Effects of 17 alpha-hydroxyprogesterone on luteinizing hormone release in the rat.

Abstract

Our previous study, in which we measured serum17α-hydroxyprogesterone (17-OHP) every 8hr during the periovulatory phase in women, suggested that17-OHP may have a physiological role in inducing and/or facilitating LH release. Therefore, this study was designed to elucidate the possible role of 17-0HP in LH release.Experiment 1: Three weeks after ovariectomy, rats were injected subcutaneously with 20 fig of estradiol benzoate (EB) at 1200hr (day0). The administration of 2mg of progesterone (P) or 17-OHP was performed systemically at 1200hr on day3. The injection of P elicited a significant increase of serum LH 6hr later as compared to the serum levels in the control group (p<0.05), but the injection of 17-OHP failed to alter serum LH significantly.Experiment 2: Three weeks after ovariectomy, rats were injected with EB at 1200 hr. Crystalline P or17-OHP was inserted through the outer cannula into the brain at1200hr on day3. All animals bearing17-0HP implants or P implants in the diagonal band of Broca (DBB) exhibited an increase in serum LH6hr after implantation. In regard to implants in the medial preoptic area (MPO), all rats with implants of 17-OHP and 4 of 7 with implants of P exhibited an increase in serum LH, whereas both17-0HP and P implantations in the septum failed to induce an increase in serum LH.Experiment3: All rats were injected subcutaneously with 20 fig of estradiol (E2) rather than with EB. Vaginal cornification was observed in 100% at 1200hr on day 3 after the injection of EB, but in only5.5% at 1200hr on day 3 after the injections of E2. In rats with implants in the DBB or the MPO, no significant increase of serum LH was observed after implantation of 17-OHP or P.These results suggest that the sites of stimulatory feedback action of 17-OHP on LH release are similar to those of P and that the stimulatory effects of 17-OHP are synergistic with estrogen in the induction of the LH surge.Results from the present study, in conjunction with our previous observation, strongly suggest a physiological role for 17-OHP in inducing the midcycle LH surge in women.