Involvement of platelet-activating factor in endotoxin-induced priming of rabbit polymorphonuclear leukocytes.

Endotoxin primes polymorphonuclear leukocytes (PMNs) for increased superoxide anion (O2-) generation in response to the chemotactic peptide, formyl-methionyl-leucyl-phenylalanine (fMLP). We have investigated the role of platelet-activating factor (PAF) in the priming of rabbit PMNs by endotoxin. In non-primed PMNs, fMLP stimulated PAF generation, but none was detected in the supernatant, whereas in primed PMNs PAF generation increased 17-fold and a large proportion of the total PAF formed was released. There was a close relationship between the concentration-response curve for increases in PAF synthesis and that for enhanced O2- generation. The possibility that PAF was causally linked to enhanced O2- generation in primed PMNs was investigated using compounds previously reported to reduce PAF generation, namely the serine protease inhibitor, tosyl-phenylalanine chloromethylketone, and the phospholipase A2 inhibitor, bromophenacylbromide. Both compounds reduced PAF generation and release in endotoxin-primed PMNs and prevented the enhancement of O2- generation. We examined the possibility that PAF acted in an autocrine fashion to enhance O2- generation following its release from fMLP-stimulated PMNs. However, rabbit PMNs, unlike human PMNs, were not primed by exogenous PAF (0.1-1000 nM) over a concentration range extending beyond that covering the concentrations of PAF determined in the supernatants of fMLP-stimulated, endotoxin-primed PMNs. We conclude that PAF release is unlikely to play a role in priming induced by endotoxin in rabbit PMNs, but our data are not inconsistent with a role for cell-associated PAF in this phenomenon. Furthermore, the release of a large amount (80%) of the total PAF from primed, but not non-primed PMNs suggests a cell-cell mediator role for PAF only in primed PMNs.