cell surface proteins that specifically bind thrombin and trigger changes in the behaviour of blood cells. There are at least two types of thrombin receptors on platelets. The higher affinity receptors mediate the inhibition of stimulated adenylate cyclase, the secretion of acid hydrolases, and the activation of phospholipase a2. The lower affinity receptors are linked to phospholipase c and trigger platelet aggregation and exposure of fibrinogen binding sites. A human platelet thrombin receptor has been cloned and is a member of the family of peptide receptors. There are also thrombin receptors on endothelial cells and smooth muscle cells.