EFFECT OF ANOXIA ON INTRACELLULAR AND EXTRACELLULAR POTASSIUM ACTIVITY IN HYPOGLOSSAL NEURONS INVITRO

1. A brain slice preparation was used to study the hypoglossal (XII) neuronal response to anoxia. Both intra- and extracellular potassium activities (K+(i), K+(o)) were measured by the use of ion-selective microelectrodes, and K+ flux was assessed by the use of pharmacologic blockers. 2. Extracellular recordings showed that a short period of anoxia (4 min) induced an increase in K+(o) of 26.4 +/- 7.5 mM (mean +/- SD, n = 20) in the XII nucleus of adult rats. 3. Intracellular recordings (n = 31) in XII neurons showed a substantial decrease in K+(i) during anoxia. Fourteen neurons were analyzed in detail and these showed that XII neurons depolarized to -25.3 +/- 7.7 mV, whereas K+(i) dropped from 93.6 +/- 14.9 to 32 +/- 9.0 mM. These results strongly suggested that K+ is lost from XII neurons during anoxia. 4. Although the extracellular space (ECS) shrank by approximately 50% during anoxia, the possibility that the increase in K+(o) and decrease in K+(i) were mainly caused by shrinkage of the ECS and swelling of intraneuronal space was excluded to a great degree because the changes in K+(i) and K+(o) during anoxia were relatively very large. 5. To study the mechanisms by which K+ is lost from XII neurons, we used several pharmacologic blockers. High concentration of ouabain (10 mM) and strophanthidin (80-mu-M) increased K+(o) from baseline (3-4 mM) to 40.9 +/- 2.5 mM (n = 6) but did not abolish an additional anoxia-induced increase in K+(o), suggesting that mechanisms other than Na+-K+-adenosine triphosphatase inhibition were also responsible for the anoxia-induced K+ leakage. 6. Glibenclamide and tolbutamide, two specific ATP-sensitive K+ channel blockers, prevented a substantial amount of K+ loss during anoxia with a dose-dependent response, whereas apamin had no effect, suggesting that anoxia-induced K- loss from XII neurons is related to the activation of ATP-sensitive but not Ca2+-dependent K+ channels. Tetrodotoxin and CoCl2, used to block synaptic transmission, decreased K+(o) accumulation, indicating that K+ loss is also related to increased neuronal activity during anoxia. 7. Intracellular recordings of membrane potential showed that the anoxia-induced depolarization in XII neurons was significantly enhanced by 25% when the perfusate contained glibenclamide (40-mu-M), demonstrating that activation of ATP-sensitive K+ channels during anoxia can limit the depolarization in XII neurons. 8. We conclude that during anoxia XII neurons lose their cytosolic K+, which substantially contributes to the increase in K+(o); loss of neuronal intracellular K+ is mediated to a large degree by the activation of ATP-sensitive K+ channels in XII neurons.