ABSTRACT
Prolactin interaction with its receptor is the first and requisite step in its action. The first reports identifying these receptors were published in 1972 to 1973, and since then, more than 400 papers have appeared on the subject. In most reports, the identification of receptor-positive tissue is based on competitive radiolabeled prolactin-binding assays of crude or partially purified plasma membranes or tissue homogenates. Autoradiographic localization following in vivo injection or in vitro incubation of cells or tissue slices with radiolabeled prolactin has also been used. Autoradiographic localization, with the controls mentioned, has been used successfully to define prolactin binding sites in subcellular fractions of liver. Although some actions of prolactin may be mediated by its initial interaction with cell surface receptors, both prolactin and its receptor are found intracellularly in a number of target tissues. Prolactin-receptor complexes, like those for other polypeptide hormones, are internalized by receptor-mediated endocytosis.
