S.R. Cockbill

Differential inhibition by low-dose aspirin of human venous prostacyclin synthesis and platelet thromboxane synthesis.

The capacity of venous tissue for prostacyclin synthesis was determined in 68 patients undergoing surgery for removal of varicose veins. A single dose of aspirin (81 mg or 300 mg) taken 14 h preoperatively strongly inhibited its synthesis, and the effect of 300 mg was still evident 48 h after ingestion. A single dose of 40 mg aspirin taken 14 h preoperatively had no effect on prostacyclin synthesis. The capacity of blood platelets to synthesise thromboxane (measured as malondialdehyde) was determined in volunteers before and at various times after ingestion of 300 mg or 40 mg aspirin. Both doses had an inhibitory effect that lasted for at least 96 h. The length of time for which the amount of thromboxane synthesised was insufficient to support platelet aggregation and the platelet release reaction depended on both the donor and the dose of aspirin. If prostacyclin and thromboxane are important in the pathogenesis of thrombosis, then doses of aspirin much lower than those used previously should be tested. The long-lasting effect of 300 mg aspirin on both venous tissue and platelets indicates that this dose is unlikely to produce a favourable prostacyclin/thromboxane balance.

A regimen for low-dose aspirin?

The effects of different regimens of 40 mg aspirin on platelet thromboxane A2 synthesis and vascular prostacyclin synthesis were determined in patients who were undergoing elective surgery for removal of varicose veins. Aspirin 40 mg taken at intervals of 48 hours consistently reduced platelet thromboxane A2 synthesis to a level at which it failed to support platelet aggregation and the associated release reaction. This effect lasted for at least 36 hours. In contrast, aspirin 40 mg every 72 hours did not have the same consistent effect. Both dose regimens led to a reduction in vascular prostacyclin synthesis 12 hours after the last dose, but 36 or 72 hours after the last dose prostacyclin synthesis was not reduced; thus the inhibition of prostacyclin synthesis was short lived. If the balance between platelet thromboxane A2 and vascular prostacyclin synthesis is important in thrombosis 40 mg aspirin every 48 hours may have the maximum antithrombotic effect.

A COMPARISON OF THE ABILITIES OF ACETYLSALICYLIC ACID, FLURBIPROFEN AND INDOMETHACIN TO INHIBIT THE RELEASE REACTION AND PROSTAGLANDIN SYNTHESIS IN HUMAN BLOOD PLATELETS

1 A quantitative comparison has been made of the abilities of acetylsalicylic acid, flurbiprofen and indomethacin to inhibit the adenosine diphosphate (ADP)‐induced platelet release reaction and to inhibit the synthesis of prostaglandins from arachidonic acid. 2 Experiments were carried out on human platelets that had been incubated with the agents in vitro and on platelets obtained from volunteers who had ingested standard doses of the drugs. 3 The results obtained for acetylsalicylic acid show that there is a close relation between the release reaction and the synthesis of prostaglandins in platelets. 4 Flurbiprofen and indomethacin appear to inhibit the release reaction rather more effectively than they inhibit the synthesis of prostaglandins. It is possible that these agents inhibit the release reaction by another mechanism.

Blood platelet behaviour during infusion of an Intralipid based intravenous feeding mixture

Blood platelet behaviour was studied in six patients during infusion of an Intralipid-based intravenous feeding mixture. Lower concentrations of sodium arachidonate were required to induce platelets to aggregate and to undergo a release reaction during intravenous feeding than were required before or after intravenous feeding. This change was not due to a change in serum albumin level. The platelet behaviour that was induced by adenosine diphosphate or by adrenaline remained unchanged during intravenous feeding.