MODULATION OF THE ACTION OF GONADOTROPIN SURGE-ATTENUATING FACTOR BY GONADOTROPIN-RELEASING-HORMONE

Gonadotrophin surge-attenuating factor (GnSAF) is a putative non-stero idal ovarian factor which attenuates the luteinizing hormone (LH) surg e in superovulated women through the reduction of the pituitary respon se to gonadotrophin-releasing hormone (GnRH). The mechanism of action of GnSAF on gonadotrophin secretion was further studied by investigati ng six normally ovulating women in two cycles- a spontaneous and a fol licle-stimulating hormone (FSH)-treated cycle. The response of the pit uitary to five consecutive pulses of GnRH was investigated in late fol licular phase (follicle size 15 mm) of both cycles. GnRH pulses, 10 mu g each, were injected i.v. every 2 h and LH was measured in blood sam ples taken before and 30, 60 and 120 min after each pulse. FSH was inj ected daily at the fixed dose of 225 IU starting on cycle day 2. Peak values of LH increment occurred 30 min after each pulse. However, maxi mal LH increment occurred in both cycles after the second GnRH dose. I n the FSH cycles the response of LH to the first three pulses was sign ificantly attenuated compared with the spontaneous cycles, while the r esponse to the fourth and fifth pulses was similar in the two cycles. In both cycles, LH increment 30 min past GnRH (net increase above the previous value),vas similar after the fourth and fifth pulses. Serum c oncentrations of oestradiol and immunoreactive inhibin, although highe r in the FSH cycles, remained stable throughout the GnRH experimental period in both cycles. These results demonstrate that multiple submaxi mal doses of GnRH can override the attenuating effect of GnSAF on LH s ecretion. From a physiological point of view, this is possibly part of the mechanism which controls the action of GnSAF at mid-cycle.