Synergistic effect of thromboxane A2 and N-formylmethionylleucylphenylalanine on platelet-activating factor synthesis in human polymorphonuclear neutrophils.

The effects of thromboxane A2 (TXA2) on the synthesis of platelet-activating factor (PAF) and leukotriene B4 (LTB4) were studied using human polymorphonuclear neutrophils (PMN). Scatchard analysis for binding experiments using [3H]S-145, a specific TXA2/prostaglandin H2 (PGH2) receptor antagonist, revealed the existence of a single class of binding sites (Kd = 83.0 +/- 2.8 nM, Bmax = 113.0 +/- 3.1 fmol/2.10(6) cells) in human PMN. Upon stimulation with a combination of U46619, a TXA2 mimetic agonist, and N-formylmethionylleucylphenylalanine (FMLP, 1 microM), the synthesis of PAF was detected, although this was not significantly enhanced by U46619 or FMLP alone. The maximal production of PAF as well as the maximal activity of acetyl-CoA acetyltransferase was observed at approx. 20 min after addition of both stimuli. The effects of U46619 plus FMLP on PAF synthesis showed dose dependence to different concentrations of U46619 (0.1-10 microM), and were completely inhibited by S-145. Contrarily, no significant amounts of LTB4 were detected by radioimmunoassay during the stimulation with U46619 and FMLP. These results suggest that TXA2 and FMLP synergistically activate human PMN to induce PAF synthesis and this effect of TXA2 is mediated through its specific receptor.