Mechanical injury alters volume activated ion channels in cortical astrocytes

Although astrocytic swelling is likely to mediate brain edema and high ICP after traumatic brain injury, the mechanism is not understood. We employed whole cell patch clamp electrophysiology and a stretch injury model to understand whether volume regulating ion currents are altered following cell injury. Mixed rat astrocytes and neurons were co-cultured on deformable silastic membranes. Mild-moderate cell injury was produced using a timed pulse of pressurized air to deform the silastic substrates by 6.5 mm. Then, ion currents were recorded with patch clamp methods. Cells were held at -65 mV and were stepped to +10 mV to monitor current changes.

In unstretched astrocytes, small amplitude currents were obtained under isotonic conditions. Hypotonic solution activated an outwardly-rectifying current which reversed near -40 mV. This current resembled a previously reported anion current whose activation may restore cell volume by mediating a net solute efflux. In contrast, stretch injured cells exhibited a large amplitude, nonrectifying current. This current was not due to non-specific ionic leakage, since it was fully suppressed by the cation channel blocker gadolinium. Activation of novel stretch-activated cation currents may exacerbate cell swelling in injured astrocytes. Stretch injured astrocytes thus express a dysfunctional cation current as opposed to an osmoregulatory anion current. This mechanism, if present in vivo, may contribute to the cytotoxic swelling seen after traumatic brain injury.