Progesterone and 17α-hydroxyprogesterone advance the estrogen-induced bioassayable luteinizing hormone surge in castrate monkeys

Recent studies have demonstrated marked disparities between bioassayable (BIO) versus radioimmunoassayable concentrations of luteinizing hormone (LH) at midcyle. Other studies in intact women and monkeys have indicated that progesterone (P) may facilitate estrogen induction of the preovulatory gonadotropin surges. We have combined the study of these observations as follows: long-term castrate female monkeys were given estradiol benzoate with and without subsequent P or 17α-hydroxyprogesterone (17-OHP) injections. Plasma was collected during a selected 24-hour interval via a chronic indwelling femoral vein cannula. Initially, estrogen-negative feedback decreased the pulsatility and circulating levels of gonadotropins. P and 17-OHP treatment (after estradiol) hastened initiation of the BIO-LH surge by up to 8 hours. Although estrogen-positive feedback alone enhanced the biologic activity of LH, P and 17-OHP expedited the midcycle-like BIO-LH surges.