SIGNIFICANT ROLE OF 5-ALPHA-REDUCTASE ON FEEDBACK EFFECTS OF ANDROGEN IN RAT ANTERIOR-PITUITARY-CELLS DEMONSTRATED WITH A NONSTEROIDAL 5-ALPHA-REDUCTASE INHIBITOR ONO-3805

A 5 alpha-reductase inhibitor with nonsteroidal structure, ONO-3805, has been used to study the role of 5 alpha-reductase on positive and negative feedback effects of androgen on gonadotropin secretion in the rat anterior pituitary gland. Initially, the potential of ONO-3805 to inhibit the formation of 5 alpha-dihydrotestosterone (DHT) was evaluated. The activity of 5 alpha-reductase in rat anterior pituitary gland was approximately one-fourth of that in rat prostate gland or epididymis, with a Michaelis' constant (K-m) value for testosterone of 5-6 x 10(-7) M. When homogenates of rat anterior pituitary glands were incubated with C-14-testosterone (C-14-T) in the presence of greater than or equal to 10(-7) M ONO-3805, the formation of labeled DHT and its metabolite 5 alpha-androstane-3 alpha, 17 beta-diol was inhibited by > 90%. The inhibition pattern was non-competitive, and the inhibition constant (Ki value) derived from Lineweaver-Burk plots was 3.9 x 10(-11) M. Dispersed pituitary cells (1-2 x 10(5)/ml) were cultured for 48 hours and then further incubated with or without androgen and/or the inhibitor for 72 hours (basal secretion). After this incubation, the media were saved, and 10 nM of luteinizing hormone-releasing hormone (LH-RH) with the same concentration of androgen and/or the inhibitor as used in the 72-hour incubation was added to the cultures for 6 hours (LH-RH-induced secretion). Secreted follicle-stimulating hormone (FSH) and luteinizing hormone (LH) were assayed by radioimmunoassay (RIA). Both testosterone (T) and DHT stimulated 72-hour basal FSH secretion from cultured pituitary cells in a dose-dependent fashion. T was less effective than DHT. The addition of inhibitor at > 10(-9) M concentration significantly reduced the stimulating effect of T on basal FSH secretion. Basal LH secretion was not affected by either T or DHT with or without the inhibitor. In the presence of 10 nM LH-RH, 6-hour secretion of both FSH and LH was significantly reduced by the addition of T or DHT at a concentration of greater than or equal to 100 pg/ml. T (10 ng/ml)-induced suppression of both FSH and LH was significantly but not completely returned by the addition of 10(-7) M Of ONO-3805. These results strongly suggest a significant role of 5 alpha-reductase on both positive and negative feedback effects of androgen in the rat anterior pituitary gland.