A study of the mechanisms by which potassium moves through brain tissue in the rat.

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Abstract
The flux of K+ produced by electric current across the pia-arachnoid surface of the neocortex of anesthetized rats was studied with K+-selective electrodes in a cup at the surface and with flame photometry. The potential differences developed across 3 regions of the rat brain (neocortex, cerebellum, hippocampus) were measured as [K+] was altered in fluid at the surface. The experimental results were related to those that would be expected if K+ moved principally by diffusion in extracellular space and if current flow through cells makes a significant contribution to K+ transfer. K+ movement produced by current across the neocortical surface accounted for 0.06 of the transfer of electric charge with small currents in either direction (.apprx. 5 .mu.A mm-2) and with larger currents out of the tissue. Large currents (.apprx. 20 .mu.A mm-2) into the tissue produced less K+ movement, but still more than the fraction 0.012 expected for purely extracellular flux. Alternating current pulses (5 Hz) with zero net transfer of charge produced no flux of K+ across the surface, while alternation with unequal durations produced the same effects as the equivalent steady charge transfer. The K+ flux lagged behind the onset and cessation of current with a time constant .apprx. 45 s, approximately as expected from calculations with a model of the tissue. A surface-negative potential shift averaging 2 mV was observed when [K+] at the brain surface was increased from 3 to 12 mM. The time for development of half of the full potential change was 20 s, with the solution changes complete in < 4 s. These results are inconsistent with the hypothesis that K+ movement through brain tissue occurs principally through intercellular clefts, except where these movements involve very localized gradients. They are consistent with the conclusion that .apprx. 5 times as much K+ flux passes through cells (probably largely glial cells) as through extracellular space, with fluxes driven by either extracellular voltage or concentration gradients.