The present invention relates to decoy peptide or polypeptide consisting of a peptide sequence represented by the following Formula I: X1-Ala-X2—X3-Ile-Glu-X4 (I). It is noteworthy that the decoy peptide or polypeptide of the present invention significantly elevates phosphorylation levels of PLB by inhibiting PP1-mediated dephosphorylation. In addition, the decoy peptide or polypeptide provides cardio-protective effects by restoring of SERCA2a activity and inotropic effect of enhancing myocardial contractility. The present invention will contribute greatly to the prevention or treatment of diseases associated with PLB.
