Biology subjects

Beard, D. A.

Publications and source records attributed to Beard, D. A..

15 recordsLinked to original sources

bioRxiv2026

Systems Analysis of Carboxylate Transport and Oxidation Pathways in Cardiac Mitochondria

Experimental assessment and computational modeling were used to analyze substrate transport, tricarboxylic acid cycle kinetics, and oxidative phosphorylation in suspensions of purified cardiac mitochondria. The kinetics of ATP synthesis and carbohydrate oxidation, including during hypoxia and reoxygenation, were investigated using various substrate combinations and conditions. Model simulations fit to transient respiration and NAD(P)H measurements reveal novel insights into pyruvate dehydrogenase regulation, regulation of mitochondrial leak, and the clearance of oxaloacetate during respiration on succinate. High concentrations of succinate induced increased mitochondrial leak respiration driven in part by ROS-activated uncoupling. Oxidative phosphorylation under succinate-fueled respiration was inhibited by rapid buildup of oxaloacetate, inhibiting succinate dehydrogenase. Malic enzyme and oxaloacetate decarboxylase activities represent potenital pathways for removal of oxaloacetate, with glutamate further enhancing clearance. The developed model captures the observed transient behaviors as well as steady-state relationships between ATP synthesis rate and phosphate metabolite levels, lending a new systems-level understanding of mitochondrial energy metabolism. In sum, these findings offer a framework for simulating and interpreting mitochondrial function in vitro and in vivo. Key PointsThis study uses experiments and computer simulations to probe the interactions between substrate transport processes, TCA cycle kinetics, redox state, and oxidative ATP synthesis in cardiac mitochondria. The developed kinetic model simulates mitochondrial metabolism in vitro and represents a framework for integrative modeling of cardiac energy metabolism. Model-based analysis identifies a kinetic model of pyruvate dehydrogenase (PDH) deactivation during leak-state respiration and activation during oxidative phosphorylation. High levels of cation leak during respiration on succinate are explained by a ROS-dependent activation of uncoupling.

biochemistry↗

bioRxiv2025

A Microstructurally-Motivated Framework to Study Autoregulation in the Coronary Circulation

Coronary autoregulation maintains relatively constant myocardial flow over a wide range of perfusion pressures through myogenic, shear-dependent, and metabolic control mechanisms. Understanding this phenomenon is challenging due to the coupled nature of these mechanisms and their heterogeneous effects throughout the coronary tree. In this study we developed a novel microstructurally-motivated model of coronary autoregulation based on constrained mixture theory, with anatomical and structural parameters calibrated through a homeostatic optimization framework. Autoregulation was simulated at three myocardial depths (subepicardium, midwall, and subendocardium), with the calibrated model accurately reproducing baseline hemodynamics and autoregulatory responses. For changes in epicardial pressure, our model reproduced experimentally measured subendocardium-to-subepicardium flow ratios (ENDO/EPI) and changes in vessel diameter, demonstrating its predictive capability. Furthermore, we extended Womersleys theory to simulate phasic coronary hemodynamics with a time-varying intramyocardial pressure. This microstructurally-motivated framework provides a mechanistic foundation for investigating coronary autoregulation and long-term vascular growth and remodeling in pathphysiological conditions. SummaryO_LICoronary autoregulation is defined as the capability of the coronary circulation to maintain the blood supply to the heart over a range of perfusion pressures. This phenomenon is facilitated through intrinsic mechanisms that control the vascular resistance by regulating the mechanical function of smooth muscle cells. Understanding the mechanisms involved in coronary autoregulation is one of the most fundamental questions in coronary physiology. C_LIO_LIThis paper presents a structurally-motivated coronary autoregulation model that uses a nonlinear continuum mechanics approach to account for the morphometry and vessel wall composition in two coronary trees in the subepicardial and subendocardial layers. C_LIO_LIThe model is calibrated against diverse experimental data from literature and is used to study heterogeneous autoregulatory response in the coronary trees. This model drastically differs from previous models, which relied on lumped parameter model formulations, and is suited to the study of long-term pathophysiological growth and remodeling phenomena in coronary vessels. C_LI

physiology↗

bioRxiv2025

Multiscale Computational Modeling of the Cardiopulmonary Consequences of Postnatal Hyperoxia with Implications for Preterm Born Children

Moderate to extreme preterm birth (<32 weeks gestation) affects cardiopulmonary structure and function and is associated with increased risk of heart failure through adulthood. The rat hyperoxia (Hx) model (term born; postnatal Hx exposure) captures biventricular changes, including at the cell- and organ-scale, and pulmonary vascular remodeling seen in preterm humans. However, synthesizing these measures across scales and organ systems is challenging. We hypothesized that in-silico modeling of biventricular mitochondrial, myofiber, and organ-scale function plus circulatory function could capture key features of cardiopulmonary abnormalities due to preterm birth. Therefore, we calibrated a multiscale model to subject-specific biventricular pressure-volume data previously obtained from Hx rats alongside normoxic (Nx) controls to investigate the abnormalities in cardiopulmonary function at multiple scales in this animal model of human preterm birth. The calibrated model demonstrates excellent agreement with the data and captures the expected pulmonary vascular changes and right ventricular dilation seen in preterm born children. Our multiscale modeling approach captures cardiopulmonary abnormalities across spatial scales and provides an innovative approach to explore the consequences of preterm birth beyond preclinical experimental data alone. This is a foundational step in understanding the impact of preterm birth on cardiopulmonary disease in childhood as well as adulthood.

bioengineering↗

bioRxiv2025

Kinetic Modeling of mant-ATP Turnover to Interpret the Biochemically Defined Myosin Super-Relaxed State

The fluorescent ATP analog mant-ATP has become a valuable tool for quantifying occupancy of the myosin super-relaxed (SRX) state, a biochemically inactive state of myosin in striated muscle. Interpretation of mant-ATP fluorescence decay kinetics is confounded by inconsistencies in state definitions and kinetic assumptions. Here, we develop a mass-action kinetic model of myosin cross-bridge cycling and mant-ATP turnover to reconcile these discrepancies and provide a mechanistic framework for interpreting SRX measurements. Our model simulates ATP label-chase experiments and demonstrates that conventional double-exponential fitting methods do not directly quantify SRX occupancy. Instead, we show that slow and fast decay phases of mant-ATP fluorescence arise from label redistribution among kinetically distinct states, not state populations in equilibrium. The model resolves several apparent paradoxes identified in recent studies by reproducing experimental observations without requiring SRX and DRX kinetic isolation or implausible equilibrium constants. Simulations further quantify the impact of experimental factors--such as ADP accumulation, photobleaching, and initial rigor state occupancy--on fluorescence kinetics and SRX estimates. These results support a revised framework for SRX quantification and suggest that label-chase experiments must be interpreted using mechanistic models to accurately assess myosin state distributions and transition kinetics. SIGNIFICANCEThe relative occupancy of myosin in the super-relaxed state (SRX) is a key determinant of basal ATP turnover in muscle. Measurement of ATP exchange using the fluorescent analog mant-ATP is used to assess the relative population of myosin in the SRX versus the disordered relaxed (DRX) state. Existing approaches to analyzing data from these experiments use double exponential fits to represent the mant-ATP decay in the chase-phase of the experiment. However, quantitative and mechanistic interpretations based on these analyses remain ambiguous. We present a mechanistic model of myosin ATP turnover that reproduces observed fluorescence decays under defined conditions. The results indicate that conventional interpretations, while qualitatively reasonable, are quantitatively inconsistent, as the observed slow and fast phases are predicted to arise from ligand redistribution rather than distinct equilibrium states. This framework enables rigorous, model-based interpretation of mant-ATP assays to help clarify how myosin kinetics are reflected in mant-ATP loading-chase experiments and experimental conditions may influence apparent SRX kinetics.

biophysics↗

bioRxiv2025

Development of a Quantitative Systems Pharmacology Model to Interrogate Mitochondrial Metabolism in Heart Failure

The metabolic hallmarks of heart failure (HF) include diminished ATP hydrolysis potential and alterations in myocardial energy substrate metabolism, such as a switch in substrate utilization away from fatty acid (FA) to carbohydrate oxidation and reduced metabolic flexibility. However, the mechanisms underlying these phenomena and their potential contributions to impaired exercise tolerance are poorly understood. We developed a comprehensive quantitative systems pharmacology (QSP) model of mitochondrial metabolism to interrogate specific pathways hypothesized to contribute to reductions in reserve cardiac power output in heart failure. The aim of this work was to understand how changes in mitochondrial function and cardiac energetics associated with heart failure may affect exercise capacity. To accomplish this task, we coupled published in silico models of oxidative phosphorylation and the tricarboxylic acid cycle with a model of {beta}-oxidation and extended the model to incorporate an updated representation of the enzyme pyruvate dehydrogenase (PDH) to account for the role of PDH in substrate selection. We tested several hypotheses to determine how metabolic dysfunction, such as a decrease in PDH activity or altered mitochondrial volume, could lead to marked changes in energetic biomarkers, such as myocardial phosphocreatine-ATP ratio (PCr/ATP). The model predicts expected changes in fuel selection and also demonstrates PDH activity is responsible for substrate-dependent switch driven by feedback from NAD, NADH, ATP, ADP, CoASH, Acetyl-CoA and pyruvate in healthy and simulated HF conditions. Through simulations, we also found elevated malonyl-coA may contribute to lower PCr/ATP ratio during exercise conditions as observed in some HF patients. Key PointsO_LIExercise intolerance is a hallmark of heart failure in patients with preserved ejection fraction C_LIO_LIWe developed a quantitative systems pharmacology modeling approach with the potential to interrogate mitochondrial pathways hypothesized to contribute to exercise intolerance C_LIO_LIThe model was developed and evaluated based on simulating in vitro experimental data C_LIO_LISubstrate selection is an emergent property of the model with an increase in ATP demand resulting in a relative increase in the use of carbohydrates to fuel oxidative phosphorylation, an effect driven by feedback regulation of pyruvate dehydrogenase C_LIO_LIThe model was used to predict the potential effects of targeted perturbations to key mitochondrial pathways C_LI

systems biology↗

bioRxiv2025

Dynamic balance of myoplasmic energetics and redox state in a fast-twitch oxidative glycolytic skeletal muscle fiber

In order to investigate the mechanisms governing energy and redox balance in skeletal muscle, we developed a computational model describing the coupled biochemical reaction network of glycolysis and mitochondrial oxidative phosphorylation (OxPhos) in fast-twitch oxidative glycolytic (FOG) muscle fibers. The model was identified against dynamic in vivo recordings of Phosphocreatine (PCr), inorganic Phosphate (Pi), and pH in rodent hindlimb muscle and verified against independent data from in vivo experiments and muscle biopsies. Step response testing revealed that mass action kinetics in combination with feedback control were sufficient to accomplish myoplasmic ATP homeostasis over a 100-fold range of ATP turnover rates. This vital emergent property of the metabolic model was associated with dynamic behaviour of intermediary metabolite concentrations similar to a second-order underdamped system that remains to be verified. The simulations additionally predicted that the lactate dehydrogenase (LDH) reaction makes substantial contributions to redox balance across the physiological range of ATP demands in this myofiber phenotype, while its role in slowing cellular acidification is minimal. Yet, LDH knock-out simulations revealed that oxidative recycling of myoplasmic NADH in and by itself sufficed to maintain redox balance over ATP turnover rates in the range of mitochondrial ATP synthesis. We conclude that aerobic lactate production in working muscles is a byproduct of the metabolic flexibility of FOG myofibers afforded by expression of high levels of LDH and OxPhos enzymes to support continual myoplasmic redox balance and ATP synthesis under conditions of high-intensity mechanical work. In the future, the presented simulation framework may be used to further enhance the understanding of how experimental observations in muscle emerge from the integrative behaviour of the metabolic network for carbohydrate metabolism in FOG myofibers.

physiology↗

bioRxiv2025

Ca2+ increases cardiac muscle viscoelasticity independent of active force development

In addition to activation of muscle contraction by Ca2+, recent studies suggest that Ca2+ also affects muscle passive mechanical properties. The goal of this study was to determine if Ca2+ regulates the stiffness of cardiac muscle, independent of active contraction. The mechanical response to stretch for mouse demembranated cardiac trabeculae was probed at different Ca2+ levels after eliminating active contraction using a combination of two myosin ATPase inhibitors: para-nitroblebbistatin (PNB, 50 M), plus mavacampten (Mava, 50 M). Myocardial force level was assessed during large stretches ({asymp} 20% initial muscle length) with a range of stretch velocities. For relaxed muscle, in response to stretch, muscle force rose to a peak and then decayed toward a lower steady-state level, consistent with the viscoelastic nature of cardiac muscle. Peak force was higher with faster stretch velocity, but the steady-state force was independent of stretch velocity, consistent with the presence of both apparent viscous and elastic components of the stretch response. In the presence of the inhibitors PNB plus Mava, when Ca2+ level was increased, active contraction was completely prevented. However, the viscoelastic force response to stretch was markedly increased by high Ca2+ and was > 6-fold higher than at low Ca2+ level. The relationship of viscous force to Ca2+ level had a similar form to the relationship of active force to Ca2+ (measured in the absence of inhibitors), suggesting a common regulatory mechanism is involved. As expected, Ca2+-activated contraction was inhibited by lowering the temperature from 21{degrees}C to 10{degrees}C. In contrast, the Ca2+-activated viscous property was not inhibited at lower temperature, further suggesting that active contraction and the viscous property involve distinct mechanisms. This study demonstrates that in addition to triggering activation of contraction, Ca2+ also increases the apparent viscous property of cardiac muscle. New and NoteworthyCa2+ is well-known to trigger activation of muscle contraction. This study demonstrates a new mechanical role for Ca2+ in cardiac muscle involving a >6-fold increase in the apparent muscle viscoelasticity. Activation of a viscous element by Ca2+ might influence the mechanical properties of activated cardiac muscle.

biophysics↗

bioRxiv2025

Theoretical Analysis of Power-Law Stress Relaxation and Calcium-Dependent Passive Mechanics in Cardiac Muscle

This study investigates the passive viscoelastic mechanical properties of cardiac muscle by introducing a theoretical model that explains the power-law kinetics of passive stress decay. The model accounts for two parallel processes contributing to passive mechanics: an elastic component and a viscoelastic component designed to simulate stress/strain-mediated unfolding of serial domains in the titin molecule. Under stress, serial globular domains within the elastic region of the titin molecule reversibly unfold. This unfolding phenomenon contributes to both hysteresis (a lag in stress between loading and unloading) and preconditioning effects in simulated striated muscle mechanics. Moreover, experimental evidence indicates that stress relaxation in cardiac muscle follows a power law, and that the muscles nonlinear stress-strain relationship and hysteresis behavior are calcium-dependent. To analyze these mechanical phenomena, we simulate the apparent viscous element as a mesoscopic-scale ensemble of chains, each composed of serial globular domains that unfold in a stress-dependent manner. Although the model was developed to represent the behavior of titin, it equivalently represents any contributing process involving a linked series of domains that undergo stress-mediated unfolding. By providing a unified basis for the observed viscoelastic and preconditioning effects, calcium dependency, and power-law stress relaxation phenomena, this study offers a novel theoretical basis for understanding and simulating the role of titin in striated muscle mechanics. 1 Key pointsO_LIPassive stress relaxation of cardiac muscle follows a power-law decay, a phenomenon that is explained using a theoretical model of dynamic unfolding of globular domains along polymer chain. C_LIO_LIThe theoretical model simulates the behavior of titin, a giant sarcomere protein linking myosin thick filaments to the Z disk and providing passive restoring force during muscle stretch. C_LIO_LIThe theoretical model is able to account the observed effects of calcium on the effective viscoelastic passive mechanics of cardiac muscle. C_LIO_LIThis model provides a theoretical basis for understanding passive visocelastic properties and titins role in striated muscle mechanics. C_LI

biophysics↗

bioRxiv2024

Deficiency of mitophagy mediator Parkin in aortic smooth muscle cells exacerbates abdominal aortic aneurysm

Abdominal aortic aneurysms (AAAs) are a degenerative aortic disease and associated with hallmarks of aging, such as mitophagy. Despite this, the exact associations among mitophagy, aging, and AAA progression remain unknown. In our study, gene expression analysis of human AAA tissue revealed downregulation of mitophagy pathways, mitochondrial structure, and function-related proteins. Human proteomic analyses identified decreased levels of mitophagy mediators PINK1 and Parkin. Aged mice and, separately, a murine AAA model showed reduced mitophagy in aortic vascular smooth muscle cells (VSMCs) and PINK1 and Parkin expression. Parkin knockdown in VSMCs aggravated AAA dilation in murine models, with elevated mitochondrial ROS and impaired mitochondrial function. Importantly, inhibiting USP30, an antagonist of the PINK1/Parkin pathway, increased mitophagy in VSMCs, improved mitochondrial function, and reduced AAA incidence and growth. Our study elucidates a critical mechanism that proposes AAAs as an age-associated disease with altered mitophagy, introducing new potential therapeutic approaches.

molecular biology↗

bioRxiv2021

Multiscale model of the physiological control of myocardial perfusion to delineate putative metabolic feedback mechanisms

Coronary blood flow is tightly regulated to ensure that myocardial oxygen delivery meets local metabolic demand via the concurrent action of myogenic, neural, and metabolic mechanisms. While several competing hypotheses exist, the specific nature of the local metabolic mechanism(s) remains poorly defined. To gain insights into the viability of putative metabolic feedback mechanisms and into the coordinated action of parallel regulatory mechanisms, we applied a multi-scale modeling framework to analyze experimental data on coronary pressure, flow, and myocardial oxygen delivery in the porcine heart in vivo. The modeling framework integrates a previously established lumped-parameter model of myocardial perfusion used to account for transmural hemodynamic variations and a simple vessel mechanics model used to simulate the vascular tone in each of three myocardial layers. Vascular tone in the resistance vessel mechanics model is governed by input stimuli from the myogenic, metabolic, and autonomic control mechanisms. Seven competing formulations of the metabolic feedback mechanism are implemented in the modeling framework, and associated model simulations are compared to experimental data on coronary pressures and flows under a range of experimental conditions designed to interrogate the governing control mechanisms. Analysis identifies a maximally likely metabolic mechanism among the seven tested models, in which production of a metabolic signaling factor is proportional to MVO2 and delivery proportional to flow. Finally, the identified model is validated based on comparisons of simulations to data on the myocardial perfusion response to conscious exercise that were not used for model identification.

physiology↗

bioRxiv2021

FMO rewires metabolism to promote longevity throughtryptophan and one carbon metabolism

Flavin containing monooxygenases (FMOs) are promiscuous enzymes known for metabolizing a wide range of exogenous compounds. In C. elegans, fmo-2 expression increases lifespan and healthspan downstream of multiple longevity-promoting pathways through an unknown mechanism. Here, we report that, contrary to its classification as a xenobiotic enzyme, fmo-2 expression leads to rewiring of endogenous metabolism principally through changes in one carbon metabolism (OCM). Using computer modeling, we identify decreased methylation as the major OCM flux modified by FMO-2 that is sufficient to recapitulate its longevity benefits. We further find that tryptophan is decreased in multiple mammalian FMO overexpression models and is a validated substrate for FMO enzymes. Our resulting model connects a single enzyme to two previously unconnected key metabolic pathways and provides a framework for the metabolic interconnectivity of longevity-promoting pathways such as dietary restriction. FMOs are well-conserved enzymes that are also induced by lifespan-extending interventions in mice, supporting a conserved and critical role in promoting health and longevity through metabolic remodeling.

molecular biology↗

bioRxiv2021

Phenotyping heart failure using model-based analysis and physiology-informed machine learning

To determine the underlying mechanistic differences between diagnoses of Heart Failure (HF) and specifically heart failure with reduced and preserved ejection fraction (HFrEF & HFpEF), a closed loop model of the cardiovascular system coupled with patient specific transthoracic echocardiography (TTE) and right heart catheterization (RHC) measures was used to identify key parameters representing cardiovascular hemodynamics. Thirty-one patient records (10 HFrEF, 21 HFpEF) were obtained from the Cardiovascular Health Improvement Project (CHIP) database at the University of Michigan. Model simulations were tuned to match RHC and TTE pressure, volume and cardiac output measures in each patient with average error between data and model of 4.87 {+/-} 2%. The underlying physiological model parameters were then plotted against model-based norms and compared between the HFrEF and HFpEF group. Our results confirm that the main mechanistic parameter driving HFrEF is reduced left ventricular contractility, while for HFpEF a much wider underlying phenotype is presented. Conducting principal component analysis (PCA), k-means, and hierarchical clustering on the optimized model parameters, but not on clinical measures, shows a distinct group of HFpEF patients sharing characteristics with the HFrEF cohort, a second group that is distinct as HFpEF and a group that exhibits characteristics of both. Significant differences are observed (p-value<.001) in left ventricular active contractility and left ventricular relaxation, when comparing HFpEF patients to those grouped as similar to HFrEF. These results suggest that cardiovascular system modeling of standard clinical data is able to phenotype and group HFpEF as different subdiagnoses, possibly elucidating patient-specific treatment strategies.

physiology↗

bioRxiv2020

A Constrained Mixture Theory Model to Study Autoregulation in the Coronary Circulation

Coronary autoregulation is a short-term response manifested by a relatively constant flow over a wide range of perfusion pressures for a given metabolic state. This phenomenon is thought to be facilitated through a combination of mechanisms, including myogenic, shear dependent, and metabolic controls. The study of coronary autoregulation is challenging due to the coupled nature of the mechanisms and their differential effects through the coronary tree. In this paper, we developed a novel framework to study coronary autoregulation based on the constrained mixture theory. This structurally-motivated autoregulation model required calibration of anatomical and structural parameters of coronary trees via a homeostatic optimization approach using extensive literature data. Autoregulation was then simulated for two different coronary trees: subepicardial and subendocardial. The structurally calibrated model reproduced available baseline hemodynamics and autoregulation data for each coronary tree. The autoregulation analysis showed that the diameter of the intermediate and small arterioles varies the most in response to changes in perfusion pressure. Finally, we demonstrated the utility of the model in two application examples: 1) response to drops in epicardial pressure, and 2) response to drug infusion in the coronary arteries. The proposed structurally-motivated model could be extended to study long-term growth and remodeling in the coronary circulation in response to hypertension, atherosclerosis, etc. Key pointsO_LICoronary autoregulation is defined as the capability of the coronary circulation to maintain the blood supply to the heart over a range of perfusion pressures. This phenomenon is facilitated through intrinsic mechanisms that control the vascular resistance by regulating the mechanical function of smooth muscle cells. Understanding the mechanisms involved in coronary autoregulation is one of the most fundamental questions in coronary physiology. C_LIO_LIThis paper presents a structurally-motivated coronary autoregulation model that uses a nonlinear continuum mechanics approach to account for the morphometry and vessel wall composition in two coronary trees in the subepicardial and subendocardial layers. C_LIO_LIThe model is calibrated against diverse experimental data from literature and is used to study heterogeneous autoregulatory response in the coronary trees. This model drastically differs from previous models, which relied on lumped parameter model formulations, and is suited to the study of long-term pathophysiological growth and remodeling phenomena in coronary vessels. C_LI

physiology↗

bioRxiv2019

Cardiac Metabolic Limitations Contribute to Diminished Performance of the Heart in Aging

Changes in the myocardial energetics associated with aging--reductions in creatine phosphate (CrP)/ATP ratio, total creatine, and ATP--mirror changes observed in failing hearts compared to healthy controls. Similarly, both aging and heart failure are associated with significant reductions in cardiac performance and maximal left ventricular cardiac power output (CPO) compared to young healthy individuals. Based on these observations, we hypothesize that reductions in the concentrations cytoplasmic adenine nucleotide, creatine, and phosphate pools that occur with aging impair the myocardial capacity to synthesize ATP at physiological free energy levels, and that the resulting changes to myocardial energetic status impair the mechanical pumping ability of the heart. The purpose of this study is to test theses hypotheses using an age-structured population model for myocardial metabolism in the adult female population and to determine the potential impact of reductions in key myocardial metabolite pools in causing metabolic/energetic and cardiac mechanical dysfunction associated with aging. To test these hypotheses, we developed a population model for myocardial energetics to predict myocardial ATP, ADP, CrP, creatine, and inorganic phosphate concentrations as functions of cardiac work and age in the adult female population. Model predictions support our hypotheses and are consistent with previous experimental observations. The major findings provide a novel theoretical and computational framework for further probing complex relationships between the energetics and performance of the heart with aging. SignificanceNormal mechanical function of the heart requires that ATP be continuously synthesized at a hydrolysis potential of roughly -60 kJ mol-1. Yet in both the aging and diseased heart the relationships between cardiac work rate and concentrations of ATP, ADP, and inorganic phosphate are altered. Important outstanding questions are: To what extent do changes in metabolite concentrations that occur in aging and heart disease affect metabolic/molecular processes in the myocardium? How are systolic and diastolic functions affected by changes in metabolite concentrations? This study addresses these questions by analyzing relationships between cardiac energy demand and supply using an age-structured population model for human myocardial energetics in women.

physiology↗

bioRxiv2017

Control of Cardiac Mitochondrial Fuel Selection by Calcium

Calcium ion concentration modulates the function of pyruvate dehydrogenase, isocitrate dehydrogenase, and -ketoglutarate dehydrogenase. Previous studies have shown that despite its ability to affect the function of these dehydrogenases, [Ca2+] does not substantially alter mitochondrial ATP synthesis in vitro under physiological sub-strate conditions. We hypothesize that, rather than contributing to respiratory control, [Ca2+] governs fuel selection. Specifically, cardiac mitochondria are able to use different primary carbon substrates to synthesize ATP aerobically. To determine if and how [Ca2+] affects the relative use of carbohydrates versus fatty acids we measured oxygen consumption and tricarboxylic acid cycle intermediate concentrations in suspensions of cardiac mitochondria with different combinations of pyruvate and palmitoyl-L-carnitine in the media at various [Ca2+] and ADP infusion rates. Results reveal that when both fatty acid and carbohydrate substrates are available, fuel selection is sensitive to both calcium and ATP synthesis rate. When no Ca2+ is added under low ATP-demand conditions, {beta}-oxidation provides roughly half of acetyl-CoA for the citrate synthase reaction with the rest coming from the pyruvate dehydrogenase reaction. Under low demand conditions with increasing [Ca2+], the fuel utilization ratio shifts to increased fractional consumption of pyruvate, with 83{+/-}10% of acetyl-CoA derived from pyruvate at the highest [Ca2+] evaluated. With high ATP demand, the majority of acetyl-CoA is derived from pyruvate, regardless of the Ca2+ level. Our results suggest that changes in work rate alone are enough to effect a switch to carbohydrate use while in vivo the rate at which this switch happens may depend on mitochondrial calcium.\n\nKey PointsO_LIDespite its effects on activity of mitochondrial dehydrogenases, Ca2+ does not substantially alter mitochondrial ATP synthesis in vitro under physiological substrate conditions. Nor does is appear to play an important role in respiratory control in vivo in the myocardium.\nC_LIO_LIWe hypothesize that Ca2+ plays a role mediating the switch in fuel selection to increasing carbohydrate oxidation and decreasing fatty acid oxidation with increasing work rate.\nC_LIO_LITo determine if and how Ca2+ affects the relative use of carbohydrates versus fatty acids in vitro we measured oxygen consumption and TCA cycle intermediate concentrations in suspensions of purified rat ventricular mitochondria with carbohydrate, fatty acid, and mixed substrates at various [Ca2+] and ATP demand rates.\nC_LIO_LIOur results suggest that changes in work rate alone are enough to effect a switch to carbohydrate use in vitro while in vivo the rate at which this switch happens may depend on mitochondrial calcium.\nC_LI

biochemistry↗