Regulation of cardiac energetics: role of redox state and cellular compartmentation during ischemia

Protection effect of nicotinamide on cardiomyoblast hypoxia/re-oxygenation injury: study of cellular mitochondrial metabolism

Nicotinamide exerts a protective effect on cardiomyoblasts against hypoxia/re-oxygenation-induced injury through reduction of reactive oxygen species generation via succinate dehydrogenase inhibition.

Modeling of spatial metabolite distributions in the cardiac sarcomere

Although a high ATP diffusion rate implies homogeneous distribution of the principal energetic currency in the cytosol, local diffusion barriers represented by macromolecular structures can render ATP concentrations to be inhomogeneous. A method is presented here that provides apparent diffusion coefficient values in local intracellular regions and allows the estimation of spatial metabolite distribution. The apparent local diffusion coefficient for ATP in cardiac myofibrils was determined from the analysis of diffusion-dependent rightward shift of the substrate dependence for actomyosin ATPase activity using the reaction-diffusion model, which accounted for the properties of phosphotransfer reactions. This functional analysis, which took into account the local diffusional ATP delivery to the active sites, provided an apparent value that was three orders of magnitude lower than that defined by direct methods for the cytosol. The low value of the diffusion coefficient was shown to define unusual properties of the intracellular space in working heart, where small reductions in ATP levels in the surrounding cytosol result in a large drop in [ATP] inside myofibrils. This drop is critical for vital cellular functions, and the analysis presented here defines its physical basis. The diffusion barriers thus defined explain the coexistence of pathological energy deficit with almost normal average ATP levels.

Parallel activation of mitochondrial oxidative metabolism with increased cardiac energy expenditure is not dependent on fatty acid oxidation in pigs

Steady state concentrations of ATP and ADP in vivo are similar at low and high cardiac workloads; however, the mechanisms that regulate the activation of substrate metabolism and oxidative phosphorylation that supports this stability are poorly understood. We tested the hypotheses that (1) there is parallel activation of mitochondrial and cytosolic dehydrogenases in the transition from low to high workload, which increases NADH/NAD+ ratio in both compartments, and (2) this response does not require an increase in fatty acid oxidation (FAO). Anaesthetized pigs were subjected to either sham treatment, or an abrupt increase in cardiac workload for 5 min with dobutamine infusion and aortic constriction. Myocardial oxygen consumption and FAO were increased 3- and 2-fold, respectively, but ATP and ADP concentrations did not change. NADH-generating pathways were rapidly activated in both the cytosol and mitochondria, as seen in a 40% depletion in glycogen stores, a 3.6-fold activation of pyruvate dehydrogenase, and a 50% increase in tissue NADH/NAD+. Simulations from a multicompartmental computational model of cardiac energy metabolism predicted that parallel activation of glycolysis and mitochondrial metabolism results in an increase in the NADH/NAD+ ratio in both cytosol and mitochondria. FAO was blocked by 75% in a third group of pigs, and a similar increase in and the NAHD/NAD+ ratio was observed. In conclusion, in the transition to a high cardiac workload there is rapid parallel activation of substrate oxidation that results in an increase in the NADH/NAD+ ratio.

Role of Cellular Compartmentation in the Metabolic Response to Stress: Mechanistic Insights from Computational Models

The mechanisms controlling ATP generation in the transition from normal resting conditions to either high work states or ischemia are poorly understood. ATP generation depends upon compartmentation between the mitochondria and cytosol of metabolic pathways and key energy transfer species that cannot be easily assessed experimentally. We developed a multicompartment mathematical model of cardiac metabolism to simulate the metabolic responses to ischemia and increased workload. The model is based on mass balances, transport, and metabolic processes in cardiac tissue, and has three distinct compartments (blood, cytosol, and mitochondria). In addition to distinguishing between cytosol and mitochondria, the model includes a cytosolic subcompartment for glycolytic metabolic channeling. The model simulations predict the rapid activation of glycogenolysis and lactate production at the onset of ischemia, and support the concept of localization of glycolysis to a cytosolic subcompartment. In addition, simulations show that mitochondrial NADH/NAD(+) is primarily determined by oxygen consumption during ischemia, while cytosolic NADH/NAD(+) and lactate production are largely a function of glycolytic flux during the initial phase, and is controlled by mitochondrial NADH/NAD(+) and the malate-aspartate shuttle during the steady state. Finally, the model predicts that metabolic activation with an abrupt increase in workload requires parallel activation of ATP hydrolysis, glycolysis, mitochondrial dehydrogenases, the electron transport chain, and ADP phosphorylation. Taken together, these studies demonstrate the importance of metabolic compartmentation in the regulation of cardiac energetics in response to acute stress, and they highlight the usefulness of computational models in this line of investigation.

Modeling cardiac ischemia

Myocardial ischemia is one of the main causes of sudden cardiac death, with 80% of victims suffering from coronary heart disease. In acute myocardial ischemia, the obstruction of coronary flow leads to the interruption of oxygen flow, glucose, and washout in the affected tissue. Cellular metabolism is impaired and severe electrophysiological changes in ionic currents and concentrations ensue, which favor the development of lethal cardiac arrhythmias such as ventricular fibrillation. Due to the burden imposed by ischemia in our societies, a large body of research has attempted to unravel the mechanisms of initiation, sustenance, and termination of cardiac arrhythmias in acute ischemia, but the rapidity and complexity of ischemia-induced changes as well as the limitations in current experimental techniques have hampered evaluation of ischemia-induced alterations in cardiac electrical activity and understanding of the underlying mechanisms. Over the last decade, computer simulations have demonstrated the ability to provide insight, with high spatiotemporal resolution, into ischemic abnormalities in cardiac electrophysiological behavior from the ionic channel to the whole organ. This article aims to review and summarize the results of these studies and to emphasize the role of computer simulations in improving the understanding of ischemia-related arrhythmias and how to efficiently terminate them.

Regulation of myocardial substrate metabolism during increased energy expenditure: insights from computational studies

In response to exercise, the heart increases its metabolic rate severalfold while maintaining energy species (e.g., ATP, ADP, and Pi) concentrations constant; however, the mechanisms that regulate this response are unclear. Limited experimental studies show that the classic regulatory species NADH and NAD+ are also maintained nearly constant with increased cardiac power generation, but current measurements lump the cytosol and mitochondria and do not provide dynamic information during the early phase of the transition from low to high work states. In the present study, we modified our previously published computational model of cardiac metabolism by incorporating parallel activation of ATP hydrolysis, glycolysis, mitochondrial dehydrogenases, the electron transport chain, and oxidative phosphorylation, and simulated the metabolic responses of the heart to an abrupt increase in energy expenditure. Model simulations showed that myocardial oxygen consumption, pyruvate oxidation, fatty acids oxidation, and ATP generation were all increased with increased energy expenditure, whereas ATP and ADP remained constant. Both cytosolic and mitochondrial NADH/NAD+ increased during the first minutes (by 40% and 20%, respectively) and returned to the resting values by 10-15 min. Furthermore, model simulations showed that an altered substrate selection, induced by either elevated arterial lactate or diabetic conditions, affected cytosolic NADH/NAD+ but had minimal effects on the mitochondrial NADH/NAD+, myocardial oxygen consumption, or ATP production. In conclusion, these results support the concept of parallel activation of metabolic processes generating reducing equivalents during an abrupt increase in cardiac energy expenditure and suggest there is a transient increase in the mitochondrial NADH/NAD+ ratio that is independent of substrate supply.

Mitochondrial medicine: a metabolic perspective on the pathology of oxidative phosphorylation disorders

The final steps in the production of adenosine triphosphate (ATP) in mitochondria are executed by a series of multisubunit complexes and electron carriers, which together constitute the oxidative phosphorylation (OXPHOS) system. OXPHOS is under dual genetic control, with communication between the nuclear and mitochondrial genomes essential for optimal assembly and function of the system. We describe the current understanding of the metabolic consequences of pathological OXPHOS defects, based on analyses of patients and of genetically engineered model systems. Understanding the metabolic consequences of OXPHOS disease is of key importance for elucidating pathogenic mechanisms, guiding diagnosis and developing therapies.

Regulation of lactate production at the onset of ischaemia is independent of mitochondrial NADH/NAD+: insights from in silico studies

Ischaemia decreases mitochondrial NADH oxidation, activates glycolysis, increases the NADH/NAD+ ratio, and causes lactate production. The mechanisms that regulate anaerobic glycolysis and the NADH/NAD+ ratio during ischaemia are unclear. Although continuous measurements of metabolic fluxes and NADH/NAD+ in cytosol and mitochondria are not possible in vivo with current experimental techniques, computational models can be used to predict these variables by simulations with in silico experiments. Such predictions were obtained using a mathematical model of cellular metabolism in perfused myocardium. This model, which distinguishes cytosolic and mitochondrial domains, incorporates key metabolic species and processes associated with energy transfer. Simulation of metabolic responses to mild, moderate and severe ischaemia in large animals showed that mitochondrial NADH/NAD+ was rapidly reset to higher values in proportion to the reduced O2 delivery and myocardial oxygen consumption . Cytosolic NADH/NAD+, however, showed a biphasic response, with a sharp initial increase that was due to activation of glycogen breakdown and glycolysis, and corresponded with lactate production. Whereas the rate of glycolysis and the malate-aspartate shuttle had a significant effect on the cytosolic NADH/NAD+, their effects on the mitochondrial NADH/NAD+ were minimal. In summary, model simulations of the metabolic response to ischaemia showed that mitochondrial NADH/NAD+ is primarily determined by O2 consumption, while cytosolic NADH/NAD+ is largely a function of glycolytic flux during the initial phase, and is determined by mitochondrial NADH/NAD+ and the malate-aspartate shuttle during the steady state.