Effects of the antiprogesterone RU486 in the early follicular phase of the menstrual cycle.

To investigate the role of progesterone (P) in the early follicular phase, the antiprogesterone effect of RU486 was examined in five normally cycling women monitored by daily hormonal levels during three consecutive cycles (control, treatment, and recovery). In addition, luteinizing hormone (LH) pulse characteristics were assessed by frequent blood sampling (10 minutes for 10 hours) on day 3 of the control and the corresponding day of treatment cycles. Administration of RU486 (3 mg/kg, orally) for the first 3 days of the menstrual cycle did not significantly alter the length of the follicular phase (13.4 +/- 1.7 to 15.2 +/- 1.3 days), the LH surge, or the luteal phase length (12.2 +/- 0.5 to 12.6 +/- 0.7 days). The intermenstrual length of the treatment cycle (29.8 +/- 1.9 days) did not differ from the control (27.6 +/- 1.8 days) or recovery cycles (29.6 +/- 2.5 days). Integrated secretion of P and estradiol (E2) did not vary during the luteal phase of the control, treatment, or recovery cycles. During RU486 treatment, LH pulse frequency, pulse amplitude, and mean LH were not altered. Whereas mean E2 levels were significantly decreased from 150.5 +/- 15.1 to 110.1 +/- 7.0 pmol/L, follicle-stimulating hormone, P, adrenocorticotropin hormone, and cortisol were not significantly altered. Thus, in spite of the transient decrement in E2 secretion during RU486 treatment, the integrity of the ovulatory menstrual cycle was maintained. We conclude that administration of the antiprogesterone RU486 at the dose used during the first 3 days of the follicular phase does not perturb menstrual cyclicity.