The acute actions of the cytokine, interferon-gamma (IFN-gamma), on intracellular calcium Ca(2+) levels in human microglia were investigated. In the presence of a calcium-containing physiological solution (Ca(2+)-PSS), IFN-gamma caused a progressive increase in Ca(2+) to a plateau level with a mean rate of increase of 0.81 +/- 0.17 nM/s and mean amplitude of 102 +/- 12 nM (n = 67 cells). Washout of the cytokine did not alter the plateau established with IFN-gamma in Ca(2+)-PSS; however, introduction of a Ca(2+)-free PSS diminished Ca(2+) to baseline levels. The decrease in Ca(2+) with Ca(2+)-free PSS would indicate that the response to IFN-gamma was mediated by an influx pathway. This result was confirmed in separate experiments showing the lack of an induced change in Ca(2+) with IFN-gamma applied in Ca(2+)-free PSS. The increase in Ca(2+) induced in Ca(2+)-PSS was reduced to near baseline levels when the external solution contained low Cl(-) in the maintained presence of IFN-gamma suggesting that cellular depolarization inhibited the cytokine mediated entry pathway. The compound SKF96365, which blocks store operated influx of Ca(2+) in human microglia, was ineffective in altering the increase in Ca(2+), however, La(3+) completely inhibited the Ca(2+) response induced by IFN-gamma. Whole-cell patch clamp studies showed no effect of IFN-gamma to alter outward currents and inward rectifier K(+) currents. The influx of Ca(2+) may serve a signaling role in microglia linking IFN-gamma to functional responses of the cells to infiltrating T lymphocytes into the central nervous system (CNS) during inflammatory processes.