Ovarian responses in macaques to pulsatile infusion of follicle-stimulating hormone (FSH) and luteinizing hormone: increased sensitivity of the maturing follicle to FSH.

This study investigated the relationship between plasma gonadotropin concentrations and the initiation and maintenance of preovulatory follicular growth in macaques. Eight adult cynomolgus monkeys were treated with a GnRH antagonist [AcD2Nal1-4ClDPhe2, DTrp3, DArg6, DAla10]GnRH X HOAc to block endogenous gonadotropin secretion. In four animals, a pulsatile infusion of human FSH and human LH (hLH) (one 3-min pulse/h) was initiated, and the amount of hFSH delivered per pulse was increased every 3-4 days until serum estradiol concentrations rose. Thereafter, the amount of FSH delivered per pulse was reduced by 12.5%/day for 5 days, whereas the amount of LH delivered per pulse was not altered. Results indicated that plasma FSH concentrations in the range of 15-20 mIU/ml were associated with the initiation of estrogen production; in addition, a progressive reduction in plasma FSH concentration to 8-10 mI/ml over the subsequent 5 days was accompanied by continued rises in estradiol concentrations and preovulatory follicular growth. In contrast, in four control animals, maintenance of plasma FSH concentrations at 8-10 mIU/ml for 13 days did not result in elevation in serum estradiol concentrations or antral follicular growth. These observations demonstrate that after stimulation by elevated FSH concentration, follicles can continue to mature in the presence of FSH concentrations which are unable to support the growth of less mature follicles. Thus, this may account for the mechanisms by which the maturing follicle continues to develop during the mid-through late follicular phase of the menstrual cycle, whereas other less mature follicles undergo atresia.