Lead exposure at an early age substantially increases lead retention in the rat

It has been hypothesized that the high rate of bone remodeling during childhood and the consequent high calcium and lead turnover result in a substantial reduction in bone lead stores so that much of the lead incorporated in bone during childhood does not persist into adulthood. We studied the effect of age at lead exposure on blood and organ concentrations of lead, calcium, and zinc 1-5 months after termination of lead ingestion. Blood and organ lead concentrations and contents 4 weeks after lead exposure ceased were significantly higher in the rats exposed beginning at 5 weeks of age than in those exposed beginning at 10 or 15 weeks old. Bone lead declined as the time since exposure increased. Despite this trend, the rats exposed when youngest had bone lead concentrations at 20 weeks after the termination of lead exposure that were higher than those of the other rats only 4 weeks after cessation of lead ingestion. Multiple regression analysis demonstrated that age at lead exposure remained a significant predictor of blood and organ lead concentrations and contents even after the inclusion of total lead consumed, body weight, and age at organ harvesting in the regression analysis. There were only small differences in organ calcium and zinc concentrations among treatment groups except for kidney calcium. The results do not support the hypothesis of rapid depletion of bone lead stores in young animals, but rather suggest that younger age at lead exposure is associated with greater lead retention and toxicity even in the absence of continued lead exposure.