Through studies of the stimulus-response coupling of GM-CSF in the neutrophils, important clues to the nature of the signal transduction of GM-CSF receptors are starting to emerge. GM-CSF receptors which have to date been identified and characterized biochemically appear to be too small to contain internal tyrosine kinase domains. Nonetheless, tyrosine phosphorylation of five separate proteins results when neutrophils are incubated with GM-CSF (73). The most appealing explanation for this interesting observation is that one or more tyrosine kinases lie distal to GM-CSF receptors in their signal transduction pathway. While less likely, the possibility exists that another class of GM-CSF receptors which exhibit intrinsic tyrosine kinase activity remains to be identified. Characterization of the specific proteins which are phosphorylated on tyrosine residues ought to provide important insights into the signal transduction of this cytokine. On the other hand, direct activation of protein kinases C has not been observed following exposure of neutrophils to GM-CSF (19,22,80). GM-CSF induces the release of small quantities of arachidonic acid from the plasma membrane of the neutrophil (22). Arachidonic acid is primarily released directly through the effects of phospholipases A2 on membrane phospholipids or indirectly through activation of phospholipases C or D, which release compounds that can serve as a source of arachidonic acid when hydrolyzed by specific enzymes. Direct activation of phospholipases C in the neutrophil by GM-CSF appears most unlikely, since GM-CSF does not elevate [Ca2+]i or induce the release of detectable quantities of inositol trisphosphates or diacylglycerols in these cells. Therefore, the possibility that GM-CSF may directly activate phospholipases A2 is quite plausible. GM-CSF directly activates Na+/H+ antiporters in the plasma membrane of the neutrophil, resulting in alkalinization of the cytoplasm of approximately 0.1-0.25 pH unit (73). The function of cytoplasmic alkalinization in the priming of mature neutrophils is unclear. However, evidence exists in other cell systems that cytoplasmic alkalinization may be important in inducing cell proliferation, since the pH optima of several enzymes necessary for DNA synthesis are slightly more alkaline than the resting pH of the cytosol. Many types of growth factors which initiate cell division through occupancy of surface receptors give rise to cytoplasmic alkalinization through just such a mechanism (52). Therefore, it seems likely that GM-cSF may activate Na+/H+ antiporters in the plasma membranes of hemic progenitor cells which bear GM-CSF receptors and which proliferate in response to this cytokine.(ABSTRACT TRUNCATED AT 400 WORDS)