Extracellular potassium influences DNA and protein syntheses and glial fibrillary acidic protein expression in cultured glial cells

Abstract

Previous reports of increases in glial cell number and expression of glial fibrillary acidic protein (GFAP) in stimulated brain regions or epileptic tissue have implicated a role for increases in extracellular potassium concentration ([K⁺]_o) in glial reactions. We examined the effects of altered [K⁺]_o on DNA and protein syntheses and GFAP expression of cultured glial cells isolated from the posthatch chick brain stem. [K⁺]_o was varied by adding both KCl and NaCl to K⁺, NaCl‐free medium to achieve final [K⁺] of 1–50 mM. DNA and protein syntheses were measured by incorporation of ³H‐thymidine and ³H‐leucine, respectively, into acid‐insoluble material. GFAP expression was measured by a dot‐immunoblotting assay. DNA synthesis in glial cells cultured in high (5–50 mM) K⁺_o was 45–60% less than that of cells cultured in low (1–3 mM) K⁺_o. Protein synthesis per cell was increased 34–44% in cells cultured in high K⁺ as compared to those cultured in low K⁺. GFAP expression was inversely related to [K⁺]_o over the 1–10 mM range. Compared to the baseline of 3 mM K⁺_o, GFAP per cell was increased 65% at 1 mM and decreased 45% at 10 mM. These data suggest that increases in glial cell number and GEAP immunoreactivity found in sites of increased neuronal activity and in pathological tissues may not be caused solely by persistent increases in [K⁺]_o. Instead, these results suggest that neuronal activity, through the release of K⁺, may have an inhibitory influence on glial proliferation and GFAP expression. In light of work by others implying a relationship between GFAP immunoreactivity and rigidity of astroglial processes together with the data presented here, we suggest that the elevated [K⁺]_o accompanying neuronal activity, by inhibiting GFAP expression, may facilitate the morphological plasticity of glial cells. Conversely, conditions of low [K⁺]_o may contribute to rigidity of astrocytic processes.