HETEROGENEITY OF PITUITARY LACTOTROPHS - IMMUNOCYTOCHEMICAL IDENTIFICATION OF FUNCTIONAL SUBTYPES

The existence of functional lactotroph subpopulations was confirmed in primary pituitary cell cultures of female rats submitted to estrogen treatment and stimulation with thyrotrophin releasing hormone (TRH) an d angiotensin II (A-II). In cell cultures of pituitary tissue, prolact in (PRL) producing cells represent about 50% of the total cell count, most of which (90%) correspond to a typical lactotroph subpopulation c haracterized by large secretory granules, 500-900 nm in diameter, and well developed rough endoplasmic reticulum (RER) and Golgi complex. Fe w atypical lactotrophs were detected with a quiescent appearance and c ontaining smaller secretory granules, often indistinguishable from gra nular content of other pituitary cells. Depletion of endogenous estrog en caused by ovariectomy (OVX) decreased the pituitary lactotroph popu lation about 34%, with a relative increase of atypical forms (56%). Re placement therapy with benzoate estradiol (EB) to OVX rats did not rev erse the proportion of typical and atypical lactotrophs gauged in cont rol pituitary glands. The predominant lactotroph population of OVX rat was an atypical PRL producing cell which displayed a quiescent appear ance compatible with a reduced secretory activity. By contrast, estrog en administration to OVX rats caused a striking development of the RER , a hypertrophy of the Golgi complex and an increased storage of matur e and immature secretory granules in the majority of lactotrophs. Thes e features are compatible with a reactivated protein synthesis. Estrog en also enhanced significantly (p < 0.05) the responsiveness of lactot rophs to A-II and the PRL secretion in both intact and OVX + EB treate d rats increased by 40% and 30% respectively. By contrast, A-II did no t produce any statistically significant response of lactotrophs from O VX female rats. At variance to this observation, in all models tested TRH increased significantly the PRL secretion (p < 0.05). The correlat ion of PRL secretion and morphology of different lactotroph subtypes a uthenticates the existence of a lactotroph subpopulation unresponsive to A-II in pituitary cell cultures from rats depleted of estrogen.