We recently identified and characterized, by RIA of bound testosterone (T), an androgen receptor in nuclei isolated from seminiferous tubules of the male rat. By use of similar methods, we now report the presence of a similar androgen receptor in salt extracts of late spermatids from stage 12 on, which were prepared by sonication of homogenized testes or mechanically dissected seminiferous tubules and isolated on a discontinuous sucrose density gradient. Purity of the spermatid preparation was approximately 99%, as established by light microscopy. The major component of a 0.4 M KC1 extract of spermatid nuclei migrated at 4.0S on a glycerol density gradient and coeluted on hydroxylapatite with a salt extract of ventral prostatic nuclei radioactively labeled with 5α-[3H]dihydrotestosterone. In both isolation systems, there were additional minor components in the spermatid macromolecular complex, as compared to the prostatic androgen receptor. A comparison was made between the bound T concentration in late spermatid nuclei (4.3 ± 1.0 pg/mg DNA) and that in the total nuclear preparation of seminiferous tubules (30.1 ± 14.2 pg/mg DNA). This 7-fold difference in receptor concentration indicates that earlier germinal elements (spermatogonia through stage 11 spermatids) must contain higher concentrations of androgen receptors than do later germinal cells, which represent approximately 60% of the total germ cell population. These data, which were collected by RIA (due to the lack of a functional nuclear exchange assay in germ cells), are the first conclusive demonstration of an androgen receptor in male germ nuclei. The results support the concept that spermatogenesis is regulated, in part, by the action of receptor-bound T in the germ cells.
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