Effects of estradiol administration in vivo on testosterone production in two populations of rat leydig cells

The effects of in vivo administration of estradiol on isolated rat testicular Leydig cells were investigated. Adult intact rats were injected s.c. with 50 μg/ 100 g B.W. of 17β-estradiol or vehicle twice daily for 2 days. Twelve hours after the last injection, collagenase dispersed interstitial cells were obtained and Leydig cells were subsequently isolated on metrizamide gradients. Two distinct peaks of specific ¹²⁵I-hCG binding corresponding to population I and II Leydig cells were observed. The hCG binding profile was unaltered as a result of estradiol treatment. Although twice as much testosterone was produced in population II, the responsiveness of the two populations to hCG or dbcAMP in vitro was identical (11-to 13-fold increase). Estradiol administration in vivo resulted in a 33-48% decrease in basal and stimulated testosterone production in both populations. These data indicate for the first time that both population I and II Leydig cells are sensitive to the direct inhibitory effects of estrogens on testosterone production. This inhibitory effect was not associated with an alteration in hCG binding capacity in either population. Therefore, we conclude that no functional difference exists between the two populations of Leydig cells with respect to the action of estrogens.