beta-Endorphin and ACTH related peptides in primary cultures of rat anterior pituitary cells: evidence for different intracellular pools.

Acid extracts of rat anterior pituitary cells and cell-derived culture media were shown to contain three forms of beta-endorphin immunoreactive peptides, corresponding in molecular size to the prohormone pro-opiomelanocortin (POMC), beta-lipotropin and 3.5 kDa beta-endorphin, and essentially two forms of adrenocorticotropin (ACTH) immunoreactivity, representing a 20 kDa intermediate fragment and 4.5 kDa ACTH. Under basal conditions the intracellular peptides contained a high proportion of the bioactive forms of beta-endorphin and ACTH whereas the extracellular peptides contained a higher proportion of the inactive precursors. When the cells were incubated for 3 h in the presence of 10(-8) M CRF, the levels of intracellular beta-endorphin and ACTH immunoreactivity were reduced by 15-30% and there was a 4-5 fold increase in the level of the secreted peptides; furthermore, unlike the peptides released under basal conditions, the peptides secreted under the influence of CRF contained much higher proportions of 4.5 kDa ACTH and 3.5 kDa beta-endorphin, reflecting the intracellular patterns of these peptides. Similar results were obtained when secretion was stimulated by 10(-7) M epinephrine, which produced a 2-fold increase in peptide release. In the presence of 10(-6) M dexamethasone the basal secretion of ACTH and beta-endorphin related peptides, and the intracellular levels of these peptides, remained unaltered. The results point to the existence of different intracellular compartments from which peptides at different states of maturation can be released selectively.