Mechanisms of Brain Ion Homeostasis during Acute and Chronic Variations of Plasma Potassium

Brain and CSF potassium concentrations are well regulated during acute and chronic alterations of plasma potassium. In a previous study, we have shown that during chronic perturbations, regulation is achieved by appropriate adaptation of potassium influx, but that the degree of such adaptation during acute perturbations is much less. To elucidate further potential regulatory mechanisms, rats were rendered acutely or chronically hyper- or hypokalemic (range 2.7–7.6 mM). Measurements were made of brain and CSF water and ion contents to examine whether regulation occurred by modulation of K⁺ uptake into parenchymal cells. Furthermore, the permeability-surface area products (PSs) of ²²Na⁺ were determined, because changes in K⁺ efflux via Na⁺,K⁺-ATPase on the brain-facing side of the blood–brain barrier might be reflected in modified Na⁺ permeability. Brain and CSF K⁺ concentrations and Na PS were all independent of chronic changes in plasma K⁺ and acute hypokalemia, suggesting that neither modulation of parenchymal K⁺ uptake nor K⁺ efflux via the Na⁺,K⁺-ATPase is involved in extracellular K⁺ regulation in these conditions. In contrast, Na PSs were increased by 40% (p < 0.05) in acute hyperkalemia. This was accompanied by a slight loss of tissue K⁺ and water from the intracellular space. These results suggest that increased potassium influx in acute hyperkalemia is compensated by stimulation of K⁺ efflux via Na⁺,K⁺-ATPase. A slight degree of overstimulation, as indicated by a net loss of tissue K⁺, leads us to hypothesize that other factors, apart from the kinetic characteristics of Na⁺,K⁺-ATPase, may regulate this enzyme at the blood–brain barrier.