Characteristics of the binding of platelet-activating factor to platelets of different animal species.

The binding of platelet-activating factor (PAF-acether) to human, rabbit and rat platelets was investigated by incubating washed platelets at 37 degrees C with radiolabeled [3H]PAF-acether. Scatchard plot analysis of the binding showed that human and rabbit platelets possess two different types of binding sites. One of them was saturable and of high affinity (KD 1.58 +/- 0.36 nM in human and 0.9 +/- 0.5 nM in rabbits), and the other one had nearly infinite binding capacity. By contrast, rat platelets only showed non-specific binding. Addition of unlabeled PAF-acether 5 min after the addition of [3H]PAF-acether showed that internalization of 66 +/- 9% of the specific binding in human and 52 +/- 6.9% in rabbit platelets had occurred. Specific binding of [3H]PAF-acether in rabbit platelets correlated well with [3H]serotonin release in response to different doses of PAF-acether and with the uptake of calcium by the platelets. By contrast, PAF-acether did not induce [3H]serotonin release or calcium uptake by rat platelets. The following data suggest that the potential of PAF-acether to activate platelets depends on the interaction with a specific membrane receptor rather than on a non-receptor-mediated alteration of the platelet membrane: (1) platelets from animal species sensitive to PAF-acether show saturability and specificity of binding; (2) platelets from one animal species non-sensitive to PAF-acether lack specific binding; (3) PAF-acether does not induce calcium uptake by platelets from an animal species which lacks specific binding sites.