Background: Metabolic remodeling in cardiomyocytes is deeply associated with the pathogenesis of heart failure (HF). Glutaminolysis is an anaplerotic pathway that incorporates alpha-ketoglutarate (alpha KG) derived from glutamine into the tricarboxylic acid (TCA) cycle. It is well known that cancer cells depend on glutamine for their increased energy demand and proliferation; however, the physiological roles of glutamine metabolism in failing hearts remain unclear. Objective: To investigate the regulatory mechanisms and biological effects of glutamine metabolism in oxidative stress-induced failing myocardium. Methods and results: The intracellular levels of glutamine, glutamate, and alpha KG were significantly decreased by H2O2 stimulation in rat neonatal cardiomyocytes (RNCMs). To better understand the metabolic flux in failing myocardium, we performed a stable isotope tracing study and found that glutaminolysis was upregulated in RNCMs under oxidative stress. Consistent with this, the enzymatic activity of glutaminase (GIs), which converts glutamine to glutamate, was augmented in RNCMs treated with H2O2. These findings suggest that glutamine anaplerosis is enhanced in cardiomyocytes under oxidative stress to compensate for the reduction of ciKG. Furthermore, the inhibition of GIs reduced cardiac cell viability, ATP production, and glutathione (GSH) synthesis in RNCMs with H2O2 stimulation. Finally, we evaluated the effects of ciKG on failing myocardium and observed that dimethyl alpha-ketoglutarate (DMKG) suppressed oxidative stress-induced cell death likely due to the enhancement of intracellular ATP and GSH levels. Conclusion: Our study demonstrates that under oxidative stress, glutaminolysis is upregulated to compensate for the loss of alpha KG and its replenishment into the TCA cycle, thereby exerting cardioprotective effects by maintaining ATP and GSH levels. Modulation of glutamine metabolism in failing hearts might provide a new therapeutic strategy for HF. (C) 2020 Elsevier Inc. All rights reserved.