Hormonal regulation of androgen-binding protein in hypophysectomized rats

We have recently reported that testosterone, glycerol, and sulfhydryl reagents must be added to the homogenizing medium in order to accurately determine androgen binding protein (ABP) in tissue samples (J Biol Chem 253: 166, 1978). These stabilizing conditions have been utilized to measure ABP in hypophysectomized rats after chronic treatment (1–4 days) with testosterone proprionate or highly purified FSH. The purpose of these experiments was to determine which hormone is primarily responsible for ABP production and transport. Administration of testosterone in vivo seemed to result in an elevation of testicular ABP binding from 0.04 to 0.21 pmol/mg protein when tissue was homogenized in Tris-EDTA buffer. However, this higher value was also obtained by homogenizing testes from control rats under stabilizing conditions (plus testosterone). Addition of testosterone to the homogenizing medium of testosterone-injected rats resulted in no further increase in ABP activity. Purified FSH resulted in a similar value of 0.25 pmol ABP/mg protein when testes were homogenized in the absence of testosterone. Including testosterone in vitro revealed a 4-fold increase in ABP activity per mg protein due to FSH treatment. Identical values were obtained in testes from rats injected with both FSH and testosterone. When rats were treated with testosterone plus increasing amounts of FSH, a dose response was observed, and ABP activity in the epididymis increased linearly over a one-log concentration of FSH. Moreover, treatment with highly purified FSH (Papkoff G4-150C) resulted in a large increase in ABP activity in both testis and epididymis (14-fold and 19-fold per organ, respectively, at 4 days), whereas no testosterone was detectable in the testis under our assay conditions (<0.03 pmol/testis). Testosterone proprionate treatment only produced a 3-fold increase of ABP activity in the testis, whereas no activity was measurable in the epididymis. These data demonstrate that the combination of hormones in vivo had neither an additive nor synergistic effect on ABP activity under our experimental conditions. Rather, purified FSH, in the absence of testosterone, will stimulate both testicular levels of ABP and its transport to the epididymis. Testosterone only stabilizes ABP activity in the testis whether administered in vivo or added in vitro before homogenization.