DEVELOPMENT OF THE PITUITARY-ADRENAL AXIS IN FETAL SHEEP TWINS

Plasma cortisol increases in fetuses at term and is important for over all development. This study was designed to determine whether cortisol increases synchronously in twin fetal sheep and whether differences b etween twins contribute to the respective timing. Catheters were surgi cally implanted in fetal arteries in twins, the amniotic sac, and a ma ternal artery and vein. Blood was drawn daily until labor was imminent or the twins were delivered. Fetal pituitaries and adrenals were remo ved for in vitro measurements. Analyses included blood gases and corti sol (daily) and plasma cortisol, adrenocorticotropic hormone (ACTH), a nd estrogens (at completion). Twins were assigned retrospectively to g roup A or B, depending on which cortisol was first elevated (group A) above baseline. Group A fetuses consistently had higher cortisol until term. All group A fetuses also first had elevated ACTH. In four of fo ur sets of twins of both sexes, the male was in group A. There were no differences between fetuses in plasma estrogens or pituitary ACTH res ponse to stimulation, but adrenal cells from group A fetuses were more responsive. These data suggest that adrenal activity is increased in one twin consistently, with the difference being attributable to the r esponsiveness of adrenal cells to ACTH rather than pituitary responsiv eness to either corticotropin-releasing hormone or vasopressin. Differ ence between sexes may also be involved.