Change of membrane fluidity of rat neutrophils accompanying Escherichia coli inoculation

Membrane fluidity of rat neutrophils was studied following Escherichia coli inoculation, and characteristic changes were observed. Membrane fluidity was assessed by the excimer-forming lipid technique using pyrenedecanoic acid and flow cytometry and expressed as the fluorescence intensity ratios of excimer and monomer pyrenedecanoic acid (IE/IM ratio). High IE/IM ratios indicated high membrane fluidity. The IE/IM ratio of rat neutrophils (0.50 +/- 0.048) increased after E. coli inoculation, reaching a maximum of almost 1.00 after 10-20 min and then returning to its starting value. Intravenous injection of heat-killed E. coli or E. coli-conditioned culture supernatants into rats induced a rapid increase of IE/IM ratios, which returned to initial levels after 20 min. The effect on membrane fluidity of in vitro neutrophil incubation with E. coli, heat-killed E. coli, or E. coli-conditioned culture supernatants was similar to that observed in vivo. Addition of 5 mM ethylenediaminetetraacetic acid (EDTA) did not affect neutrophil membrane fluidity. Addition of either 5 micrograms/ml cytochalasin B or 10(-5) M colchicine did not directly affect neutrophil membrane fluidity but did block the change observed following incubation with bacteria.