Endotoxin protects against hyperoxic decrease in membrane fluidity in endothelial cells but not in fibroblasts.

We evaluated the ability of endotoxin to protect against hyperoxic depression of plasma membrane fluidity in endothelial cells and fibroblasts in culture. Second- to-fifth passage porcine aortic endothelial cells and human newborn foreskin fibroblasts with 20 ng/ml of endotoxin or diluent in the culture medium were exposed to 20% O2 (control) or 95% O2 (hyperoxic) in 5% CO2 for 4 hours. After exposure, cells were labeled with 1,6-diphenyl-1,3,5-hexatriene (DPH), an aromatic hydrocarbon that partitions into the hydrophobic core of lipid bilayer membranes, or transparinaric acid (TPA), a natural, conjugated fatty acid that orients parallel to fatty acyl chains of membrane phospholipids. Membrane fluidity was monitored by measuring changes in the steady state fluorescence anisotropies (rs) for DPH and for TPA by using fluorescence spectroscopy. Reductions in membrane fluidity increase the value of rs. Addition of endotoxin to the culture medium of control endothelial cells and fibroblasts had no effect on rs for DPH or TPA. In hyperoxic endothelial cells, rs for DPH and rs for TPA were increased (p less than 0.001). Addition of endotoxin to the medium of hyperoxic endothelial cells prevented the increases in rs for DPH and TPA. Hyperoxia increased rs for DPH (p less than 0.003) but not rs to TPA in fibroblasts, and endotoxin failed to prevent this increase. These results indicate that hyperoxia decreases plasma membrane fluidity in endothelial cells and fibroblasts and demonstrate that endotoxin prevents the decrease in plasma membrane fluidity in endothelial cells, but not in fibroblasts. These membrane-protective effects may represent an alternative mechanism by which endotoxin protects against hyperoxic cellular injury, and this mechanism may be specific for hyperoxic injury to endothelial cells.