Effects of the lysophospholipids sphingosine-1-phosphate and lysophosphatidic acid were studied in cultured murine microglia using the patch-clamp and video imaging techniques. Both lysophospholipids induced transient membrane hyperpolarization and K(+) current activation. The lysophospholipid-induced K(+) current was blocked by charybdotoxin or iberiotoxin, but was unaffected by apamin. In recordings with 1 microM intracellular free Ca(2+), Ca(2+)-dependent K(+) currents of microglia showed a similar pharmacological profile to lysophospholipid-induced currents. The Ca(2+)-dependent K(+) channels activated in microglia by lysophospholipids are most likely encoded by the IKCa1 channel gene. The presence of IKCa1 mRNA in microglia was demonstrated by reverse transcriptase-polymerase chain reaction studies. Ca(2+) imaging experiments revealed increases in the intracellular free Ca(2+) concentration of microglia to a mean value of about 400 nM after application of 1 microM sphingosine-1-phosphate or 1 microM lysophosphatidic acid. We suggest that the transient membrane hyperpolarization seen in microglia following exposure to sphingosine-1-phosphate or lysophosphatidic acid is caused by activation of IKCa1 Ca(2+)-dependent K(+) channels. Increases in the concentration of intracellular free Ca(2+) evoked by the lysophospholipids are sufficient to activate microglial Ca(2+)-dependent K(+) channels.