The effect of acetylsalicylate on aggregation and arachidonate metabolism by human platelets suspended in plasma or buffer

Acetylsalicylate inhibits prostaglandin and thromboxane production by human platelets suspended in plasma or buffer. Acetylsalicylate inhibits arachidonate-induced aggregation of human platelets suspended in plasma, but the effect of acetylsalicylate on arachidonate-induced aggregation of human washed platelets in buffer has not been reported. We have therefore studied this in relation to arachidonate metabolism in human platelets suspended in plasma or buffer. Platelets suspended in plasma and in buffer were both prepared from each donor, who had not taken acetylsalicylate or like-acting drugs for at least 7 days. Acetylsalicylate was 1500 times less potent in inhibiting arachidonate-induced aggregation in buffer (IC50 = 27.3 +/- 7.5 (s.e.m.)mM) than it was in plasma (IC50 = 18.3 +/- 6.0 microM); whereas it was only 4 times less potent in inhibiting thromboxane production in buffer (IC50 = 110 +/- 51.0 microM) than in plasma (IC50 = 25.3 +/- 8.9 microM). The acetylsalicylate concentration required to inhibit aggregation in buffer was sufficient to inhibit 12-hydroxyeicosatetraenoic acid production whereas the concentration that inhibited thromboxane production in buffer was not. These results indicate that arachidonate-induced aggregation of platelets in buffer may depend on product(s) of lipoxygenase rather than of cyclooxygenase, and is hence insensitive to inhibition by acetylsalicylate compared with arachidonate-induced aggregation of platelets in plasma.