Attenuating activity of the ovary on LH response to GnRH during the follicular phase of the cycle.

OBJECTIVE

Oestradiol sensitizes the pituitary to GnRH, while gonadotrophin surge attenuating factor (GnSAF) may oppose this action. Using the LH response to GnRH during treatment with FSH as an in vivo bioassay for GnSAF, we tested the hypothesis that the augmented LH response to GnRH in the late follicular phase is related to reduced production of GnSAF from the ovulatory follicle.

DESIGN

Prospective intervention study.

PATIENTS

Ten healthy, normally cycling women.

MEASUREMENTS

The LH response to 10 μg GnRH i.v. (ΔLH) was investigated on days 2 and 3 and on days v (follicle size 16-17 mm) and v + 1 of cycle 1 (control) and cycle 2. On days 2 and v, a single s.c. injection of either normal saline (cycle 1) or 450 IU recombinant FSH (cycle 2) was given after the end of the GnRH experiment.

RESULTS

FSH injection increased both serum oestradiol and inhibin B. In cycle 1, ΔLH remained unchanged from days 2 to 3 but increased significantly from days v to v + 1. In contrast, in cycle 2, ΔLH decreased significantly from days 2 to 3 (P < 0·05) and showed a nonsignificant increase from day v to day v + 1. The percentage difference in ΔLH between cycle 1 and cycle 2 was similar on days 3 (-66·9 ± 17·5%) and v + 1 (-65·2 ± 3·6%).

CONCLUSIONS

These results suggest that during the follicular phase of the menstrual cycle, GnSAF is produced by small antral follicles, while the contribution of the ovulatory follicle is minimal.