The role of thromboxane A2 in endotoxin-induced aggregation of guinea-pig platelets in vitro.

Large concentrations of endotoxin lipopolysaccharides (LPS) E. coli O127:B8 and E. coli O26:B6 were needed for induction of platelet aggregation in citrated platelet rich plasma (PRP) from normal guinea-pigs. When guinea-pigs were actively immunized with LPS E. coli O127:B8 their PRP became more sensitive to the aggregatory effects of this type of LPS. The increase in sensitivity was rather selective since the responses to ADP and LPS E. coli O26:B6 remained unchanged. In endotoxin-stimulated PRP from normal guinea-pigs thromboxane A2 (TXA2) was not detectable by bioassay. Thus biosynthesis of TXA2 seemed to be of little importance in endotoxin-induced platelet responses in normal citrate PRP. LPS E. coli O127:B8, but not LPS E. coli O26:B6, was a potent inducer of the biosynthesis of TXA2 in sensitized PRP. Under those conditions endotoxin-induced aggregation seemed to be dependent on the endogenous biosynthesis of TXA2 by the platelets.