Effect of 17 β-Estradiol on Phosphoinositide Metabolism and Prolactin Secretion in Anterior Pituitary Cells

Abstract

The present study was undertaken to investigate the effect of 17 beta-estradiol (E2) administration on in vitro prolactin (PRL) release and intracellular phosphoinositide metabolism. The incorporation of [3H]inositol (Ins) into phosphatidylinositol (PtdIns), phosphatidylinositol-4-phosphate [PtdIns(4)P] and phosphatidylinositol-4,5-bisphophate [PtdIns(4,5)P2], and the generation of inositol phosphate (InsPx) following thyrotropin-releasing hormone (TRH) stimulation were studied in primary cultures of anterior pituitary cells obtained from ovariectomized rats. Administration of polyestradiol phosphate (PEP) to ovariectomized rats produced a significant increase (p less than 0.05) in serum PRL levels. This treatment also enhanced significantly (p less than 0.01) the in vitro release of PRL in a progressive manner during 24, 48 and 72 h of culture of dispersed anterior pituitary cells. The radioisotopic labeling by [3H]Ins of all species of phosphoinositides was progressive throughout 72 h of culture, and a good correlation was observed between intracellular phosphoinositide synthesis and PRL release from these cells. PEP treatment enhanced significantly (p less than 0.05-0.01) [3H]Ins incorporation into PtdIns and PtdIns(4)P after 48 and 72 h of culture, although it did not alter [3H]Ins incorporation into PtdIns(4,5)P2. Furthermore, this treatment caused a small, but significant increase (p less than 0.01) in InsPx generation following TRH stimulation. However, the increased [3H]Ins incorporation into phosphoinositide and InsPx generation that we observed after TRH stimulation was significantly (p less than 0.01) less than the increased amount of in vitro PRL release following PEP treatment. There was no significant correlation between the percentage increases in PRL release and phosphoinositide metabolism following the same treatment. These data suggest that phosphoinositide metabolism is enhanced in the anterior pituitary cells of ovariectomized rats by treatment with PEP, but this system does not appear to be tightly coupled or causally related to the much greater production of PRL release.