Astrocyte-Selective Volume Increase in Elevated Extracellular Potassium Conditions Is Mediated by the Na(+)/K(+) ATPase and Occurs Independently of Aquaporin 4

Astrocytes and neurons have been shown to swell across a variety of different conditions, including increases in extracellular potassium concentration (^[K(+)](o)). The mechanisms involved in the coupling of K(+) influx to water movement into cells leading to cell swelling are not well understood and remain controversial. Here, we set out to determine the effects of ^[K(+)](o) on rapid volume responses of hippocampal CA1 pyramidal neurons and stratum radiatum astrocytes using real-time confocal volume imaging. First, we found that elevating [K(+)](o) within a physiological range (to 6.5 mM and 10.5 mM from a baseline of 2.5 mM), and even up to pathological levels (26 mM), produced dose-dependent increases in astrocyte volume, with absolutely no effect on neuronal volume. In the absence of compensating for addition of KCl by removal of an equal amount of NaCl, neurons actually shrank in ^[K(+)](o), while astrocytes continued to exhibit rapid volume increases. Astrocyte swelling in ^[K(+)](o) was not dependent on neuronal firing, aquaporin 4, the inwardly rectifying potassium channel Kir 4.1, the sodium bicarbonate cotransporter NBCe1, , or the electroneutral cotransporter, sodium-potassium-chloride cotransporter type 1 (NKCC1), but was significantly attenuated in 1 mM barium chloride (BaCl(2)) and by the Na(+)/K(+) ATPase inhibitor ouabain. Effects of 1 mM BaCl(2) and ouabain applied together were not additive and, together with reports that BaCl(2) can inhibit the NKA at high concentrations, suggests a prominent role for the astrocyte NKA in rapid astrocyte volume increases occurring in ^[K(+)](o). These findings carry important implications for understanding mechanisms of cellular edema, regulation of the brain extracellular space, and brain tissue excitability.