The distribution of zinc between the mother and the fetoplacental unit, and its placental transfer, were studied using stable and isotopic zinc in unanaesthetized pregnant guinea pigs and an in situ perfusion preparation. The concentration of stable zinc in fetal plasma and skeletal muscle was higher than that in the maternal tissues: 2.0 compared with 1.4 μg/ml and 84 with 49 ng/mg dry weight, respectively. The placenta and maternal and fetal liver had similar zinc concentrations: 90, 75 and 88 ng/mg dry weight, respectively. The ability of the placenta to concentrate ⁶⁵Zn, measured 1 h after a single intravenous injection into the unanaesthetized mother, was comparable with that of the maternal liver. Maternal-fetal mass transfer of zinc was directly related to maternal plasma zinc concentrations from 0.7 to 24.1 μg/ml (b = 2 ng·min^––1·g^––1·μg^––1, r = 0.92). At physiological plasma levels, the calculated transfer would supply the fetus with 0.12 mg zinc/day, similar to the accretion rate over the last trimester. Placental transfer of zinc was not influenced by the concentration of zinc in the placental perfusate. Extraction of zinc from the perfusate was also slow, and partly by absorption. Matemofetal transfer of zinc was directly related to both uterine and umbilical blood flows. The high concentration of zinc in the syncytium, relative to both maternal and fetal plasma levels, suggests active uptake at the maternal surface, combined with a slow release into the fetus, down a concentration gradient.

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