Factors Affecting the Potassium Content of Incubated Brain Slices

When rat cerebral cortex slices are incubated in glucose–saline medium in the presence of oxygen, the slices lose half of their potassium almost immediately and then reconcentrate it to some extent. Uptake of potassium proceeds further if glutamate is added to the medium but, since the apparent intracellular space is also increased by glutamate, the intracellular potassium concentration is not increased by glutamate. The difference between the potassium concentrations in the incubated slice and in the medium reaches a maximum when the medium contains about 5 mM potassium. In the absence of oxygen, under ordinary conditions, slices lose potassium rapidly but the final concentration reached remains higher than that of the medium and this difference is marked if the concentration in the medium is high. The greatly depleted potassium content of slices which have been incubated anaerobically can be largely restored by subsequent aerobic incubation. The rate and extent of this restoration decreases with the length of time during which the slices have been deprived of oxygen. If the medium contains pyruvate or if the slices have been subjected to a previous period of aerobiosis, anaerobic glycolysis is stimulated and concomitantly the potassium concentration in the slices increases. The effects of pyruvate and previous aerobiosis are additive. The sum of sodium and potassium in slices incubated under various conditions does not vary greatly but the sum is significantly increased in the presence of glutamate, in phosphate instead of bicarbonate-buffered medium, and, especially, in medium containing a high potassium concentration. The fluid taken up in swelling has approximately the same total cation content as that of the medium.