An evaluation of some commercial lead(II)-selective electrodes

Molecular determinants of Pb2+ interaction with NMDA receptor channels

Lead (Pb2+) is a potent neurotoxin that acts as a non-competitive, voltage-independent antagonist of the NMDA receptor (NR) channel. Pb2+ action partially overlaps with that of zinc (Zn2+), but precise coincidence with Zn2+ binding site is debated. We investigated the site of Pb2+ interaction in NR channels expressed in Xenopus laevis oocytes from the clones zeta1, epsilon1 or epsilon2 and mutated epsilon1 or epsilon2 forms. For each epsilon subunit we chose two mutations that have been identified as 'strong mutations' for Zn2+ binding and examined the effect of Pb2+ on channels that contained those mutations. In epsilon1-containing channels, mutations D102A and H128A caused a decrease of Pb2+ inhibition with a 10-fold (D102A) and four-fold (H128A) shift of IC50. In epsilon2-containing channels, the most effective mutation in removing Pb2+ inhibition was H127A, with a five-fold increase of IC50, while D101A was virtually ineffective. Other mutations, D104A, T103A, and T233A, were less effective. The double mutation D101AH127A, while reducing Zn2+ inhibition by nearly nine-fold, caused a minor (less than two-fold) shift in Pb2+ IC50. Competition experiments showed that increasing doses of Zn2+ reduced the apparent affinity for Pb2+ in epsilon1-containing receptors, but not in epsilon2-containing receptors. In addition the effect of Pb2+ on epsilon2-containing channels was additive with that of ifenprodil, with no competition for the site. Although none of the mutations that we have tested abolished the block by Pb2+, our results indicate that the action of this toxic metal on NR channels is more dependent on the receptor composition than previously thought, because Zn2+ is able to displace Pb2+ from its binding site in epsilon1-containing channels, but not in epsilon2-containing channels.

Molecular targets of lead in brain neurotoxicity

The detrimental effects of lead poisoning have been well known since ancient times, but some of the most severe consequences of exposure to this metal have only been described recently. Lead [Pb(II)] affects the higher functions of the central nervous system and undermines brain growth, preventing the correct development of cognitive and behavioral functions. As an established neurotoxin, Pb(II) crosses the blood-brain barrier rapidly and concentrates in the brain. The mechanisms of lead neurotoxicity are complex and still not fully understood, but recent findings recognized that both Ca(II) dependent proteins and neurotransmitters receptors represent significant targets for Pb(II). In particular, acute and chronic exposure to lead would predominantly affect two specific protein complexes: protein kinase C and the N-methyl-D-aspartate subtype of glutamate receptor. These protein complexes are deeply involved in learning and cognitive functions and are also thought to interact significantly with each other to mediate these functions. This review outlines the most recent hypotheses and evidences that link lead poisoning to impairment of these protein functions, as well as the in vitro experimental approaches that are most likely to provide information on basic mechanicistic processes.

Lead inhibition of NMDA channels in native and recombinant receptors

NMDA channels are key targets for lead (Pb2+) neurotoxicity and Pb2+-induced inhibition of NMDA current is age- and subunit-dependent. In rat cerebellar granule cells maintained in high KCl, glycine affinity as well as sensitivity to ifenprodil change significantly with the days in vitro, indicating a reduction of NR2B subunit expression. Pb2+ blocked NMDA current with IC50 approximately 4 microM and this effect decreased significantly during the second week in vitro. In Xenopus laevis oocytes expressing recombinant NR1-NR2A, NR1-NR2B or NR1-NR2C receptors, Pb2+ inhibited glutamate-activated currents with IC50 of 3.3, 2.5 and 4.7 microM respectively. These data indicate that Pb2+ action is dependent on subunit composition and suggest that down-regulation of the NR2B subunit is correlated to a diminished sensitivity to Pb2+ inhibition.

Multiple pathways of Pb(2+) permeation in rat cerebellar granule neurones

The pathways of lead (Pb(2+)) uptake were studied in fura-2-loaded cerebellar granule cells from 8-day-old rats. In a nominal Ca-free external bath, Pb(2+) (5-50 microM) determined an increase of the fluorescence emission ratio (R = E(340)/E(380)) even in the absence of any specific stimulus. This rise was dose-dependent, was not significantly affected by mM Mg(2+) or Ca(2+), but it was readily reversed by the membrane-permeant heavy metal chelator tetrakis(2-pyridylmethyl) ethylene-diamine (TPEN, 100 microM), indicating that it was due to Pb(2+) influx. The rate of rise, dR/dt, was increased up to a factor of 5 by depolarizing high-KCl solution, indicating a sizeable permeation through voltage-dependent channels. This effect was neither antagonized by nimodipine, nor enhanced by BayK8644, but it was slackened by omega-agatoxin IVA (200 nM), suggesting an involvement of non-L-type calcium channels. Pb(2+) influx was also stimulated by glutamic acid or NMDA in the presence of 10-30 microM glycine, but only in Mg-free solution, suggesting that glutamate channels of the NMDA type are an additional pathway of Pb(2+) uptake. Pb(2+) caused a time-, dose- and stimulus-dependent saturation of the dye, whose intracellular concentration is approximately 10 microM, indicating that intracellular Pb(2+) can readily reach a concentration in the micromolar range. These results indicate that the particular vulnerability of neurones to Pb(2+) poisoning is linked to the presence of specific transport systems, which mediate the rapid uptake of Pb(2+) into the neurone.

Organic and inorganic lead inhibit neurite growth in vertebrate and invertebrate neurons in culture

Neurons from brains of chick embryos and pond snails (Lymnaea stagnalis) were cultured for 3 to 4 d in the presence of no toxins, inorganic lead (PbCl2), or organic lead (triethyl lead chloride). In chick neurons, inorganic lead reduced the percentage of cells that grew neurites (IC50 = 270 microM total lead, approximately 70 nM free Pb2+) but did not reduce the number of neurites per cell or the mean neurite length. Triethyl lead reduced the percentage of cells that grew neurites (IC50 = 0.24 microM) and the mean neurite length (extrapolated IC50 = 3.6 microM) but did not reduce the number of neurites per cell. In Lymnaea neurons, inorganic lead reduced the percentage of cells that grew neurites (IC50 = 13 microM total lead; approximately 10 nM free Pb2+). Triethyl lead reduced the percentage of cells that grew neurites (IC50 = 0.4 microM) and exerted significant toxicity at 0.2 microM. The two forms of lead affected neurite growth in qualitatively different ways, which suggests that their mechanisms of action are different.

Influence of lead ions on cation permeability in human red cell ghosts

Intracellular Pb2+ ions can replace Ca2+ ions in stimulating the Ca-dependent K permeability of human red blood cells. In metabolically depleted resealed ghosts, the threshold for stimulation of 86Rb efflux by internal Pb2+ is around 5 X 10(-10) M, and stimulation is half-maximal at about 2 X 10(-9) M, and maximal at 10(-8) M Pb2+. There is no effect on 22Na efflux in this concentration range. 86Rb efflux is antagonized by internal Mg2+ ions, and by the channel-blocking drugs quinidine and diS-C2(5), as observed for the Ca-dependent K permeability in red cells. In ghosts containing EDTA, which prevents any internal effects of Pb2+ ions, external Pb2+ increases both 22Na and 86Rb permeability when its concentration exceeds 6 X 10(-7) M. This effect is seemingly unrelated to the Ca-dependent K permeability. This work makes extensive use of Pb2+ ion buffers, and gives information about their preparation and properties.

Active transport of lead by human red blood cells

Human red cells suspended in lead-citrate buffers (2.6 microM Pb2+) take up much less Pb than predicted from studies of equilibrium binding of Pb to haemolysates. Pb uptake is increased by ATP depletion, or by loading at 0 degrees C. Tracer studies with 203Pb indicate that the low uptake at 37 degrees C in the presence of substrate is not due to membrane impermeability to Pb. Cold-loaded cells extrude Pb against a concentration gradient at 37 degrees C when glucose is present. These results suggest that the cellular loading of Pb is dependent on the balance between an inward leak and an outward pump. The extrusion of Pb from the cells is possibly brought by the Ca pump.