We have utilized the fluorescent chelate probe chlorotetracycline to investigate the possible involvement of membrane calcium in the response of rabbit peritoneal neutrophils to chemotactic factors. Two chemotactic factors, the small molecular weight fragment of the fifth component of complement C5a and the synthetic peptide formyl-methionyl-leucyl-phenylalanine (F-Met-Leu-Phe), were tested and found to decrease the fluorescence of cell-associated chlorotetracycline in a manner strongly suggesting stimulus-induced displacement of membrane calcium. The time-course, concentration dependence, and receptor specificity of the calcium redistribution induced by the stimuli are consistent with its early role in the initiation of the various neutrophil functions. F-Met-Leu-Phe and C5a appear to interact with the same pool of membrane calcium and to release it to the cytoplasmic side of the plasma membrane. Intracellular calcium then binds back to the membrane(s) from where it can be displaced by additional stimulation. The release of membrane calcium, experimentally defined here, appears to play a central role in the initiation of the various neutrophil functions.

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