Phospholipase A2 stimulation by methyl mercury in neuron culture

In primary prelabeled cultures of cerebellar granule cells, methyl mercury (MeHg) induced a concentration- and time-dependent release of [3H]arachidonic acid. MeHg-induced [3H]arachidonate release was partially dependent on the extracellular Ca2+ concentration. MeHg at 10-20 microM also stimulated basal 45Ca2+ uptake after 20 min of incubation at 37 degrees C, and at 10 microM inhibited K+ depolarization-stimulated uptake. MeHg stimulated [3H]arachidonate uptake, but had no effect on the rate of phospholipid reacylation. Phospholipase A2 (PLA2) activation preceded cytotoxicity, but at higher concentrations of MeHg such dissociation was not evident. Inhibition of MeHg-induced PLA2 activation by 100 microM mepacrine failed to modify cytotoxicity. MeHg-induced lipoperoxidation, measured as the production of thiobarbituric acid-reacting products, was inhibited by alpha-tocopherol without inhibition of [3H]arachidonate release. The absence of alpha-tocopherol inhibition of MeHg-induced arachidonate release precludes a causal role for lipoperoxide-induced PLA2 activation in this system. Moreover, MeHg induced an increased susceptibility of unilamellar vesicles to exogenous PLA2 in the presence of low Ca2+ concentrations without evidence of lipid peroxidation. [3H]Arachidonate incorporation into granule neuron phospholipids was analyzed by isocratic HPLC analysis. Relatively high proportional incorporation was found in the combined phosphatidylcholine fractions and phosphatidylinositol. With MeHg, an increase in the relative specific activity of incorporation was found in the phosphatidylinositol fraction, indicating a preferential turnover in this phospholipid species in the presence of MeHg.