Coculture of anterior and posterior pituitary cells: selective stimulation of lactotrophs.

There is extensive evidence that the posterior pituitary (PP) participates in the regulation of PRL secretion. We recently reported that a putative PRL-releasing factor is localized, and possibly produced, in the intermediate lobe of the PP. The aim of the present investigation was to determine whether cultured PP cells affect anterior pituitary (AP) function in terms of cell content and cumulative release of PRL. Anterior and posterior pituitaries from adult male rats were dispersed with trypsin and cultured either alone or together for 4 and 8 days in serum-free medium. The concentrations of PRL, GH, and LH in cell extracts and culture media were measured by RIA. Coculturing of AP and PP cells at different plating densities resulted in a 2-fold rise in PRL cell content after 4 days. The cumulative release of PRL in the cocultures was significantly increased only after 8 days. LH and GH were affected slightly, or not at all. Medium conditioned by PP cells mimicked the effects of coculture on the cumulative release of PRL, but not on the cell content. Short-term incubation with TRH induced a much larger release of PRL from AP + PP cocultures than from AP cells cultured alone. In conclusion, these data suggest that 1) PP cells stimulate the production and release of PRL in a hormone-specific manner, and 2) coculturing of AP + PP cells augments the responsiveness of lactotrophs to secretagogues such as TRH. We propose that at least two factors, one of which might be PRL-releasing factor, are involved in these effects.