Background and Purpose: Spermidine, a natural polyamine, is abundant in mammalian cells and is involved in cell growth, proliferation, and regeneration. Recently, oral spermidine supplements were cardioprotective in age‐related cardiac dysfunction, through enhancing autophagic flux. However, the effect of spermidine on myocardial injury and cardiac dysfunction following myocardial infarction (MI) remains unknown. Experimental Approach: We determined the effects of spermidine in a model of MI, Sprague–Dawley rats with permanent ligation of the left anterior descending artery, and in cultured neonatal rat cardiomyocytes (NRCs) exposed to angiotensin II (Ang II). Cardiac function in vivo was assessed with echocardiography. In vivo and in vitro studies used histological and immunohistochemical techniques, along with western blots. Key Results: Spermidine improved cardiomyocyte viability and decreased cell necrosis in NRCs treated with angiotensin II. In rats post‐MI, spermidine reduced infarct size, improved cardiac function, and attenuated myocardial hypertrophy. Spermidine also suppressed the oxidative damage and inflammatory cytokines induced by MI. Moreover, spermidine enhanced autophagic flux and decreased apoptosis both in vitro and in vivo. The protective effects of spermidine on cardiomyocyte apoptosis and cardiac dysfunction were abolished by the autophagy inhibitor chloroquine, indicating that spermidine exerted cardioprotective effects at least partly through promoting autophagic flux, by activating the AMPK/mTOR signalling pathway. Conclusions and Implications: Our findings suggest that spermidine improved MI‐induced cardiac dysfunction by promoting AMPK/mTOR‐mediated autophagic flux.

Polydatin (PD), a resveratrol (RES) glycoside, has a stronger antioxidative effect than RES. It is known that RES is an autophagic enhancer and exerts a cardioprotective effect against ischaemia/reperfusion (I/R) injury. However, the effect of PD post-treatment on myocardial I/R injury remains unclear. In the present study, we investigated the influences of PD post-treatment on myocardial I/R injury and autophagy. C57BL/6 mice underwent left coronary artery (LCA) occlusion and cultured neonatal rat cardiomyocytes (NRCs) subjected to hypoxia were treated with vehicle or PD during reperfusion or re-oxygenation. We noted that PD enhanced autophagy and decreased apoptosis during I/R or hypoxia/reoxygenation (H/R), and this effect was antagonized by co-treatment with adenovirus carrying short hairpin RNA for Beclin 1 and 3-methyladenine (3-MA), an autophagic inhibitor. Compared with vehicle-treated mice, PD-treated mice had a significantly smaller myocardial infarct size (IS) and a higher left ventricular fractional shortening (LVFS) and ejection fraction (EF), whereas these effects were partly reversed by 3-MA. Furthermore, in the PD-treated NRCs, tandem fluorescent mRFP-GFP-LC3 assay showed abundant clearance of autophagosomes with an enhanced autophagic flux, and co-treatment with Bafilomycin A1 (Baf), a lysosomal inhibitor, indicated that PD promoted the degradation of autolysosome. In addition, PD post-treatment reduced mitochondrial membrane potential and cellular reactive oxygen species (ROS) production in NRCs, and these effects were partially blocked by Baf. These findings indicate that PD post-treatment limits myocardial I/R injury by promoting autophagic flux to clear damaged mitochondria to reduce ROS and cell death.