Calcium dependence of rapid astrocyte death induced by transient hypoxia, acidosis, and extracellular ion shifts.

Exposure to hypoxic, acidic, ion-shifted Ringer (HAIR) for 15-40 min has been shown to cause rapid astrocyte death upon reperfusion with normal media. The ion shifts of the HAIR solution included a rise in extracellular K(+) (e.g., [K(+)](o)) and a fall in [Na(+)](o), [Cl(-)](o), and [Ca(2+)](o), characteristic of ischemic-traumatic brain insults. We investigated the ionic basis of the HAIR-induced injury. After HAIR exposure, reperfusion in 0 Ca(2+)/EGTA media completely protected astrocytes. Preincubation of cells in BAPTA-AM ester was also protective, indicating that the injury was triggered by Ca(2+) influx during reperfusion. Neither nimodipine, CNQX, APV, nor TTX reduced injury. Astrocyte death could be blocked by 100 microM Ni(2+) or 100 microM benzamil, suggesting involvement of Na(+)-Ca(2+) exchange. KB-R7943, which preferentially inhibits reverse Na(+)-Ca(2+) exchange, also protected astrocytes. Elevation of [K(+)](o) was not necessary for astrocyte death. However, when [Na(+)](o) was maintained at 151 mM throughout the HAIR protocol, cell death was markedly reduced. We postulate that [Na(+)](o) shifts aid reversal of Na(+)-Ca(2+) exchange by favoring cytosolic Na(+) loading. Possible means of astrocytic Na(+) accumulation are discussed.