Effects of maternal betamethasone administration on fetal and maternal blood pressure and heart rate in the baboon at 0.7 of gestation

OBJECTIVE

We sought to determine the effects of the intramuscular maternal administration of betamethasone to the pregnant baboon at 0.7 of gestation on fetal blood pressure and heart rate.

STUDY DESIGN

We treated pregnant baboons at 0.7 of gestation with intramuscular betamethasone (n = 4), at a weight-adjusted dose equivalent to the daily dose administered to women in preterm labor or with saline solution (n = 5). Four injections were given at 12-hour intervals. Fetal and maternal blood pressure and heart rate were recorded continuously. Within-group differences and between-group differences were analyzed with repeated measures analysis of variance.

RESULTS

Fetal blood pressure increased significantly after betamethasone treatment. Fetal heart rate, maternal blood pressure, and heart rate did not change.

CONCLUSION

Exposure of the developing primate fetus to exogenous glucocorticoid at 0.7 of gestation elevates fetal blood pressure. These findings confirm and extend the observations in the fetal sheep. Further studies are needed to evaluate the mechanisms that are involved and possible long-term consequences of these cardiovascular effects of antenatal glucocorticoid exposure in the fetal primate.

Prostaglandin dehydrogenase mRNA in baboon intrauterine tissues in late gestation and spontaneous labor

The present study was designed to characterize prostaglandin dehydrogenase (PGDH) mRNA expression in critical intrauterine tissues of pregnant baboons in late gestation and at spontaneous labor. In addition, we determined regulatory effects of betamethasone in vivo on chorionic and placental PGDH mRNA expression. PGDH mRNA was present in chorion, decidua, lower uterine segment, fundal myometrium, and cervix in late gestation but undetectable in amnion. PGDH mRNA significantly decreased in decidua and cervix during late gestation and in chorion and fundus during spontaneous labor. PGDH mRNA in lower uterine segment, decidua, cervix, and placenta was unchanged during spontaneous labor from late gestation levels. Betamethasone had no effect on chorionic and placental PGDH mRNA expression. In summary, our data suggest that PGDH mRNA expression is tightly controlled in gestation- and tissue-specific manners. Decreased chorionic and fundal PGDH abundance during labor and decreased decidua and cervical PGDH mRNA in late gestation allow local uterine prostaglandin accumulation and assist prostaglandin transfer to myometrium. Local differences in PGDH function may regulate tissue- and region-specific requirements for prostaglandins to promote and complete labor.

Inhibitory effect of iloprost on the contractility of lower uterine segment myometrium from rhesus monkeys in normal-term and androstenedione-induced preterm labor

OBJECTIVE

Iloprost, a combined EP(1) stimulatory and IP inhibitory receptor agonist, was tested in vitro on myometrium from the lower uterine segment of pregnant rhesus monkeys to compare its effects in spontaneous labor and in labor induced by the administration of androstenedione to the mother.

METHODS

Pregnant rhesus monkeys carrying fetuses of known gestational age were instrumented under halothane general anesthesia with femoral artery and vein catheters and uterine electromyogram leads. Experimental animals were infused with androstenedione from 139 days' gestation. Control animals were infused with intralipid vehicle from 139 days' gestation. Lower uterine segment myometrium was removed from control animals either before labor began (n = 6) or in spontaneous labor (n = 4) and from animals undergoing premature labor induced by androstenedione (n = 4). Myometrial contractility in response to iloprost was evaluated using a superfusion system in vitro.

RESULTS

Iloprost was inhibitory on myometrium obtained from the lower uterine segment from androstenedione-treated animals as well as vehicle-infused animals in spontaneous term labor. In contrast, iloprost had no effect on myometrial strips from control animals not in labor.

CONCLUSION

These findings indicate up-regulation of IP receptors which inhibit myometrial contractility and/or down-regulation of EP(1) receptors which stimulate myometrial contractility in the lower uterine segment during labor. A relative increase in inhibitory responses in the lower uterine segment during labor may enable this region to dilate to allow passage of the fetus.

Timing of the switch from myometrial contractures to contractions in late-gestation pregnant rhesus monkeys as recorded by myometrial electromyogram during spontaneous term and androstenedione-induced labor

Pregnant rhesus monkeys were studied to determine the precise time in relation to photoperiod of the onset, and the nature, of the switch in myometrial activity patterns from contractures to contractions. We investigated both spontaneous term labor and androstenedione-induced preterm labor. Under general anesthesia at 127 +/- 2 days gestation (dGA) (mean +/- SEM), 16 pregnant rhesus monkeys were instrumented with maternal femoral arterial and venous catheters and myometrial electromyogram electrodes. Eight animals (group I) received continuous i.v. infusion of intralipid (n = 7) or saline (n = 1) that was started at 143.3 +/- 2 dGA and maintained until the spontaneous onset of term labor. Nine animals (group II) received continuous i.v. infusion of androstenedione that was started at 139 +/- 0.4 dGA and maintained until the onset of prematurely induced labor. Myometrial activity was recorded continuously. All monkeys in both groups demonstrated nocturnal switches in myometrial activity from contractures to contractions. The mean time of onset of the switch in group I and group II monkeys was similar, occurring at 0.7 +/- 0.4 h or 0.8 +/- 0.5 h, respectively, after the onset of darkness. Group II monkeys demonstrated greater regularity in both the time of onset and the repetitive occurrence each night once the switch occurred, as well as greater consistency in duration in their switch patterns, than did group I monkeys.

The oxytocin antagonist atosiban prevents androstenedione-induced myometrial contractions in the chronically instrumented, pregnant rhesus monkey

We tested the hypothesis that increased oxytocin is a necessary mechanism for the mediation of androstenedione (delta 4A)-induced myometrial contractions by investigating the effects of maternal treatment with the oxytocin antagonist atosiban on in vivo delta 4A-induced contractions. In four monkeys (group I), maternal estradiol, oxytocin, and myometrial contractions were assessed at baseline and after continuous iv delta 4A administration. Similar measurements were made in three monkeys (group II) that received the same delta 4A infusion regimen, but in addition were treated daily with atosiban. Maternal estradiol and oxytocin levels and contractions were also assessed in four additional monkeys (controls; group III), in which the delta 4A vehicle, intralipid, was infused iv continuously. In group I, delta 4A induced myometrial contractions and increased maternal estradiol and oxytocin to term concentrations. No myometrial contractions occurred in group II monkeys after combined delta 4A and atosiban treatment despite estradiol being elevated to concentrations similar to those measured in group I monkeys. Atosiban had no effect on maternal heart rate or blood pressure. Maternal estradiol, oxytocin, and number of myometrial contractions remained unchanged from baseline values in control monkeys. In conclusion, oxytocin is a necessary part of the mechanisms mediating delta 4A-induced myometrial contractions. delta 4A promotes myometrial contractions via similar mechanisms that mediate spontaneous term contractions in pregnant monkeys.

Production of premature delivery in pregnant rhesus monkeys by androstenedione infusion

The endocrine mechanism involved in term and preterm delivery in primates, including pregnant women, are poorly understood. In the term monkey, fetal plasma androgen concentration rises to two hundred times the maternal concentration which remains unchanged. Placental conversion of androgen to estrogen results in increased maternal plasma estrogen concentration at term in both pregnant nonhuman primates and women. In the present study, continuous infusion of androstenedione to 0.8 gestation monkeys resulted in the premature occurrence of labor-type myometrial activity and increases in maternal plasma estrogen, oxytocin and amnion fibronectin concentrations similar to those measured at normal-term labor. Androstenedione induction of these normal-term biochemical and endocrine changes accompanied by fetal membrane rupture, cervical dilatation and live delivery provides a rich opportunity to study the molecular and physiological mechanisms of both term and preterm labor in primates.

Daily and hourly temporal association between delta4-androstenedione-induced preterm myometrial contractions and maternal plasma estradiol and oxytocin concentrations in the 0.8 gestation rhesus monkey

OBJECTIVE

Our purpose was to investigate the temporal relationship between delta4-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 delta4-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 delta4-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 delta4-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 delta4-androstenedione treatment and (2) delta4-androstenedione-induced preterm myometrial contractions are closely associated in time with physiologic increases in maternal plasma oxytocin concentrations.

Effect of androstenedione administration on the maternal hypothalamo-pituitary-adreno-placental axis in the pregnant rhesus monkey

To assess the interaction among androgens, placenta, and the hypothalamo-pituitary-adrenal axis we studied effects of androstenedione administered intravascularly to the pregnant monkey on maternal plasma CRH, ACTH, dehydroepiandrosterone sulfate (DHEAS), cortisol, and estradiol concentrations. Ten monkeys (128 +/- 3 days gestation; mean +/- SEM) were instrumented under general halothane anesthesia with maternal femoral artery and venous catheters and uterine electromyogram electrodes. At 137-144 days gestation, baseline maternal femoral artery samples for CRH, ACTH, DHEAS, cortisol, and estradiol measurements were taken at 1.5-h intervals for 7 h starting 2 h before darkness. On the following day, a continuous iv androstenedione infusion (0.3 mg/kg.min at 0.25 ml/h) in 10% intralipid was started at 0930 h in four monkeys; the other six animals received vehicle alone at the same rate starting at the same time. Maternal blood sampling was repeated 1 and 3 days after androstenedione or vehicle administration. Maternal plasma CRH, ACTH, DHEAS, cortisol, and estradiol levels were unaffected by intralipid. In contrast, androstenedione infusion produced a sustained increase in maternal plasma estradiol and a sustained fall in maternal plasma ACTH, but did not affect maternal plasma CRH, DHEAS, or cortisol concentrations. These results provide evidence for negative feedback regulation by androgens at the hypothalamo-pituitary-adrenal axis in the pregnant monkey. Lack of inhibition of maternal plasma CRH after androstenedione administration supports differential regulation of hypothalamic and placental CRH by androgens.

Corticotropin-releasing hormone and its receptor distribution in fetal membranes and placenta of the rhesus monkey in late gestation and labor

Maternal plasma corticotropin-Releasing Hormone (CRH) rises from midgestation to term and increases further during labor in pregnant women. The primate placenta contains both the CRH peptide and its gene and is the likely source of circulating CRH. In the present study, we examined CRH messenger RNA (mRNA) and peptide expression in fetal membranes and the placenta of 14 pregnant rhesus monkeys (140-161 days gestational age) using Northern blot analysis, in situ hybridization, and immunocytochemistry to define the cellular distribution of CRH during the last third of pregnancy and in relation to onset of both spontaneous term and androstenedione-induced preterm labor. To localize the target tissues for placental CRH, CRH receptor gene expression was also studied in the fetal membranes and placenta. CRH mRNA in the placenta was of similar molecular size to hypothalamic CRH. Placental CRH mRNA increased significantly during both spontaneous term and androstenedione-induced preterm labor (P < 0.05). Placental CRH peptide detected by CRH immunostaining also increased, matching the changes of CRH mRNA. In situ hybridization and immunocytochemistry demonstrated that syncytiotrophoblast cells are the major cell type expressing CRH mRNA and producing CRH protein. CRH mRNA was not detected in either amnion or chorion. CRH receptor complementary DNA and oligo probes that successfully hybridized CRH receptor mRNA in the fetal rhesus monkey hypothalamus failed to reveal the existence of CRH receptor mRNA in amnion, chorion, and placenta by Northern blot hybridization. In conclusion, placental but not fetal membrane syncytiotrophoblasts are the source of CRH production in the pregnant rhesus monkey. The significant increase in CRH peptide and mRNA content in both spontaneous term and androstenedione-induced preterm labor indicates a role for CRH in the process of parturition. The lack of CRH receptor mRNA in either the fetal membranes or the placenta suggests that placental CRH exerts its action at sites other than these tissues.

Regulation of the switch from myometrial contractures to contractions in late pregnancy: studies in the pregnant sheep and monkey

Myometrial contractility occurs throughout pregnancy and characteristic patterns of myometrial activity exist according to the endocrine status and the relationship to parturition. These characteristic patterns differ between species, yet certain common features can be observed. Throughout pregnancy, myometrial activity is of the contractures type, long-lasting, low-amplitude epochs of activity switching to contraction-type activity at term. This switch from contractures to contractions tends to occur at night and is related to alteration in maternal plasma oestrogen concentrations, and maternal oxytocin function. Studies in several animal species support the hypothesis that maternal oestrogen prepares the myometrium for a periodic signal that causes the switch from contractures to contractions. Several lines of evidence implicate oxytocin in the switch. These studies show that the detailed preparation for parturition takes longer than previously considered and is brought about by a carefully regulated sequence of events in which oestrogen production by the placenta plays a central role.

Comparison of the myometrial response to oxytocin during daylight with the response obtained during the early hours of darkness in the fetectomized rhesus monkey at 160-172 days gestational age

Six rhesus monkeys, fetectomized at 122-129 days gestational age (dGA), were studied to evaluate photoperiod-dependent differences in the myometrial sensitivity to exogenous oxytocin in the absence of a live fetus. One to two weeks before the study, at 150-162 dGA, sixteen maternal arterial samples were taken every 4 h to assess plasma concentrations as well as the presence of 24-h rhythmicity in plasma estradiol and cortisol. At 160-172 dGA, oxytocin was administered via the maternal inferior vena cava to elicit regular myometrial contractions. Each animal was studied twice: once during daylight and once during the early hours of darkness. Oxytocin was infused in a pulsatile fashion during the first minute out of every five over four consecutive 30-min intervals, during which the dose rate was increased after each 6-h oxytocin pulse (400, 800, 2000, and 4000 pg.kg-1.min-1). The overall response of the myometrium of the fetectomized animals, evaluated as the number of contractions per pulse of oxytocin, was higher during the daytime study (p = 0.006). The response to oxytocin in intact pregnant animals is greatest during the early hours of darkness. Comparison of the overall myometrial response of the fetectomized monkeys with the response of intact pregnant animals showed that the daytime responses were similar, but that the nighttime response of the fetectomized animals was significantly lower (p < 0.0002). These contrasting findings may be due to differences in the maternal neuroendocrine environment depending on the presence or absence of a fetal adrenal steroid contribution acting directly on the myometrium or acting indirectly through provision of precursors for estrogen formation.(ABSTRACT TRUNCATED AT 250 WORDS)