Modulation of calcium signaling and LH secretion by progesterone in pituitary gonadotrophs and clonal pituitary cells.

In estradiol-treated pituitary cells, progesterone enhances gonadotropin-releasing hormone (GnRH)-induced LH secretion from cultured rat pituitary cells during short-term treatment but attenuates this response during prolonged treatment. In the present study, the effects of gonadal 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, respectively, and pretreated for 48 h with 1 nM estradiol with or without 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 lineage, GnRH elicited biphasic Ca2+ signals composed of an initial peak response followed by a prolonged plateau phase. The amplitudes of both the extracellular Ca(2+)-independent spike phase and the extracellular Ca(2+)-dependent plateau phase were enhanced or inhibited by short- or long-term progesterone treatment, respectively. In single pituitary gonadotrophs, GnRH (0.5 nM) elicited oscillatory responses due to intermittent 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 agonist-induced responses, the transient [Ca2+]i responses of pituitary cells and individual gonadotrophs to high K+ were enhanced or inhibited after short- or long-term progesterone treatment, respectively. These actions were correlated with the effects of progesterone on K(+)-induced LH secretion. The [Ca2+]i and LH secretory responses to phorbol ester treatment were also enhanced by short-term exposure of the cells to progesterone. The results demonstrate that the stimulatory and inhibitory effects of progesterone on agonist-induced Ca2+ signaling result from changes in Ca2+ mobilization and entry, and contribute to the modulatory actions of the steroid on GnRH-induced LH secretion.