Measurement of circulating prostacyclin

Prostacyclin (PGI 2 ) is a potent inhibitor of platelet aggregation and a vasodilator that is synthesized from prostaglandin endo-peroxides by vascular endothelial and smooth muscle cells 1,2 . Its action on platelets is mediated by cyclic AMP 3–5 . Circulating PGI 2 has been detected through its inhibitory effect on the accumulation of platelets on collagen strips superfused with blood from anaesthetized and heparinized cats 6 and rabbits 7 and this led to the suggestion that PGI 2 is released continuously into blood passing through the lungs and functions as a circulating hormone that regulates platelet aggregability 6–9 . Because of its chemical instability, no more sensitive method of detecting circulating PGI 2 has been developed and there is doubt over whether PGI 2 is present in more physiological conditions 10,11 . Although 6-keto-PGF 1 α , the stable non-enzymatic breakdown product of PGI 2 , has been assayed in human 12,13 and rabbit 14,15 plasma, uncertainty over the fraction present as PGI 2 in the circulation has limited the value of the results obtained. Finally, it has been suggested that 6-keto-PGE 1 , a metabolic product of PGI 2 or 6-keto-PGF 1 α that can inhibit platelet aggregation 16,17 , may also function as a circulating hormone 18 . To resolve these uncertainties, we have now developed a sensitive bioassay for PGI 2 -like compounds in blood. The results indicate that fresh arterial blood from physiologically normal rabbits contains insufficient PGI 2 or 6-keto-PGE 1 to affect platelet function.