Phospholipid Metabolism in Neural Microvascular Endothelial Cells after Exposure to Lead in Vitro

Abstract

Lead (Pb 2+) is known to alter the permeability of brain capillaries. A possible mechanism for this alteration may be related to the ability of Pb 2+ to substitute for Ca 2+. Products derived from phospholipid metabolism, namely eicosanoids and diacylglycerol, control endothelial permeability, are partly regulated by intracellular Ca 2+, and thus may be sensitive to Pb 2+. We asked in this study whether Pb 2+ increased arachidonic acid release or stimulated phosphatidylcholine breakdown in an in vitro model of brain capillaries, namely cultured bovine retinal endothelial (BRE) cells. Pb 2+ stimulated arachidonic acid release and phosphatidylcholine and phosphatidylinositol metabolism in the presence of ionomycin, but not by itself. More arachidonic acid was released than phosphorylcholine in BRE cells stimulated with ionomycin and Pb 2+, but the magnitudes of these responses were similar in cells exposed to ionomycin plus Ca 2+. Ionomycin plus Pb 2+ or plus Ca 2+ resulted in the activation of phospholipase A 2, since an increase in lysophosphatidylcholine and arachidonic acid was observed. Protein kinase C was not required for arachidonic acid release because release was observed in cells with a down-regulated enzyme. Ionomycin plus other metals (La 3+, Cd 2+, or Mg 2+) did not result in arachidonic acid release, but Cd 2+ or Co 2+ inhibited arachidonic acid release by more than 80% when cells were exposed to ionomycin with either Pb 2+ or Ca 2+. Thapsigargin or maitotoxin plus Ca 2+ increased arachidonic acid release that was inhibited by the receptor-dependent calcium channel antagonist SK&F 96365 but not by the voltage-dependent calcium channel antagonist nifedipine. However, thapsigargin or maitotoxin plus Pb 2+ failed to stimulate arachidonic release. Since in this in vitro model Pb 2+ stimulated phospholipid metabolism solely in the presence of an ionophore, the increase in permeability observed in Pb 2+-exposed animals is probably not due to a release of metabolites of arachidonic acid.