Actin polymerization occurs in the process of neutrophil migration toward chemotactic stimuli, and rapid increase of Ca
2+ concentration is also observed after stimulation of chemotactic substances such as formyl peptides. We monitored the changes in intracellular actin concentration ([actin]
i) and Ca
2+ concentration ([Ca
2+]
i) simultaneously in living guinea pig neutrophils, to investigate the relationship between Ca
2+ and actin in the mechanism of chemotaxis. To induce chemotaxis, we used N-formyl-methionylleucylphenylalanine (FMLP) and a unique chemotactic peptide [Lys
7] I-CP which causes only chemotaxis but no additional responses, such as superoxide production. To monitor the changes in [actin]
i and [Ca
2+]
i simultaneously, rabbit muscle actin was labeled with a fluorescent dye, iodoacetamido morpholino rhodamine (IMR), and injected into neutrophils together with fluo3, a fluorescent Ca
2+ indicator, and monitored with confocal laser microscope. Rapid increases in [actin]
i and [Ca
2+]
i were observed in the focal plane of FMLP-stimulated neutrophils, but not observed in [Lys
7] I-CP-stimulated neutrophils. However, in both cases, injected actin mostly coexisted with higher concentration of Ca
2+. These results suggest that the inerease in [Ca
2+]
i is not strongly related to chemotaxis, but is needed for actin polymerization in the process of chemotaxis.
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