Prolactin transit through mammary epithelial cells and appearance in milk

In lactating mammary epithelial cells, prolactin (PRL) binds to its receptors, is endocytosed and carried to the milk. In order to study the transit of the hormone and its receptor respectively, the intracellular pathway of PRL and ot two monoclonal antibodies against PRL-receptor (PRL-R), labelled with biotin and colloidal gold, were monitored in incubated fragments of enzymatically dissociated mammary cells of lactating rabbits. PRL was internalised in endosomes and carried to microvesicular bodies, lysosomes, Golgi apparatus and secretory vesicles containing casein micelles. After 60 min of incubation at 37 degrees C, PRL was released in the incubation medium. M110 anti PRL-R was internalised in endosomes and detected mainly in microvesicular bodies during a one hour incubation. In contrast, A917 anti PRL-R also internalised in endosomes and in microvesicular bodies, was carried out to the Golgi apparatus and to the lumen of the acini after 5 min of incubation at 37 degrees C. These results suggest that an intracellular sorting occurs in the presence of the hormone or the different antibodies. The fatty acid composition of the mammary epithelial cell membranes influences the activity of these cells. To examine the effect of this membrane composition on the transit of PRL, the intracellular pathway of the hormone was studied in mammary cells of lactating rats previously fed with lipid deficient diets. Plasma levels of PRL were not modified in rats receiving a deficient diet compared to controls. Labelled PRL was accumulated inside the microvesicular bodies during a one-hour incubation at 37 degrees C. However, PRL was always detectable in milk, suggesting that the intracellular transit of PRL could be slowed down but not inhibited. Possible relationships between endocytosis of PRL and its secretagogue effect are discussed.