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

Abstract

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 (BaCl2) and by the Na+/K+ ATPase inhibitor ouabain. Effects of 1 mM BaCl2 and ouabain applied together were not additive and, together with reports that BaCl2 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.

Rapid volume pulsation of the extracellular space coincides with epileptiform activity in mice and depends on the NBCe1 transporter

“…Modest elevations in [K + ] ECS (Walch et al . 2020) or hypoosmolar challenges (Andrew et al . 2007) have been shown to induce astrocytic‐selective swelling, although there are some competing observations (Murphy et al .…”

From Physiology to Pathology of Cortico-Thalamo-Cortical Oscillations: Astroglia as a Target for Further Research

“…The combined use of cell-specific conditional knock-out of these channels and pharmacological models of SWDs could shine new light on the topic. K + uptake is associated with cellular swelling due to Na + /K + -pump dependent water influx (113). Although the exact molecular mechanisms are still under debate, water fluxes across the astrocytic membrane are associated with K + homeostasis and they influence local interstitial osmolarity as well as seizure generation and progression (114)(115)(116)(117)(118)(119)(120).…”

“…Although the exact molecular mechanisms are still under debate, water fluxes across the astrocytic membrane are associated with K + homeostasis and they influence local interstitial osmolarity as well as seizure generation and progression (114)(115)(116)(117)(118)(119)(120). Given the accumulating evidence against the predominant contribution of the astroglial water channel aquaporin-4 (AQP4) in water homeostasis (113,121), the use of AQP4 conditional knock-out as a model of disrupted water homeostasis has been recently challenged. Nevertheless, water homeostasis impairment and the resulting volume and osmolarity dysregulation should affect neural network excitability.…”

“…K + uptake is associated with cellular swelling due to Na + /K + -pump dependent water influx ( 113 ). Although the exact molecular mechanisms are still under debate, water fluxes across the astrocytic membrane are associated with K + homeostasis and they influence local interstitial osmolarity as well as seizure generation and progression ( 114 – 120 ).…”

Potential of Multiscale Astrocyte Imaging for Revealing Mechanisms Underlying Neurodevelopmental Disorders

“…Recently, we postulated that diffusion MRI can detect the astrocyte volume change because astrocytes undergo dramatic volume changes as a result of fluid flux associated with extracellular K + release and subsequent activation of the astrocyte Na–K–Cl cotransporter (NKCC1), Kir4.1 inwardly rectifying potassium channel, and/or the Na + /K + ATPase [ 168 , 169 , 170 , 171 , 172 , 173 ]. In addition, AQP4 activity may contribute to these volume changes by mediating ISF–CSF exchange [ 174 ].…”