Release of azurophilic granule contents in fMLP-stimulated neutrophils requires two activation signals, one of which is a rise in cytosolic free Ca2+

We have used a continuous spectrofluorimetric method to analyse the role of cytosolic free Ca2+ ([Ca2+]i) in the lysosomal enzyme release from the azurophilic granules in human neutrophils stimulated with f-Met-Leu-Phe (fMLP) in the presence of cytochalasin B. Measurements were performed with the beta-glucuronidase substrate 4-methylumbelliferyl-beta-D-glucuronide. We found that the transient rise in [Ca2+]i induced by fMLP is a necessary signal to obtain maximal degranulation. When this Ca2+ transient is prevented by the Ca2+ chelator BAPTA, degranulation can still be induced by a stimulated Ca2+ influx, albeit to a lower extent. We also studied the degranulation process in the neutrophils of a patient with a generalized chemotactic defect. Release of beta-glucuronidase from the patient's neutrophils could not be induced despite the occurrence of a normal Ca2+ response and normal degranulation of specific granules. We conclude that, besides an increase in [Ca2+]i, an additional signal is required for the fusion of azurophilic granules with the plasma membrane in human neutrophils.