Background Platelet-mediated mechanisms have been implicated in intimal lesion formation following vascular injury. Although the participation of peptide growth factors has been suspected in this process, little has been known about the possible mitogenic role of other platelet factors that are released at sites of vascular injury.Methods and Results We tested the hypothesis that platelet products, which are not peptide growth factors, are important modulators of the platelet-induced smooth muscle cell (SMC) proliferative response by acting as growth amplification factors. In these studies, cell proliferation was assessed by (Hydrogen-3)thymidine incorporation, flow cytometry, and direct cell counting. We examined the potential mitogenicity of several platelet products, including serotonin, ADP, norepinephrine, histamine, platelet-activating factor, the thromboxane A2mimetic U46619, and bradykinin. Of the platelet products tested, serotonin and ADP induced a synergistic response with peptide growth factors. This synergy was greatest at low growth-factor concentrations. Addition of nonaggregated platelets to quiescent SMC cultures strongly stimulated cell proliferation. Since the addition of suramin to platelet-treated cultures markedly inhibited SMC proliferation, peptide growth factors are most likely the primary mitogens mediating this response. However, platelet-induced proliferation was also markedly reduced by the serotonin antagonists ketanserin and LY53857 (44%), and by the ADP antagonist apyrase (35%).Conclusions Therefore, serotonin and ADP contribute significantly, in synergy with peptide growth factors, to the platelet-induced SMC proliferative response. We propose that in vivo serotonin and ADP act as amplification factors for SMC proliferation at sites of vascular injury. (Circulation. 1994;90: 1908-1918.)