Administration of an aromatase inhibitor during the late follicular phase of gonadotropin-treated cycles in rhesus monkeys: Effects on follicle development, oocyte maturation, and subsequent luteal function

Local modulation of follicular and gametogenic functions by ovarian androgens and estrogens in mammalian species has been proposed. This study examined the effects of elevated androgen/estrogen ratios during follicular maturation in vivo by inhibiting aromatase activity in rhesus monkeys. To obviate steroid feedback effects, gonadotropin-treated animals were used. Beginning at menses (day 1), animals received human (h) FSH (60 IU/day, im) on days 1-6, followed by hFSH plus hLH (60 IU/day, im) on days 7-9 to promote the growth of multiple follicles. Ovulatory maturation was induced by hCG (1000 IU, im) on day 10. On days 8-10, four animals received an aromatase inhibitor, 1,4,6-androstatrien-3,17-dione (ATD; 1-1.25 g, orally, twice/ day), while five served as controls and received no further treatment. Within 8 h of ATD treatment, a 63% reduction in serum estradiol levels relative to control values was evident, which reached maximal suppression (84%) by day 10. A marked elevation (17-fold) in serum androstenedione and a lesser increase (2.6-fold) in serum testosterone occurred with aromatase inhibition, yielding androstenedione/estradiol (18.0) and testosterone/estradiol (1.9) ratios greater than those in controls (0.6 and 0.3, respectively). ATD treatment did not alter follicular diameters or the total number of follicles per animal (20 ± 3) relative to control values (16 ± 3). Of the total cohort classified, the proportion of oocytes collected at prophase I was greater (P < 0.05) after ATD treatment (31%) than in controls (11%). Completion of oocyte meiosis to metaphase II was retarded (P < 0.05) in ATD-treated (4%) compared to control (26%) animals. Furthermore, the in vitro fertilization rate of metaphase II oocytes from ATD-treated animals (9%) was reduced (P < 0.05) relative to that in controls (25%). While basal progesterone production by luteinizing granulosa cells in vitro was similar between groups, the addition of hCG in vitro enhanced progesterone secretion by cells from ATD-treated animals (3.1 ±0.3-fold over basal) to a greater extent (P = 0.05) than in controls (1.5 ± 0.3-fold). Progesterone receptor was detected by immunocytochemistry in nuclei of luteinizing granulosa cells from ATD-treated animals as well as controls. Serum progesterone profiles and the length of the luteal phase were similar between groups. Thus, acute elevation of serum androgen/estrogen ratios in vivo during follicular maturation was detrimental to the gametogenic functions of the primate follicle, but did not alter follicular growth, events of early luteinization, or subsequent luteal function. Increasing levels of circulating estradiol during the late follicular phase are not required to sustain follicular maturation or granulosa cell responsiveness to an ovulatory stimulus. Whether abrogation of oocyte nuclear maturation after aromatase inhibition in vivo is due to androgen excess and/or a reduction in estradiol remains to be determined.