In vivo and in vitro effects of phenytoin on anion and cation transport systems of the rat brain

Various parameters of anion and cation transport were measured in the cerebral cortex of neonatal (3 days old) and adult rats following acute and chronic treatment with PHT. Acutely, phenytoin (PHT) significantly inhibited the enzyme Na+, K+-ATPase in both neonatal and adult rats. This effect was accompanied by a significant increase in cerebral cortical sodium (Na+) content and a decrease in potassium (K+) content only in neonatal animals. Chronic treatment (bid and qid for 7 days) of adult rats with PHT significantly reduced the Na+ content without affecting the whole homogenate Na+, K+-ATPase activity. The activity of this enzyme was markedly increased in the myelin- (glial product) and slightly decreased in the synaptosomal- (neuronal) fractions following chronic (qid for 7 days) PHT treatment. These results suggest that PHT differentially affects the two forms (neuronal and glial) of the enzyme Na+, K+-ATPase. The possible relevance of this hypothesis in relationship to the anticonvulsant and excitatory properties of PHT is discussed. Chronic (bid and qid for 7 days) PHT treatment increased both DNA content and activity of the glial marker enzyme carbonic anhydrase. Activity of the mitochondrial enzyme HCO3-C1-ATPase was also increased following chronic PHT treatment. These two enzymes are intimately involved in the regulation of HCO3[-]-C1- transport across glial cell and mitochondrial membranes, and these results suggest that PHT is able to beneficially effect glial regulatory processes. The ability to enhance glial regulation of extracellular fluid anions and cations provides new and important insights into the mechanism of the anticonvulsant action of PHT. The activity of enzymes involved in anion and cation transport, the concentration of intracellular potassium (K+i), and the trans-membrane potential (Em) was determined following acute and chronic exposure of primary astroglial cultures to micro-molar concentrations of phenytoin (PHT). Na+, K+-ATPase activity of homogenates of cultured glial cells was determined in the presence of increasing K+ concentration (1-20 mM). Acutely, PHT had little effect on the K+-activation pattern of Na+, K+-ATPase. In contrast, the percent of Na+, K+-ATPase activated by elevating the K+ concentration was dose-dependently increased by chronic PHT treatment. This effect was accompanied by a marked increase in K+i and a significant membrane hyper polarization. The acute effect of PHT on the Em was biphasic, characterized by membrane hyper polarization at concentrations of 1 x 10[-6] to 1 x 10[-5] M. At concentrations between 1 x 10[-5] M and 1 x 10[-4] M, the Em progressively returned to control values. These results suggest that glial cells acutely and chronically treated with therapeutic concentrations of PHT have an enhanced capacity to control elevated extracellular potassium. Return of the Em to control values at PHT concentrations greater than 1 x 10[-5] M suggest that these cells are less able to regulate extracellular potassium. These data can partially explain the excitatory effects of PHT at high therapeutic concentrations.