Prostacyclin Stimulation of the Activation of Blood Coagulation Factor X by Platelets

When platelets were incubated with prostacyclin, prostaglandin E1, or prostaglandin D2 at concentrations insufficient to increase the level of adenosine 3',5'-monophosphate (cyclic AMP), coagulation factor X was activated by a platelet cysteine protease. Prostacyclin or prostaglandin E1 at higher concentrations increased the cyclic AMP level and inhibited the activation of factor X by platelets. Inhibition of platelet adenylate cyclase by 2',5'-dideoxyadenosine allowed the activation of the protease at higher concentrations of the autocoids. Prostaglandins A1, A2, B1, B2, E2, F2 alpha, 6-keto-prostaglandin F1 alpha, and thromboxane B2, which do not affect platelet cyclic AMP level, did not stimulate the protease.