Arresten, a cleaved fragment of type IV collagen α1 chain, exerts anti-angiogenic and anti-tumor effects by binding to α₁β₁ integrin. Although arresten is highly expressed in normal rat myocardium, its functions have not been clarified. This study investigated the effects of arresten on functions of cardiac fibroblasts and left atria. Rat cardiac fibroblasts were stimulated with recombinant rat arresten. Matrix metalloproteinase (MMP)-9 secretion and phosphorylation of extracellular signal-regulated kinase (ERK) were evaluated by Western blotting. Cell migration was measured by Boyden chamber assay. Isometric contraction in mouse isolated left atria treated with recombinant rat arresten was measured. In cardiac fibroblasts, arresten (250, 500 ng/ml, 24 h) promoted MMP-9 secretion and migration. Arresten (250 ng/ml, 60, 180 min) induced ERK activation and inhibition of ERK activation by PD98059 (20 µM) suppressed the arresten (250 ng/ml, 24 h)-induced MMP-9 secretion and migration. In isolated left atria, arresten (10-500 ng/ml) induced positive inotropic effect, which was suppressed by obtustatin (50 ng/ml), an α₁β₁ integrin inhibitor. We for the first time clarified the functions of arresten in cardiac fibroblasts and isolated left atria.