Developmental Aspects of the Pituitary-Adrenal Axis Response to Hemorrhagic Stress in Lamb Fetuses In Utero

Plasma ACTH and corticosteroid concentrations were measured by radioimmunoassay in chronically catheterized fetuses of 32 pregnant sheep. Fetal plasma ACTH levels 38+/-5 pg/ml (means+/-SEM) were slightly (P < 0.05) lower than maternal 54+/-4 pg/ml levels. No general rise in fetal plasma ACTH concentration was noted before 140 days gestation; however, fetal plasma corticoid levels began to increase after about 125 days. This suggested that an increase in fetal adrenal responsiveness to endogenous ACTH occurred during gestation.Hemorrhage of 15% of estimated blood volume decreased mean arterial pressure from 54+/-3 to 36+/-3 torr and increased plasma ACTH from 30+/-5 to 130+/-30 pg/ml in fetuses older than 0.80 gestation. In fetuses younger than 0.67 gestation, 15% hemorrhage caused no change in plasma ACTH levels despite a significant fall in mean arterial pressure. This suggests that system(s) subserving the ACTH response to mild hemorrhage are either absent or nonfunctional in the younger fetuses. The hemorrhage-induced increase in plasma ACTH levels was associated with a small rise in plasma corticoids in fetuses younger than 0.94 gestation. In older fetuses, a similar increase in plasma ACTH was associated with a pronounced increase in plasma corticoid levels. This also suggests that an increase in adrenal responsiveness to endogenous ACTH occurs during gestation. No detectable changes in maternal plasma ACTH or corticoids were found in response to fetal hemorrhage, thus the fetal pituitary-adrenal axis can autonomously respond to stress.