Functional Variations among Prolactin Cells from Different Pituitary Regions*

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Abstract
Reverse hemolytic plaque assays were used to compare the responsiveness of cells from different pituitary regions to the modulatory effects of human pancreatic GH-releasing factor (GRF), TRH, and dopamine (DA). Tissues from the peripheral rim (outer zone) and the central region (inner zone) of adenohypophyses from day 10 lactating rats were dispersed with trypsin, and the cells were placed into culture. On the following day, these cells were subjected to GH plaque assays (conducted in the presence or absence of GRF) and PRL plaque assays (performed with or without TRH and DA). Cells from both zones responded similarly to GRF with a rapid acceleration of GH plaque formation. However, the rate of PRL plaque formation in response to TRH and DA differed between cells from these regions. For outer zone cells, plaque development increased greatly with TRH treatment, but was only moderately affected by DA. Plaque formation from inner zone cells was influenced slightly by TRH, but markedly inhibited by DA. These results suggests that PRL, but not GH, cells from these pituitary regions are differentially responsive to at least two hypothalamic secretagogues. We then performed fixed sequential plaque assays to determine whether the proportions of cells that released PRL only (classical mammotropes) or those that released both GH and PRL (mammosomatotropes) also differed between the inner and outer zones. Using this approach, we found that the outer zone contained a much larger proportion of dual hormone secretors than did the inner zone. These results, when taken together with the responsiveness differences discussed above, raise the possibility that the release of PRL from mammotropes and mammosomatotropes is regulated differently and that the ratio of these two cell types may dictate, in part, the manner in which a specific region of the pituitary responds to hypothalamic input.