Evidence for two mechanisms of thrombin-induced platelet activation: one proteolytic, one receptor mediated.
The possibility that thrombin-induced platelet reactivity could occur via both a receptor-related and a proteolytic process was examined. Thrombin elicited the formation of considerably more [32P]phosphatidic acid (an index of phospholipase C catalysed phosphoinositide metabolism) than did platelet activating factor, 5-hydroxytryptamine, ADP, and the thromboxane A2 analogue EP171, when these agents were added either alone or in combination. Co-addition of thrombin and EP171 did not evoke significantly more [32P]phosphatide acid than did thrombin alone. The protease inhibitor leupeptin, decreased but did not abolish [32P]phosphatidic acid formation elicited by either thrombin alone or thrombin in combination with EP171. The serine protease, trypsin, stimulated an increase in [32P]phosphatidic acid and this effect was additive with that of EP171. This augmentation by trypsin of EP171-induced [32P]phosphatidic acid formation was inhibited by leupeptin. These results are consistent with the concept that thrombin-induced activation of phospholipase C occurs by two distinct mechanisms: one via proteolysis, which is sensitive to leupeptin, and the other via receptor activation, a process shared by EP171. The individual components of this dual mechanism can be mimicked by the co-addition of a receptor-directed agonist (EP171) and a proteolytic agent (trypsin).
McNicol
A
A
Department of Pharmacology, University of Glasgow, Scotland.
Gerrard
J M
JM
MacIntyre
D E
DE
D000244:Adenosine Diphosphate / Q000494:pharmacology; D006801:Humans; D066298:In Vitro Techniques; D007976:Leupeptins / Q000494:pharmacology; D010716:Phosphatidylinositols / Q000378:metabolism; D010972:Platelet Activating Factor / Q000494:pharmacology; D015539:Platelet Activation / Q000187:drug effects*; D011956:Receptors, Cell Surface / Q000378:metabolism*; D018179:Receptors, Thrombin; D012701:Serotonin / Q000494:pharmacology; D013917:Thrombin / Q000494:pharmacology*; D013928:Thromboxane A2 / Q000494:pharmacology / Q000031:analogs & derivatives; D014357:Trypsin / Q000494:pharmacology
D007976:Leupeptins; D010716:Phosphatidylinositols; D010972:Platelet Activating Factor; D011956:Receptors, Cell Surface; D018179:Receptors, Thrombin; C053315:EP 171; D012701:Serotonin; D013928:Thromboxane A2; D000244:Adenosine Diphosphate; D014357:Trypsin; D013917:Thrombin; C032854:leupeptin