Cell surface expression of fMet-Leu-Phe receptors on human neutrophils. Correlation to changes in the cytosolic free Ca2+ level and action of phorbol myristate acetate

Abstract

We have studied how cytosolic free Ca2+ ([Ca2+](i)) changes and phorbol myristate acetate (PMA) exposure affects ligand-independent cell surface expression of fMet-Leu-Phe receptors on human neutrophils. Mere incubation primed neutrophils to double their binding of fMet-Leu-Phe. This spontaneous increase of peptide binding was unaffected by changes in this extracellular calcium concentration. However, depression of the [Ca2+](i) 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. Normalization of the [Ca2+](i) 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 [Ca2+](i) 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+](i). These observations suggest the existence of a dual regulatory mechanism for up- and down-regulation of fMet-Leu-Phe receptors on the cell surface of human neutrophils.