Metabolism of estrogens in the gastrointestinal tract of swine. I. Instilled estradiol.

One minute after instillation of 14C-estradiol-17 beta (14C-E2 17 beta) into selected sections of the gastrointestinal tract of swine, radioactive estradiol metabolites were present in blood collected from the portal and jugular veins. Ether was used to extract free but not conjugated estrogens. The percentage of plasma radioactivity that was ether extractable (EE) was low in portal plasma and even lower in jugular plasma following instillation of 14C-E2 17 beta into the stomach, ileum and colon. EE radioactivity was not detectable in either portal or jugular plasma when estradiol was instilled into the duodenum or jejunum. Therefore, estrogens were conjugated either in the lumen of the gastrointestinal tract or as they crossed the intestinal mucosa. The liver played only a minor role in conjugation of these steroids, since the estrogen metabolites present in portal plasma were very similar to those in jugular plasma, and metabolites in the urine were similar to those in plasma. The principal estrogen conjugate found in both portal and jugular plasma, regardless of the gastrointestinal section into which 14C-E2 17 beta was instilled, was estrone glucuronide. There was no uniform metabolic pattern observed in the metabolites of estradiol that remained in the lumen of each gastrointestinal section; however, many metabolic transformations occurred. We concluded that almost all estrogens absorbed were metabolized during the absorption process. The liver was active only in the metabolism of estrogens that escaped conjugation in the intestinal mucosa.