title: Dynamic volume changes in astrocytes are an intrinsic phenomenon mediated by bicarbonate ion flux.
abstract: 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 (HCO(3-)) ions as swelling was significantly decreased when the influx of HCO(3-) was reduced. We found: 1) in HCO(3-) 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 HCO(3-) efflux is known to occur through the γ-amino-butyric acid (GABA) channel, we performed a series of experiments to determine if astrocytes were capable of HCO(3-) 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.
authors.affiliation: Department of Physiology, University of Saskatchewan, Saskatoon, Saskatchewan, Canada.
authors.forename: Clare M
authors.initials: CM
authors.lastname: Florence
authors.forename: Landon D
authors.initials: LD
authors.lastname: Baillie
authors.forename: Sean J
authors.initials: SJ
authors.lastname: Mulligan
mesh_terms: D000818:Animals; D001253:Astrocytes / Q000166:cytology* / Q000378:metabolism*; D001639:Bicarbonates / Q000378:metabolism*; D001692:Biological Transport; D048429:Cell Size; D021621:Imaging, Three-Dimensional; D007473:Ion Channels / Q000378:metabolism; D008297:Male; D011188:Potassium / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D005680:gamma-Aminobutyric Acid / Q000378:metabolism
chemical_list: D001639:Bicarbonates; D007473:Ion Channels; D005680:gamma-Aminobutyric Acid; D011188:Potassium