Inhibition of prostaglandin E1-responsive platelet adenylate cyclase by heparin: a study of the mechanism of inhibition and its relevance to platelet aggregation

摘要

>1 Heparin can produce platelet aggregation in vitro and in vivo; it has been proposed that this may be due to the reported inhibition of the prostaglandin E1 (PGE1)-stimulated adenylate cyclase of the platelet by heparin.>2 The effect of heparin on the cyclic adenosine 3′,5′-monophosphate (cyclic AMP) response to PGE1 was measured in intact and broken platelets both in vitro and in platelets obtained from normal subjects during intravenous infusion with herapin.>3 In platelet lysates, heparin produced a dose-related inhibition of PGE1-stimulated adenylate cyclase. The maximum response to PGE1 was reduced, with half-maximal inhibition occurring at 3 μg/ml heparin. This inhibition could be prevented by protamine sulphate.>4 Heparin did not affect PGE1-stimulated cyclic AMP production in intact platelets either in vitro or in platelets taken during the infusion of 5,000iu heparin over 2h to 2 normal volunteers. Similarly, preincubation of platelets with heparin for up to 3h at 37°C did not affect platelet adenylate cyclase.>5 The effects of heparin were very similar to those of fluoride on the platelet adenylate cyclase: heparin and fluoride increased basal enzyme activity slightly (3-4 fold) but their effects were not additive; both inhibited the response to PGE1 by approximately 50% when added directly to the assay and the inhibitory effects of the two were not additive; preincubation of membranes with either heparin or fluoride produced an irreversible state of inhibition.>6 As heparin inhibits PGE1-stimulated adenylate cyclase activity only in broken platelets, we suggest that the aggregatory effects of heparin are probably independent of any action on cyclic AMP production.