Concentration-sensitive Na⁺ channel (Na_C) is involved in the regulation of proliferation in rat C6 glioma cells

Abstract

The concentration-sensitive Na⁺ channel (Na_C; c:concentration) works as a Na⁺ sensor and it opens when [Na⁺]_o changes. The present study was carried out to clarify the function of the Na_C as one of the regulating factors in cell growth, using rat C6 glioma cells, since they have Na_C in quantity. The image analysis of Na⁺ dynamics, using a Na⁺ indicator (SBFI) and ARGUS-50 (Hamamatsu Photonics), revealed an elevation of [Na⁺]_i when [Na⁺]_o was raised from normal (140 mM) to 190 mM. This increase was augmented when Na⁺ efflux was suppressed by inhibitors of the Na⁺ pump (ouabain) or of the Na⁺/K⁺/Cl⁻ cotransporter (bumetanide). Osmolarity alteration was not important because addition of mannitol to the external solution did not introduce any changes in [Na⁺]_i. The expression of immediate early gene egr-1, measured by the real-time PCR method, was reduced when [Na⁺]_o was raised or when [Na⁺]_i was elevated by a Na⁺ ionophore, monensin (Cell Biol Int 29:261-268, 2005). These procedures suppressed the rate of cell proliferation. When the expression of Na_C was selectively inhibited by RNA interference (RNAi) techniques, both [Na⁺]_i and the growth rate of C6 cells were less affected by [Na⁺]_o changes, indicating that Na_C was involved in cell growth (by controlling gene expression through introduction of Na⁺ into the cell). It is concluded that Na⁺ ions enter C6 glioma cells mainly through Na_C, and Na⁺ ions regulate cell growth by controlling expression of proliferation-related genes. [J Physiol Sci. 2006;56 Suppl:S112]