ENDOTOXIN-INDUCED PLATELET-AGGREGATION IN HEPARINIZED EQUINE WHOLE-BLOOD IN-VITRO

Endotoxaemia is a leading cause of death among horses. Thrombocytopeni a is a common finding in clinical and experimentally-induced cases of endotoxaemia and can lead to coagulopathies, including disseminated in travascular coagulopathy which is usually fatal. In this study it was shown that endotoxin (3 ng ml(-1) to 25 mu g ml(-1)) can aggregate equ ine platelets in heparinised whole blood in vitro. The endotoxin-induc ed aggregation (EIA) was shown to be dependent on the presence of leuc ocytes in the blood and did not occur when detoxified endotoxin was us ed, suggesting that lipid A was necessary for the response. Aspirin (1 mmol litre(-1)) had no effect on EIA whereas apyrase (40 mu g ml(-1)) completely abolished it and CV3988 (3 to 30 mu mol litre(-1)) (a comp etitive antagonist of platelet-activating factor) inhibited the respon se in a concentration-dependent manner. It is concluded that endotoxin activates equine platelets at low concentrations through an indirect mechanism that involves calcium, leucocytes, adenine nucleotides and p latelet-activating factor.