Protease-activated receptors 1 and 4 mediate activation of human plateletsby thrombin

Because of the role of thrombin and platelets in myocardial infarction and other pathological processes, identifying and blocking the receptors by whi ch thrombin activates platelets has been an important goal. Three protease- activated receptors (PARs) for thrombin - PAR1, PAR3, and PAR4 - are now kn own. PAR1 functions in human platelets, and the recent observation that a P AR4-activating pep tide activates human platelets suggests that PAR4 also a cts in these cells. Whether PAR1 and PAR4 account for activation of human p latelets by thrombin, or whether PARS or still other receptors contribute, is unknown. We have examined the roles of PAR1, PARS, and PAR4 in platelets . PAR1 and PAR4 mRNA and protein were detected in human platelets. Activati on of either receptor was sufficient to trigger platelet secretion and aggr egation. Inhibition of PAR1 alone by antagonist, blocking antibody, or dese nsitization blocked platelet activation by 1 nM thrombin but only modestly attenuated platelet activation by 30 nM thrombin. Inhibition of PAR4 alone using a blocking antibody had little effect at either thrombin concentratio n. Strikingly, simultaneous inhibition of both PAR1 and PAR4 virtually abla ted platelet secretion and aggregation, even at 30 nM thrombin. These obser vations suggest that PAR1 and PAR4 account for most, if not all, thrombin s ignaling in platelets and that antagonists that block these receptors might be useful antithrombotic agents.