Macrophage activation is accompanied by reductions in prostacyclin generation and platelet-activating factor (PAF) levels, but the mechanism has not been identified. We have investigated the involvement of glucocorticoid-sensitive mechanisms and endogenous PAF in this phenomenon. The synthetic glucocorticoid, dexamethasone (1 μM) reduced basal prostacyclin and PAF generation in resident, but not in activated macrophages. PAF receptor antagonists or dexamethasone reduced the basal, but not stimulation-induced prostacyclin generation in resident macrophages. Dexamethasone and PAF antagonists did not have synergistic inhibitory effects on prostacyclin generation by resident macrophages, and the reduction was less than that which accompanies macrophage activation. Thus, differences in the effects of PAF receptor antagonists and dexamethasone compared with macrophage activation on zymosan-induced prostacyclin synthesis indicate that factors other than PAF or glucocorticoid-sensitive mechanisms contribute to this phenomenon. Subsequent experiments with culture media conditioned by resident macrophages indicated the presence of a soluble factor which increases prostacyclin generation and appears to be absent in media conditioned by activated macrophages.

We have examined species differences in the mobilization of arachidonic acid and generation of prostacyclin in non-activated and activated peritoneal macrophages in vitro. Mobilization of 3H-arachidonic acid was reduced in rat activated macrophages compared with that in non-activated macrophages, but a similar difference was not observed in guinea-pig macrophages. In guinea-pig peritoneal macrophages, exposure to formyl-Methionyl-Leucyl-Phenylalanine (fMLP), platelet-activating factor (Paf), zymosan and A23187 increased the generation of prostacyclin. In contrast, in rat peritoneal macrophages, fMLP and Paf did not stimulate the mobilization of arachidonic acid or the generation of prostacyclin, whereas both zymosan and A23187 were effective stimuli. Pretreatment of either rats or guinea-pigs by intraperitoneal injection of C. Parvum reduced prostacyclin generation by peritoneal macrophages in vitro. We conclude that there may be species differences in receptor populations between guinea-pig and rat peritoneal macrophages. However, the reduction in eicosanoid generation induced by the inflammatory stimulus, C. Parvum is not species-dependent.

The cellular signalling mechanisms initiated by platelet-activating factor (PAF) for stimulation of superoxide anion (O^–₂) generation in peritoneal macrophages were investigated by monitoring intracellular calcium ([Ca²⁺]_i) levels. In rat peritoneal macrophages, exogenous PAF elicited an increase in [Ca²⁺]_i, but there was no O^–₂ generation. PAF elicited similar increases in [Ca²⁺]_i in guinea pig macrophages, which were accompanied by O^-₂ generation. Removal of external calcium revealed that the major component of the PAF-induced increase in [Ca²⁺] _i, in both rat and guina pig peritoneal macrophages, was dependent on transmembrane Ca²⁺ flux. Pertussis toxin pretreatment had no effect on basal or PAF-induced increases in [Ca²⁺]_i. In guinea pig macrophages, removal of external calcium or pertussis toxin pretreatment reduced O^–₂ generation by approximately 50% and the combination of these pretreatments caused almost complete suppression of the response. Thus, additional second mesenger systems are activated in guinea pig macrophages following PAF receptor activation which appear to be essential for O^–₂ generation.

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