INTRACELLULAR ANTAGONISTIC INTERACTION BETWEEN GNRH AND GONADOTROPIN SURGE-INHIBITING ATTENUATING FACTOR BIOACTIVITY DOWNSTREAM OF 2ND-MESSENGER INVOLVED IN THE SELF-PRIMING PROCESS/

The antagonistic control of LH concentrations by GnRH and the putative ovarian factor gonadotrophin surge-inhibiting or -attenuating factor (GnSIF/AF) was studied by perifusion of female rat pituitary glands in vitro. LH release and GnRH self-priming were measured in response to five (0.32-10 nmol l(-1)) or three (10 nmol l(-1)) 10 min pulses of Gn RH. In the latter case, pulses were preceded by five 10 min pulses of either I mmol 8-bromo-cAMP l(-1) plus 10 mmol theophylline l(-1) or I mu mol phorbol 12-myristate 13-acetate l(-1) The stimulations were car ried out in the presence or absence of steroid-free bovine follicular fluid, which possesses GnSIF/AF bioactivity, to study the effect of Gn SIF/AF on the self-priming process during successive stimulations by G nRH. First, the effect of follicular fluid was studied on GnRH-induced LH release from pituitary glands collected during the ovarian cycle. Only when a clear self-priming effect was evident, as on day 2 of dioe strus and the day of pro-oestrus, follicular fluid antagonized the sel f-priming effect of GnRH. Second, when,glands on day 2 of dioestrus we re used, self-priming was displayed by various combinations of GnRH an d follicular fluid. The pulses with the lowest concentrations of GnRH together with a high concentration of follicular fluid, however, led t o stable low amplitude LH pulses. Finally, priming of the pituitary LH response to GnRH with 8-bromo-cAMP plus theophylline or phorbol 12-my ristate 13-acetate was inhibited by follicular fluid. These results co nfirm the control of LH release by GnRH and GnSIF/AF bioactivity. The effect of GnSIF/AF in follicular fluid is most pronounced on the days before the LH surge, showing evidence of its important role in maintai ning low LH concentrations during this period. GnSIF/AF neutralizes do wnstream actions of second messengers involved in GnRH self-priming.