EFFECTS OF RAT CORTICOTROPIN-RELEASING FACTOR, ARGININE-VASOPRESSIN AND OXYTOCIN ON THE SECRETIONS OF ADRENOCORTICOTROPIC HORMONE AND CORTICOSTERONE IN THE FETAL-RAT IN LATE-GESTATION - IN-VIVO AND IN-VITRO STUDIES

The effects of rat corticotrophin-releasing factor (rCRF), arginine vasopressin (AVP) and oxytocin (OT) were investigated in vivo in 21-day-old rat fetuses injected through the umbilical vein and in vitro on perifused anterior pituitary glands from 21-day-old rat fetuses. In vivo, rCRF (1.25 pmol.50 mu l(-1).fetus(-1)), AVP (5 pmol.50 mu l(-1).fetus(-1)) alone and rCRF in association with AVP or oxytocin (12.5 pmol.50 mu l(-1).fetus(-1)) increased plasma adrenocorticotrophic hormone (ACTH) and corticosterone levels only 30 min after the start of injection. During the first 10 min of the sampling period, the injection of these peptides alone or in combination and the injection of saline decreased the plasma ACTH concentration, which was lower than that of uninjected fetuses, but had no effect on the plasma corticosterone concentration. In vitro, the release of ACTH by perifused anterior pituitary glands was increased strongly by rCRF (4 pmol/0.5 ml) but only slightly by AVP (92 pmol/0.5 ml) and oxytocin (198 pmol/0.5 ml). Arginine vasopressin and oxytoxin potentiated the release of ACTH stimulated by rCRF in vitro but not in vivo. Our results suggest that rCRF is the major peptide that controls ACTH secretion in the fetal rat at term. In conclusion, the rise of the ACTH level observed only 30 min after injection of rCRF or AVP suggests the existence of a factor able to inhibit the ACTH response after injection of these peptides. This factor might be elicited by the blood volume expansion.