Characterization of the prostanoid receptors mediating inhibition of PAF-induced aggregation of guinea-pig eosinophils

1. Prostanoids induce a wide range of biological actions which are mediated by specific membrane-bound receptors. We have recently shown that the E-type prostaglandins, PGE₁ and PGE₂, effectively inhibit eosinophil aggregation induced by platelet-activating factor (PAF). In an attempt to determine which prostanoid receptor(s) were involved, we investigated the effects of a range of selective prostanoid agonists and antagonists on eosinophil homotypic aggregation induced by PAF.
2. Both PGE₁ and PGE₂ (10⁻¹⁰ to 10⁻⁶ M) induced a concentration-related inhibition of the aggregation response induced by PAF. PGE₁ was more effective than PGE₂ but PGE₂ was slightly more potent than PGE₁ (approximate IC₅₀ values for PGE₁ and PGE₂ of 1.5×10⁻⁸ M and 5×10⁻⁹ M, respectively).
3. The EP₂-selective agonists, 11-deoxy-PGE₁, butaprost and AH13205, and the EP₂/EP₃-selective agonist, misoprostol, also inhibited PAF-induced aggregation. The rank order of potency for EP₂-selective agonists was 11-deoxy-PGE₁ > misoprostol > butaprost = AH13205. The protein kinase A inhibitor, KT5720 (10⁻⁶ M), reversed the inhibitory effects of 11-deoxy-PGE₁ (10⁻⁶ M) and AH13205 (10⁻⁵ M).
4. The EP₁/EP₃-selective agonist, sulprostone, and the EP₁-selective agonist, 17-phenyl-ω-trinor PGE₂, had no significant inhibitory activity when tested at concentrations up to 10⁻⁶ M. The EP₄-receptor antagonist, AH23848B, had no effect on PAF-induced aggregation and did affect the inhibitory activity of PGE₁.
5. The IP-selective agonist, cicaprost (up to 10⁻⁶ M), and the IP/EP₁-receptor agonist, iloprost (up to 10⁻⁵ M), had no significant effect on PAF-induced eosinophil aggregation. However, iloprost significantly augmented the inhibitory effects of a maximally inhibitory concentration of PGE₂.
6. PGD₂ (10⁻⁵ M) had no effect on eosinophil aggregation and the inhibitory activity of PGE₁ on PAF-induced eosinophil aggregation was not altered by the DP-selective receptor antagonist, BWA868C.
7. The results presented here suggest that the inhibition of PAF-induced eosinophil aggregation by prostanoids is mediated by the occupation of EP₂-receptors. It is important to note that the effects of naturally occuring prostanoids, such as PGE₂, on eosinophil aggregation occur at low concentrations highlighting a potential role for EP₂ receptors in regulating eosinophil function in vivo.