Acute actions of tumor necrosis factor-alpha on intracellular Ca2+ and K+ currents in human microglia

The effects of acute application of the pro-inflammatory cytokine tumor nec rosis factor-alpha (TNF alpha) on levels of intracellular Ca2+ ([Ca2+]i) an d on whole-cell outward and inward K+ currents were studied in cultured hum an microglia. TNF alpha elicited a linear increase in [Ca2+]i to a plateau level in microglia bathed in either standard physiological saline solution or Ca2+-free physiological saline solution. The rate of increase of [Ca2+]i or the level of [Ca2+]i attained was not significantly altered in the abse nce of external Ca2+ indicating that Ca2+ influx did not contribute appreci ably to the cytokine-induced rise in [Ca2-]i. This point was directly confi rmed using Mn2+ quenching where no change in signal fluorescence was observ ed with TNF alpha treatment of microglia in Ca2+-free physiological saline solution. The rate of increase of [Ca2+]i induced by TNF alpha in Ca2+-free physiological saline solution was not altered by prior application of ATP to deplete inositol triphosphate stores indicating that these stores did no t contribute to the cytokine response. In whole-cell patch clamp recordings . the acute treatment of human microglia with TNF alpha led to the expressi on of an outward K+ current in one-third (14 of 41) of cells. This current was activated at potentials positive to -30 mV. showed rapid kinetics of ac tivation with no evident inactivation and had an I-V relation exhibiting ou tward rectification. Analysis of tail currents showed reversal of the outwa rd K+ current near -70 mV and tetraethylammonium (10 mM) inhibited the outw ard K+ current to 24% of control level. Acute application of TNF alpha. had no effect to alter inward rectifier currents generated from voltage ramps. The signaling pathways involving TNF alpha modulation of [Ca2+]i and K+ cha nnels in human microglia may contribute to functional and pathological acti ons of the cytokine in the brain. (C) 2001 IBRO, Published by Elsevier Scie nce Ltd. All rights reserved.