Heart failure is a major public health problem without adequate therapies. Loss of myocardial Ca2+ homeostasis and mitochondrial Ca2+ overload are fundamental events driving heart failure progression, but no currently available therapies prevent excessive mitochondrial Ca2+ entry. In 2011, after a 50 year search, two groups independently identified the molecular basis for the mitochondrial Ca2+ uniporter (MCU), the main pathway for Ca2+ entry into mitochondria. We developed new, myocardial-selective transgenic and inducible knock out mouse models of MCU inhibition to test this concept in vivo. Our new mice with myocardial MCU inhibition are viable and our preliminary data show they are resistant to myocardial death after isoproterenol infusion. Here we propose to establish how MCU inhibition contributes to myocardial physiology and disease. The multifunctional Ca2+ and calmodulin-dependent protein kinase II (CaMKII) contributes to heart failure by promoting defective intracellular Ca2+ handling, including mitochondrial Ca2+ overload, but the molecular targets for cardiomyopathic actions of CaMKII are uncertain. During the original period of this competing renewal, we found that CaMKII is present in mitochondria, that mitochondrial CaMKII inhibition reduces MCU- mediated mitochondrial Ca2+ entry and protects against mitochondrial Ca2+ overload in clinically-relevant models of heart failure (Joiner Nature 2012). We identified key sites on MCU (serines 57 and 92) that are essential for CaMKII agonist actions. Thus, MCU is the first validated CaMKII target protein in mitochondria. Here we propose to test the role of mitochondrial CaMKII at MCU in myocardial physiology and disease. The overall goal of this competitive renewal application is to determine the importance of MCU and mitochondrial CaMKII-dependent MCU phosphorylation for myocardial metabolism and disease using 3 specific aims. 1. Determine the effect of MCU inhibition on myocardial physiology; 2. Determine the effect of MCU inhibition on myocardial responses to pathological stress; 3. Determine the effects of CaMKII-dependent MCU phosphorylation on metabolism and disease.