Abstract Desensitization of b1-adrenergic receptors (b1ARs) is a hallmark of heart failure (HF), in which b1ARs on the plasma membrane (PM-b1AR) undergo endocytosis. The loss of PM-b1AR leads to impaired cAMP-PKA activity associated with reduced ejection fraction in HF. Despite decades of efforts that have primarily aimed to restore the cell surface b1AR signaling in failing hearts, effective pharmacotherapy for HF remains an unmet clinical need. Distinct from most prior studies that focused on bAR at the PM, accumulating evidence of bAR signaling inside myocyte indicates potential implication in HF. We aim to reveal an internal b1AR signaling at the sarcoplasmic reticulum (SR) critical for regulating cardiac contractility, which is desensitized in HF. We recently reported a pool of functional b1ARs on the SR (SR-b1AR). The SR-b1AR is activated by catecholamines entering cells via primarily organic cation transporter 3. We found that monoamine oxidase A (MAOA), an enzyme responsible for the degradation of catecholamines, is significantly increased in human HF. Thus, the elevated expression of MAOA essentially restricts the access of catecholamines to the SR-b1AR in HF despite increased sympathetic drive. In this proposal, we hypothesize that 1) activation of the SR-b1AR is fine-tuned by MAOA for enhancing cardiac E-C coupling in physiological stress; 2) the increased expression of MAOA in HF exacerbates b1AR signaling desensitization and contributes to depressed contractility. We propose that MAOA inhibition re- sensitizes the SR-b1AR signaling in failing myocytes and rescues E-C coupling and cardiac contractility in HF. We will apply integrated experimental approaches to test the hypotheses. Aim 1 MAOA controls the access of catecholamines to the SR-b1AR and cardiac inotropy. Aim 2. Cardiac b1ARs undergo translocation from the PM to the SR under chronic adrenergic stimulation in HF. Aim 3. Inhibition of MAOA rescues the SR-b1AR signaling and cardiac contractility in HF. Distinct from most studies focusing on adrenergic signaling on the PM, this study aims to unravel a novel local SR adrenergic signaling in the regulation of cardiac E-C coupling and present MAOA as a potential therapeutic target to rescue depressed cardiac contractility in HF.