The equilibration time course of [K⁺]₀ in cat cortex

Summary

Steady state and transient values of intracortical potassium were measured with K⁺ sensitive microelectrodes. Resting intracortical K⁺ activity is low and resembles that of cerebrospinal fluid. Elevation of intracortical K⁺ was brought about by electrophoretic injection of K⁺ by a constant current source from a KCl containing micropipette at fixed distances from the recording electrode. The intracortical K⁺ responses to electrophoretic K⁺ injection were compared with those in a medium of 150 mM/l NaCl plus 3 mM/l KCl. The dependence of intracortical K⁺ steady state levels on electrophoretic currents is nearly linear, but the K⁺ response in the cortex was about six times higher than in saline. Half times (T_1/2) of the rising and falling phases of K⁺ during current steps were found to be prolonged by the same degree in the cortex. The distribution of [K⁺]₀ appears to be dominated by free diffusion with an apparent diffusion coefficient of 1/6 that in the medium. Primarily diffusional redistribution may also apply to K⁺ which is released by direct cortical stimulation. K⁺ released by brief stimulation distributes faster than K⁺ during and after prolonged continuous stimulation with average T_1/2 of 1.2 and 3.0 sec respectively in accordance with diffusion from instantaneous and continuous point sources. For small [K⁺]₀ changes, deviations from diffusional kinetics were found to be about one-fifth of absolute [K⁺]₀ values and became predominant at times longer than 10 T_1/2. They can be ascribed to K⁺ uptake mechanisms. DC recorded cortical surface potentials reveal close relations to the slopes of intracortical potassium activity.