Contributions of the Na + /K + ‐ATPase, NKCC1, and Kir4.1 to hippocampal K + clearance and volume responses

Network activity in the brain is associated with a transient increase in extracellular K + concentration. The excess K + is removed from the extracellular space by mechanisms proposed to involve Kir4.1‐mediated spatial buffering, the Na + /K + /2Cl − cotransporter 1 (NKCC1), and/or Na + /K + ‐ATPase activity. Their individual contribution to [K + ] o management has been of extended controversy. This study aimed, by several complementary approaches, to delineate the transport characteristics of Kir4.1, NKCC1, and Na + /K + ‐ATPase and to resolve their involvement in clearance of extracellular K + transients. Primary cultures of rat astrocytes displayed robust NKCC1 activity with [K + ] o increases above basal levels. Increased [K + ] o produced NKCC1‐mediated swelling of cultured astrocytes and NKCC1 could thereby potentially act as a mechanism of K + clearance while concomitantly mediate the associated shrinkage of the extracellular space. In rat hippocampal slices, inhibition of NKCC1 failed to affect the rate of K + removal from the extracellular space while Kir4.1 enacted its spatial buffering only during a local [K + ] o increase. In contrast, inhibition of the different isoforms of Na + /K + ‐ATPase reduced post‐stimulus clearance of K + transients. The astrocyte‐characteristic α2β2 subunit composition of Na + /K + ‐ATPase, when expressed in Xenopus oocytes, displayed a K + affinity and voltage‐sensitivity that would render this subunit composition specifically geared for controlling [K + ] o during neuronal activity. In rat hippocampal slices, simultaneous measurements of the extracellular space volume revealed that neither Kir4.1, NKCC1, nor Na + /K + ‐ATPase accounted for the stimulus‐induced shrinkage of the extracellular space. Thus, NKCC1 plays no role in activity‐induced extracellular K + recovery in native hippocampal tissue while Kir4.1 and Na + /K + ‐ATPase serve temporally distinct roles. GLIA 2014;62:608–622