Adrenocorticotrophic hormone in the anterior and neurointermediate lobes of the rat during the perinatal period: polymorphism, biological and immunological activities of ACTH.

When acid extracts (0.1 N HCl) of the anterior or neurointermediate lobes of fetal and newborn rat pituitary were subjected to gel filtration on Sephadex G-50 fine columns, polymorphism of ACTH was revealed by radioimmunoassay. At every stage of pregnancy and after birth three main ACTH forms were observed. The first form ('big' ACTH) eluted near the void volume of the column; its apparent molecular weight was 45,000. The second form ('little' ACTH) coeluted with synthetic human ACTH1-39 and 125I human ACTH. As the latter marker eluted more slowly than the former, 'little' ACTH, which showed an apparent molecular weight near 5,000, could not be a single entity. The third form ('intermediate' ACTH) eluted between 'big' and 'little' ACTH; its apparent molecular weight was near 13,000. During the perinatal period, the relative proportions between these different molecular forms of ACTH changed in both anterior and neurointermediate lobes. Thus the gradual increase of 'intermediate' and 'little' forms was accompanied by a decrease of 'big' ACTH. The perfusion of fetal adrenal glands with the same immunological quantities of 'big', 'intermediate' and 'little' ACTH showed that the 'little' and 'intermediate' forms have greater biological potency in eliciting corticosterone secretion than the 'big' ACTH form.