A series of studies measured endogenous androgen concentrations by radioimmunoassay within nuclei prepared from seminiferous tubules of the adult male rat. Testosterone was the principal androgen in the nuclei and occurred in high concentration (0.3 ng/mg DNA). This concentration was maintained even after substantial purification of the nuclei, indicating that the concentrations of testosterone measured were not the result of cytoplasmic contamination. Testosterone was actively retained in these nuclei following hypophysectomy of the animal: nuclear testosterone concentration fell to 20% of that in intact animals, while serum testosterone concentration fell to 3%. About 70% of the testosterone within the nuclei could be extracted with buffers containing KCl. Data from titration of testosterone in the salt extract of nuclei and in their residue generated a regression line which passed through the origin. Initial observations showed that salt-extracted testosterone could be separated into bound and free fractions by chromatography and that the bound portion was approx. 55% of total. It is suggested that this methodology is appropriate for the study of the relation of androgen binding to spermatogenesis in the seminiferous tubules of normal animals.
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