Activated neutrophils aggregate, generate superoxide (O-2), and degranulate. The role of Ca as "second messenger" in neutrophil activation was examined using as agonist the chemotactic peptide fMet-Leu-Phe and its antagonist t-butoxycarbonyl-Phe-Leu-Phe-Leu-Phe to systematically vary the time of receptor occupancy. Release of enzymes from specific and azurophil granules showed a finite requirement for receptor occupancy; the cells were committed to full degranulation after 10 s of receptor-agonist interaction. In contrast, continuous occupation of the receptor by agonist was required to initiate and maintain O-2 generation and aggregation. Cytosolic Ca ( Quin2 fluorescence) increased immediately in response to fMet-Leu-Phe, requiring less than 2 s of agonist-receptor interaction to initiate an optimal response. Mobilization of membrane-associated Ca (chlorotetracycline fluorescence) also demonstrated a finite time requirement; the cells were fully committed after 10 s of agonist-receptor interaction. Increased Ca permeability (45Ca uptake) was fully launched after 15 s of agonist-receptor interaction. The data indicate that Ca movements ( quin2 , chlorotetracycline fluorescence, 45Ca uptake) are both necessary and sufficient to account for degranulation by neutrophils activated by fMet-Leu-Phe. However, neutrophil aggregation and the generation and release of O-2 in response to the same stimulus require a further unknown factor(s) associated with receptor occupancy to maintain these responses.
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