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.