Interactions of Testosterone and Estradiol-17β on the Reproductive Tract of the Male Rat1

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Abstract
The effects of subcutaneous sustained-release implants of testosterone and estradiol-17β, given either alone or in combination to adult male rats, on the weights of testis and sex accessory tissues, the testicular content of spermatids and spermatozoa and the serum concentrations of testosterone, estradiol-17β and gonadotropins were investigated. Increasing amounts of testosterone, in the absence of added estradiol, caused a biphasic response (decline followed by a partial recovery) in testicular weight and in the number of spermatids and spermatozoa/testis. Serum testosterone initially remained unchanged and subsequently rose. This pattern was reflected by the weights of sex accessory tissues. Before serum testosterone rose, serum LH decreased to undetectable levels. Serum estradiol-17β levels were not affected by increasing doses of testosterone. Low doses of estradiol-17β (0.1 cm and 0.3 cm implants), in the absence of added testosterone, had no significant effect on serum estradiol-17β levels, on testicular weights or on the testicular content of spermatids and spermatozoa, but did cause a fall in serum LH and testosterone levels and in the weights of sex accessory tissues. Higher doses of estradiol-17β resulted in increased serum estradiol-17β levels and either a decrease or a sustained low level for all other measured parameters (weights of testes and sex accessory tissues and testicular content of spermatozoa and spermatids). Depending on the doses, the combinations of testosterone and estradiol-17β resulted in 2 types of interaction: 1) at low doses, these 2 steroids acted synergistically to decrease testicular content of spermatids and spermatozoa and testicular weights but had little or no effect on serum testosterone and estradiol-17β or weights of sex accessory tissues; 2) at higher doses, there was an apparent direct antagonism between these 2 compounds on all tissue components measured.