Release of big and small molecular forms of prolactin: dependence upon dynamic state of the lactotroph.

Secretion of different molecular forms of prolactin was studied from a multidisciplinary standpoint in three different experimental rat models which covered the broadest range of lactotrophic activity. The results obtained allowed the recognition of common secretory patterns in stimulated, inhibited and hyperstimulated lactotrophic activity. Polymeric (big) prolactin is stored exclusively in membrane-bound secretory granules and appears to be a convenient biochemical marker with which to quantify the pool of hormone stored in the cell cytoplasm, while monomeric (small) prolactin represents a pool of newly synthesized hormone which is loosely coupled to organelles involved in its intracellular processing. Most of the prolactin in stimulated lactotrophs is processed through a regulated pathway and the hormone released by exocytosis of secretory granules. Interruption of stimulation resulted in accumulation of secretory granules and polymeric prolactin. In hyperstimulated lactotrophs the polymerization and aggregation of prolactin into secretory granules was bypassed and most of the hormone released directly after its synthesis. The pattern of prolactin secretion could be closely correlated with the dynamic state of the lactotroph.