Identification, cellular and subcellular distribution of 21 and 72 kDa proteins (tuberalins?) secreted by specific cells of the pars tuberalis

The cell types of the pars tuberalis (PT) are the follicular cells, the par s distalis cells and the so-called PT-specific cells. The latter are distin ct endocrine cells displaying melatonin receptors. Although the nature of t he secretory product(s) of the PT-specific cells has not yet been clarified , the function of these cells has started to be unfolded. For practical rea sons, previous authors have designated the, as yet, unidentified PT hormone (s) as tuberalin(s). PT-specific cells synthesise the common a subunit of t he pars distalis glycoprotein hormones, and it has been suggested that tube ralin would correspond to the beta chain of a specific glycoprotein secrete d by these cells. The aims of the present investigation were to identify the compounds secret ed by the specific cells of bovine PT, and to establish their cellular and subcellular distribution. For this purpose, proteins secreted into the cult ure medium of PT explants were separated by electrophoresis and used to rai se antibodies. Two of these proteins, with an apparent molecular mass of 21 and 72, generated antibodies (Ab-21 and Ab-72) that differentially immunor eacted with PT-specific cells. These two antibodies were used for immunoblo tting of conditioned medium and of PT explants, and for light and electron microscopy immunocytochemistry. In immunoblots, Ab-21 reacted with compound s of 21, 22, 47 and 52 kDa, whereas Ab-72 revealed a compound of 72 kDa onl y. Ab-72 immunoreactive material corresponded to a protein, here designated as tuberalin I, secreted by a small population of PT-specific cells (type 2 cells), and stored in 140 nm secretory granules. Immunoreactive tuberalin I was missing from bovine pars distalis and from rat PT. The predominant p opulation of PT-specific cells (type 3 cells) secreted and stored, within 2 80 nm secretory granules, an Ab-21 immunoreactive protein, here designated as tuberalin II. All cells of rat PT immunoreacted with Ab-21. In the cells of bovine and rat PT, immunoreactive tuberalin II was mostly confined to a paranuclear spot; this spot also bound wheat germ agglutinin and reacted w ith an antibody against the a chain of glycoprotein pars distalis hormones. It is suggested that tuberalin II would correspond to the beta chain of a s pecific glycoprotein secreted by type 3 PT-specific cells. In bovine PT, th e cells displaying immunoreactive tuberalins I and II did not react with an y of the antibodies against pars distalis hormones.