THE VASCULAR DISTRIBUTION OF THE PLATELET-ACTIVATING-FACTOR RECEPTOR

Although the platelet-activating factor (PAF) is the most active infla mmatory mediator known to date, little is known about its effects on t he vascular endothelium and about the cellular and subcellular distrib ution of its receptor, already identified as a membrane protein of sim ilar to 39 kDa, To better understand its functions we derided: i) to s tudy PAF effects on a model microvascular bed (the rat cremaster), ii) to raise monoclonal antibodies against synthetic peptides reproducing short segments (14 and 16 amino acids) at the N and C terminal parts of PAF-receptor (PAF-R), iii) to determine the distribution of PAF-R o n a number of microvascular beds. Topical application of the PAF on th e cremaster led promptly to: i) opening of the venular and capillary e ndothelial junctions; ii) fenestration of the endothelium and iii) swe lling, clustering and fusion of endothelial plasmalemmal vesicles, Wit h the anti-N terminal antibody, we localized PAF-R by immunofluorescen ce on semithin frozen sections of lung, heart, diaphragm, kidney, and brain specimens, With the exception of brain, the signal was restricte d primarily to the vascular endothelium. Using immunogold procedures, we localized the PAF-R in small clusters on endothelial surfaces and f ound it associated preferentially with the plasmalemma proper, rather than to any differentiated microdomain, A morphometric analysis reveal ed a greater signal density at the level of the venular endothelium th an at the level of the endothelium of any other segment of the microva sculature. With th, same antibody, we immunoprecipitated PAF-R from wh ole homogenates of the same tissues, The results obtained were in gene ral agreement with the immunofluorescence tests.