Chemoattractant-induced respiratory burst: Increases in cytosolic Ca2+ concentrations are essential and synergize with a kinetically distinct second signal

The role of the cytosolic free Ca2+ concentration ([Ca2+](e)) and its relationship to other second messengers in the signalling between chemoattractant [e.g. N-formyl-L-methionyl-L-leucyl-L-phenylalanine (fMLP)] receptors and the NADPH oxidase is still poorly understood. In this study, we have used thapsigargin, an inhibitor of the Ca2+-ATPase of intracellular stores, as a tool to selectively manipulate Ca2+ release from intracellular stores and Ca2+ influx across the plasma membrane. We thereby temporarily separated the Ca2+ signal from other signals generated by fMLP and analysed the consequences on the respiratory burst. Under all conditions investigated, the extent of fMLP-induced respiratory burst activation was critically determined by [Ca2+](e) elevation. fMLP was unable to activate the respiratory burst without [Ca2+](e) elevation. Thapsigargin-induced Ca2+ influx activated the respiratory burst in the absence of fMLP, but only to approx. 20% of the values observed in the presence of fMLP. The second signal generated by fMLP did not activate the respiratory burst by itself, but acted in synergy with [Ca2+](e) elevation. The second signal was long lasting(> 15 min) provided that there was no rise in [Ca2+](e) and that the receptor was continuously occupied. The second signal was inactivated by high [Ca2+](e) elevation. Our results demonstrate that [Ca2+](e) elevations are an essential step in the signalling between the fMLP receptor and NADPH oxidase. They also provide novel information about the properties of the second Ca2+-independent signal that activates the respiratory burst in synergy with [Ca2+](e).