Prolactin is transported across the epithelium of the jejunum and ileum of the suckling rat.

Milk prolactin is transferred from the gastrointestinal tract to the circulation of the suckling rat. To identify the site of prolactin penetration and to determine the mechanism by which the hormone traverses the mucosal barrier, we followed the uptake of prolactin from ligated loops of jejunum or ileum in vivo by three methods: autoradiography, transport of prolactin-gold conjugates, and immunocytochemistry. Autoradiographic studies demonstrated specific binding sites for 125I-prolactin on apical membranes of the jejunum and ileum. Excess cold prolactin reduced radiolabel in apical and basal compartments. Gel autoradiography of portal sera showed the presence of intact prolactin and a prolactin fragment following jejunal transport but only a prolactin fragment following ileal transport. Uptake of prolactin-gold conjugates demonstrated that, in the jejunum, label was present at the luminal surface, within endosomal compartments and lysosomes, in basal coated and smooth vesicles, within basal coated pits, and beyond the basolateral surface. In the ileum, label was found at the luminal surface; within the tubulocisternae, endosomal vesicles, lysosomes, and basal smooth vesicles; and beyond the basolateral surface. Immunoreactive prolactin was present throughout the transepithelial pathways. This study demonstrates that prolactin is selectively and nonselectively absorbed in the jejunum and ileum and that the hormone is directed either to the lysosome for degradation or across the epithelium by means of a transcellular pathway.