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.