TESTOSTERONE AUTOREGULATION OF ITS BIOSYNTHESIS IN THE RAT TESTIS - INHIBITION OF 17-ALPHA-HYDROXYLASE ACTIVITY

Using the in vitro perfused rat testis, the effects of testosterone (T ) on its own biosynthesis, and in particular on the inhibition of spec ific steroidogenic step(s) in the biosynthetic pathway from cholestero l to T, were examined. Rat testes perfused in vitro for 1 hour with me dium containing 1.5 mu M T secreted significantly less T than control testes in response to physiologic or maximal luteinizing hormone (LH) stimulation. To locate the site(s) of this rapid inhibition, the effec ts of arterial T infusion on steroidogenesis by testes also infused wi th pregnenolone (PREG), progesterone (FROG), 17 alpha-hydroxyprogester one (17-PROG), or androstenedione (ADIONE) were measured by summing al l the possible reaction products from each substrate. This approach al lowed us to examine the effect of T in situ on the reactions: LH-stimu lated PREG secretion; PREG to FROG; FROG to 17-PROG; 17-FROG to ADIONE ; and ADIONE to T. Only FROG to 17-FROG (17 alpha-hydroxylase activity ) was inhibited by arterial T infusion. A kinetic examination of the F ROG to 17-PROG reaction demonstrated that the specific inhibition by T was competitive. The apparent k(m) for FROG in this system was 16.0 m u M, whereas the apparent k(i) of T was 1.6 mu M, indicating a relativ ely high degree of sensitivity of the reaction to T. Taken together, t hese data confirm that T is able to regulate its own synthesis and ind icate that this autoregulation is the result of rapid, specific inhibi tion by T of 17 alpha-hydroxylase activity.