Rat prolactin in serum, milk, and mammary tissue: characterization and intracellular localization

To study the localization of PRL in mammary epithelial cells (MEC) and to characterize PRL forms in serum, milk, and mammary tissue, two groups of lactating rats, a control group and a bromocriptine-treated group, were compared. In serum and milk from control rats, two forms of PRL (25 and 23 kDa) were detected by immunoblotting. In bromocriptine-treated rats, only the 25-kDa form was present. In mammary tissues from control rats, 25-, 23-, and 14-kDa forms of the hormone were detectable, whereas only 25- and 14-kDa forms were present in bromocriptine-treated rats. Immunofluorescence and immunogold electron microscopy revealed that in MEC from control rats, PRL was located in the organelles involved in endocytosis and was also present in rough endoplasmic reticulum, Golgi apparatus, secretory vesicles, and lumen of the acini. In bromocriptine-treated rats, a decrease in the labeling was evidenced in endosomes and multivesicular bodies. On the contrary, labeling associated with the rough endoplasmic reticulum and secretory vesicles was increased. Thus, even when the amount of circulating PRL detected by RIA was strongly decreased, PRL was always detectable in MEC and in the lumen of the acini, suggesting an active participation of MEC in the transport of PRL to milk.