The interaction of hCG, hydroxysteroids and interstitial fluid on rat Leydig cell steroidogenesis in vitro.

Rat testicular interstitial fluid and hydroxycholesterol both stimulated testosterone production by isolated Leydig cells in vitro in a dose-dependent manner, but the dose-response lines were not parallel. The addition of cycloheximide blocked the stimulation by interstitial fluid but not that of hydroxycholesterol. Use of the compounds SU 10603 and cyanoketone (which inhibit 3 beta-hydroxysteroid dehydrogenase and 17 alpha-hydroxylase respectively) or aminoglutethimide (which acts on the cholesterol side-chain cleavage enzyme) showed that the stimulatory factor(s) in interstitial fluid stimulated steroidogenesis at the cholesterol side-chain cleavage enzyme, before the conversion of pregnenolone. This enzyme is rate-limiting in the synthesis of testosterone by Leydig cells and a site of action of LH; therefore, these results support the view that an interstitial fluid factor may be involved in the paracrine regulation of testicular steroidogenesis.