DIFFERENCES IN BINDING OF PLATELET FACTOR-4 TO VASCULAR ENDOTHELIUM IN-VIVO AND ENDOTHELIAL-CELLS IN-VITRO

The binding of fluorescein-labelled recombinant human platelet factor 4 (rhPF4) to the vasculature of the hamster cheek pouch in vivo was co mpared with that to cultured endothelial cells (EC) from human umbilic al veins (HUVEC) and arteries (HUAEC) and from human aorta (HAEC). In vivo data: systemically injected rhPF4 rapidly disappeared from plasma in a biphasic pattern (t(1/2) = 2 and 41 min). High intensity non-uni form binding of rhPF4 occurred at short specific sites along both arte rioles and venules. The length of the intense sites was 76 +/- 46 mu m and their frequency was 10 +/- 4 per cm(2) cheek pouch. Heparin was i njected at 4 and 9 min, but not 30 min, post-rhPF4 displaced most of t he high intensity labelling indicating internalization with time. Neit her pretreatment with more than 50-fold excess of unlabelled rhPF4 nor histamine- or LTB(4)-induced vascular macromolecular leakage changed the frequency of short intense sites. In vitro data: uniform time-depe ndent intense binding of rhPF4 occurred in a similar fashion in subcon fluent HUVEC, HUAEC and HAEC. All cell types showed nuclear staining, demonstrating internalization. When heparin was given to EC prior to r hPF4, binding was delayed in time but not blocked. In conclusion, rhPF 4 does not bind uniformly with high intensity along pre- and post-capi llary vessels of the hamster cheek pouch in vivo as predicted by the r hPF4-labelling of subconfluent (migrating/proliferating) human EC in v itro. The short infrequent sites of intense rhPF4-labelling in vivo ma p represent regions of endothelial cell migration/proliferation simila r to subconfluent EC in culture.