THE TRANSFER OF ZINC ACROSS THE TERM DUALLY PERFUSED HUMAN PLACENTAL LOBULE

Net zinc transfer between fetal and maternal circulation has been described in the in vitro perfused lobule of the human term placenta. The transfer results in a 3 .mu.M differential between fetal and maternal perfusates which is comparable to estimated values in vivo. Tissue zinc concentration is more than 10 times perfusate concentration and is correlated with zinc flux across the fetal borders of the lobule. Open-circuit studies on net transport of zinc, and upon flow of 65Zn relative to tritiated water (3H2O), indicate these fluxes depend upon the nonprotein-bound (free) zinc gradient across the fetal-facing membranes. On the other hand movement across the maternal surface is not related to diffusion gradients of free zinc and is either independent of, or inversely related to, the level of tissue zinc. It is concluded that the uptake of zinc from the maternal perfusate may be coupled to metabolism whereas its transfer to fetal circulation is linked to the level of free zinc in the tissue.