Relative androgenic and myotropic activity plots of 19-nortestosterone

Abstract

In order to determine the androgenic and myotropic activities of 19-nortestosterone relative to those of testosterone, a graphical analysis of the response of RNA/DNA ratio in the seminal vesicles and levator ani muscle of adult male castrated rats to various doses of the phenylpropionate esters of these steroids was employed. The relative androgenic and myotropic activity plots obtained allowed the determination of the relative androgenic ( RAA N ) and myotropic ( RMA N ) activities of nortestosterone independently of the steroid doses applied for treatment. The results show that (I) RAA N value correlates positively with the duration of the presence of measurable steroid levels (i.e. testosterone and 19-nortestosterone) in the blood plasma, while RMA N does not; (II) The decrease of the myotropic/androgenic index ( RMA N / RAA N ) with more prolonged plasma levels of steroids can entirely be ascribed to the increase of RMA N values; (III) When either the androgenic or the myotropic activity is regarded, there is no difference between the maximal responses to nortestosterone and testosterone. The findings confirm the close correlation between receptor binding and biological effect of nortestosterone and suggest an essential role of 5α-reductive steroid metabolism in the mechanism of myotropic-androgenic dissociation characteristic of the effect of 19-nortestosterone as well as other anabolic steroids.