Effect of glutamate and extracellular calcium on uptake of inorganic lead (Pb2+) in immortalized mouse hypothalamic GT1–7 neuronal cells

We have previously shown that although glutamate alone has no effects on viability of mouse hypothalamic GT1–7 cells, it clearly enhances Pb2+-induced cytotoxicity. It is likely that Pb2+ must enter cells to exert most of its toxic effects. Pb2+ is known to substitute for Ca2+ in many cellular processes. Therefore, we studied the uptake mechanisms of Pb2+ into GT1–7 neuronal cells with a special focus on the role of extracellular calcium (Ca2+), voltage-sensitive calcium channels (VSCCs) and glutamate. Basal uptake of Pb2+ (1 μM or 10 μM), i.e. without any external stimulus, clearly increased in nominally Ca2+-free buffer and was partially abolished by 13 mM Ca2+ when compared to uptake in the presence of a physiological concentration of extracellular Ca2+ (1.3 mM). Depolarization by 25 mM K+, or antagonists of VSCCs, verapamil (10 μM) or flunarizine (10 μM) had no clear effect on basal Pb2+ uptake. Glutamate (1 mM) increased Pb2+ uptake, but only when cells were treated with 1 μM Pb2+ in the presence of 1.3 mM Ca2+. Our data suggest that Pb2+ competes for the same cellular uptake pathways with Ca2+, although not via VSCCs. In addition, enhancement of Pb2+-induced neurotoxicity by glutamate may be due to increased neuronal uptake of Pb2+.