Effects of phorbol ester on intracellular Ca2+ and membrane currents in cultured human microglia

The effects of protein kinase C (PKC) activation by phorbol ester on intracellular Ca2+ concentration ([Ca2+](i)) and membrane currents in human microglia grown in culture were investigated. Treatment of microglia with phorbol myristate acetate (PMA) resulted in a large increase in [Ca2+](i) in cells loaded with fura-2. The increased levels of [Ca2+](i) were not altered following removal of the phorbol ester. In Ca2+-free medium, application of PMA did not increase [Ca2+](i). In addition, PMA application in standard Ca2+-solution containing lanthanum (1.8 mM) had no effect on the microglial response to PMA, suggesting that the phorbol ester actions were due to transmembrane influx of Ca2+ but not through voltage-gated Ca2+ channels, Whole-cell patch clamp measurements demonstrated that PMA potentiated an outward K+ current and inhibited an inward rectifier K+ current. This study is the first demonstration that PKC activation by phorbol ester leads to increased intracellular [Ca2+] and changes in membrane currents in human microglia.