Astrocytes are reported to have critical functions in ischemic
brain injury including protective effects against
ischemia-induced neuronal dysfunction. Na-K
ATPase maintains ionic gradients in astrocytes and is suggested as an
indicator of ischemic injury in glial cells. Here, we examined the role of the Na-K
ATPase in the pathologic process of ischemic injury of primary cultured astrocytes. Chemical
ischemia was induced by
sodium azide and
glucose deprivation.
Lactate dehydrogenase assays showed that the cytotoxic effect of chemical
ischemia on astrocytes began to appear at 2 h of
ischemia. The expression of Na-K
ATPase α1
subunit protein was increased at 2 h of chemical
ischemia and was decreased at 6 h of
ischemia, whereas the expression of α1 subunit
mRNA was not changed by chemical
ischemia. Na-K
ATPase activity was time-dependently decreased at 1, 3, and 6 h of chemical
ischemia, whereas the
enzyme activity was temporarily recovered to the control value at 2 h of chemical
ischemia. Cytotoxicity at 2 h of chemical
ischemia was significantly blocked by reoxygenation for 24 h following
ischemia. Reoxygenation following chemical
ischemia for 1 h significantly increased the activity of the Na-K
ATPase, while reoxygenation following
ischemia for 2 h slightly decreased the
enzyme activity. These results suggest that the critical time for
ischemia-induced cytotoxicity of astrocytes might be 2 h after the initiation of ischemic insult and that the increase in the expression and activity of the Na-K
ATPase might play a protective role during ischemic injury of astrocytes.