Effects of ATP and elevated K+ on K+ currents and intracellular Ca2+ in human microglia

We have used whole-cell patch-clamp recordings and calcium microfluorescenc e measurements to study the effects of ATP and elevated external K+ on prop erties of human microglia. The application of ATP (at 0.1 mM) led to the ac tivation of a transient inward non-selective cationic current at a cell hol ding potential of -60 mV and a delayed, transient expression of an outward K+ current activated with depolarizing steps applied from holding level. Th e ATP response included an increase in inward K+ conductance and a depolari zing shift in reversal potential as determined using a voltage ramp wavefor m applied from -120 to -50 mV. Fura-2 microspectrofluorescence measurements showed intracellular calcium to be increased following the application of ATP, This response was characterized by an initial transient phase, which p ersisted in Ca2+-free media and was due to release of Ca2+ from intra cellu lar storage sites. The response had a later plateau phase, consistent with Ca2+ influx. In addition, ATP-induced changes in intracellular Ca2+ exhibit ed prominent desensitization. Elevated external K+ (at 40 mM) increased inw ard K+ conductance and shifted the reversal potential in the depolarizing d irection, with no effect on outward K+ current or the level of internal Ca2 The results of these experiments show the differential responses of human m icroglia to ATP and elevated K+, two putative factors associated with neuro nal damage in the central nervous system. (C) 1999 IBRO. Published by Elsev ier Science Ltd.