[The enterohepatic circulation of estradiol and ethinylestradiol in the rat].

After intraduodenal administration of 6,7-3H-estradiol to rats about 56% of the radioactive dose is excreted in the bile within 4.5 h. The bulk of this radioactivity is confined to glucuronide conjugates. By using different methods it is shown that 12-16% of the administered estradiol dose is excreted in bile as glucuronides of estrone and estradiol. Similarly, after i.v. administration of 6,7-3H-ethinylestradiol 12% of this dose appears in bile of rats as ethinylestradiolgucuronide. Thes conjugates of the biologically active estrogens (estrone, estradiol, ethinylestradiol), upon removal of the glucuronic acid moiety by action of gut bacterial enzymes, may make available the active estrogenic compounds to the enterohepatic circulation. Therefore, enterohepatic circulation of estrogens may be of some biological importance in the rat.