Plasticity in the adrenocorticotropin-related peptides produced by primary cultures of neonatal rat pituitary.

Intermediate pituitary lobe cells from newborn rats were maintained in culture to determine the extent to which they continue to exhibit the tissue-specific properties of the newborn and adult intermediate pituitary lobes. At all times examined these cultures contained mostly alpha MSH-sized and corticotropin-like intermediate lobe peptide-sized peptides; the alpha MSH-sized peptides were predominantly diacetyl-ACTH-(1-13)NH2. After 6 days in culture, the intermediate pituitary lobe cells retained the ability to synthesize diacetyl-ACTH-(1-13)NH2. Compared to that of the adult, the newborn anterior pituitary lobe is enriched in high mol wt forms of ACTH-related molecules. Therefore, the ability of newborn anterior pituitary lobe cell cultures to develop the adult processing pattern in culture was investigated. After 6 days in culture, peptides the size of alpha MSH predominated rather than ACTH-(1-39), which is the major form found in the adult. Immunocytochemical studies showed that all cultured newborn corticotropes strongly stained for alpha MSH-related material. The alpha MSH-sized molecules were identified as ACTH-(1-13)NH2 by reverse phase HPLC. In 6-day-old cultures of neonatal anterior pituitary lobes grown in the presence of a synthetic glucocorticoid, dexamethasone, the amount of alpha MSH-sized material was diminished, and instead, precursor forms of the ACTH-related peptides were detected. In biosynthetic labeling experiments, the ratio of newly synthesized ACTH-(1-13)NH2 to ACTH-(1-39) was greatly reduced by treatment of the cells with dexamethasone. The extensive cleavage of ACTH-(1-39) and its regulation by dexamethasone are unique to newborn anterior pituitary lobe corticotropes; such plasticity is not observed in cultures of adult tissue.