Oxidant injury in PC12 cells--a possible model of calcium "dysregulation" in aging: I. Selectivity of protection against oxidative stress

Abstract

Previous research has suggested that the initial effects of cellular free radical neurotoxic insult involve large increases in intracellular Ca2+. However, the exact role of oxidative stress on the various parameters involved in these increases has not been specified. The present experiments were performed to examine these parameters in PC12 cells exposed to 5, 25, or 300 microM H2O2 for 30 min in growth medium alone or containing either nifedipine (L-type Ca2+ antagonist), conotoxin (N-type antagonist), Trolox (vitamin E analogue), or alpha-phenyl-n-tert-butylnitrone (nitrone trapping agent; PBN). The concentrations of H2O2 were chosen by examining the degree of cell killing induced by exposure to graded concentrations of H2O2. The 5 and 25 microM concentrations of H2O2 produced no significant cell killing at either 30 min or 24 h after treatment, whereas the 300 microM concentration produced a moderate degree of cell killing that did not increase between the two times. Fluorescent imaging was used to visualize intracellular Ca2+ changes in fura-2-loaded cells. Baseline (pre-30 mM KCl) Ca2+ levels were increased significantly by H2O2 treatment (e.g., 300 microM, 200%), but the rise in the level of free intracellular Ca2+ after KCl stimulation (i.e., peak) was decreased (e.g., 300 microM, 50%) and the cell's ability to sequester or extrude the excess Ca2+ (i.e., Ca2+ recovery time) after depolarization was decreased significantly. All compounds prevented baseline Ca2+ increases and, with the exception of conotoxin, antagonized the peak decreases in Ca2+. It is interesting that after 300 microM H2O2 exposure, only Trolox was partially effective in preventing these deficits in recovery. Conotoxin increased the decrement recovery in the absence of H2O2. However, in cells exposed to 5 or 25 microM H2O2, conotoxin as well as the other agents were effective in preventing the deficits in recovery.