Measurement of steroidogenesis in rodent Leydig cells: a comparison between pregnenolone and testosterone production.

The efficiency and specificity of inhibition of pregnenolone metabolism in mature, immature rat Leydig cells, mouse and tumour Leydig cells by SU-10603, a 17 alpha-hydroxylase inhibitor and epostane (WIN-32729), a 3 beta-hydroxysteroid dehydrogenase inhibitor, were studied. Metabolism of [14C]pregnenolone by mature rat Leydig cells was inhibited for more than 95% in the presence of 20 microM SU-10603 and 5 microM epostane. The sum of the different steroids produced by Leydig cells from immature rats incubated in the presence of a 5 alpha-reductase inhibitor was only 80% of pregnenolone production in the presence of SU-10603 and epostane. Pregnenolone metabolism could also be inhibited in tumour Leydig cells but not in mouse Leydig cells. Pregnenolone and testosterone production by Leydig cells from mature rats were similar when steroidogenesis is maximally stimulated by luteinizing hormone (LH). However, in the presence of LH and bovine serum albumin (bSA), or 22 R-hydroxycholesterol and bSA, pregnenolone production was 1.7- and 6-fold higher respectively, than testosterone production. The data show that for measuring the steroidogenic activity of Leydig cells estimation of pregnenolone production is more reliable than measuring testosterone production. At high activities of the cholesterol side-chain cleavage (CSCC) the conversion of pregnenolone into testosterone may become the rate-limiting step for testosterone production. Under all conditions the conversion of cholesterol into pregnenolone is the (hormonal regulated) rate-determining step for steroidogenesis.