Paraquat-induced membrane dysfunction in pulmonary microvascular endothelial cells

Membrane dysfunction monitored by lactate dehydrogenase release from cultured pulmonary microvascular endothelial cells of pigs, which were exposed to paraquat at different concentrations (0.1-2 mM), was examined. Paraquat caused a time-dependent increase in lactate dehydrogenase release. Lactate dehydrogenase releases after 72 hr, 32, 58, and 84% by 0.1, 0.5, and 2 mM paraquat, respectively, were well correlated with cell viability measured by cell adherence. In contrast, reductions of two tetrazolium compounds were depleted profoundly by 72 hr after exposure to 0.5 mM paraquat, suggesting depletion of intracellular reductive substances. Extracellular hydrogen peroxide began to significantly increase 56 hr or 32 hr after exposure to 0.5 mM or 1.5 mM paraquat, respectively, preceding the initial increase of lactate dehydrogenase release (64 hr by 0.5 mM or 48 hr by 1.5 mM). Lactate dehydrogenase release 72 hr after exposure to 0.5 mM paraquat was prevented strongly by catalase (1000 units/ml), but weakly by superoxide dismutase (1000 units/ml). These enzymes failed to restore the reduced acid phosphatase activity. Also, 0.1 mM desferal or alpha,alpha'-dipyridyl protected lactate dehydrogenase release. Similarly, 1 mM thiourea or dimethylthiourea, and 0.5 mM alpha-tocopherol or trolox, were effective, but diethylenetriaminepentaacetic acid (0.1 mM) and probucol (5 or 10 microM) were ineffective. Exposure of 0.5 or 1.5 mM paraquat suppressed levels of lipid peroxidation. These results indicate that membrane dysfunction by paraquat is ascribed to an iron-catalyzed reaction of extracellularly increased hydrogen peroxide. A deleterious species for the membrane dysfunction is discussed.