Properties of voltage-gated potassium currents of microglia differentiated with granulocyte/macrophage colony-stimulating factor.

Voltage-gated whole-cell currents were recorded from cultured microglial cells which had been developed in the presence of the macrophage/microglial growth factor granulocyte/macrophage colony-stimulating factor. Outward K+ currents (IK) were most prominent in these cells. IK could be activated at potentials more positive than -40 mV. Half-maximal activation of IK was achieved at -13.8 mV and half-maximal inactivation of IK was determined at -33.8 mV. The recovery of IK from inactivation was described by a time constant of 7.9 sec. For a tenfold change in extracellular K+ concentration the reversal potential of IK shifted by 54 mV. Extracellularly applied 10 mM tetraethylammonium chloride reduced IK by about 50%, while 5 mM 4-aminopyridine almost completely abolished IK. Several divalent cations (Ba2+, Cd2+, Co2+, Zn2+) reduced current amplitudes and shifted the activation curve of IK to more positive values. Charybdotoxin (IC50 = 1.14 nM) and noxiustoxin (IC50 = 0.89 nM) blocked IK in a concentration-dependent manner, whereas dendrotoxin and mast cell degranulating peptide had no effect on the current amplitudes. The outward K+ currents showed a frequency dependence when depolarizing pulses were applied at a frequency of 1 Hz. A frequency-independent outward current (IK') characterized by the same activation behavior as IK was detected. IK' was blocked completely by 10 nM charybdotoxin or by 10 nM noxiustoxin. In contrast to its effect on IK, 10 mM tetraethylammonium chloride did not reduce IK'.