rac-1-O-Myristoyl-2-O-acetylglycerol, rac-1-O-palmitoyl-2-O-acetylglycerol, and rac-1-O-oleoyl-2-O-acetylglycerol acted like phorbol myristate acetate and mezerein in stimulating human neutrophil aggregation. Responses to these agents were equally influenced by cytochalasin B, extracellular calcium and magnesium, arachidonate antimetabolites, and procedures that rendered the cells desensitized to other agonists. The compounds also inhibited the binding of [3H]-phorbol myristate acetate to its receptor on neutrophils. Thus, these agents are biologically homologous. They act by binding to a common receptor. This receptor may function physiologically as a transducer for endogenous glycerides that form in cells challenged by other stimuli.

The bioflavinoid quercetin was found to exert at least three separate effects on human polymorphonuclear leukocytes. (1) Concentrations of approximately 100 microM inhibited the membrane-associated magnesium adenosine triphosphatase by 60%-80% in either broken cell preparations or intact cells. Lineweaver-Burk plots showed the inhibition to be uncompetitive in nature. (2) Similar concentrations of quercetin inhibited respiratory burst activity of the cells as measured by oxygen consumption, glucose oxidation, or iodination of protein. All inhibitions were dose-dependent and were observed with either opsonized zymosan or phorbol myristate acetate as stimulus. (3) Quercetin likewise inhibited the transport of the nonmetabolizable hexose, 3H-2- deoxyglucose. These observations are most consistent with the hypothesis that quercetin exerts a generalized effect at the level of the cell membrane of the neutrophi.

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Chemotactic factors and arachidonic acid aggregate neutrophils; indomethacin and 5,8,11,14-eicosatetraynoic acid, two blockers of cellular arachidonate metabolism, inhibit these responses. Additionally, A23187, an ionophore which specifically transports bivalent cations into cells, also aggregates the neutrophils, and this response, as well as the response to chemotactic factors, requires the presence of extracellular calcium and magnesium. In this report these relationships were further studied. It was found that human neutrophil aggregation stimulated by arachidonic acid required calcium and magnesium. Furthermore, cells preincubated with arachidonate for 4 min before exposure to the bivalent cations did not aggregate before or after this exposure and, following this exposure, were refractory to subsequent stimulation by more arachidonate, a chemotactic tripeptide (formylmethionylleucylphenylalanine), or A23187, i.e., these cells had become nonselectively desensitized to the aggregating agents. Finally, indomethacin and 5,8,11,14-eicoasatetraynoic acid inhibited the aggregation response stimulated by A23187 and their potency in doing so paralleled their potency in inhibiting the responses induced by arachidonate or the chemotactic tripeptide. Thus, the neutrophil aggregation responses induced by arachidonate, chemotactic factor, and A23187 were similarly influenced by preincubating the cells with arachidonate, had similar requirements for calcium and magnesium, and were similarly inhibited by blockers of arachidonate metabolism. It appears that bivalent cations and arachidonic acid play essential and, perhaps, interacting roles in the aggregation response to diverse stimuli.