Introduction

Thrombin is one of the most potent agonists that platelets will encounter in vivo, but unlike most of the others it is a protease. For years after thrombin was shown to be a platelet activator as well as an effector in the clotting cascade, the precise mechanism by which it activates platelets remained obscure. Binding studies demonstrated high affinity interactions with several sites on the platelet surface, including glycoprotein (GP) Ibα, but efforts to establish that any of these constituted a receptor in the signalling sense were not entirely successful. Substrates on the platelet surface for proteolytic cleavage by thrombin were also identified, including GP V, but cleavage of these sites did not appear to be required for platelet activation by thrombin. Before discussing the receptors that have been identified, it is worth considering what some of the criteria might be for establishing a protein as a true signalling receptor for thrombin. Such criteria would include (i) demonstrating its presence on the platelet surface, (ii) showing that it was a substrate for thrombin or closely associated with a substrate for thrombin, (iii) demonstrating an association of the candidate receptor with mediators or effectors for intracellular signalling cascades, (iv) showing that expression of the candidate receptor could render a cell that was otherwise unresponsive to thrombin capable of responding, and (v) showing that blocking, dismantling or otherwise removing the candidate receptor would reduce platelet responses to thrombin.