Effect of follicle stimulating hormone or human chorionic gonadotrophin treatment on the production of gonadotrophin surge attenuating factor (GnSAF) during the luteal phase of the human menstrual cycle.

Abstract

Although there is much in-vivo evidence for the existence of a gonadotrophin surge attenuating factor (GnSAF), its source and identity remain unknown. We have studied the control of GnSAF production by FSH and hCG during the luteal phase of the cycle. Normally cycling women were investigated in three cycle. Starting on day 5 after the midcycle LH peak, the women received i.m. injections of placebo (1st cycle control), hCG at a dose of 750 IU per day (2nd cycle) and FSH at a dose of 225 IU per day (3rd cycle) for five consecutive days. The response of LH to a single i.v. dose of 10 microg GnRH (GnSAF bioactivity) was investigated several times during the experimental period. Six normally ovulating women with long-standing unexplained infertility were studied. The women were used as their own controls during the cycle treated with placebo. Pituitary response to GnRH was calculated as the net increase in LH at 30 minutes (deltaLH) above the basal value. Serum concentrations of FSH and hCG increased significantly during the second and 3rd cycles respectively. Compared with the control cycles, deltaLH was significantly attenuated as early as 12 hours from the onset of FSH injections. In contrast, basal concentrations of oestradiol (E2) and immunoreactive inhibin started to increase 48 hours after the first injection of FSH, while progesterone values remained similar to those in the controls. During treatment with hCG, no attenuation was seen in deltaLH values, while those of E2, progesterone and inhibin showed a significant increase. These results demonstrate that during the luteal phase of the human menstrual cycle, FSH, but not LH, stimulates the production of gonadotrophin surge attenuating factor. It is suggested that the source of gonadotrophin surge attenuating factor at that stage of the cycle is a cohort of small follicles rather than the corpus luteum.