EFFECTS OF CORTICOTROPIN-RELEASING FACTOR, ARGININE-VASOPRESSIN AND OXYTOCIN ON THE POLYMORPHISM OF PLASMA ADRENOCORTICOTROPIC HORMONE IN THE RAT FETUS IN LATE PREGNANCY

The effects of rat corticotropin-releasing factor (rCRF, 1.25 pmol/50 mu/fetus), arginine vasopressin (AVP, 5 pmol/50 mu l/fetus) and oxytocin (OT, 12.5 pmol/50 mu l/fetus) alone or in association were investigated in 21-day-old rat fetuses injected intravenously through the umbilical vein. Blood samples were collected 15 and 30 min after injection for the determination of corticosterone concentration and the different plasma molecular adrenocorticotropic hormone (ACTH) forms isolated by chromatography on Sephadex G(50) fine. All the plasma samples chromatographed 15 and 30 min after injection of the different peptides showed 3 different molecular ACTH forms: big ACTH (>20,000 molecular weight), intermediate ACTH (=13,000) and little ACTH (=4,500). The injection of rCRF or AVP alone and rCRF in association with AVP or OT increased the concentrations of big ACTH 15 min and little ACTH 30 min after injection. The injection of OT alone or in association with AVP did not change the concentration of the 3 molecular ACTH forms 15 and 30 min after injection. The rise of big ACTH 15 min after injection was not associated with a significant increase in plasma corticosterone concentration, whereas the increase in little ACTH 30 min later enhanced plasma corticosterone concentration. Our results suggest that rCRF or AVP alone and rCRF in association with AVP or OT injected intravenously in the fetal rat produced a selective release of the molecular ACTH forms and the increase in the plasma corticosterone concentration occurred when the proportion of little ACTH which is predominant ACTH form in the fetal rat was enhanced.