By using tritiated 1-O-alkyl-2-O-acetyl-sn-glycero-3-phosphocholine (3H-PAF), we have directly identified its specific binding sites on rabbit platelet plasma membranes. The equilibrium dissociation constant for 3H-PAF is 1.36 (+/- 0.05) X 10(-9) M at 0 degrees C. The number of binding sites is 1.61 (+/- 0.34) X 10(12)/mg of membrane, which corresponds to approximately 150-300 receptors/platelet (depending on membrane vesicle orientation). Binding of 3H-PAF to rabbit platelet plasma membrane is rapid (t1/2 less than 5 min at 0 degrees C) and reversible. For a series of PAF analogues, their affinity for the receptor sites parallels with their relative potency to induce platelet aggregation. PAF can cause contraction of smooth muscle of heart, parenchymal strip, trachea, and ileum. Specific PAF receptor binding was demonstrated with purified plasma membrane from several smooth muscles and from polymorphonuclear leukocytes but not from presumably PAF nonresponsive cells such as erythrocytes and alveolar macrophages. It is likely that the interaction of PAF with these binding sites initiates the specific responses of platelets, polymorphonuclear leukocytes, and smooth muscles.
The six (racemic or meso) isomers of 3,4-dimethyl-2,5-bis(3,4-dimethoxyphenyl)tetrahydrofuran and four corresponding desmethyl analogues were prepared and assayed as inhibitors of platelet activating factor (PAF) receptor binding to rabbit platelet plasma membranes. The inhibition by these isomers is stereodependent and varies with the gross shape of the molecules as determined by the molecular mechanics program MM2. The most potent PAF antagonist in this group of compounds is trans-2,5-bis(3,4,5-trimethoxyphenyl)tetrahydrofuran (L-652,731, 14) with an IC50 of 0.02 microM.
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