Department of Physiology, University of Saskatchewan, Saskatoon, Saskatchewan, Canada.
PLoS One. 2012;7(11):e51124. doi: 10.1371/journal.pone.0051124. Epub 2012 Nov 30.
Astrocytes, the major type of non-neuronal cells in the brain, play an important functional role in extracellular potassium (K(+)) and pH homeostasis. Pathological brain states that result in K(+) 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(+) (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.
星形胶质细胞是大脑中主要的非神经元细胞类型，在细胞外钾离子(K(+))和 pH 值稳态中发挥重要的功能作用。已经表明，导致K(+)和 pH 值失调的病理性脑状态会导致星形胶质细胞肿胀。然而，在生理条件下星形胶质细胞体积是否发生变化尚不清楚。在这项研究中，我们使用双光子成像技术可视化海马 CA1 区放射层中星形胶质细胞的实时体积变化。观察到星形胶质细胞在 K(+)浓度（3mM）的小生理增加下肿胀 19.0±0.9%。星形胶质细胞肿胀是由碳酸氢根（HCO(3-))离子内流介导的，因为当 HCO(3-)内流减少时，肿胀显著减少。我们发现：1）在无 HCO(3-)的细胞外溶液中，星形胶质细胞肿胀 5.4±0.7%，2）当钠-碳酸氢盐共转运体（NBC）的活性被阻断时，星形胶质细胞肿胀 8.3±0.7%，3）在存在细胞外碳酸酐酶（CA）抑制剂的情况下，星形胶质细胞肿胀 11.4±0.6%。因为已知通过γ-氨基丁酸（GABA）通道会发生显著的 HCO(3-)外排，所以我们进行了一系列实验来确定星形胶质细胞是否能够通过 GABA 通道激活进行 HCO(3-)介导的体积收缩。星形胶质细胞在 GABA(A) 通道激动剂 muscimol 的作用下收缩 -7.7±0.5%的对照值。在膜通透性 CA 抑制剂乙酰唑胺（ACTZ）存在的情况下，星形胶质细胞从 GABA(A)通道激活引起的收缩显著减少至对照值的-5.0±0.6%。这些动态的星形胶质细胞体积变化可能代表了星形胶质细胞调节生理脑功能的一种以前未被重视但基本的机制。
