Oxidant Injury in PC12 Cells—A Possible Model of Calcium "Dysregulation" in Aging: II. Interactions with Membrane Lipids

Abstract: In a model recently developed to study the parameters altering vulnerability to oxidative stress, it was shown via image analysis that H₂O₂-exposed PC12 cells exhibited increased levels of intracellular Ca²⁺ (baseline), decreases in K⁺-stimulated Ca²⁺ levels (peak), and decreased poststimulation Ca²⁺ clearance (recovery). The present experiments were performed to determine if the response patterns in these parameters to oxidative stress would be altered after modification of membrane lipid composition induced by incubating the PC12 cells with 660 µ M cholesterol (CHL) in the presence or absence of 500 µ M sphingomyelin (SPH) before low (5 µ M ) or high (300 µ M ) H₂O₂ exposure. Neither CHL nor SPH had synergistic effects with high concentrations of H₂O₂ on baseline. However, CHL in the presence or absence of SPH reversed the effect of low concentrations of H₂O₂ on baseline. SPH decreased significantly the cell's ability to clear excess Ca²⁺ in the presence or absence of H₂O₂ and increased significantly the level of conjugated dienes (CDs). It is surprising that in the cells pretreated with CHL, the CD levels were not significantly different from controls. However, in the presence of SPH, the effects of CHL on CDs were altered. These results suggest that the ratios of membrane lipids could be of critical importance in determining the vulnerability to oxidative stress and Ca²⁺ translocation in membranes. This may be of critical importance in aging where there is increased membrane SPH and significant loss of calcium homeostasis.