Possible Role of Cortisol in the Stimulation of Cortisol-Binding Capacity in the Plasma of Fetal Sheep*

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Abstract
In sheep, the prepartum rise in fetal plasma cortisol (F) is associated with an increase in the corticosteroid-binding capacity (CBC) of the plasma. Whether CBC changed during preterm labor, induced by administering ACTH to the fetus was examined. The role of F in such changes was examined. Beginning on day 127, chronically catheterized fetal sheep were divided into 6 groups; saline control infusions (n = 4); pulsatile ACTH (P-ACTH; 66 ng/min for 15 min every 2 h; n = 5); P-ACTH (as 2) plus metopirone (MET; 500 mg/24 h no. = 5); P-ACTH and MET (as 3) plus F (13.3 .mu.g/min for 15 min every 2 h; n = 3); P-ACTH and MET (as 3) plus F (133 .mu.g/min for 15 min every 2 h; n = 4); P-ACTH and MET (3) plus dexamethasone (0.56 .mu.g/min for 15 min every 2 h for 50 h, then 5.6 .mu.g/min for 15 min every 2 h until 100 h; n = 4). All infusions continued for 100 h. Fetal blood samples were collected at 8 h intervals for determination of plasma F concentration and CBC. Estrone and progesterone were measured in 8 h samples of maternal femoral arterial blood. P-ACTH resulted in a significant increase in CBC within 24 h; values at 72-96 h were 2- to 3-fold greater than controls. Rises in CBC were positively correlated with the changes in plasma F in the P-ACTH-treated fetuses and preceded changes in plasma progesterone or estrogens. MET treatment the P-ACTH-induced increases in F and CBC. Exogenous F and dexamethasone overcame the MET-induced inhibition of P-ACTH-induced increases in CBC. In fetal sheep, CBC is elevated by P-ACTH treatment; F may mediate this stimulation of its own binding protein.

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