Dynamic Volume Changes in Astrocytes Are an Intrinsic Phenomenon Mediated by Bicarbonate Ion Flux

Astrocytes, the major type of non-neuronal cells in the brain, play an important functional role in extracellular potassium ([K+](o)) and pH homeostasis. Pathological brain states that result in [K+](o) and pH dysregulation have been shown to cause astrocyte swelling. However, whether astrocyte volume changes occur under physiological conditions is not known. In this study we used two-photon imaging to visualize real-time astrocyte volume changes in the stratum radiatum of the hippocampus CA1 region. Astrocytes were observed to swell by 19.0 +/- 0.9% in response to a small physiological increase in the concentration of [K+](o) (3 mM). Astrocyte swelling was mediated by the influx of bicarbonate (HCO3-) ions as swelling was significantly decreased when the influx of HCO3- was reduced. We found: 1) in HCO3- free extracellular solution astrocytes swelled by 5.4 +/- 0.7%, 2) when the activity of the sodium-bicarbonate cotransporter (NBC) was blocked the astrocytes swelled by 8.3 +/- 0.7%, and 3) in the presence of an extracellular carbonic anhydrase (CA) inhibitor astrocytes swelled by 11.4 +/- 0.6%. Because a significant HCO3- efflux is known to occur through the gamma-amino-butyric acid (GABA) channel, we performed a series of experiments to determine if astrocytes were capable of HCO3- mediated volume shrinkage with GABA channel activation. Astrocytes were found to shrink -7.7 +/- 0.5% of control in response to the GABA(A) channel agonist muscimol. Astrocyte shrinkage from GABA(A) channel activation was significantly decreased to -5.0 +/- 0.6% of control in the presence of the membrane-permeant CA inhibitor acetazolamide (ACTZ). These dynamic astrocyte volume changes may represent a previously unappreciated yet fundamental mechanism by which astrocytes regulate physiological brain functioning.