CHANGES IN MEMBRANE-PROPERTIES DURING ENERGY DEPLETION-INDUCED CELL INJURY STUDIED WITH FLUORESCENCE MICROSCOPY

The changes in membrane structural properties occurring during the pro cess of ATP depletion-induced cell injury in adherent human astrocytom a cells (UC-11 MG) were studied With two epifluorescence techniques: 1 ) steady-state fluorescence anisotropy (r) to examine microstructural changes in the membrane phospholipids and 2) fluorescence redistributi on after photobleaching (FRAP) to examine membrane fluidity changes. A new method for r measurement was established that provides the unique advantage of simultaneously monitoring both vertical and horizontal p olarized fluorescence emissions needed for the calculation of r. In th is study, r in the astrocytoma cells labeled with 1-(4-trimethylammoni um phenyl)-6-phenyl-1,3,5-hexatriene p-toluenesulfonate was shown to r emain stable for up to 90 min. However, when the cells were treated wi th 75 mu M iodoacetic acid (IAA), a metabolic inhibitor that induces r apid depletion of cellular ATP, r continually decreased, indicating a decrease in membrane lipid order and perturbation of the bilayer struc ture. This decrease in r could be prevented by the pretreatment of cel ls with lipophilic antioxidants such as tirilazad or gossypol. Tirilaz ad itself caused a significant increase in r, suggesting that tirilaza d intercalates into the membrane bilayer and profoundly increases the lipid order in uninjured cells. Gossypol, however, did not exhibit thi s property. Further investigations into these phenomena with FRAP conf irmed the r results and indicated that membrane fluidity increased whi le its structure became less rigid during the process of ATP-induced c ell injury. In addition, lipophilic antioxidants prevented the membran e structural aberrations induced by IAA. Experimental results suggest that different mechanisms of cytoprotective action may exist for tiril azad and the antioxidant gossypol. Gossypol appears to prevent or dela y the observed cell injury entirely because of its antioxidant action, whereas tirilazad's protection is mediated not only via its antioxida nt activity, but also by its ability to increase cell membrane lipid o rder.