Specific receptor sites for 1-0-alkyl-2-0-acetyl-sn-glycero-3-phosphocholine (platelet activating factor) on rabbit platelet and guinea pig smooth muscle membranes

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Abstract
Tritiated 1-O-alkyl-2-O-acetyl-sn-glycero-3-phosphocholine (3H-PAF) was used to directly identify its specific binding sites on rabbit platelet plasma membranes. The equilibrium dissociation constant for 3H-PAF is 1.36 (.+-. 0.05) .times. 10-9 m at 0.degree. C. The number of binding sites is 1.61 (.+-. -0.34) .times. 1012/mg of membrane, which corresponds to .apprx. 150-300 receptors/platelet (depending on membrane vesicle orientation). Binding of 3H-PAF to rabbit platelet plasma membrane is rapid (t1/2 [half-time] < 5 min at 0.degree. C) and reversible. For a series of PAF analogs, 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. The interaction of PAF with these binding sites may initiate the specific responses of platelets, polymorphonuclear leukocytes and smooth muscles.