Ammonium triggers calcium elevation in cultured mouse microglial cells by initiating Ca2+ release from thapsigargin-sensitive intracellular stores

Microglial cells are thought to serve as sensors for pathologic events in the brain. In the present study we demonstrate that these cells respond with an increase in intracellular calcium concentration ([Ca2+]i) to intracellular alkaline shifts induced by either application of NH3/NH4+ or by an extracellular alkaline shift. The cytoplasmic pH (pHi) and [Ca2+]i in cultured mouse microglial cells were studied employing the fluorescent probes BCECF and fura-2, respectively. Application of NH3/NH4+ caused an initial rapid alkalinization followed by a slow recovery towards the resting level, while application of alkaline (pH 8.2) solution triggered a slower rise in pHi. The [Ca2+]i elevation triggered by NH3/NH4+ and extracellular alkaline shift were caused by different mechanisms: extracellular alkalinization induced a transmembrane Ca2+ entry, whereas NH3/NH4+ triggered Ca2+ release from thapsigargin- and ATP-sensitive intracellular pools. The mobilization of intracellular Ca2+ caused by NH3/NH4+ was blocked by a specific inhibitor of phospholipase C, U-73122, but was not affected by an inhibitor of G-protein, pertussis toxin. This implies that NH3/NH4 interacts with phospholipase C and leads to an increase in the intracellular level of inositol 1,4,5-trisphosphate (InsP3). In contrast to a previous study using a microglial cell line, application of NH3/NH4+ did not result in a release of tumor necrosis factor alpha (TNF-alpha), a marker of microglial activation, in the primary microglial cells. This implies that ammonium does not lead to activation of microglia in the culture model.