Zinc levels in the rat fetal liver are not determined by transport across the placental microvillar membrane or the fetal liver plasma membrane.

During pregnancy, zinc (Zn) levels in the rat fetal liver increase markedly. Why they do so or how the increase is regulated is unknown. We firstly investigated whether the increase occurs as a result of increased Zn transfer across the placenta and then tested whether the regulation occurred at the level of the microvillar membrane of the placenta or the fetal liver plasma membrane. Rats at different stages of gestation were injected with 7.5 microCi 65Zn in 100 microliters rat serum and killed after 1 h. 65Zn levels in the fetus remained constant at equivalent to the amount in 50 microliters maternal plasma per fetus until Day 18; at this time they increased to equivalent to 1.33 ml and then continued to increase until term. We isolated placental microvillar vesicles from placentas at each stage of gestation, characterized them, and measured Zn uptake. Zn uptake rates did not change during pregnancy. Similarly, we isolated vesicles from fetal liver plasma membrane and measured Zn uptake. Again, the uptake properties did not change during pregnancy. The data suggest that some other step in the transport process is rate limiting and that the increase in Zn levels in the fetal liver that occurs during pregnancy is possibly a result, rather than a cause, of metallothionein induction.