Effects of platelet activating factor on rat platelets in vivo.

Platelet activating factor (PAF) is not able to aggregate rat platelets in vitro. Due to the agonist effects of PAF on multiple cells, a possible role of PAF in the activation of rat platelets in vivo where different cells may influence each other was investigated. The pulmonary microembolization of 51Cr-labelled activated platelets was used as in vivo model. This model allowed us to monitor platelet behaviour by means of non-invasive methods. In contrast to results obtained in vitro, PAF activated rat platelets in vivo. The pulmonary microembolization of the platelets was dose-dependent and rapidly reversible. About 0.5 microgram/kg PAF caused a half-maximal rise of platelet-bound radioactivity in the thorax. Activation of the platelets by PAF was followed by extreme desensitization, so that a second injection of PAF did not provoke a significant response of the platelets. Platelet function was, however, not completely impaired because they still accumulated in the thorax after the application of ADP (50 micrograms/kg). That pulmonary entrapment had taken place was shown by a 3-fold increase in lung specific radioactivity. This was accompanied by a short-lasting thrombocytopenia. The PAF antagonist, WEB 2170 (30 micrograms/kg), significantly inhibited the microembolization of the platelets induced by PAF. Under in vitro conditions leukocytes purified from rat blood and activated by PAF were able to induce platelet aggregation. These results demonstrate that a PAF-specific activation of rat platelets is achievable in vivo which is probably mediated by other cells.