Identification of RBK1 potassium channels in C6 astrocytoma cells

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Abstract
Ionic currents in C6 astrocytoma cells were studied using the patch clamp technique under the whole cell configuration. A delayed rectifier K+ current with an amplitude of ∼ 1 nA at +50 mV was observed in 86% (92/107) of the cells examined. This K+ current resembled the delayed rectifier present in type‐1 and type‐2 astrocytes in vitro and could be inhibited by a variety of K+ channel blockers, including TEA (IC50: 0.5 mM), 4‐aminopyridine (IC50: 0.2 mM), MCD peptide (IC50 : 52 nM), dendrotoxin I (IC50: 9 nM), and charybdotoxin (74% inhibition at 50 nM). Northern blot analysis, cloning of cDNA and subsequent sequencing showed that the C6 cell delayed rectifier K+ channel is equivalent to the RBK1 K+ channel derived from a rat brain cDNA library. The level of RBK1 transcripts in C6 cells was comparable to that reported in rat brain. The C6 delayed rectifier K+ channel is probably a homomeric RBK1 K+ channel judging from its pharmacological properties which are similar to the RBK1 channel expressed in Xenopus oocytes. Some C6 cells also expressed a transiently activated outward K+ current (IA). This current was found in less than 50% of the cells and in general contributed no more than 8% of the total outward current. No voltage‐dependent inward Na+ or Ca2+ currents or inwardly rectifying K+ currents were observed in over 100 C6 cells examined. The present results show that the dominant voltage gated ionic current in C6 cells is the RBK1 delayed rectifier K+ channel. Since C6 cells express RBK1 mRNA at a level comparable to its rat brain counterpart, these cells may be a valuable system to study the RBK1 channel and its gene expression.