Endotoxin-mediated inhibition of human platelet aggregation

A variety of endotoxins, when added to human platelet-rich plasma (PRP) or to suspensions of washed platelets (WP), demonstrated an inhibitory effect on platelet aggregation induced by various aggregating agents. Endotoxin blocked the release of 14C serotonin from platelets but had no influence on cyclic AMP production. Endotoxin did not interfere with thromboxane generation by platelets. However, endotoxin-treated platelets failed to respond to added thromboxane. The inhibitory effect of endotoxin on platelet aggregation was more pronounced in the presence of ionophore A23187 as compared to other aggregating agents and was effectively reversed by calcium but not by magnesium, another divalent cation. Furthermore, endotoxin failed to inhibit the ristocetin-induced agglutination of formaldehyde-fixed platelets; a non-calcium dependent phenomenon. These findings appear to suggest that endotoxin-mediated inhibitory activity of platelet aggregation is related to the interference in the role of calcium. The antiaggregatory activity of endotoxin appears to be due to a direct and rapid action on platelets and not due to a non-specific binding, as the effect was not abolished by washing the endotoxin-incubated platelets. Endotoxin-mediated alteration of platelet function may contribute to bleeding diathesis in septecemic and endotoxemic patients.