1231-1236,
1986[Medline].
60.
Zimmerman, G. A.,
D. E. Lorant,
T. M. McIntyre,
and
S. M. Prescott.
Juxtacrine intercellular signalling: another way to do it.
Am. J. Respir. Cell Mol. Biol.
9:
573-577,
1993[Medline].