Abstract: | Human and rabbit platelets fully aggregated byplatelet-activating factor (PAF) underwent slow disaggregation but wererapidly disaggregated by the PAF receptor antagonists WEB-2086,Y-24180, SM-12502, and CV-3988. Whereas the1-alkyl-2-3H]acetyl-sn-glycero-3-phosphocholine(3H]acetyl-PAF)specifically bound to platelet receptors underwent slow and spontaneousdissociation, it dissociated promptly from its receptor when WEB-2086was added, in parallel with platelet disaggregation and disappearanceof P-selectin on the cell surface. Extracellular3H]acetyl-PAF wasrapidly deacetylated by normal rabbit platelets; some of the3H]acetyl-PAF wasbound to the cells and a very small amount of 3H]acetate wasdetected in the cells. In contrast, when1-3H]alkyl-2-acetyl-sn-glycero-3-phosphocholinewas added to the platelets, the radioactivity was rapidly incorporatedinto the 1-alkyl-2-acyl-sn-glycero-3-phosphocholinefraction. These results indicate that1) continuous binding of PAF to itsreceptor is necessary for prolonged platelet aggregation, which may bemediated through an unknown signaling system for a long-term cellresponse rather than a transient signaling system, and2) most of the3H]acetyl-PAF boundto platelets is metabolized extracellularly by ecto-type PAFacetylhydrolase, with the lyso-PAF generated being incorporated rapidlyinto the cells and converted to1-alkyl-2-acyl-sn-glycero-3-phosphocholine. |