Differential effects of single and repeated administration of gonadotropins on testosterone production and steroidogenic enzymes in Leydig cell populations.

ment with 2.5 or 5 IU of hCG/ day for 6 days (1.5- and 1.3-fold, respectively). The activity of several key steroidogenic enzymes was measured following treatment with 5 IU of hCG/day for 6 days. A5-3/3-Hydroxysteroid dehydrogenase-isomerase activity and 17a-hydroxylase activity both increased approximately 2-fold in all three Leydig cell populations following hCG treatment, while Cl7-Cz0 lyase activity increased 4.4-fold in population IA, 2.7-fold in population IB, and 1.8-fold in popdation 11. The 17-ketosteroid reductase activity did not change, except in population I1 in which it decreased by 63%. 5a-Reductase activity similarly did not change except in population IA in which it decreased by 77%. There were no changes in apparent K,,, values following injection of 5 IU of hCG/day with the exception of a 2.6fold increase in the apparent K,,, of 17a-hydroxylase in population IA (0.017 to 0.045 p~). Seventy-two hours after a single injection of 150 pg of luteinizing hormone, 50 IU of hCG, or 100 IU of hCG, there were marked decreases in the activities of 17a-hydroxylase and CI.I-C~O lyase in all three Leydig cell populations. A5-3/3Hydroxysteroid dehydrogenase-isomerase activity in