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

Abstract

The role of the cytosolic free Ca²⁺ concentration ([Ca²⁺]_c) 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⁺]_c elevation. fMLP was unable to activate the respiratory burst without [Ca²⁺]_c 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⁺]_c elevation. The second signal was long lasting (> 15 min) provided that there was no rise in [Ca²⁺]_c and that the receptor was continuously occupied. The second signal was inactivated by high [Ca²⁺]_c elevation. Our results demonstrate that [Ca²⁺]_c 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⁺]_c.