Follicular arrest during the midfollicular phase of the menstrual cycle: a gonadotropin-releasing hormone antagonist imposed follicular-follicular transition.

The functional dependency of the dominant follicle on pulsatile gonadotropin inputs was evaluated by using a GnRH antagonist as a probe. Hormonal dynamics, particularly the relationship of FSH, estradiol, and inhibin, during and after the withdrawal of GnRH receptor blockade achieved by treatment with Nal-Glu GnRH antagonist (50 micrograms/kg, im) for 3 days in the midfollicular phase of the cycle (days 7-9) were ascertained. Daily blood samples were obtained for LH, FSH, estradiol (E2), progesterone, and immunoreactive inhibin (i-INH) measurements by RIA during 2 consecutive (control and treatment) cycles in 12 women. In 5 women, LH pulsatility was assessed by 10-min blood sampling for 12 h before, during, and after Nal-Glu treatment. The administration of Nal-Glu prolonged both follicular phase (14.0 +/- 0.5 vs. 19.7 +/- 0.8 days; P less than 0.0001) and total cycle length (28.1 +/- 0.5 vs. 34.1 +/- 1.2 days; P less than 0.0001). Gonadotropin suppression (50-60%) was achieved, as reflected by a marked decrease in mean LH levels (14.3 +/- 1.9 to 5.4 +/- 0.5; P less than 0.01) and LH pulse amplitude (5.5 +/- 0.7 to 2.4 +/- 0.3 IU/L; P less than 0.01) in response to Nal-Glu antagonist. The number of LH pulses was reduced (36%), but pulses remained discernible. Concentrations of FSH (10.8 +/- 1.4 to 5.9 +/- 0.4 IU/L; P less than 0.05), E2 (322.7 +/- 71.9 to 84.8 +/- 7.7 pmol/L; P less than 0.01) and i-INH (284.0 +/- 25.9 to 164.4 +/- 7.5 U/L; P less than 0.01) decreased concomitantly. Within 24-48 h of the last injection of Nal-Glu, all hormones had returned to pretreatment levels. This was followed by normal functional expression of follicular growth and maturation, as reflected by an increase in E2 and i-INH levels, timely ovulation, and normal luteal function. These findings indicate that an approximately 50% decline in gonadotropin support to the dominant follicle leads to functional arrest, but not demise, of the developing follicle(s) without triggering new folliculogenesis. The follicular apparatus retained its ability to reinitiate its original functionality once appropriate gonadotropin inputs were reinstated.