Background. In the setting of arterial injury, thrombin contributes to the hemostatic process by activating the coagulation cascade and platelets. We hypothesized that thrombin also contributes to hemostasis by inducing vasospasm. The purpose of this investigation was to characterize the cellular signaling mechanisms that modulate thrombin-induced vascular smooth muscle contractions. Methods. Contractile responses of intact bovine carotid artery smooth muscles were determined in a muscle bath. Contractile responses were correlated with phosphorylation events as determined with whole cell phosphorylation and two-dimensional gel electrophoresis and with immunoblotting of glycerol-urea or two-dimensional gels. Results. Thrombin (1 to 1000 units/ml) induced sustained vascular smooth muscle contractions of similar magnitude as the potent contractile agonist, endothelin. Thrombin- induced contractions were associated with increases in the phosphorylation of the myosin light chains (MLC20) and in the tyrosine phosphorylation of mitogen-activated protein kinase. Conclusions. These data suggest that thrombin is a potent physiologic contractile agonist that may modulate some forms of vasospasm. Thrombin-induced contractions are associated with the activation of two cellular signaling pathways, the myosin light chain kinase and the mitogen-activated protein kinase pathways.
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