Aberrant -adrenergic signaling and depressed calcium homeostasis, associated with an imbalance of protein kinase A and phosphatase-1 activities, are hallmarks of heart failure. Phosphatase-1 is restrained by its endogenous inhibitor, protein phosphatase inhibitor-1 (PPI-1). We assessed 352 normal subjects, along with 959 patients with heart failure and identified a polymorphism in PPI-1 (G147D) exclusively in black subjects. To determine whether the G147D variant could affect cardiac function, we infected adult cardiomyocytes with adenoviruses expressing D147 or wild-type (G147) PPI-1. Under basal conditions, there were no significant differences in fractional shortening or contraction or relaxation rates. However, the enhancement of contractile parameters after isoproterenol stimulation was significantly blunted in D147 compared with G147 and control myocytes. Similar findings were observed in calcium kinetics. The attenuated -agonist response was associated with decreased (50%) phosphorylation of phospholamban (PLN) at serine 16, whereas phosphorylation of troponin I and ryanodine receptor was unaltered. These findings suggest that the human G147D PPI-1 can attenuate responses of cardiomyocytes to -adrenergic agonists by decreasing PLN phosphorylation and therefore may contribute to deteriorated function in heart failure.—Chen, G., Zhou, X., Nicolaou, P., Rodriguez, P., Song, G., Mitton, B., Pathak, A., Zachariah, A., Fan, G.-C., Dorn, G. A., II., Kranias, E. G. A human polymorphism of protein phosphatase-1 inhibitor-1 is associated with attenuated contractile response of cardiomyocytes to -adrenergic stimulation.