The prostanoid receptor(s) that mediates inhibition of bacterial lipopolysaccharide (LPS)‐induced tumour necrosis factor‐α (TNFα) generation from human peripheral blood monocytes was classified by use of naturally occurring and synthetic prostanoid agonists and antagonists.

In human monocytes that were adherent to plastic, neither prostaglandin D₂ (PGD₂), prostaglandin E₂ (PGE₂), prostaglandin F_2α (PGF_2α) nor the stable prostacyclin and thromboxane mimetics, cicaprost and U‐46619, respectively, promoted the elaboration of TNFα‐like immunoreactivity, as assessed with a specific ELISA, indicating the absence of excitatory prostanoid receptors on these cells.

Exposure of human monocytes to LPS (3 ng ml⁻¹, ∼ EC₈₄) resulted in a time‐dependent elaboration of TNFα which was suppressed in cells pretreated with prostaglandin E₁ (PGE₁), PGE₂ and cicaprost. This effect was concentration‐dependent with mean pIC₅₀ values of 7.14, 7.34 and 8.00 for PGE₁, PGE₂ and cicaprost, respectively. PGD₂, PGF_2α and U‐46619 failed to inhibit the generation of TNFα at concentrations up to 10 μM.

With respect to PGE₂, the EP‐receptor agonists, 16,16‐dimethyl PGE₂ (non‐selective), misoprostol (EP₂/EP₃‐selective), 11‐deoxy PGE₁ (EP₂‐selective) and butaprost (EP₂‐selective) were essentially full agonists as inhibitors of LPS‐induced TNFα generation with mean pIC₅₀ values of 6.21, 6.02, 5.67 and 5.59, respectively. In contrast to the results obtained with butaprost and 11‐deoxy PGE₁, another EP₂‐selective agonist, AH 13205, inhibited TNFα generation by only 21% at the highest concentration (10 μM) examined. EP‐receptor agonists which have selectivity for the EP₁‐ (17‐phenyl‐ω‐trinor PGE₂) and EP₃‐receptor (MB 28,767, sulprostone) were inactive or only weakly active as inhibitors of TNFα generation.

Pretreatment of human monocytes with the TP/EP₄‐receptor antagonist, AH 23848B, at 10, 30 and 100 μM suppressed LPS‐induced TNFα generation by 10%, 28% and 77%, respectively, but failed to shift significantly the location of the PGE₂ concentration‐response curves.

Given that AH 13205 was a poor inhibitor of TNFα generation, studies were performed to determine if it was a partial agonist and whether it could antagonize the inhibitory effect of PGE₂. Pretreatment of human monocytes with 10 and 30 μM AH 13205 inhibited the generation of TNFα by 31% and 53%, respectively, but failed to shift significantly the location of the PGE₂ concentration‐response curves at either concentration examined.

Since PGD₂ and 17‐phenyl‐ω‐trinor PGE₂ (EP₁‐agonist) did not suppress TNFα generation, the EP₁/EP₂/DP‐receptor antagonist, AH 6809, was employed to assess if EP₂‐receptors mediated the inhibitory effect of PGE₂. Pretreatment of human monocytes with 10 μM AH 6809 did not affect LPS‐induced TNFα generation but produced a parallel 3.5 fold rightwards shift of the PGE₂ concentration‐response curve.

Collectively, these data suggest that human peripheral blood monocytes express at least two distinct populations of inhibitory prostanoid receptors that mediate inhibition of LPS‐induced TNFα generation. One of these probably represents IP receptors based upon the selectivity of cicaprost for this subtype. The other population has the pharmacology of EP‐receptors, but the rank order of potency for a range of synthetic EP‐receptor agonists was inconsistent with an interaction with any of the currently defined subtypes. Given the pharmacological behaviour of butaprost, AH 6809 and AH 23848B in these cells, we propose that multiple (EP₂‐ and/or EP₄‐ and/or IP) or novel EP‐receptors mediate the …