Ceiling of stimulus induced rises in extracellular potassium concentration in the cerebral cortex of cat

Levels of extracellular potassium activity (aK) during repetitive electrical stimulation were measured with ion sensitive microelectrodes in the somatosensory cortex of the cat to determine maximum values ("ceiling" levels) under different experimental conditions. The maximal values of aK were 10.2 mequiv./1 during stimulation of the cortical surface (CS) or of the nucleus ventroposterolateralis thalami (VPL) and during selfsustained afterdischarges (SAD). Similarly, peak values were 6.5 mequiv./1 for the nucleus ventrolateralis anterior and 4 mequiv./1 for the nucleus centromedianus as well as for the nucleus cuneatus. The rise in aK during a test stimulus with constant intensity and frequency was inversely related to the level of aK produced by a preceding stimulation. Also rise in aK during SAD was smaller when it started from an enhanced level of aK. During repetitive stimulation of CS or VPL a rise in aK was not observed when aK was increased to levels above 10 mequiv./1 by superfusion with potassium enriched solutions. An electrophoretically evoked K+ test signal was reduced between 10 and 48% when applied during stimulus induced increased levels of aK. Stimulus induced potassium changes could become negative when aK was increased to levels above 7 mequiv./1 by local electrophoresis, while the stimulus induced increase in neuronal discharge rate did not disappear or reverse. Amplitudes of ECoG and local evoked potentials were reduced as aK increased during stimulation or superfusion. It is suggested that the ceiling in its steady state is maintained by an active K+ uptake mechanism which balances extra releases of K+. Decreased release of K+ at increased levels of aK may in addition limit the rise in aK.