Synergy between thrombin and serotonin in inducing vascular smooth muscle cell proliferation

Previous studies have indicated that apart from playing an important role i n hemostasis and thrombosis, thrombin may also contribute to the developmen t of postangioplasty restenosis caused by the stimulation of vascular smoot h muscle cell (VSMC) proliferation. Because thrombin generation in vivo is accompanied by platelet activation and release of smooth muscle cell (SMC) growth factors such as serotonin, we examined the possible interaction betw een these two compounds on VSMC proliferation. Thrombin (0.01 to 100 nmol/L ), thrombin receptor-activating peptide (0.1 to 1000 mu mol/L), and seroton in (5HT; 0.1 to 1000 mu mol/L) increased tritiated thymidine incorporation into the DNA of canine aortic VSMCs in a dose-dependent manner. When thromb in and 5HT were added together at sub-threshold concentrations, they acted synergistically in inducing tritiated thymidine incorporation. These findin gs were paralleled by a 90% +/- 5% increase in the cell number at 48 hours, as compared with a 37% +/- 2% increase with 50 mu mol/L serotonin and a 13 % +/- 3% increase with 0.1 nmol/L thrombin, We also demonstrated that a bri ef exposure to thrombin (1 hour) is sufficient to show its potentiating eff ect on serotonin. The mitogenic effect of serotonin and its synergistic int eraction with thrombin on VSMC proliferation was abolished by serotonin typ e 2 receptor antagonist LY281067. Similarly, gamma-hirudin--a direct thromb in inhibitor--blocked the mitogenic effect of thrombin and its synergistic interaction with serotonin. When LY281067 and gamma-hirudin were used toget her, they abolished the mitogenic effects of both the agonists. Because clo t-bound active thrombin can escape inactivation by anti-thrombin, this thro mbin may potentiate the mitogenic effect of serotonin and keep the VSMCs in a proliferative state for a long period of time. These findings support th e use of 5HT2 receptor antagonists in combination with thrombin inhibitors in the prevention of SMC proliferation after coronary angioplasty.