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.
Department of Cellular and Molecular Medicine, Faculty of Health and Medical Sciences, University of Copenhagen, Copenhagen, Denmark.
Brian Roland
BR
Larsen
Mette
M
Assentoft
Maria L
ML
Cotrina
Susan Z
SZ
Hua
Maiken
M
Nedergaard
Kai
K
Kaila
Juha
J
Voipio
Nanna
N
MacAulay
D000818:Animals; D000831:Animals, Newborn; D002034:Bumetanide / Q000494:pharmacology; D002478:Cells, Cultured; D002540:Cerebral Cortex / Q000166:cytology / Q000187:drug effects; D004305:Dose-Response Relationship, Drug; D018690:Excitatory Amino Acid Agonists / Q000494:pharmacology; D045604:Extracellular Fluid / Q000378:metabolism; D006624:Hippocampus / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D066298:In Vitro Techniques; D008297:Male; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D009865:Oocytes; D011188:Potassium / Q000378:metabolism* / Q000494:pharmacology; D024661:Potassium Channels, Inwardly Rectifying / Q000378:metabolism*; D051381:Rats; D017207:Rats, Sprague-Dawley; D049994:Sodium Potassium Chloride Symporter Inhibitors / Q000494:pharmacology; D000254:Sodium-Potassium-Exchanging ATPase / Q000378:metabolism*; D064506:Solute Carrier Family 12, Member 2 / Q000378:metabolism*; D014982:Xenopus laevis; D000098854:Kcnj10 Channel
D018690:Excitatory Amino Acid Agonists; D000098854:Kcnj10 Channel; D024661:Potassium Channels, Inwardly Rectifying; D049994:Sodium Potassium Chloride Symporter Inhibitors; D064506:Solute Carrier Family 12, Member 2; D002034:Bumetanide; D000254:Sodium-Potassium-Exchanging ATPase; D011188:Potassium
cell volume changes in mammalian brain; extracellular ion homeostasis; ion transport