Deep Isoflurane Anesthesia Is Associated with Alterations in Ion Homeostasis and Specific Na+/K+-ATPase Impairment in the Rat Brain.

BACKGROUND: Maintenance of ion homeostasis is essential for normal brain function. Inhalational anesthetics are known to act on various receptors, but their effects on ion homeostatic systems, such as sodium/potassium-adenosine triphosphatase (Na+/K+-ATPase), remain largely unexplored. Based on reports demonstrating global network activity and wakefulness modulation by interstitial ions, the hypothesis was that deep isoflurane anesthesia affects ion homeostasis and the key mechanism for clearing extracellular potassium, Na+/K+-ATPase.

METHODS: Using ion-selective microelectrodes, this study assessed isoflurane-induced extracellular ion dynamics in cortical slices of male and female Wistar rats in the absence of synaptic activity, in the presence of two-pore-domain potassium channel antagonists, during seizures, and during spreading depolarizations. The specific isoflurane effects on Na+/K+-ATPase function were measured using a coupled enzyme assay and studied the relevance of the findings in vivo and in silico.

RESULTS: Isoflurane concentrations clinically relevant for burst suppression anesthesia increased baseline extracellular potassium (mean ± SD, 3.0 ± 0.0 vs. 3.9 ± 0.5 mM; P < 0.001; n = 39) and lowered extracellular sodium (153.4 ± 0.8 vs. 145.2 ± 6.0 mM; P < 0.001; n = 28). Similar changes in extracellular potassium and extracellular sodium and a substantial drop in extracellular calcium (1.5 ± 0.0 vs. 1.2 ± 0.1 mM; P = 0.001; n = 16) during inhibition of synaptic activity and two-pore-domain potassium suggested a different underlying mechanism. After seizure-like events and spreading depolarization, isoflurane greatly slowed extracellular potassium clearance (63.4 ± 18.2 vs. 196.2 ± 82.4 s; P < 0.001; n = 14). Na+/K+-ATPase activity was markedly reduced after isoflurane exposure (greater than 25%), affecting specifically the α2/3 activity fraction. In vivo, isoflurane-induced burst suppression resulted in impaired extracellular potassium clearance and interstitial potassium accumulation. A computational biophysical model reproduced the observed effects on extracellular potassium and displayed intensified bursting when Na+/K+-ATPase activity was reduced by 35%. Finally, Na+/K+-ATPase inhibition with ouabain induced burst-like activity during light anesthesia in vivo.

CONCLUSIONS: The results demonstrate cortical ion homeostasis perturbation and specific Na+/K+-ATPase impairment during deep isoflurane anesthesia. Slowed potassium clearance and extracellular accumulation might modulate cortical excitability during burst suppression generation, while prolonged Na+/K+-ATPase impairment could contribute to neuronal dysfunction after deep anesthesia.