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 alpha 2 beta 2 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