PROSTACYCLIN AND THROMBOXANE A2 GENERATION IN VIVO IN MAN -EFFECT OF LOW DOSE ASPIRIN

Abstract

The effect of a low-dose aspirin regimen on platelet and vascular prostaglandin metabolism was studied in vivo in man. In a double-blind placebo-controlled cross-over study, 7 healthy male volunteers were treated with aspirin (35 mg.day−1 or placebo for 7 days. After a washout period of 2 weeks, the subject were crossed to the alternate treatment. 12 hours after the last dose of aspirin or placebo formation of thromboxane A2 (TxA2) and prostacyclin (FGI2) was measured in blood emerging from a standardized injury of the microvasculature made to determine bleeding time. TxA2 and PGI2 were measured as their stable degradation products, thromboxane B2 (TxB2) and 6-keto-prostaglandin F1α (6-keto PGF1α), using radioimmunoassay procedures. When subjects were treated with placebo, there was a rapid and substantial generation of both TxA2 and PGI2 at the site of plate-let-vessel wall interaction. This was reflected by an increase of TxB2 from 2.8±1 ng/ml and of 6-keto-PGF1α from 38.6±14.6pg/ml in the first minute to 4.5±0.6 ng/ml TxB2 and 154±50 pg/ml 6-keto-PGF1α after 4 minutes. Low-dose aspirin caused a significant inhibition of both TxA2 and PGI2 generation in bleeding time blood as represented by 65-92% and 81-84% inhibition of TxB2 and 6-keto-PGF1α Respectively throughout the 4 minute study period. We conclude that (a) rapid activation of both platelet prostaglandin metabolism and vascular PGI2 biosynthesis occurs at the site of platelet-vessel wall interaction and (b) low-dose aspirin results in a significant inhibition of platelet and vascular cyclo-oxygenase activity. Thus, our data fail to confirm the concept of a differential effect of low-dose aspirin on platelet and vascular prostaglandin synthesis in vivo in man.