Glial Versus Neuronal Na /K -ATPase in Activity-Evoked K Clearance and Their Sensitivity to Elevated Extracellular K.

Neuronal activity in the central nervous system is associated with a [K] transient that is swiftly cleared from the extracellular space, predominantly by the Na/K-ATPase. The temporal contribution of the glial (α2β2) and the neuronal (α3β1) isoform complexes remains unresolved due to the lack of an isoform-specific inhibitor. The role of the two main brain isoform complexes in spreading depression (SD) also remains unresolved, but an SD-mediated increase in [K] may suppress Na/K-ATPase activity and thereby promote SD propagation. As demonstrated here, inhibitor assays of purified recombinant human and heterologously expressed rat Na/K-ATPase isoforms demonstrated significant selectivity for inhibition of α2β2 compared to α3β1 isoform complexes by a cyclobutyl perhydro-1,4-oxazepine derivative of digoxin (DcB). This phenomenon was utilized to demonstrate the temporal role of α2β2 and α3β1 in [K] clearance in electrically stimulated rat hippocampal slices, as monitored with ion-sensitive microelectrodes. The observations demonstrate a role of α2β2 in regulating the [K] during electrical stimulus of hippocampal slices, whereas α3β1 serves to restore [K] to baseline post-stimulus. SD can be triggered by elevated [K] but elevated [K] did not reduce the activity of the Na/K-ATPase or the glutamate transporters in hippocampal brain slices or upon heterologous expression of individual isoforms in Xenopus oocytes. Our results demonstrate the temporal contribution of the glial and neuronal Na/K-ATPase isoform complexes to clearance of [K] but do not support the concept that direct effects of elevated [K] on Na/K-ATPase activity or glutamate transport underlie SD propagation.