PLASMA OXYTOCIN AND NOCTURNAL UTERINE ACTIVITY - MATERNAL BUT NOT FETAL CONCENTRATIONS INCREASE PROGRESSIVELY DURING LATE PREGNANCY AND DELIVERY IN RHESUS-MONKEYS

OBJECTIVE: The purpose of the study was to determine whether rising ma ternal or fetal plasma oxytocin concentrations could be responsible fo r the increasing levels of nocturnal uterine activity on the nights pr eceding delivery. STUDY DESIGN: Chronically catheterized pregnant rhes us monkeys were exposed to a 16-hour light, 8-hour dark photoperiod (d ark 11 PM to 7 AM). Uterine activity and maternal arterial plasma oxyt ocin concentrations were measured concurrently at weekly intervals in late gestation, on the night preceding term delivery (158 to 167 days, n = 4), and during delivery (149 to 170 days, n = 6). Fetal carotid a rterial plasma oxytocin levels were measured during episodes of noctur nal uterine activity in six animals. The effect of oxytocin infusions into the fetus (30 to 480 ng/kg/hr) on uterine activity and on materna l and fetal plasma oxytocin levels was also determined (n = 3). RESULT S: Maximal nocturnal oxytocin concentrations in the maternal plasma ro se progressively during late gestation from 9.9 +/- 3.5 pg/ml at 130 t o 139 days to 28.7 +/- 9.8 pg/ml on the night preceding term delivery (p < 0.005); a significant increase in nocturnal uterine activity acco mpanied this rise (p < 0.001). Maternal oxytocin concentrations were e levated during labor and increased further at delivery (62.5 +/- 5.5 p g/ml, p < 0.05). There was no increase in fetal plasma oxytocin during nocturnal uterine activity (3.1 +/- 0.2 pg/ml) or during labor. Fetal oxytocin infusions raised fetal plasma oxytocin concentrations sixtyf old but had no effect on maternal plasma oxytocin concentrations or on uterine activity. CONCLUSIONS: Elevated maternal plasma oxytocin conc entrations are responsible, at least in part, for the increasing magni tude of nocturnal uterine activity episodes as term approaches and for the elevated uterine activity before delivery at night. Fetal plasma oxytocin does not contribute to nocturnal uterine activity or to mater nal plasma oxytocin concentrations.