Pulsatile oxytocin administered to ewes at 120 to 140 days gestational age increases the rate of maturation of the fetal electrocorticogram and nuchal activity.

Spontaneous, long lasting epochs of myometrial contractility, contractures, occur throughout the majority of pregnancy in sheep. Contractures are temporally related to a switch in fetal electroencephalogram (ECoG) patterns from low to high voltage. In late gestation, fetal ECoG increases in voltage. We have previously suggested that contractures may influence fetal ECoG maturation. In the present study, we hypothesized that a sustained increase in the frequency of myometrial contractures in pregnant sheep at 120-140 days gestation would accelerate maturation of the fetal ECoG. Five pregnant ewes were pulsed with oxytocin 600 microU.kg-1.min-1 intravenously for five minutes in every 30 minutes from 127.8 +/- 1.5 days gestational age for a minimum of six days. Six control ewes received pulses of saline. Fetuses of all eleven ewes were instrumented with bilateral electrodes to record fetal ECoG and nuchal muscle activity. Fetal high voltage (HV) ECoG increased in amplitude in both groups but the rate of increase was faster in the fetuses of ewes receiving oxytocin. There were no differences between the two groups in the duration of HV ECoG. The percentage increase in the amount of time the fetal nuchal muscles were active compared with the baseline day before infusion was only significant in the oxytocin infused group on the first day of oxytocin infusion. These findings support the hypothesis that myometrial activity during pregnancy has the capacity to influence fetal neural development.

Oxytocin given in a pulsatile manner to the ewe at 120 to 140 days' gestational age increases fetal sheep plasma cortisol.

A single intravenous injection of oxytocin into pregnant sheep (123 to 144 days' gestational age) causes a bout of myometrial activity and an increase in fetal plasma corticotropin levels. We hypothesized that a sustained increased frequency of myometrial contractures accelerates the normal increase in fetal adrenal secretion in sheep in the last 3 weeks of gestation. To test this hypothesis, pulses of saline solution (group 1, 9 ewes and 10 fetuses) or oxytocin (group 2, 11 ewes and 12 fetuses) 600 or 960 microU/kg/min for 5 of every 30 minutes were infused into the maternal jugular vein for 6 days beginning at day 128 +/- 2 (mean +/- SD) days' gestational age. Total myometrial activity increased to 160% of baseline in group 2 by day 6. Myometrial activity did not change throughout the study in group 1. Maternal plasma cortisol concentrations did not rise during oxytocin infusion when compared with their own baselines (group 2) and were not different from concentrations in saline-infused ewes (group 1). By day 4 of oxytocin administration fetal plasma cortisol concentrations had risen significantly above baseline in group 2 (p less than 0.05). Fetal plasma cortisol concentrations did not rise in group 1. Corticotropin levels were not elevated in the fetal carotid arterial plasma of either group. A small but significant decrease occurred in fetal carotid arterial PO2 in group 2 by day 6 but not in group 1. In conclusion, increased myometrial activity produced by pulsed oxytocin is accompanied by increased fetal plasma cortisol concentrations, demonstrating that long-term alteration of myometrial activity affects fetal adrenal function over several days at this critical period of gestation.