Metabolic Response to Stress by the Immature Right Ventricle Exposed to Chronic Pressure Overload

Analyses of mitochondrial metabolism in diseases: a review on 13C magnetic resonance tracers

Metabolic diseases such as obesity, type 2 diabetes, and cardiovascular diseases have become a global health concern due to their widespread prevalence and profound impact on life expectancy, healthcare expenditures, and the overall economy. Devising effective treatment strategies and management plans for these diseases requires an in-depth understanding of the pathophysiology of the metabolic abnormalities associated with each disease. Mitochondrial dysfunction is intricately linked to a wide range of metabolic abnormalities and is considered an important biomarker for diseases. However, assessing mitochondrial functions in viable tissues remains a challenging task, with measurements of oxygen consumption rate (OCR) and ATP production being the most widely accepted approaches for evaluating the health of mitochondria in tissues. Measurements of cellular metabolism using carbon-13 (or 13C) tracers have emerged as a viable method for characterizing mitochondrial metabolism in a variety of organelles ranging from cultured cells to humans. Information on metabolic activities and mitochondrial functions can be obtained from magnetic resonance (MR) analyses of 13C-labeled metabolites in tissues and organs of interest. Combining novel 13C tracer technologies with advanced analytical and imaging tools in nuclear magnetic resonance spectroscopy (NMR) and magnetic resonance imaging (MRI) offers the potential to detect metabolic abnormalities associated with mitochondrial dysfunction. These capabilities would enable accurate diagnosis of various metabolic diseases and facilitate the assessment of responses to therapeutic interventions, hence improving patient health and optimizing clinical outcomes.

Mitochondrial Integrity Is Critical in Right Heart Failure Development

Molecular processes underlying right ventricular (RV) dysfunction (RVD) and right heart failure (RHF) need to be understood to develop tailored therapies for the abatement of mortality of a growing patient population. Today, the armament to combat RHF is poor, despite the advancing identification of pathomechanistic processes. Mitochondrial dysfunction implying diminished energy yield, the enhanced release of reactive oxygen species, and inefficient substrate metabolism emerges as a potentially significant cardiomyocyte subcellular protagonist in RHF development. Dependent on the course of the disease, mitochondrial biogenesis, substrate utilization, redox balance, and oxidative phosphorylation are affected. The objective of this review is to comprehensively analyze the current knowledge on mitochondrial dysregulation in preclinical and clinical RVD and RHF and to decipher the relationship between mitochondrial processes and the functional aspects of the right ventricle (RV).

Right ventricular energy metabolism in a porcine model of acute right ventricular pressure overload after weaning from cardiopulmonary bypass

Acute right ventricular pressure overload (RVPO) occurs following congenital heart surgery and often results in low cardiac output syndrome. We tested the hypothesis that the RV exhibits limited ability to modify substrate utilization in response to increasing energy requirements during acute RVPO after cardiopulmonary bypass (CPB). We assessed the RV fractional contributions (Fc) of substrates to the citric acid cycle in juvenile pigs exposed to acute RVPO by pulmonary artery banding (PAB) and CPB. Sixteen Yorkshire male pigs (median 38 days old, 12.2 kg of body weight) were randomized to SHAM (Ctrl, n = 5), 2-h CPB (CPB, n = 5) or CPB with PAB (PAB-CPB, n = 6). Carbon-13 (13 C)-labeled lactate, medium-chain, and mixed long-chain fatty acids (MCFA and LCFAs) were infused as metabolic tracers for energy substrates. After weaning from CPB, RV systolic pressure (RVSP) doubled baseline in PAB-CPB while piglets in CPB group maintained normal RVSP. Fc-LCFAs decreased significantly in order PAB-CPB > CPB > Ctrl groups by 13 C-NMR. Fc-lactate and Fc-MCFA were similar among the three groups. Intragroup analysis for PAB-CPB showed that the limited Fc-LCFAs appeared prominently in piglets exposed to high RVSP-to-left ventricular systolic pressure ratio and high RV rate-pressure product, an indicator of myocardial oxygen demand. Acute RVPO after CPB strongly inhibits LCFA oxidation without compensation by lactate oxidation, resulting in energy deficiency as determined by lower (phosphocreatine)/(adenosine triphosphate) in PAB-CPB. Adequate energy supply but also metabolic interventions may be required to circumvent these RV energy metabolic abnormalities during RVPO after CPB.

Characterization of main pulmonary artery and valve annulus region of piglets using echocardiography, uniaxial tensile testing, and a novel non-destructive technique

Characterization of cardiovascular tissue geometry and mechanical properties of large animal models is essential when developing cardiovascular devices such as heart valve replacements. These datasets are especially critical when designing devices for pediatric patient populations, as there is often limited data for guidance. Here, we present a previously unavailable dataset capturing anatomical measurements and mechanical properties of juvenile Yorkshire (YO) and Yucatan (YU) porcine main pulmonary artery (PA) and pulmonary valve (PV) tissue regions that will inform pediatric heart valve design requirements for preclinical animal studies. In addition, we developed a novel radial balloon catheter-based method to measure tissue stiffness and validated it against a traditional uniaxial tensile testing method. YU piglets, which were significantly lower weight than YO counterparts despite similar age, had smaller PA and PV diameters (7.6-9.9 mm vs. 10.1-12.8 mm). Young's modulus (stiffness) was measured for the PA and the PV region using both the radial and uniaxial testing methods. There was no significant difference between the two breeds for Young's modulus measured in the elastic (YU PA 84.7 ± 37.3 kPa, YO PA 79.3 ± 15.7 kPa) and fibrous regimes (YU PA 308.6 ± 59.4 kPa, YO PA 355.7 ± 68.9 kPa) of the stress-strain curves. The two testing techniques also produced similar stiffness measurements for the PA and PV region, although PV data showed greater variation between techniques. Overall, YU and YO piglets had similar PA and PV diameters and tissue stiffness to previously reported infant pediatric patients. These results provide a previously unavailable age-specific juvenile porcine tissue geometry and stiffness dataset critical to the development of pediatric cardiovascular prostheses. Additionally, the data demonstrates the efficacy of a novel balloon catheter-based technique that could be adapted to non-destructively measure tissue stiffness in situ.

Physiology of the Right Ventricle Across the Lifespan

The most common cause of heart failure in the United States is ischemic left heart disease; accordingly, a vast amount of work has been done to elucidate the molecular mechanisms underlying pathologies of the left ventricle (LV) as a general model of heart failure. Until recently, little attention has been paid to the right ventricle (RV) and it has commonly been thought that the mechanical and biochemical properties of the RV are similar to those of the LV. However, therapies used to treat LV failure often fail to improve ventricular function in RV failure underscoring, the need to better understand the unique physiologic and pathophysiologic properties of the RV. Importantly, hemodynamic stresses (such as pressure overload) often underlie right heart failure further differentiating RV failure as unique from LV failure. There are significant structural, mechanical, and biochemical properties distinctive to the RV that influences its function and it is likely that adaptations of the RV occur uniquely across the lifespan. We have previously reviewed the adult RV compared to the LV but there is little known about differences in the pediatric or aged RV. Accordingly, in this mini-review, we will examine the subtle distinctions between the RV and LV that are maintained physiologically across the lifespan and will highlight significant knowledge gaps in our understanding of pediatric and aging RV. Consideration of how RV function is altered in different disease states in an age-specific manner may enable us to define RV function in health and importantly, in response to pathology.

Pharmacokinetic and Biochemical Profiling of Sodium Dichloroacetate in Pregnant Ewes and Fetuses

Sodium dichloroacetate (DCA) is an investigational drug that shows promise in the treatment of acquired and congenital mitochondrial diseases, including myocardial ischemia and failure. DCA increases glucose utilization and decreases lactate production, so it may also have clinical utility in reducing lactic acidosis during labor. In the current study, we tested the ability of DCA to cross the placenta and be measured in fetal blood after intravenous administration to pregnant ewes during late gestation and labor. Sustained administration of DCA to the mother over 72 hours achieved pharmacologically active levels of DCA in the fetus and decreased fetal plasma lactate concentrations. Multicompartmental pharmacokinetics modeling indicated that drug metabolism in the fetal and maternal compartments is best described by the DCA inhibiting lactate production in both compartments, consistent with our finding that the hepatic expression of the DCA-metabolizing enzyme glutathione transferase zeta1 was decreased in the ewes and their fetuses exposed to the drug. We provide the first evidence that DCA can cross the placental compartment to enter the fetal circulation and inhibit its own hepatic metabolism in the fetus, leading to increased DCA concentrations and decreased fetal plasma lactate concentrations during its parenteral administration to the mother. SIGNIFICANCE STATEMENT: This study was the first to administer sodium dichloroacetate (DCA) to pregnant animals (sheep). It showed that DCA administered to the mother can cross the placental barrier and achieve concentrations in fetus sufficient to decrease fetal lactate concentrations. Consistent with findings reported in other species, DCA-mediated inhibition of glutathione transferase zeta1 was also observed in ewes, resulting in reduced metabolism of DCA after prolonged administration.

Metabolic Response to Stress by the Immature Right Ventricle Exposed to Chronic Pressure Overload

Background

The right ventricle exposed to chronic pressure overload exhibits hypertrophy and decompensates when exposed to stress. We hypothesize that impaired ability to increase myocardial oxidative flux through pyruvate dehydrogenase leads to hypertrophied right ventricular (RV) dysfunction when exposed to hemodynamic stress, and pyruvate dehydrogenase stimulation can improve RV function.

Methods and Results

Infant male Yorkshire piglets (13.5±0.6 kg weight, n=19) were used to assess substrate fractional contribution to the citric acid cycle after sustained pulmonary artery banding (PAB). Carbon 13–labeled glucose, lactate, and leucine, oxidative substrate tracers for the citric acid cycle, were infused into the right coronary artery on 7 to 10 days after PAB. RV systolic pressure, RV free wall thickness, and individual cardiomyocyte cell size after PAB were significantly elevated compared with the sham group. Both fractional glucose and lactate oxidations in the PAB group were >2‐fold higher than in the sham group. Pigs with overdrive atrial pacing (≈80% increase in heart rate) stress after PAB showed only a 22% increase in rate‐pressure product from baseline before atrial pacing and limited carbohydrate oxidation rate in the right ventricle. Intracoronary infusion of dichloroacetate, a pyruvate dehydrogenase agonist, produced higher rate‐pressure product (59% increase) in response to increased workload by atrial pacing in association with a marked increase in lactate oxidation.

Conclusions

The immature hypertrophied right ventricle shows limited ability to increase carbohydrate oxidation in response to tachycardia stress leading to energy supply/utilization imbalance and decreased systolic function. Enhanced pyruvate dehydrogenase activation by dichloroacetate increases energy supply and preserves hypertrophied RV contractile function during hemodynamic stress.