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

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.