OVARIAN-STEROIDS MODULATE GONADOTROPIN-RELEASING HORMONE-INDUCED BIPHASIC LUTEINIZING-HORMONE SECRETORY RESPONSES AND INOSITOL PHOSPHATE ACCUMULATION IN RAT ANTERIOR-PITUITARY-CELLS AND ALPHA-T3-1 GONADOTROPHS

The ovarian steroids estradiol and progesterone act as important modul ators of GnRH-induced luteinizing hormone (LH) secretion from anterior pituitary cells. Recently, we demonstrated that the steroids are able to influence GnRH-stimulated Ca2+ mobilization from extra- and intrac ellular sources. Here we investigated the actions of estradiol and pro gesterone on GnRH-induced biphasic LH secretory responses in the model of perifused female rat pituitary cells. A 20 min GnRH stimulus elici ted biphasic LH responses composed of an initial peak followed by a pr olonged plateau phase. Both phases were equally enhanced by long-term (48 h) estradiol treatment. This action was facilitated by subsequent short-term progesterone treatment. In contrast, combined treatment wit h estradiol and progesterone for 48 h led to inhibited LH secretory pr ofiles. To determine the steroid actions on the extracellular Ca2+ ind ependent component of LH secretion we performed experiments using cell s that were perifused with Ca2+ deficient medium. Under these conditio ns the cells responded exclusively with a single peak phase of LH secr etion, which was augmented or inhibited by estradiol and progesterone treatment as described above. To test the hypothesis that an effect of estradiol and progesterone on GnRH-induced polyphophoinositide hydrol ysis is responsible for their modulatory actions on Ca2+ signals and L H secretion we measured inositol phosphate (IP) accumulation after dif ferent steroid treatment paradigms in rat pituitary cells and alpha T3 -1 immortalized gonadotrophs. GnRH-induced IP production was enhanced by long-term estradiol treatment. Short-term exposure of estradiol-pri med cells to progesterone did not lead to significant changes of IP pr oduction. The long-term progesterone treatment paradigm enhanced GnRH- induced IP formation, while it decreased Ca2+ signals and LH secretion . alpha T3-1 cells were used to perform more detailed analysis of IP f ormation. The actions of estradiol and progesterone on the production of inositol mono-, bis-, and trisphosphates were similar to those obse rved in the mixed cell population. It is concluded that estradiol and progesterone modulate both peak and plateau phases of GnRH-stimulated LH secretory responses, effects which are associated with their impact on Ca2+ signals. Our findings argue against a role of IP modulation i n the mechanism of progesterone actions on Ca2+ signaling and LH secre tion in gonadotrophs. Such a mechanism might be involved in the positi ve effects of estradiol in these cells.