Role of metallothionein in human placenta and rats exposed to cadmium.

A strong positive relationship between zinc and copper and metallothionein (MT) in placentas has been found, but a negative one between cadmium and MT. In rats given cadmium i.p. as the chloride, liver cadmium is lower and kidney cadmium higher than in cadmium-treated non-pregnant rats, suggesting that pregnancy enhances mobilization of cadmium from liver to kidney. The cadmium concentration of digested whole fetuses is not significantly increased in offspring of dams given cadmium i.p. as the chloride or CdMT, but the placental levels of cadmium and MT are increased. The placenta therefore acts as a barrier to maternal-fetal cadmium transfer. The way in which cadmium is retained in the placenta but zinc and copper are transferred to the fetus is not understood, since all are bound to MT. Focal renal tubular necrosis and placental necrosis occur at the same level of cadmium exposure, suggesting a similar threshold to cadmium toxicity for the two organs.