RAPID AUGMENTATION OF PROLACTIN CELL NUMBER AND SECRETORY CAPACITY BY AN ESTROGEN-INDUCED FACTOR RELEASED FROM THE NEUROINTERMEDIATE LOBE

17-beta-Estradiol (E2) has been shown to exert an acute stimulatory effect on PRL secretion via an indirect action involving the neurointermediate lobe (NIL). In the present study we used a reverse hemolytic plaque assay to determine whether this effect was manifested as an augmentation of the number of PRL secretors and/or an increase in the amount of hormone released per PRL cell. Cultures of anterior pituitary (AP) and NIL cells from ovariectomized rats were cultured overnight, exposed to the treatment (E2 or vehicle) for 3 h, and then subjected to a reverse hemolytic plaque assay that was carried out in the presence or absence of TRH. Concurrent exposure of AP cells to E2 and NIL cells evoked an 11-12% increase in the overall proportion of PRL-secreting cells. This was true when the AP and NIL cells were incubated as a mixed culture and when the two cell types were maintained in the same petri dish but on separate plastic supports, and TRH did not significantly influence this response. The effect of E2 on the number of PRL secretors was negated when NIL cells were not present throughout the experiment or when they were removed from the cultures just before commencement of E2 treatment. Simultaneous treatment with E2 and NIL cells also significantly augmented the sizes of PRL plaques produced under basal conditions by AP cells. Taken together, these results demonstrate that E2 stimulates NIL cells to release an activity that enhances PRL secretion in two ways: 1) by recruiting additional PRL cells into the secretory pool, and 2) by augmenting the secretory capacity of individual PRL cells.