Luteinizing hormone secretion from wild-type and progesterone receptor knockout mouse anterior pituitary cells

The progesterone receptor (PR) has a central role in the hypothalamo-pituit ary events culminating in the preovulatory LH surge, and mice with genetica lly ablated PR provide a model for dissecting cellular pathways subserving this role. The aims of this study were to determine l)whether the GnRH self -priming response and acute progesterone augmentation of secretagogue-stimu lated LH secretion are present in cultured wild-type (WT) mouse pituitary c ells, and 2) whether the PR is essential for self-priming by comparing the responses in PR knockout (PRKO) cells. Pituitary cells from ovariectomized WT or PRKO mice cultured +/- 17 beta -estradiol (E-2) for 3 days were chall enged with hourly pulses of 1 nM GnRH or 54 mM K+. A background of E-2 had no effect on the initial LH secretory response for either WT or PRKO cells. However, for subsequent GnRH pulses, E-2 was permissive for the GnRH self- priming response in WT cells. PRKO cells exhibited a blunted GnRH self-prim ing response. Exposure to progesterone for 90 min before secretagogue stimu lation resulted in a modest (1.5-fold) augmentation of the LH response to G nRH but not K+ pulses in WT cells; progesterone had no effect in PRKO cells . Unlike in the rat, the PR antagonists RU486 or ZK98299 failed to prevent potentiation of LH secretory responses to multiple GnRH pulses in WT cells. Although RU486 blocked progesterone augmentation of the initial GnRH pulse , it was ineffective in blocking progesterone's action after multiple GnRH pulses. In WT cells, 8-bromo-cAMP (8-Br-cAMP) was able to substitute for th e GnRH priming pulse; 8-Br-cAMP also augmented GnRH-stimulated secretion in PRKO cells but less effectively. 8-Br-cAMP augmented K+-stimulated LH secr etion in WT and PRKO cells equally. These results suggest that, although mo use gonadotropes show GnRH self-priming, they have adapted strategies diffe rent than rat cells for amplifying the GnRH signal as shown by the residual self-priming in PRKO cells, the modest or absent augmentation by acute pro gesterone of GnRH- or K+-stimulated secretion in WT cells, and the reduced ability of PR antagonists to interfere with GnRH self-priming and progester one augmentation. We speculate that the adaptations could involve, at least in part, differences in the ratio of PR isoforms.