Neurotoxicity in rats chronically exposed to lead ingestion: Measurement of intracellular concentrations of free calcium and lead ions in resting or depolarized brain slices

The intracellular free calcium (Ca²⁺) or lead (Pb(i)²⁺) ions were measured by using ¹⁹F-NMR and 1,2-bis(2-amino-5-fluorophenoxy)ethane-N,N,N',N'-tetraacetic acid in brain slices obtained from control rats and rats chronically exposed to 1 mg/kg.day or 100 mg/kg.day dose of Pb acetate ingestion. The basal Ca(i)²⁺ values obtained in this study were comparable to the values reported previously for brain neurons by using the Fura-2 method (Singh, 1993a). The concentrations of total Pb in blood samples obtained from 2-week old rats exposed to 1 and 100 mg/kg.day doses of lead acetate were 31 and 521 μg/dl respectively. The concentrations of total Pb in brain samples obtained from 2-week old rats exposed to 1 and 100 mg/kg.day doses of lead acetate were 150 and 2700 ng/g respectively. The concentration of Pb in neonatal and adult samples did not differ significantly. Free Pb²⁺ was not detected in samples obtained from the rats exposed to a Pb dose of 1 mg/kg.day; however, a small concentration (ranging from 100 to 300 pM) of the ion was detected in samples obtained from the rats exposed to a Pb dose of 100 mg/kg.day. This suggests that either a small proportion of total Pb is ionized in brain or there is an efflux of Pb²⁺ from intracellular fluid into the incubation medium. Depolarization caused significant increase in Ca(i)²⁺ in cortical slices obtained both from control and Pb-exposed rats. However, depolarization increased Pb(i)²⁺ only in samples obtained from rats exposed to a Pb dose of 100 mg/kg.day. Chronic Pb exposure significantly reduced the effects of depolarization on Ca(i)²⁺ levels. Since there is very little extracellular Pb²⁺ remaining in the slices, the observed decrease in depolarization-induced Ca(i)²⁺ may not be because of the inhibition by Pb²⁺ in the influx of Ca²⁺ but because of long-term changes in cellular metabolism.