MODULATION OF CALCIUM SIGNALING AND LH-SECRETION BY PROGESTERONE IN PITUITARY GONADOTROPHS AND CLONAL PITUITARY-CELLS

In estradiol-treated pituitary cells, progesterone enhances gonadotrop in-releasing hormone (GnRH)-induced LH secretion from cultured rat pit uitary cells during short-term treatment but attenuates this response during prolonged treatment. In the present study, the effects of gonad al steroids on GnRH-induced cytoplasmic calcium ([Ca2+](i)) responses in gonadotrophs were analyzed in rat pituitary cells and immortalized (alpha T3-1) murine gonadotrophs. Ca2+ responses were measured in cell suspensions and single gonadotrophs, loaded with Fura-2 or Indo-1, re spectively, and pretreated for 48 h with 1 nM estradiol with or withou t 100 nM progesterone, or for 48 h with 1 nM estradiol and then for 3 h with 100 nM progesterone. In cells of the alpha T3-1 gonadotroph lin eage, GnRH elicited biphasic Ca2+ signals composed of an initial peak response followed by a prolonged plateau phase. The amplitudes of both the extracellular Ca2+-independent spike phase and the extracellular Ca2+-dependent plateau phase were enhanced or inhibited by short- or l ong-term progesterone treatment, respectively. In single pituitary gon adotrophs, GnRH (0.5 nM) elicited oscillatory responses due to intermi ttent release and uptake of Ca2+ from intracellular stores. Treatment with progesterone shifted the oscillatory signal toward biphasic (3 h) or subthreshold (48 h) response profiles, revealing a steroid-induced change in the pattern of Ca2+ mobilization. In addition to these agon ist-induced responses, the transient [Ca2+](i) responses of pituitary cells and individual gonadotrophs to high K+ were enhanced or inhibite d after short- or long-term progesterone treatment, respectively. Thes e actions were correlated with the effects of progesterone on K+-induc ed LH secretion. The [Ca2+](i) and LH secretory responses to phorbol e ster treatment were also enhanced by short-term exposure of the cells to progesterone. The results demonstrate that the stimulatory and inhi bitory effects of progesterone on agonist-induced Ca2+ signaling resul t from changes in Ca2+ mobilization and entry, and contribute to the m odulatory actions of the steroid on GnRH-induced LH secretion.