Cell surface expression of fMet-Leu-Phe receptors on human neutrophils

We have studied how cytosolic free Ca2 " ([Ca2+Ii) changes and phorbol myristate acetate (PMA) exposure affects ligand-in-dependent cell surface expression offMet-Leu-Phe receptors on human neutrophils. Mere incubation primed neutrophils to dou-ble their binding of fMet-Leu-Phe. This spontaneous increase of peptide binding was unaffected by changes in the extracellular calcium concentration. However, depression of the [Ca2"1, totally abolished the increased binding of fMet-Leu-Phe. Scatchard-Plot analysis revealed that the observed increase of peptide binding was due to an increased number of receptors. Normal-ization of the [Ca2"1 in cells where it was initially depressed resulted in a slow but progressive increase in fMet-Leu-Phe binding. The rate of receptor recruitment could be enhanced by rapidly increasing the [Ca2J1, by addition of ionomycin. Addition of PMA to cells with near maximal receptor expression led to a marked reduction of fMet-Leu-Phe binding without affecting [Ca2+Ii. These observations suggest the exis-tence of a dual regulatory mechanism for up- and down-regulation of fMet-Leu-Phe receptors on the cell surface of human neutro-phils