Glucose-insulin therapy, plasma substrate levels and cardiac recovery after cardiac ischemic events

Insulin and glycolysis dependency of cardioprotection by nicotinamide riboside

Decreased nicotinamide adenine dinucleotide (NAD+) levels contribute to various pathologies such as ageing, diabetes, heart failure and ischemia-reperfusion injury (IRI). Nicotinamide riboside (NR) has emerged as a promising therapeutic NAD+ precursor due to efficient NAD+ elevation and was recently shown to be the only agent able to reduce cardiac IRI in models employing clinically relevant anesthesia. However, through which metabolic pathway(s) NR mediates IRI protection remains unknown. Furthermore, the influence of insulin, a known modulator of cardioprotective efficacy, on the protective effects of NR has not been investigated. Here, we used the isolated mouse heart allowing cardiac metabolic control to investigate: (1) whether NR can protect the isolated heart against IRI, (2) the metabolic pathways underlying NR-mediated protection, and (3) whether insulin abrogates NR protection. NR protection against cardiac IRI and effects on metabolic pathways employing metabolomics for determination of changes in metabolic intermediates, and 13C-glucose fluxomics for determination of metabolic pathway activities (glycolysis, pentose phosphate pathway (PPP) and mitochondrial/tricarboxylic acid cycle (TCA cycle) activities), were examined in isolated C57BL/6N mouse hearts perfused with either (a) glucose + fatty acids (FA) ("mild glycolysis group"), (b) lactate + pyruvate + FA ("no glycolysis group"), or (c) glucose + FA + insulin ("high glycolysis group"). NR increased cardiac NAD+ in all three metabolic groups. In glucose + FA perfused hearts, NR reduced IR injury, increased glycolytic intermediate phosphoenolpyruvate (PEP), TCA intermediate succinate and PPP intermediates ribose-5P (R5P) / sedoheptulose-7P (S7P), and was associated with activated glycolysis, without changes in TCA cycle or PPP activities. In the "no glycolysis" hearts, NR protection was lost, whereas NR still increased S7P. In the insulin hearts, glycolysis was largely accelerated, and NR protection abrogated. NR still increased PPP intermediates, with now high 13C-labeling of S7P, but NR was unable to increase metabolic pathway activities, including glycolysis. Protection by NR against IRI is only present in hearts with low glycolysis, and is associated with activation of glycolysis. When activation of glycolysis was prevented, through either examining "no glycolysis" hearts or "high glycolysis" hearts, NR protection was abolished. The data suggest that NR's acute cardioprotective effects are mediated through glycolysis activation and are lost in the presence of insulin because of already elevated glycolysis.

Energy substrate metabolism and mitochondrial oxidative stress in cardiac ischemia/reperfusion injury

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.

Plasma fatty acid levels in children during extracorporeal membrane oxygenation support--a pilot study

Plasma fatty acids levels are markedly elevated in patients with myocardial ischemic-reperfusion injury including those after cardiopulmonary bypass (CPB). High levels of fatty acids have detrimental effects on myocardial function. Extracorporeal membrane oxygenation (ECMO) is like CPB, but much longer, to provide a life-saving support for patients with cardiac arrest. We measured plasma fatty acid levels in children during ECMO support. Five children (aged .3-36 months, median 20 months) receiving venoarterial ECMO support after cardiac arrest in 2010 and 2011 were enrolled. The study was initiated at 32-56 hours after the start of ECMO support as a result of the complicated clinical scenario. Fatty acids were measured at 8-hour intervals for 1-3 days. The dosage of inotropes and vasoactive agents was recorded concurrently. The duration of ECMO ranged from 70 to 240 hours (median 177 hours). Four patients were successfully weaned off ECMO support. One died after termination of ECMO. Levels of fatty acids were elevated compared with the normal values. Overall, fatty acid levels continuously decreased over time (p < .0001), the mean being 1.03 +/- .33 mmol/L in 30-50 hours, 1.01 +/- .57 in 50-70 hours, .81 +/- .32 in 70-90 hours, and .63 +/- .23 hours. No correlation was found between fatty acid levels and other clinical variables, including age, dosage of inotropes and vasoactive agents, or ECMO duration. Plasma fatty acids levels are elevated in children during ECMO support and continuously decrease over time. Fatty acid levels may be markedly higher in the immediate hours after the initiation of ECMO. Data from more patients are needed to understand the profiles of fatty acids and the correlations with clinical variables. Metabolic manipulations to decrease fatty acids might improve myocardial recovery in patients undergoing ECMO support.