Inward rectifying potassium channels in human malignant glioma cells

Human glioma cells obtained from established cell lines (Tp-276MG, Tp-301MG, Tp-378MG, Tp-483MG and U-251MG) were analyzed for the presence of ion channels with the tight-seal voltage clamp technique. The current-voltage relation revealed a marked inward rectification at hyperpolarizing voltages, due to the presence of inward rectifying K-channels in cells from all studied cell lines. These channels were conducting when the membrane potential was more negative than the K-equilibrium potential. The slope conductance for the inward K-currents (gKi) was affected both by [K+]i and [K+]o. gKi was proportional to [K+]o raised to 0.35 or 0.50, of which the larger value was measured in the presence of low [K+]i (25 mM). The rectification was not significantly different in cells perfused with Mg-free EDTA-buffered internal solution. Tl+ was 3.5 times more permanent than K+. gKi was blocked by Cs+ (1 mM) in a voltage-dependent way (more effective in the hyperpolarized membrane), and by Na+ (154 mM) depending on voltage and time. From measurements of unitary current events in membrane patches (outside out or cell attached) the conductance of the single inward rectifying channel was estimated to be 27 +/- 7 pS. This type of ion channel may be important for K-uptake by glial cells and hence for the K-homeostasis in the brain.