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Daily and hourly temporal association between Δ4-androstenedione - induced preterm myometrial contractions and maternal plasma estradiol and oxytocin concentrations in the 0.8 gestation rhesus monkey

      Abstract

      OBJECTIVE: Our purpose was to investigate the temporal relationship between Δ4-androstenedione-induced preterm switching of myometrial activity patterns from contractures to contractions and maternal plasma estradiol and oxytocin concentrations in the 0.8 gestation rhesus monkey. STUDY DESIGN: Eight rhesus monkeys (132 to 136 days' gestation) were instrumented under halothane with femoral artery and vein catheters and uterine electromyogram electrodes. At 138 to 142 days' gestation baseline maternal femoral artery blood samples for estradiol and oxytocin measurement were taken at 30-minute intervals for 7 hours, starting 2 hours before the onset of darkness. The day after baseline sampling a continuous intravenous Δ4-androstenedione infusion (0.3 mg · kg-1 · hr-1 in 10% intralipid at 0.25 ml · hr-1) was started in four monkeys, while four monkeys were infused intravenously with intralipid alone. The sampling regimen was then repeated at 1 and 3 days after the start of the Δ4-androstenedione or intralipid infusion. Contractions were counted and estradiol and oxytocin were measured by radioimmunoassay. RESULTS: Androstenedione promoted a premature nocturnal increase in myometrial contractions in conjunction with an increase in maternal plasma concentrations of estradiol and oxytocin, which were of similar magnitude to those measured in spontaneous term labor. The increase in maternal estradiol preceded the increase in maternal oxytocin levels and myometrial contractions. The onset of the increase in maternal plasma oxytocin was closely associated with the appearance of myometrial contractions after Δ4-androstenedione treatment. In contrast, no sustained premature contractions or changes in estradiol and oxytocin occurred in intralipid-treated monkeys. CONCLUSIONS: We conclude that in the 0.8 gestation rhesus monkey (1) the increase in maternal plasma estradiol precedes the increase in maternal plasma oxytocin after Δ4-androstenedione treatment and (2) Δ4-androstenedione-induced preterm myometrial contractions are closely associated in time with physiologic increases in maternal plasma oxytocin concentrations. (AM J OBSTET GYNECOL 1996;174:1050-5.)

      Keywords

      The development of continuous measurements of uterine electromyographic activity with concomitant changes in intraamniotic pressure in nonhuman primate species by several groups of investigators has allowed the description of two types myometrial activity patterns: low amplitude, long-lasting contractures and prominent, short-lasting contractions.
      • Honnebier MBOM
      • Nathanielsz PW
      Primate parturition and the role of the maternal circadian system.
      In most species studied to date, including the human,
      • Schulman H
      • Romney SL
      Variability of uterine contractions in normal human parturition.
      the baboon,
      • Morgan MA
      • Silavin SL
      • Wentworth RA
      • et al.
      Different patterns of myometrial activity and 24-h rhythms in myometrial contractility in the gravid baboon during the second half of pregnancy.
      • Wilson Jr, L
      • Parsons MT
      • Ouano L
      • Flouret G
      A new tocolytic agent: development of an oxytocin antagonist for inhibiting uterine contractions.
      and the rhesus monkey,
      • Honnebier MBOM
      • Figueroa JP
      • Rivier J
      • Vale W
      • Nathanielsz PW
      Studies on the role of oxytocin in late pregnancy in the pregnant rhesus monkey, I: plasma concentrations of oxytocin in the maternal circulation throughout the 24h day and the effect of the synthetic oxytocin antagonist [1-β-Mpa(β-(CH2)5)1,(Me(Tyr2, Orn8] OT on spontaneous nocturnal myometrial contractions.
      • Mecenas CA
      • Honnebier MBOM
      • Owiny JR
      • Nathanielsz PW
      Continuous intravenous infusion of Δ4-androstenedione to pregnant rhesus monkeys at 139 days gestational age produces premature delivery.
      • Novy MJ
      • Haluska GJ
      Endocrine and paracrine control of parturition in rhesus monkeys.
      although myometrial contractures occur throughout the majority of pregnancy, the switch to myometrial contractions is an inherent prerequisite for spontaneous labor and delivery.
      In the preparturient primate, including women, the switch from contractures to contractions has three special characteristics. First, the switch occurs around the onset of darkness.
      • Morgan MA
      • Silavin SL
      • Wentworth RA
      • et al.
      Different patterns of myometrial activity and 24-h rhythms in myometrial contractility in the gravid baboon during the second half of pregnancy.
      • Honnebier MBOM
      • Figueroa JP
      • Rivier J
      • Vale W
      • Nathanielsz PW
      Studies on the role of oxytocin in late pregnancy in the pregnant rhesus monkey, I: plasma concentrations of oxytocin in the maternal circulation throughout the 24h day and the effect of the synthetic oxytocin antagonist [1-β-Mpa(β-(CH2)5)1,(Me(Tyr2, Orn8] OT on spontaneous nocturnal myometrial contractions.
      • Tambyraja RL
      • Hobel CJ
      Characterization of 24 hr. uterine activity (UA) in the second half of the human pregnancy [Abstract 516].
      Second, it is reversible. The night when spontaneous delivery occurs is generally preceded by a variable number of nights during which the switch from contractures to contractions reverts to contractures after a few hours of contraction activity.
      • Honnebier MBOM
      • Figueroa JP
      • Rivier J
      • Vale W
      • Nathanielsz PW
      Studies on the role of oxytocin in late pregnancy in the pregnant rhesus monkey, I: plasma concentrations of oxytocin in the maternal circulation throughout the 24h day and the effect of the synthetic oxytocin antagonist [1-β-Mpa(β-(CH2)5)1,(Me(Tyr2, Orn8] OT on spontaneous nocturnal myometrial contractions.
      • Taylor NF
      • Martin MC
      • Nathanielsz PW
      • Seron-Ferre M
      The fetus determines circadian oscillation of myometrial electromyographic activity in the pregnant rhesus monkey.
      Finally, the switch is progressive, the number of nighttime contractions augmenting on consecutive nights until spontaneous delivery finally occurs.
      • Taylor NF
      • Martin MC
      • Nathanielsz PW
      • Seron-Ferre M
      The fetus determines circadian oscillation of myometrial electromyographic activity in the pregnant rhesus monkey.
      The mechanisms mediating the switch from contractures to contractions during spontaneous labor have been the focus of a number of investigations. In the primate a central role for mediating the switch from contractures to contractions has been attributed to estrogen-induced increases in maternal plasma oxytocin levels. Accordingly, the onset of spontaneous nighttime contractions is closely associated in time with elevated maternal plasma oxytocin concentrations,
      • Honnebier MBOM
      • Figueroa JP
      • Rivier J
      • Vale W
      • Nathanielsz PW
      Studies on the role of oxytocin in late pregnancy in the pregnant rhesus monkey, I: plasma concentrations of oxytocin in the maternal circulation throughout the 24h day and the effect of the synthetic oxytocin antagonist [1-β-Mpa(β-(CH2)5)1,(Me(Tyr2, Orn8] OT on spontaneous nocturnal myometrial contractions.
      • Hirst JJ
      • Haluska GJ
      • Cook MJ
      • Novy MJ
      Plasma oxytocin and nocturnal uterine activity: maternal but not fetal concentrations increase progressively during late pregnancy and delivery in rhesus monkeys.
      and use of oxytocin antagonists will abolish myometrial contractions in the monkey
      • Honnebier MBOM
      • Figueroa JP
      • Rivier J
      • Vale W
      • Nathanielsz PW
      Studies on the role of oxytocin in late pregnancy in the pregnant rhesus monkey, I: plasma concentrations of oxytocin in the maternal circulation throughout the 24h day and the effect of the synthetic oxytocin antagonist [1-β-Mpa(β-(CH2)5)1,(Me(Tyr2, Orn8] OT on spontaneous nocturnal myometrial contractions.
      • Hirst JJ
      • Haluska GJ
      • Cook MJ
      • Hess DL
      • Novy MJ
      Comparison of plasma oxytocin and catecholamine concentrations with uterine activity in pregnant rhesus monkeys.
      and in the baboon.
      • Wilson Jr, L
      • Parsons MT
      • Ouano L
      • Flouret G
      A new tocolytic agent: development of an oxytocin antagonist for inhibiting uterine contractions.
      Although precise recording of myometrial activity is not possible in human pregnancy, oxytocin antagonists do appear to decrease myometrial activity in pregnant women.
      • Andersen LF
      • Lyndrup J
      • Akerlund M
      • Melin P
      Oxytocin receptor blockade: a new principle in the treatment of preterm labor?.
      We have recently developed a model for inducing premature labor in the rhesus monkey at 0.8 gestation that reproduces the characteristics of the primate switch from myometrial contractures to contractions. Thus administration of Δ4-androstenedione to the pregnant monkey at 0.8 gestation will induce premature nighttime, reversible and progressive epochs of myometrial contractions, an increase in maternal plasma estradiol and oxytocin concentrations, and live delivery.
      • Mecenas CA
      • Honnebier MBOM
      • Owiny JR
      • Nathanielsz PW
      Continuous intravenous infusion of Δ4-androstenedione to pregnant rhesus monkeys at 139 days gestational age produces premature delivery.
      • Figueroa JP
      • Honnebier MBOM
      • Binienda Z
      • Wimsatt J
      • Nathanielsz PW
      Effect of 48-hour intravenous -androstenedione infusion on the pregnant rhesus monkey in the last third of gestation: changes in maternal plasma estradiol concentrations and myometrial contractility.
      However, the temporal relationship between the increases in estradiol and oxytocin in maternal plasma and between increased maternal oxytocin and Δ4-androstenedione - induced premature myometrial contractions has not been investigated to date. In the current study we investigated the effect of Δ4-androstenedione treatment on the following events in the 0.8 gestation rhesus monkey: (1) the daily temporal relationship between maternal plasma estradiol and oxytocin and (2) the hourly temporal relationship between the onset of the increase in maternal plasma oxytocin levels and premature myometrial contractions.

      MATERIAL AND METHODS

      Care of animals

      Eight pregnant rhesus monkeys of known gestational age were obtained from the California Regional Primate Research Center (Davis, Calif.) and acclimated to laboratory conditions and to the tether restraint system, as previously described.
      • Honnebier MBOM
      • Figueroa JP
      • Rivier J
      • Vale W
      • Nathanielsz PW
      Studies on the role of oxytocin in late pregnancy in the pregnant rhesus monkey, I: plasma concentrations of oxytocin in the maternal circulation throughout the 24h day and the effect of the synthetic oxytocin antagonist [1-β-Mpa(β-(CH2)5)1,(Me(Tyr2, Orn8] OT on spontaneous nocturnal myometrial contractions.
      • Taylor NF
      • Martin MC
      • Nathanielsz PW
      • Seron-Ferre M
      The fetus determines circadian oscillation of myometrial electromyographic activity in the pregnant rhesus monkey.
      • Figueroa JP
      • Honnebier MBOM
      • Binienda Z
      • Wimsatt J
      • Nathanielsz PW
      Effect of 48-hour intravenous -androstenedione infusion on the pregnant rhesus monkey in the last third of gestation: changes in maternal plasma estradiol concentrations and myometrial contractility.
      • Binienda Z
      • Massmann A
      • Wimsatt J
      • et al.
      In brief, after a full physical examination the animals were housed in individual cages in rooms with controlled light/dark cycles (14 hours light and 10 hours dark) and placed in quarantine. During quarantine the animals were jacketed and familiarized with the tether through which vascular catheters and electrode leads were to be connected after surgical instrumentation. One week later the tether was fixed to a box on the back of the jacket. The animals were fed daily (Purina 5045 High Protein Monkey Chow, Purina, St. Louis; and fresh fruits), and water was continuously available.

      Surgical instrumentation and postsurgical management

      The monkeys were instrumented between 118 and 136 days' gestation (term is approximately 163 days, see Table I) by techniques previously described.
      • Honnebier MBOM
      • Figueroa JP
      • Rivier J
      • Vale W
      • Nathanielsz PW
      Studies on the role of oxytocin in late pregnancy in the pregnant rhesus monkey, I: plasma concentrations of oxytocin in the maternal circulation throughout the 24h day and the effect of the synthetic oxytocin antagonist [1-β-Mpa(β-(CH2)5)1,(Me(Tyr2, Orn8] OT on spontaneous nocturnal myometrial contractions.
      • Taylor NF
      • Martin MC
      • Nathanielsz PW
      • Seron-Ferre M
      The fetus determines circadian oscillation of myometrial electromyographic activity in the pregnant rhesus monkey.
      • Figueroa JP
      • Honnebier MBOM
      • Binienda Z
      • Wimsatt J
      • Nathanielsz PW
      Effect of 48-hour intravenous -androstenedione infusion on the pregnant rhesus monkey in the last third of gestation: changes in maternal plasma estradiol concentrations and myometrial contractility.
      • Binienda Z
      • Massmann A
      • Wimsatt J
      • et al.
      Following food deprivation for 24 hours, polyvinyl catheters (inner diameter 0.04 inch, outer diameter 0.07 inch; Tygon, Baxter, N.J.) were placed in the dorsal aorta and inferior vena cava through the left femoral artery and vein, respectively, while animals were under general anesthesia (15 mg · kg-1 ketamine for induction, 1% to 2% halothane in oxygen for maintenance). In addition, three pairs of multistranded electrodes (AS 632, Cooner Wire, Chatsworth, Calif.) were sewn on different sites of the anterior surface of the uterine body to monitor myometrial electromyographic activity. Catheters were filled with heparinized saline solution (25 IU ml-1) and tunneled subcutaneously with the electrode leads to exit between the shoulder blades. After surgery the monkey was rejacketed and maintained on the tether-and-swivel system, which allowed the animal to be free ranging and permitted passage of vascular catheters and electromyographic leads to the top of the cage.
      • Honnebier MBOM
      • Figueroa JP
      • Rivier J
      • Vale W
      • Nathanielsz PW
      Studies on the role of oxytocin in late pregnancy in the pregnant rhesus monkey, I: plasma concentrations of oxytocin in the maternal circulation throughout the 24h day and the effect of the synthetic oxytocin antagonist [1-β-Mpa(β-(CH2)5)1,(Me(Tyr2, Orn8] OT on spontaneous nocturnal myometrial contractions.
      • Taylor NF
      • Martin MC
      • Nathanielsz PW
      • Seron-Ferre M
      The fetus determines circadian oscillation of myometrial electromyographic activity in the pregnant rhesus monkey.
      • Figueroa JP
      • Honnebier MBOM
      • Binienda Z
      • Wimsatt J
      • Nathanielsz PW
      Effect of 48-hour intravenous -androstenedione infusion on the pregnant rhesus monkey in the last third of gestation: changes in maternal plasma estradiol concentrations and myometrial contractility.
      • Binienda Z
      • Massmann A
      • Wimsatt J
      • et al.
      The tether was attached to the box within the animal's jacket. At least 5 days was allowed to elapse before commencement of all experiments, during which time antibiotics (oxacillin, intravenous infusion of 100 mg · kg-1 · day-1) and analgesics (buprenorphine, intraarterial infusion of 15 μg · kg-1 · day-1 for 2 days) were administered to the mother.

      Treatment groups and blood sampling

      Catheters were maintained patent by continuous infusion of heparinized saline solution (25 IU ml-1 at 0.5 ml · hr-1) from surgery until the commencement of one of two treatments at 138 to 145 days' gestation (Table I). In four monkeys (control group) the saline solution administration was switched to a continuous infusion of intralipid vehicle alone (intralipid 10%, Kabi Vitrum, Alameda, Calif.) set at the same rate and in the other four monkeys (experimental group) Δ4-androstenedione (4-androstenedione-3,17-dione, Sigma, St. Louis) dissolved in intralipid was infused continuously at 0.3 mg · kg-1 · hr-1.
      Table IAnimals used during experimental protocol
      GroupWeight (kg)Surgery (days' gestation)Start of infusion (days' gestation)Δ4-Androstenedione (mg · kg-1 · hr-1)
      Control
       Rh15.91181450.0
       Rh27.31321380.0
       Rh35.91321380.0
       Rh46.81361420.0
       Mean ± SEM6.5 ± 0.3129 ± 4141 ± 20.0 ± 0
      Experimental
       Rh55.91211390.30
       Rh66.81181390.30
       Rh710.91361420.30
       Rh85.61321400.30
       Mean ± SEM7.3 ± 1.2127 ± 4140 ± 10.30 ± 0
      Four monkeys (control group) were treated with intralipid and four (experimental group) with Δ4-androstenedione.
      In all animals 15 blood samples were taken at 30-minute intervals for 7 hours during a baseline period starting 2 hours before the onset of darkness. In addition, 0.3 ml of arterial blood was taken at the beginning and end of the sampling period for measurement of pH, PCO2, and PO2. These sampling procedures were then repeated 1 and 3 days after the start of intralipid or Δ4-androstenedione treatment.
      All arterial blood samples for hormone analyses (4 ml) were collected under aseptic techniques and transferred into chilled polypropylene collection tubes. These were centrifuged at 4° C at 1200g for 5 minutes. Plasma was removed, aliquoted, flash-frozen, and stored at -20° C until assayed. Remaining red blood cells from the centrifuged tubes were resuspended in heparinized saline solution (25 IU ml-1) and returned to the animal's arterial circulation.

      Hormone analyses

      Measurement of estradiol and oxytocin concentrations in rhesus monkey plasma was determined by radioimmunoassay. These assays have been validated for use in rhesus monkeys and the assay procedures previously described in detail. Maternal plasma estradiol was measured on duplicate 200 μl plasma aliquots.
      • Binienda Z
      • Massmann A
      • Wimsatt J
      • et al.
      Oxytocin was extracted with acetone from 1 ml plasma and duplicate 200 μl aliquots were measured.
      • Honnebier MBOM
      • Figueroa JP
      • Rivier J
      • Vale W
      • Nathanielsz PW
      Studies on the role of oxytocin in late pregnancy in the pregnant rhesus monkey, I: plasma concentrations of oxytocin in the maternal circulation throughout the 24h day and the effect of the synthetic oxytocin antagonist [1-β-Mpa(β-(CH2)5)1,(Me(Tyr2, Orn8] OT on spontaneous nocturnal myometrial contractions.
      The lower limit of detection of the assays, defined as 90% B/Bo, was 12 pg · ml-1 for estrogen and 1 pg · ml-1 for oxytocin. Intraassay and interassay coefficients of variation were 8.5% and 7.0% for estradiol and 8.8% and 12.7% for oxytocin, respectively.

      Analyses of myometrial activity

      Uterine activity was recorded continuously throughout the study period. Recording of the myometrial electromyographic activity was performed with a computer-based data acquisition system. The signal was sampled at 32 Hz and integrated, and the average over 8 seconds was digitized and stored with a time signal.
      • Honnebier MBOM
      • Nathanielsz PW
      Primate parturition and the role of the maternal circadian system.
      A switch in myometrial electromyographic activity to contractions was defined if at least six contractions, each lasting about 1 minute, occurred sequentially and the bout of contraction activity lasted ≥30 minutes. Individual contractions were counted visually.

      Statistical analyses

      Blood gases and pH are expressed as the mean ± SEM of each of the experimental periods: baseline, day 1, and day 3. For daily associations of plasma concentrations of estradiol and oxytocin to myometrial contraction activity, the mean of all 15 hormone samples and the total number of contractions within the 7-hour experimental period were calculated. For close association of plasma oxytocin to myometrial contraction activity, each sample taken at 30-minute intervals was plotted along with the number of contractions per 30-minute period for each animal.
      Data for statistical analyses that were not normally distributed were log-transformed for endocrine parameters and square root transformed for contraction counts. Repeated-measures analysis of variance with Dunnetts multiple comparison tests was used to compare all measured variables between baseline and 1 or 3 days for both the control and experimental groups of animals. Significance was accepted when p < 0.05.

      RESULTS

      Arterial blood gases and pH

      Mean arterial blood gases and pH values were similar between intralipid- and Δ4-androstenedione - treated monkeys during the baseline period. Furthermore, there were no differences in any of these variables measured after intralipid or Δ4-androstenedione treatment (Table II).
      Table IIArterial blood gas and pH values in monkeys during baseline and 1 and 3 days after start of intralipid (control, n = 4) or Δ4-androstenedione (experimental, n = 4) treatment
      GrouppHPaCO2 (mm Hg)PaO2 (mm Hg)
      Control
       BL7.424 ± 0.0234.7 ± 0.8111.5 ± 2.5
       Day 17.452 ± 0.0132.7 ± 0.1113.1 ± 3.3
       Day 37.428 ± 0.0134.6 ± 0.8110.2 ± 1.2
      Experimental
       BL7.442 ± 0.0235.7 ± 1.499.7 ± 3.4
       Day 17.459 ± 0.0135.9 ± 1.4107.4 ± 3.1
       Day 37.439 ± 034.4 ± 2.3108.1 ± 7.2
      Values shown are mean ± SEM. BL, Baseline.

      Daily association of endocrine parameters to myometrial contractions

      In control monkeys mean plasma estradiol and oxytocin levels and mean number of contractions remained unchanged from baseline 1 and 3 days after the start of the intralipid infusion (Fig. 1).
      Figure thumbnail gr1
      Fig. 1Daily temporal relationship between maternal plasma concentration of estradiol (E2) and oxytocin (OT) and number of contractions per sampling period during baseline (BL) and 1 to 3 days after intralipid (control, n = 4, white bars) or androstenedione (experimental, n = 4, black bars) treatment. Asterisk, p < 0.05.
      In contrast, mean plasma estradiol increased after 1 day and remained elevated after 3 days of Δ4-androstenedione treatment (Fig. 1). The increase in plasma oxytocin concentration by the third day after Δ4-androstenedione was concurrent with an increase in the number of myometrial contractions.

      Hourly association of plasma oxytocin concentrations to myometrial contractions

      The relationship between changes in plasma oxytocin concentrations with the switch in myometrial activity to contractions in either day 1 or day 3 after Δ4-androstenedione treatment is shown for two individual animals in Fig. 2, A. In addition, some control animals did occasionally undergo short periods of contraction activity that were not sustained during baseline and after intralipid treatment. An example of one intralipid-infused animal showing the relationship between these bouts of contractions and plasma oxytocin concentrations is shown in Fig. 2, B.
      Figure thumbnail gr2a
      Fig. 2Close relationship between maternal plasma oxytocin (OT) and uterine activity in two androstenedione-treated monkeys (A) and one monkey treated with intralipid (B). Blood samples for oxytocin measurement were taken every 30 minutes. Uterine activity is shown as raw data collected every 8 seconds and as number of contractions counted per 30-minute periods. UEMG, Uterine electromyographic activity.
      Figure thumbnail gr2b
      Fig. 2Close relationship between maternal plasma oxytocin (OT) and uterine activity in two androstenedione-treated monkeys (A) and one monkey treated with intralipid (B). Blood samples for oxytocin measurement were taken every 30 minutes. Uterine activity is shown as raw data collected every 8 seconds and as number of contractions counted per 30-minute periods. UEMG, Uterine electromyographic activity.
      In all four Δ4-androstenedione - treated monkeys and in control monkeys, which occasionally exhibited bouts of contractions, there was a close temporal association between the switch in myometrial activity to contractions and the increase in plasma oxytocin concentrations.

      COMMENT

      Because premature birth occurs in 7% of pregnancies and is responsible for 50% of long-term neurologic deficits and 75% of neonatal mortality,
      a physiologic model for providing insight into complications during pregnancy that would lead to prematurity is of paramount importance. Although such a model has been established in sheep in which infusion of cortisol or corticotropin to the fetus at about 120 days' gestation (term 147 days) consistently induces premature delivery,
      • Liggins GC
      • Fairclough RJ
      • Grieves SA
      • Kendall JZ
      • Knox BS
      The mechanism of initiation of parturition in the ewe.
      no such model exists to study premature birth in the primate.
      Such a successful model in the primate must induce preterm labor by similar mechanisms mediating primate spontaneous term labor. Current evidence suggests that increased placental estrogen synthesis during gestation promotes parturition in both sheep
      • Liggins GC
      • Fairclough RJ
      • Grieves SA
      • Kendall JZ
      • Knox BS
      The mechanism of initiation of parturition in the ewe.
      and primates,
      • Myers DA
      • Nathanielsz PW
      Biologic basis of term and preterm labor.
      although the source of this estrogen increase has been demonstrated to be very different between the two species. This difference has been attributed to the lack of P450 17α enzyme expression in the primate placenta.
      • Albrecht ED
      • Pepe GJ
      Placental steroid hormone biosynthesis in primate pregnancy.
      In sheep this enzyme promotes estrogen synthesis from progesterone.
      • Myers DA
      • Nathanielsz PW
      Biologic basis of term and preterm labor.
      Increased fetal plasma cortisol induces P450 17α placental expression. In primates increased placental estrogen production cannot be induced either from progesterone
      • Albrecht ED
      • Pepe GJ
      Placental steroid hormone biosynthesis in primate pregnancy.
      or by maternally administered cortisol.
      • Novy MJ
      • Walsh SW
      Dexamethasone and estradiol treatment in pregnant rhesus macaques: effects on gestational length, maternal plasma hormones, and fetal growth.
      However, the primate placenta does express the enzyme P450 aromatase, which can promote estrogen synthesis from aromatizable placental precursors such as Δ4-androstenedione.
      • Albrecht ED
      • Pepe GJ
      Placental steroid hormone biosynthesis in primate pregnancy.
      We have hypothesized that in the primate increased fetal adrenal androgen production leads to increased placental estrogens, their synthesis contributing to increased maternal plasma oxytocin concentrations and myometrial contractions. Accordingly, estrogens have been demonstrated to stimulate a number of maternal systems that are fundamental to activation of spontaneous myometrial contraction activity in the primate. For example, estrogens will increase oxytocin synthesis by the hypothalamus
      • Amico JA
      • Hempel J
      An oxytocin precursor intermediate circulates in the plasma of humans and rhesus monkeys administered estrogen.
      and by intrauterine tissues
      • Miller FD
      • Chibbar R
      • Mitchell BF
      Synthesis of oxytocin in amnion, chorion and decidua: a potential paracrine role for oxytocin in the onset of human parturition.
      and lead to an increase in myometrial gap junctions.
      • Garfield RE
      • Kannan MS
      • Daniel EE
      Gap junction formation in myometrium: control by estrogens, progesterone and prostaglandins.
      In spite of well-known descriptions of the uterotonic action of oxytocin, early doubts about its role in human parturition arose largely because some investigators reported an increase in human maternal plasma and amniotic fluid oxytocin at term and during labor but others failed to demonstrate these increases. This failure to demonstrate prepartum changes in oxytocin may reflect procedural problems rather than the lack of a rise in oxytocin. Two potential sources of negative results may have been the lack of specific assays with adequate sensitivity and sampling regimens that did not allow for the 24-hour rhythm that exists in maternal plasma oxytocin in the pregnant primate.
      • Seron-Ferre M
      • Taylor NF
      • Martin MC
      • Leake RD
      Development of a circadian variation of plasma oxytocin concentration in the late gestation rhesus monkey.
      More recent reports using systematic and frequent blood sampling regimens for oxytocin measurement have now demonstrated the development of such a 24-hour pattern in oxytocin concentrations in both late-gestation monkeys
      • Honnebier MBOM
      • Figueroa JP
      • Rivier J
      • Vale W
      • Nathanielsz PW
      Studies on the role of oxytocin in late pregnancy in the pregnant rhesus monkey, I: plasma concentrations of oxytocin in the maternal circulation throughout the 24h day and the effect of the synthetic oxytocin antagonist [1-β-Mpa(β-(CH2)5)1,(Me(Tyr2, Orn8] OT on spontaneous nocturnal myometrial contractions.
      • Taylor NF
      • Martin MC
      • Nathanielsz PW
      • Seron-Ferre M
      The fetus determines circadian oscillation of myometrial electromyographic activity in the pregnant rhesus monkey.
      • Seron-Ferre M
      • Taylor NF
      • Martin MC
      • Leake RD
      Development of a circadian variation of plasma oxytocin concentration in the late gestation rhesus monkey.
      and women,
      • Fuchs A-R
      • Behrens O
      • Liu H-C
      Correlation of nocturnal increase in plasma oxytocin with a decrease in plasma estradiol/progesterone ratio in late pregnancy.
      with the highest concentrations occurring in the early hours of darkness. In addition, the peak plasma nocturnal oxytocin levels are associated to the nocturnal transition to contraction-type myometrial activity in the pregnant monkey,
      • Honnebier MBOM
      • Figueroa JP
      • Rivier J
      • Vale W
      • Nathanielsz PW
      Studies on the role of oxytocin in late pregnancy in the pregnant rhesus monkey, I: plasma concentrations of oxytocin in the maternal circulation throughout the 24h day and the effect of the synthetic oxytocin antagonist [1-β-Mpa(β-(CH2)5)1,(Me(Tyr2, Orn8] OT on spontaneous nocturnal myometrial contractions.
      and the magnitude of oxytocin secretory peaks is increased with advancing gestation.
      • Hirst JJ
      • Haluska GJ
      • Cook MJ
      • Novy MJ
      Plasma oxytocin and nocturnal uterine activity: maternal but not fetal concentrations increase progressively during late pregnancy and delivery in rhesus monkeys.
      These studies thus provide a possible explanation for the apparent lack of increase in maternal plasma and amniotic fluid oxytocin concentrations reported by investigators who only took samples in the morning. In addition, abolition of spontaneous myometrial contractions by oxytocin antagonists in the pregnant monkey,
      • Honnebier MBOM
      • Figueroa JP
      • Rivier J
      • Vale W
      • Nathanielsz PW
      Studies on the role of oxytocin in late pregnancy in the pregnant rhesus monkey, I: plasma concentrations of oxytocin in the maternal circulation throughout the 24h day and the effect of the synthetic oxytocin antagonist [1-β-Mpa(β-(CH2)5)1,(Me(Tyr2, Orn8] OT on spontaneous nocturnal myometrial contractions.
      • Hirst JJ
      • Haluska GJ
      • Cook MJ
      • Hess DL
      • Novy MJ
      Comparison of plasma oxytocin and catecholamine concentrations with uterine activity in pregnant rhesus monkeys.
      baboon,
      • Wilson Jr, L
      • Parsons MT
      • Ouano L
      • Flouret G
      A new tocolytic agent: development of an oxytocin antagonist for inhibiting uterine contractions.
      and woman
      • Andersen LF
      • Lyndrup J
      • Akerlund M
      • Melin P
      Oxytocin receptor blockade: a new principle in the treatment of preterm labor?.
      produced renewed support for a role for oxytocin in regulating myometrial contractions and labor in the primate. Additional data reporting an increase in oxytocin receptors in the human uterus during pregnancy and parturition
      • Fuchs A-R
      • Fuchs F
      Endocrinology of human parturition: a review.
      and an increase in oxytocin peptide and message in the amnion, chorion, and decidua in the rat
      • Lefebvre DL
      • Giaid A
      • Bennett H
      • Lariviere R
      • Zingg HH
      Oxytocin gene expression in rat uterus.
      and human
      • Miller FD
      • Chibbar R
      • Mitchell BF
      Synthesis of oxytocin in amnion, chorion and decidua: a potential paracrine role for oxytocin in the onset of human parturition.
      confirm an association between oxytocin and parturition.
      We have previously demonstrated that acute intravenous administration of Δ4-androstenedione for 48 hours to the preparturient monkey leads to increased maternal plasma estradiol concentrations throughout the day and an increase in nocturnal myometrial contractions.
      • Figueroa JP
      • Honnebier MBOM
      • Binienda Z
      • Wimsatt J
      • Nathanielsz PW
      Effect of 48-hour intravenous -androstenedione infusion on the pregnant rhesus monkey in the last third of gestation: changes in maternal plasma estradiol concentrations and myometrial contractility.
      The appearance of nocturnal myometrial contractions in spite of elevated maternal plasma estradiol throughout the day has been attributed to the 24-hour pattern in circulating maternal plasma oxytocin with peak concentrations occurring at night
      • Honnebier MBOM
      • Figueroa JP
      • Rivier J
      • Vale W
      • Nathanielsz PW
      Studies on the role of oxytocin in late pregnancy in the pregnant rhesus monkey, I: plasma concentrations of oxytocin in the maternal circulation throughout the 24h day and the effect of the synthetic oxytocin antagonist [1-β-Mpa(β-(CH2)5)1,(Me(Tyr2, Orn8] OT on spontaneous nocturnal myometrial contractions.
      • Taylor NF
      • Martin MC
      • Nathanielsz PW
      • Seron-Ferre M
      The fetus determines circadian oscillation of myometrial electromyographic activity in the pregnant rhesus monkey.
      • Seron-Ferre M
      • Taylor NF
      • Martin MC
      • Leake RD
      Development of a circadian variation of plasma oxytocin concentration in the late gestation rhesus monkey.
      • Fuchs A-R
      • Behrens O
      • Liu H-C
      Correlation of nocturnal increase in plasma oxytocin with a decrease in plasma estradiol/progesterone ratio in late pregnancy.
      coupled with an increase in nocturnal myometrial sensitivity to oxytocin demonstrated in late-gestation monkeys and in pregnant women.
      • Honnebier MBOM
      • Nathanielsz PWN
      Primate parturition and the role of the maternal circadian system.
      If the Δ4-androstenedione treatment to the 0.8 gestation rhesus monkey is sustained, then the regularly recurring and augmenting switch from contractures to contractions occurs for several nights, accompanied by an increase in maternal estradiol concentrations, finally resulting in live delivery of the fetal rhesus monkey.
      • Mecenas CA
      • Honnebier MBOM
      • Owiny JR
      • Nathanielsz PW
      Continuous intravenous infusion of Δ4-androstenedione to pregnant rhesus monkeys at 139 days gestational age produces premature delivery.
      The current study confirms sustained elevations in maternal plasma estradiol levels after Δ4-androstenedione treatment and reports further characterization of the model showing that the increase in maternal estradiol precedes an increase in maternal oxytocin by about 24 to 48 hours. These premature increases in maternal plasma estradiol and oxytocin after Δ4-androstenedione treatment are of similar magnitude to the increase in estradiol
      • Novy MJ
      • Walsh SW
      Dexamethasone and estradiol treatment in pregnant rhesus macaques: effects on gestational length, maternal plasma hormones, and fetal growth.
      and oxytocin
      • Honnebier MBOM
      • Figueroa JP
      • Rivier J
      • Vale W
      • Nathanielsz PW
      Studies on the role of oxytocin in late pregnancy in the pregnant rhesus monkey, I: plasma concentrations of oxytocin in the maternal circulation throughout the 24h day and the effect of the synthetic oxytocin antagonist [1-β-Mpa(β-(CH2)5)1,(Me(Tyr2, Orn8] OT on spontaneous nocturnal myometrial contractions.
      measured during spontaneous term myometrial contractions in the monkey. Furthermore, the hourly changes in maternal plasma oxytocin and myometrial contractions after Δ4-androstenedione treatment demonstrate a close association between the onset of the increase in both these variables. The daily temporal relationship between physiologic changes in maternal plasma estradiol and oxytocin and the hourly temporal relationship between plasma oxytocin and myometrial contractions are consistent with the hypothesis that estrogen-induced increases in maternal plasma oxytocin contribute to mediating both spontaneous term and Δ4-androstenedione - induced preterm myometrial contractions.

      Acknowledgements

      We thank James Owiny, BVM, PhD, for running the estradiol assay. We are grateful to Karen Moore for her help with the manuscript.

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