Mechanical strain injury increases intracellular sodium and reverses Na super(+)/Ca super(2+) exchange in cortical astrocytes

Traditionally, astrocytes have been considered less susceptible to injury than neurons. Yet, we have recently shown that astrocyte death precedes neuronal death in a rat model of traumatic brain injury (TBI) (Zhao et al.: Glia 44:140-152, [2003). A main mechanism hypothesized to contribute to cellular injury and death after TBI is elevated intracellular calcium ([Ca super(2+)] sub(i)). Since calcium regulation is also influenced by regulation of intracellular sodium ([Na super(+)] sub(i)), we used an in vitro model of strain-induced traumatic injury and live-cell fluorescent digital imaging to investigate alterations in [Na super(+)] sub(i) in cortical astrocytes after injury. Changes in [Na super(+)] sub(i), or [Ca super(2+)] sub(i) were monitored after mechanical injury or L-glutamate exposure by ratiometric imaging of sodium-binding benzofuran isophthalate (SBFI-AM), or Fura-2-AM, respectively. Mechanical strain injury or exogenous glutamate application produced increases in [Na super(+)] sub(i) that were dependent on the severity of injury or concentration. Injury-induced increases in [Na super(+)] sub(i) were significantly reduced, but not completely eliminated, by inhibition of glutamate uptake by DL-threo- beta -benzyloxyaspartate (TBOA). Blockade of sodium-dependent calcium influx through the sodium-calcium exchanger with 2-[2-[4-(4-Nitrobenzyloxy)phenyl]ethyl]isothiourea mesylate (KB-R7943) reduced [Ca super(2+)] sub(i) after injury. KB-R7943 also reduced astrocyte death after injury. These findings suggest that in astrocytes subjected to mechanical injury or glutamate excitotoxicity, increases in intracellular Na super(+) may be a critical component in the injury cascade and a therapeutic target for reduction of lasting deficits after traumatic brain injury.