An inhibitor of cyclic AMP-dependent protein kinase enhances the superoxide production of human neutrophils stimulated by N-formyl-methionyl-leucyl-phenylalanine

Intact human neutrophils produced superoxide (O₂^–) by the stimulation with N-formyl-methionyl-leucyl-phenylalanine (fMLP) even when the extracellular Ca²⁺ was absent (0.56±0.13 nmol/min per 10⁶ cells). The production by fMLP was enhanced more than twice in the presence of the extracellular Ca²⁺. Moreover, the O₂^– production by fMLP in the presence of extracellular Ca²⁺ was enhanced nearly three times by the treatment of cells with H-89, an inhibitor of cyclic AMP-dependent protein kinase (PKA). The enhancement was not observed when the extracellular Ca²⁺ was depleted from the reaction mixture. In addition, H-89 did not enhance fMLP-induced O₂^– production of electropermeabilized neutrophils in which the intracellular Ca²⁺ concentration was fixed to about 100 nM. These observations suggest that not only Ca²⁺ influx but the inhibition of PKA is necessary for the maximum O₂^– production by fMLP and that the O₂^– production is partially suppressed by the activation of PKA induced by fMLP.