Disrupted SR–Mitochondria Coupling Drives Ischemia–Reperfusion Vulnerability in the Middle-Aged Rat Heart

Background: Myocardial ischemia–reperfusion (IR) injury is associated with dysregulated Ca2+ handling and oxidative stress, particularly in the middle-aged heart. Sarcoplasmic reticulum (SR)–mitochondria communication via mitochondria-associated membranes (MAMs) is essential for coordinating Ca2+ transfer and redox signaling; however, its role in IR injury in the middle-aged myocardium remains incompletely understood. This study investigated changes in cardiac MAM protein composition and associated functional and oxidative parameters during ischemia and IR. Methods: Middle-aged rat hearts were subjected to global ischemia or IR using the Langendorff perfusion model. Mitochondrial, MAM, and homogenate fractions were analyzed using biochemical, proteomic, and functional assays to assess Ca2+-handling proteins, redox enzymes, lipid peroxidation markers, and mitochondrial antioxidant defenses. Results: Myocardial ischemia and IR disrupted SR–mitochondria communication in middle-aged hearts, leading to impaired Ca2+ handling, redox imbalance, and reduced contractile recovery. Ischemia induced significant MAM remodeling, characterized by reduced mitofusin 2 levels and increased enrichment of voltage-dependent anion channel 1. These changes were associated with disturbed mitochondrial Ca2+ signaling, impaired SR Ca2+ sequestration. Although mitochondrial antioxidant defenses, including MnSOD, were largely preserved, IR was associated with compartment-specific redox alterations within MAMs, as inferred from altered redox enzyme activity and enhanced lipid peroxidation. Conclusions: Disruption of SR–mitochondria coupling and MAM-associated redox regulation represents a key mechanism underlying increased vulnerability to IR injury in the middle-aged heart. Targeting MAM integrity and modulating Ca2+-redox cross-talk may improve cardiac resilience in elderly populations.