O2 deprivation in the central nervous system: on mechanisms of neuronal response, differential sensitivity and injury

O2 occlusion and cyanide induced immediate relaxation and contraction of murine skeletal muscle

The acute changes of muscle tone and membrane current upon occlusion of oxygenation (O2 occlusion) were studied in vitro in mouse diaphragms. O2 occlusion immediately produced a contraction and a relaxation, respectively, in ryanodine- and high K(+)-contracted muscles while a biphasic change (an initial decrease then a late increase) of muscle tone was produced in muscles contracted with caffeine. The O2 occlusion effects were reversed after reoxygenation. CN- produced similar acute changes of muscle tone and abolished O2 occlusion effects. The O2 occlusion-induced relaxation in high K+ medium was converted into a contraction by 3,4-diaminopyridine and by low Cl- Tyrode's. O2 occlusion induced a small outward current and membrane hyperpolarization at a rate slower than the changes of muscle tone. Glybenclamide inhibited all of the changes induced by O2 occlusion. It is possible that the K+ and Cl- permeabilities of sarcoplasmic reticulum are highly sensitive to hypoxic challenge and related to the immediate changes of muscle tone after O2 occlusion.

Short periods of hypoxia activate a K+ current in central neurons

The effect of hypoxia on ionic currents was studied in acutely dissociated substantia nigra neurons. Using an external solution containing 0 mM Na+ and 0.5 mM Co2+, we found that overall whole-cell outward currents increased by 15-20% during 3-4 min of hypoxia. This current was voltage sensitive and could be completely blocked by TEA- and Cs+, suggesting that this is a K+ current. In cell-attached patches with 150 mM K+ in the external solution, we recorded a large-conductance outward current which was not observed during baseline and was reversibly activated by hypoxia. These results therefore provide the first direct evidence for the activation of K+ channels during O2 deprivation in central neurons.