The use of stable prostaglandins to investigate prostacyclin (PGI2)-binding sites and PGI2-sensitive adenylate cyclase in human platelet membranes.

Prostacyclin, (PGI2) is a potent but unstable inhibitor of platelet aggregation, probably acting through stimulation of adenylate cyclase. A stable analogue of prostacyclin with antiaggregatory properties, 5,6-dihydro-PGI2 (6 beta-PGI1), and PGE1 can compete for the binding sites labelled by 3H-PGI2 in human platelet membranes (the affinity being PGI2 greater than PGE1 greater than 6 beta-PGI1). Both 6 beta-PGI1 and PGE1, as well as PGI2, bind to two classes of binding sites. 6 beta-PGI1 and PGE1 activate adenylate cyclase to the same extent as PGI2, with a rank order of potency which parallels that observed in binding experiments. The stimulation of this enzyme is brought about by interaction of each of these prostanoids with two different classes of components. The comparison of binding and adenylate cyclase data suggests that the sites to which PGI2, 6 beta-PGI1 and PGE1 bind might be coupled to the activation of adenylate cyclase. Since 6 beta-PGI1 seems to act through the same molecular mechanisms as PGI2, because of its stability it is an useful tool to investigate the mode of action of prostacyclin in platelets.