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Pregnancy |
a The Perinatal Research Centre, CIHR Group in Perinatal Health and Disease, Departments of Obstetrics and Gynaecology, Pediatrics, and Physiology, University of Alberta, Edmonton, Alberta, Canada T6G 2S2
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSResultsDISCUSSIONREFERENCES |
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concentration, reaching a peak on Gestational Day (GD) 18. This peak coincided with the final descent in plasma progesterone and birth on GD 19.3 ± 0.2. Implantation of a progesterone-releasing pellet in intact pregnant dams on GD 16 delayed birth at term until GD 20.9 ± 0.4 and inhibited the GD 18 increase in placental PGF2 levels in conjunction with a delayed fall in plasma progesterone that reached its lowest level 1 day after term birth. The mRNA levels of uterine activation proteins, connexin-43 (CX-43), oxytocin receptor, PGF2 receptor (FP), and PGHS-2, and the concentration of uterine PGF2 all increased at normal term birth. At progesterone-delayed term birth on GD 19.3, even though tissue PGF2 concentrations were at the same high levels observed at normal term birth, CX-43 and FP mRNA levels were lower than those at normal term birth, thereby possibly contributing to the delay of birth. These data are consistent with the hypotheses that fetal placental PGs affect the timing of birth by hastening luteolysis, that uterine activation initiates labor, and that birth may be delayed by blocking or decreasing the expression of two of the uterine activation proteins.
parturition, placenta, progesterone, uterus
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSResultsDISCUSSIONREFERENCES |
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, the luteolytic agent [3, 4]. In PGHS-1-deleted dams, birth is delayed at normal term, confirming the importance of this enzyme in maternal uterine tissues for normal labor initiation. Ultimately, low levels of progesterone lead to an increase in the mRNA expression of the uterine activation proteins [5, 6] connexin-43 (CX-43), oxytocin receptor (OTR), PGF2 receptor (FP), and PGHS-2 and a consequent increase in the uterine tissue levels of PGF2, thereby eliciting normal term birth [5]. Administration of ethanol to dams or ovariectomy both hasten the fall in plasma progesterone, causing preterm birth, whereas administration of progesterone to dams overcomes ethanol-induced preterm birth and restores gestational length to normal [7]. Thus, a considerable amount of maternal control over the timing of birth is evident in mice, and a clear relationship exists between the requirement for mRNA expression of the uterine activation proteins and the need for appropriate levels of uterine tissue concentrations of the contractile stimulant PGF2.
Little attention has been afforded, however, to the role of mouse fetal tissue PG synthetic enzyme expression and PGs and their possible contribution to birth. Considerable evidence in a variety of species supports the concept that the fetus plays an important role in the timing of its birth [8, 9], especially through the regulation of PG synthesis in the fetal membranes or placenta. A role for fetal PGs in the timing of birth in the mouse does appear to exist; the timing of birth is normal and newborn survival is high when the fetuses in PGHS-1-deleted dams are PGHS-1 replete, whereas implantation of PGHS-1-deleted dams with PGHS-1-deleted blastocysts delays birth significantly and leads to >80% newborn mortality [2]. Yet, the possible role for fetal PGs in the timing of birth is not well understood in the mouse, and the relationship between the role of fetal PGs and the apparent maternal control of parturition is not appreciated. The purpose of this study, therefore, was to examine the mRNA expression of two PG synthetic enzymes, PGHS-1 and PGHS-2, in the mouse fetal membranes and placenta near term and to determine the PG levels in these tissues. The potential relationship between fetal PGs and maternal regulation of birth was then examined by manipulating the levels of maternal progesterone and observing the effects upon the timing of birth and the mRNA expression of uterine activation proteins and uterine PGF2 concentrations.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSResultsDISCUSSIONREFERENCES |
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Nulliparous 10- to 12-wk-old female C57BL/6J mice (Jackson Laboratories, Bar Harbor, ME) were bred for these experiments. These animals were housed in a temperature- and humidity-controlled room, with a daily 12L:12D cycle (lights-on at 0600 h). One to four females were placed in each cage containing one male for breeding daily between 0900 and 1100 h. After this time, females were examined for the presence of a seminal plug. Successfully bred females were weighed, housed individually, and then weighed daily after GD 10 to monitor for pregnancy progression. The day on which the seminal plug was present was considered GD 0. The University of Alberta animal care committee approved all procedures.
Experiment 1
The fetal membranes and placentas were collected at 0900 h on GD 14, 16, and 17.5 and from dams in labor at the delivery of the first pup (term, GD 19.3 ± 0.1). Pregnant dams were killed by cervical dislocation. The abdomen was opened, and the uterus was carefully dissected and washed in a physiological (0.9%) saline solution. The uterus was then cut open, taking care not to break the membrane sacs. Each placenta was gently pulled away from the uterine wall, the sac was broken, and the membranes and placenta were removed from the fetus. The umbilical cord was cut close to the placenta. The membranes were then removed at their interface with the placenta. The decidua normally stays adhered to the uterine wall, but some decidual tissue may have been attached to the placentas. The tissues were snap frozen in liquid nitrogen and stored at -80°C until assayed.
Experiment 2
Pregnant dams were separated into two groups, those into which a progesterone pellet was implanted and those that did not receive a pellet and were considered controls. Three-week-release progesterone pellets (2.5 mg progesterone/pellet; Innovative Research of America, Sarasota, FL) were implanted on GD 15. Once the dams were brought to a stage of surgical anesthesia (by exposure to metofane in a nose cone in a sterile environment), determined by the toe-pad reflex, the pellet was placed subcutaneously and the animals were monitored until fully recovered. The times of gestation at which the fetal membranes and placenta were sampled were altered from experiment 1 to increase the collection frequency between GD 16 and 19 when the changes appeared to be occurring; sample times were GD 16, 17, and 18 and at the delivery of the first pup (term). Otherwise, tissue collections were performed as described for experiment 1. In preliminary experiments, placebo pellets were implanted in five control dams. These animals delivered at exactly the same time as untreated control dams (GD 19.3 ± 0.2). Therefore, untreated control dams were used in all experiments reported here.
Experiment 3
Three experimental treatments were used to test the effects of progesterone on the expression of uterine activation proteins and PGF2 concentrations in the uterine tissue. The first group was untreated control dams who were killed by decapitation to collect trunk blood for progesterone assay at GD 15, 16, 16.5, 17, 17.5, 17.75, 18, 18.25, and 18.5 and at delivery (term), and tissues were collected for subsequent assay. The second group of pregnant mice was anesthetized to a surgical plane on GD 16. Small incisions were made through the skin of the back and the peritoneal cavity at the approximate location of the ovaries. The ovaries were captured with forceps, and uterine tissue was pinched below each ovary with forceps to prevent bleeding. The ovaries were severed with scissors for the ovariectomy group or were held between forceps and then tucked back inside the animal for the sham-operated group. Incisions were stapled, and dams were allowed to recover from surgery. Animals were killed at GD 16 and 16.25 and at the delivery of the first pup (preterm, GD 17.0 ± 0.3). The third treatment group consisted of intact dams that received progesterone pellets (as described previously). Dams were killed at GD 16, 17, 18, 19.3, and 20 and at the delivery of the first pup (post-term, GD 20.9 ± 0.4), and tissues were collected for subsequent assay as described. Trunk blood was collected into heparinized tubes and centrifuged at 12 000 x g for 10 min, and plasma was removed and stored at -80°C until extracted for progesterone.
RNA Extraction and Generation of RNA Probes
Total RNA was extracted as previously described [5]. The generation of the probes for CX-43, OTR, FP, PGHS-2, and cyclophilin was described in detail [5]. Linearized pGT-PGHS-1-Mouse plasmid was transcribed to make a murine PGHS-1 antisense RNA probe. The RNA probe was made as previously described [5].
RNase Protection Assay
Twenty micrograms of total tissue RNA was hybridized to either 5 x 105, 1 x 106, or 2 x 106 cpm of the appropriate probe in 30 µl hybridization buffer for 16 h at 55°C. Yeast tRNA was processed in the same manner as a negative control. Following hybridization, samples were digested with 2.3 µg/ml RNase A and 300 U RNase T1 in 300 µl RNase digestion buffer (10 mM Tris-HCl, pH 7.5, 300 mM NaCl, 5 mM EDTA) for 30 min at 30°C. RNases were removed by treatment with 25 µg/ml proteinase K in the presence of 0.6 % SDS for 20 min at 37°C. Samples were phenol/chloroform extracted and precipitated with isopropanol using yeast tRNA as a carrier. Samples were centrifuged for 10 min at 14 000 x g, resuspended in 8 µl formamide RNA buffer, and electrophoresed on a 6% acrylamide, 8 M urea gel. The gel was dried, exposed to film for either 15 h (PGHS and uterine activation proteins) or 1 h (cyclophilin, within the linear range) and analyzed by autoradiography. The size of the protected band for PGHS-1 was 231 nucleotides. Autoradiograms were analyzed by densitometry using the Fluor-S-Max multi-imager (BioRad, Hercules, CA). Background was subtracted for each lane. Densitometric measurements of PGHS and uterine activation protein mRNA were normalized to those of cyclophilin, and values are expressed as the mean percentage of cyclophilin ± SEM.
Progesterone Determination
Progesterone was extracted from and concentrations measured in the plasma using a 3H RIA kit from ICN Diagnostics (Costa Mesa, CA) as described previously [7].
PG Extraction
PGs were extracted from fetal membrane, placental, and uterine tissue for subsequent mass determination. Tissue samples were kept on ice throughout the extraction procedure. Frozen fetal membranes or placentas from a dam were pooled, or uterine tissue was pulverized in liquid nitrogen and 50 mg of the tissue was homogenized in 500 µl of 100% ethanol in glass test tubes. In addition, two blanks containing only ethanol were homogenized to ensure that the presence of tissue did not affect recovery values. Prior to homogenization, 2500 cpm of 3H-PGE2 was added to each test tube for recovery calculations. Recovery was 89.6% ± 1.2%. Following homogenization, 4 ml of 50 mM citrate buffer (pH 3.5) was added to each sample to bring the ethanol concentration to <15%. The samples were then inverted several times, placed at room temperature for 5 min, and centrifuged at 12 000 x g for 25 min. Ninety percent of the supernatant was withdrawn for sample purification. The tubes were then inverted on an absorbent surface to remove any residual supernatant. The remaining protein pellet was capped, stored at 4°C, and assayed for protein mass within 24 h using the Micro BCA Protein Assay Reagent Kit (Pierce, Rockford, IL).
Sample Purification
Samples were purified according to a protocol similar to that of the PGE2 and PGF2 enzyme immunoassay (EIA) kits (Cayman Chemical Company, Ann Arbor, MI). C-18 solid phase extraction (SPE) Sep-Pak cartridges were attached to 5-ml syringes and activated with methanol followed by 5 ml milli-Q water. The sample (90% of the supernatant above) was passed slowly through the column. The column was then washed again with milli-Q water and hexane. Finally, the PGs were eluted with ethyl acetate containing 1% methanol. The ethyl acetate was then evaporated under a gentle stream of nitrogen. Each sample was reconstituted in 450 µl EIA buffer. Fifty microliters of the sample was removed for scintillation counting. The remaining sample was aliquoted (30 µl) and stored at -80° C until the assay.
PG Measurement
PGE2 and PGF2 levels in fetal membrane and placental tissues were determined using an EIA kit (Cayman) specific to each PG. Absorbance values were measured at 412 nm. The intra- and interassay coefficients of variation (COVs) for the PGE2 EIA were 10% and 14%, respectively. The intra- and interassay COVs for the PGF2 EIA were 9% and 8%, respectively. Uterine tissue PGF2 concentration was determined by RIA [5]. The intra- and interassay COVs were 11.7% and 13.9%, respectively. The only difference in the two immunoassays was that the EIA was more sensitive than the RIA by a factor of 10. All values were normalized to protein and are expressed as pg prostaglandin/mg protein.
Statistical Analyses
The data were analyzed using a Student t-test, one-way ANOVA, or two-way ANOVA as appropriate. When a significant F value was obtained, posthoc multiple range analysis was performed using the Tukey test. Differences were considered significant at P 
0.05.
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Results |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSResultsDISCUSSIONREFERENCES |
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The values of fetal membrane PGHS-1 and PGHS-2 mRNA normalized to cyclophilin mRNA during late gestation are shown in Figure 1A. PGHS-1 mRNA expression was significantly higher (P < 0.05) at term birth than at GD 14, whereas PGHS-2 mRNA relative abundance was unchanged between GD 14 and GD 17.5 but then rose significantly (P < 0.05) at birth on GD 19.3.
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The values of placental PGHS-1 and -2 normalized to cyclophilin mRNA are shown in Figure 1B. There was no significant increase in the relative abundance of PGHS-1 mRNA (P > 0.05). In contrast, PGHS-2 mRNA expression increased significantly (P < 0.05) between GD 16 and GD 17.5. No further significant increase was observed at term labor.
Experiment 2: Fetal Membrane and Placental PG Concentrations
Figure 2, A and B, illustrates the effect of gestational age and maternal progesterone supplementation on fetal membrane PGE2 and PGF2 levels, respectively. In the untreated controls, neither PGE2 nor PGF2 concentrations changed between GD 16 and term birth or expected term birth on GD 19.3 except for a significant (P < 0.05) decrease on GD 18, which differed from values at all other time points measured. Progesterone supplementation had no effect on these fetal membrane PG values.
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Figure 2C illustrates the effect of gestational age and progesterone supplementation on placental PGE2 levels. There was no change in placental PGE2 concentrations from GD 16 to term birth or expected term birth, although an increase (P < 0.05) in PGE2 levels was observed between GD 16 and GD 17 in control animals but was lost by GD 18. Maternal progesterone supplementation did, however, significantly lower (P < 0.05) placental PGE2 concentrations on GD 18 only.
Unique for the PGs, the placental PGF2 concentrations increased (P < 0.05) 8-fold from GD 16 to a peak on GD 18 and then dropped slightly (P < 0.05) at the time of term labor, although the concentrations were still significantly greater than those on GD 16 (P < 0.05). In progesterone-supplemented dams, no change in PGF2 levels occurred between GD 16 and expected term birth, although a marked suppression of PGF2 concentration was evident at GD 18 (P < 0.05). This suppression of PGF2 levels yielded a 30-fold difference between this value and those observed in the untreated control group on GD 18.
Experiment 3: Plasma Progesterone Profiles
The three delivery dates in the ovariectomized, control, and progesterone-supplemented dams were significantly different from each other (ANOVA, Tukey test, P < 0.05). Because maternal progesterone supplementation suppressed the increase in placental PGF2 and extended the length of pregnancy, in the next series of experiments (on separate animals) we studied the effects of progesterone supplementation or removal by ovariectomy on maternal uterine activation and PGF2 concentrations. However, before these experiments were conducted the plasma profiles of progesterone during gestation in intact control dams, ovariectomized dams, and intact dams with progesterone implants were examined (Fig. 3). In the intact control dams, mean plasma progesterone levels of 117 ng/ml decreased significantly (P < 0.05) from GD 15 to GD 16 and then stabilized at approximately 30–50 ng/ml until after GD 18, when the final decrease to parturition levels of 3.9 ± 1 ng/ml (P < 0.05) occurred at the time of normal birth on GD 19.3 ± 0.1. In the ovariectomized dams, plasma progesterone values fell quickly after removal of the ovaries on GD 16.0 to parturition levels of 3.4 ± 1.3 ng/ml (P < 0.05), and birth occurred at GD 17.3 ± 0.3. There was no difference in plasma progesterone values at the time of preterm and term births. The plasma progesterone levels in intact dams receiving a progesterone-releasing pellet were higher by an average 35 ± 11 ng/ml than those in control dams (Student t-test, P < 0.05), and delivery was delayed until GD 20.9 ± 0.4 when plasma progesterone levels were 33.7 ± 4.9 ng/ml. These values were higher than those at preterm or term birth (P < 0.001). Observational data suggested that the duration of labor was also increased at postterm birth.
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Uterine Activation Protein mRNA Expression and Tissue PGF2 Concentrations
In the normal intact dams, a similar and consistent increase in the relative abundance for the uterine mRNA of CX-43, OTR, FP, and PGHS-2 and the uterine tissue concentrations of PGF2 occurred during late gestation between GD 17 and term birth on GD 19.3 (P < 0.05 for all five values, Fig. 4, A–E). Ovariectomy also increased the uterine tissue mRNA relative abundance for all four uterine activation proteins at the time of preterm delivery compared with their age-matched control dams at GD 17 (P < 0.05; Fig. 5, A–D). However, preterm birth occurred despite the fact that uterine tissue PGF2 concentrations decreased relative to those on GD 17 (Fig. 5E).
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Progesterone supplementation produced a variable effect on uterine activation protein gene expression at the time of expected term delivery on GD 19. CX-43 and FP mRNA relative abundances were significantly lower (P < 0.05) than those in uterine tissues from control dams delivering at term (Fig. 6, A and C), but there were no differences in the mRNA levels of OTR or PGHS-2 compared with values at term (Fig. 6, B and D). Additionally, uterine tissue concentrations of PGF2 were as high at delayed term birth as they were at actual term birth (Fig. 6E). When the delayed dams did deliver postterm, the relative abundance levels for OTR, FP, and PGHS-2 mRNA were as high or higher than those in uterine tissues of control dams that delivered at normal term (P < 0.05; Fig. 6, B–D). Only CX-43 mRNA levels were lower than at expected term birth or normal term birth (P < 0.05; Fig. 6A), reaching levels equivalent to those of a pregnant uterus on GD 16. Similarly, uterine tissue PGF2 concentrations at postterm birth were also very low relative to those at normal term birth (P < 0.05) and were comparable to those of an intact pregnant dam at GD 17 (Fig. 6E).
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The differences in mRNA for uterine activation proteins and uterine tissue PGF2 concentrations at birth or at delayed birth are summarized in Table 1. At normal term, all factors are increased relative to their values at GD 17. However, preterm and postterm birth occur when PGF2 levels are lower than or equal to those at GD 17, and postterm birth occurs in the absence of an increase in CX-43 mRNA expression relative to that on GD 17. At expected term birth in the progesterone-supplemented dams, activities of two of the uterine activation proteins, CX-43 and FP, were lower than those at normal term birth, and PGF2 concentrations were as high as those at term birth. Of all the mRNA species for the uterine activation proteins tested, only FP mRNA was elevated at term birth and decreased at delayed birth.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSResultsDISCUSSIONREFERENCES |
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in the mouse fetal membranes and placenta during late gestation and at term and the effects of progesterone supplementation on these concentrations. It is the first exploration of the effects of a low level of progesterone supplementation on the simultaneous expression of mRNA for four uterine activation proteins and the timing of birth. It also provides further evidence for a key role for the FP receptor in the regulation of birth in mice. Together, these data provide new information about the regulation of birth in mice and suggest a possible role for placental PGs in that regulation.
There is very little information regarding the levels of PGF2 or PGE2 in murine fetal membranes and placenta. Anton et al. [10] indicated that the periembryonic tissues of the fetal mouse (fetal membranes, placenta, and uterus) are capable of producing PGE2 from endogenous and exogenous arachidonic acid on GD 10. This present study appears to be the first to demonstrate the levels of these PGs over the last 3 days of gestation in mouse fetal membranes and placenta.
The late gestation patterns of PGHS-1 and -2 expression in the mice in this study were similar to those in sheep and humans, but the changes in PG levels differed considerably from those seen in these two species. PGHS-2 mRNA expression increases before labor in sheep placenta, and both PGHS-2 and PGHS-1 mRNA expression increase in human amnion and chorion before labor at term [11–13]. Increases in concentration and output of PGs from the fetal lamb membranes, amnion, and chorioallantois occur as birth approaches [14]. Similarly, in the human amnion and chorion, an increase in PGE2 content is evident in late gestation [15] and is correlated with PG output from exogenous and endogenous precursors in these tissues and with amniotic fluid levels [16–20].
The increases in mouse fetal membrane and placental PGHS-1 and -2 mRNA abundance are consistent with in situ hybridization data from other laboratories [1, 2, 21]. PGHS-1 was expressed strongly in the uterine luminal epithelium and decidua, there was some expression in the labyrinthine section of the placenta, and very low (if any) expression was found in the myometrium and none in the yolk sac (vitelline membrane). PGHS-2 mRNA is localized primarily to uterine myometrial circular smooth muscle and decidua. In fetal tissues, PGHS-2 mRNA is found in the yolk sac and giant cell, spongiotrophoblast, and labyrinthine areas of the placenta. The amniotic membrane was reported to be devoid of PGHS-1 or -2 mRNA [2]. However, we did observe PGHS-1 mRNA in the fetal membranes, perhaps because of the greater sensitivity of the RNase protection assay. In spite of the demonstrated changes of PGHS-1 and -2 mRNA in the fetal membranes, we observed no increases in PGE2 or PGF2 concentrations in these tissues and no increase in PGE2 in the placenta before birth. The placental increases in PGF2, therefore, likely reflect changes in enzymes that are distal to PGHS.
The PG rise from GD 16 to GD 18 in the placenta was selective for PGF2 but not PGE2, suggesting that expression and/or protein levels of the PGF synthase (or reductase) enzyme may have been enhanced. PGF synthase protein has been detected by Western immunoblot in mouse placenta [22], although neither the gestational profile nor regulation of PGF synthase is known. This increase in PGF2 probably was not due to a decrease in the activity of PG dehydrogenase (PGDH), which metabolizes PGs, because PGE2 levels would have increased similarly. The lack of change in either PG level during late gestation in the fetal membranes suggests that PGHS was not translated into active protein or that PGDH activity changed, keeping the levels of primary PGs constant. Similarly, the consistent decrease in PG levels in the fetal membranes on GD 18 may be due to an effect at the arachidonic acid deacylation step or the PGHS step of synthesis, because both PGs decreased. Alternatively and perhaps more realistically, these decreases may have been due to acute changes in the vascular tone and thereby delivery of substrates or oxygen to these tissues.
The plasma progesterone concentrations in late gestation mice have been determined in several other studies, and there is significant variation in these values that may be due to the strain of mice, frequency of sampling, or the assay employed [23–28]. Even so, our plasma concentrations (Fig. 3) are consistent with many of these previously determined values, especially those of Soares and Talamantes [26], who obtained samples frequently (every 4 h). Our data and those of these other studies demonstrate a significant fall in plasma concentrations on GD 17 or 18, which coincides with the increase in placental PGF2 from GD 16 to GD 18. Given this timing before labor onset, this rise in placenta PGs may lead to a hastening of luteolysis, the fall of plasma progesterone, and the onset of labor. Progesterone supplementation prevented this rise in placental PGF2 (Fig. 2D) and delayed the timing of the nadir of plasma progesterone levels (Fig. 3), also suggesting that luteolysis may have been delayed because the placental PG values were lowered. Progesterone can regulate PG synthesis at the phospholipase A2, PGHS, and PGDH levels of the PG synthetic pathway [7, 21, 29–32]. However, differences in tissue responsiveness may exist; uterine tissue concentrations of PGF2 in progesterone-supplemented dams did not decrease at expected term labor relative to normal term labor concentrations (Fig. 4E).
The data of Reese et al. [2] also support a role for a fetal PG contribution to birth in mice. Fetuses that were homozygous replete for PGHS-1 and were transferred to the uterus of a homozygous-deleted PGHS-1 dam had nearly normal gestational lengths (1 day longer than normal) and a high rate of survival. Conversely, PGHS-1 knockout dams with fetuses that were without or haploid for PGHS-1 delivered 2 days later than normal and 
80% died. We observed a trend for an increase in placental PGHS-1 during late gestation, suggesting that the sum of PGs from both PGHS-1 and -2 catalytic activities may be required for maximal effect in the terminal events of the corpus luteum. Reese et al. also administered PGs (PGF2 was most effective) to PGHS-1-deleted dams, causing luteolysis and shortening the delay in birth by 1 day, thereby vastly improving pup survival. The uterine decidua and epithelium would have diminishing levels of PG for luteolysis by GD 18 because mRNA levels for uterine PGHS-1 are falling by then [1, 21]. The uterine myometrium probably is not contributing PGs because its PGHS-2 mRNA and PG levels do not increase until later, closer to birth [1, 2, 5]. The placenta, therefore, is a reasonable source of PGs for luteolysis at this time in gestation.
Birth was delayed because the release of progesterone from the pellet was enough to lower the gene expression of two of the four uterine activation proteins in the uterus (Table 1). Two other uterine activation proteins had mRNA expression levels equal to those at term birth, and the PGF2 concentrations were similarly high. These data suggest that the progesterone concentrations required to regulate the expression of the uterine activation proteins vary according to the protein. The postterm data indicate that the lack of an increase in only CX-43 mRNA compared with that at GD 17 does not prevent birth, even though PGF2 levels were lower than those at term birth. Rather, when two of the uterine activation protein genes have decreased expression (or fail to increase to parturient levels), birth is delayed. If blockage of two uterine activation proteins is important in delaying birth, this could explain the failure of birth to be delayed in FP gene-deleted dams when they were ovariectomized 1 day prior to term birth [21]. In that mouse study, only one uterine activation gene was absent (FP), whereas following ovariectomy all the other genes are apparently expressed (Table 1). There appear to be redundant systems to effect birth, but when two of these are inhibited (or not activated), then pregnancy is maintained.
Our ovariectomy data confirm the observations regarding the relationship between the uterine FP mRNA and PGF2 concentrations when preterm birth was induced in mice by maternal intake of ethanol [5]. The mRNA abundance for FP was elevated, whereas there was no difference in uterine concentrations of PGF2 compared with gestational age-matched mice that were pregnant but not in labor. At both ovariectomy-induced preterm labor and at postterm labor in this study, the uterine PGF2 concentrations were lower than those at GD 17 in nonlaboring pregnant dams, yet the FP mRNA expression levels were increased (Table 1). To our knowledge, these two studies are the only ones to produce data that suggest that an increase in uterine sensitivity to PGs, rather than a rise in PG concentrations, may be causal for labor. Conversely, Smith et al. [33] demonstrated that labor was associated with a much lower level of the inhibitor EP2 receptor mRNA in baboon uterus. It will be interesting to determine whether these same relationships exist in other species, including humans.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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2 Correspondence: David M. Olson, Perinatal Research Centre, 220 HMRC, University of Alberta, Edmonton, AB, Canada T6G 2S2. FAX: 780 492 1308; david.olson{at}ualberta.ca
3 Current address: Health Canada, Ottawa, ON, Canada
Received: 26 June 2002.
First decision: 25 July 2002.
Accepted: 5 September 2002.
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