Is Cytosolic Ionized Calcium Regulating Neutrophil Activation?

The concentration of cytosolic ionized calcium, Ca2+i, was measured in intact neutrophils by use of a fluorescent indicator trapped in the icytoplasm. A given rise of Ca2+i elicited by the chemotactic peptide formylmethionylleucylphenylalanine (FMLP) was associated with a much greater degree of superoxide generation and myeloperoxidase secretion than was the same or larger Ca2+i produced by a specific calcium ionophore, ionomycin, which bypasses cell surface receptors. Thus, FMLP appears to generate some important excitatory signal in addition to a rise in Ca2+i and exocytosis and superoxide generation in neutrophils may not be simply dependent on Ca2+i as is widely supposed.