Protein phosphatase 1 (PP1) has emerged as a nodal regulator of function and survival in the heart. Indeed, the increased activity of this enzyme in failing hearts contributes to depressed SR Ca-cycling and deteriorative remodeling. PP1 is negatively regulated by endogenous inhibitor-1, which is considered a promising therapeutic target. Increases in inhibitor-1 activity and decreases in PP1 protect against ischemia/reperfusion injury, chronic isoproterenol stimulation and heart failure progression. We recently identified a polymorphism (G109E) in the inhibitor-1 gene in heart failure patients with a frequency of 6%. Expression of G109E (called mutant) in rat cardiomyocytes resulted in ~20% decreases in contractile parameters, Ca-transients and sarcoplasmic reticulum Ca-load. This depressed function was rescued by isoproterenol. Interestingly, when subjected to stress conditions (2 Hz +/- Iso), the mutant cells were more susceptible to aftercontractions. Similar findings were obtained by expression of G109E in inhibitor-1 knockout cardiomyocytes in the absence of endogenous protein. The underlying mechanisms included reduced binding of mutant to PP1, increased PP1 activity and hyper-phosphorylation of S2814 in ryanodine receptor (RyR), promoting aberrant SR Ca-release. These findings were also reflected by in vivo cardiac overexpression of G109E. Contractile and Ca-kinetic parameters were depressed by ~30% in mutant cardiomyocytes, while isoproterenol relieved these inhibitory effects. Stress conditions were associated with induction of Ca waves and aftercontractions in G109E cells. Furthermore, serial caffeine/Iso injections in vivo, elicited higher incidence of ventricular ectopy (bigeminy, trigeminy and non-sustained ventricular tachycardia) in mutant mice, whereas WTs had normal rhythm. Our findings suggest that G109E and increased PP1 may dephosphorylate the RyR and promote Ca-leak, in agreement with previous reports. Subsequently, the increased Ca-levels activate CAMK, leading to hyperphosphorylation of RyR and further increases in Ca-leak. Thus, inhibitor-1 is critical in cardiac function and represents a key control in balancing phosphatase/kinase activities to stabilize Ca-cycling in the heart.