Changes in Astroglial K upon Brief Periods of Energy Deprivation in the Mouse Neocortex.

Malfunction of astrocytic K regulation contributes to the breakdown of extracellular K homeostasis during ischemia and spreading depolarization events. Studying astroglial K changes is, however, hampered by a lack of suitable techniques. Here, we combined results from fluorescence imaging, ion-selective microelectrodes, and patch-clamp recordings in murine neocortical slices with the calculation of astrocytic [K]. Brief chemical ischemia caused a reversible ATP reduction and a transient depolarization of astrocytes. Moreover, astrocytic [Na] increased by 24 mM and extracellular [Na] decreased. Extracellular [K] increased, followed by an undershoot during recovery. Feeding these data into the Goldman-Hodgkin-Katz equation revealed a baseline astroglial [K] of 146 mM, an initial K loss by 43 mM upon chemical ischemia, and a transient K overshoot of 16 mM during recovery. It also disclosed a biphasic mismatch in astrocytic Na/K balance, which was initially ameliorated, but later aggravated by accompanying changes in pH and bicarbonate, respectively. Altogether, our study predicts a loss of K from astrocytes upon chemical ischemia followed by a net gain. The overshooting K uptake will promote low extracellular K during recovery, likely exerting a neuroprotective effect. The resulting late cation/anion imbalance requires additional efflux of cations and/or influx of anions, the latter eventually driving delayed astrocyte swelling.