Cardiac energetics alteration in a chronic hypoxia rat model: A non-invasive in vivo 31P magnetic resonance spectroscopy study

BACKGROUND: Energetics alteration plays a crucial role in the myocardial injury process in chronic hypoxia diseases (CHD). 31P magnetic resonance spectroscopy (MRS) can investigate alterations in cardiac energetics in vivo. OBJECTIVE: To characterize the potential value of 31P MRS in evaluating cardiac energetics alteration of chronic hypoxic rats (CHRs). METHODS: Twenty-four CHRs were induced by SU5416 combined with hypoxia and divided into four groups according to the modeling time of one, two, three and five weeks, respectively. Control group also contains six rats. 31P MRS was performed weekly and the ratio of concentrations of phosphocreatine (PCr) to adenosine triphosphate (ATP) (PCr/ATP) was obtained. In addition, the cardiac structure index and systolic function parameters, including the right ventricular ejection fraction (RVEF), right ventricular end-diastolic volume index (RVEDVi), right ventricular end-systolic volume index (RVESVi), and the left ventricular function parameters, were measured. RESULTS: Decreased resting cardiac PCr/ATP ratio in CHRs was observed at the first week, compared to the control group (2.90±0.35 vs. 3.31±0.45, p = 0.045), while the RVEF, RVEDVi, and RVESVi decreased at the second week (p < 0.05). The PCr/ATP ratio displayed a significant correlation with RVEF (r = 0.605, p = 0.001), RVEDVi, and RVESVi (r = –0.661, r = –0.703; p < 0.001). CONCLUSIONS: 31P MRS can easily detect the cardiac energetics alteration in a CHR model before the onset of ventricular dysfunction. The decreased PCr/ATP ratio likely reveales myocardial injury and cardiac dysfunction.

Altered Cardiac Energetics and Mitochondrial Dysfunctionin Hypertrophic Cardiomyopathy

Background: Hypertrophic cardiomyopathy (HCM) is a complex disease partly explained by the effects of individual gene variants on sarcomeric protein biomechanics. At the cellular level, HCM mutations most commonly enhance force production, leading to higher energy demands. Despite significant advances in elucidating sarcomeric structure-function relationships, there is still much to be learned about the mechanisms that link altered cardiac energetics to HCM phenotypes. In this work, we test the hypothesis that changes in cardiac energetics represent a common pathophysiologic pathway in HCM. Methods: We performed a comprehensive multi-omics profile of the molecular (transcripts, metabolites, and complex lipids), ultrastructural, and functional components of HCM energetics using myocardial samples from 27 HCM patients and 13 normal controls (donor hearts). Results: Integrated omics analysis revealed alterations in a wide array of biochemical pathways with major dysregulation in fatty acid metabolism, reduction of acylcarnitines, and accumulation of free fatty acids. HCM hearts showed evidence of global energetic decompensation manifested by a decrease in high energy phosphate metabolites [ATP, ADP, and phosphocreatine (PCr)] and a reduction in mitochondrial genes involved in creatine kinase and ATP synthesis. Accompanying these metabolic derangements, electron microscopy showed an increased fraction of severely damaged mitochondria with reduced cristae density, coinciding with reduced citrate synthase (CS) activity and mitochondrial oxidative respiration. These mitochondrial abnormalities were associated with elevated reactive oxygen species (ROS) and reduced antioxidant defenses. However, despite significant mitochondrial injury, HCM hearts failed to upregulate mitophagic clearance. Conclusions: Overall, our findings suggest that perturbed metabolic signaling and mitochondrial dysfunction are common pathogenic mechanisms in patients with HCM. These results highlight potential new drug targets for attenuation of the clinical disease through improving metabolic function and reducing mitochondrial injury.

The β3 Adrenergic Receptor Antagonist L-748,337 Attenuates Dobutamine-Induced Cardiac Inefficiency While Preserving Inotropy in Anesthetized Pigs

Excessive myocardial oxygen consumption (MVO2) is considered a limitation for catecholamines, termed oxygen cost of contractility. We hypothesize that increased MVO2 induced by dobutamine is not directly related to contractility but linked to intermediary myocardial metabolism. Furthermore, we hypothesize that selective β3 adrenergic receptor (β3AR) antagonism using L-748,337 prevents this. In an open-chest pig model, using general anesthesia, we assessed cardiac energetics, hemodynamics and arterial metabolic substrate levels at baseline, ½ hour and 6 hours after onset of drug infusion. Cardiac efficiency was assessed by relating MVO2 to left ventricular work (PVA; pressure–volume area). Three groups received dobutamine (5 μg/kg/min), dobutamine + L-748,337 (bolus 50 μg/kg), or saline for time-matched controls. Cardiac efficiency was impaired over time with dobutamine infusion, displayed by persistently increased unloaded MVO2 from ½ hour and 47% increase in the slope of the PVA–MVO2 relation after 6 hours. Contractility increased immediately with dobutamine infusion ( dP/ dt max; 1636 ± 478 vs 2888 ± 818 mmHg/s, P < 0.05) and persisted throughout the protocol (2864 ± 1055 mmHg/s, P < 0.05). Arterial free fatty acid increased gradually (0.22 ± 0.13 vs 0.39 ± 0.30 mM, P < 0.05) with peak levels after 6 hours (1.1 ± 0.4 mM, P < 0.05). By combining dobutamine with L-748,337 the progressive impairment in cardiac efficiency was attenuated. Interestingly, this combined treatment effect occurred despite similar alterations in cardiac inotropy and substrate supply. We conclude that the extent of cardiac inefficiency following adrenergic stimulation is dependent on the duration of drug infusion, and β3AR blockade may attenuate this effect.

Metabolic and Proteomic Defects in Human Hypertrophic Cardiomyopathy

Rationale: Impaired cardiac energetics in hypertrophic cardiomyopathy (HCM) is thought to result from increased ATP utilization at the sarcomere and is believed to be central to pathophysiology. However, the precise defects in cardiac metabolism and substrate availability in human HCM have not been defined. Objective: The purpose of this study is to define major disease pathways and determine the pool sizes of intermediary metabolites in human HCM. Methods and Results: We conducted paired proteomic and metabolomic analyses of septal myectomy samples from patients with HCM and compared results to non-failing control human hearts. Increased abundance of extracellular matrix and intermediate filament / Z-disc proteins, and decreased abundance of proteins involved in fatty acid oxidation and cardiac energetics was evident in HCM compared to controls. Acyl carnitines, byproducts of fatty acid oxidation, were markedly depleted in HCM samples. Conversely, the ketone body 3-hydroxybutyrate, lactate, and the 3 branched chain amino acids, were all significantly increased in HCM hearts, suggesting that they may serve as alternate fuel sources for the production of ATP. ATP, nicotinamide adenine dinucleotide (NADH), NADP and NADPH, and acetyl CoA were also severely depleted in HCM hearts. Based on measurements from human skinned muscle fibers, the magnitude of observed reduction in ATP content in the HCM hearts would be expected to decrease the rate of cross-bridge detachment, implying a direct effect of energy depletion on myofilament function that could contribute to diastolic dysfunction. Conclusions: HCM hearts display profound deficits in cardiac energetics, marked by depletion of fatty acid derivatives and compensatory increases in other metabolites that could serve as alternate fuel sources. These results lend support to the paradigm that energy depletion contributes to the pathophysiology of HCM and also have important therapeutic implications for the future design of metabolic modulators to treat HCM.

Effects of Sodium‐Glucose Linked Transporter 2 Inhibition With Ertugliflozin on Mitochondrial Function, Energetics, and Metabolic Gene Expression in the Presence and Absence of Diabetes Mellitus in Mice

Background Inhibitors of the sodium‐glucose linked transporter 2 improve cardiovascular outcomes in patients with or without type 2 diabetes mellitus, but the effects on cardiac energetics and mitochondrial function are unknown. We assessed the effects of sodium‐glucose linked transporter 2 inhibition on mitochondrial function, high‐energy phosphates, and genes encoding mitochondrial proteins in hearts of mice with and without diet‐induced diabetic cardiomyopathy. Methods and Results Mice fed a control diet or a high‐fat, high‐sucrose diet received ertugliflozin mixed with the diet (0.5 mg/g of diet) for 4 months. Isolated mitochondria were assessed for functional capacity. High‐energy phosphates were assessed by 31 P nuclear magnetic resonance spectroscopy concurrently with contractile performance in isolated beating hearts. The high‐fat, high‐sucrose diet caused myocardial hypertrophy, diastolic dysfunction, mitochondrial dysfunction, and impaired energetic response, all of which were prevented by ertugliflozin. With both diets, ertugliflozin caused supernormalization of contractile reserve, as measured by rate×pressure product at high work demand. Likewise, the myocardial gene sets most enriched by ertugliflozin were for oxidative phosphorylation and fatty acid metabolism, both of which were enriched independent of diet. Conclusions Ertugliflozin not only prevented high‐fat, high‐sucrose–induced pathological cardiac remodeling, but improved contractile reserve and induced the expression of oxidative phosphorylation and fatty acid metabolism gene sets independent of diabetic status. These effects of sodium‐glucose linked transporter 2 inhibition on cardiac energetics and metabolism may contribute to improved structure and function in cardiac diseases associated with mitochondrial dysfunction, such as heart failure.

Beneficial and Therapeutic Potential of Ketone Bodies (KB) in Clinical Practice

Various discussion exists concerning ketone bodies (KB) for beneficial effects. In 2021, the American College of Cardiology (ACC) has presented the therapeutic potential of KB for cardiovascular (CV) disease. KB cover 10-15% of cardiac production of ATP, elevation of cardiac energetics, and reduction of cardiac remodeling, inflammation, and oxidative stress.

The Pitfalls of in vivo Cardiac Physiology in Genetically Modified Mice – Lessons Learnt the Hard Way in the Creatine Kinase System

In order to fully understand gene function, at some point, it is necessary to study the effects in an intact organism. The creation of the first knockout mouse in the late 1980’s gave rise to a revolution in the field of integrative physiology that continues to this day. There are many complex choices when selecting a strategy for genetic modification, some of which will be touched on in this review, but the principal focus is to highlight the potential problems and pitfalls arising from the interpretation of in vivo cardiac phenotypes. As an exemplar, we will scrutinize the field of cardiac energetics and the attempts to understand the role of the creatine kinase (CK) energy buffering and transport system in the intact organism. This story highlights the confounding effects of genetic background, sex, and age, as well as the difficulties in interpreting knockout models in light of promiscuous proteins and metabolic redundancy. It will consider the dose-dependent effects and unintended consequences of transgene overexpression, and the need for experimental rigour in the context of in vivo phenotyping techniques. It is intended that this review will not only bring clarity to the field of cardiac energetics, but also aid the non-expert in evaluating and critically assessing data arising from in vivo genetic modification.