Role of the plasma membrane in signal transduction in human polymorphonuclear leukocytes

To more closely examine the role of the cell surface in transmembrane signal transduction in human neutrophils, sealed right-side-out membrane vesicles free of organellar membrane components were used as models of the plasma membrane. These vesicles, incubated with a fluorescent analogue of the chemotactic peptide fMLP, bound this ligand similarly in extent and kinetics to intact neutrophils. Vesicles responded to this stimulation with a slow increase in internal [Ca++] which was inhibited by EGTA but not by verapamil; the cytosolic Ca++ transient seen in intact cells within 10 sec of stimulation was absent in vesicles. The vesicles also maintained a transmembrane potential (psi) and were depolarized by the K+ ionophore valinomycin. However, unlike intact cells which hyperpolarized and then depolarized in response to fMLP, the vesicles demonstrated only a sustained hyperpolarization. Vesicles also differed from intact cells by not producing superoxide (O2-) in response to fMLP. Finally, fMLP caused dramatic alterations in membrane vesicle lipid metabolism: at early time points (within 5-10 sec), there was a transient production of diacylglycerol (DAG) concomitant with inositol lipid breakdown, with no apparent hydrolysis of non-inositol phospholipids. For up to 5 min after stimulation, there was no increase in the levels of phosphatidic acid or of inositol lipids. Thus, a significant portion of the signalling pathway in neutrophils is located at the cell surface or in the plasma membrane and functions independently of intracellular components. Furthermore, the plasma membrane is intimately involved in events occurring during both the early (DAG generation) and late (slow, prolonged rise in [Ca++]) phases of cellular response. In contrast, several of the responses to fMLP (the Ca++ transient, depolarization, generation of O2-, recycling of lipid metabolites) involve signalling machinery not constitutively resident on the neutrophil surface.