Control of Exocytosis in Early Neutrophil Activation

Exocytosis of human neutrophil secretory vesicles, gelatinase granules, specific granules, and azurophil granules was measured during modulation of the intracellular free calcium concentration. A strict rank order of exocytosis of the four compartments was observed when cytosolic calcium was elevated by an ionophore in Fura-2-loaded cells. The rank order was: secretory vesicles, gelatinase granules, specific granules, and azurophil granules. Secretory vesicles were exceptionally sensitive to cytosolic free calcium, being completely mobilized after small cytosolic calcium transients. Kinetic studies with FMLP or ionomycin as stimulus showed that the same rank order was valid concerning the rate of exocytosis. In contrast to the other granules, a major part of the secretory vesicles were exocytosed after FMLP stimulation when intracellular calcium was chelated with bis-(2-amino-5-methylphenoxy)-ethane-N,N,N',N'-tetraacetic acid tetraacetoxymethyl ester. These data indicate that, in contrast to other neutrophil granules, secretory vesicles are mobilized completely after small isolated elevations of cytosolic calcium. On the other hand, complete elimination of the FMLP-derived calcium transient does not inhibit exocytosis of secretory vesicles markedly. Thus, the effect of FMLP on secretory vesicles is signaled via cytosolic free calcium and an alternative pathway activated in parallel. This may be the basis for the unique position of this compartment as a readily mobilizable store of components important in early neutrophil activation.