Gestational changes in the metabolism of progesterone by the fetal calf adrenal

The metabolism of [3H]progesterone by the fetal calf adrenal was examined in homogenates and microsomal fractions of adrenals obtained from mid- or late gestational fetal calves. Throughout the course of gestation, there was no detectable difference in the formation of corticosterone from progesterone by adrenal homogenates. In contrast, cortisol production in these same homogenates increased 7-fold from 0.3-2.2% at the end of gestation. Similar results were obtained using microsomal fractions prepared from these homogenates: the 17 alpha-hydroxylation of progesterone (i.e. 17 alpha-hydroxyprogesterone and 11-deoxycortisol) increased from 0.7% to 26% at the end of gestation. The results suggest that the increased cortisol observed in the fetal circulation toward the end of gestation is the result of a rapid increase in the capacity of the fetal calf adrenal to form 17 alpha-hydroxycorticosteroids. The underlying mechanism for this change is discussed.