Lipopolysaccharide and interleukin-6 enhance lead entry into cerebellar neurons: application of a new and sensitive flow cytometric technique to measure intracellular lead and calcium concentrations.

The distribution of intracellular ionized lead (Pb) and calcium in dissociated cerebellar cells of ten-day-old mice was measured by flow cytometry. There are no fluorescent probes specific for lead, whereas commonly used fluorescent calcium indicators bind heavy metals with greater affinity than they do calcium, which impedes discrimination of lead- and calcium-induced fluorescence changes. Therefore, we developed a method to determine [Pb2+]i and [Ca2+]i by employing a combination of the calcium indicator fluo-3 and the heavy-metal chelator TPEN. Using these methods, we studied the effects of multiple in vivo exposure (five subcutaneous injections over 10 days) to lipopolysaccharide (LPS, 100 ng/g), recombinant mouse interleukin-6 (IL-6, 5 ng/g) and/or inorganic lead (lead, 2.5 micrograms/g) on lead and calcium concentrations. Control cells had [Cai] of 112 nM. Lead exposure alone had little effect on [Ca2+]i and resulted in a mean [Pb2+]i of about 7 pM, and did not alter cell volume. A significant fraction of cells (about 44% of living cells) from animals treated with lead plus LPS were swollen, as determined by analysis of the light scattering pattern, and there was a small increase in the number of dead cells, identified with the nucleic acid stain, 7-aminoactinomycin. While [Ca2+]i was not significantly increased in animals treated with either only LPS or IL-6, lead and calcium concentrations were increased in animals exposed to lead and LPS or IL-6 in both the non-swollen and swollen cells, with a mean value of (Pb2+)i of 32 pM and (Ca2+)i of 155 nM in cells not swollen. Electrophysiological analysis showed that LPS injections caused decreases in the membrane potential of endothelial cells of the blood-brain barrier (BBB) and lead potentiated the effect of LPS. IL-6 mimicked the effects of LPS, but was less potent. Thus these experiments indicate a synergistic interaction between lead and cytokines on biophysical properties of both neurons and endothelial cells of the BBB.