Adrenocorticotropin and cortisol levels in the plasma of bovine fetuses and neonates.

Plasma ACTH and cortisol levels in the bovine fetuses over the period of 5 to 9 months of gestation and in the neonates immediately after birth and at 5 days old were studied. In the bovine fetuses, the plasma ACTH levels ranged from 60.8 +/- 17.8 to 71.3 +/- 19.7 pg/ml over the period of 5 to 7 months of gestation. It increased rapidly to 239.2 +/- 261.5 pg/ml at 8 months and to 406.9 +/- 409.4 pg/ml at 9 months of gestation. In the neonates immediately after birth it decreased to 182.3 +/- 110.7 pg/ml. The plasma cortisol levels ranged from 3.23 +/- 2.12 to 3.85 +/- 2.52 ng/ml over the period of 5 to 8 months of gestation and increased to 8.10 +/- 4.88 ng/ml at 9 months of gestation. It then increased rapidly to 88.35 +/- 42.78 ng/ml in the neonates immediately after birth. The correlation between plasma ACTH and cortisol levels in the fetuses of 5 to 7 months of gestation was not significant, but in the fetuses of 8 and 9 months of gestation and neonates were significant. However, especially immediately after birth, the increase in plasma cortisol occurred without a concomitant rise in plasma ACTH. According to these findings, it is suggested that the pituitary-adrenocortical axis in bovine fetus matures in the later stage of gestation and an increase of sensitivity in the fetal adrenal gland to ACTH may serve as a trigger for the onset of parturition.