Comparison between the binding of 19-nortestosterone, 5alpha-dihydrotestosterone and testosterone in rat prostate and bulbocavernosus/levator ani muscle.

Specific binding of (3H)19-nortestosterone in the 100000 g cytosol of the rat bulbocavernosus/levator ani muscle (BCLA) and prostate was demonstrated by agargel electrophoresis at low temperature and compared qualitatively and quantitatively with the binding of tritiated testosterone and 5alpha-dihydrotestosterone (5alpha-DHT). Both tissues showed a greater binding affinity for 5alpha-DHT than for 19-nortestosterone, with testosterone binding the least well of the three. The relative binding affinities in the BCLA and prostate were: 19-nortestosterone:testosterone=1-4, 1.-nortestosterone: 5alpha-DHT = 0.7 The differences were statistically significant (P less than 0-02). The concentrations of receptor sites for 5alpha-DHT were 171 +/- 20 (S.D.) fmol/mg prostatic cytosol protein and 24 +/- 4 (S.D.) fmol/mg BCLA cytosol protein. The in-vitro metabolism of the three steroids in both tissues was aldo investigated by thin-layer chromatography. After incubating for 2 h at 0 degrees C the prostate was shown to reduce 26% of the 5alpha-DHT to androstanediols whilst the BCLA showed a 5% conversion. Testosterone was converted by the prostate to 5alpha-DHT (10%) and the androstanediols (6%) whilst the BCLA showed little activity in this respect. Comparing these in-vitro data with in-vivo findings from the literature, in both organs there is a positive correlation of the extent of binding in vitro to the stimulation of growth in vivo, bearing in mind that testosterone is metabolized to 5alpha-DHT in the prostate whilst in the BCLA, 5alpha-reductase is essentially absent.