Abstract

The serine proteases thrombin and trypsin are both powerful platelet agonists that act by cleaving the terminal portion of the thrombin receptor and allowing the new C-terminal to autostimulate the receptor. Synthetic peptides, termed thrombin receptor-activating peptides (TRAPs), have been shown to mimic many of the effects of thrombin. Here we have compared the effects of inhibitors on platelet aggregation and [¹⁴C]-arachidonic acid release in response to thrombin, trypsin and TRAP. Pretreatment of human platelets with BW755C (80 μM), which inhibits both cyclooxygenase and lipoxygenase, blocked trypsin (15–20 nM)- or TRAP (4–6 μM)-induced aggregation, but not thrombin (0.06–0.1 U/ml)-induced aggregation. The protease inhibitor leupeptin (10 μg/ml) abolished trypsin-induced aggregation and returned [¹⁴C]-arachidonic acid release from [¹⁴C]-arachidonic acid-prelabeled platelets to control levels. In contrast, leupeptin did not affect either aggregation or [¹⁴C]-arachidonic acid release in platelets stimulated by TRAP. Thrombin-induced aggregation and [¹⁴C]-arachidonic acid release were only partially inhibited by leupeptin. These data are consistent with the activation of platelets by both trypsin and TRAP occurring via the proteolytic receptor, whereas thrombin-induced platelet activation appears to occur by a dual mechanism of action. One component of thrombin-induced platelet activation is by a proteolytic action on the moderate-affinity receptor. This effect is sensitive to inhibition by leupeptin and is mimicked by trypsin and TRAP. The other component of thrombin is nonproteolytic and may occur by an action at a high-affinity receptor such as glycoprotein Ib.