Characterization of receptors for platelet-activating factor in guinea pig lung membranes

Platelet-activating factor (PAF) is a potent contractile agonist in guinea pig peripheral lung strips. Whether PAF acts via specific receptor sites in the lung has not been fully established. To determine if specific receptor sites could be demonstrated in guinea pig peripheral lung tissue, we performed direct radioligand binding studies in tissue homogenates with [3H]C16-PAF (1-O-hexadecyl-sn-glyceryl-3-phosphorylcholine) and the PAF antagonists [3H]WEB 2086 and [3H]RP52770. These studies demonstrated binding sites for [3H]C16-PAF of high affinity (Kd of 2.6 nM from saturation isotherms and 0.9 nM from kinetic experiments) and a mean density of 200 fmol/mg protein. Binding was inhibited to the same degree with unlabeled C16-PAF, WEB 2086, and RP52770, all with pseudo-Hill slopes of unity. [3H]WEB 2086 binding demonstrated a receptor density similar to that of [3H]C16-PAF (226 fmol/mg protein) and was inhibited to the same degree by unlabeled C16-PAF, C18-PAF, WEB 2086, and RP52770. Although antagonist inhibition yielded pseudo-Hill slopes near unity, agonist inhibition slopes were shallow, suggesting two types or states for the PAF receptors. Direct binding studies with [3H]RP52770 revealed a much larger density of binding sites (1,200 fmol/mg protein), and this binding was not inhibited with C16-PAF, C18-PAF, WEB 2086, or lyso-PAF. These results indicate that guinea pig lung has specific binding sites for [3H]C16-PAF, that WEB 2086 is an effective antagonist of C16- and C18-PAF binding at these sites, and that RP52770 binds to the PAF site but, in addition, binds to another site with a much greater density.