Thrombin is involved in tissue repair through its proteolytic activation of a specific thrombin receptor (PAR-1). Previous studies have shown that serine proteases and their inhibitors are involved in neuromuscular junction plasticity. We hypothesized that thrombin could also be involved during skeletal muscle inflammation. Thus we investigated the expression of PAR-1 in human myoblasts and myotubes in vitro and its regulation by injury-related factors. The functionality of this receptor was tested by measuring thrombin's ability to elicit Ca2+ signals. Western blot analysis and immunocytochemistry demonstrated the presence of PAR-1 in myoblasts but not in myotubes unless they were treated by tumor necrosis factor-alpha (10 ng/ml), interleukin-1beta (5 ng/ml), or transforming growth factor-beta(1) (10 ng/ml). The addition of 10 nM alpha-thrombin evoked a strong Ca2+ signal in myoblasts while a limited response in myotubes was observed. However, in the additional presence of injury-related factors, the amplitude of the Ca2+ response was significantly enhanced, representing 88, 65, 48% of their respective basal level, compared to 27% of that obtained in controls. Moreover, immunochemical studies on human skeletal muscle biopsies of patients suffering from inflammatory myopathies showed an overexpression of PAR-1. These results suggest that PAR-1 synthesis may be induced in response to muscle injury, thereby implicating thrombin signaling in certain muscle inflammatory diseases.