Abstract
Astrocytes express potassium and water channels to support dynamic regulation of potassium homeostasis. Potassium kinetics can be modulated by aquaporin-4 (AQP4), the essential water channel for astrocyte water permeability regulation. We investigated whether extracellular potassium ([K+]o) can regulate astrocyte water permeability and the mechanisms of such an effect. Studies were performed on rat primary astrocytes and a rat astrocyte cell line transfected with AQP4. We found that 10mM [K+]o caused an immediate, more than 40%, increase in astrocyte water permeability which was sustained in 5min. The water channel AQP4 was a target for this regulation. Potassium induced a significant increase in intracellular cAMP as measured with a FRET based method and with enzyme immunoassay. We found that protein kinase A (PKA) could phosphorylate AQP4 in vitro. Further elevation of [K+]o to 35mM induced a global intracellular calcium response and a transient water permeability increase that was abolished in 5min. When inwardly rectifying potassium (Kir)-channels were blocked, 10mM [K+]o also induced a calcium increase and the water permeability increase no longer persisted. In conclusion, we find that elevation of extracellular potassium regulates AQP4 and astrocyte water permeability via intracellular signaling involving cAMP. A prolonged increase of astrocyte water permeability is Kir-channel dependent and this response can be impeded by intracellular calcium signaling. Our results support the concept of coupling between AQP4 and potassium handling in astrocytes.
Highlights
Extracellular potassium in the brain has to be tightly controlled
A small but significant water permeability was observed after 5min of 10mM potassium (P = 0.045), n = 43–130. (H) 35mM potassium did not have any effect on water permeability in AQP4-negative astrocyte cell line (1min, p = 0.62; 5min p = 0.89), n = 33– 119
We report that extracellular potassium regulates astrocyte water permeability in a concentration and time-dependent manner that involves Cyclic AMP (cAMP) and that can be modified by Kir-channels via intracellular calcium
Summary
Astrocytes are essential for brain potassium homeostasis and are responsible for regulating potassium dynamics following neuronal synaptic activity [1,2]. When the systems for maintaining potassium homeostasis are disrupted or overwhelmed, extracellular potassium concentrations can reach values as high as 30 to 80mM [7]. This can be predicted to occur in pathological conditions and leads to severely compromised CNS function [8,9]. We previously reported that APQ4 water permeability in astrocytes can be dynamically regulated [16] It has not been demonstrated whether astrocyte AQP4 responds to changes in extracellular potassium. The effect can be modulated by calcium when such signaling is triggered by extracellular potassium
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