Mechanisms of physiological and pharmacological sex hormone action on the mammalian liver.

Androgen and oestrogen receptors have been demonstrated in mammalian liver, but since it is generally accepted that they are probably non-functional at endogenous steroid concentrations, it is not apparent how they mediate physiological influences on this organ. Nor is it certain to what extent pharmacological actions of sex hormones reflect overstimulation of physiological routes or whether alternative mechanisms become available once threshold values have been reached. In this presentation an attempt has been made to answer some of these questions using data obtained from a study of the regulation of the activities of microsomal 3 alpha-hydroxysteroid dehydrogenase (3 alpha-HSDH) and 5 alpha-reductase in rat liver. Androgens exert their primary physiological and pharmacological influences at the level of the hypothalamus. Oestrogens can elicit three different types of effect-physiological, antiandrogenic and pharmacological--of which the first two involve primary effects on the pituitary. Hepatic oestrogen receptors only become activated when oestrogen concentrations reach pharmacological levels. Progestins probably have no physiological influence on the livers of non-pregnant rats. Their pharmacological actions may either be traced back to secondary androgenic (e.g. medroxyprogesterone acetate, levonorgestrel) or oestrogenic (e.g. norethynodrel, lynestrenol) properties, involving the routes described above, or to independent effects on the central nervous system (e.g. cyproterone acetate modulation of 5 alpha-reductase activity).