Relationship between testosterone binding globulin and the failure of androgens to suppress serum gonadotropin concentrations in long-term castrated adult male rhesus monkeys ( Macaca mulatta)

Abstract

To determine whether the failure of androgen administration to suppress gonadotropin secretion in long-term orchidectomized adult rhesus monkeys could result from increased androgen binding to testosterone binding globulin (TeBG), the effects of castration and androgen administration on TeBG were studied. TeBG binding capacity, expressed as μg dihydrotesterone (DHT) bound/dl, was significantly greater (P < 0.01) in chronically castrated monkeys (7.1 ± 2.0; M ± SD; n = 3) than in intact animals (3.4 ± 0.7; n = 10). Castration, however, did not influence the binding affinity of TeBG for (DHT) at 0°C, the electrophoretic mobility of TeBG in polyacrylamide gels, or the half time of dissociation of the TeBG-DHT complex at 0°C. Three animals orchidectomized 3–8 years earlier were treated with testosterone and dihydrotesiosterone each for 2 months, using Silastic capsules. While these androgen treatments reproduced intact serum androgen concentrations of 9–20 ng/ml, serum LH and FSH levels remained in the castrate range. By contrast, TeBG binding capacity was reduced within 1–2 weeks to levels observed in intact animals. These findings indicate that failure of androgen to suppress the elevated serum LH and FSH concentrations of long-term orchidectomized rhesus monkeys cannot be attributed to its increased binding to circulating TeBG.