Serum and ovarian progesterone levels and in vitro production of progesterone by preovulatory follicles were measured on proestrus in pregnant mare's serum gonadotropin (PMSG) primed immature rats in which the luteinizing hormone (LH) surge and ovulation were blocked by administration of the antiandrogen hydroxyflutamide. Serum progesterone levels observed at 12:00 on proestrus were significantly elevated, twofold above those observed in vehicle-treated controls, by in vivo administration of 5 mg hydroxyflutamide 4 h earlier. In control rats, proestrous progesterone did not increase until 16:00, in parallel with rising LH levels of the LH surge. No LH surge occurred in the hydroxyflutamide-treated rats, ovulation was blocked, and serum progesterone declined throughout the afternoon of proestrus, from the elevated levels present at 12:00. Administration of human chorionic gonadotropin (hCG) at 11:00 advanced the elevation of serum progesterone by 2 h in vehicle-treated controls and prevented the decline in progesterone levels in hydroxyfiutamide-treated rats. The patterns of change in ovarian tissue concentrations with time and treatment were essentially similar to those observed for serum progesterone. In in vitro experiments, progesterone secretion during 24 h culture of preovulatory follicles obtained on PMSG-induced proestrus was significantly increased, sixfold, by addition to the culture media of 370 μM but not of 37 μM hydroxyflutamide. Testosterone (50 nM) and hCG (20 mIU/mL) caused 26- and 14-fold increases, respectively, in progesterone secretion by cultured follicles. Hydroxyflutamide significantly reduced the stimulatory effect of testosterone but not of hCG on progesterone secretion in vitro. These results suggest that the antiandrogen hydroxyflutamide stimulates progesterone secretion, both in vivo and in vitro, through an initial androgen-agonistic action, before its antagonistic action is expressed. Its androgen-antagonistic action is responsible for its ability to inhibit testosterone-induced progesterone secretion in vitro. Its failure to reduce hCG-stimulated progesterone secretion in vivo and in vitro indicates that the latter stimulation is exerted independently of, and not as a consequence of, androgen action. The decrease in serum progesterone levels on the afternoon of proestrus therefore appears to be a consequence rather than a cause of the absence of an LH surge in the hydroxyflutamide-treated rats. It is concluded that the inhibitory effect of hydroxyflutamide on the preovulatory LH surge and ovulation is due not to inhibition of progesterone secretion at the ovarian level but most likely to neuroendocrine site(s) of action of the inhibitor.Key words: antiandrogen, hydroxyflutamide, progesterone, luteinizing hormone, ovulation, human chorionic gonadotropin.
The activity and kinetic properties of mevalonate kinase in superovulated rat ovary