Interaction of lead and zinc in cultured astroglia

Astroglia take up lead (Pb) in vivo and in vitro. In view of the fact that zinc affects both tissue deposition of Pb and clinical signs of Pb intoxication, the present study was carried out to test the effects of various Zn levels on lead toxicity in astroglia. Primary cultures of astroglia from 1- to 3-day-old neonatal rats were divided into three groups and cultured in Waymouth's 752/l medium with 0, 1, or 2 microM ZnCl2. Each group was further divided into two subgroups which were treated with either 0, 29.9, or 32.5 mumol of Pb acetate. Cultures were assayed for viability and metal content after 1 and 3 days of continuous exposure to Pb (designated days 1 and 3) as well as 10, 17, and 24 days after the initiation of a 3-day exposure to Pb. The Trypan blue dye exclusion viability assay showed no significant differences between controls and Pb-treated groups except on day 3, at which time the 0 and 2 microM Zn groups treated with Pb had reduced viability. 3H-Leucine incorporation into acid-precipitable proteins (cpm/micrograms protein) was unaffected by Pb or Zn except on days 1 and 17, when cultures given 2 microM Zn and no Pb showed increased incorporation. Pb-treated cultures showed a reduction in cell number which was partially offset in a dose-dependent manner by the presence of Zn in the medium but not enough to mask completely the reduction caused by Pb. Pb produced the following effects on intracellular trace metal concentrations: (1) increased intracellular [Pb]. (2) increased intracellular [Fe], (3) increased intracellular [Cu], and increased intracellular [Zn]. By day 24, intracellular Cu concentrations were normal, but intracellular [Zn] and [Pb] remained elevated in all Pb-treated subgroups. Furthermore, intracellular Fe levels remained increased in the Pb-treated subgroup cultured with 0 microM Zn. Zinc showed a protective effect by (1) reducing intracellular Pb levels and (2) delaying or preventing the Pb-induced increase in intracellular [Fe] and [Zn] but not the increase in intracellular [Cu]. These effects became more pronounced with increasing extracellular Zn concentrations, although intracellular Zn levels did not increase in response to extracellular levels. Increased dietary zinc in rats is known to reduce Pb accumulation in organs. Our results extend this observation to cells in culture and, furthermore, suggest that the Pb-Zn interaction is complex and not simply a substitution of Pb by Zn at the point of absorption through the plasma membrane.