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Hepatic microsomal 11β-hydroxysteroid dehydrogenase activity is higher in male than in female rat liver. Gonadectomy on day 25 of life only affects the activity in the adult male animal, causing a decrease towards the normal female level. Administration of testosterone to gonadectomized rats of either sex causes the induction of typical male activity levels. On the basis of these experiments, this enzyme activity may be classified as androgen-dependent. However, 11β-hydroxysteroid dehydrogenase differs from other known androgen-dependent activities in that administration of oestradiol to gonadectomized animals of either sex causes a further significant repression of the activity to levels close to the limits of detection. Hypophysectomy on day 50 of life does not affect the activity in 75 day-old male rats, but causes the appearance of typically male activity levels in females. These results indicate that the hypophysis exerts a repressive influence on hepatic 11β-hydroxysteroid dehydrogenase in female rats. The fact that this activity is not influenced by androgen or oestrogen administration once the pituitary has been removed demonstrates the obligatory role of the hypophysis for sex hormone action.

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