Abstract

N-Formyl-L-methionyl-L-leucyl-L-phenylalanine (fMet-Leu-Phe) stimulation of human neutrophils leads to a rapid increase of the cytosolic free Ca2+ concentration, [Ca2+]i, which is significantly reduced by removal of extracellular calcium. In the present study we show that fMet-Leu-Phe-induced [Ca2+]i increases are, in part, mediated by an increase of the plasma membrane permeability to Ca2+. This conclusion is based on the following evidence. In the presence of extracellular calcium, addition of La3+ reduced the fMet-Leu-Phe-induced [Ca2+]i increase to approximately the same level as that observed in the absence of extracellular calcium. A net increase of the plasma membrane permeability for Mn2+ could be observed after fMet-Leu-Phe stimulation, as revealed by intracellular quenching of the quin2 signal. The influx of Mn2+, like that of Ca2+, was inhibited by La3+ and was more pronounced in the absence of extracellular Ca2+, suggesting competition for the same pathway. Temporal dissociation of intracellular Ca2+ release from stores and Ca2+ influx from the medium could be demonstrated by readdition of calcium to cells stimulated in the absence of this cation. This second [Ca2+]i increase could be abolished either by giving the specific chemotactic peptide receptor antagonist, BOC-Met-Leu-Phe, or Co2+. We could also show that the fMet-Leu-Phe-dependent Ca2+ influx was not due to the activation of voltage-dependent calcium channels since depolarization either by K+ or gramicidin D did not affect the resting [Ca2+]i, nor did it affect a subsequent [Ca2+]i increase induced by fMet-Leu-Phe. Furthermore, nifedipine and verapamil, at concentrations known to block classical voltage-dependent calcium channels, had no significant effects on the Ca2+ influx induced by fMet-Leu-Phe. We suggest that fMet-Leu-Phe promotes influx of Ca2+ ions across the plasma membrane of human neutrophils by opening of receptor-dependent calcium channels.