Significant Role of 5α‐Reductase on Feedback Effects of Androgen in Rat Anterior Pituitary Cells Demonstrated With a Nonsteroidal 5α‐Reductase Inhibitor ONO‐3805

Abstract

ABSTRACT: A 5α‐reductase inhibitor with nonsteroidal structure, ONO‐3805, has been used to study the role of 5α‐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 5α‐dihydrotestosterone (DHT) was evaluated. The activity of 5α‐reductase in rat anterior pituitary gland was approximately one‐fourth of that in rat prostate gland or epididymis, with a Michaelis' constant (Km) value for testosterone of 5–6 × 10−7 M. When homogenates of rat anterior pituitary glands were incubated with 14C‐testosterone (14C‐T) in the presence of ≥ 10−7 M ONO‐3805, the formation of labeled DHT and its metabolite 5α‐androstane‐3α, 17β‐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 × 10−11 M. Dispersed pituitary cells (1–2 × 105/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 ≥ 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 5α‐reductase on both positive and negative feedback effects of androgen in the rat anterior pituitary gland.