Ca is essential in cardiac electrophysiology, contraction, energetics and nuclear transcription. Calmodulin (CaM) and Ca/CaM-dependent protein kinase (CaMKII) are also important mediators of Ca signaling in myocytes. CaMKII can phosphorylate and modulate function of Na, Ca and K channels, ryanodine receptor (RyR) and IP3 receptor channels, the phospholamban-SERCA complex and myofilaments. Some of these pathways may contribute to decreased cardiac function and enhanced propensity for arrhythmias in hypertrophy and heart failure (HF). Since CaMKII expression and activation state is increased in HF, these pathways may be important in contributing to the development and consequences of HF and may represent important therapeutic targets. CaMKII effects on cardiac Na channels and RyRs may be particularly important in HF and arrhythmias, and these acquired CaMKII-dependent effects can recapitulate genetic mutations in these channels that are associated with long QT (LQT), Brugada syndromes and Catecholaminergic Polymorphic Ventricular Tachycardia (CPVT). In particular CaMKII can phosphorylate NaV1.5 and cause both enhanced late INa (as observed in LQT3) and also loss of Na channel availability (as observed in NaV1.5 mutants linked to Brugada and short QT syndromes) which the outcome dependent on heart rate. RyR phosphorylation by CaMKII increase diastolic sarcoplasmic reticulum (SR) Ca leak (as occurs in CPVT-linked mutations in Ryr2 and calsequestrin 2). This altered RyR gating can lead to increase delayed afterdepolarizations (DADs) and serve as a source of triggered arrhythmias as well as cause reduced SR Ca content available for release in HF myocytes. Thus, CaMKII activation in HF and arrhythmogenic conditions can mediate acquired forms of cardiac arrhythmias and contractile dysfunction in pathologic conditions. CaM and FKBP12.6 both bind to the RyR in cardiac myocytes and can reduce RyR opening and SR Ca leak. We have directly measured the binding kinetics and functional impact of both CaM and FKBP12.6 to RyR in functioning myocytes, using fluorescence confocal imaging and fluorescence resonance energy transfer (FRET). FKBP12.6 binds with very high affinity (Kd~1 nM) but has only modest functional effects, whereas CaM binds with Kd ~20 nM and strongly suppresses diastolic SR Ca leak and Ca spark frequency. Moreover, knock-in mice expressing a mutant RyR2 that cannot bind CaM have much higher SR Ca leak, triggered activity and arrhythmias at the whole animal level. This corresponds to human CPVTs that are linked to CaM point mutations which we have shown to bind to RyR2 with higher affinity than wild type CaM, but which also fail to quiet RyR2 like WT CaM (thus explaining the human CPVT phenotype). Pathological conditions that promote SR Ca leak (heart failure, oxidative stress and CaMKII activation) cause a shift in myocyte RyR2 conformation that has reduces CaM affinity and enhances binding of the conformation-sensitive peptide DPc10. Dantrolene and increased CaM concentration can shift this conformational state back to normal and also suppress diastolic SR Ca leak and arrhythmias. This raises the possibility of a novel RyR-related therapeutic strategy, namely new molecules that, like dantrolene, can shift the RyR conformational state from pathophysiological to physiological.