AROMATASE AND TESTOSTERONE RECEPTOR IN THE LIVER OF THE FEMALE GREEN FROG, RANA-ESCULENTA

In the green frog, Rana esculenta, a peculiar feature of female reprod uctive endocrinology is an high level of circulating testosterone. Alt hough several hypotheses have been set out to explain this phenomenon, the testosterone specific roles in female anuran have not been yet fu lly explored. This study results propose a testosterone implication in liver vitellogenin synthesis control, since in ovariectomized frogs t he hormone induces an increase of circulating vitellogenin. The testos terone action could depend on its local conversion to 17 beta-estradio l by aromatase which is present in frog liver tissue. Liver aromatase activity ranges from 7.5 to 26 fmoles E-2 formed/mg protein/h and resu lts higher as long as liver is engaged in vitellogenin synthesis. Arom atase activity seems depend on testosterone since it decreases after o variectomy and is restored by testosterone injection in ovariectomized frogs. In green frog liver, testosterone binding molecules are presen t both in cytosol and nuclei. These molecule binding properties (K-d a nd B-max in nM range; t 1/2 = 85 min; specificity) are in line with th ose of testosterone receptor of other lower vertebrate target tissue. In liver nuclei, testosterone receptor level undergoes modification th roughout the sexual cycle which almost coincides with that of plasma t estosterone level and liver aromatase activity. This could indicate th at the testosterone induction of liver aromatase in frogs is via the t estosterone receptor, as reported for aromatase of mammalian brain tis sues.