[K+]o clearance in cortex: a new analytical model.

1. It has been suggested that passive diffusion is the principal mechanism of the clearance of locally elevated extracellular potassium, K+o, in the cerebral cortex. This concept was based on the assumption that elevated K+ could be modeled as a point source. In the present study, the functional role of passive diffusion was reevaluated in the anesthetized cat cortex following local electrical stimulation. 2. The initial spatial distribution of extracellular potassium activity, [K+]o, elevated by monopolar stimulation, could be modeled by a two-dimensional Gaussian function at and below the 500-micron cortical depth. Azimuthal symmetry around the stimulating electrode was assumed and cylindrical spatial coordinates were used. 3. The observed clearance of transiently elevated [K+]o as a function of space and time was much more rapid than that predicted by an analytical model consisting of the homogeneous diffusion equation whose initial condition was the Gaussian spatial distribution of [K+]o at the onset of the clearance process. 4. It is concluded that passive diffusion does not significantly contribute to the rapid clearance of locally elevated extracellular potassium in the cortex. Active uptake of potassium by cortical cells should be more seriously considered as being primarily responsible for the potassium clearance.