Dose dependent transfer of 203lead to milk and tissue uptake in suckling offspring studied in rats and mice.

The dose-dependent transfer of 203Pb to milk and uptake in suckling rats and mice during a three-day nursing period was studied. On day 14 of lactation, the dams were administered a single intravenous dose of lead, labelled with 203Pb, in four or five doses from 0.0005 to 2.0 mg Pb/kg b.wt. There was a linear relationship between Pb levels in plasma and milk of both species. The Pb milk:plasma ratios at 24 hr after administration were 119 and 89 in mice and rats, respectively. At 72 hr the Pb milk:plasma ratio had decreased to 72 in mice and 35 in rats. The tissue levels of lead in the suckling rats and mice were also linearly correlated with lead concentration in milk at 72 hr, showing that milk could be used as an indicator of lead exposure to the suckling offspring. It is concluded that lead is transported into rat and mouse milk to a very high extent and the excretion into milk is more efficient in mice than in rats. On the other hand, rat pups had higher lead levels in tissues than mice pups, which might be due to a higher bioavailability and/or a lower excretion of lead in rat pups. Thus, lead in breast milk could be used as a biological indicator of lead exposure in the mother as well as in the suckling offspring.