A B S T R A C T Aspirin inhibits platelet frinction by permanently acetylating the cyclooxygenase that forms prostaglandins. We determined the sensitivity of platelets to aspirin in normal subjects by measuring [3H-acetyl]aspirin-susceptible cyclooxygenase in washed platelets obtained at various times after aspirin ingestion. A single 325-mg aspirin dose inactivated 89% of platelet cyclooxygenase. The inhibition persisted for 2 days suggesting that oral aspirin also inactivated megakaryocyte cyclooxygenase. Thereafter, active enzyme returned with a time-course reflecting platelet turnover (life-span 8.2+2 days). Single doses of 20-650 mg aspirin resulted in 34->95% inhibition after 24 h. Daily doses of 20-325 mg aspirin for brief periods produced 61->95% inactivation when measured 24 h after cessation of the drug. Platelet cyclooxygenase is more sensitive to inactivation by aspirin than enzyme in sheep seminial vesicles.

Since platelet cyclo-oxygenase is much more sensitive to inactivation by aspirin than is the enzyme in the arterial wall and low doses of aspirin may prevent thrombosis by blocking thromboxane synthesis, we conducted a randomized, double-blind trial of aspirin (160 mg per day) vs. placebo in 44 patients on chronic hemodialysis. The study was continued until there were 24 patients with thrombi and both groups had been under observation for a mean of nearly five months. Thrombi occurred in 18 of 25 (72 per cent) of patients given placebo and 16 of 19 (32 per cent) of those given aspirin (P less than 0.01). The incidence of thrombosis was reduced from 0.46 thrombi per patient month in the placebo group to 0.16 thrombi per patient month in the aspirin group (p less than 0.005). A dose of 160 mg of aspirin per day is an effective, nontoxic antithrombotic regimen in patients on hemodialysis.

The rate of acetylation of fatty acid cyclooxygenase (prostaglandin synthase, EC 1.14.99.1) by [acetyl-3H-aspirin was measured in microsomes from human aortas and coronary arteries and intact and disrupted human platelets. We also measured the inhibition by aspirin of prostacyclin generation from exogenous arachidonic acid in shredded human aorta. Cyclooxygenase in human aorta and coronary artery microsomes is approximately 1/250th as sensitive to aspirin as enzyme in intact platelets, and 1/60th as sensitive to aspirin as enzyme measured in a platelet microsomal preparation. On the basis of the in vitro data presented, we predict that small oral doses of aspirin are sufficient to inhibit platelet prostaglandin production but are not sufficient to substantialFy affect aorta or coronary artery prostaglandin production.

This study does not support the hypothesis that corticosteroid stimulation of PMN donors is associated with an increased risk of developing a PSC.