1-0-alkyl-2-acetyl-sn-glycero-3-phosphorylcholine (Platelet Activating Factor): Interactions with Sheep Platelets
Platelet Activating Factor (PAF) seems to link platelet function with allergy and inflammation. Early events associated with the action of PAF on platelets are poorly understood, but PAF appears to interact with a platelet membrane receptor. Two photoaffinity-labelling analogs of PAF, diazoacetyl PAF and trifluorodiazoacetyl PAF, were designed for examining the interaction between PAF and its platelet receptor. These photoactive compounds were selected on the basis of the structure-activity relationship of PAF, and the general guidelines of the photoaffinity labelling technique. Some progress has been made in their synthesis and characterization. The standard platelet aggregation assay was unreliable in testing the activity of PAF analogs on sheep platelets. The response of sheep platelets to PAF was characterized in terms of aggregation, serotonin release, ATP release and phosphatidic acid formation but also with a new and more satisfactory method based on platelet shape change. The development of the latter was based on microscopic and rheooptic observations but is also supported by theoretical considerations. A shape change parameter (SCP) was defined and, therefore, the method is called the SCP assay. The SCP assay demonstrated, for the first time, that platelet shape change is a response sensitive to small changes in nanomolar PAF concentrations (less than an order of magnitude). The SCP assay proved superior to the aggregation assay for the characterization of the photoaffinity-labelling analogs in a comparison of sensitivity to PAF, PAF analogs and modulators of platelet activation. A wider range of PAF concentrations could be distinguished with the SCP assay, and it was more sensitive to structural differences between lyso PAF, PAF, propanoyl PAF, butanoyl PAF, hexanoyl PAF and oleoyl PAF than the aggregation assay. The order of relative activities observed with analogs did not differ between the assays. The SCP assay extended the range of detectable PAF levels to concentrations lower than those measurable by aggregation, serotonin release or ATP release. Modulators of platelet activation such as prostacyclin, TMB-8 and indomethacin, had similar effects on the magnitude of response by aggregation and SCP assay. Both ATP release and SCP were inhibited in proportion to the concentration of prostacyclin. Neither aggregation nor SCP was affected by indomethacin at concentrations that inhibit the response to thrombin and collagen, supporting the definition of PAF as mediator of a "third pathway" of platelet activation. The relationships of shape change and platelet aggregation to platelet activation suggest that the new assay will be of more value than the aggregation assay in characterizing the initial interaction of PAF with platelets.
Biochemistry, Platelet Activating Factor; Blood Platelets
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