The Relative Transfer of Estrogens and their Glucuronides across the Placenta in the Guinea Pig1

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Abstract
The transport of estrogens across the placenta was studied using the following technique: The guinea pig fetus was removed by hysterotomy and the fetal placenta perfused in situ leaving the maternal circulation intact. This eliminated the complicating feature of fetal metabolism. With this technique it was possible to demonstrate the rapid transfer of estriol-16-C14 and the ether-soluble metabolites of estradiol-16-C14 across the placenta in both directions. However, the placenta was relatively impermeable to the water-soluble estrogens found in urine, 80-90% of which are glucuronides. The possible significance of protein-binding and of glucuronide conjugation in transport and excretion is briefly discussed.