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Reperfusion paradox: A novel mode of glial cell injury

We have attempted to reconstruct in vitro the events that may occur in vivo during reperfusion injury after ischemia in the central nervous system. The phenomenon is induced by previous exposure to low calcium solutions (“calcium paradox”) before the reperfusion episode. Intracellular calcium alterations during reperfusion of human astrocytoma U1242MG cells have been investigated with microspectrofluorimetry using the calcium‐sensitive dye fura‐2. Cells were perfused in calcium‐free buffer solution for 30 min and then re‐exposed to the control buffer solution (1.5 mM CaCl2). [Ca2+]i increased up to 3.5 times control levels during the reperfusion period. The mechanism of the increase was also investigated. Addition of TTX (2 μM) or choline chloride sodium substitution during perfusion with low calcium prevented the [Ca2+]i increase during reperfusion. Reperfusion increases in [Ca2+]i were exacerbated by low potassium in the perfusion medium, but unaltered by the calcium channel blockers cadmium (100 μM) and nickel (100 μM). In a similar manner, flunarizine (10 μM) and cadmium (100 μM) were unable to modify reperfusion [Ca2+]i alterations. Low sodium in the reperfusion medium produced significant increases in [Ca2+]i if preceded by low potassium and calcium perfusion. The viability of cells after 24 h of incubation after the insult produced by exposure to Ca2+‐free media for 30 min was also investigated. Compared with control groups, the groups treated with Ca2+‐free media for 30 min had a decreased number of surviving cells and morphological alterations indicative of cell pathology. The relative number of cytotoxic cells was increased by maneuvers (low potassium perfusion) that presumably blocked the Na/K ATPase. These data support the hypothesis that calcium entry during reperfusion is via the Na/Ca exchanger and suggest a mechanism by which glia could become irreversibly injured in vivo. Such delayed pathological processes could exacerbate already described mechanisms of cell death in the central nervous system.