Inhibin and estradiol alter gonadotropes differentially in ovine pituitary cultures: Changing gonadotrope numbers and calcium responses to gonadotropin-releasing hormone

Both inhibin (IN) and estradiol (E) use separate and distinct mechanisms to decrease the production of FSH 60%-80% while increasing receptors for GnRH (GnRH-Rec) 400%-600% in ovine pituitary cultures. To investigate how these hormones act to create outcomes that appear similar, individual cells in ovine pituitary cultures were analyzed for changes in GnRH-stimulated calcium signaling, GnRH binding, and gonadotropin content under IN or E treatments. Calcium mobilization studies showed that 10 nM GnRH increased intracellular calcium ([Ca++](i)) in 9.2% +/- 0.8% of cells in control ovine pituitary cultures. After treatment with 10 nM E for 48 h, there was a small increase in the number of cells responding to GnRH (12.9%+/-1.4% responded) and a major 5-fold increase in [Ca++](i) response to GnRH as compared with untreated cells. By contrast, IN did not alter cellular calcium responses to GnRH but markedly increased the number of cells responding to GnRH (a 3.7-fold increase to 33.8+/-2.6%). Cytochemical studies measuring GnRH:GnRH-Rec complexes, FSH, and LH showed that E had no effect on the number of pituitary cells containing FSH, LH, or GnRH-Rec. In contrast, similar studies using IN showed that the number of cells containing GnRH-Rec and LH significantly increased (>50%), whereas the number of FSH cells decreased (36%), and a lower than normal percentage of the remaining FSH cells carried GnRH-Rec. Thus, the net effect of IN on gonadotropes was to dramatically decrease the ratio of FSH:LH cells that contained GnRH-Rec, from 1:1(untreated) to 1:8 (IN-treated) and to decrease the percentage of LH/FSH colocalization. In summary, these data indicate that E primarily operates on a fixed, preset number of ovine gonadotropes to inhibit FSH production and increase responsiveness to GnRH. IN, however, seems to change the very nature of ovine gonadotropes by completely turning off FSH synthesis in some cells, lowering GnRH-Rec in other FSH cells, and, finally, inducing LH and GnRH-Rec in newly recruited gonadotropes.