Calcium efflux and influx in f-met-leu-phe (fMLP)-activated human neutrophils are chronologically distinct events

The kinetics of efflux of calcium mobilized from intracellular stores following activation of human neutrophils with the synthetic chemotactic tripeptide, fMLP (1 mu M), as well as that of the subsequent store-operated influx of this cation, has been measured by radiometric procedures using Ca-45. These procedures enabled distinction between net efflux and influx of Ca-45. Preincubation of neutrophils in medium containing Ca-45 as the sole source of Ca2+, followed by activation with fMLP, resulted in a rapid efflux of the cation, which coincided with its release from intracellular stores. Efflux terminated at approximate to 30 s after addition of fMLP to neutrophils and resulted in the loss of 42 +/- 3% (P < 0.005) of cell-associated Ca-45. Net influx of Ca-45, which was insensitive to the voltage-dependent Ca2+ channel blockading agent, verapamil (20 mu M), could only be detected at 30-60 s after the addition of fMLP to neutrophils, and proceeded for about 5 min, resulting in intracellular concentrations of Ca2+ which were 27 +/- 3% (P < 0.05) higher than preactivation levels, These results demonstrate that the efflux of cytoplasmic Ca2+ mobilized from intracellular stores during activation of neutrophils by fMLP, and the subsequent influx of extracellular Ca2+ to replete these stores, are chronologically distinct events in fMLP-activated neutrophils.