Question: Which protein is the main inhibitor of protein phosphatase 1 (PP1)?

Context:
The type 1 protein phosphatase (PP1) is a critical negative regulator of Ca(2+) cycling and contractility in the cardiomyocyte. In particular, it mediates restoration of cardiac function to basal levels, after beta-adrenergic stimulation, by dephosphorylating key phospho-proteins. PP1 is a holoenzyme comprised of its catalytic and auxiliary subunits. These regulatory proteins dictate PP1's subcellular localization, substrate specificity and activity. Amongst them, inhibitor-1 is of particular importance since it has been implicated as an integrator of multiple neurohormonal pathways, which finely regulate PP1 activity, at the level of the sarcoplasmic reticulum (SR). In fact, perturbations in the regulation of PP1 by inhibitor-1 have been implicated in the pathogenesis of heart failure, suggesting that inhibitor-1-based therapeutic interventions may ameliorate cardiac dysfunction and remodeling in the failing heart. 
Aberrant beta-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)
Using this technology, we showed that inhibition of the protein phosphatase 1, by its constitutively active inhibitor-1, significantly increases cardiac contractility and calcium handling. 
Thus, long-term cardiac specific overexpression of the protein phosphatase 1 inhibitor-1 and the associated increases in cardiac contractility appear to herald changes in a rather small number of proteins, which may reflect important compensatory adaptations in a hyperdynamic heart [corrected]
Inhibitor-1 becomes a potent inhibitor of protein phosphatase 1 when phosphorylated by cAMP-dependent protein kinase at Thr(35). Moreover, Ser(67) of inhibitor-1 serves as a substrate for cyclin-dependent kinase 5 in the brain.
Inhibitor 1 (I-1) is a protein inhibitor of protein phosphatase 1 (PP1), a major eukaryotic Ser/Thr phosphatase. Nonphosphorylated I-1 is inactive, whereas phosphorylated I-1 is a potent PP1 inhibitor. 
Ser67-phosphorylated inhibitor 1 is a potent protein phosphatase 1 inhibitor.
PP1 activity was inhibited by protamine, heparin, okadaic acid (IC50 50 nM) and mammalian inhibitor-1 (IC50 2 nM). On the other hand. 
The cDNA encoding human brain protein phosphatase inhibitor-1 (I-1) was expressed in Escherichia coli. Following PKA phosphorylation at a threonine, recombinant human I-1 was indistinguishable from rabbit skeletal muscle I-1 as a potent and specific inhibitor of the type-1 protein serine/threonine phosphatase (PP1). N-Terminal phosphopeptides of I-1 that retained the selectivity of intact human I-1 highlighted a functional domain that mediates PP1 inhibition.
Protein phosphatase inhibitor-1 was purified from bovine adipose tissue. The protein had an apparent molecular mass of 32 kDa by SDS/PAGE and a Stokes' radius of 3.4 nm. It was phosphorylated by cAMP-dependent protein kinase on a threonyl residue; this phosphorylation was necessary for inhibition of protein phosphatase-1.
Bovine adipose tissue inhibitor-1 was compared directly with rabbit skeletal muscle inhibitor-1 and with a 32000-Mr, dopamine- and cAMP-regulated phosphoprotein from bovine brain (DARPP-32), also an inhibitor of protein phosphatase-1. 
Protein inhibitor-1 (I-1) specifically inhibits protein phosphatase 1 (PP1), the predominant PLB phosphatase in heart.
Inhibitor 1 (I-1) is a protein inhibitor of protein phosphatase 1 (PP1), the predominating Ser/Thr phosphatase in the heart.
Inhibitor-1 (I-1) is a selective inhibitor of protein phosphatase-1 (PP1) and regulates several PP1-dependent signaling pathways, including cardiac contractility and regulation of learning and memory.
Inhibitor 1 (I-1) is a protein inhibitor of protein phosphatase 1 (PP1), a major eukaryotic Ser/Thr phosphatase.
Inhibitor 1 (I-1) is a protein inhibitor of protein phosphatase 1 (PP1), the predominating Ser/Thr phosphatase in the heart
Protein inhibitor-1 (I-1) specifically inhibits protein phosphatase 1 (PP1), the predominant PLB phosphatase in heart

Answer: Inhibitor 1 (I-1) is a protein inhibitor of protein phosphatase 1 (PP1), a major eukaryotic Ser/Thr phosphatase. Nonphosphorylated I-1 is inactive, whereas phosphorylated I-1 is a potent PP1 inhibitor.   
Protein Phosphatase-1 is restrained by its endogenous inhibitor, protein phosphatase inhibitor-1 (PPI-1). Inhibition of the protein phosphatase 1, by inhibitor-1, significantly increases cardiac contractility and calcium handling. Inhibitor-1 becomes a potent inhibitor of protein phosphatase 1 when phosphorylated by cAMP-dependent protein kinase.
Exact answer: protein phosphatase inhibitor 1, inhibitor 1, PPI-1, I1