Abstract
The direct involvement of the pituitary-adrenal axis in birth has been well established, at least in sheep, and its removal prolongs pregnancy. As part of the process the fetal sheep adrenal grows rapidly during the 10-15 d prepartum and is associated with a large rise in the plasma corticosteroid concentration. This does not seem to result from an increased ACTH secretion. The fetal adrenal in vivo seems refractory to circulating ACTH and shows poor response to elevation of plasma concentration. Thus the signal for the adrenal hypertrophy and the initiation of parturition remains unclear. The responsiveness of the fetal adrenal to ACTH has been re-examined using isolated adrenal cells. The study shows that in the fetal sheep these are not inherently unresponsive to ACTH, but that high-molecular-weight forms of ACTH block the action of ACTH. These peptides may be responsible for controlling the activity of the adrenal in situ.
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