Increase in the response to adrenocorticotrophin of isolated maternal adrenal cells from sheep in late pregnancy.

Adrenal cells were prepared from non-pregnant (anoestrous) sheep, from ewes at days 50, 100 and 130 of pregnancy and at term, and from animals at 1-5 days post partum. The ability of the cells to respond to adrenocorticotrophin (ACTH1-24), alpha-melanocyte-stimulating hormone (alpha-MSH), or combinations of these peptides has been examined in vitro. There was a progressive rise in the basal output of cortisol during pregnancy and in the absence of adrenocorticotrophin the cortisol output from adrenal cells of late pregnant and post-partum sheep the amount of ACTH required to produce half the maximum output of steroid (ED50) was 8 pg/ml. The ED50 increased in early pregnancy to 112 pg/ml and then fell to < 5 pg/ml between day 100 and term. At term both the stimulation ratio and the absolute increment in cortisol output elicited by a maximal concentration of ACTH were greater than at any other time tested in pregnant or non-pregnant sheep. Cortisol output during pregnancy was not increased by alpha-MSH, although at term the stimulatory effect of ACTH1-24 was partially antagonized by alpha-MSH. These results suggest that there may be an increase in the responsiveness of the maternal adrenal during pregnancy, although the factor(s) responsible remains unknown.