A common low-affinity binding site for primary prostanoids on bovine aortic endothelial cells

[³H]PGE₂ and [³H]PGF(2α) were shown to bind with similar binding capacity and dissociation constants to bovine aorta endothelial cells. The similarity in the binding parameters suggests that both agonists may bind to the same binding site. Displacement of [³H]PGE₂ performed with PGE₂, PGF(2α) or U-46619, a thromboxane agonist, shows that all three prostanoids displaced the bound [³H]PGE₂ with comparable potency (IC₅₀ = 10^-7 M). These results indicated that the three different prostanoids, which serve as specific agonists to different prostanoid receptors, also compete for the same binding site in bovine endothelial cells with similar affinity. Comparison of the displacement of [³H]PGE₂ or [³H]PGF(2α) by a number of prostaglandin agonists and antagonists further supports the notion that the natural prostanoids bind with similar affinities to the same binding site. Thus, sulprostone, an EP₁/EP₃ agonist, displaced bound [³H]PGE₂ and [³H]PGF(2α) with IC₅₀ of about 10^-7 M. On the other hand, thromboxane antagonists (BAY u-3405 and GR-32191B), EP₁ specific antagonist (SC-19220) EP₁/DP antagonist (AH-6809) and iloprost, a stable prostacyclin agonist, failed to displace bound [³H]PGF(2α) or [³H]PGF(2α) at a concentration range of 10^-9-10^-6 M. Gradual increase of sodium fluoride (NaF), a general activator of G binding proteins, or incubation of permeabilized cells with GTPγS resulted in a decrease in [³H]PGE₂ binding, suggesting that the binding site represents a low-affinity common prostanoid receptor which, similar to other prostanoid receptors, is probably coupled with G binding proteins.