Role of different Ca2+ sources in the superoxide production of human neutrophil granulocytes

The role of different Ca2+ sources in the activation of the NADPH oxidase was investigated in human neutrophil granulocytes. Selective depletion of the stimulus-responsive intracellular Ca2+-pool and the consequent opening of the store-dependent Ca2+ channel of the plasma membrane was achieved with thapsigargin, an inhibitor of microsomal Ca2+-ATPase. Low concentration (10–100 nM) of thapsigargin did not induce any O2•−-production, indicating that elevation of [Ca2+]ic to similar level and probably via similar route as following stimulation of chemotactic receptors, by itself is not sufficient to activate the NADPH oxidase. In significantly higher concentration (1–10 μM) thapsigargin did induce O2•−-generation but this effect was not the result of elevation of [Ca2+]ic. In the absence of external Ca2+ a gradual decrease of the responsive Ca2+ pool was accompanied by a gradual decrease of the rate and duration of the respiratory response stimulated by formyl-methionyl-leucyl-phenylalanin. Maximal extent of receptor-initiated O2•−-production could only be obtained when the intracellular [Ca2+] was higher than the resting level. Under this condition Ca2+ originating from intracellular or external source was equally effective in supporting the biological response.