Surgical disconnection of the hypothalamus from the fetal pituitary abolishes the corticotrophic response to intrauterine hypoglycemia or hypoxemia in the sheep during late gestation.

We have investigated the ACTH and cortisol responses to acute episodes of hypoxemia or hypoglycemia in fetal sheep in which the hypothalamus and pituitary were surgically disconnected at between 112 and 123 days gestation. Before 130 days gestation, basal plasma concentrations of ACTH were significantly greater in the hypothalamo-pituitary disconnected (HPD) fetuses than in the intact fetal sheep (126-130 days; 105.0 +/- 11.4 ng/liter, HPD group; 64.0 +/- 9.5 ng/liter, intact group). After 130 days, however, there was no difference between plasma ACTH concentrations in the HPD (136-140 days; 154.7 +/- 16.7 ng/liter HPD group; 113.6 +/- 19.1 ng/liter, intact group) and intact fetal sheep, and in both groups the mean ACTH concentrations were significantly greater after 136 days gestation than before 130 days. In the HPD group, however, while the plasma ACTH concentrations were elevated there was no prepartum increase in the plasma concentrations of cortisol. A decrease in the fetal arterial blood PO2 by approximately 50% for 30 min between 123 and 132 days, stimulated a significant increase in fetal ACTH and cortisol concentrations in the intact but not in the HPD fetuses. In the intact group, plasma ACTH concentrations were also significantly increased (P less than 0.001) above control values (98.2 +/- 21.2 ng/liter) at 120 min after the start of an iv infusion of insulin (1 IU/60 min) (517.2 +/- 160.5 ng/liter) and were still elevated at 60 min after the end of the infusion period (1248.1 +/- 643.2 ng/liter). In the HPD fetuses, however, there was no significant change in plasma ACTH concentrations during or after the insulin infusion. In both the HPD and intact groups, there was a significant increase in plasma ACTH concentrations above control values (62.5 +/- 8.5 ng/liter intact; 135.0 +/- 30.6 ng/liter, HPD) after intrafetal administration of CRF (+10 min; 117.5 +/- 8.1 ng/liter, intact; 225.3 +/- 33.1 ng/liter, HPD) indicating that the secretory capacity of the pituitary corticotrophs was not reduced by the HPD procedure. Our results demonstrate that an intact hypothalamic-pituitary connection is required to generate a normal prepartum increase in fetal cortisol concentrations and is essential for an appropriate fetal pituitary-adrenal response to intrauterine hypoxemia and hypoglycemia.