Activation of the respiratory burst in eosinophil leucocytes—a transduction sequence decoupled from cytosolic Ca2+ rise

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Abstract
The activation of the respiratory burst by complement factor 5a (C5a), platelet‐activating factor (PAF), formyl‐Met‐Leu‐Phe (fMLP) and neutrophil‐activating peptide IL‐8 was explored in eosinophils from patients with the hypereosinophilic syndrome. The amplitude of the response increased with increasing concentrations of C5a and PAF, but the time for its induction was unaffected by the amount of stimulus applied. Respiratory burst activity resulting from phorbol 12‐myristate, 13‐acetate (PMA)‐mediated activation of protein kinase C (PKC) produced longer onset times, which shortened with increasing PMA concentrations. Total inhibition of the C5a‐ and PMA‐mediated burst could be achieved with the PKC inhibitor staurosporine at concentrations of 100 and 5 nM, respectively. Calcium depletion abolished agonist‐induced rises in cytosolic free calcium ([Ca2+]i) and respiratory burst activity, but not PMA‐mediated NADPH‐oxidase activation. While PMA reduced elevations in [Ca2+]i, it restored the burst response to agonists in Ca2+‐depleted eosinophils. These results agree with the agonist‐induced activation of the NADPH‐oxidase via PKC, but suggest a parallel, Ca2+‐, phospholipase C‐ and PKC‐independent signal transduction pathway. Data obtained with B. pertussis toxin showed that the respiratory burst in eosinophils is blocked by ADP‐ribosylation of Gi‐proteins, but that in the presence of PMA portions of the agonist response could be recovered.