Capacitative Ca2+ influx and activation of the neutrophil respiratory burst. Different regulation of plasma membrane- and granule-localized NADPH-oxidase.

The neutrophil NADPH-oxidase may be activated in the plasma membrane, resulting in release of oxygen metabolites extracellularly, or in the granule or phagosomal membranes, giving intracellular production of oxidants. An increase in [Ca2+]i mediated through binding of fMLF to its receptor is part of a signaling cascade that activates the plasma membrane-localized oxidase. In contrast, a rise in [Ca2+]i induced by a Ca2+ ionophore results in activation of the intracellular pool of oxidase. We mimicked fMLF-induced emptying of intracellular Ca2+ stores with thapsigargin. This induced a pronounced intracellular oxidase activity but no extracellular release of oxidants. The thapsigargin-induced effect was dependent on capacitative Ca2+ influx, because the effect was inhibited dose-dependently by EGTA and the Ca2+ channel blocker La3+. At La3+ concentrations between 200 and 400 microM, thapsigargin also induced a massive extracellular production of superoxide anion. No other channel blockers tested induced a similar effect. We conclude that elevation in [Ca2+]i by capacitative Ca2+ influx induces NADPH-oxidase activation at an intracellular site. Further, activation of the plasma membrane-localized NADPH-oxidase is regulated by a more complex Ca2+ signaling, involving capacitative Ca2+ influx and possibly the specific action of La3+-sensitive Ca2+ channels.