Corticotropin-releasing factor in the ovine fetus and pregnant ewe: role of placenta.

In the sheep, maternal plasma adrenocorticotropic hormone and cortisol are increased in late pregnancy, and fetal plasma cortisol and adrenocorticotropic hormone rise precipitously in late gestation. To test whether the ovine placenta secretes corticotropin-releasing factor (CRF) into either the maternal or fetal circulation, pregnant ewes and their fetuses were prepared with femoral arterial catheters and uterine and umbilical venous catheters. Samples were taken from all sites before and during hypoxia. There was no difference in CRF concentration across the placenta in the mothers or the fetuses under resting or hypoxemic conditions, but maternal and fetal arterial plasma CRF concentrations increased between 128 and 145 days. In a second study, maternal and fetal femoral venous plasma CRF concentrations were measured 1-19 days before spontaneous parturition. The mean concentration increased 8.6 +/- 0.6 pg/ml 11-19 days before parturition to 13.0 +/- 1.0 and 13.2 +/- 1.4 pg/ml in fetuses 4-8 and 1-3 days before parturition, respectively. Maternal plasma concentrations did not significantly increase in the days closer to parturition. These studies demonstrate that there are low but measurable CRF concentrations in fetal and maternal sheep plasma but that these are not the result of tonic placental secretion of CRF.