Transfer of Estriol and Estriol Conjugates Across the Human Placenta Perfusedin Situat Midpregnancy1

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Abstract
Following the delivery of fetuses at laparotomy (18–20 weeks' gestation) the placentas were perfused in situ with labeled estriol (E3), estriol-3-sulfate (E3-3S) and estriol-16-glucosiduronate (E3-16G). In each experiment the placenta was perfused for a single cycle with 2 forms of E3, one labeled with 3H and the other with 14C, and the radioactive material present in the maternal urine, placenta and perfusate was identified and quantitated. Twenty-three to 38% of the radioactivity perfused as E3 was recovered from the mother's urine during the subsequent 12 hr. The corresponding figures for E3-3S and E3-16G were less than 3%. In 2 experiments in which 3H-E3-3S and l4C-E3-16G were compared directly, the percentage of 3H transferred was 2–3 times greater than 14C. A significant part of the E3-3S, but little if any of E3-16G, was converted by the placentas into E3. This provides an additional mechanism for the disposal of the sulfate not available to the glucosiduronate. Because of differences in specific activity, larger amounts of the glucosiduronate were transferred. Interpretation of these observations in terms of relative rates of transfer of the 2 conjugates is not possible because of inadequate information concerning the endogenous levels of the 2 conjugates and the saturation characteristics of the transport mechanisms. A concept is presented describing the metabolism and transfer of estriol and its conjugates in the fetal-placental unit at midpregnancy.