CHEMOATTRACTANT-INDUCED RESPIRATORY BURST - INCREASES IN CYTOSOLIC CA2-SIGNAL( CONCENTRATIONS ARE ESSENTIAL AND SYNERGIZE WITH A KINETICALLY DISTINCT 2ND)

The role of the cytosolic free Ca2+ concentration ([Ca2+](e)) and its relationship to other second messengers in the signalling between chem oattractant [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 i ntracellular 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 ge nerated by fMLP and analysed the consequences on the respiratory burst . Under all conditions investigated, the extent of fMLP-induced respir atory burst activation was critically determined by [Ca2+](e) elevatio n. 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 o bserved in the presence of fMLP. The second signal generated by fMLP d id 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 wa s continuously occupied. The second signal was inactivated by high [Ca 2+](e) elevation. Our results demonstrate that [Ca2+](e) elevations ar e an essential step in the signalling between the fMLP receptor and NA DPH oxidase. They also provide novel information about the properties of the second Ca2+-independent signal that activates the respiratory b urst in synergy with [Ca2+](e).