Significance of Plasma Lead Levels in Normal and Lead-Intoxicated Children*

Plasma lead (Pb) levels have been measured in normal and lead-intoxicated children, newborns, and children with sickle cell disease. The results in all groups were contant over a wide range of red cell Pb concentration. These results support the thesis that the red cell represents a large repository for Pb, maintaining plasma Pb concentration within closely defined limits, and that methods other than measurements of plasma Pb will be necessary to uncover a presumably dynamic transport system between red cell and plasma. Indeed, we have demonstrated in vitro that ionized calcium (Ca(2+)) lowers red cell Pb content according to a linear dose-response curve. Ca(2+) may thereby control Pb transport from red cell to plasma, and fluctuations in the concentration of Ca(2+) in serum and extracellular fluid may influence the toxic activities of Pb. In bone organ culture, changes in the concentration of Ca(2+) and phosphate in the medium alter the release of previously incorporated (210)Pb from fetal rat bones in response to parathyroid hormone (PTH). Therefore, both PTH and the ionic milieu of the medium apparently regulate bone Pb metabolism.We would expect that understanding further the dynamics of Pb transport in plasma and bone may lead to a more exact definition of the real hazards of low level Pb toxicity in children.