Maintenance of a constant brain extracellular potassium.

The development of potassium specific ion exchanger microelectrodes has enabled investigators to measure directly brain extracellular potassium ion activity. Although serum potassium in various species ranges between 3.5 and 6 mEq/l, brain extracellular potassium is maintained at a level close to 3 mEq/l independent of fluctuations in serum values. Despite this buffering of the internal brain environment by extracerebral changes, local variations in extracellular potassium occur in response to evoked neuronal activity, seizures, and spreading depression. Mechanisms involved in the maintenance of this ionic homeostasis in the brain include mediated transport at the level of the cerebral capillary and the choroid plexus epithelium. In addition, there are ouabain-sensitive clearance mechanisms presumably involving Na,K-ATPase that participate in the removal of excess potassium. The relative roles of simple diffusion, high glial cell conductance of potassium, and active ionic pumps in restoring basal potassium levels after activity are still controversial.