The heart is the most metabolically flexible organ with respect to the use of substrates available in different states of energy metabolism. Cardiac mitochondria sense substrate availability and ensure the efficiency of oxidative phosphorylation and heart function. Mitochondria also play a critical role in cardiac ischemia/reperfusion injury, during which they are directly involved in ROS-producing pathophysiological mechanisms. This review explores the mechanisms of ROS production within the energy metabolism pathways and focuses on the impact of different substrates. We describe the main metabolites accumulating during ischemia in the glucose, fatty acid, and Krebs cycle pathways. Hyperglycemia, often present in the acute stress condition of ischemia/reperfusion, increases cytosolic ROS concentrations through the activation of NADPH oxidase 2 and increases mitochondrial ROS through the metabolic overloading and decreased binding of hexokinase II to mitochondria. Fatty acid-linked ROS production is related to the increased fatty acid flux and corresponding accumulation of long-chain acylcarnitines. Succinate that accumulates during anoxia/ischemia is suggested to be the main source of ROS, and the role of itaconate as an inhibitor of succinate dehydrogenase is emerging. We discuss the strategies to modulate and counteract the accumulation of substrates that yield ROS and the therapeutic implications of this concept.

The accumulation of long-chain fatty acids (FAs) and their CoA and carnitine esters is observed in the ischaemic myocardium after acute ischaemia/reperfusion. The aim of the present study was to identify harmful FA intermediates and their detrimental mechanisms of action in mitochondria and the ischaemic myocardium. In the present study, we found that the long-chain acyl-CoA and acylcarnitine content is increased in mitochondria isolated from an ischaemic area of the myocardium. In analysing the FA derivative content, we discovered that long-chain acylcarnitines, but not acyl-CoAs, accumulate at concentrations that are harmful to mitochondria. Acylcarnitine accumulation in the mitochondrial intermembrane space is a result of increased carnitine palmitoyltransferase 1 (CPT1) and decreased carnitine palmitoyltransferase 2 (CPT2) activity in ischaemic myocardium and it leads to inhibition of oxidative phosphorylation, which in turn induces mitochondrial membrane hyperpolarization and stimulates the production of reactive oxygen species (ROS) in cardiac mitochondria. Thanks to protection mediated by acyl-CoA-binding protein (ACBP), the heart is much better guarded against the damaging effects of acyl-CoAs than against acylcarnitines. Supplementation of perfusion buffer with palmitoylcarnitine (PC) before occlusion resulted in a 2-fold increase in the acylcarnitine content of the heart and increased the infarct size (IS) by 33%. A pharmacologically induced decrease in the mitochondrial acylcarnitine content reduced the IS by 44%. Long-chain acylcarnitines are harmful FA intermediates, accumulating in ischaemic heart mitochondria and inducing inhibition of oxidative phosphorylation. Therefore, decreasing the acylcarnitine content via cardioprotective drugs may represent a novel treatment strategy.

Background and purpose:  Mildronate [3‐(2,2,2‐trimethylhydrazinium) propionate] is an anti‐ischaemic drug whose mechanism of action is based on its inhibition of L‐carnitine biosynthesis and uptake. As L‐carnitine plays a pivotal role in the balanced metabolism of fatty acids and carbohydrates, this study was carried out to investigate whether long‐term mildronate treatment could influence glucose levels and prevent diabetic complications in an experimental model of type 2 diabetes in Goto‐Kakizaki (GK) rats.Experimental approach:  GK rats were treated orally with mildronate at doses of 100 and 200 mg·kg⁻¹ daily for 8 weeks. Plasma metabolites reflecting glucose and lipids, as well as fructosamine and β‐hydroxybutyrate, were assessed. L‐carnitine concentrations were measured by ultra performance liquid chromatography with tandem mass spectrometry. An isolated rat heart ischaemia‐reperfusion model was used to investigate possible cardioprotective effects. Pain sensitivity was measured with a tail‐flick latency test.Key results:  Mildronate treatment significantly decreased L‐carnitine concentrations in rat plasma and gradually decreased both the fed‐ and fasted‐state blood glucose. Mildronate strongly inhibited fructosamine accumulation and loss of pain sensitivity and also ameliorated the enhanced contractile responsiveness of GK rat aortic rings to phenylephrine. In addition, in mildronate‐treated hearts, the necrosis zone following coronary occlusion was significantly decreased by 30%.Conclusions and implications:  These results demonstrate for the first time that in GK rats, an experimental model of type 2 diabetes, mildronate decreased L‐carnitine contents and exhibited cardioprotective effects, decreased blood glucose concentrations and prevented the loss of pain sensitivity. These findings indicate that mildronate treatment could be beneficial in diabetes patients with cardiovascular problems.