Gonadotropic Regulation of Aromatase Activity in the Adult Rat Testis

Publisher Website
Google Scholar
Abstract
Aromatase activity during gonadotropin action in vivo and in vitro was examined in purified Leydig cells to further define the early effects of LH [luteinizing hormone] and for elucidation of the enzymatic processes involved in the development of late lesions of androgen biosynthetic pathway. Aromatase was measured by the tritiated water release method using [1.beta.-3H]testosterone as substrate. Enzyme activity was proportional to the amount of cells (0.1-1.0 .times. 106) incubated, increased with the incubation time (2-60 min), was inhibited by androstatriendione (ED50, 5.0 .mu.M), and showed a Km for testosterone of 1.69 .mu.M. Aromatase activity was stimulated (10-20%; P < 0.05) 1 h after treatment of rats with a single s.c. dose of 5 .mu.g hCG [human chorionic gonadotropin]. This activation preceded the late steroidogenic lesion at the site of 17.alpha.-hydroxylase and 17,20-desmolase activity by 2-5 h. A radioimmunoassay of improved sensitivity (0.5 pg) was developed to detect the very low cellular and secretory levels of estradiol. The testicular contents of testosterone and estradiol showed small increases (P < 0.05) within 40 and 60 min after hCG treatment, respectively. Testicular testosterone levels reached a peak by 1 h after injection and preceded the peak level of estradiol formation by 2 h. After in vitro treatment of cultured Leydig cells with 100 ng hCG, the aromatase activity was significantly increased within 30 min (P < 0.05) and then returned to control levels for up to 16 h of culture. A similar temporal pattern for enzyme activation was observed after treatment of cultures with 8-bromo-cAMP or forskolin (23-27% above control; P < 0.05), while cholera toxin stimulated aromatase activity at 2 h. Net testosterone accumulation in the incubation medium increased 30 min after the hCG treatment and reached a plateau by 4 h. A small but significant increase in estradiol levels (P < 0.05) was also observed at 30 min, remaining constant until 120 min, which was followed by a sharp rise parallel to that of testosterone. Apparently, the estradiol-mediated desensitization of the Leydig cell observed after hCG administration is consistent with an early cAMP-dependent activation of aromatase and a further rise in estradiol formation due to increased substrate availability.

This publication has 21 references indexed in Scilit: